Meniscal repair by fibrocartilage? An experimental study in the dog

Successful Total Meniscus Reconstruction Using a Novel Fiber-Reinforced Scaffold: A 16- and 32-Week Study in an Ovine Model

BACKGROUND

Meniscus injuries in the United States result in an estimated 850,000 surgical procedures each year. Although meniscectomies are the most commonly performed orthopaedic surgery, little advancement has been made in meniscus replacement and regeneration, and there is currently no total meniscus replacement device approved by the Food and Drug Administration.

HYPOTHESIS

A novel fiber-reinforced meniscus scaffold can be used as a functional total meniscus replacement.

STUDY DESIGN

Controlled laboratory study.

METHODS

A tyrosine-derived, polymer fiber-reinforced collagen sponge meniscus scaffold was evaluated mechanically (tensile and compressive testing) and histologically after 16 and 32 weeks of implantation in an ovine total meniscectomy model (N = 20; 16 implants plus 4 meniscectomies, divided equally over the 2 time periods). The extent of cartilage damage was also measured on tibial plateaus by use of toluidine blue surface staining and on femoral condyles by use of Mankin scores on histological slides.

RESULTS

Scaffolds induced formation of neomeniscus tissue that remained intact and functional, with breaking loads approximating 250 N at both 16 and 32 weeks compared with 552 N for native menisci. Tensile stiffness values (99 and 74 N/mm at 16 and 32 weeks, respectively) were also comparable with those of the native meniscus (147 N/mm). The compressive modulus of the neomeniscus tissue (0.33 MPa at both 16 and 32 weeks) was significantly increased compared with unimplanted (time 0) scaffolds (0.15 MPa). There was histological evidence of extensive tissue ingrowth and extracellular matrix deposition, with immunohistochemical evidence of types I and II collagen. Based on significantly decreased surface damage scores as well as Mankin scores, the scaffold implants provided greater protection of articular cartilage compared with the untreated total meniscectomy.

CONCLUSION

This novel fiber-reinforced meniscus scaffold can act as a functional meniscus replacement, with mechanical properties similar to those of the native meniscus, while protecting the articular cartilage of the knee from the extensive damage after a total meniscectomy.

CLINICAL RELEVANCE

This meniscus replacement scaffold has the potential to improve surgical treatment and provide better long-term outcomes for those suffering from severe meniscus damage.

Biomechanical properties of murine meniscus surface via AFM-based nanoindentation

This study aimed to quantify the biomechanical properties of murine meniscus surface. Atomic force microscopy (AFM)-based nanoindentation was performed on the central region, proximal side of menisci from 6- to 24-week old male C57BL/6 mice using microspherical tips (Rtip≈5µm) in PBS. A unique, linear correlation between indentation depth, D, and response force, F, was found on menisci from all age groups. This non-Hertzian behavior is likely due to the dominance of tensile resistance by the collagen fibril bundles on meniscus surface that are mostly aligned along the circumferential direction. The indentation resistance was calculated as both the effective modulus, Eind, via the isotropic Hertz model, and the effective stiffness, Sind = dF/dD. Values of Sind and Eind were found to depend on indentation rate, suggesting the existence of poro-viscoelasticity. These values do not significantly vary with anatomical sites, lateral versus medial compartments, or mouse age. In addition, Eind of meniscus surface (e.g., 6.1±0.8MPa for 12 weeks of age, mean±SEM, n=13) was found to be significantly higher than those of meniscus surfaces in other species, and of murine articular cartilage surface (1.4±0.1MPa, n=6). In summary, these results provided the first direct mechanical knowledge of murine knee meniscus tissues. We expect this understanding to serve as a mechanics-based benchmark for further probing the developmental biology and osteoarthritis symptoms of meniscus in various murine models.

The collagen meniscus implant

Lesions in the meniscus occur particularly in young, active patients in the nonvascularized area which, consequently have a bad intrinsic healing capacity. This has a large impact on the mobility and function of the knee joint. Lesions, and partial lesions, lead to the progression of osteoarthritis over time in a large proportion of patients. The only clinical treatment for severe cases so far is an allograft donor meniscus, which is used mostly in patients with severe osteoarthritis with a completely destroyed meniscus. However, this technique still has to be considered as experimental and, thus, is not yet used on a routine basis. Various technical solutions have been advocated to repair meniscus lesions. One solution is to perform a partial meniscectomy and insert a collagen meniscus implant (CMI) at the site of the lesion. However, the initial mechanical properties of the collagen scaffold are inferior to the native meniscus. Therefore, it is only possible to perform a CMI implantation if the peripheral rim of the meniscus is still intact. Histology of preclinical and clinical biopsies of the implanted CMI demonstrated a repopulation of the scaffold by fibrous tissue and in time a remodeling of the fibrous tissue into fibrocartilaginous-like tissue. Based on histology, the ingrowth of new tissue into the CMI might occur by a process of synovial overgrowth, but other mechanisms of revitalization are also possible. Although some clinical studies demonstrated improvement in outcome scores, the number of patients was small in all studies and the positive effect on the prevention of progression of osteoarthritis was not compared with control groups.

Current strategies in meniscal regeneration

The meniscus plays an important role in the biomechanics and tribology of the knee joint. Damage to or disease of the meniscus is now recognized to predispose to the development of osteoarthritis. Treatment of meniscal injury through arthroscopic surgery has become one of the most common orthopedic surgical procedures, and in the United States this can represent 10 to 20% of procedures related to the knee. The meniscus has a limited healing capacity constrained to the vascularized periphery and therefore, surgical repair of the avascular regions is not always feasible. Replacement and repair of the meniscus to treat injuries is being investigated using tissue engineering strategies. Promising as these approaches may be, there are, however, major barriers to overcome before translation to the clinic.

Analysis of frictional behavior and changes in morphology resulting from cartilage articulation with porous polyurethane foams

Porous polyurethane foams (PUR) have been extensively evaluated as meniscal replacement materials and show great promise enabling infiltration of cells and fibrocartilage formation in vivo. Similar to most materials, PUR demonstrates progressive degeneration of opposing cartilage; however, the damage mechanism is impossible to determine because no information exists on the frictional properties of PUR-cartilage interfaces. The goals of this study were to characterize the frictional behavior of a cartilage-PUR interface across a range of articulating conditions and assess the resulting morphological changes to the cartilage surface following articulation. Articular cartilage was oscillated against PUR or stainless steel using phosphate-buffered saline (PBS) and synovial fluid as lubricants. Following friction testing, cartilage and PUR samples were analyzed with environmental scanning electron microscopy and histological staining to determine changes in tissue morphology. Stribeck-surface analysis demonstrated distinct lubrication modes; however, boundary mode lubrication was dominant in cartilage-PUR interfaces and the low-friction pressure-borne lubrication mechanism present in native joints was absent. Microscopy noted obvious wear, with disruption of the collagen architecture and concomitant proteoglycan loss in cartilage articulated against PUR. These data collectively point to the importance of frictional properties as design parameters for implants and materials for soft tissue replacement.

Candidate bone-tissue-engineered product based on human-bone-derived cells and polyurethane scaffold

Biodegradable polyurethanes (PURs) have recently been investigated as candidate materials for bone regenerative medicine. There are promising reports documenting the biocompatibility of selected PURs in vivo and the tolerance of certain cells toward PURs in vitro - potentially to be used as scaffolds for tissue-engineered products (TEPs). The aim of the present study was to take a step forward and create a TEP using human osteogenic cells and a polyurethane scaffold, and to evaluate the quality of the obtained TEP in vivo. Human-bone-derived cells (HBDCs) were seeded and cultured on polyurethane scaffolds in a bioreactor for 14 days. The TEP examination in vitro was based on the evaluation of cell number, cell phenotype and cell distribution within the scaffold. TEPs and control samples (scaffolds without cells) were implanted subcutaneously into SCID mice for 4 and 13 weeks. Explants harvested from the animals were examined using histological and immunohistochemical methods. They were also tested in mechanical trials. It was found that dynamic conditions for cell seeding and culture enable homogeneous distribution, maintaining the proliferative potential and osteogenic phenotype of the HBDCs cultured on polyurethane scaffolds. It was also found that HBDCs implanted as a component of TEP survived and kept their ability to produce the specific human bone extracellular matrix, which resulted in higher mechanical properties of the harvested explants when preseeded with HBDCs. The whole system, including the investigated PUR scaffold and the method of human cell seeding and culture, is recommended as a candidate bone TEP.

Influence of the meniscus on friction and degradation of cartilage in the natural knee joint

BACKGROUND

Total meniscectomy has been shown to cause early-onset arthritis in the underlying cartilage and bone in the knee joint, demonstrating that the meniscus plays an important protective role in the load carrying capacity. Relationships between friction and wear in synovial joints are complex due to the biphasic nature of articular cartilage and the time dependency of tribological responses. Determination of friction and wear in the whole natural joint in vitro or in vivo is technically difficult and the tribological effect of meniscectomy has not been previously studied in an articulating knee joint.

OBJECTIVE

The aim of this study was to use a tribological simulation of the medial compartmental bovine knee, to investigate friction and wear, with and without the meniscus. We hypothesised that meniscectomy would lead to elevated contact stress and frictional coefficient across the joint.

METHODS

Skeletally mature bovine medial compartmental knee joints were dissected and mounted in a pendulum friction simulator, which was used to apply physiologically relevant loading and motion. Wear was quantified using micro-MRI scans and surface profilometry.

RESULTS

Knees tested with the intact meniscus showed no change in surface roughness and no detectable cartilage loss or deformation. However, increased contact stress and frictional coefficient upon removal of the meniscus, led to immediate surface fibrillation, biomechanical wear and permanent deformation of cartilage.

CONCLUSIONS

This study presents, for the first time, an in vitro model simulation system to investigate the tribological effects of meniscectomy and meniscus repair and regeneration.

An allogenic cell-based implant for meniscal lesions

BACKGROUND

Meniscal tears in the avascular zones do not heal. Although tissue-engineering approaches using cells seeded onto scaffolds could expand the indication for meniscal repair, harvesting autologous cells could cause additional trauma to the patient. Allogenic cells, however, could provide an unlimited amount of cells.

HYPOTHESIS

Allogenic cells from 2 anatomical sources can repair lesions in the avascular region of the meniscus.

STUDY DESIGN

Controlled laboratory study.

METHODS

Both autologous and allogenic chondrocytes were seeded onto a Vicryl mesh scaffold and sutured into a bucket-handle lesion created in the medial menisci of 17 swine. Controls consisted of 3 swine knees treated with unseeded implants and controls from a previous experiment in which 4 swine were treated with suture only and 4 with no treatment. Menisci were harvested after 12 weeks and evaluated histologically for new tissue and percentage of interface healing surface; they were also evaluated statistically.

RESULTS

The lesions were closed in 15 of 17 menisci. None of the control samples demonstrated healing. Histologic analysis of sequential cuts through the lesion showed formation of new scar-like tissue in all experimental samples. One of 8 menisci was completely healed in the allogenic group and 2 of 9 in the autologous group; the remaining samples were partially healed in both groups. No statistically significant differences in the percentage of healing were observed between the autologous and allogenic cell-based implants.

CONCLUSION

Use of autologous and allogenic chondrocytes delivered via a biodegradable mesh enhanced healing of avascular meniscal lesions.

CLINICAL RELEVANCE

This study demonstrates the potential of a tissue-engineered cellular repair of the meniscus using autologous and allogenic chondrocytes.

Meniscal Repair With Fibrocartilage Engineering

Injuries to the knee meniscus, particularly those in the avascular region, pose a complex problem and a possible solution is tissue engineering of a replacement tissue. Tissue engineering of the meniscus involves scaffold selection, addition of cells, and stimulation of the construct to synthesize, maintain, or enhance matrix production. An acellular collagen implant is currently in clinical trials and there are promising results with other scaffolds, composed of both polymeric and natural materials. The addition of cells to these constructs may promote good matrix production in vitro, but has been studied in a limited manner in animal studies. Cell sources ranging from fibroblasts to stem cells could be used to overcome challenges in cell procurement, expansion, and synthetic capacity currently encountered in studies with fibrochondrocytes. Manipulation of construct culture with exogenous growth factors and mechanical stimulation will also likely play a role in these strategies.

Replacement of the knee meniscus by a porous polymer implant: a study in dogs

BACKGROUND

Meniscectomy will lead to articular cartilage degeneration in the long term. Therefore, the authors developed an implant to replace the native meniscus.

HYPOTHESIS

The porous polymer meniscus implant develops into a neomeniscus and protects the cartilage from degeneration.

STUDY DESIGN

Controlled laboratory study.

METHODS

In a dog model, a porous polymer scaffold with optimal properties for tissue infiltration and regeneration of a neomeniscus was implanted and compared with total meniscectomy. The tissue infiltration and redifferentiation in the scaffold, the stiffness of the scaffold, and the articular cartilage degeneration were evaluated.

RESULTS

Three months after implantation, the implant was completely filled with fibrovascular tissue. After 6 months, the central areas of the implant contained cartilage-like tissue with abundant collagen type II and proteoglycans in their matrix. The foreign-body reaction remained limited to a few giant cells in the implant. The compression modulus of the implant-tissue construct still differed significantly from that of the native meniscus, even at 6 months. Cartilage degeneration was observed both in the meniscectomy group and in the implant group.

CONCLUSION

The improved properties of these polymer implants resulted in a faster tissue infiltration and in phenotypical differentiation into tissue resembling that of the native meniscus. However, the material characteristics of the implant need to be improved to prevent degeneration of the articular cartilage.

CLINICAL RELEVANCE

The porous polymer implant developed into a polymer-tissue construct that resembled the native meniscus, and with improved gliding characteristics, this prosthesis might be a promising implant for the replacement of the meniscus.

Tensile and Compressive Properties of the Medial Rabbit Meniscus

Quantification of the material properties of the meniscus is of paramount importance, creating a ‘gold-standard’ reference for future tissue engineering research. The purpose of this study was to determine the compressive and circumferential tensile properties in the rabbit meniscus. Creep and recovery indentation experiments were performed on the meniscus using a creep indentation apparatus and analysed via a finite element optimization method to determine the compressive material properties at six topographical locations. Tensile properties of samples taken circumferentially from the rabbit meniscus were also examined. Results show that the femoral side of the anterior portion exhibits the highest aggregate modulus (510 ± 100 kPa) and shear modulus (240 ± 40 kPa), while the lowest aggregate modulus (120 ± 30 kPa) and shear modulus (60 ± 20 kPa) were found on the femoral side of the posterior location. Values of 156.6 ± 48.9 MPa for Young's modulus and of 21.6 ± 7.0 MPa for the ultimate tensile strength of were found from the tensile samples, which are similar to the values found in other animal models. These baseline values of material properties will be of help in future tissue engineering efforts.

Biomechanical characteristics of the normal medial and lateral porcine knee menisci

The purpose of this investigation was to examine the compressive properties of the porcine meniscus at a variety of topographical locations using a creep indentation experiment. Three different solution techniques were used to analyse the creep response of the tissue. Specifically, the indentation stiffness, aggregate modulus, permeability, Poisson's ratio, and shear modulus were determined at six different testing locations (anterior, central, and posterior regions; femoral and tibial sides) of both the medial and lateral porcine menisci. Results indicate topographical variations among the testing locations, with the femoral-anterior portion of the medial meniscus having the highest indentation stiffness (350+/-110 kPa), aggregate modulus (270+/-90 kPa), and shear modulus (140+/-40 kPa). The tibial-posterior region of the medial meniscus exhibited the lowest indentation stiffness (170+/-40 kPa), aggregate modulus (130+/-30 kPa), and shear modulus (60+/-20 kPa). No statistical differences were found at the six tested locations of the lateral meniscus.

Prosthetic replacement of the medial meniscus in cadaveric knees: does the prosthesis mimic the functional behavior of the native meniscus?

Meniscus replacement by a polymer meniscus prosthesis in dogs resulted in generation of new meniscal tissue.

HYPOTHESIS

Optimal functioning of the prosthesis would involve realistic deformation and motion patterns of the prosthesis during knee joint motion.

STUDY DESIGN

Controlled laboratory study.

METHODS

The movements of the meniscus were determined during knee joint flexion and extension with and without internal and external tibial torque by means of roentgen stereophotogrammetric analysis. Subsequently, the meniscus in 6 human cadaveric knee joints was replaced by a meniscus prosthesis.

RESULTS

All different parts of the meniscus showed a posterior displacement during knee joint flexion. The anterior horn was more mobile than the posterior horn. The prosthesis mimicked the movements of the meniscus. However, the excursions of the prosthesis on the tibial plateau were less. The knee joint laxity was not significantly higher after replacement with the meniscus prosthesis.

CONCLUSIONS

The prosthesis approximated the behavior of the native meniscus. Improvement in both the gliding characteristics of the prosthetic material and the fixation of the prosthesis may improve the function.

CLINICAL RELEVANCE

The meniscus prosthesis needs to be optimized to achieve a better initial function in the knee joint.

Tissue engineering of the meniscus

Meniscus lesions are among the most frequent injuries in orthopaedic practice and they will inevitably lead to degeneration of the knee articular cartilage. The fibro-cartilage-like tissue of the meniscus is notorious for its limited regenerative capacity. Tissue engineering could offer new treatment modalities for repair of meniscus tears and eventually will enable the replacement of a whole meniscus by a tissue-engineered construct. Many questions remain to be answered before the final goal, a tissue-engineered meniscus is available for clinical implementation. These questions are related to the selection of an optimal cell type, the source of the cells, the need to use growth factor(s) and the type of scaffold that can be used for stimulation of differentiation of cells into tissues with optimal phenotypes. Particularly in a loaded, highly complex environment of the knee, optimal mechanical properties of such a scaffold seem to be of utmost importance. With respect to cells, autologous meniscus cells seems the optimal cell source for tissue engineering of meniscus tissue, but their availability is limited. Therefore research should be stimulated to investigate the suitability of other cell sources for the creation of meniscus tissue. Bone marrow stroma cells could be useful since it is well known that they can differentiate into bone and cartilage cells. With respect to growth factors, TGF-beta could be a suitable growth factor to stimulate cells into a fibroblastic phenotype but the problems of TGF-beta introduced into a joint environment should then be solved. Polyurethane scaffolds with optimal mechanical properties and with optimal interconnective macro-porosity have been shown to facilitate ingrowth and differentiation of tissue into fibro-cartilage. However, even these materials cannot prevent cartilage degeneration in animal models. Surface modification and/or seeding of cells into the scaffolds before implantation may offer a solution for this problem in the future.This review focuses on a number of specific questions; what is the status of the development of procedures for lesion healing and how far are we from replacing the entire meniscus by a (tissue-engineered) prosthesis. Subquestions related to the type of scaffold used are: is the degree of tissue ingrowth and differentiation related to the initial mechanical properties and if so, what is the influence of those properties on the subsequent remodelling of the tissue into fibro-cartilage; what is the ideal pore geometry and what is the optimal degradation period to allow biological remodelling of the tissue in the scaffold. Finally, we will finish with our latest results of the effect of tear reconstruction and the insertion of prostheses on articular cartilage degradation.

A porous polymer scaffold for meniscal lesion repair--a study in dogs

Meniscal lesions often occur in the avascular area of the meniscus with little chance of spontaneous repair. An access channel in the meniscal tissue can function as an entrance for ingrowing repair tissue from the vascular periphery of the meniscus to the lesion in the avascular zone which again induced healing of the lesion. Implantation of a porous polymer in a full-thickness access channel induced healing. However, a better integration between meniscal tissue and the implant might be achieved with the combination of the newly developed porous polymers and a modified surgical technique. This might improve meniscal lesion healing and the repair of the access channel with neo-meniscal tissue. Longitudinal lesions were created in the avascular part of 24 canine lateral menisci and a partial-thickness access channel was formed to connect the lesion with the meniscal periphery. In 12 menisci, the access channel was left empty (control group), while in the remaining 12 menisci the polymer implant was sutured into the access channel. Repair of the longitudinal lesions was achieved with and without polymer implantation in the partial-thickness access channel. Polymer implants induced fibrous ingrowth with cartilaginous areas, which resembled neo-meniscal tissue. Implantation did not prevent articular cartilage degeneration.

Presence and mechanism of knee articular cartilage degeneration after meniscal reconstruction in dogs

OBJECTIVE

Partial meniscectomy is the golden standard for treating a bucket-handle tear in the meniscus of the knee, but it inevitably leads to articular cartilage degeneration. Surgical creation of an access channel between the lesion and the vascularized synovial lining is intended to induce ingrowth of repair tissue and thus avoid degeneration of articular cartilage.

DESIGN

The presence and mechanism of cartilage degeneration were evaluated in 24 canine menisci after a longitudinal lesion and access channel had been created in the avascular part of the meniscus. In 12 menisci the channel was implanted with a porous polymer scaffold, while the remaining 12 were left empty. Evaluation was performed using routine histology and antibodies directed against denatured type II collagen (Col2-3/4M).

RESULTS

Articular degeneration was apparent in the polymer implant group and the empty channel group. This consisted of fibrillation, loss of chondrocytes and decreased proteoglycan content. Areas of fibrillated cartilage always showed positive labeling with the collagen degradation antibody Col2-3/4M. Collagen degradation was also visible in non-fibrillated areas. The upper zone of the cartilage showed swelling especially in the implant group, with empty cell lacunae and moderate levels of Col2-3/4M antibody labeling.

DISCUSSION

This reconstruction technique cannot be considered superior to partial meniscectomy. We propose that degradation of the collagen type II network is a result of cartilage fibrillation and vice versa.

Articular cartilage repair: basic science and clinical progress. A review of the current status and prospects

OBJECTIVE

To review the basic scientific status of repair in articular cartilage tissue and to assess the efficiency of current clinical therapies instigated for the treatment of structural lesions generated therein as a result of trauma or during the course of various diseases, notably osteoarthritis (OA). Current scientific trends and possible directions for the future will also be discussed.

DESIGN

A systematic and critical analysis is undertaken, beginning with a description of the spontaneous repair responses in different types of lesion. Surgical interventions aimed at inducing repair without the use of active biologics will then be considered, followed by those involving active biologics and those drawing on autogenic and allogeneic tissue transplantation principles. Cell transplantation approaches, in particular novel tissue engineering concepts, will be critically presented. These will include growth-factor-based biological treatments and gene transfection protocols. A number of technical problems associated with repair interventions, such as tissue integration, tissue retention and the role of mechanical factors, will also be analysed.

RESULTS

A critical analysis of the literature reveals the existence of many novel and very promising biologically-based approaches for the induction of articular cartilage repair, the vast majority of which are still at an experimental phase of development. But prospective, double-blinded clinical trials comparing currently practiced surgical treatments have, unfortunately, not been undertaken.

CONCLUSION

The existence of many new and encouraging biological approaches to cartilage repair justifies the future investment of time and money in this research area, particularly given the extremely high socio-economic importance of such therapeutic strategies in the prevention and treatment of these common joint diseases and traumas. Clinical epidemiological and prospective trials are, moreover, urgently needed for an objective, scientific appraisal of current therapies and future novel approaches.

Tissue ingrowth and degradation of two biodegradable porous polymers with different porosities and pore sizes

Commonly, spontaneous repair of lesions in the avascular zone of the knee meniscus does not occur. By implanting a porous polymer scaffold in a knee meniscus defect, the lesion is connected with the abundantly vascularized knee capsule and healing can be realized. Ingrowth of fibrovascular tissue and thus healing capacity depended on porosity, pore sizes and compression modulus of the implant. To study the lesion healing potential, two series of porous polyurethanes based on 50/50 epsilon-caprolactone/L-lactide with different porosities and pore sizes were implanted subcutaneously in rats. Also, in vitro degradation of the polymer was evaluated. The porous polymers with the higher porosity, more interconnected macropores, and interconnecting micropores of at least 30 microm showed complete ingrowth of tissue before degradation had started. In implants with the lower macro-porosity and micropores of 10-15 microm degradation of the polymer occurred before ingrowth was completed. Directly after implantation and later during degradation of the polymer, PMN cells infiltrated the implant. In between these phases the foreign body reaction remained restricted to macrophages and giant cells. We can conclude that both foams seemed not suited for implantation in meniscal reconstruction while either full ingrowth of tissue was not realized before polymer degradation started or the compression modulus was too low. Therefore, foams must be developed with a higher compression modulus and more connections with sufficient diameter between the macropores.

Toward tissue engineering of the knee meniscus

This review details current efforts to tissue engineer the knee meniscus successfully. The meniscus is a fibrocartilaginous tissue found within the knee joint that is responsible for shock absorption, load transmission, and stability within the knee joint. If this tissue is damaged, either through tears or degenerative processes, then deterioration of the articular cartilage can occur. Unfortunately, there is a dearth in the amount of work done to tissue engineer the meniscus when compared to other musculoskeletal tissues, such as bone. This review gives a brief overview of meniscal anatomy, biochemical properties, biomechanical properties, and wound repair techniques. The discussion centers primarily on the different components of attempting to tissue engineer the meniscus, such as scaffold materials, growth factors, animal models, and culturing conditions. Our approach for tissue engineering the meniscus is also discussed.

Biomechanical factors in tissue engineered meniscal repair

Damage to the meniscus after trauma or injury is associated with detrimental changes in joint function that can lead to pain, disability, and degenerative joint changes. Recently, tissue engineering strategies for meniscal repair have been suggested including using biocompatible grafts as a substrate for regeneration, and cellular supplementation to promote remodeling and healing. Little is known, however, about the contributions of these novel repair strategies to restoration of normal meniscal function. Biomechanical factors play a role in the design and synthesis of tissue engineered biomaterials and bioreactors, and also are important for evaluating the efficacy of these new strategies for restoring normal meniscal function. In this report, an overview is presented of biomechanical factors that are critical to meniscal function followed by a review of biomechanical considerations for the design and evaluation of tissue engineered strategies for meniscal repair. Recommendations for future study of biomechanical factors in tissue engineered meniscal repair also are provided.

The menisci of the knee joint. Anatomical and functional characteristics, and a rationale for clinical treatment

The menisci and their insertions into bone (entheses) represent a functional unit. Thanks to their firm entheses, the menisci are able to distribute loads and therefore reduce the stresses on the tibia, a function which is regarded essential for cartilage protection and prevention of osteoarthrosis. The tissue of the hypocellular meniscal body consists mainly of water and a dense elaborate type I collagen network with a predominantly circumferential alignment. The content of different collagens, proteoglycans and nonproteoglycan proteins shows significant regional variations probably reflecting functional adaptation. The meniscal horns are attached via meniscal insertional ligaments mainly to tibial bone. At the enthesis, the fibres of the insertional ligaments attach to bone via uncalcified and calcified fibrocartilages. This anatomical configuration of gradual transition from soft to hard tissue, which is identical to other ligament entheses, is certainly essential for normal mechanical function and probably protects this vulnerable transition between 2 biomechanically different tissues from failure. Clinical treatment of meniscal tears needs to be based on these special anatomical and functional characteristics. Partial meniscectomy will preserve some of the load distribution function of the meniscus only when the meniscal body enthesis entity is preserved. Repair of peripheral longitudinal tears will heal and probably preserve the load distribution function of the meniscus, whereas radial tears through the whole meniscal periphery or more central and complex tears may be induced to heal, but probably do not preserve the load distribution function. There is no proof that replacement of the meniscus with an allograft can reestablish some of the important meniscal functions, and thereby prevent or reduce the development of osteoarthrosis which is common after meniscectomy. After implantation, major problems are the remodelling of the graft to inferior structural, biochemical and mechanical properties and its insufficient fixation to bone which fails to duplicate a normal anatomical configuration and therefore a functional meniscal enthesis.

Meniscal tissue regeneration in porous 50/50 copoly(L-lactide/epsilon-caprolactone) implants

Porous materials of a high-molecular-weight 50/50 copolymer of L-lactide and epsilon-caprolactone with different compression moduli were used for meniscal repair. In contrast to the previously used 4,4'-diphenylmethane and 1,4-trans-cyclohexane diisocyanates containing polyurethanes, degradation products of the copolymer are non-toxic. Two series of porous materials with compression moduli of 40 and 100 kPa respectively were implanted in the knees of dogs using a new, less traumatizing suturing technique. A porous aliphatic polyurethane series with compression modulus of 150 kPa was implanted for comparison. Adhesion of the implant to meniscal tissue was found to be essential for healing of the longitudinal lesion. Copolymer implants showed better adhesion, probably due to the higher degradation rate of the copolymer. Fibrocartilage formation was found to be affected by the compression modulus of the implant. Implants with a modulus of 40 kPa did not show ingrowth of fibrocartilage, whereas implants with compression moduli of 100 and 150 kPa yielded 50-70 and 80-100% fibrocartilage respectively. During degradation the copolymer phase separated into a crystalline phase containing mainly L-lactide and an amorphous phase containing mainly epsilon-caprolactone. The copolymer degraded through bulk degradation.

Meniscal replacement using a porous polymer prosthesis: a preliminary study in the dog

A porous polyurethane prosthesis was used to replace the lateral meniscus in the dog. After an initial ingrowth of fibrous tissue, the prostheses became filled with tissue strongly resembling normal meniscal fibrocartilage. Although less severe than seen after total meniscectomy, cartilage degeneration was frequent, possibly because tissue ingrowth in the prostheses occurred too slowly. Porous polymers can be useful for replacement of the meniscus, provided that chemical and physical properties are optimized.

Changes in the content of the fibrillar collagens and the expression of their mRNAs in the menisci of the rabbit knee joint during development and ageing

The menisci are first seen as triangular aggregations of cells in the 20-day rabbit fetus. At 25-days, a matrix that contains types I, III and V collagens has formed. These collagens are also found in the 1-week neonatal meniscus, but by 3 weeks, type II collagen is present in some regions. By 12 to 14 weeks, typically cartilaginous areas with large cells in lacunae are found and by 2 years, these occupy the central regions of the inner two-thirds of the meniscus. The surface layers of the meniscus contain predominantly type I collagen. From 12 to 14 weeks onwards, there is little overlap between the regions with types I or II collagens, that is, these are discrete regions of type I-containing fibrocartilage and type II-containing cartilage. Types III and V collagens are found throughout the menisci, particularly in the pericellular regions. All the cells in the fetal and early neonatal menisci express the mRNA for type I collagen. At 3 weeks postnatal, cells that express type I collagen mRNA are found throughout the meniscus, but type II collagen mRNA is expressed only in the regions of developing cartilage. At 12- to 14-weeks, only type II collagen mRNA is expressed, except at the periphery next to the ligament where a few cells still express type I collagen mRNA. Rabbit menisci, therefore, undergo profound changes in their content and arrangement of collagens during postnatal development.

The effect of cast immobilization on meniscal healing. An experimental study in the dog

A 1.5-cm longitudinal, full-thickness incision was made in the vascularized portion of the medial meniscus in 20 adult dogs and anatomically repaired. Postoperatively, the animals were either placed in a long leg cast (N = 9) or mobilized immediately (N = 11). The animals were sacrificed at 2 weeks (6 dogs), 4 weeks (6 dogs), or 10 weeks (8 dogs). Five medial menisci from the nonoperated side were used as controls. Collagen content was measured using a digital image analysis system, and the collagen percentage in the repair tissue in each postoperative treatment group was compared. In the 2-week and 4-week groups, there was no statistically significant difference in the percentage of collagen between those animals immobilized versus those that had early mobilization. The animals in the 10-week group that were mobilized had a significantly greater collagen percentage in the healing meniscal incision than those that were cast immobilized (44.6% +/- 10% versus 27.0% +/- 11%, P < 0.0001). There was no significant difference in the collagen percentages between the mobilized 10-week group and the contralateral control menisci group. All other menisci had a decreased collagen percentage compared with the controls. Prolonged immobilization decreases collagen formation in healing menisci. Thus, our results suggest that patients undergoing isolated meniscal repair either be immediately mobilized after surgery or immobilized for short periods only.

Interspecies variation of compressive biomechanical properties of the meniscus

Various animal models have been used to investigate the normal and reparative properties of the knee meniscus. Yet, only limited data on meniscal biomechanical properties of various animals are available. It was therefore the objective of this study to compare measurements of meniscal biomechanical properties between six species: human, bovine, monkey, canine, sheep, and porcine. Uniaxial confined compression tests were conducted on 1-mm-thick, 4-mm-diameter meniscal discs, and the viscoelastic creep deformation was obtained. Two biomechanical parameters, the aggregate modulus (HA) and permeability (K), were found by implementing the linear biphasic theory and a newly developed nonlinear regression scheme. A one-way analysis of variance was conducted along with Student-Newman-Keuls comparison tests to assess the differences in these parameters among the species. Sheep menisci exhibited HA and K values most similar to human menisci. The water content of each specimen was also measured and correlated significantly with K. The interspecies variations found in material properties of the knee meniscus indicate the need for caution in extrapolating data on the biomechanical behavior of the human meniscus from animal models.

Porous polymer implants for repair of full-thickness defects of articular cartilage: an experimental study in rabbit and dog

Full-thickness defects of articular cartilage were repaired by implantation of porous polymer implants in rabbits and dogs. The quality of the repair tissue was determined by collagen typing with antibodies. Implants with varying pore sizes and chemical composition were used. The effect of loading and motion was determined by inserting implants higher than, level with and lower than the surrounding cartilage. It appeared that healing took place by formation of fibrocartilaginous repair tissue containing both type I and type II collagen. Hyaline cartilage was observed in a minority of the rabbits used but not in the dog. Fibrocartilage formation in the dog was simulated by implantation of a porous polymer. Chemical composition of the polymer did not alter the results, neither did loading of the implant. It is concluded that the formation of fibrocartilaginous repair cartilage is stimulated by implantation of a porous polymer. This tissue seemed to function adequately in the dog but did show signs of degeneration in the rabbit.