The nutrition of the human meniscus: A computational analysis investigating the effect of vascular recession on tissue homeostasis

Revealing the complexity of meniscus microvasculature through 3D visualization and analysis

Three-dimensional information is essential for a proper understanding of the healing potential of the menisci and their overall role in the knee joint. However, to date, the study of meniscal vascularity has relied primarily on two-dimensional imaging techniques. Here we present a method to elucidate the intricate 3D meniscal vascular network, revealing its spatial arrangement, connectivity and density. A polymerizing contrast agent was injected into the femoral artery of human cadaver legs, and the meniscal microvasculature was examined using micro-computed tomography at different levels of detail and resolution. The 3D vascular network was quantitatively assessed in a zone-base analysis using parameters such as diameter, length, tortuosity, and branching patterns. The results of this study revealed distinct vascular patterns within the meniscus, with the highest vascular volume found in the outer perimeniscal zone. Variations in vascular parameters were found between the different circumferential and radial meniscal zones. Moreover, through state-of-the-art 3D visualization using micro-CT, this study highlighted the importance of spatial resolution in accurately characterizing the vascular network. These findings, both from this study and from future research using this technique, improve our understanding of microvascular distribution, which may lead to improved therapeutic strategies.

Assessing the role of surface layer and molecular probe size in diffusion within meniscus tissue

Diffusion within extracellular matrix is essential to deliver nutrients and larger metabolites to the avascular region of the meniscus. It is well known that both structure and composition of the meniscus vary across its regions; therefore, it is crucial to fully understand how the heterogenous meniscal architecture affects its diffusive properties. The objective of this study was to investigate the effect of meniscal region (core tissue, femoral, and tibial surface layers) and molecular weight on the diffusivity of several molecules in porcine meniscus. Tissue samples were harvested from the central area of porcine lateral menisci. Diffusivity of fluorescein (MW 332 Da) and three fluorescence-labeled dextrans (MW 3k, 40k, and 150k Da) was measured via fluorescence recovery after photobleaching. Diffusivity was affected by molecular size, decreasing as the Stokes' radius of the solute increased. There was no significant effect of meniscal region on diffusivity for fluorescein, 3k and 40k dextrans (p>0.05). However, region did significantly affect the diffusivity of 150k Dextran, with that in the tibial surface layer being larger than in the core region (p = 0.001). Our findings contribute novel knowledge concerning the transport properties of the meniscus fibrocartilage. This data can be used to advance the understanding of tissue pathophysiology and explore effective approaches for tissue restoration.

Outcomes After Meniscal Root Repair in Patients With and Without Advanced Patellofemoral Chondromalacia: Comparison at 2-Year Follow-up

Background

Meniscal root repair can improve patient outcomes significantly; however, several contraindications exist, including arthritic change to the medial or lateral tibiofemoral compartments.

Purpose/Hypothesis

The purpose of this study was to evaluate the outcomes of meniscal root repair in patients with advanced patellofemoral chondromalacia (PFC). It was hypothesized that the presence of advanced PFC would not significantly affect the postoperative outcomes.

Study Design

Cohort study; Level of evidence, 3.

Methods

A retrospective review was conducted of patients who underwent meniscal root repair with at least 2 years of follow-up data. Patients with chondromalacia as determined by arthroscopic visualization (defined as Outerbridge grade 3 or 4) were placed in the PFC group; patients with Outerbridge grade 0 to 2 chondromalacia were placed in the non-PFC group. Outcomes were measured by the Lysholm knee scoring scale and the International Knee Documentation Committee (IKDC) Subjective Knee Form. Clinical outcomes including complications were also recorded. Quantitative data between the groups were analyzed using the 2-tailed independent-samples t test.

Results

Overall, this study included 81 patients (35 in the PFC group, 46 in the non-PFC group). The mean follow-up times were 25.1 months in the PFC group and 24.8 months in the non-PFC group. In both groups, Lysholm and IKDC scores improved significantly with 24-month Lysholm scores averaging 85.86 in the PFC group and 86.61 in the non-PFC group (P = .62) and 24-month IKDC scores averaging 77.66 for the PFC group and 79.59 for the non-PFC group (P = .45). The cohorts demonstrated similar rates of retear, arthrofibrosis, infection, and progression to total knee arthroplasty.

Conclusion

The presence of advanced PFC was not associated with inferior outcomes in patients who underwent posterior meniscal root repair, and rates of recurrent tears, postoperative infection, arthrofibrosis, and conversion to total knee arthroplasty were similar between the study groups. These findings suggest that PFC may not significantly alter the results of meniscal root repair and should not be considered an absolute contraindication for this procedure.

Strain-Dependent Diffusivity of Small and Large Molecules in Meniscus

Due to lack of full vascularization, the meniscus relies on diffusion through the extracellular matrix to deliver small (e.g., nutrients) and large (e.g., proteins) to resident cells. Under normal physiological conditions, the meniscus undergoes up to 20% compressive strains. While previous studies characterized solute diffusivity in the uncompressed meniscus, to date, little is known about the diffusive transport under physiological strain levels. This information is crucial to fully understand the pathophysiology of the meniscus. The objective of this study was to investigate strain-dependent diffusive properties of the meniscus fibrocartilage. Tissue samples were harvested from the central portion of porcine medial menisci and tested via fluorescence recovery after photobleaching to measure diffusivity of fluorescein (332 Da) and 40 K Da dextran (D40K) under 0%, 10%, and 20% compressive strain. Specifically, average diffusion coefficient and anisotropic ratio, defined as the ratio of the diffusion coefficient in the direction of the tissue collagen fibers to that orthogonal, were determined. For all the experimental conditions investigated, fluorescein diffusivity was statistically faster than that of D40K. Also, for both molecules, diffusion coefficients significantly decreased, up to ∼45%, as the strain increased. In contrast, the anisotropic ratios of both molecules were similar and not affected by the strain applied to the tissue. This suggests that compressive strains used in this study did not alter the diffusive pathways in the meniscus. Our findings provide new knowledge on the transport properties of the meniscus fibrocartilage that can be leveraged to further understand tissue pathophysiology and approaches to tissue restoration.

Review of Meniscus Anatomy and Biomechanics

PURPOSE OF REVIEW

Anatomic repair of meniscal pathology is critical for restoring native joint biomechanics and kinematics for patients who suffer from meniscal tears. The purpose of this review was to summarize the pertinent anatomy, biomechanics, and kinematics of the meniscus to guide surgeons during meniscal repair procedures.

RECENT FINDINGS

Over the past decade, there has been a growing trend to save the meniscus whenever possible. The goal of repair should be to recreate native anatomy as close as possible to recapitulate normal mechanics. Studies describing the quantitative and qualitative relationship of the meniscus roots, ligaments, and attachments are key in guiding any meniscus repair. This review summarizes these relationships, with particular emphasis on meniscal roots and other key attachments to the meniscus. The composition, embryology, vascularization, biomechanics, in vivo kinetics, and in vivo kinematics of the meniscus are also discussed in this review. Meniscal tears can cause profound functional, biomechanical, and kinematic derangements within the knee joint leading to accelerated degeneration of the articular cartilage. A strong understanding of the quantitative and qualitative relationships of the meniscus and its attachments with key arthroscopic landmarks will allow a surgeon to anatomically repair meniscal pathology in order to restore native joint biomechanics.

Increased Vascularity in the Neonatal versus Adult Meniscus: Evaluation with Magnetic Resonance Imaging

Objective. Quantification of meniscus vascularity has been limited with previous techniques, and minimal data exist describing differential vascular zones in the skeletally immature meniscus. The objective of this study is to use quantitative contrast-enhanced magnetic resonance imaging (MRI) to compare meniscal vascularity in neonatal specimens with adults. We hypothesized that the developing meniscus has greater and more uniform vascularity throughout all zones. Design. Ten fresh-frozen human cadaveric knees (5 neonatal, age 0-6 months; 5 adult, 34-67 years) underwent gadolinium-enhanced MRI using an established vascularity quantification protocol. Regions of interest corresponding to peripheral and central zones of the meniscus were identified on pre-contrast coronal images, and signal enhancement within the same regions (normalized against background tissue) was compared between pre- and post-contrast images. Results. The medial and lateral menisci had similar distribution of perfusion (45.8% ± 8.1% medial vs. 54.2% ± 8.1% lateral in neonatal knees; 50.6% ± 11.3% medial vs. 49.4% ± 11.3% lateral in adult knees, P = 0.47). Increased perfusion was demonstrated in the periphery compared with the central zone (2.3:1 in neonatal knees and 3.25:1 in adult knees, P = 0.31). Neonatal specimens demonstrated 6.0-fold greater overall post-contrast meniscal signal enhancement compared with adults (P < 0.0001), with the 0-month specimen demonstrating the greatest proportional signal enhancement. Conclusions. While blood flow to the periphery is greater than to central zones in all menisci, younger menisci receive proportionally greater overall blood flow compared to adults, including to the central zone, suggesting that the immature meniscus is a more biologically active tissue than its adult counterpart.

Osteochondral Autograft Transplant as a Potential Salvage Procedure for Articular Cartilage Defects of the Lateral Compartment in Lateral Meniscus-Deficient Knees: Results From a Country With Limited Availability of Meniscal Transplant

Background:

The treatment of a meniscus-deficient knee is challenging, especially when patients are young and active and are not favorable candidates for prosthetic joint replacement.

Hypothesis:

We hypothesized that osteochondral autologous transplant (OAT) alone can be considered a salvage treatment for patients with cartilage damage of the lateral compartment of the knee, even with lateral meniscal deficiency, if the knee alignment is close to neutral.

Study Design:

Case series; Level of evidence, 4.

Methods:

Patients with lateral meniscal deficiency, whose femorotibial angle was 170° to 180° and who underwent OAT on the lateral compartment of the knee without concomitant realignment osteotomy, were retrospectively included in this study. The International Knee Documentation Committee (IKDC) subjective score and the Japanese Orthopaedic Association score for knee osteoarthritis (JOA knee score) were recorded. The International Cartilage Repair Society (ICRS) cartilage repair assessment was used to evaluate the repaired cartilage at second-look arthroscopy.

Results:

The study included 10 patients (mean ± SD age, 31.7 ± 19.7 years; 3 men and 7 women) who had ICRS grade 4 cartilage lesions (mean size, 3.5 ± 1.7 cm²); the mean follow-up was 73.8 ± 42.5 months. From preoperative assessment to final follow-up, the mean IKDC subjective score improved significantly from 53.5 ± 10.0 to 85.4 ± 10.1, and the mean JOA knee score improved significantly from 81.0 ± 8.4 to 95.6 ± 5.3 (P = .004 for both). One patient with a femorotibial angle of 170° underwent revision distal femoral osteotomy owing to prolonged symptoms and progression of the valgus deformity, and 2 other patients with femorotibial angles of 170° and 171° also exhibited progression of valgus malalignment or low clinical scores postoperatively. Patients with a favorable femorotibial angle (174°-178°) exhibited relieved symptoms and preservation of femorotibial angle alignment within 1° of change at follow-up. At second-look arthroscopy (n = 8 patients), 6 patients had an ICRS score of nearly normal or normal.

Conclusion:

In the study patients, for which a meniscal allograft was unavailable, the OAT procedure was able to relieve the symptoms associated with cartilage lesions, even with lateral meniscal deficiency, when the femorotibial angle alignment was close to neutral.

Protein synthesis rates of muscle, tendon, ligament, cartilage, and bone tissue in vivo in humans

Skeletal muscle plasticity is reflected by a dynamic balance between protein synthesis and breakdown, with basal muscle tissue protein synthesis rates ranging between 0.02 and 0.09%/h. Though it is evident that other musculoskeletal tissues should also express some level of plasticity, data on protein synthesis rates of most of these tissues in vivo in humans is limited. Six otherwise healthy patients (62±3 y), scheduled to undergo unilateral total knee arthroplasty, were subjected to primed continuous intravenous infusions with L-[ring-¹³C₆]-Phenylalanine throughout the surgical procedure. Tissue samples obtained during surgery included muscle, tendon, cruciate ligaments, cartilage, bone, menisci, fat, and synovium. Tissue-specific fractional protein synthesis rates (%/h) were assessed by measuring the incorporation of L-[ring-¹³C₆]-Phenylalanine in tissue protein and were compared with muscle tissue protein synthesis rates using a paired t test. Tendon, bone, cartilage, Hoffa’s fat pad, anterior and posterior cruciate ligament, and menisci tissue protein synthesis rates averaged 0.06±0.01, 0.03±0.01, 0.04±0.01, 0.11±0.03, 0.07±0.02, 0.04±0.01, and 0.04±0.01%/h, respectively, and did not significantly differ from skeletal muscle protein synthesis rates (0.04±0.01%/h; P>0.05). Synovium derived protein (0.13±0.03%/h) and intercondylar notch bone tissue protein synthesis rates (0.03±0.01%/h) were respectively higher and lower compared to skeletal muscle protein synthesis rates (P<0.05 and P<0.01, respectively). Basal protein synthesis rates in various musculoskeletal tissues are within the same range of skeletal muscle protein synthesis rates, with fractional muscle, tendon, bone, cartilage, ligament, menisci, fat, and synovium protein synthesis rates ranging between 0.02 and 0.13% per hour in vivo in humans.

Clinical trial registration: NTR5147

Effect of Sustained Joint Loading on TMJ Disc Nutrient Environment

The temporomandibular joint (TMJ) disc nutrient environment profoundly affects cell energy metabolism, proliferation, and biosynthesis. Due to technical challenges of in vivo measurements, the human TMJ disc extracellular nutrient environment under load, which depends on metabolic rates, solute diffusion, and disc morphometry, remains unknown. Therefore, the study objective was to predict the TMJ disc nutrient environment under loading conditions using combined experimental and computational modeling approaches. Specifically, glucose consumption and lactate production rates of porcine TMJ discs were measured under varying tissue culture conditions (n = 40 discs), and mechanical strain-dependent glucose and lactate diffusivities were measured using a custom diffusion chamber (n = 6 discs). TMJ anatomy and loading area were obtained from magnetic resonance imaging of healthy human volunteers (n = 11, male, 30 ± 9 y). Using experimentally determined nutrient metabolic rates, solute diffusivities, TMJ anatomy, and loading areas, subject-specific finite element (FE) models were developed to predict the 3-dimensional nutrient profiles in unloaded and loaded TMJ discs (unloaded, 0% strain, 20% strain). From the FE models, glucose, lactate, and oxygen concentration ranges for unloaded healthy human TMJ discs were 0.6 to 4.0 mM, 0.9 to 5.0 mM, and 0% to 6%, respectively, with steep gradients in the anterior and posterior bands. Sustained mechanical loading significantly reduced nutrient levels (P < 0.001), with a critical zone in which cells may die representing approximately 13.5% of the total disc volume. In conclusion, this study experimentally determined TMJ disc metabolic rates, solute diffusivities, and disc morphometry, and through subject-specific FE modeling, revealed critical interactions between mechanical loading and nutrient supply and metabolism for the in vivo human TMJ disc. The results suggest that TMJ disc homeostasis may be vulnerable to pathological loading (e.g., clenching, bruxism), which impedes nutrient supply. Given difficulties associated with direct in vivo measurements, this study provides a new approach to systematically investigate homeostatic and degenerative mechanisms associated with the TMJ disc.