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Surgical Options for Meniscal Replacement

Brophy, Robert H. MD; Matava, Matthew J. MD

JAAOS - Journal of the American Academy of Orthopaedic Surgeons: May 2012 - Volume 20 - Issue 5 - p 265–272
doi: 10.5435/JAAOS-20-05-265
Orthopaedic Advances
Free

As a result of biologic issues and technical limitations, repair of the meniscus is indicated for unstable, peripheral vertical tears; most other types of meniscal tears that are degenerative, significantly traumatized, and/or located in an avascular area of the meniscus are managed with partial meniscectomy. Options to restore the meniscus range from allograft transplantation to the use of synthetic technologies. Recent studies demonstrate good long-term outcomes from meniscal allograft transplantation, although the indications and techniques continue to evolve and the long-term chondroprotective potential has yet to be determined. Several synthetic implants, none of which has US Food and Drug Administration approval, have shown some promise for replacing part or all of the meniscus, including the collagen meniscal implant, hydrogels, and polymer scaffolds.

From the Department of Orthopedic Surgery, Washington University, St. Louis, MO.

Dr. Brophy or an immediate family member serves as a board member, owner, officer, or committee member of the American Orthopaedic Society for Sports Medicine and serves as a paid consultant to or is an employee of Genzyme and Stryker. Dr. Matava or an immediate family member serves as a paid consultant to or is an employee of ISTO Technologies and Schwartz Biomedical; has received nonincome support (such as equipment or services), commercially derived honoraria, or other non-research-related funding (such as paid travel) from Arthrex and Breg; and serves as a board member, owner, officer, or committee member of the Southern Orthopaedic Association.

The meniscus plays a vital role in protecting the health and function of the knee joint. Meniscal tears are common; management is reserved for symptomatic tears. When nonsurgical treatment fails to address symptoms, surgery is indicated.

Primary surgical options include partial meniscectomy or meniscal repair. Because of biologic and technical issues, meniscal repair is typically limited to unstable, vertical, peripheral tears; therefore, most meniscal surgeries are partial meniscectomies.1 Each year, 690,000 partial meniscectomies and approximately 1 million additional knee arthroscopies (most of which involve at least some débridement of the meniscus) are performed in the United States.2 Surgeons who perform these procedures should attempt to preserve as much intact meniscal tissue as possible because altered biomechanics following partial meniscectomy increase articular cartilage contact pressures and can hasten joint degeneration.1,3,4

Several options exist for restoring the deficient meniscus, from allograft transplantation to synthetic technologies. Recent studies have demonstrated good short-term outcomes from meniscal allograft transplantation, although the indications and techniques are evolving. Despite the improvement in clinical symptoms, however, the long-term chondroprotective potential of meniscal transplantation has yet to be determined. Several synthetic implants have been developed to replace part of or the entire meniscus, including the collagen meniscal implant, hydrogels, and polymer scaffolds. These devices have shown some promise in recent studies, although clinical experience with their use is limited.

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Meniscal Transplantation

Meniscal transplantation has become an accepted management option for selected symptomatic patients who have undergone a complete or nearcomplete meniscectomy. The primary indication for a meniscal transplant is pain localized to the involved compartment. So-called prophylactic transplantation in young, athletic, asymptomatic patients is not indicated because significant potential complications are associated with the procedure. In addition, evidence is lacking that meniscal transplantation definitively prevents long-term arthrosis.

Patient age of <50 years is the accepted upper limit for meniscal transplantation; however, no lower age limit has been established other than skeletal maturity. To date, no studies have been published evaluating age as it affects outcome. At the time of transplantation, there should be only mild preexisting arthrosis and no focal lesion higher than grade III by the International Knee Documentation Committee (IKDC) classification.5 Any focal cartilage lesions should be addressed either before or concurrent with the transplantation. Contraindications to meniscal transplantation may relate to the knee and overall patient health (Table 1).

Table 1

Table 1

A review of existing clinical trials consistently confirms that several management principles should be mandatory for surgeons performing this procedure. These principles include the reestablishment (if necessary) of normal knee alignment and stability; the implantation of a sizematched, nonirradiated graft with secure fixation of the meniscal horns (Figure 1); and a return to only light sports activities to optimize the chances for graft survival.

Figure 1 Photograph of a meniscal allograft.

Figure 1 Photograph of a meniscal allograft.

Several factors at least theoretically influence clinical outcome following transplantation. These can be categorized into knee-specific factors (ie, chondral damage, ligamentous stability, axial alignment, prior surgery), graft-specific factors (ie, medial versus lateral side, method of preservation, secondary sterilization, sizing method), surgeon-specific factors (ie, surgeon experience, insertion method, graft fixation, concomitant procedures), and rehabilitation-specific factors (ie, range of motion, weight bearing, continuous passive motion, return to activities).

Accurate sizing of the allograft is likely the most critical technical factor affecting clinical success. Knee joint tolerance to size mismatch is not completely understood, although estimates suggest that a size tolerance of 5% is acceptable. However, to date, no study has evaluated size tolerance as an outcome measure. Not definitively answered is the question of who is responsible for accurate size calculation, the tissue bank or the surgeon. Shaffer et al6 found that MRI was slightly more accurate than plain radiography in determining accurate sizing. However, measurement accuracy within 2 mm was found in only 35% of their specimens.

Historically, most meniscal transplants have been harvested in an aseptic fashion and then processed as either prolonged fresh grafts (4°C), deep-frozen grafts (−80°C), lyophilized grafts (freeze-dried), or cryopreserved grafts (ie, slow-freezing the graft to −196°C in an anhydrous environment to prevent intracellular water crystallization). Initially it was assumed that preservation methods that maintain cell viability (ie, fresh and cryopreserved) would enhance graft function. However, basic science data in animal models7,8 have shown a relatively rapid repopulation of donor DNA with recipient DNA, thereby raising questions about the necessity of cell viability—and its inherent cost—on graft survival and, consequently, on clinical outcome. As a result, most surgeons use either prolonged fresh or deepfrozen grafts.

Meniscal allografts are associated with the potential for transmission of not only bacterial organisms but also viral and fungal pathogens. Because the transplanted tissue has the potential to transmit bacterial, fungal, and viral infection (including hepatitis and the human immunodeficiency virus [HIV]), graft sterilization has been used as a means of reducing this risk. Although gamma radiation has been used as a common method of secondary sterilization of allograft tissues, the data are limited in regard to the effect of gamma radiation on meniscal allografts. Gamma radiation at dosages 1.0 to 1.5 Mrad (10,000 to 15,000 Gy) has been shown to inactivate large classes of these microorganisms.9,10 Concern regarding the effect of irradiation on the mechanical function of the meniscus11 has led to a diminished clinical use of any form of meniscal allograft irradiation because doses higher than those typically used (ie, 1.5 to 2.0 Mrad) are needed to eradicate HIV.12 A critical review of the existing literature makes it difficult to formulate conclusions as to the necessity of secondary sterilization.

Although there is no ideal method of meniscal allograft fixation, commonly accepted principles include the use of strong sutures placed in a vertical-mattress fashion, tied over the joint capsule, with accurate reestablishment of the native meniscal horn insertions. Ideally, some peripheral rim tissue should be left to decrease peripheral extrusion of the implant and to provide firm tissue through which to pass the fixation sutures. Success rates with early efforts at meniscal transplantation were likely hampered by the failure to attach either one or both of the meniscal horns to their original locations. The greatest area of innovation and advancement has been in terms of securing these horns so as to preserve the so-called hoop stress of the normal meniscus. Basic science data have confirmed the importance of horn fixation to prevent extrusion of the meniscus with weight bearing.13-15

It is unclear from the available data whether there is any difference between bony fixation or soft-tissue fixation of the meniscal horns (assuming secure fixation with either method). Methods that have been used include the use of bone tunnels (for either bone plug or soft-tissue fixation) and a variety of bonebridge techniques (eg, Keyhole [Arthrex, Naples, FL], Dovetail [Arthrex]) that leave the anterior and posterior horns attached to the meniscal cartilage and to each other. The bone-bridge methods are optimal for lateral meniscal transplants because the lateral meniscal horns are approximately 1 cm apart. A bone-bridge construct is contraindicated for medial meniscal transplantation with associated anterior cruciate ligament reconstruction because of the proximity of the two grafts. To date, there have been no comparative trials demonstrating one method of fixation to be superior to another.

Recommended methods of rehabilitation following meniscal transplantation vary in the published literature. Patellar mobilization, therapeutic modalities, and quadriceps/hamstring strengthening are initiated immediately postoperatively. Bracing is often recommended for the first 6 weeks to allow protected range of motion (except deep flexion) so as to enhance the biologic milieu of the healing transplant. Despite a lack of supportive data, limited weight bearing has been recommended while the transplanted meniscus heals to the periphery. Most patients are able to bear full weight on the involved limb by 8 weeks. Jogging is allowed at 3 to 4 months, with progression to running, cutting, and sports-specific activities at 4 to 6 months, as tolerated.

A low-grade effusion is often seen for the first year following transplantation and may necessitate a slower rehabilitation regimen. Meniscal transplantation is not analogous to a large meniscal repair, in which return to high-level sports is a realistic goal. Based on the available literature, it is not possible to determine the ideal progression of weight bearing or the influence of knee motion on allograft healing and biomechanics. Until such studies are performed, return to strenuous sports cannot be recommended after meniscal transplantation.

In general, published data regarding outcomes following transplantation are limited by the absence of control groups, the evaluation of mixed patient populations, inconsistent or nonexistent exclusion criteria for transplantation, and variability in the use of validated outcome measures.5 Nevertheless, a growing body of literature provides short- to longterm results of meniscal transplantation (Table 2).

Table 2

Table 2

The success of the procedure may be highly dependent on the side of involvement; Verdonk and colleagues25,28 found a 72% success rate for medial meniscal allografts but a 63% success rate for lateral grafts. However, Cole et al26 found that patient satisfaction was 93% for lateral meniscal transplants but only 68% for medial transplants.

Ultimately, the long-term effect of meniscal transplantation on the radiographic progression of osteoarthritis remains to be determined. An increase in the degree of osteoarthritis was seen in 42% of patients in a recent systematic review.5 MRI has been used to provide a more objective evaluation of the transplanted graft; Potter et al36 reported meniscal degeneration in 63% of patients. All patients in their series with graft extrusion were symptomatic. In addition, all patients with meniscal degeneration on MRI reported pain but no locking, whereas all patients with meniscal fragmentation reported both pain and locking. In a similar study, Lee et al37 compared the clinical outcome of extruded and nonextruded meniscal allografts. They reported a mean extrusion of 3 mm but no effect of meniscal extrusion on knee function. The degree of meniscal extrusion remained stable after 1 year.

Second-look arthroscopy has been performed in several studies17,19,20,24 to define more accurately the status of the transplant as well as the adjacent articular cartilage. However, the arthroscopic assessment of a meniscal allograft cannot be used to determine its histologic architecture, cellular repopulation, or vascular supply because the gross inspection of the graft does not necessarily correlate with its biomechanical function.

Complications have been noted following transplantation; graft tears are the most common (8.2%).5 Infection rates appear to be similar to those following routine knee arthroscopy (ie, <1%). When infection has occurred, it is difficult to determine whether it was because of a contaminated graft or from the surgical procedure.

Several drawbacks and risks to transplantation exist. General matters of concern include the fact that there are a limited number of available grafts in tissue banks and that the grafts are relatively expensive. Anatomic risks relate to the accuracy of implantation, the reapproximation of the native meniscal horn insertion, the unknown effect of different graft-preservation and -fixation techniques on clinical outcome, and the neurovascular risks associated with an inside-out meniscal repair. As with the transplantation of any human tissue, there is also the risk of infectious disease transmission; bacterial infection is the most likely (0.5%).5 To date, no instance of HIV infection following meniscal transplantation has been reported. The possibility nevertheless exists; the reported risk of HIV following the transplantation of musculoskeletal tissue has been estimated at approximately 1 in 1.6 million.38 There is also the potential for an immune response resulting from class I and II histocompatibility antigens following transplantation. Rodeo et al39 found B lymphocyte and cytotoxic T cells in 9 of 12 deep-frozen grafts. In their series, the clinical outcome was not related to this immune response, although the histologic scores were better in patients without this response.

Despite the limitations, meniscal grafts appear to decrease tibiofemoral pain, with subjective improvement in symptoms noted in approximately 70% of patients.17,19,24,25,28 Similarly, there seems to be an increase in activity level over the short term. These grafts may provide some chondroprotective effects and improve joint stability over the short term. However, most grafts demonstrate some degree of shrinkage on MRI as well as partial or complete extrusion, variable degrees of signal changes, and tears. As a result, most transplants will likely fail over the long term. Nevertheless, this procedure can be effective in terms of improving symptoms and function before the patient becomes an appropriate candidate for arthroplasty.

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Meniscal Replacement

Alternative options for meniscal replacement are being developed. Although none of the technologies listed below is currently approved by the Food and Drug Administration (FDA) for clinical use in the United States, orthopaedic surgeons should be aware of options that may become available.

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Collagen Meniscal Implant

The collagen meniscal implant (CMI) is a bioresorbable collagen matrix designed to serve as a template for ingrowth of new meniscal tissue (Figure 2). The CMI requires a meniscal rim and intact anterior and posterior meniscal horns for attachment.40,41 Following numerous in vitro and in vivo analyses, the CMI underwent phase I42 and phase II40 feasibility studies. Medium-term follow-up of the phase II cohort demonstrated good clinical outcomes, with an average defect fill of 69% on secondlook arthroscopy at a minimum of 5 years postimplantation.41 Rodkey et al43 reported the medium-term results of a prospective, randomized trial that compared the CMI with partial meniscectomy. This study involved 311 patients with either a symptomatic medial meniscal tear (acute group) or previous partial medial meniscectomy (chronic group). No differences were seen in the acute arm of the study, which compared these treatment approaches in patients with no previous surgery on the meniscus. In the chronic group, which involved patients who had a history of at least one previous partial meniscectomy, those who received the CMI had a greater recovery of activity and fewer reoperations.

Figure 2

Figure 2

Two recent studies have reported long-term (>10 years) results using the CMI. A series of 25 patients with a minimum 10-year follow-up reported sustained pain relief and functional improvement with very little joint space narrowing and a failure rate of 8%.44 A comparative longterm study with a minimum 10-year follow-up reported improved pain, activity level, and radiologic outcomes in patients treated with the CMI compared with patients treated with partial meniscectomy alone.45 This study was limited by selection bias, however, because the patients chose which surgery would be performed. Furthermore, it is difficult to interpret the radiologic outcome because joint space width was reported at the final evaluation, not the change in joint space over the course of the study period. Nevertheless, a growing body of evidence indicates that the CMI can at least be tolerated over the medium and long term and may provide some chondroprotection following a partial meniscectomy with intact meniscal roots. The CMI does not currently have FDA approval for use in the United States.

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Hydrogels

Hydrogels are biocompatible, biphasic materials with load rate-dependent mechanical properties; they have been proposed as a possible solution to meniscal replacement. Two small-animal models have demonstrated that hydrogel menisci could be durable.46,47 However, in a largeanimal model, hydrogel menisci demonstrated increased cartilage degeneration by 4 months compared with controls; the hydrogel menisci also developed radial splits in the posterior one third within 1 year of implantation.48 Although hydrogels with improved material properties and surface characteristics may eventually prove to be useful, there are no clinical studies to date of hydrogels used for meniscal replacement.

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Polymer Scaffold

A novel synthetic, biodegradable, acellular scaffold composed of aliphatic polyurethane (Actifit; Orteq Bioengineering, London, UK) has recently become available in Europe as an alternative option for partial meniscal replacement (Figure 3). Consisting of polycaprolactone and urethane segments, the scaffold degrades slowly over 5 years, starting with hydrolysis of the ester bonds in the polycaprolactone segments.49 The more stable urethane segments may be phagocytized over time by macrophages or giant cells; alternatively, they may integrate into the surrounding tissue.50,51 The scaffold has been shown to improve contact area and pressure in a sheep cadaver model.52 A preclinical canine study found tissue ingrowth onto the scaffold and integration with the surrounding capsule by 6 months.53 A recent case series reported tissue ingrowth within 3 months and viable fibrochondroblast-like cells after 12 months in human medial and lateral meniscal defects.54 Two-year results in a case series of 52 patients (34 medial meniscus, 18 lateral meniscus) demonstrated clinically and statistically significant improvements from baseline in all clinical outcomes, including IKDC score, Knee injury and Osteoarthritis Outcome Score, and Lysholm score.55 More than 90% of patients demonstrated stable or improved International Cartilage Repair Society articular cartilage scores on MRI at 24 months compared with baseline. Actifit does not currently have FDA approval for use in the United States.

Figure 3

Figure 3

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Stem Cells

Interest has been growing in the use of stem cells in orthopaedic surgery. Several recent reports have been published of experimental models using mesenchymal stem cells to replace meniscal tissue.56-60 These typically involve a stem cell-seeded scaffold modulated to generate meniscus-like tissue. Studies have demonstrated regeneration of the meniscus using a seeded scaffold in rat,58 rabbit,61,62 and pig models.57 The latter study demonstrated a substantial improvement in meniscal healing compared with that in controls, which essentially exhibited no healing. However, the mechanical properties of the repair tissue were inferior to those of the intact meniscus. Stem cell-based implants are still in the investigative stage but may become available for preclinical studies in the near future. Human trials likely are a distant prospect.

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Summary

The meniscus plays an important role in protecting the health of the knee joint. Once the meniscus has been torn and is removed from the joint, options are limited to replace this tissue. Meniscal allograft transplantation is a viable option showing increasing evidence of clinical utility, although the high cost, limited availability, and risk of disease transmission from allografts may preclude their widespread use. Options such as the CMI or Actifit scaffold are under investigation as potential partial meniscal replacements. In the future, stem cells may provide an alternative, potentially autogenous, source of meniscal tissue to regenerate the resected segment. Even with these advances, however, surgeons should continue to attempt meniscal repair whenever feasible and resect as little meniscal tissue as possible in tears that are deemed irreparable.

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References

Evidence-based Medicine: Levels of evidence are described in the table of contents. In this article, references 4 and 43 are level I studies. Reference 45 is a level II study. Reference 37 is a level III study. References 3, 16-26, 28-36, 40-42, 44, 54, and 55 are level IV studies.

References printed in bold type are those published within the past 5 years.

1. McDermott ID, Amis AA: The consequences of meniscectomy. J Bone Joint Surg Br 2006;88(12):1549-1556.
2. Cullen KA, Hall MJ, Golosinskiy A: Ambulatory surgery in the United States, 2006. Natl Health Stat Report 2009; 20(11):1-25.
3. Andersson-Molina H, Karlsson H, Rockborn P: Arthroscopic partial and total meniscectomy: A long-term follow-up study with matched controls. Arthroscopy 2002;18(2):183-189.
4. Hede A, Larsen E, Sandberg H: Partial versus total meniscectomy: A prospective, randomised study with longterm follow-up. J Bone Joint Surg Br 1992;74(1):118-121.
5. Matava MJ: Meniscal allograft transplantation: A systematic review. Clin Orthop Relat Res 2007;455:142-157.
6. Shaffer B, Kennedy S, Klimkiewicz J, Yao L: Preoperative sizing of meniscal allografts in meniscus transplantation. Am J Sports Med 2000;28(4):524-533.
7. Arnoczky SP, McDevitt CA, Schmidt MB, Mow VC, Warren RF: The effect of cryopreservation on canine menisci: A biochemical, morphologic, and biomechanical evaluation. J Orthop Res 1988;6(1):1-12.
8. Jackson DW, Whelan J, Simon TM: Cell survival after transplantation of fresh meniscal allografts: DNA probe analysis in a goat model. Am J Sports Med 1993; 21(4):540-550.
9. Christensen EA, Kristensen H, Sehestad K: Radiation sterilization, in Russel AD, Hugo WB, Ayliffe GAJ, ed: Principles and Practices of Disinfection, Preservation, and Sterilization. Oxford, UK, Blackwell Scientific Publications, 1982, pp 513-533.
10. Wright K, Trump J: Co-operative studies in the use of ionizing radiation for sterilization and preservation of biological tissue: Twenty years' experience, in Sterilization and Preservation of Biological Tissues by Ionizing Radiation. Vienna, Austria, International Atomic Energy Agency, 1969, pp 107-118.
11. Yahia LH, Drouin G, Zukor D: The irradiation effect on the initial mechanical properties of meniscal grafts. Biomed Mater Eng 1993;3(4):211-221.
12. Fideler BM, Vangsness CT Jr, Moore T, Li Z, Rasheed S: Effects of gamma irradiation on the human immunodeficiency virus: A study in frozen human bone-patellar ligamentbone grafts obtained from infected cadavera. J Bone Joint Surg Am 1994; 76(7):1032-1035.
13. Alhalki MM, Howell SM, Hull ML: How three methods for fixing a medial meniscal autograft affect tibial contact mechanics. Am J Sports Med 1999;27(3): 320-328.
14. Chen MI, Branch TP, Hutton WC: Is it important to secure the horns during lateral meniscal transplantation? A cadaveric study. Arthroscopy 1996; 12(2):174-181.
15. Paletta GA Jr, Manning T, Snell E, Parker R, Bergfeld J: The effect of allograft meniscal replacement on intraarticular contact area and pressures in the human knee: A biomechanical study. Am J Sports Med 1997;25(5):692-698.
16. Stollsteimer GT, Shelton WR, Dukes A, Bomboy AL: Meniscal allograft transplantation: A 1- to 5-year follow-up of 22 patients. Arthroscopy 2000;16(4): 343-347.
    17. Rath E, Richmond JC, Yassir W, Albright JD, Gundogan F: Meniscal allograft transplantation: Two- to eightyear results. Am J Sports Med 2001; 29(4):410-414.
    18. Ryu RK, Dunbar V WH, Morse GG: Meniscal allograft replacement: A 1-year to 6-year experience. Arthroscopy 2002; 18(9):989-994.
      19. van Arkel ER, de Boer HH: Survival analysis of human meniscal transplantations. J Bone Joint Surg Br 2002;84(2):227-231.
      20. Wirth CJ, Peters G, Milachowski KA, Weismeier KG, Kohn D: Long-term results of meniscal allograft transplantation. Am J Sports Med 2002; 30(2):174-181.
      21. Yoldas EA, Sekiya JK, Irrgang JJ, Fu FH, Harner CD: Arthroscopically assisted meniscal allograft transplantation with and without combined anterior cruciate ligament reconstruction. Knee Surg Sports Traumatol Arthrosc 2003;11(3): 173-182.
        22. Sekiya JK, Giffin JR, Irrgang JJ, Fu FH, Harner CD: Clinical outcomes after combined meniscal allograft transplantation and anterior cruciate ligament reconstruction. Am J Sports Med 2003;31(6):896-906.
          23. Graf KW Jr, Sekiya JK, Wojtys EM, Department of Orthopaedic Surgery, University of Michigan Medical Center, Ann Arbor, Michigan, USA: Long-term results after combined medial meniscal allograft transplantation and anterior cruciate ligament reconstruction: Minimum 8.5-year follow-up study. Arthroscopy 2004; 20(2):129-140.
            24. Noyes FR, Barber-Westin SD, Rankin M: Meniscal transplantation in symptomatic patients less than fifty years old. J Bone Joint Surg Am 2004;86(7):1392-1404.
            25. Verdonk PC, Demurie A, Almqvist KF, Veys EM, Verbruggen G, Verdonk R: Transplantation of viable meniscal allograft: Survivorship analysis and clinical outcome of one hundred cases. J Bone Joint Surg Am 2005;87(4):715-724.
            26. Cole BJ, Dennis MG, Lee SJ, et al: Prospective evaluation of allograft meniscus transplantation: A minimum 2-year follow-up. Am J Sports Med 2006;34(6):919-927.
            27. Sekiya JK, Ellingson CI: Meniscal allograft transplantation. J Am Acad Orthop Surg 2006;14(3):164-174.
              28. Verdonk PC, Verstraete KL, Almqvist KF, et al: Meniscal allograft transplantation: Long-term clinical results with radiological and magnetic resonance imaging correlations. Knee Surg Sports Traumatol Arthrosc 2006; 14(8):694-706.
              29. Rue JP, Yanke AB, Busam ML, McNickle AG, Cole BJ: Prospective evaluation of concurrent meniscus transplantation and articular cartilage repair: Minimum 2-year follow-up.Am J Sports Med2008;36(9):1770-1778.
                30. Alentorn-Geli E, Seijas Vázquez R, García Balletbó M, et al: Arthroscopic meniscal allograft transplantation without bone plugs. Knee Surg Sports Traumatol Arthrosc 2010;19:174-182.
                  31. Ha JK, Shim JC, Kim DW, Lee YS, Ra HJ, Kim JG: Relationship between meniscal extrusion and various clinical findings after meniscus allograft transplantation. Am J Sports Med 2010; 38(12):2448-2455.
                    32. LaPrade RF, Wills NJ, Spiridonov SI, Perkinson S: A prospective outcomes study of meniscal allograft transplantation. Am J Sports Med 2010; 38(9):1804-1812.
                      33. Vundelinckx B, Bellemans J, Vanlauwe J: Arthroscopically assisted meniscal allograft transplantation in the knee: A medium-term subjective, clinical, and radiographical outcome evaluation. Am J Sports Med 2010;38(11):2240-2247.
                        34. Ha JK, Sung JH, Shim JC, Seo JG, Kim JG: Medial meniscus allograft transplantation using a modified bone plug technique: Clinical, radiologic, and arthroscopic results. Arthroscopy 2011; 27(7):944-950.
                          35. Zhang H, Liu X, Wei Y, et al: Meniscal allograft transplantation in isolated and combined surgery.Knee Surg Sports Traumatol Arthrosc2012;20(2):281-289.
                            36. Potter HG, Rodeo SA, Wickiewicz TL, Warren RF: MR imaging of meniscal allografts: Correlation with clinical and arthroscopic outcomes. Radiology 1996; 198(2):509-514.
                            37. Lee DH, Kim SB, Kim TH, Cha EJ, Bin SI: Midterm outcomes after meniscal allograft transplantation: Comparison of cases with extrusion versus without extrusion.Am J Sports Med2010;38(2): 247-254.
                            38. Buck BE, Malinin TI, Brown MD: Bone transplantation and human immunodeficiency virus: An estimate of risk of acquired immunodeficiency syndrome (AIDS). Clin Orthop Relat Res 1989;240:129-136.
                            39. Rodeo SA, Seneviratne A, Suzuki K, Felker K, Wickiewicz TL, Warren RF: Histological analysis of human meniscal allografts: A preliminary report. J Bone Joint Surg Am 2000;82(8):1071-1082.
                            40. Rodkey WG, Steadman JR, Li ST: A clinical study of collagen meniscus implants to restore the injured meniscus. Clin Orthop Relat Res 1999;(367 suppl): S281-S292.
                            41. Steadman JR, Rodkey WG: Tissueengineered collagen meniscus implants: 5- to 6-year feasibility study results. Arthroscopy 2005;21(5):515-525.
                            42. Stone KR, Steadman JR, Rodkey WG, Li ST: Regeneration of meniscal cartilage with use of a collagen scaffold: Analysis of preliminary data. J Bone Joint Surg Am 1997;79(12):1770-1777.
                            43. Rodkey WG, DeHaven KE, Montgomery WH III, et al: Comparison of the collagen meniscus implant with partial meniscectomy: A prospective randomized trial.J Bone Joint Surg Am2008;90(7):1413-1426.
                            44. Monllau JC, Gelber PE, Abat F, et al: Outcome after partial medial meniscus substitution with the collagen meniscal implant at a minimum of 10 years' follow-up. Arthroscopy 2011;27(7):933-943.
                            45. Zaffagnini S, Marcheggiani Muccioli GM, Lopomo N, et al: Prospective longterm outcomes of the medial collagen meniscus implant versus partial medial meniscectomy: A minimum 10-year follow-up study.Am J Sports Med2011; 39(5):977-985.
                            46. Kobayashi M, Toguchida J, Oka M: Development of an artificial meniscus using polyvinyl alcohol-hydrogel for early return to, and continuance of, athletic life in sportspersons with severe meniscus injury: I. Mechanical evaluation. Knee 2003;10(1):47-51.
                            47. Kobayashi M, Toguchida J, Oka M: Preliminary study of polyvinyl alcoholhydrogel (PVA-H) artificial meniscus. Biomaterials 2003;24(4):639-647.
                            48. Kelly BT, Robertson W, Potter HG, et al: Hydrogel meniscal replacement in the sheep knee: Preliminary evaluation of chondroprotective effects. Am J Sports Med 2007;35(1):43-52.
                            49. Tateishi T, Chen G, Ushida T, Murato T, Mizuno S: Lactide copolymers for scaffolds, in Lewandrowski KU, Wise DL, Trantolo DJ, Gresser JD, Yasemski MJ, Altobeli DE, eds: Tissue Engineering. Boca Raton, FL, CRC Press, 2002, pp 111-122.
                            50. van Minnen B, van Leeuwen MB, Kors G, Zuidema J, van Kooten TG, Bos RR: In vivo resorption of a biodegradable polyurethane foam, based on 1,4butanediisocyanate: A three-year subcutaneous implantation study.J Biomed Mater Res A2008;85(4):972-982.
                            51. Zuidema J, van Minnen B, Span MM, Hissink CE, van Kooten TG, Bos RR: In vitro degradation of a biodegradable polyurethane foam, based on 1,4butanediisocyanate: A three-year study at physiological and elevated temperature. J Biomed Mater Res A 2009;90(3):920-930.
                            52. Brophy RH, Cottrell J, Rodeo SA, Wright TM, Warren RF, Maher SA: Implantation of a synthetic meniscal scaffold improves joint contact mechanics in a partial meniscectomy cadaver model.J Biomed Mater Res A2010;92(3):1154-1161.
                            53. Tienen TG, Heijkants RG, de Groot JH, et al: Replacement of the knee meniscus by a porous polymer implant: A study in dogs. Am J Sports Med 2006;34(1):64-71.
                            54. Verdonk R, Verdonk P, Huysse W, Forsyth R, Heinrichs EL: Tissue ingrowth after implantation of a novel, biodegradable polyurethane scaffold for treatment of partial meniscal lesions.Am J Sports Med2011;39(4):774-782.
                            55. Verdonk P, Beaufils P, Bellemans J, et al: Successful treatment of painful irreparable partial meniscal defects with a polyurethane scaffold: Two-year safety and clinical outcomes. Am J Sports Med 2012; Feb 9 [Epub ahead of print].
                            56. Angele P, Johnstone B, Kujat R, et al: Stem cell based tissue engineering for meniscus repair. J Biomed Mater Res A 2008;85(2):445-455.
                            57. Dutton AQ, Choong PF, Goh JC, Lee EH, Hui JH: Enhancement of meniscal repair in the avascular zone using mesenchymal stem cells in a porcine model.J Bone Joint Surg Br2010;92(1): 169-175.
                            58. Izuta Y, Ochi M, Adachi N, Deie M, Yamasaki T, Shinomiya R: Meniscal repair using bone marrow-derived mesenchymal stem cells: Experimental study using green fluorescent protein transgenic rats. Knee 2005;12(3):217-223.
                            59. Murphy JM, Fink DJ, Hunziker EB, Barry FP: Stem cell therapy in a caprine model of osteoarthritis. Arthritis Rheum 2003;48(12):3464-3474.
                            60. Zellner J, Mueller M, Berner A, et al: Role of mesenchymal stem cells in tissue engineering of meniscus.J Biomed Mater Res A2010;94(4):1150-1161.
                            61. Kang SW, Son SM, Lee JS, et al: Regeneration of whole meniscus using meniscal cells and polymer scaffolds in a rabbit total meniscectomy model. J Biomed Mater Res A 2006;78(3):659-671.
                            62. Lu HD, Cai DZ, Wu G, Wang K, Shi DH: Whole meniscus regeneration using polymer scaffolds loaded with fibrochondrocytes.Chin J Traumatol2011;14(4):195-204.
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