Synovial tissue from sites of joint pain in knee osteoarthritis patients exhibits a differential phenotype with distinct fibroblast subsets

Mechanical stress overload promotes NF-κB/NLRP3-mediated osteoarthritis synovitis and fibrosis through Piezo1

Mechanical stress is a pivotal factor in the development of knee osteoarthritis (KOA). Piezo1, an innovative mechanosensitive ion channel, plays a key role in detecting variations in mechanical stress and transforming them into electrical signals. This research focuses on examining how Piezo1 influences synovial inflammation and fibrosis induced by mechanical stress in KOA, as well as delving into the potential underlying mechanisms. In vivo, pathological changes and immunohistochemical staining were conducted on both normal and overexercise rat synovial tissues to analyze the expression of Piezo1 and the NF-κB/NLRP3 pathways. In vitro utilized a cell stretcher to replicate the mechanical conditions seen in KOA. Levels of pro-inflammatory cytokines and fibrosis-related markers were assessed to investigate the impact of Piezo1 on mechanical stress in fibroblast-like synoviocytes (FLS). Subsequently, following cell stretching interventions, the effects on synovial inflammation and fibrosis were observed with the use of the Piezo1 inhibitor GsMTx4 or the NLRP3 inhibitor MCC950. Mechanical stress significantly promoted the activation of Piezo1, increased the phosphorylation ratio of p65, and elevated the levels of NLRP3, caspase-1, ASC, GSDMD, IL-1β, IL-18, IL-6, and TNF-α. Both in vitro and in vivo, mechanical stress also promoted the occurrence and development of synovial fibrosis, with significant increases in the expression levels of fibrosis-related markers. Under mechanical stress overload, upregulation of Piezo1 can promote the secretion of pro-inflammatory cytokines and the fibrotic process in synovium through the NF-κB/NLRP3 signaling pathway.

Association of Synovial Innate Immune Exhaustion With Worse Pain in Knee Osteoarthritis

OBJECTIVE

Uncontrolled pain remains a major clinical challenge in the management of knee osteoarthritis (OA), the most common disabling joint disease. Worse pain is associated with synovial innate immune cell infiltration (synovitis), but the role of innate immune-regulatory cells in pain is unknown. Our objective was to identify synovial innate immune cell subsets and pathophysiologic mechanisms associated with worse pain in patients with knee OA.

METHODS

Synovial tissue biopsies from 122 patients with mild-to-severe knee OA pain (Knee Injury and OA Outcome Score [KOOS]) were analyzed to identify associations between synovial histopathology and worse pain. We then used spatial transcriptomics and proteomics of synovial tissue microenvironments (n = 32), followed by single-cell RNA sequencing (n = 8), to identify synovial cell composition and cell-cell communication networks in patients with more severe OA pain.

RESULTS

Histopathological signs of synovial microvascular dysfunction and perivascular edema were associated with worse KOOS pain (-10.76; 95% confidence interval [CI] -18.90 to -2.61). Patients with worse pain had fewer immune-regulatory macrophages, expanded fibroblast subsets, and enrichment in neurovascular remodeling pathways. Synovial macrophages from patients with worse pain expressed markers of immune exhaustion and decreased phagocytic function (-19.42%; 95% CI -35.96 to -2.89) and their conditioned media increased neuronal cell stress in dorsal root ganglia.

CONCLUSION

Although synovitis increases during OA, our findings suggest that exhaustion, dysfunction, and loss of immune-regulatory macrophages is associated with worse pain and may be an important therapeutic target.

Constructing a cross-tissue human knee single-cell atlas identified osteoarthritis reduces regenerative tissue stem cells while increasing inflammatory pain macrophages

Osteoarthritis (OA) affects the entire knee joint; however, cross-tissue molecular mechanisms are poorly understood due to a lack of comprehensive, integrated analysis. We constructed the first comprehensive single-cell RNA sequencing knee OA atlas from articular cartilage, meniscus, synovium, and subchondral bone which showed active communication between them. Healthy synovium and meniscus contain the largest populations of tissue stem cells (TSCs) and immune cells that are altered in OA. The regenerative TSCs expressing SDF1, SOX9, CD146, PDGFRB, and CD105 decrease during OA, whereas osteogenic TSCs expressing osteogenic differentiation-related factor NT5E (CD73) are increased. In OA, the balance between regenerative and osteogenic TSCs shifts in the OA state with an increased number of osteogenic TSCs. We also report an increased level of quadruple-positive inflammatory (IL1B-IL6-NOS2-TNF) and pain marker (P2RX7) specific macrophages in OA. Fibroblasts are enriched in OA-synovium and may contribute to fibrosis. Importantly, OA cartilage contains unique MMP13-producing detrimental chondrocytes along with RUNX2-producing chondrocytes that worsen OA pathophysiology. This atlas provides a novel avenue for potential therapeutic applications in human knee OA and other musculoskeletal diseases and injuries, targeting synovium and meniscus to intervene in OA-specific molecular and cellular alterations.

Recent advances in omics and the integration of multi-omics in osteoarthritis research

Osteoarthritis (OA) is a complex disorder driven by the combination of environmental and genetic factors. Given its high global prevalence and heterogeneity, developing effective and personalized treatment methods is crucial. This requires identifying new disease mechanisms, drug targets, and biomarkers. Various omics approaches have been applied to identify OA-related genes, pathways, and biomarkers, including genomics, epigenomics, transcriptomics, proteomics, and metabolomics. These omics studies have generated vast datasets that are shaping the field of OA research. The emergence of high-resolution methodologies, such as single-cell and spatial omics techniques, further enhances our ability to dissect molecular complexities within the OA microenvironment. By integrating these multi-layered datasets, researchers can uncover central signaling hubs and disease mechanisms, ultimately facilitating the development of targeted therapies and precision medicine approaches for OA treatment.

Generation of cytotoxic aptamers specifically targeting fibroblast-like synoviocytes by CSCT-SELEX for treatment of rheumatoid arthritis

OBJECTIVES

Rheumatoid arthritis (RA) is an autoimmune disease characterised by aggressive fibroblast-like synoviocytes (FLSs). Very few RA patients-derived FLSs (RA-FLSs)-specific surface signatures have been identified, and there is currently no approved targeted therapy for RA-FLSs. This study aimed to screen therapeutic aptamers with cell-targeting and cytotoxic properties against RA-FLSs and to uncover the molecular targets and mechanism of action of the screened aptamers.

METHODS

We developed a cell-specific and cytotoxic systematic evolution of ligands by exponential enrichment (CSCT-SELEX) method to screen the therapeutic aptamers without prior knowledge of the surface signatures of RA-FLSs. The molecular targets and mechanisms of action of the screened aptamers were determined by pull-down assays and RNA sequencing. The therapeutic efficacy of the screened aptamers was examined in arthritic mouse models.

RESULTS

We obtained an aptamer SAPT8 that selectively recognised and killed RA-FLSs. The molecular target of SAPT8 was nucleolin (NCL), a shuttling protein overexpressed on the surface and involved in the tumor-like transformation of RA-FLSs. Mechanistically, SAPT8 interacted with the surface NCL and was internalised to achieve lysosomal degradation of NCL, leading to the upregulation of proapoptotic p53 and downregulation of antiapoptotic B-cell lymphoma 2 (Bcl-2) in RA-FLSs. When administrated systemically to arthritic mice, SAPT8 accumulated in the inflamed FLSs of joints. SAPT8 monotherapy or its combination with tumour necrosis factor (TNF)-targeted biologics was shown to relieve arthritis in mouse models.

CONCLUSIONS

CSCT-SELEX could be a promising strategy for developing cell-targeting and cytotoxic aptamers. SAPT8 aptamer selectively ablates RA-FLSs via modulating NCL-p53/Bcl-2 signalling, representing a potential alternative or complementary therapy for RA.

New mesenchymal stem/stromal cell-based strategies for osteoarthritis treatment: targeting macrophage-mediated inflammation to restore joint homeostasis

Macrophages are pivotal in osteoarthritis (OA) pathogenesis, as their dysregulated polarization can contribute to chronic inflammatory processes. This review explores the molecular and metabolic mechanisms that influence macrophage polarization and identifies potential strategies for OA treatment. Currently, non-surgical treatments for OA focus only on symptom management, and their efficacy is limited; thus, mesenchymal stem/stromal cells (MSCs) have gained attention for their anti-inflammatory and immunomodulatory capabilities. Emerging evidence suggests that small extracellular vesicles (sEVs) derived from MSCs can modulate macrophage function, thus offering potential therapeutic benefits in OA. Additionally, the transfer of mitochondria from MSCs to macrophages has shown promise in enhancing mitochondrial functionality and steering macrophages toward an anti-inflammatory M2-like phenotype. While further research is needed to confirm these findings, MSC-based strategies, including the use of sEVs and mitochondrial transfer, hold great promise for the treatment of OA and other chronic inflammatory diseases.

Machine learning analysis of FOSL2 and RHoBTB1 as central immunological regulators in knee osteoarthritis synovium

BackgroundKnee osteoarthritis is a debilitating disease with a complex pathogenesis. Synovitis, which refers to inflammation of the synovial membrane surrounding the joint, is believed to play an important role in the development and progression of knee osteoarthritis. To better understand the molecular mechanisms underlying knee osteoarthritis, we conducted a comprehensive analysis of gene expression in knee osteoarthritis synovium using machine learning.MethodsDifferentially expressed genes between knee osteoarthritis and control synovial tissues were analyzed using the GSE55235 dataset. We employed several machine learning algorithms, including least absolute shrinkage and selection operator and support vector machine-recursive feature elimination, to screen for key genes. Then, we validated the key genes using an external dataset (GSE51588) and an in vitro knee osteoarthritis animal model. CIBERSORT was used to compare immune cell infiltration levels between knee osteoarthritis and control synovial tissues and determine their relationship with the key genes. Finally, we performed a Connectivity Map analysis to screen for potential small-molecule compounds. Moreover, we conducted single-cell RNA sequencing analysis using knee joint tissues to annotate different subtypes of cells.ResultsA total of 930 differentially expressed genes were identified. Least absolute shrinkage and selection operator regression and support vector machine-recursive feature elimination identified FOSL2 and RHoBTB1 as key genes. The expression levels of both genes were further validated in the GSE51588 dataset as well as verified through an in vitro experiment involving a knee osteoarthritis mouse model. Multiple significant correlation pairs were found between the immune cell infiltration levels. We unveiled the genetic basis of knee osteoarthritis using genome-wide association study and specific signaling pathways through gene set enrichment analysis. The GeneCards database was used to obtain 3032 pathogenic genes associated with knee osteoarthritis, and we found that RHoBTB1 expression was significantly negatively correlated and FOSL2 expression was significantly positively correlated with interleukin-1β expression. We predicted several small-molecule compounds based on Connectivity Map analysis. Finally, single-cell RNA sequencing analysis revealed the expression levels of the two key genes in chondrocytes and tissue stem cells.ConclusionFOSL2 and RHoBTB1 may play key roles in the pathogenesis of knee osteoarthritis, exhibiting correlations with immune cell infiltration levels. These findings indicate that these genes have potential as therapeutic targets. However, further research and validation are necessary to confirm their exact roles and therapeutic potential in knee osteoarthritis.

Metabolic Reprogramming in Stromal and Immune Cells in Rheumatoid Arthritis and Osteoarthritis: Therapeutic Possibilities

Metabolic reprogramming of stromal cells, including fibroblast-like synoviocytes (FLS) and chondrocytes, as well as osteoclasts (OCs), are involved in the inflammatory and degenerative processes underlying rheumatoid arthritis (RA) and osteoarthritis (OA). In RA, FLS exhibit mTOR activation, enhanced glycolysis and reduced oxidative phosphorylation, fuelling inflammation, angiogenesis, and cartilage degradation. In OA, chondrocytes undergo metabolic rewiring, characterised by mTOR and NF-κB activation, mitochondrial dysfunction, and increased glycolysis, which promotes matrix metalloproteinase production, extracellular matrix (ECM) degradation, and angiogenesis. Macrophage-derived immunometabolites, including succinate and itaconate further modulate stromal cell function, acting as signalling molecules that modulate inflammatory and catabolic processes. Succinate promotes inflammation whilst itaconate is anti-inflammatory, suppressing inflammatory joint disease in models. Itaconate deficiency also correlates inversely with disease severity in RA in humans. Emerging evidence highlights the potential of targeting metabolic processes as promising therapeutic strategies for connective tissue disorders.

Advances in Extracellular Matrix-Associated Diagnostics and Therapeutics

The extracellular matrix (ECM) is the common denominator of more than 50 chronic diseases. Some of these chronic pathologies lead to enhanced tissue formation and deposition, whereas others are associated with increased tissue degradation, and some exhibit a combination of both, leading to severe tissue alterations. To develop effective therapies for diseases affecting the lung, liver, kidney, skin, intestine, musculoskeletal system, heart, and solid tumors, we need to modulate the ECM's composition to restore its organization and function. Across diverse organ diseases, there are common denominators and distinguishing factors in this fibroinflammatory axis, which may be used to foster new insights into drug development across disease indications. The 2nd Extracellular Matrix Pharmacology Congress took place in Copenhagen, Denmark, from 17 to 19 June 2024 and was hosted by the International Society of Extracellular Matrix Pharmacology. The event was attended by 450 participants from 35 countries, among whom were prominent scientists who brought together state-of-the-art research on organ diseases and asked important questions to facilitate drug development. We highlight key aspects of the ECM in the liver, kidney, skin, intestine, musculoskeletal system, lungs, and solid tumors to advance our understanding of the ECM and its central targets in drug development. We also highlight key advances in the tools and technology that enable this drug development, thereby supporting the ECM.

Association Between Synovial NTN4 Expression and Pain Scores, and Its Effects on Fibroblasts and Sensory Neurons in End-Stage Knee Osteoarthritis

Osteoarthritis (OA) is a chronic joint disease marked by synovial inflammation, cartilage degradation, and persistent pain. Although Netrin-4 (NTN4) has been implicated in pain modulation in rheumatoid arthritis (RA), its role in OA pain remains less understood. Previous research has documented that NTN4 promotes axonal growth in rodent-derived neurons; however, its effects on human sensory neurons are yet to be fully explored. NTN4 also plays a multifactorial role in various non-neuronal cells, such as endothelial cells, tumor cells, and stromal cells. Nevertheless, its specific impact on synovial fibroblasts, which are key components of the synovium and have been linked to OA pain, is still unclear. This study examined the correlation between NTN4 expression levels and pain severity in OA, specifically investigating its effects on human iPSC-derived sensory neurons (iPSC-SNs) and synovial fibroblasts from OA patients. Our findings indicate a positive correlation between synovial NTN4 expression and pain severity. Recombinant human Netrin-4 (rh-NTN4) was also shown to enhance neurite outgrowth in human iPSC-SNs, suggesting a potential role in neuronal sensitization. Additionally, rh-NTN4 stimulated the production of pro-inflammatory cytokines (IL-6, IL-8) and chemokines (CXCL1, CXCL6, CXCL8) in synovium-derived fibroblastic cells, implicating it in synovial inflammation. Collectively, these results suggest that NTN4 may contribute to KOA pathology by promoting synovial inflammation and potentially sensitizing sensory neurons, thereby influencing the mechanisms of underlying pain.

Cell culture expansion media choice affects secretory, protective and immuno-modulatory features of adipose mesenchymal stromal cell-derived secretomes for orthopaedic applications

Introduction

Mesenchymal stromal cells (MSCs) gained attention for their anti-inflammatory and trophic properties, with musculoskeletal diseases and osteoarthritis (OA) being among the most studied conditions. Alongside cells, their released factors and extracellular vesicles (EVs), overall termed "secretome", are actively sifted being envisioned as the main therapeutic actors. In addition to standard supplementation given by foetal bovine serum (FBS) or human platelet lysate (hPL), new good manufacturing practice (GMP)-compliant serum/xeno (S/X)-free media formulations have been proposed, although their influence on MSCs phenotype and potential is scarcely described. The aim of this study is therefore to evaluate, in the OA context, the differences in secretome composition and potential after adipose-MSCs (ASCs) cultivation in both standard (FBS and hPL) and two next generation (S/X) GMP-ready supplements.

Methods

Immunophenotype and secretory ability at soluble protein and EV-related levels, including embedded miRNAs, were analysed in the secretomes by means of flow cytometry, nanoparticle tracking analysis, high throughput ELISA and qRT-PCR arrays. Secretomes effect was tested in in vitro models of chondrocytes, lymphocytes and monocytes to mimic the OA microenvironment.

Results

Within a conserved molecular signature, a divergent fingerprint emerged for ASCs' secretomes collected after expansion in standard FBS/hPL or next-generation S/X formulations. Regarding soluble factors, a less protective feature for those in the secretome collected after ASCs were cultured in S/X media emerged. Moreover, the overall message for EV-miRNAs was characterized by a preponderance of protective signals in FBS and hPL conditions in a context of general safeguard given by ASCs released molecules. This dichotomy was reflected on secretomes' potential in vitro, with expansion in hPL resulting in the most effective secretome for chondrocytes and in FBS for immune cells.

Conclusions

These data open the question about the implications from using new media for MSCs expansion for clinical application. Although the undeniable advantages for GMP compliant processes, this study results suggest that new media formulations would deserve a deep characterization to drive the choice of the most effective one tailored to each specific application.

Osteoarthritis

Osteoarthritis is a heterogeneous whole-joint disease that can cause pain and is a leading cause of disability and premature work loss. The predominant disease risk factors - obesity and joint injury - are well recognized and modifiable. A greater understanding of the complex mechanisms, including inflammatory, metabolic and post-traumatic processes, that can lead to disease and of the pathophysiology of pain is helping to delineate mechanistic targets. Currently, management is primarily focused on alleviating the main symptoms of pain and obstructed function through lifestyle interventions such as self-management programmes, education, physical activity, exercise and weight management. However, lack of adherence to known effective osteoarthritis therapeutic strategies also contributes to the high global disease burden. For those who have persistent symptoms that are compromising quality of life and have not responded adequately to core treatments, joint replacement is an option to consider. The burden imparted by the disease causes a substantial impact on individuals affected in terms of quality of life. For society, this disease is a substantial driver of increased health-care costs and underemployment. This Primer highlights advances and controversies in osteoarthritis, drawing key insights from the current evidence base.

Pioglitazone Regulates Chondrocyte Metabolism and Attenuates Osteoarthritis by Activating Peroxisome Proliferator-Activated Receptor Gamma

Osteoarthritis presents a significant clinical challenge due to its high prevalence and the resultant impairment of patients' motor function. Osteoarthritic chondrocytes are characterised by inflammation and metabolic disturbances. Pioglitazone, an agonist of peroxisome proliferator-activated receptor γ (PPAR-γ), has been demonstrated to exert anti-inflammatory effects across various diseases. This study aims to investigate the potential protective effects of Pioglitazone on osteoarthritic chondrocytes. An in vitro chondrocyte inflammation model was established utilising IL-1β. The impact of Pioglitazone on chondrocyte inflammation and extracellular matrix synthesis was evaluated through enzyme-linked immunosorbent assay, immunofluorescence staining and Alcian blue staining. The affinity of Pioglitazone for PPAR-γ was investigated using molecular docking techniques. Alterations in chondrocyte glycolysis and oxidative phosphorylation were examined using the Seahorse XF Analyser, and the influence of Pioglitazone on glucose uptake and the mitochondrial electron transport chain was further analysed. Pioglitazone was gavaged in a mouse OA model established by anterior cruciate ligament transection to evaluate the therapeutic efficacy of Pioglitazone. Our findings indicate that Pioglitazone mitigates chondrocyte inflammation and osteoarthritis in murine models by inhibiting the expression of inflammatory mediators such as TNF-α, IL-6 and PGE2, and by preventing the degradation of aggrecan and collagen II. Furthermore, Pioglitazone significantly upregulated the expression of glucose transporter 1 and stabilised the mitochondrial proton delivery chain in a PPAR-γ-dependent manner, thereby enhancing chondrocyte glucose uptake, glycolysis, and oxidative phosphorylation. These effects were partially reversed by the PPAR-γ antagonist GW9662. Pioglitazone can confer chondroprotective benefits in osteoarthritis by activating PPAR-γ.

Complement 3 and 4 impact in osteoarthritis

Aim: To investigate the association between serum complement 3 and 4 (C3 and C4) levels and clinical characteristics in osteoarthritis (OA) patients.Methods: This retrospective study included 361 OA patients divided into groups based on complement levels. Clinical data and laboratory test results, including C3, C4, ESR and CRP levels, were analyzed using non-parametric tests, Spearman correlation and multivariate regression.Results: The AC-OA group had lower PA, C3, C4, ESR, CRP and IgA levels compared with controls. C3 levels were positively correlated with ESR (r = 0.260, p < 0.001) and CRP (r = 0.243, p < 0.001). C4 levels also correlated with ESR (r = 0.175, p = 0.001) and CRP (r = 0.263, p < 0.001) and were significantly associated with the number of affected joints.Conclusion: Serum C4 levels are effective indicators of disease activity in OA, particularly in terms of joint involvement and CRP levels, while C3 levels showed no significant association. These findings suggest a potential role for C4 in predicting OA activity.

CD34hi subset of synovial fibroblasts contributes to fibrotic phenotype of human knee osteoarthritis

Osteoarthritis (OA) shows various clinical manifestations depending on the status of its joint components. We aimed to identify the synovial cell subsets responsible for OA pathophysiology by comprehensive analyses of human synovium samples in single-cell resolution. Two distinct OA synovial tissue groups were classified by gene expression profiles in RNA-Seq: inflammatory and fibrotic. The inflammatory group exhibited high expression of inflammatory cytokines, histologically inflammatory infiltrate, and a more severe pain score. The fibrotic group showed higher expression of fibroblast growth factor (FGFs) and bone morphogenetic proteins (BMPs), showed histologically perivascular fibrosis, and showed a lower pain score. In single-cell RNA-Seq (scRNA-Seq) of synovial cells, MERTKloCD206lo macrophages and CD34hi fibroblasts were associated with the inflammatory and fibrotic groups, respectively. Among the 3 fibroblast subsets, CD34loTHY1lo and CD34loTHY1hi fibroblasts were influenced by synovial immune cells, whereas CD34hi fibroblasts were influenced by mural and endothelial cells. Particularly, in CD34hi fibroblast subsets, CD34hiCD70hi fibroblasts promoted proliferation of Tregs, potentially suppressing synovitis and protecting articular cartilage. Elucidation of the mechanisms underlying the regulation of these synovial cell subsets may lead to novel strategies for OA therapeutics.

Effect of Antiseptic Irrigation Solutions on Primary Human Knee Fibroblasts Cultured in Human Platelet Lysate

BACKGROUND

Native and periprosthetic joint infections are devastating conditions fraught with patient morbidity and mortality. Aseptic and septic joints are often debrided and irrigated to decrease bacterial loads when preventing or treating infection. However, the effect of clinically used irrigation solutions on the native cellular components of the synovial joint is unknown.

METHODS

Patients consented, and their suprapatellar knee tissue was surgically excised for fibroblast isolation. Cultured knee fibroblasts were treated with normal saline for three minutes or one of the following experimental solutions: acetic acid, chlorhexidine-gluconate, Dakin's solution, hydrogen peroxide, or povidone-iodine. The exposure time for the antiseptic solutions was one and three minutes. At 24 hours after irrigation treatment, metabolic activity was measured via MTS [3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium] activity assay, and deoxyribonucleic acid content was assessed by Hoechst staining as a surrogate for cell number. Phase-contrast imaging elucidated proliferation potential, progressive cell loss, and cell morphology over a 5-day period. All experiments were repeated in triplicate.

RESULTS

All experimental antiseptic irrigation solutions, regardless of application time, caused a significant reduction in metabolic activity and DNA content, indicating extensive cell death. Phase-contrast imaging showed halted cellular proliferation, progressive cell loss, and distinct changes in cellular morphology, indicating decreased cellular viability and progressive cell death.

CONCLUSIONS

All antiseptic irrigation solutions investigated in this study were severely cytotoxic to human knee fibroblasts regardless of their chemical composition. The concentrations of these solutions are commonly used in orthopaedic surgery. Although these solutions have high bactericidal properties, it may be beneficial to use them in combination at lower doses to retain their effect on bacteria while remaining benign to native synovial cells.

Symptoms and joint degeneration correlate with the temperature of osteoarthritic knees: an infrared thermography analysis

PURPOSE

This study aim was to analyze the joint temperature of patients affected by bilateral knee osteoarthritis (OA) using infrared thermography to investigate whether thermographic imaging patterns are influenced by the severity of symptoms and joint degeneration.

METHODS

Sixty-sixpatients ranging from 43 to 78 years old (63.3 ± 8.8 years) with bilateral knee OA and one symptomatic knee were enrolled. Thermograms of the two knees were captured using a thermographic camera FLIR T1020 and analyzed with the ResearchIR software to calculate the temperature of the overall knee and the four regions of interest (ROIs): patella, suprapatellar, medial, and lateral areas.

RESULTS

The temperature of knees affected by OA was influenced by joint degeneration level and symptoms: patients with higher OA grade in the symptomatic knees presented higher total knee temperatures compared to the asymptomatic ones (p = 0.002), as well as in the patellar (p = 0.005), lateral (p = 0.002), and medial (p = 0.001) areas. On the other hand, patients with the same OA level in the two knees presented a higher temperature in the symptomatic knee only in the medial area (p = 0.037). Symptomatic knees demonstrated a different pattern compared to asymptomatic knees, with the medial area presenting the highest temperature changes (p = 0.020). Patients reporting prevalent pain in the lateral knee area presented higher differences in total knee temperature (0.7 ± 0.7 °C) than patients with pain in the medial area (0.1 ± 0.5 °C) (p = 0.023).

CONCLUSION

The temperature of knees affected by OA is influenced by the degree of joint degeneration and by the presence of symptoms, with higher temperatures found in symptomatic joints, especially with prevalent lateral knee pain, and in more severe OA. Symptomatic knees demonstrated a different pattern compared to asymptomatic knees, with the medial area presenting the highest temperature changes.

Activation of the proton-sensing GPCR, GPR65 on fibroblast-like synoviocytes contributes to inflammatory joint pain

Inflammation is associated with localized acidosis, however, attributing physiological and pathological roles to proton-sensitive receptors is challenging due to their diversity and widespread expression. Here, agonists of the proton-sensing GPCR, GPR65, were systematically characterized. The synthetic agonist BTB09089 (BTB) recapitulated many proton-induced signaling events and demonstrated selectivity for GPR65. BTB was used to show that GPR65 activation on fibroblast-like synoviocytes (FLS), cells that line synovial joints, results in the secretion of proinflammatory mediators capable of recruiting immune cells and sensitizing sensory neurons. Intra-articular injection of BTB resulted in GPR65-dependent sensitization of knee-innervating neurons and nocifensive behaviors in mice. Stimulation of GPR65 on human FLS also triggered the release of inflammatory mediators and synovial fluid samples from human osteoarthritis patients were shown to activate GPR65. These results suggest a role of GPR65 in mediating cell-cell interactions that drive inflammatory joint pain in both mice and humans.

Single-cell profiling uncovers synovial fibroblast subpopulations associated with chondrocyte injury in osteoarthritis

Background

Chondrocytes and synovial cells participate in the pathogenesis of osteoarthritis (OA). Nonetheless, the interactions and correlations between OA synovial cells and chondrocytes remain unclear. This study aims to elucidate the interactions and correlations between OA synovial cells and chondrocytes, so as to deepen understanding of OA pathogenesis.

Methods

Single-cell sequencing analysis was employed to analyze clusters of synovial and chondrocyte cells within the OA dataset. Through cell interaction analysis, the potential interactions between these two cell types were further explored. Differential gene expression analysis was used to examine the differences among synovial-related cell clusters.

Results

The study identified specific characteristics of synovial fibroblasts through single-cell sequencing analysis. Subsequent cell interaction analysis revealed interactions and correlations between synovial fibroblast clusters and cell clusters in both damaged and non-damaged cartilages. CILP+ fibroblasts showed significant interactions with non-damaged chondrocytes, while POSTN+ fibroblasts exhibited significant interactions with damaged chondrocytes. Furthermore, differential gene expression analysis revealed that genes such as PRELP, CLU, COMP, TNFRSF12A, INHBA, CILP, and SERPINE2, were significantly upregulated in CILP+ fibroblasts. These genes are involved in promoting cell proliferation, inhibiting inflammatory pathways, and stabilizing cell structure, thereby exerting reparative and protective effects on chondrocytes. In contrast, COL6A3, COL6A1, COL1A2, COL1A1, COL3A1, TGF-β1, MMP2, AEBP1, SPARC, FNDC1, and POSTN were upregulated in POSTN+ fibroblasts. These genes may contribute to chondrocyte damage and further degeneration by promoting chondrocyte catabolism, driving inflammation, activating inflammatory pathways, and facilitating chondrocyte apoptosis and destruction.

Conclusion

Our study elucidated the interactions and correlations between OA synovial cells and chondrocytes. CILP+ synovial fibroblasts may exert reparative and protective effects on chondrocytes of patients with OA by promoting cell proliferation, inhibiting inflammation, and stabilizing cellular structures, thereby potentially mitigating the progression of cartilage lesions in affected patients. In contrast, POSTN+ synovial fibroblasts may exacerbate chondrocyte deterioration in patients with OA by enhancing degradation, inflammation, and apoptosis, thereby exacerbating cartilage lesions. Investigating the underlying molecular mechanisms between OA synovial cells and chondrocytes refines the understanding of OA pathogenesis and provides valuable insights for the clinical diagnosis and treatment of OA.

Obesity-induced fibrosis in osteoarthritis: Pathogenesis, consequences and novel therapeutic opportunities

Osteoarthritis (OA) is a significant global burden, affecting more than half a billion people across the world. It is characterized by degeneration and loss of articular cartilage, synovial inflammation, and subchondral bone sclerosis, leading to pain and functional impairment. After age, obesity is a major modifiable risk factor for OA, and it has recently been identified as a chronic disease by the World Health Organization (WHO). Obesity is associated with high morbidity and mortality, imposing a significant cost on individuals and society. Obesity increases the risk of knee OA through increased joint loading, altered body composition, and elevated pro-inflammatory adipokines in the systemic circulation. Moreover, obesity triggers fibrotic processes in different organs and tissues, including those involved in OA. Fibrosis in OA refers to the abnormal accumulation of fibrous tissue within and around the joints. It can be driven by increased adiposity, low-grade inflammation, oxidative stress, and metabolic alterations. However, the clinical outcomes of fibrosis in OA are unclear. This review focuses on the link between obesity and OA, explores the mechanism of obesity-driven fibrosis, and examines potential therapeutic opportunities for targeting fibrotic processes in OA.

BDNF sensitizes bone and joint afferent neurons at different stages of MIA-induced osteoarthritis

There is emerging evidence that Brain Derived Neurotrophic Factor (BDNF), and one of its receptors TrkB, play important roles in the pathogenesis of osteoarthritis (OA) pain. Whilst these studies clearly highlight the potential for targeting BDNF/TrkB signaling to treat OA pain, the mechanism for how BDNF/TrkB signaling contributes to OA pain remains unclear. In this study, we used an animal model of mono-iodoacetate (MIA)-induced OA, in combination with electrophysiology, behavioral testing, Western blot analysis, and retrograde tracing and immunohistochemistry, to identify roles for BDNF/TrkB signaling in the pathogenesis of OA pain. We found that: 1) TrkB is expressed in myelinated medium diameter neurons that innervate the knee joint and bone in naïve animals; 2) peripheral application of BDNF increases the sensitivity of Aδ, but not C knee joint and bone afferent neurons, in response to mechanical stimulation, in naïve animals; 3) BDNF expression increases in synovial tissue in early MIA-induced OA, when pathology is confined to the joint, and in the subchondral bone in late MIA-induced OA, when there is additional damage to the surrounding bone; and 4) TrkB inhibition reverses MIA-induced changes in the sensitivity of Aδ but not C knee joint afferent neurons early in MIA-induced OA, and Aδ but not C bone afferent neurons late in MIA-induced OA. Our findings suggest that BDNF/TrkB signaling may have a role to play in the pathogenesis of OA pain, through effects on knee joint afferent neurons early in disease when there is inflammation confined to the joint, and bone afferent neurons late in disease when there is involvement of damage to subchondral bone. Targeted manipulation of BDNF/TrkB signaling may provide therapeutic benefit for the management of OA pain.

Immunomodulation and fibroblast dynamics driving nociceptive joint pain within inflammatory synovium: Unravelling mechanisms for therapeutic advancements in osteoarthritis

OBJECTIVE

Synovitis is a widely accepted sign of osteoarthritis (OA), characterised by tissue hyperplasia, where increased infiltration of immune cells and proliferation of resident fibroblasts adopt a pro-inflammatory phenotype, and increased the production of pro-inflammatory mediators that are capable of sensitising and activating sensory nociceptors, which innervate the joint tissues. As such, it is important to understand the cellular composition of synovium and their involvement in pain sensitisation to better inform the development of effective analgesics.

METHODS

Studies investigating pain sensitisation in OA with a focus on immune cells and fibroblasts were identified using PubMed, Web of Science and SCOPUS.

RESULTS

In this review, we comprehensively assess the evidence that cellular crosstalk between resident immune cells or synovial fibroblasts with joint nociceptors in inflamed OA synovium contributes to peripheral pain sensitisation. Moreover, we explore whether the elucidation of common mechanisms identified in similar joint conditions may inform the development of more effective analgesics specifically targeting OA joint pain.

CONCLUSION

The concept of local environment and cellular crosstalk within the inflammatory synovium as a driver of nociceptive joint pain presents a compelling opportunity for future research and therapeutic advancements.

Trained immunity of synovial macrophages is associated with exacerbated joint inflammation and damage after Staphylococcus aureus infection

OBJECTIVES

Investigate whether and which synoviocytes would acquire trained immunity characteristics that could exacerbate joint inflammation following a secondary Staphylococcus aureus infection.

METHODS

Lipopolysaccharide (LPS) and S. aureus were separately or double injected (21 days of interval) into the tibiofemoral joint cavity of male C57BL/6 mice. At different time points after these stimulations, mechanical nociception was analyzed followed by the analysis of signs of inflammation and damage in the affected joints. The trained immunity markers, including the glycolytic and mTOR pathway, were analyzed in whole tissue or isolated synoviocytes. A group of mice was treated with Rapamycin, an mTOR inhibitor before LPS or S. aureus stimulation.

RESULTS

The double LPS - S. aureus hit promoted intense joint inflammation and damage compared to single joint stimulation, including markers in synoviocyte activation, production of proinflammatory cytokines, persistent nociception, and bone damage, despite not reducing the bacterial clearance. The double LPS - S. aureus hit joints increased the synovial macrophage population expressing CX3CR1 alongside triggering established epigenetic modifications associated with trained immunity events in these cells, such as the upregulation of the mTOR signaling pathway (p-mTOR and HIF1α) and the trimethylation of histone H3. Mice treated with Rapamycin presented reduced CX3CR1+ macrophage activation, joint inflammation, and bone damage.

CONCLUSIONS

There is a trained immunity phenotype in CX3CR1+ synovial macrophages that contributes to the exacerbation of joint inflammation and damage during septic arthritis caused by S. aureus.

Sexual dimorphism of the synovial transcriptome underpins greater PTOA disease severity in male mice following joint injury

OBJECTIVE

Osteoarthritis (OA) is a disease with sex-dependent prevalence and severity in both human and animal models. We sought to elucidate sex differences in synovitis, mechanical sensitization, structural damage, bone remodeling, and the synovial transcriptome in the anterior cruciate ligament rupture (ACLR) mouse model of post-traumatic OA (PTOA).

DESIGN

Male and female 12-week-old C57/BL6J mice were randomized to Sham or noninvasive ACLR with harvests at 7d or 28d post-ACLR (n = 9 per sex in each group - Sham, 7d ACLR, 28d ACLR). Knee hyperalgesia, mechanical allodynia, and intra-articular matrix metalloproteinase (MMP) activity (via intravital imaging) were measured longitudinally. Trabecular and subchondral bone (SCB) remodeling and osteophyte formation were assessed by µCT. Histological scoring of PTOA, synovitis, and anti-MMP13 immunostaining were performed. NaV1.8-Cre;tdTomato mice were used to document localization and sprouting of nociceptors. Bulk RNA-seq of synovium in Sham, 7d, and 28d post-ACLR, and contralateral joints (n = 6 per group per sex) assessed injury-induced and sex-dependent gene expression.

RESULTS

Male mice exhibited more severe joint damage at 7d and 28d and more severe synovitis at 28d, accompanied by 19% greater MMP activity, 8% lower knee hyperalgesia threshold, and 43% lower hindpaw withdrawal threshold in injured limbs compared to female injured limbs. Females had injury-induced catabolic responses in trabecular and SCB, whereas males exhibited 133% greater normalized osteophyte volume relative to females and sclerotic remodeling of trabecular and SCB. NaV1.8+ nociceptor sprouting in SCB and medial synovium was induced by injury and comparable between sexes. RNA-seq of synovium demonstrated similar injury-induced transcriptomic programs between the sexes at 7d, but only female mice exhibited a transcriptomic signature indicative of synovial inflammatory resolution by 28d, whereas males had persistent pro-inflammatory, pro-fibrotic, pro-neurogenic, and pro-angiogenic gene expression.

CONCLUSION

Male mice exhibited more severe overall joint damage and pain behavior after ACLR, which was associated with persistent activation of synovial inflammatory, fibrotic, and neuroangiogenic processes, implicating persistent synovitis in driving sex differences in murine PTOA.

CRISPR-mediated Sox9 activation and RelA inhibition enhance cell therapy for osteoarthritis

Osteoarthritis (OA) is a painful and debilitating disease affecting over 500 million people worldwide. Intraarticular injection of mesenchymal stromal cells (MSCs) shows promise for the clinical treatment of OA, but the lack of consistency in MSC preparation and application makes it difficult to further optimize MSC therapy and to properly evaluate the clinical outcomes. In this study, we used Sox9 activation and RelA inhibition, both mediated by the CRISPR-dCas9 technology simultaneously, to engineer MSCs with enhanced chondrogenic potential and downregulated inflammatory responses. We found that both Sox9 and RelA could be fine-tuned to the desired levels, which enhances the chondrogenic and immunomodulatory potentials of the cells. Intraarticular injection of modified cells significantly attenuated cartilage degradation and palliated OA pain compared with the injection of cell culture medium or unmodified cells. Mechanistically, the modified cells promoted the expression of factors beneficial to cartilage integrity, inhibited the production of catabolic enzymes in osteoarthritic joints, and suppressed immune cells. Interestingly, a substantial number of modified cells could survive in the cartilaginous tissues including articular cartilage and meniscus. Together, our results suggest that CRISPR-dCas9-based gene regulation is useful for optimizing MSC therapy for OA.

The Genetics and Functional Genomics of Osteoarthritis

Osteoarthritis is the most prevalent whole-joint degenerative disorder, and is characterized by the degradation of articular cartilage and the underlying bone structures. Almost 600 million people are affected by osteoarthritis worldwide. No curative treatments are available, and management strategies focus mostly on pain relief. Here, we provide a comprehensive overview of the available human genetic and functional genomics studies for osteoarthritis to date and delineate how these studies have helped shed light on disease etiopathology. We highlight genetic discoveries from genome-wide association studies and provide a detailed overview of molecular-level investigations in osteoarthritis tissues, including methylation-, transcriptomics-, and proteomics-level analyses. We review how functional genomics data from different molecular levels have helped to prioritize effector genes that can be used as drug targets or drug-repurposing opportunities. Finally, we discuss future directions with the potential to drive a step change in osteoarthritis research.

Research progress on nanotechnology of traditional Chinese medicine to enhance the therapeutic effect of osteoarthritis

Osteoarthritis (OA) is a prevalent chronic condition that primarily impacts the articular cartilage and surrounding bone tissue, resulting in joint inflammation and structural deterioration. The etiology of OA is multifaceted and intricately linked to the oxidative stress response of joint tissue. Oxidative stress (OS) in OA leads to the creation of reactive oxygen species (ROS) and other oxidizing agents, resulting in detrimental effects on chondrocytes. This oxidative damage diminishes the flexibility and robustness of cartilage, thereby expediting the progression of joint deterioration. Therefore, the antioxidant effect is crucial in the treatment of OA. Currently, a considerable number of components found in traditional Chinese medicine (TCM) have been scientifically demonstrated to exhibit remarkable antioxidant and anti-inflammatory properties. Nevertheless, the utilization of this program is considerably constrained as a result of intrinsic deficiencies, notably stability concerns. The successful amalgamation of TCM components with nanotechnology has properly tackled these concerns and enhanced the efficacy of therapeutic results. The objective of this study is to delineate the antioxidant characteristics of nano-TCM and assess the current inventory of literature pertaining to the application of nano-TCM in the treatment of OA. In conclusion, this paper will now turn to the constraints and potential avenues for the advancement of nano-TCM within the realm of OA therapy.

Current evidence on the role of fibroblasts in large-vessel vasculitides: From pathogenesis to therapeutics

Large-vessel vasculitides (LVV) comprise a group of chronic inflammatory diseases of the aorta and its major branches. The most common forms of LVV are giant cell arteritis (GCA) and Takayasu arteritis (TAK). Both GCA and TAK are characterized by granulomatous inflammation of the vessel wall accompanied by a maladaptive immune and vascular response that promotes vascular damage and remodeling. The inflammatory process in LVV starts in the adventitia where fibroblasts constitute the dominant cell population. Fibroblasts are traditionally recognized for synthesizing and renewing the extracellular matrix thereby being major players in maintenance of normal tissue architecture and in tissue repair. More recently, fibroblasts have emerged as a highly plastic cell population exerting various functions, including the regulation of local immune processes and organization of immune cells at the site of inflammation through production of cytokines, chemokines and growth factors as well as cell-cell interaction. In this review, we summarize and discuss the current knowledge on fibroblasts in LVV. Furthermore, we identify key questions that need to be addressed to fully understand the role of fibroblasts in the pathogenesis of LVV.

Advancing skeletal health and disease research with single-cell RNA sequencing

Orthopedic conditions have emerged as global health concerns, impacting approximately 1.7 billion individuals worldwide. However, the limited understanding of the underlying pathological processes at the cellular and molecular level has hindered the development of comprehensive treatment options for these disorders. The advent of single-cell RNA sequencing (scRNA-seq) technology has revolutionized biomedical research by enabling detailed examination of cellular and molecular diversity. Nevertheless, investigating mechanisms at the single-cell level in highly mineralized skeletal tissue poses technical challenges. In this comprehensive review, we present a streamlined approach to obtaining high-quality single cells from skeletal tissue and provide an overview of existing scRNA-seq technologies employed in skeletal studies along with practical bioinformatic analysis pipelines. By utilizing these methodologies, crucial insights into the developmental dynamics, maintenance of homeostasis, and pathological processes involved in spine, joint, bone, muscle, and tendon disorders have been uncovered. Specifically focusing on the joint diseases of degenerative disc disease, osteoarthritis, and rheumatoid arthritis using scRNA-seq has provided novel insights and a more nuanced comprehension. These findings have paved the way for discovering novel therapeutic targets that offer potential benefits to patients suffering from diverse skeletal disorders.

Identification and validation of immune-related genes in osteoarthritic synovial fibroblasts

Objective

OA was generally considered as a non-inflammatory disease dominated by articular cartilage degeneration. However, the role of synovitis in OA pathogenesis has received increasing attention. Recent studies support that OA patients have a pro-inflammatory/catabolic synovial environment similar to RA patients, promoting the occurrence and development of OA. Therefore, we investigated the co-immune-related genes and pathways of OA and RA to explore whether part of the pathogenesis of RA synovitis can be used to explain OA synovitis.

Methods

Data of GSE29746 and GSE12021 were downloaded from the Gene Expression Omnibus (GEO) database. Compared with control group, differentially expressed genes (DEGs) of OA and RA groups were screened separately by R software, Venny website was used to screen co-DEGs. Metascape was used to screen the common enriched terms and pathways between OA and RA. STRING website and Cytoscape software were used to map protein-protein interaction (PPI) networks and screen co-hub genes. GSE29746 was selected as the test dataset, and GSE12021 as the validation dataset for validate the co-hub genes. The results were validated by western blotting (WB) and real-time quantitative polymerase chain reaction (qPCR) of clinical synovial samples.

Results

We identified 573 OA-related DEGs, 148 RA-related DEGs, and 52 co-DEGs, revealing 14 common enriched terms, most of which were related to immune inflammation. IL7R was the only upregulated co-hub gene between OA and RA in the PPI network, consistent with the validation dataset. IL7R was highly expressed in clinical osteoarthritic synovial samples (P < 0.001).

Conclusion

Our findings suggested that IL7R is a critical co-DEG in OA and RA and confirmed the involvement of immune inflammation in disease pathogenesis. Furthermore, it confirms the role of IL7R in synovial inflammation in RA and OA synovitis and provides evidence for further investigation of OA immune inflammation.

Synovial fibroblast gene expression is associated with sensory nerve growth and pain in rheumatoid arthritis

It has been presumed that rheumatoid arthritis (RA) joint pain is related to inflammation in the synovium; however, recent studies reveal that pain scores in patients do not correlate with synovial inflammation. We developed a machine-learning approach (graph-based gene expression module identification or GbGMI) to identify an 815-gene expression module associated with pain in synovial biopsy samples from patients with established RA who had limited synovial inflammation at arthroplasty. We then validated this finding in an independent cohort of synovial biopsy samples from patients who had early untreated RA with little inflammation. Single-cell RNA sequencing analyses indicated that most of these 815 genes were most robustly expressed by lining layer synovial fibroblasts. Receptor-ligand interaction analysis predicted cross-talk between human lining layer fibroblasts and human dorsal root ganglion neurons expressing calcitonin gene-related peptide (CGRP+). Both RA synovial fibroblast culture supernatant and netrin-4, which is abundantly expressed by lining fibroblasts and was within the GbGMI-identified pain-associated gene module, increased the branching of pain-sensitive murine CGRP+ dorsal root ganglion neurons in vitro. Imaging of solvent-cleared synovial tissue with little inflammation from humans with RA revealed CGRP+ pain-sensing neurons encasing blood vessels growing into synovial hypertrophic papilla. Together, these findings support a model whereby synovial lining fibroblasts express genes associated with pain that enhance the growth of pain-sensing neurons into regions of synovial hypertrophy in RA.

Finding new analgesics: Computational pharmacology faces drug discovery challenges

Despite the worldwide prevalence and huge burden of pain, pain is an undertreated phenomenon. Currently used analgesics have several limitations regarding their efficacy and safety. The discovery of analgesics possessing a novel mechanism of action has faced multiple challenges, including a limited understanding of biological processes underpinning pain and analgesia and poor animal-to-human translation. Computational pharmacology is currently employed to face these challenges. In this review, we discuss the theory, methods, and applications of computational pharmacology in pain research. Computational pharmacology encompasses a wide variety of theoretical concepts and practical methodological approaches, with the overall aim of gaining biological insight through data acquisition and analysis. Data are acquired from patients or animal models with pain or analgesic treatment, at different levels of biological organization (molecular, cellular, physiological, and behavioral). Distinct methodological algorithms can then be used to analyze and integrate data. This helps to facilitate the identification of biological molecules and processes associated with pain phenotype, build quantitative models of pain signaling, and extract translatable features between humans and animals. However, computational pharmacology has several limitations, and its predictions can provide false positive and negative findings. Therefore, computational predictions are required to be validated experimentally before drawing solid conclusions. In this review, we discuss several case study examples of combining and integrating computational tools with experimental pain research tools to meet drug discovery challenges.

Three decades of advancements in osteoarthritis research: insights from transcriptomic, proteomic, and metabolomic studies

OBJECTIVE

Osteoarthritis (OA) is a complex disease involving contributions from both local joint tissues and systemic sources. Patient characteristics, encompassing sociodemographic and clinical variables, are intricately linked with OA rendering its understanding challenging. Technological advancements have allowed for a comprehensive analysis of transcripts, proteomes and metabolomes in OA tissues/fluids through omic analyses. The objective of this review is to highlight the advancements achieved by omic studies in enhancing our understanding of OA pathogenesis over the last three decades.

DESIGN

We conducted an extensive literature search focusing on transcriptomics, proteomics and metabolomics within the context of OA. Specifically, we explore how these technologies have identified individual transcripts, proteins, and metabolites, as well as distinctive endotype signatures from various body tissues or fluids of OA patients, including insights at the single-cell level, to advance our understanding of this highly complex disease.

RESULTS

Omic studies reveal the description of numerous individual molecules and molecular patterns within OA-associated tissues and fluids. This includes the identification of specific cell (sub)types and associated pathways that contribute to disease mechanisms. However, there remains a necessity to further advance these technologies to delineate the spatial organization of cellular subtypes and molecular patterns within OA-afflicted tissues.

CONCLUSIONS

Leveraging a multi-omics approach that integrates datasets from diverse molecular detection technologies, combined with patients' clinical and sociodemographic features, and molecular and regulatory networks, holds promise for identifying unique patient endophenotypes. This holistic approach can illuminate the heterogeneity among OA patients and, in turn, facilitate the development of tailored therapeutic interventions.

The role of inflammatory mediators and matrix metalloproteinases (MMPs) in the progression of osteoarthritis

Osteoarthritis (OA) is a chronic musculoskeletal disorder characterized by an imbalance between (synthesis) and catabolism (degradation) in altered homeostasis of articular cartilage mediated primarily by the innate immune system. OA degenerates the joints resulting in synovial hyperplasia, degradation of articular cartilage with damage of the structural and functional integrity of the cartilage extracellular matrix, subchondral sclerosis, osteophyte formation, and is characterized by chronic pain, stiffness, and loss of function. Inflammation triggered by factors like biomechanical stress is involved in the development of osteoarthritis. In OA apart from catabolic effects, anti-inflammatory anabolic processes also occur continually. There is also an underlying chronic inflammation present, not only in cartilage tissue but also within the synovium, which perpetuates tissue destruction of the OA joint. The consideration of inflammation in OA considers synovitis and/or other cellular and molecular events in the synovium during the progression of OA. In this review, we have presented the progression of joint degradation that results in OA. The critical role of inflammation in the pathogenesis of OA is discussed in detail along with the dysregulation within the cytokine networks composed of inflammatory and anti-inflammatory cytokines that drive catabolic pathways, inhibit matrix synthesis, and promote cellular apoptosis. OA pathogenesis, fluctuation of synovitis, and its clinical impact on disease progression are presented here along with the role of synovial macrophages in promoting inflammatory and destructive responses in OA. The role of interplay between different cytokines, structure, and function of their receptors in the inter-cellular signaling pathway is further explored. The effect of cytokines in the increased synthesis and release of matrix-decomposing proteolytic enzymes, such as matrix metalloproteinase (MMPs) and a disintegrin-like and metalloproteinase with thrombospondin motif (ADAMTS), is elaborated emphasizing the potential impact of MMPs on the chondrocytes, synovial cells, articular and periarticular tissues, and other immune system cells migrating to the site of inflammation. We also shed light on the pathogenesis of OA via oxidative damage particularly due to nitric oxide (NO) via its angiogenic response to inflammation. We concluded by presenting the current knowledge about the tissue inhibitors of metalloproteinases (TIMPs). Synthetic MMP inhibitors include zinc binding group (ZBG), non-ZBG, and mechanism-based inhibitors, all of which have the potential to be therapeutically beneficial in the treatment of osteoarthritis. Improving our understanding of the signaling pathways and molecular mechanisms that regulate the MMP gene expression, may open up new avenues for the creation of therapies that can stop the joint damage associated with OA.

Fatty acid binding protein 5 inhibition attenuates pronociceptive cytokine/chemokine expression and suppresses osteoarthritis pain: A comparative human and rat study

OBJECTIVE

Osteoarthritis (OA) is often accompanied by debilitating pain that is refractory to available analgesics due in part to the complexity of signaling molecules that drive OA pain and our inability to target these in parallel. Fatty acid binding protein 5 (FABP5) is a lipid chaperone that regulates inflammatory pain; however, its contribution to OA pain has not been characterized.

DESIGN

This combined clinical and pre-clinical study utilized synovial tissues obtained from subjects with end-stage OA and rats with monoiodoacetate-induced OA. Cytokine and chemokine release from human synovia incubated with a selective FABP5 inhibitor was profiled with cytokine arrays and ELISA. Immunohistochemical analyses were conducted for FABP5 in human and rat synovium. The efficacy of FABP5 inhibitors on pain was assessed in OA rats using incapacitance as an outcome. RNA-seq was then performed to characterize the transcriptomic landscape of synovial gene expression in OA rats treated with FABP5 inhibitor or vehicle.

RESULTS

FABP5 was expressed in human synovium and FABP5 inhibition reduced the secretion of pronociceptive cytokines (interleukin-6 [IL6], IL8) and chemokines (CCL2, CXCL1). In rats, FABP5 was upregulated in the OA synovium and its inhibition alleviated incapacitance. The transcriptome of the rat OA synovium exhibited >6000 differentially expressed genes, including the upregulation of numerous pronociceptive cytokines and chemokines. FABP5 inhibition blunted the upregulation of the majority of these pronociceptive mediators.

CONCLUSIONS

FABP5 is expressed in the OA synovium and its inhibition suppresses pronociceptive signaling and pain, indicating that FABP5 inhibitors may constitute a novel class of analgesics to treat OA.

Cerebral Mechanism of Tuina on the Descending Pain Inhibitory System in Knee Osteoarthritis: Protocol for a Randomized Controlled Trial

BACKGROUND

Knee osteoarthritis (KOA) is reputedly the most common musculoskeletal disease of the lower limbs and the main cause of pain and disability among older individuals. Pain is the most significant and widespread symptom of KOA. The descending pain inhibitory system has a cardinal role in normal pain consciousness, and its malfunction may be one of the pathophysiological mechanisms in KOA. Crucially, the rostral ventromedial medulla (RVM) and periaqueductal gray (PAG), as important components of the descending pain inhibitory system, directly modulate the activity of the spinal neurons involved in pain transmission. Tuina, a manual therapy, is effective and safe for reducing clinical symptoms of KOA; however, the mechanism that influences pain through the descending pain inhibitory system in KOA is unclear.

OBJECTIVE

This study aims to investigate the modulatory implications of Tuina on the RVM and PAG, which have critical roles in the descending pain inhibitory system in patients with KOA.

METHODS

This randomized controlled parallel trial will be conducted at the Tuina Clinic of the Third Affiliated Hospital of Henan University of Chinese Medicine (Zhengzhou, China). Patients with KOA will be randomly assigned (1:1) to 6 weeks of health education or Tuina. All patients in both groups will accept a resting-state functional magnetic resonance scan at the beginning and end of the experiment, and the resting-state functional connectivity and the voxel-based morphometry analysis will be performed to detect the RVM and PAG function and structure changes. The clinical outcome assessments will be (1) the pressure pain thresholds, (2) the Numerical Rating Scale, (3) the Hamilton Depression Scale (HAMD), and (4) the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC). Considering that this trial is a study of resting-state functional magnetic resonance imaging technology, resting-state functional connectivity and voxel-based morphometry are the primary outcomes, and clinical outcome assessments are secondary outcomes. Adverse events will be documented and assessed throughout. All main analyses will be carried out on the basis of the intention-to-treat principle. The outcome evaluators and data statisticians will be masked to the treatment group assignment to reduce the risk of bias.

RESULTS

This trial was approved by the ethics committee of the Third Affiliated Hospital of Henan University of Chinese Medicine. Enrollment began in December 2023, and the results of this trial are expected to be submitted for publication in May 2025.

CONCLUSIONS

This trial will identify a possible relationship between function and structure changes of RVM and PAG and the improvement of clinical variables, elucidating the effect of Tuina on the descending pain inhibitory system of patients with KOA. This trial will provide much-needed knowledge for Tuina for patients with KOA.

TRIAL REGISTRATION

Chinese Clinical Trial Registry ChiCTR2300070289; https://www.chictr.org.cn/showproj.html?proj=182570.

INTERNATIONAL REGISTERED REPORT IDENTIFIER (IRRID)

PRR1-10.2196/52820.

Comparative analysis of late-stage rheumatoid arthritis and osteoarthritis reveals shared histopathological features

OBJECTIVES

Osteoarthritis (OA) is a debilitating and heterogeneous condition, characterized by various levels of articular cartilage degradation, osteophytes formation, and synovial inflammation. Multiple evidences suggest that synovitis may appear early in the disease development and correlates with disease severity and pain, therefore representing a relevant therapeutic target. In a typical synovitis-driven joint disease, namely rheumatoid arthritis (RA), several pathotypes have been described by our group and associated with clinical phenotypes, disease progression, and response to therapy. However, whether these pathotypes can be also observed in the OA synovium is currently unknown.

METHODS

Here, using histological approaches combined with semi-quantitative scoring and quantitative digital image analyses, we comparatively characterize the immune cell infiltration in a large cohort of OA and RA synovial tissue samples collected at the time of total joint replacement.

RESULTS

We demonstrate that OA synovium can be categorized also into three pathotypes and characterized by disease- and stage-specific features. Moreover, we revealed that pathotypes specifically reflect distinct levels of peripheral inflammation.

CONCLUSIONS

In this study, we provide a novel and relevant pathological classification of OA synovial inflammation. Further studies investigating synovial molecular pathology in OA may contribute to the development of disease-modifying therapies.

Diverse Response to Local Pharmacological Blockade of Sirt1 Cleavage in Age-Induced versus Trauma-Induced Osteoarthritis Female Mice

Objective: Previous studies have shown that the cleavage of Sirt1 contributes to the development of osteoarthritis (OA). In fact, OA was effectively abrogated by the intra-articular (IA) administration of two compounds, one blocking Sirt1 cleavage (CA074me) and the other activating Sirt1 (SRT1720), using a post-traumatically induced model (PTOA) in young female mice. In this study, we attempted to understand if this local treatment is effective in preventing age-associated OA (AOA) progression and symptoms. Design: A group of 17-month-old female C57BL/6J mice were IA administered with CA074me and/or SRT1720 or their combination. Joint histopathological analysis and bone histomorphometry were carried out, with an assessment of knee mechanical hyperalgesia. A serum analysis for NT/CT Sirt1 was carried out along with immunohistochemistry for articular cartilage to detect p16INK4A or γH2A.X. Similarly, meniscal cartilage was monitored for Lef1 and Col1a1 deposition. The data were compared for young female mice subjected to post-traumatic OA (PTOA). Results: Similar to PTOA, combination-treated AOA exhibited improved knee hyperalgesia, yet structural improvements were undetected, corresponding to unchanged NT/CT Sirt1 serum levels. Both AOA and PTOA exhibited unchanged staining for nuclear p16INK4A or γH2A.X and lacked a correlation with OA severity. Contrarily to PTOA, the combination treatment with AOA did not exhibit a local reduction in the Lef1 and Col1 targets. Conclusions: When targeting Sirt1 cleavage, the PTOA and AOA models exhibited a similar pain response to the combination treatment; however, they displayed diverse structural outcomes for joint-related damage, related to Lef1-dependent signaling. Interestingly, nuclear p16INK4A was unaffected in both models, regardless of the treatment's effectiveness. Finally, these findings highlight the variations in the responses between two highly researched OA preclinical models, reflecting OA pathophysiology heterogeneity and variations in gender-related drug-response mechanisms.

Downregulation of Gadd45β alleviates osteoarthritis by repressing lipopolysaccharide-induced fibroblast-like synoviocyte inflammation, proliferation and migration

OBJECTIVE

Gadd45β have a regulatory role in cellular inflammation, proliferation and migration. However, the role of Gadd45β in synovial inflammation in osteoarthritis (OA) remains to be explored. This study aimed to ascertain whether Gadd45β is involved in OA synovial inflammation.

METHODS

The rat model was induced by sodium iodoacetate and the cellular model was constructed with lipopolysaccharide (LPS)-induced fibroblast-like synoviocytes (FLSs). siRNA was applied to interfere with the expression of intracellular Gadd45β. Real-time quantitative polymerase chain reaction (RT-qPCR) and western blotting were used to detect the expression of Gadd45β mRNA and protein. The inflammation, proliferation, and migration of OA-FLSs were detected by enzyme-linked immunosorbent assay, cell scratch assay, 5-ethynyl-2'-deoxyuridine assay, etc. The effect of downregulation of Gadd45β on the nuclear factor-κB (NF-κB) pathway was investigated.

RESULTS

Expression of Gadd45β in OA rat synovial tissues and OA-FLSs was increased, and LPS treatment promoted cell proliferation and enhanced cell migration. Gadd45β interference inhibited the inflammation, proliferation and migration of cells induced by LPS. LPS promoted P65 expression in the nucleus and activated the NF-κB signaling pathway, whereas si-Gadd45β reversed this situation.

CONCLUSIONS

si-Gadd45β inhibited the inflammatory response, proliferation and migration of FLSs, and activation of the NF-κB signaling pathway, which could delay the progression of OA. Hence, it may become a potential therapeutic target for OA.

Single-cell RNA sequencing in osteoarthritis

Osteoarthritis is a progressive and heterogeneous joint disease with complex pathogenesis. The various phenotypes associated with each patient suggest that better subgrouping of tissues associated with genotypes in different phases of osteoarthritis may provide new insights into the onset and progression of the disease. Recently, single-cell RNA sequencing was used to describe osteoarthritis pathogenesis on a high-resolution view surpassing traditional technologies. Herein, this review summarizes the microstructural changes in articular cartilage, meniscus, synovium and subchondral bone that are mainly due to crosstalk amongst chondrocytes, osteoblasts, fibroblasts and endothelial cells during osteoarthritis progression. Next, we focus on the promising targets discovered by single-cell RNA sequencing and its potential applications in target drugs and tissue engineering. Additionally, the limited amount of research on the evaluation of bone-related biomaterials is reviewed. Based on the pre-clinical findings, we elaborate on the potential clinical values of single-cell RNA sequencing for the therapeutic strategies of osteoarthritis. Finally, a perspective on the future development of patient-centred medicine for osteoarthritis therapy combining other single-cell multi-omics technologies is discussed. This review will provide new insights into osteoarthritis pathogenesis on a cellular level and the field of applications of single-cell RNA sequencing in personalized therapeutics for osteoarthritis in the future.

Activation of vascular endothelial cells by synovial fibrosis promotes Netrin-1-induced sensory nerve sprouting and exacerbates pain sensitivity

Synovial fibrosis is one of the most dominant histopathological changes in osteoarthritis of the knee (KOA), and activation of vascular endothelial cells in synovial fibrosis is both an important factor in mediating pain in KOA and a major contributor to the generation of pain signals. At the same time, angiogenesis and nerve fibres are more likely to underlie the pathology of pain induced by synovial fibrosis. In the present study, we established a co-culture model of human umbilical vein endothelial cells (HUVECs) with dorsal root ganglion (DRG) and detected tissue and cellular Netrin-1, vascular cell adhesion molecule-1 (VCAM-1), intercellular cell adhesion molecule-1 (ICAM-1), growth-associated protein-43 (GAP43), colorectal cancer deleted (DCC), uncoordinated 5 (UNC5), and the related expression of calcitonin gene-related peptide (CGRP), substance P (SP) and nerve growth factor (NGF) in supernatant by ELISA to investigate the intervention of vascular endothelial cell activation on sensory nerve sprouting exacerbating peripheral pain sensitivity and to investigate the effect of Netrin-1 from the perspective of Netrin-1 secretion to illustrate its effector mechanism.

Identification of anterior cruciate ligament fibroblasts and their contribution to knee osteoarthritis progression using single-cell analyses

The mechanical properties of the anterior cruciate ligament (ACL) in the knee have been highlighted, but its role in the regulation of the joint microenvironment remains unclear, especially in the progression of Knee Osteoarthritis (KOA). Here, single-cell RNA sequencing (scRNA-seq) and single-cell assay for transposase-accessible chromatin sequencing (scATAC-seq) data were integrated to reveal the transcriptional and epigenomic landscape of ACL in normal and OA states. We identified a novel subpopulation of fibroblasts in ACL, which provides new insights into the role of the ACL in knee homeostasis and disease. Degeneration of the ACL during OA mechanically alters the knee joint homeostasis and influences the microenvironment by regulating inflammatory- and osteogenic-related factors, thereby contributing to the progression of KOA. Additionally, the specific mechanism by which these Inflammation-associated Fibroblasts (IAFs) regulate KOA progression was uncovered, providing new foundation for the development of targeted treatments for KOA.

CD39+CD55- Fb Subset Exhibits Myofibroblast-Like Phenotype and Is Associated with Pain in Osteoarthritis of the Knee

Recent studies utilizing single-cell analysis have unveiled the presence of various fibroblast (Fb) subsets within the synovium under inflammatory conditions in osteoarthritis (OA), distinguishing them from those in rheumatoid arthritis (RA). Moreover, it has been reported that pain in knee OA patients is linked to specific fibroblast subsets. Single-cell expression profiling methods offer an incredibly detailed view of the molecular states of individual cells. However, one limitation of these methods is that they require the destruction of cells during the analysis process, rendering it impossible to directly assess cell function. In our study, we employ flow cytometric analysis, utilizing cell surface markers CD39 and CD55, in an attempt to isolate fibroblast subsets and investigate their relationship with OA pathology. Synovial tissues were obtained from 25 knee OA (KOA) patients. Of these, six samples were analyzed by RNA-seq (n = 3) and LC/MS analysis (n = 3). All 25 samples were analyzed to estimate the proportion of Fb (CD45-CD31-CD90+) subset by flow cytometry. The proportion of Fb subsets (CD39+CD55- and CD39-CD55+) and their association with osteoarthritis pathology were evaluated. CD39+CD55- Fb highly expressed myogenic markers such as CNN1, IGFBP7, MYH11, and TPM1 compared to CD39-CD55+ Fb. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis of upregulated differentially expressed genes (DEGs) in CD39+CD55- Fb identified the Apelin pathway and cGMP-PKC-signaling pathway as possibly contributing to pain. LC/MS analysis indicated that proteins encoded by myogenic marker genes, including CNN1, IGFBP7, and MYH11, were also significantly higher than in CD39-CD55+ Fb. CD39-CD55+ Fb highly expressed PRG4 genes and proteins. Upregulated DEGs were enriched for pathways associated with proinflammatory states ('RA', 'TNF signaling pathway', 'IL-17 signaling pathway'). The proportion of CD39+CD55- Fb in synovium significantly correlated with both resting and active pain levels in knee OA (KOA) patients (resting pain, ρ = 0.513, p = 0.009; active pain, ρ = 0.483, p = 0.015). There was no correlation between joint space width (JSW) and the proportion of CD39+CD55- Fb. In contrast, there was no correlation between the proportion of CD39-CD55+ Fb and resting pain, active pain, or JSW. In conclusion, CD39+CD55- cells exhibit a myofibroblast phenotype, and its proportion is associated with KOA pain. Our study sheds light on the potential significance of CD39+CD55- synovial fibroblasts in osteoarthritis, their myofibroblast-like phenotype, and their association with joint pain. These findings provide a foundation for further research into the mechanisms underlying fibrosis, the impact of altered gene expression on osteoarthritic joints, and potential therapeutic strategies.

The potential role of synovial cells in the progression and treatment of osteoarthritis

Osteoarthritis (OA), the commonest arthritis, is characterized by the progressive destruction of cartilage, leading to disability. The Current early clinical treatment strategy for OA often centers on anti-inflammatory or analgesia medication, weight loss, improved muscular function and articular cartilage repair. Although these treatments can relieve symptoms, OA tends to be progressive, and most patients require arthroplasty at the terminal stages of OA. Recent studies have shown a close correlation between joint pain, inflammation, cartilage destruction and synovial cells. Consequently, understanding the potential mechanisms associated with the action of synovial cells in OA could be beneficial for the clinical management of OA. Therefore, this review comprehensively describes the biological functions of synovial cells, the synovium, together with the pathological changes of synovial cells in OA, and the interaction between the cartilage and synovium, which is lacking in the present literature. Additionally, therapeutic approaches based on synovial cells for OA treatment are further discussed from a clinical perspective, highlighting a new direction in the treatment of OA.

Artemin sensitizes nociceptors that innervate the osteoarthritic joint to produce pain

OBJECTIVE

There have been significant developments in understanding artemin/GFRα3 signaling in recent years, and there is now accumulating evidence that artemin has important roles to play in pain signaling, including that derived from joint and bone, and that associated with osteorthritis (OA).

METHODS

A total of 163 Sprague-Dawley rats were used in this study. We used an animal model of mono-iodoacetate (MIA)-induced OA, in combination with electrophysiology, behavioral testing, Western blot analysis, and retrograde tracing and immunohistochemistry, to identify roles for artemin/GFRα3 signaling in the pathogenesis of OA pain.

RESULTS

We have found that: 1) GFRα3 is expressed in a substantial proportion of knee joint afferent neurons; 2) exogenous artemin sensitizes knee joint afferent neurons in naïve rats; 3) artemin is expressed in articular tissues of the joint, but not surrounding bone, early in MIA-induced OA; 4) artemin expression increases in bone later in MIA-induced OA when pathology involves subchondral bone; and 5) sequestration of artemin reverses MIA-induced sensitization of both knee joint and bone afferent neurons late in disease when there is inflammation of knee joint tissues and damage to the subchondral bone.

CONCLUSIONS

Our findings show that artemin/GFRα3 signaling has a role to play in the pathogenesis of OA pain, through effects on both knee joint and bone afferent neurons, and suggest that targeted manipulation of artemin/GFRα3 signaling may provide therapeutic benefit for the management of OA pain.

DATA AVAILABILITY

Data are available on request of the corresponding author.

Synovium is a sensitive tissue for mapping the negative effects of systemic iron overload in osteoarthritis: identification and validation of two potential targets

BACKGROUND

The prevention and treatment of osteoarthritis (OA) pose a major challenge in its research. The synovium is a critical tissue in the systematic treatment of OA. The present study aimed to investigate potential target genes and their correlation with iron overload in OA patients.

METHODS

The internal datasets for analysis included the microarray datasets GSE46750, GSE55457, and GSE56409, while the external datasets for validation included GSE12021 and GSE55235. The GSE176308 dataset was used to generate single-cell RNA sequencing profiles. To investigate the expression of the target genes in synovial samples, quantitative reverse transcription-PCR, western blotting, and immunohistochemical assay were conducted. ELISA was used to detect the levels of ferritin and Fe2+ in both serum and synovium.

RESULTS

JUN and ZFP36 were screened from the differentially expressed genes, and their mRNA were significantly reduced in the OA synovium compared to that in normal synovium. Subsequently, complex and dynamically evolving cellular components were observed in the OA synovium. The mRNA level of JUN and ZFP36 differed across various cell clusters of OA synovium and correlated with immune cell infiltration. Moreover, ferritin and Fe2+ were significantly increased in the serum and synovium of OA patients. Further, we found that JUN elevated and ZFP36 decreased at protein level.

CONCLUSIONS

The synovium is a sensitive tissue for mapping the adverse effects of systemic iron overload in OA. JUN and ZFP36 represent potential target genes for attenuating iron overload during OA treatment. Some discrepancies between the transcription and protein levels of JUN suggest that post-transcriptional modifications may be implicated. Future studies should also focus on the roles of JUN and ZFP36 in inducing changes in cellular components in the synovium during OA pathogenesis.

Single-cell and bulk tissue sequencing unravels the heterogeneity of synovial microenvironment in arthrofibrosis

Arthrofibrosis (AF) is a debilitating complication that occurs after trauma or surgery, leading to functional impairment and surgical failures worldwide. This study aimed to uncover the underlying mechanism of AF. A total of 141 patients were enrolled, and synovial samples were collected from both patients and animal models at different time points. Single-cell RNA-sequencing (scRNA-seq) and bulk tissue RNA sequencing (bulk-seq) were employed to profile the distinct synovial microenvironment. This study revealed changes in cell proportions during AF pathogenesis and identified Engrailed-1 (EN1) as a key transcription factor strongly associated with disease severity and clinical prognosis. Additionally, the researchers discovered a specific type of synovial fibroblast called DKK3-SLF, which played a critical role in driving AF development. These findings shed light on the composition and heterogeneity of the synovial microenvironment in AF, offering potential avenues for identifying therapeutic targets and developing clinical treatments for AF and other fibrotic diseases.

Identification of four-gene signature to diagnose osteoarthritis through bioinformatics and machine learning methods

BACKGROUND

Although osteoarthritis (OA) is one of the most prevalent joint disorders, effective biomarkers to diagnose OA are still unavailable. This study aimed to acquire some key synovial biomarkers (hub genes) and analyze their correlation with immune infiltration in OA.

METHODS

Gene expression profiles and clinical characteristics of OA and healthy synovial samples were retrieved from the Gene Expression Omnibus (GEO) database. Hub genes for OA were mined based on a combination of weighted gene co-expression network analysis (WGCNA), the least absolute shrinkage and selection operator (LASSO), support vector machine recursive feature elimination (SVM-RFE), and random forest (RF) algorithms. A diagnostic nomogram model for OA prediction was developed based on the hub genes. Receiver operating characteristic curves (ROC) were performed to confirm the abnormal expression of hub genes in the experimemtal and validation datasets. qRT-PCR using patients' samples were conducted as well. In addition, the infiltration level of 28 immune cells in the expression profile and their relationship with hub genes were analyzed using single-sample GSEA (ssGSEA).

RESULTS

4 hub genes (ZBTB16, TNFSF11, SCRG1 and KDELR3) were obtained by WGCNA, lasso, SVM-RFE, RF algorithms as potential biomarkers for OA. The immune infiltration analyses revealed that hub genes were most correlated with regulatory T cell and natural killer cell.

CONCLUSION

A machine learning model to diagnose OA based on ZBTB16, TNFSF11, SCRG1 and KDELR3 using synovial tissue was constructed, providing theoretical foundation and guideline for diagnostic and treatment targets in OA.

Association Between Power Doppler Ultrasound Signals and Chronic Pain After Total Knee Arthroplasty: A Cross-Sectional Explorative Study

Purpose

Some patients experience chronic postsurgical pain (CPSP) after total knee arthroplasty (TKA) despite the absence of clinical or radiographic abnormalities. Postoperative synovitis as a cause of CPSP after TKA has received limited research attention. This study aimed to investigate the relationship between synovitis after TKA and CPSP.

Patients and Methods

A total of 111 knees of 85 patients, with at least 1-year post-TKA follow-up, were assessed retrospectively and cross-sectionally. Power Doppler (PD) ultrasonography was used to detect the synovial hypervascularity associated with synovitis. The knee joint was divided into 15 areas, and PD signals were graded semi-quantitatively (0-3) in each area, the sum of which was defined as the total PD score. Clinical information regarding CPSP, including the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) pain subscales, was recorded. The relationship between pain and PD ultrasonography findings was accessed. Patients were divided into two groups (CPSP+ and CPSP- groups) based on pain severity. Clinical information, including PD ultrasonography findings and other possible causes, was compared between the groups.

Results

The WOMAC pain subscale was significantly correlated with the total PD score and maximum PD grade (r=0.3977, p<0.0001; r=0.2797, p=0.0029; respectively). The CPSP+ group had a significantly higher maximum PD grade and total PD score than the CPSP- group (median [interquartile range]: 2 [1, 2] vs 1 [1, 2], p=0.0001; 6 [2, 11] vs 2 [1, 4], p=0.0002; respectively). Multiple and logistic regression analyses showed that the total PD score was an independent factor for the WOMAC pain subscale (β=0.3822, 95% confidence interval [CI]=0.1460, 0.6184, p=0.00176) and CPSP (odds ratio=1.19, 95% CI=1.01, 1.41, p=0.0424).

Conclusion

This study indicated a possible association between the total PD score and chronic pain after TKA; however, further studies are needed to corroborate these findings.