Migration of cells from human anterior cruciate ligament explants into collagen-glycosaminoglycan scaffolds

Primary anterior cruciate ligament repair: Current concepts

The renewed interest in ACL repair over the last two decades stems from advances in modern arthroscopic techniques and clinical studies that have provided evidence that the ACL can reliably heal, and patients can return to sport at a comparable rate to ACL reconstruction patients. The ability to maintain and utilize native ACL tissue, with proprioceptive capabilities, and the smaller drill tunnels needed to repair an ACL leads to an overall less invasive procedure and improved early rehabilitation. Additionally, repair avoids a variety of comorbidities associated with autograft harvest. This current concept review details modern techniques of ACL repair and their current studies, a review on the use of biologic enhancement in ACL repair, and other considerations to appropriately integrate ACL repair into the sports medicine orthopaedic surgeon's practice.

Advanced Gene Therapy Strategies for the Repair of ACL Injuries

The anterior cruciate ligament (ACL), the principal ligament for stabilization of the knee, is highly predisposed to injury in the human population. As a result of its poor intrinsic healing capacities, surgical intervention is generally necessary to repair ACL lesions, yet the outcomes are never fully satisfactory in terms of long-lasting, complete, and safe repair. Gene therapy, based on the transfer of therapeutic genetic sequences via a gene vector, is a potent tool to durably and adeptly enhance the processes of ACL repair and has been reported for its workability in various experimental models relevant to ACL injuries in vitro, in situ, and in vivo. As critical hurdles to the effective and safe translation of gene therapy for clinical applications still remain, including physiological barriers and host immune responses, biomaterial-guided gene therapy inspired by drug delivery systems has been further developed to protect and improve the classical procedures of gene transfer in the future treatment of ACL injuries in patients, as critically presented here.

Effects of radiation dose and nitrogen purge on collagen scaffold properties

Sterilization of medical devices is commonly performed using radiation methods. However, collagen materials can be damaged when using standard radiation doses (25 kGy). Small increases of radiation dose can allow for increases in the acceptable initial bioburden load of aseptically manufactured devices while maintaining required sterility assurance levels, which is often critical in early stage translational settings. In this study, we hypothesized that small increases in radiation dose from 15 to 20 kGy would result in significant changes to several key characteristics of collagen scaffolds. Scaffolds were manufactured by lyophilizing the pepsin digest of dense bovine connective tissue in cylindrical molds and were irradiated at either 0, 15, 17.5, or 20 kGy with an additional group packaged in nitrogen and irradiated at 17.5 kGy. Groups were evaluated for changes to the soluble collagen and glycosaminoglycan mass fractions, protein banding patterns in electrophoresis, a collagen fragmentation assay, and resistance to enzymatic degradation. All parameters were statistically analyzed using one-way analysis of variance with Tukey's correction for multiple comparisons. The soluble collagen mass fraction was significantly decreased in the 20 kGy group; however, there was no significant effect of radiation dose or a nitrogen-rich environment on the other measured parameters, including protein banding patterns, fragmented collagen content, and resistance to enzymatic degradation.Statement of Clinical Significance: Collagen scaffolds have proven useful in clinical applications but can be damaged by standard radiation doses. Low-dose sterilization may be a viable alternative that minimally impacts key properties of these scaffolds.

Optimizing outcomes of ACL surgery-Is autograft reconstruction the only reasonable option?

Anterior cruciate ligament (ACL) injuries occur at a high frequency in the United States with approximately 400,000 ACL reconstructions being performed each year. While ACL reconstruction is our current gold standard of treatment, it does not restore joint motion, or prevent the premature development of posttraumatic osteoarthritis (PTOA) in many patients. Thus, new treatments for an ACL injury, which are less invasive and minimize patient morbidity, including cartilage damage, are highly desirable. We have used a tissue-engineered approach to stimulate ligament healing, to improve upon current treatment options. In this review, we describe and discuss our work moving a tissue engineering strategy from the concept to bench, preclinical, clinical trials and ultimately FDA 510(k) de Novo approval, providing clinicians and patients with a viable alternative to ACL reconstruction.

Bridge-Enhanced Anterior Cruciate Ligament Repair Is Not Inferior to Autograft Anterior Cruciate Ligament Reconstruction at 2 Years: Results of a Prospective Randomized Clinical Trial

BACKGROUND

Preclinical studies suggest that for complete midsubstance anterior cruciate ligament (ACL) injuries, a suture repair of the ACL augmented with a protein implant placed in the gap between the torn ends (bridge-enhanced ACL repair [BEAR]) may be a viable alternative to ACL reconstruction (ACLR).

HYPOTHESIS

We hypothesized that patients treated with BEAR would have a noninferior patient-reported outcomes (International Knee Documentation Committee [IKDC] Subjective Score; prespecified noninferiority margin, -11.5 points) and instrumented anteroposterior (AP) knee laxity (prespecified noninferiority margin, +2-mm side-to-side difference) and superior muscle strength at 2 years after surgery when compared with patients who underwent ACLR with autograft.

STUDY DESIGN

Randomized controlled trial; Level of evidence, 1.

METHODS

One hundred patients (median age, 17 years; median preoperative Marx activity score, 16) with complete midsubstance ACL injuries were enrolled and underwent surgery within 45 days of injury. Patients were randomly assigned to receive either BEAR (n = 65) or autograft ACLR (n = 35 [33 with quadrupled semitendinosus-gracilis and 2 with bone-patellar tendon-bone]). Outcomes-including the IKDC Subjective Score, the side-to-side difference in instrumented AP knee laxity, and muscle strength-were assessed at 2 years by an independent examiner blinded to the procedure. Patients were unblinded after their 2-year visit.

RESULTS

In total, 96% of the patients returned for 2-year follow-up. Noninferiority criteria were met for both the IKDC Subjective Score (BEAR, 88.9 points; ACLR, 84.8 points; mean difference, 4.1 points [95% CI, -1.5 to 9.7]) and the side-to-side difference in AP knee laxity (BEAR, 1.61 mm; ACLR, 1.77 mm; mean difference, -0.15 mm [95% CI, -1.48 to 1.17]). The BEAR group had a significantly higher mean hamstring muscle strength index than the ACLR group at 2 years (98.2% vs 63.2%; P < .001). In addition, 14% of the BEAR group and 6% of the ACLR group had a reinjury that required a second ipsilateral ACL surgical procedure (P = .32). Furthermore, the 8 patients who converted from BEAR to ACLR in the study period and returned for the 2-year postoperative visit had similar primary outcomes to patients who had a single ipsilateral ACL procedure.

CONCLUSION

BEAR resulted in noninferior patient-reported outcomes and AP knee laxity and superior hamstring muscle strength when compared with autograft ACLR at 2-year follow-up in a young and active cohort. These promising results suggest that longer-term studies of this technique are justified.

REGISTRATION

NCT02664545 (ClinicalTrials.gov identifier).

Ultrastructural characterization of cells in the tibial stump of ruptured human anterior cruciate ligament, their changes and significance with duration of injury

Fibroblasts and myofibroblasts have been known to be present in both ruptured and intact human anterior cruciate ligament (ACL), and although their relevant histology and immunochemistry have been studied in the past, ultrastructural features of these cells are largely lacking. Therefore, we aim to characterise the ultrastructural details of these cells with the help of transmission electron microscopy (TEM) and to study the changes and their significance with duration of injury. Samples from 60 ruptured human ACL undergoing surgery were obtained and categorised according to duration of injury and observed under TEM with main focus on the following ultrastructural features: cellular morphology, presence of rough endoplasmic reticulum, Golgi apparatus, lamina, myofilaments, and presence of myofibroblasts. These features were further correlated with the duration of injury and association, if any, determined using appropriate statistical analysis. A total of 54 male and 6 female patients with mean duration of the injury of 23.01 ± 26.09 weeks (2-108 weeks) were included in the study and categorised into five groups based on duration of injury as follows: I (< 6 weeks), II (7-12 weeks), III (13-20 weeks), IV (21-50 weeks) and V (> 50 weeks). There was a significant association between the above-mentioned ultrastructural features and the duration of injury (p < 0.05) except for the presence of ovoid fibroblast cells (p = 0.53). Furthermore, number of myofibroblasts and cells with Golgi apparatus and rough endoplasmic reticulum was seen to peak at 13-20 weeks following injury. We describe ultrastructural features of fibroblast of different morphology along with myofibroblasts in the ligaments following injury, the changes in which might have a potential bearing on ligament healing.

Ultrastructural and histological changes in tibial remnant of ruptured anterior cruciate ligament stumps: a transmission electron microscopy and immunochemistry-based observational study

OBJECTIVES

Anterior cruciate ligament (ACL) rupture is a common injury and has a non-union rate of 40-100%. Important cellular events, such as fibroblast proliferation, angiogenesis and change in collagen fibril thickness in the ACL remnant, as described in other dense connective tissue, might have an implication in graft recovery following ACL reconstruction. Thus we conducted a study with an aim to characterize the ultrastructural and histological features of ruptured ACL tibial stump and correlate the same with the duration of injury.

MATERIALS AND METHODS

This was a prospective observational study in which 60 ruptured human ACLs were evaluated for collagen fibril thickness, blood vessel density (per mm²) and fibroblast density (per mm²) with the help of transmission electron microscopy, immunohistochemistry via CD34 antibody staining and light microscopy (H&E staining). The findings were correlated with duration of injury.

RESULTS

Fifty-four male and six female patients with a mean duration of the injury of 23.01 weeks (SD = 26.09; range 2-108 weeks) were included for the study and were divided on the basis of duration of injury as follows: Group I (≤ 6 weeks; N = 16), Group II (7-12 weeks; N = 18), Group III (13-20 weeks; N = 7), Group IV (21-50 weeks; N = 12), Group V (> 50 weeks; N = 7). A significant correlation was seen with blood vessel density (r = 0.303, p = 0.01) and fibroblast density (r = - 0.503, p = 0.001). Thickness of collagen fibril did not correlate with the duration of injury (r = 0.15, p = 0.23). The thickness of the collagen reached its peak after 50 weeks following injury, whereas highest density of blood vessel and fibroblast was seen at 12-20 weeks. Matched pair analysis revealed a significant decrease in collagen fibril thickness and an increase in fibroblast density at 7-12 weeks.

CONCLUSION

Following injury to ACL, the ruptured tibial stump undergoes a series of changes at the cellular level vis-à-vis changes in collagen fibril thickness, vascular density and fibroblast density that possibly suggest an intrinsic healing response. This further may have implications on the functional outcome following ACL reconstruction with remnant preservation.

LEVEL OF EVIDENCE

III.

Combined Anterior Cruciate Ligament Repair and Anterolateral Ligament Reconstruction

There has been a renewed interest in anterior cruciate ligament (ACL) repairs over the last decade with some early promising results in the right patient population. Additionally, the anterolateral ligament has been extensively studied and has recently been shown to have a protective effect on standard ACL reconstructions in a clinical trial. Given its protective effect on ACL reconstructions, we believe this phenomenon is also relevant to ACL repairs and can decrease rerupture rates. In this publication, we demonstrate a surgical technique for ACL repair using an internal brace combined with an anterolateral ligament reconstruction using a gracilis autograft.

Bench-to-bedside: Bridge-enhanced anterior cruciate ligament repair

Anterior cruciate ligament (ACL) injuries are one of the most well-known orthopaedic injuries and are treated with one of the most common orthopaedic procedures performed in the United States. This surgical procedure, ACL reconstruction, is successful at restoring the gross stability of the knee. However, the outcomes of ACL reconstruction can be limited by short and long-term complications, including muscle weakness, graft rupture, and premature osteoarthritis. Thus, new methods of treating this injury are being explored. This review details the pathway of how a tissue engineering strategy can be used to improve the healing of the ACL in preclinical studies and then translated to patients in an FDA-approved clinical study. This review paper will outline the clinical importance of ACL injuries, history of primary repair, the pathology behind failure of the ACL to heal, pre-clinical studies, the FDA approval process for a high risk medical device, and the preliminary results from a first-in-human study. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:2606-2612, 2017.

The past, present and future of ligament regenerative engineering

Regenerative engineering has been defined as the convergence of Advanced Materials Sciences, Stem Cell Sciences, Physics, Developmental Biology and Clinical Translation for the regeneration of complex tissues and organ systems. Anterior cruciate ligament (ACL) reconstruction necessitates the regeneration of bone, ligament and their interface to achieve superior clinical results. In the past, the ACL has been repaired with the use of autologous and allogeneic grafts, which have their respective drawbacks. Currently, investigations on the use of biodegradable matrices to achieve knee stability and permit tissue regeneration are making promising advancements. In the future, utilizing regenerative biology cues to induce an endogenous regenerative response may aid the enhancement of clinical ACL reconstruction outcomes.

A Comparative Animal Study of Tendon Grafts Healing After Remnant-Preserving Versus Conventional Anterior Cruciate Ligament Reconstruction

BACKGROUND The aim of this study was to determine if anterior cruciate ligament (ACL) reconstruction by remnant preservation promotes cell proliferation, vascularization, proprioception recovery, and improved biomechanical properties of the tendon grafts. MATERIAL AND METHODS 75 New Zealand rabbits were randomly assigned into the control group (group A), conventional ACL reconstruction group (group B), ACL reconstruction using remnant preservation and graft through remnant sleeve technique group (group C), and ACL reconstruction using remnant preservation and remnant tensioning technique group (group D). The remnant and healing of tendon grafts in groups C and D were observed at 3, 6, and 12 weeks after surgery, and the mRNA expression levels of VEGF, NT-3 and GAP-43 in ACL (group A) or tendon graft samples (groups B, C, and D) were determined by real-time PCR. Tendon graft cell count, microvessel density (MVD), and proprioceptors were determined by H&E staining, CD34, and S-100 immunohistochemical staining. The biomechanical properties of the tendon graft at week 12 in groups B, C, and D were examined by using a tensile strength test. RESULTS Remnant and tendon grafts were not healed at 3, 6, and 12 weeks after the operation in groups C and D. VEGF, NT-3, and GAP-43 mRNA expressions in groups B, C, and D were higher than those in group A (P<0.05), but no significant difference was observed between groups B, C, and D (P>0.05). Furthermore, tendon graft cell count, MVD, proprioception, and biomechanical properties showed no significant differences (P>0.05) among groups B, C, and D at various time points. CONCLUSIONS There was no significant difference in cell proliferation, vascularization, proprioception recovery, or biomechanical properties of the tendon grafts between remnant-preserving and conventional ACL reconstruction methods.

Effect of low-temperature ethylene oxide and electron beam sterilization on the in vitro and in vivo function of reconstituted extracellular matrix-derived scaffolds

Reconstituted extracellular matrix (ECM)-derived scaffolds are commonly utilized in preclinical tissue engineering studies as delivery vehicles for cells and growth factors. Translation into clinical use requires identifying a sterilization method that effectively removes bacteria but does not harm scaffold function. To determine effectiveness of sterilization and impact on ECM scaffold integrity and function, low-temperature ethylene oxide and 15 kGy electron beam irradiation techniques were evaluated. Scaffold sterility was assessed in accordance to United States Pharmacopeia Chapter 71. Scaffold matrix degradation was determined in vitro using enzymatic resistance tests and gel electrophoresis. Scaffold mechanics including elastic modulus, yield stress and collapse modulus were tested. Lastly, 14 Yorkshire pigs underwent ACL transection and bio-enhanced ACL repair using sterilized scaffolds. Histologic response of ligament, synovium, and lymph nodes was compared at 4, 6, and 8 weeks. Ethylene oxide as well as electron beam irradiation yielded sterile scaffolds. Scaffold resistance to enzymatic digestion and protein integrity slightly decreased after electron beam irradiation while ethylene oxide altered scaffold matrix. Scaffold elastic modulus and yield stress were increased after electron beam treatment, while collapse modulus was increased after ethylene oxide treatment. No significant changes in ACL dimensions, in vivo scaffold resorption rate, or histologic response of synovium, ligament, and lymph nodes with either terminal sterilization technique were detectable. In conclusion, this study identifies two methods to terminally sterilize an ECM scaffold. In vitro scaffold properties were slightly changed without significantly influencing the biologic responses of the surrounding tissues in vivo. This is a critical step toward translating new tissue engineering strategies to clinical trials.

Bio-enhanced repair of the anterior cruciate ligament

Suture repair of the anterior cruciate ligament (ACL) has been widely abandoned in favor of ACL reconstruction, largely because of the high rates of failure and unreliability of the outcomes after suture repair. However, there have been recent basic science studies that suggest that combining a suture repair with a biological adjunct may improve the results of suture repair of the ACL, with several studies in large animal models showing equivalent strength of an ACL treated with bio-enhanced repaired to that of an ACL graft at 3, 6, and 12 months after surgery. In addition, the groups treated with bio-enhanced repair had significantly less osteoarthritis when compared with the animals undergoing ACL reconstruction. These findings have led to a renewed interest in bio-enhanced primary repair as a way to make repair of the ACL a viable option for a select group of patients in the future.

Myofibroblast differentiation: main features, biomedical relevance, and the role of reactive oxygen species

SIGNIFICANCE

Myofibroblasts are prototypical fibrotic cells, which are involved in a number of more or less pathological conditions, from foreign body reactions to scarring, from liver, kidney, or lung fibrosis to neoplastic phenomena. The differentiation of precursor cells (not only of fibroblastic nature) is characterized by a complex interplay between soluble factors (growth factors such as transforming growth factor β1, reactive oxygen species [ROS]) and material properties (matrix stiffness).

RECENT ADVANCES

The last 15 years have seen very significant advances in the identification of appropriate differentiation markers, in the understanding of the differentiation mechanism, and above all, the involvement of ROS as causative and persistence factors.

CRITICAL ISSUES

The specific mechanisms of action of ROS remain largely unknown, although evidence suggests that both intracellular and extracellular phenomena play a role.

FUTURE DIRECTIONS

Approaches based on antioxidant (ROS-scavenging) principles and on the potentiation of nitric oxide signaling hold much promise in view of a pharmacological therapy of fibrotic phenomena. However, how to make the active principles available at the target sites is yet a largely neglected issue.

Human anterior cruciate ligament fibroblasts from immature patients have a stronger in vitro response to platelet concentrates than those from mature individuals

A number of recently published studies have established a substantial age dependence of the response of ACL fibroblasts to stimulation by platelet-rich plasma (PRP). Further in-depth research of this age dependence revealed negative effects on both histological and biomechanical results in a large animal model. However, while it has been postulated that this association could affect potential human applications negatively too it remains to be proven that the same effects occur in human cells. Thus it was the objective of this study to search for age dependence in human fibroblasts before further human experiments are done. Human fibroblasts were obtained from 10 immature and adolescent patients, based on a-priori power calculations, and cultured in a collagen-PRP composite. Three parameters that are pivotal for defect remodeling and wound healing-cell migration, cell proliferation, and scaffold contraction-were chosen as endpoints. Both migration and proliferation were significantly higher in immature cells, but no differences were seen in wound contraction. The former findings suggest that immature patients respond more favorably to treatment with PRP, which consequently might translate into better results in ACL tissue engineering.

Ligament-derived matrix stimulates a ligamentous phenotype in human adipose-derived stem cells

Human adipose stem cells (hASCs) can differentiate into a variety of phenotypes. Native extracellular matrix (e.g., demineralized bone matrix or small intestinal submucosa) can influence the growth and differentiation of stem cells. The hypothesis of this study was that a novel ligament-derived matrix (LDM) would enhance expression of a ligamentous phenotype in hASCs compared to collagen gel alone. LDM prepared using phosphate-buffered saline or 0.1% peracetic acid was mixed with collagen gel (COL) and was evaluated for its ability to induce proliferation, differentiation, and extracellular matrix synthesis in hASCs over 28 days in culture at different seeding densities (0, 0.25 x 10(6), 1 x 10(6), or 2 x 10(6) hASC/mL). Biochemical and gene expression data were analyzed using analysis of variance. Fisher's least significant difference test was used to determine differences between treatments following analysis of variance. hASCs in either LDM or COL demonstrated changes in gene expression consistent with ligament development. hASCs cultured with LDM demonstrated more dsDNA content, sulfated-glycosaminoglycan accumulation, and type I and III collagen synthesis, and released more sulfated-glycosaminoglycan and collagen into the medium compared to hASCs in COL (p <or= 0.05). Increased seeding density increased DNA content incrementally over 28 days in culture for LDM but not COL constructs (p <or= 0.05). These findings suggest that LDM can stimulate a ligament phenotype by hASCs, and may provide a novel scaffold material for ligament engineering applications.

Immature animals have higher cellular density in the healing anterior cruciate ligament than adolescent or adult animals

There has been recent interest in the biologic stimulation of anterior cruciate ligament (ACL) healing. However, the effect of age on the ability of ligaments to heal has not yet been defined. In this study, we hypothesized that skeletal maturity would significantly affect the cellular and vascular repopulation rate of an ACL wound site. Skeletally Immature (open physes), Adolescent (closing physes), and Adult (closed physes) Yucatan minipigs underwent bilateral ACL transection and suture repair using a collagen-platelet composite. The response to repair was evaluated histologically at 1, 2, and 4 weeks. All three groups of animals had completely populated the ACL wound site with fibroblasts at 1 week. The Immature animals had a higher cellular density in the wound site than the Adult animals at weeks 2 and 4. Cells in the Immature ligament wounds were larger and more ovoid than in the Adult wounds. There were no significant differences in the vascular density in the wound site. Animal age had a significant effect on the density of cells populating the ACL wound site. Whether this observed cellular difference has an effect on the later biomechanical function of the repaired ACL requires further study.

Age dependence of expression of growth factor receptors in porcine ACL fibroblasts

Tissue engineering approaches that harness the stimulatory power of platelet-rich plasma have produced encouraging results in anterior cruciate ligament (ACL) repair. However, a number of recent studies have demonstrated age-dependent differences in cellular responses to such an approach. Identifying the reasons for these differences would allow counteracting them and consequently improve outcomes. In this study we hypothesized that these age-related effects are caused by differences in the expression of the receptors for growth factors released from platelet-rich plasma (PRP). Porcine ACL fibroblasts from a predetermined number of animals of different ages were obtained, and mRNA levels of the receptors of platelet-derived growth factor (PDGF), transforming growth factor beta (TGF-beta), fibroblast growth factor (FGF), and vascular endothelial growth factor (VEGF) were determined. Expression levels were compared across age groups (young and adolescent) and regressed on age in days. While no significant difference was seen across groups, the regression analysis showed decreases in receptor expression with increasing age. These differences were statistically significant for TGF-beta receptor 1, FGF receptor, and VEGF receptor 2; and borderline significant for TGF-beta receptor 3 and PDGF receptor. The only receptor that was not associated with age was VEGF receptor 1, a regulator of VEGF receptor 2. These findings suggest that the decrease in growth factor receptor expression as a likely reason for reduced PRP action with increasing age.

Translational studies in anterior cruciate ligament repair

Translational research, which can be explained as the principle of combining advances in both basic research and clinical understanding in a bedside-to-bench-to-bedside approach, has become one of the central themes of present-day medical research. One orthopedic problem that has strongly benefited from such an approach is tissue-engineering-enhanced primary repair of the anterior cruciate ligament. Recent years have shown a clearer definition of the clinical problem and established an underlying mechanistic cause of the incapacity of the anterior cruciate ligament to heal-the premature loss of provisional scaffold in the wound site. These clinical findings were then translated into a research objective, namely, to replace the missing scaffold with a biomaterial with appropriate structural and bio-stimulatory characteristics. Subsequently, a tissue-engineering-based treatment using a collagen-platelet composite was developed and tested in vitro. After proofing the efficacy of this new treatment in the laboratory, it was translated into a potential clinical application, which showed highly successful results in structural integrity and biomechanical capacity in large animal testing. This approach of defining the scientific mechanism underlying a clinical observation and then using that information to design new therapies is but one example of how translational research in tissue engineering can help define and develop new treatments for challenging problems faced by patients.

Current status and potential of primary ACL repair

Anterior cruciate ligament (ACL) rupture occurs in hundreds of thousands of active adolescents and young adults each year. Despite current treatment, posttraumatic osteoarthritis following these injuries is common in these young patients. Thus, there is widespread clinical and scientific interest in improving patient outcomes and preventing osteoarthritis. The current emphasis on the removal of the torn ACL and subsequent replacement with a tendon graft (ACL reconstruction) stems from adherence to a long-held and widely accepted doctrine that the ACL has only a limited healing response and, therefore, cannot heal or regenerate with suture repair. Recent work has shown that, despite an active biologic response in the ACL after injury, the two ends of the torn ligament never reconnect. Additional studies have detailed findings after placement of a substitute provisional scaffold in the wound site of the ACL injury to bridge the gap and initiate healing of the ruptured ligament after primary repair. This technique, called enhanced primary repair, has significant potential advantages over current ACL reconstruction techniques, including the preservation of the complex attachment sites and innervation of these structures, thus retaining much of the biomechanical and proprioceptive function of these tissues. This manuscript summarizes the recent in vitro and in vivo studies in the area of enhancing ACL healing using biologic supplementation. Subsequent work in this area may lead to the development of a novel approach to treat this important injury.

Therapeutic ultrasound stimulation of tendon cell migration

Ultrasound is a therapeutic agent commonly used to treat sports-related tendinopathy. Tendon healing requires tendon cells migration to the repair site, followed by the proliferation and synthesis of extracellular matrix. This study was designed to determine the effect of ultrasound on migration of tendon cells intrinsic to rat Achilles tendon. Furthermore, the existence of a correlation between this effect and the expression of the contractile actin isoform, alpha-smooth muscle (SM) actin, which is associated with cell mobility, was also examined. Cell migration was evaluated by transwell filter migration assay. The mRNA expressions of alpha-SM actin were determined by reverse transcription-polymerase chain reaction. Dose-dependent ultrasound enhancement of tendon cells migration through the transwell filter was demonstrated. Using immunofluorescence stain for alpha-SM actin, the percentages of alpha-SM actin-positive cells of total cells, nonmigrated cells, and migrated cells on the filter were calculated. Ultrasound-treated cells which had migrated to the bottom side of the filter were more likely to express alpha-SM actin than migrated control cells and nonmigrated cells. However, there was no change of mRNA and protein expression of alpha-SM actin as well as expression of FAK and p-FAK. In conclusion, ultrasound stimulates tendon cell migration in association with increased expression of alpha-SM actin of tendon cells.

Age-dependent expression of VEGF isoforms and receptors in the rabbit anterior cruciate ligament

Vascular endothelial growth factor (VEGF) gene gives rise to several distinct isoforms of VEGF. Those isoforms differ in biochemical and biological properties, and it has been reported that their expression patterns are tissue and age specific as well. We investigated the expression levels of VEGF isoforms (VEGF121, VEGF165, VEGF183, VEGF189) and its receptors (VEGFR-1, flt-1 and VEGFR-2, flk-1/KDR) in the anterior cruciate ligament (ACL) of 2- to 3-week-, 2-month-, and 18-month-old New Zealand White rabbits using Sybr green Real-Time RT-PCR. VEGF isoforms and both receptors were expressed in the ACL at all investigated ages. VEGF121 was found to be the most abundant isoform at the ages under investigation, followed by VEGF165, VEGF189 and VEGF183. All isoforms showed decreased expression levels with age, however the larger membrane bound isoforms, VEGF183 and VEGF189, showed the most striking age-associated decrease in expression level. VEGFR-1 expression levels increased with age, while the expression level of VEGFR-2 expression was highest at 2-3 weeks and was significantly lower at 2 and 18 months of age. Distinct age-associated differences in the expression level of VEGF isoforms as well as their receptors suggest differential physiological functions during development, maturation and ageing of the ACL.

Development of a useful technique to discriminate anterior cruciate ligament cells and mesenchymal stem cells—The application of cell electrophoresis

Mesenchymal stem cells (MSCs) can differentiate into multiple nonhematopoietic cell lineages, including osteoblasts, chondrocytes, and ligament cells. The purpose of this study is to identify the difference between MSCs and anterior cruciate ligament (ACL) cells for the application of distinguishing these two cells during the process of MSCs differentiating into ACL cells. Although culture of MSCs and ACL cells have been studied extensively, it was found that these two cells could not be distinguished from their appearance, expression of surface antigens (including CD105, CD34, CD45, CD29, CD44, and CD71), alpha-smooth muscle actin, and mRNAs for type I collagen, type III collagen, and tenascin-C, based on a series of traditional methods for cell identification. Cell electrophoresis, measuring the electrophoretic mobility (EPM) of cells, was proposed to investigate the discrepancy in surface charge properties of MSCs and ACL cells. Surprisingly, the EPM value of MSCs is significantly greater than that of ACL cells (p < 0.001). Although cell electrophoresis cannot determine the specific surface protein, it can reflect the net surface charge density of cell membrane, which can be influenced by the dissociation of functional groups of peripheral membrane proteins. Therefore, it is suggested that cell electrophoresis, while simple and cheap in manipulation, can serve as a useful research tool to assist in identification of MSCs differentiating into ACL cells.

Effects of applied DC electric field on ligament fibroblast migration and wound healing

Applied electric fields (static and pulsing) are widely used in orthopedic practices to treat nonunions and spine fusions and have been shown to improve ligament healing in vivo. Few studies, however, have addressed the effect of electric fields (EFs) on ligament fibroblast migration and biosynthesis. In the current study, we applied static and pulsing direct current (DC) EFs to calf anterior cruciate ligament (ACL) fibroblasts. ACL fibroblasts demonstrated enhanced migration speed and perpendicular alignment to the applied EFs. The motility of ligament fibroblasts was further modulated on type I collagen. In addition, type I collagen expression increased in ACL fibroblasts after exposure to pulsing EFs. In vitro wound-healing studies showed inhibitory effects of static EFs, which were alleviated with a pulsing EF. Our results demonstrate that applied EFs augment ACL fibroblast migration and biosynthesis and provide potential mechanisms by which EFs may be used for enhancing ligament healing and repair.

Tissue engineering for anterior cruciate ligament reconstruction: a review of current strategies

The anterior cruciate ligament (ACL) is one the most commonly injured ligaments of the knee. Chronic ACL insufficiency can result in episodic instability, chondral and meniscal injury, and early osteoarthritis. The intra-articular environment of the ligament precludes normal healing and surgical replacement of the injured ligament is often mandated to restore stability. Current surgical strategies include the use of local autograft or allograft tissues for ligament reconstruction. These procedures have yielded superior long-term clinical results yet have the potential for serious associated morbidities. Existing limitations have prompted ongoing research designed to engineer a replacement ligament that will parallel the native ACL in both its biologic properties and mechanical durability. Ligament engineering necessitates the use of appropriate source cells and a growth matrix to support cell proliferation and collagen synthesis. The identification of appropriate growth modulators including both biochemical factors and mechanical stimuli are requisites for successful tissue growth. The characterization of the elements essential for successful graft development represents a significant challenge for investigators. This review examines the current literature regarding the potential and limitations of ligament engineering and describes the development of a novel 3-dimensional scaffold and bioreactor system at our institution.

The effect of thrombin on ACL fibroblast interactions with collagen hydrogels

Premature loss of provisional scaffold formation has been identified as one of the factors responsible for poor healing of intraarticular tissues. To address this deficiency, substitute provisional scaffolds are being developed. The function of these scaffolds can be enhanced by the addition of specific extracellular matrix proteins. In this study, it was hypothesized that the addition of thrombin to a provisional scaffold material would result in increases in cell proliferation, collagen production, and cell migration within the scaffold. These three parameters are thought to be critical components of wound healing. Gels containing fibrin and collagen supplemented with either 0, 10.5, 21, or 42 U/mL of thrombin were placed in contact with explants of tissue from the anterior cruciate ligament. The addition of thrombin stimulated cell migration at low concentrations and impaired migration at higher concentrations, and had no significant effect on cell proliferation or collagen production. The use of all concentrations of thrombin resulted in mechanically weaker gels. Thus, the use of thrombin to optimize a collagen-platelet rich plasma (PRP) provisional scaffold must be done with caution, and use of high concentrations of thrombin (>42 IU/mL) should be avoided specifically in situations where gel strength or cell ingrowth is important. Use of low concentrations of thrombin (10.5 IU/mL) may be beneficial in applications where a faster set time and enhanced cell migration are desirable and the gel mechanical strength is of secondary importance.

The central ACL defect as a model for failure of intra-articular healing

Intra-articular soft tissues, such as the anterior cruciate ligament (ACL), fail to heal in contrast to the extra-articular medial collateral ligament (MCL), which undergoes classic healing. The goal of this study was to validate a model for failure of intra-articular healing that could be used in the future to test new repair strategies. We conducted a two-part experiment, the first part ex vivo, and the second in vivo. Our initial ex vivo experiments were used to determine the optimal width of the central defect in the canine ACL that would produce reproducible structural properties at time zero. The second experimental series used this optimal scalpel blade width to create a central defect in the canine ACL followed by measurement of structural properties in the ACL after either a 3- or 6-week in vivo healing period. A 3.5-mm beaver blade resulted in a maximum tolerated load of 56.8 +/- 4.7% (mean +/- SEM) of control at time zero. After the 3- and 6-week in vivo healing periods, the maximum load was 74.6 +/- 5.3 at 3 weeks and 64.9 +/- 3.8% at 6 weeks compared to control. Thus, biomechanical parameters tested at 6 weeks after creation of a defect showed no significant gains from defects tested immediately after the creation of injury. The centrally placed ACL defect in this canine model demonstrates failure to mechanically heal, which should prove suitable for future in vivo evaluation of the biomechanical and histological response to tissue engineering repair strategies for intra-articular soft tissues.

Enhanced repair of the anterior cruciate ligament by in situ gene transfer: evaluation in an in vitro model

The inability of the ruptured anterior cruciate ligament (ACL) of the knee joint to heal spontaneously presents numerous clinical problems. Here we describe a novel, gene-based approach to augment ACL healing. It is based upon the migration of cells from the ruptured ends of the ligament into a collagen hydrogel laden with recombinant adenovirus. Cells entering the gel become transduced by the vector, which provides a basis for the local synthesis of gene products that aid repair. Monolayers of bovine ACL cells were readily transduced by first-generation, recombinant adenovirus, and transgene expression remained high after the cells were incorporated into collagen hydrogels. Using an in vitro model of ligament repair, cells migrated from the cut ends of the ACL into the hydrogel and were readily transduced by recombinant adenovirus contained within it. The results of experiments in which GFP was used as the transgene suggest highly efficient transduction of ACL cells in this manner. Moreover, during a 21-day period GFP+ cells were observed more than 6 mm from the severed ligament. This distance is ample for the projected clinical application of this technology. In response to TGF-beta1 as the transgene, greater numbers of ACL cells accumulated in the hydrogels, where they deposited larger amounts of type III collagen. These data confirm that it is possible to transduce ACL cells efficiently in situ as they migrate from the ruptured ACL, that transduction does not interfere with the cells' ability to migrate distances necessary for successful repair, and that ACL cells will respond in a suitable manner to the products of the transgenes they express. This permits optimism over a possible clinical use for this technology.

The 2003 Nicolas Andry Award. Orthopaedic gene therapy

We review progress in the field of orthopaedic gene therapy since the concept of using gene transfer to address orthopaedic problems was initiated approximately 15 years ago. The original target, arthritis, has been the subject of two successful Phase I clinical trials, and additional human studies are pending in rheumatoid arthritis and osteoarthritis. The repair of damaged musculoskeletal tissues also has proved to be a fruitful area of research, and impressive enhancement of bone healing has been achieved in preclinical models. Rapid progress also is being made in the use of gene transfer to improve cartilage repair, ligament healing, and restoration of various additional tissues, including tendon and meniscus. Other applications include intervertebral disc degeneration, aseptic loosening, osteoporosis, genetic diseases, and orthopaedic tumors. Of these various orthopaedic targets of gene therapy, tissue repair is likely to make the earliest clinical impact because it can be achieved with existing technology. Tissue repair may become one of the earliest clinical successes for gene therapy as a whole. Orthopaedics promises to be a leading discipline for the use of human gene therapy.

Tissue engineering of ligaments

Tissue engineering is emerging as a significant clinical option to address tissue and organ failure by implanting biological substitutes for the compromised tissues. As compared to the transplantation of cells alone, engineered tissues offer the potential advantage of immediate functionality. Engineered tissues can also serve as physiologically relevant models for controlled studies of cells and tissues designed to distinguish the effects of specific signals from the complex milieu of factors present in vivo. A high number of ligament failures and the lack of adequate options to fully restore joint functions have prompted the need to develop new tissue engineering strategies. We discuss the requirements for ligament reconstruction, the available treatment options and their limitations, and then focus on the tissue engineering of ligaments. One representative tissue engineering system involving the integrated use of adult human stem cells, custom-designed scaffolds, and advanced bioreactors with dynamic loading is described.

Interspecies variation in the fibroblast distribution of the anterior cruciate ligament

BACKGROUND

The future of the treatment of a ruptured anterior cruciate ligament is likely to involve cell-based therapies. These therapies are intrinsically dependent on the cellular distributions in the ligament. Thus, when selecting an animal model for testing of these new treatment methods, it is important to select a model that has similar cellular distributions to that of the normal human anterior cruciate ligament.

HYPOTHESIS

There are interspecies differences in the histology of the anterior cruciate ligament.

STUDY DESIGN

A descriptive histological study comparing the cell and vessel distribution of normal human anterior cruciate ligaments with that of 3 animal models and anterior cruciate ligaments from osteoarthritic human knees.

METHODS

The histology of each of the anterior cruciate ligament sources was quantified in terms of cell number density, expression of alpha-smooth muscle actin, blood vessel density, and cell nuclear morphology using standardized histomorphometric techniques.

RESULTS

The normal human anterior cruciate ligament was similar to the canine anterior cruciate ligament and the anterior cruciate ligament from patients with osteoarthritis with respect to cell density, blood vessel density, and cell nuclear shape. The normal anterior cruciate ligament had significantly fewer vessels than the bovine anterior cruciate ligament and rounder cells than the bovine and ovine anterior cruciate ligaments.

CONCLUSIONS

There is significant interspecies variation in the histology of the anterior cruciate ligament, with the canine anterior cruciate ligament most similar to the human anterior cruciate ligament.

CLINICAL RELEVANCE

This finding may have an effect on the accuracy of testing of new, cell-based treatments for the ruptured anterior cruciate ligament, such as guided tissue regeneration.

Alpha-smooth muscle actin in pathological human disc nucleus pulposus cells in vivo and in vitro

That a contractile actin isoform has been found in cells of other cartilage tissues in healing and disease states prompted this investigation of the presence of alpha-smooth muscle actin (alpha-SMA) in pathological human intervertebral disc tissue. The presence of this isoform has been reported in human intervertebral disc specimens obtained at autopsy from subjects for whom there were no reported symptoms. An objective of this study was to evaluate the cell density and percentage of alpha-SMA-containing cells in pathological nucleus pulposus tissue obtained from lumbar disc surgery from 17 patients. Additionally, explants of nucleus pulposus material were cultured to determine how alpha-SMA expression changed with time in vitro. Seventy-six 5-mm diameter explants (approximately 2 mm thick) pooled from six lumbar surgeries were cultured for 1, 2, 4, or 6 weeks. Microtomed sections of paraffin-embedded specimens were stained with hematoxylin and eosin or a monoclonal antibody to alpha-SMA. Histologically, cells were categorized as to alpha-SMA phenotype (positive or negative), and the areal cell density was determined. The evaluation of the cultured nucleus pulposus explants also included documentation of the percentage of cells that were round or elongated and the percentage of the cells that were part of a group (group: >/= 2 cells). Every nucleus pulposus section exhibited the presence of alpha-SMA-containing cells, which accounted for approximately 24 percent of the cells in vivo. In vivo, the cell density was significantly higher in older individuals (p = 0.02). The average time for cell outgrowth from the explants was 8.6 days. Approximately 10-15 percent of the cells in the explants stained positive for alpha-SMA. The time in culture had no significant effect on any of the outcome measures except the percentage of alpha-SMA-containing cells that were round (p = 0.008), with values decreasing through 4 weeks and then slightly rising at 6 weeks. The role of alpha-SMA in intervertebral disc pathology warrants further investigation.

Expression of α-Smooth Muscle Actin by and Contraction of Cells Derived from Synovium

Cells derived from synovium have drawn interest as donor cells for articular cartilage tissue engineering because they have been implicated in certain cartilage repair processes in vivo and the chondrogenic potential of the cells has been demonstrated in vitro. Studies have demonstrated that several other types of musculoskeletal connective tissue cells--including chondrocytes, fibrochondrocytes, ligament fibroblasts and osteoblasts, and mesenchymal stem cells can express the gene for the contractile actin isoform, alpha-smooth muscle actin (SMA), and can contract analogs of extracellular matrix in vitro. Although the physiological roles of SMA-enabled contraction of these cells have yet to be established, cell-mediated contraction of scaffolds employed for tissue engineering can alter the pore diameter of the matrix and distort its overall shape, and thus needs to be addressed. Toward this goal, the objective of this study was to investigate the expression of SMA by synovial cells and to evaluate their contraction of collagen-glycosaminoglycan (GAG) scaffolds. Synovial membranes obtained from the knees (stifle joints) of six adult dogs were evaluated for the presence of SMA by immunohistochemistry. Cells isolated from the synovial tissue were expanded through seven passages in monolayer culture, with samples from each passage allocated for Western blot analysis of SMA. Cells from passage 4 were seeded into porous type I collagen-GAG matrices and cultured for 4 weeks. Synovial cell-mediated contraction of the scaffolds was determined by measuring the diameters of the cell-seeded scaffolds and nonseeded controls every other day. Synovium-derived cells cultured as micropellets or in collagen-GAG matrices were incubated in chondrogenic medium with and without fetal bovine serum and evaluated for chondrogenesis by type II collagen immunohistochemistry. Immunohistochemistry revealed the presence of SMA in some cells (less than 10% of the cells) in the intimal layer of synovium from four of the five animals analyzed. Western blot analysis demonstrated a regular increase in the amount of SMA in the synovium-derived cells with passage number. Synovial cell-mediated contraction of the collagen-GAG scaffolds reached a value of 43% of the original diameter after 4 weeks, comparable to that found with other musculoskeletal cell types. Incubation of micropellet cultures of synovium-derived cells with chondrogenic medium revealed trace amounts of type II collagen production by immunohistochemistry. The findings of this study indicate that control of SMA-enabled contraction may be important when employing synovial cells for cartilage repair procedures, and warrant further investigation into the physiological role of SMA expression in synovial cells.

Biocompatible collagen scaffolds from a human amniotic membrane: physicochemical and in vitro culture characteristics

A reconstituted collagen membrane from human amnion has been investigated as a source of collagen matrix, which could be used as a substratum for culturing human fibroblasts. The suitability of pepsin-solubilized reconstituted human amniotic membrane, before and after cross-linking with chitosan, as a dermal matrix for culturing fibroblast was assessed by morphologic, physicochemical, cytotoxic and histochemical methods. Measurement of thermodynamic behaviour, by differential scanning calorimetric (DSC) and thermogravimetric analysis (TGA), and tensile strength suggested that the cross-linked membrane had sufficient elasticity to serve as an efficient dermal substrate for in vitro culture of fibroblasts. Fibroblasts cultured on the chitosan cross-linked collagen membrane had good adherence, retaining their morphology as indicated by microscopic analysis. Proliferation of fibroblasts. observed on this membrane affirms its non-toxic nature. These results support the application of reconstituted human amniotic collagen membrane as collagenous scaffolds to culture fibroblasts in vitro.

Inhibition of tendon cell migration by dexamethasone is correlated with reduced alpha-smooth muscle actin gene expression: a potential mechanism of delayed tendon healing

Local corticosteroid injection is commonly used to treat sports-related tendon injuries. However, isolated cases of tendon rupture following injection suggest that this treatment may impair the healing process. Tendon healing requires the migration of tendon cells to the repair site, followed by the proliferation and synthesis of the extracellular matrix. This study was designed to determine the effect of dexamethasone on the migration of tendon cells intrinsic to rat Achilles tendon at concentrations similar to those typically used for local injection treatment. Furthermore, the existence of a correlation between this effect and the expression of the contractile actin isoform, alpha-smooth muscle (SM) actin, which is associated with cell motility, was also examined. Using cultured tendon cells, migration was evaluated by counting the number of initial outgrowths from the tendon explants and by transwell filter migration assay. The distribution and assembly of alpha-SM actin were assessed by immunocytochemistry. The mRNA and protein expressions of alpha-SM actin were determined by reverse transcription-polymerase chain reaction (RT-PCR) and Western blot analysis. Dose-dependent dexamethasone inhibition was demonstrated for both tendon cells outgrowth from the explants, ex vivo, and migration of tendon cells through the transwell filter, in vitro. Immunocytochemical staining revealed significant decreases in both the amount and assembly of alpha-SM actin in cells. Suppression of mRNA expression and protein level of alpha-SM actin was revealed from RT-PCR and Western blot analyses. In conclusion, dexamethasone inhibits tendon cell migration that is correlated with decreased gene expression of alpha-SM actin.

The effect of selected growth factors on human anterior cruciate ligament cell interactions with a three-dimensional collagen-GAG scaffold

Our work focuses on development of a collagen-glycosamimoglycan (CG) scaffold to facilitate ligament healing in the gap between the ruptured ends of the human anterior cruciate ligament (ACL). In the present investigation, we evaluated the effects of selected growth factors on human ACL cell responses important in tissue regeneration, namely cell migration, proliferation, collagen production, and expression of alpha-smooth muscle actin (SMA).

METHODS

Explants from six human ACLs were cultured on top of a CG scaffold. Culture conditions were with either 2% FBS (control), or 2% FBS supplemented with TGF-beta1, PDGF-AB, EGF, or FGF-2. Histologic cell distribution, total DNA content, proliferation rate, rate of collagen synthesis, scaffold diameter and percentage of SMA positive cells were determined at two, three and four weeks.

RESULTS

The addition of TGF-beta1 to the culture medium resulted in increased cell number, increased collagen production and increased expression of SMA within the scaffold. Supplementation with PDGF-AB resulted in increased cell proliferation rates within the scaffold and increased collagen production. The addition of FGF-2 resulted in increased cell proliferation rates and slowed rates of scaffold shrinkage when compared with the control group.

DISCUSSION

This work suggests that certain growth factors can alter the biologic functions of human ACL cells in a CG scaffold implanted as a bridge at the site of an ACL rupture. Based on these findings, the addition of selected growth factors to an implantable CG scaffold may facilitate ligament healing in the gap between the ruptured ends of the human ACL.

Expression of smooth muscle actin in cells involved in distraction osteogenesis in a rat model

Distraction osteogenesis has proven to be of great value for the treatment of a variety of musculoskeletal problems. Little is still known, however, about the phenotypic changes in the cells participating in the bone formation process, induced by the procedure. Recent findings of the expression of a contractile muscle actin isoform, alpha-smooth muscle actin (SMA), in musculoskeletal connective tissue cells prompted this immunohistochemical study of the expression of SMA in cells participating in distraction osteogenesis in a rat model. The tissues within and adjacent to the distraction site could be distinguished histologically on the basis of cell morphology, density, and extracellular matrix make-up. The percentage of SMA-containing cells within each tissue zone was graded from 0 to 4. The majority of the cells in each of the zones stained positive for SMA within five days of the distraction period. The SMA-containing cells included those with elongated morphology in the center of the distraction site and the active osteoblasts on the surfaces of the newly forming bone. These finding warrant further investigation of the role of this contractile actin isoform in distraction osteogenesis and investigation of the effects of modulation of this actin isoform on the procedure.

Future direction of the treatment of ACL ruptures

The future of treatment of the ACL rupture is changing as our understanding of the biology surrounding the ACL continues to increase. It is our expectation that clinically applicable treatments, including the repair of the ACL and the development of a biologically engineered ACL, will occur in the next decade.

Cell outgrowth from the human ACL in vitro: regional variation and response to TGF-beta1

One of the new methods being developed to stimulate healing of the human anterior cruciate ligament (ACL) after rupture is the implantation of a biodegradable scaffold which the host cells invade, populate and remodel. One of the cellular behaviors critical to the success of this method is cell outgrowth from the ligament remnants onto an adjacent scaffold. As morphological differences have been previously reported in the proximal and distal human ACL, the primary aim of this study was to determine if the cells from the proximal and distal ACL had different outgrowth behaviors as well. A second aim was to determine whether TGF-beta1, reported to be a mitogen for fibroblasts, would affect the outgrowth behaviors from the proximal and distal ACL. To achieve these aims, explants from both the proximal and distal human ACL were placed into culture under various conditions and outgrowth measured for 42 days. In explants cultured with 10% fetal bovine serum (high serum group) the explants from the proximal ACL had an earlier start of outgrowth than the distal explants (p < 0.015), and outgrowth rates were similar in the two groups. In explants cultured with 2% fetal bovine serum (low serum group), the explants from the proximal ACL had an earlier start to outgrowth (p < 0.003) as well as a faster rate of outgrowth (p < 0.004) than the distal explants. The addition of TGF-beta1 to the low serum cultures significantly slowed the rate of outgrowth from both groups of ACL explants (p < 0.025). These results suggest that outgrowth behaviors are different in the proximal and distal human ACL, and that TGB-beta1 has an inhibitory effect on cell outgrowth from ACL explants. However, this study is only a first step, and additional experiments are needed to further optimize tissue engineering parameters for enhancement of the repair of the ACL.

Novel cell-scaffold interactions encountered in tissue engineering: contractile behavior of musculoskeletal connective tissue cells

Methods employed in the course of tissue engineering often offer unique opportunities to observe cell-matrix interactions that cannot otherwise be viewed. These observations may provide insights into cell behavior than can contribute important new knowledge about cell biology. One such set of observations led to the discoveries that musculoskeletal connective tissue cells express a contractile muscle actin isoform, alpha-smooth muscle actin, and can contract. This knowledge may help to explain how these cells generate forces required for certain physiological and pathological functions, and this information may inform future approaches to regulate this function to advance tissue engineering. Tissue engineering science is thus emerging as an importance force that can both contribute to cell and molecular biology and add to the fund of knowledge supporting the production of tissue in vitro or in vivo to improve the management of a wide variety of disorders.

Expression of smooth muscle actin in connective tissue cells participating in fracture healing in a murine model

The role of alpha-smooth muscle actin (SMA)-expressing fibroblasts in the contraction of skin wounds has been known for three decades. Recent studies have demonstrated that osteoblasts can also express the gene for this contractile muscle actin isoform and can contract a collagen-glycosaminoglycan analog of extracellular matrix in vitro. These findings provided rationale for the hypothesis that SMA-expressing cells contribute to fracture healing by drawing the bone ends together. To begin to test this hypothesis, immunohistochemistry was employed to evaluate the distribution of connective tissue cells expressing SMA in a mouse model of successful fracture healing. The results demonstrated that the majority of the cells comprising the mesenchymal tissue interposed between the fracture ends contained SMA after 7 and 21 days, supporting the working hypothesis. Most of the osteoblasts lining the surfaces of newly forming bone and the chondrocytes comprising the cartilaginous callus also expressed this contractile actin isoform. The maximal SMA expression extended from 7 to 21 days postfracture. The finding of high levels of SMA expression in connective tissue cells participating in fracture healing suggests that SMA-enabled contraction may be playing a role in the healing process. These results warrant further study of the specific SMA-dependent cell behavior.

Expression of smooth muscle actin in osteoblasts in human bone

It is well known that certain connective tissue cells (viz., dermal fibroblasts) can express the gene for a muscle actin--alpha-smooth muscle actin--and can contract. This process contributes to skin wound closure and is responsible for Dupuytren's contracture. The objective of this study was to determine if human osteoblasts can also express the gene for alpha-smooth muscle actin. Immunohistochemistry using a monoclonal antibody for alpha-smooth muscle actin was performed on human cancellous bone samples obtained from 20 individuals at the time of total joint arthroplasty. The percentages of resting and active osteoblasts on the bone surfaces containing this muscle actin isoform were evaluated. Explants of human bone were also studied for the expression of alpha-smooth muscle actin in the tissue and in the outgrowing cells with time in culture. Western blot analysis was performed to quantify the alpha-smooth muscle actin content of the outgrowing cells relative to smooth muscle cell controls. Nine +/- 2% (mean +/- SEM; n = 20) of the cells classified as inactive osteoblasts and 69 +/- 3% (n = 19) of the cells identified as active osteoblasts on the bone surface contained alpha-smooth muscle actin. This difference was highly statistically significant (Student's t test, p < 0.0001). Similar profiles of alpha-smooth muscle actin-expressing cells were found in explants cultured for up to 12 weeks. Cells forming a layer on the surface of the explants and growing out from them in monolayer also contained alpha-smooth muscle actin by immunohistochemistry and Western blot analysis. Human osteoblasts can express the gene for alpha-smooth muscle actin. This expression should be considered a phenotypic characteristic of this cell type, conferred by its progenitor cells: bone marrow stromal-derived stem cells, and perhaps pericytes and smooth muscle cells.

Biology and biomechanics

Increased participation by the general population in athletic activities leads to increased trauma to bones, joint surfaces, and soft tissues. Management and treatment of these injuries has significantly improved over the past few decades. The application of knowledge gained from basic science research in biology and biomechanics has continuously contributed to that. Biological advances have been made in the field of gene therapy, cell therapy, and tissue engineering. Certainly, the greatest focus is bone and cartilage research that will lead to improved fracture repair in the traumatic injured population, as well as prevention of early osteoarthritic changes in the injured athletic population. In biomechanical research, contributions have been made to further understand kinematic behavior of joints that will lead to improved ligament reconstruction techniques and rehabilitation regimens. Various fixation techniques and several different ligament reconstruction techniques have been studied and validated. In the future, improved understanding of ligament healing, graft incorporation, and revascularization will lead to improved outcome of surgical reconstruction techniques in orthopaedic sports medicine. Exciting research has been performed over the past years and will be reviewed in this article.

The future of anterior cruciate ligament surgery

Future anterior cruciate ligament surgery techniques will evolve from emphasizing the technical factors involved in successful ligament reconstruction to emphasizing the biomechanical, neuromuscular, and biologic factors, which will enhance healing. Advances in computer and robotic technology will help the surgeon perform anterior cruciate ligament reconstruction. The importance of the anterior cruciate ligament and its relationship with other anatomic and neuromuscular structures of the knee has been well researched over the past decade; the next decade will combine this knowledge with technological and biological advancements.

The migration of cells from the ruptured human anterior cruciate ligament into collagen-glycosaminoglycan regeneration templates in vitro

Guided tissue regeneration of the ruptured anterior cruciate ligament (ACL) offers the potential benefits of retaining the complex footprints of the ACL and the proprioceptive nerve fibers of the tissue. For this approach to be successful, ACL cells must retain the ability to migrate into an adjacent regeneration template, or scaffold, after ligament rupture. Ruptured ACLs were obtained from the knees of four men, ages 25-35, at the time of ACL reconstruction. Explants of ACL tissue were taken from three locations along the longitudinal axis of the remnant: the rupture site, the middle of the remnant, and far from the rupture site. These three areas have been found to be distinct histologically, with the region far from the rupture site having a histologic appearance similar to the intact ligament. Explants from each area were cultured in conventional tissue culture dishes (2-D culture) and on porous collagen-glycosaminoglycan (CG) scaffolds. Two-dimensional outgrowth was measured 3 times a week, and the 3-D explant/scaffold constructs were examined at 1, 2, 3 and 4 weeks to assess outgrowth of cells into the scaffold. The cell number density and expression of a-smooth muscle actin (SMA) were determined at each time point. The decrease in the diameter of the scaffolds and non-seeded controls were determined as a function of time in culture. The outgrowth of cells onto the tissue culture dishes was observed to begin as early as 3 days and as late as 21 days, with outgrowth first detected at an average of 6.8 +/- 2.0 days after explantation. In general, there was a larger area of outgrowth at the 2-week time point from explants with higher cell number density and higher blood vessel density. The 2-week area of outgrowth also correlated with the percentage of SMA-positive cells in the explant. In the experimental constructs with CG scaffolds, fibroblasts were noted to migrate from the human ACL explants into the templates at the earliest time point recorded (I week). The migration and proliferation of cells from the explants in the CG matrices resulted in an increase in the cell density in the scaffolds with time. There was a significant effect of the location from which the explant was taken on cell density in the scaffold, with a higher density of cells migrating from the explants from the rupture site of the ACL specimens. The percentage of cells staining positive for the SMA isoform varied from 0 to 50% of cells in the scaffold. Scaffolds co-cultured with explants showed a reduction in diameter that was significantly affected by time in culture and the location in the ACL from which the explant was taken. The percentage contraction attributed to the cells was 15% at 2 weeks, and increased to 27% for the injury-zone explant at 4 weeks. There was a significant correlation of the cell-mediated contraction of the matrices at 4 weeks with the cell density in the scaffolds, but not with the number of SMA-positive cells in the scaffolds. These data demonstrate that cells in the human ACL retain their ability to migrate into an adjacent CG scaffold in vitro, weeks after complete rupture. Moreover, the ACL-derived cells can express a contractile actin isoform and can contract a CG analog of extracellular matrix.