Meniscal Allografts—Where Do We Stand?
Scott A. Rodeo
The importance of the meniscus to the well-being of the knee has been well documented. This importance has provided the rationale for preserving these structures whenever possible. While meniscal repair has become an accepted mode of treatment for selected meniscal injuries, it is not applicable in every instance and partial or even total meniscectomy may still be necessary. Meniscal transplantation has offered a potential solution to this problem. The laboratory studies of the mid-1980s have provided the impetus for the clinical use of meniscal allografts, and subsequent clinical and basic science investigations have further refined the application of these allografts. This article will review the current clinical status of meniscal transplantation.
CLINICAL INDICATIONS
Transplantation of the meniscus was initially undertaken in patients with arthrotic knees and who had undergone previous total meniscectomy. The goal of transplantation was to prevent, and possibly even reverse, the progressive joint degeneration that predictably follows meniscectomy. Clinical experience with meniscal transplantation has redefined the indications for this procedure. However, at this time, there is still limited information available in the literature on the results of this procedure and, as a result, the indications continue to evolve. At present, there are considered to be four specific clinical situations in which meniscal transplantation may be considered.
Articular Cartilage Damage
The most common indication for meniscal transplantation is in patients with symptoms referable to a meniscus-deficient tibiofemoral compartment. The most common symptoms are pain and swelling, with mechanical symptoms (catching, locking) less frequently reported. The results of meniscal transplantation have been poor in cases of advanced joint degeneration, and thus most authors currently recommend limiting transplantation to those patients with no more than grade II chondral degeneration (Refs. 9, 19, 51; F. R. Noyes and S. D. Barber-Westin, unpublished data, 1995; S. Rodeo et al., unpublished data, 1998). The presence of a large area of subchondral bone exposure (grade IV lesions) with radiographic joint-space narrowing and malalignment is generally a contraindication to this procedure. The knee should be stable or able to be stabilized by appropriate ligament reconstruction. The mechanical axis of the limb should not go through the involved compartment (for example, isolated medial meniscal transplantation should not be performed in a varus knee).
There is a lack of information as to the critical size and location of hyaline cartilage lesions, which are contraindications for meniscal transplantation. For example, a small, focal lesion may permit loadbearing around its periphery and thus not become a deleterious mechanical environment. There are usually varying degrees of cartilage damage on different parts of the articular surfaces, making it difficult to accurately grade such surfaces and difficult to interpret published reports. Many patients with meniscal deficiency demonstrate focal erosive lesions of hyaline cartilage, most commonly on the flexion weightbearing zone of the femur and tibia (Ref. 16; F. R. Noyes and S. D. Barber-Westin, unpublished data, 1995). Such lesions may be detected by MRI (Fig. 1) and may result in early joint-space narrowing as viewed on flexion weightbearing radiographs. Since the posterior aspect of the meniscus is loaded in flexion,[55] the presence of focal erosive lesions on the flexion weightbearing zone of the femur and the posterior tibia should be carefully evaluated. Furthermore, there is a meniscal weightbearing and a nonmeniscal weightbearing area on the femoral condyle and tibial plateau. The ideal candidate for transplantation would be a person with articular cartilage degeneration in the meniscal weightbearing zone of the femoral condyle or tibial plateau.
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Another consideration is the combination of meniscal transplantation with a cartilage-resurfacing procedure, such as osteochondral autograft transplantation, osteochondral allograft transplantation, or autologous chondrocyte implantation. In support of this strategy, Zukor et al.[58] and Garrett[19] have reported meniscal transplantation in conjunction with osteochondral allograft resurfacing of major chondral defects. Concomitant meniscal transplantation may help to protect the healing cartilage surface by aiding in stress transmission across the involved compartment. Similarly, a healthier articular surface is likely to improve the healing and ultimate survival of a meniscal transplant. Consideration may be made for staging these procedures, with the cartilage resurfacing procedure done first and meniscal transplantation done later, if there is successful improvement of the articular surface. There is very little clinical evidence currently available to support these complicated procedures, and thus they are not routinely recommended. If axial malalignment is present, osteotomy would be the procedure of choice.
Articular Cartilage Damage with Axial Malalignment
Degeneration of one tibiofemoral compartment is often associated with a varus or valgus deformity. In the younger, active patient, an osteotomy is often indicated in these situations to shift the mechanical axis to the more normal compartment. There is usually some degree of meniscal deficiency in the involved compartment; in fact, the progressive varus or valgus deformity may be a result of a previous meniscectomy. Most clinicians have performed an osteotomy alone, without concomitant meniscal transplantation. However, it is well known that recurrence of symptoms after osteotomy is often due to gradual progression of arthrosis in the compartment that was unloaded.[22] Consideration may be made for concomitant or staged meniscal transplantation if it could be demonstrated that meniscal transplantation could delay such recurrence. However, it must be noted that there are currently very few data to support concomitant osteotomy and meniscal transplantation. Such a procedure would be considered in very few select candidates, such as a young patient with articular cartilage that was predominantly intact.
Meniscal transplantation at the same time as the osteotomy may be technically challenging since drill holes in the proximal tibia are required if the meniscus is transplanted with bone plugs attached to the anterior and posterior horns. Careful placement of any internal fixation devices for the osteotomy is necessary to avoid the bone tunnels. Alternatively, meniscal transplantation can be performed as a staged procedure after the osteotomy has healed.
Ligament Instability
It is common to have ACL insufficiency associated with early arthrosis because of a previous meniscectomy. Several different reconstructive strategies have been reported, including ACL reconstruction alone,[45,46] ACL reconstruction combined with osteotomy,[35] or osteotomy alone.[22] The majority of the meniscal transplantations reported to date have been done with concomitant ACL reconstruction. It has been demonstrated that isolated ACL reconstruction in the arthritic knee can provide pain relief.[46] The mechanism of such pain relief remains unclear, as studies in this area have not been conclusive. It has been hypothesized that pain relief results from diminished shear loading of the articular surfaces once stability is restored. It is not yet known if concomitant meniscal transplantation can improve results as there are no studies that have compared isolated ACL reconstruction with ACL
reconstruction with concomitant meniscal transplantation in patients with similar degrees of arthrosis and meniscal deficiency. Outcome studies of meniscal repair have clearly demonstrated superior rates of healing in stable knees, and thus it is widely accepted that ligament stabilization should be performed during meniscal repair in the unstable knee.[8,34] Likewise, ligament stabilization is recommended if meniscal transplantation is being undertaken in an unstable knee.
Biomechanical studies have demonstrated that the medial meniscus is a secondary restraint to anterior tibial translation in the ACL-deficient knee.[3,31] Thus, medial meniscal transplantation at the time of ACL reconstruction may help to protect the ACL graft. Support for this theory comes from a recent cadaveric study that reported significant increases in the in situ forces in an ACL graft in medial meniscus-deficient knees compared with meniscus-intact knees (C. D. Papageorgiou et al., unpublished data, 2000). If there is slightly increased residual anterior tibial translation after the ACL reconstruction, especially as may occur after revision ACL reconstructions, the presence of a functioning medial meniscus may be important to prevent high loads on the ACL graft. In support of this strategy, Garrett[20] has reported significantly improved KT-1000 arthrometer results for ACL reconstructions performed with concomitant medial meniscal transplantation compared with a group of patients who underwent isolated ACL reconstruction with persistent medial meniscal deficiency, van Arkel and de Boer[51] also reported improved anterior stability after meniscal transplantation. Further evidence for the role of the medial meniscus in knee stability is provided in a recent report by Shelbourne and Gray,[45] who demonstrated greater laxity as measured by KT-1000 arthrometer findings after ACL reconstruction in patients who had undergone previous medial meniscectomy compared with knees with intact menisci.
In contrast, the lateral meniscus has not been demonstrated to act as a secondary restraint to anterior tibial translation in the ACL-deficient knee in cadaveric studies,[30] and clinical follow-up has demonstrated no difference in KT-1000 arthrometer results after ACL reconstruction in lateral meniscus-deficient knees compared with knees with intact menisci.[45]
The absence of both the medial and lateral menisci may result in slightly increased varus/valgus rotation. Meniscal transplantation may be considered in this setting, especially if collateral ligament repair or reconstruction is performed. It has been noted in such patients that replacing both the medial and lateral meniscus may help improve varus and valgus laxity. Markolf et al.[32] demonstrated greater varus/valgus laxity in the ACL-deficient and medial meniscus-deficient knee compared with the ACL-deficient knee with an intact medial meniscus.
Early Meniscal Transplantation
Eventual joint degeneration is a well-documented sequela of meniscectomy, with more rapid degeneration in the lateral compartment than in the medial compartment.[16] It is possible that the best time to perform meniscal transplantation is at the time of meniscectomy, before development of significant hyaline cartilage degeneration. The goal of meniscal replacement in this situation would be to prevent or delay degenerative joint changes. However, this cannot be routinely recommended until further longterm results are available. It is possible that such prophylactic meniscal transplantation may play a role, especially in the lateral compartment where there is a more rapid progression to degeneration after meniscectomy. This may be considered after resection of a symptomatic discoid lateral meniscus (Fig. 2), an irreparable bucket-handle tear that involves most of the lateral meniscus, or with a radial split tear, in which the loss of hoop stress transmission is functionally equivalent to a total lateral meniscectomy.
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Although it is becoming evident that meniscal transplantation should probably be performed earlier than it has been in most reported series (before the development of significant degenerative changes), the difficult question is how early to consider this procedure. Patients typically have no symptoms in the early years after meniscectomy in the otherwise normal knee, and meniscal transplantation cannot be currently recommended for a patient without symptoms. An objective indicator is needed for the detection of early compartment degeneration, to allow for detection of pathologic changes before the development of significant cartilage injury. High-resolution MRI or flexion weightbearing radiographs may be useful.[39,43] Bone scan may also demonstrate early evidence of compartment overload.[15] It is possible that, in the future, synovial fluid analyses of collagen or proteoglycan breakdown products or matrix metalloproteinase activity will be able to provide very early evidence of cartilage breakdown.[7] Such information may assist in the identification of the appropriate time to consider meniscal replacement in a patient with known meniscal deficiency.
CONTRAINDICATIONS
Given the poor results of meniscal transplantation in the setting of advanced degenerative changes, most authors currently recommend limiting transplantation to patients with early articular cartilage degeneration and relatively normal axial alignment as shown on long-standing cassette radiographs (Refs. 9, 19; F. R. Noyes and S. D. Barber-Westin, unpublished data, 1995; S. Rodeo et al., unpublished data, 1998). The presence of diffuse subchondral bone exposure is a contraindication to this procedure. The location of chondral lesions is probably as important as the size and depth, although there is currently little information available. Since most failures of meniscal transplantation occur because of progressive degeneration of the posterior part of the transplanted meniscus, the presence of full-thickness articular cartilage lesions on the flexion weightbearing zone of the femoral condyle or tibia that are greater than 10 to 15 mm in width or length should be considered a contraindication to meniscal transplantation (F. R. Noyes and S. D. Barber-Westin, unpublished data, 1995; S. Rodeo et al., unpublished data, 1998).
In addition to articular cartilage degeneration, the clinician should also consider changes in bone morphology. Previous reports have demonstrated poorer results when there is remodeling and flattening of the femoral condyle (F. R. Noyes and S. D. Barber-Westin, unpublished data, 1995; S. Rodeo et al., unpublished data, 1998). This should be carefully evaluated on radiographs made with the knee in full extension as well as on flexion weightbearing radiographs.[43] Uncorrected knee instability or axial malalignment is also a contraindication to meniscal transplantation.
PATIENT EVALUATION IN CONSIDERATION FOR MENISCAL TRANSPLANTATION
A careful history and physical examination are the most important factors in the initial assessment of a patient’s suitability for meniscal transplantation. Examination should focus on the presence of effusion, joint-line tenderness, crepitus, stability, and axial alignment. Review of previous operative reports may be helpful in determining the nature, degree, and duration of meniscal and chondral injury. Further evaluation must include standing radiographs, including flexion views to examine the flexion weightbearing zone of the femoral condyle or tibia, and standing hip-to-ankle views for assessment of the mechanical axis. High-resolution MRI with appropriate pulse sequences (proton-density weighted, fast-spin-echo) allows evaluation of the amount of remaining meniscus and the status of the hyaline cartilage and underlying subchondral bone.[39] Gait analysis may be useful in the assessment of compartment overload, especially in the knee with malalignment, but this has not been routinely used.[36]
A particular challenge is early detection of the onset of joint degeneration in patients who are known to be meniscus-deficient, such as a young patient who has undergone total lateral meniscectomy for a torn discoid meniscus. Magnetic resonance imaging is probably the most sensitive tool for following such patients because it will allow assessment of subchondral marrow edema, subchondral bone remodeling, as well as early softening and fibrillation of hyaline cartilage.[39] Flexion weightbearing radiographs are also useful for early detection of degenerative changes.[43] Bone scan may also be useful for early detection of compartment overload.[15] Further studies are required to determine the most sensitive indicator of early arthrosis.
GRAFT PROCESSING AND GRAFT SIZING
Meniscal allografts may be fresh, cryopreserved, fresh-frozen, or lyophilized. Fresh and cryopreserved allografts contain viable cells at the time of transplantation, while fresh-frozen and lyophilized tissues are acellular. Fresh grafts require transplantation within several days of graft procurement, resulting in difficult logistics.[54] It is not known what proportion of the cells in a fresh transplant survive after transplantation, and for how long these cells survive. Jackson et al.[23] used DNA probe analysis in a goat model and found that all of the donor cells in a fresh meniscal transplant were rapidly replaced by host cells. Their study examined fresh transplants; no similar studies have examined the fate of viable cells in cryopreserved grafts. At this time it is unclear if the additional cost associated with the use of cryopreserved grafts will be justified by improved results. Lyophilized grafts have been found to undergo shrinkage, and thus are not currently recommended (G. Peters et al., unpublished data, 2000). At the author’s institution, fresh-frozen grafts are currently used.
The tissue may be secondarily sterilized using gamma irradiation or ethylene oxide. Ethylene oxide is not recommended since one of the byproducts (ethylene chlorohydrin) has been found to induce synovitis.[24] Gamma irradiation may be used to eliminate viral DNA, but the dosage required to eliminate viral DNA (at least 3.0 mrad) may adversely affect the material properties of the meniscus (E. A. Yoldas et al., unpublished data, 1998). The author currently recommends use of fresh-frozen, nonirradiated allografts.
Appropriate size-matching of the meniscal transplant to the recipient knee is likely to be critical for optimal mechanical function. The graft may be sized based on intraoperative measurements or radiographic measurements of either the meniscus or the tibial plateau. There are currently no data available regarding the accuracy of intraoperative measurements of meniscal dimensions. This method would also necessitate a two-stage approach. Imaging methods that may be used to size the transplant include plain radiographs, MRI, and CT scans.[44] There is no agreement in the literature as to the most accurate imaging modality.[20,25,29,44] Direct measurement of the involved meniscus is often not possible because of previous meniscectomy. As a result, some authors have recommended sizing based on measurement of meniscus size in the contralateral knee using MRI, but studies have demonstrated variability between right and left knees in the same person, indicating that opposite menisci are not necessarily mirror images of each other.[26,44]
For this reason, most tissue banks currently size the meniscus based on bone measurements. Studies have demonstrated a consistent relationship between meniscus size and radiographic landmarks.[38,50] However, these studies have reported significant variability in the relationship of meniscal length and width to tibial plateau dimensions. For example, Pollard et al.[38] reported measurement error up to 8.4%, or 3.8 mm. A recent study compared MRI and plain radiography and found MRI to be slightly more accurate[44]; however, only 35% of images measured within 2 mm of actual meniscus dimensions. Importantly, the tolerance of the joint compartment to meniscus size mismatch is not known. Further studies are required to improve the reliability of graft sizing and to define the tolerance to size mismatch. At this time, the author uses plain radiographs (taken with a size marker to aid in correcting for magnification) and MRI to determine tibial plateau dimensions and then obtains a matching tibial plateau (or hemiplateau) graft with attached meniscus. This can be done for use with fresh-frozen grafts; however, if only the meniscus is supplied by the tissue bank (such as when using a fresh or cryopreserved graft), it will be necessary to use some formula to derive meniscal dimensions from the bone measurements. It is recommended that the clinician familiarize himself or herself with the technique used at the tissue bank supplying the grafts. Careful attention should be paid to obtaining a properly sized graft.
SURGICAL TECHNIQUE
Meniscal grafts may be transplanted using either open or arthroscopically assisted techniques. It is beyond the scope of this review to describe the technical details of each technique, but comparable results have been reported with both techniques (F. R. Noyes and S. D. Barber-Westin, unpublished data, 1995; G. Peters et al., unpublished data, 2000; S. Rodeo et al., unpublished data, 1998; E. A. Yoldas et al., unpublished data, 1998). Perhaps more important than the particular surgical technique used is the selection of appropriate sites for anchoring the anterior and posterior horns. The clinician needs to be familiar with normal meniscal anatomy and the relationship of the insertion sites to other intraarticular structures, including the tibial spines and cruciate ligaments. The horn insertion sites often are identifiable by a small stump of remaining meniscus.
The graft may be inserted either as a free graft, with separate bone plugs attached to the horns (Fig. 3), or by using the “keyhole” technique, in which the graft contains a common bone bridge attached to both anterior and posterior horns (Fig. 4). This bone bridge is then inserted into a similarly shaped slot in the recipient tibia. The usual diameter of the bone plugs is 9 mm. Implantation with attached bone appears to result in more secure anchorage of the horn attachment sites. Cadaveric models have demonstrated superior load transmission with meniscal horn bone plug fixation compared with no bone plugs.[2,10,37] The author currently recommends the use of bone plugs for medial transplants and a bone bridge for lateral transplants. If concomitant ACL reconstruction is performed with medial meniscal transplantation, the starting point for the ACL tibial tunnel on the outside of the tibia is moved slightly more medially to allow central placement of the two smaller tunnels for the anterior and posterior horn meniscal bone plugs. For combined lateral meniscal transplantation and ACL reconstruction, the keyhole technique is used, followed by reaming of the ACL tibial tunnel. In so doing, the ACL tibial tunnel may partially violate the meniscal bone slot, but the overall integrity of the slot usually remains intact. The other option is to consider staged ACL reconstruction and lateral meniscal transplantation. If both medial and lateral transplants are being performed, an arthrotomy is used and the grafts are implanted with one common bone bridge containing the attachments of both menisci (Fig. 5).
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The graft must also be securely sutured to the capsule using standard meniscal repair techniques. The author uses an inside-out technique to suture the posterior aspect and an outside-in approach for the anterior part of the transplant (Fig. 6). A posterior incision is used for placement of a retractor to protect the posterior neurovascular structures during the inside-out suture placement. Consideration may be made for use of absorbable meniscal fixation devices, but studies have demonstrated that the holding strength of these devices is inferior to vertical mattress sutures.[1,13]
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RESULTS OF MENISCAL TRANSPLANTATION
There is very little objective information available in the literature on the results of meniscal transplantation. It is difficult to interpret and compare the reports that are available because of variability in a number of important factors. Most series are small with limited follow-up. One of the principal limitations is the fact that the majority of the patients reported underwent other concomitant procedures, making it difficult to evaluate the contribution of the meniscal transplant to the overall result. Graft processing techniques (cryopreserved versus fresh-frozen, irradiated versus nonirradiated), surgical technique (whether implanted with attached bone or not), degree of arthrosis, and method of evaluation have not been uniform. For these reasons, it is difficult to draw definitive conclusions from reports in the literature.
Because the majority of patients reported in the literature have undergone concomitant procedures with meniscal transplantation, there is no way to demonstrate objectively that any change in symptoms is the result of the meniscal transplant. Accordingly, the most definitive information about the status of the meniscal allograft has been gained from studies that have directly evaluated the meniscus using either arthroscopy or MRI. Potter et al.[40] demonstrated that MRI provides accurate assessment of meniscal position, the horn and capsular attachments, meniscal degeneration, and the adjacent articular cartilage, and correlates well with arthroscopic evaluation of the transplant. Noyes and Barber-Westin (unpublished data, 1995) and Garrett[19] have demonstrated that clinical evaluation using only symptoms and physical examination does not allow reliable assessment of the status of the meniscus. Thus, this review will focus principally on direct objective evaluations of the transplanted meniscus. A summary of the results of meniscal transplantation will be presented based on a comprehensive review of the results published in the literature as well as reports from national meetings as of June 2000.
Based on a comprehensive literature review by the author, a total of 1599 meniscal transplants in 1551 patients have been reported in published series or at national meetings to date (Refs. 6, 9, 11, 19, 20, 29, 33, 47, 51, 53, 54, 58; W. Del Pizzo, unpublished data, 1996; E. M. Goble and K. J. Nelson, unpublished data, 1998; D. D. Mack et al., unpublished data, 1997; F. R. Noyes and S. D. Barber-Westin, unpublished data, 1995; E. Rath et al., unpublished data, 2000; S. Rodeo et al., unpublished data, 1998; E. A. Yoldas et al., unpublished data, 1998). However, direct objective evaluation of the transplant (arthroscopy, arthrotomy, MRI, or arthrogram) has been reported for only 366 menisci in 338 patients (Refs. 6, 9, 19, 33, 47, 51, 53, 58; W. Del Pizzo, unpublished data, 1996; E. M. Goble and K. J. Nelson, unpublished data, 1998; B. H. Min et al., unpublished data, 1999; F. R. Noyes and S. D. Barber-Westin, unpublished data, 1995; E. Rath et al., unpublished data, 2000; S. Rodeo et al., unpublished data, 1998; R. Ryu, unpublished data, 1998). The average age of these patients was 33, ranging from 10 to 68, and most procedures were performed in male patients. These series include fresh, fresh-frozen, cryopreserved, freeze-dried, irradiated, and nonirradiated menisci, both with and without attached bone plugs. Most transplantations (1463) were performed in combination with other procedures, most commonly ACL reconstruction, and the degree of preexisting arthrosis varied from grade I to grade IV (subchondral bone exposure).
Like any new or experimental procedure, meniscal transplantation was initially used as a salvage procedure in young, active patients with fairly advanced joint degeneration. This is a difficult group of patients, for whom there are no highly predictable reconstructive options. Predictably, the results in these patients have been variable. The majority of the patients in these series (60% to 100%) reported improvements in pain, swelling, and function, and there are significant improvements reported on standardized knee scales. However, objective evaluation usually demonstrated some degree of degeneration of the transplant. Overall, the results are related to the degree of arthrosis at the time of transplantation, with poorer results in patients with more advanced degeneration in the involved compartment (Ref. 19; F. R. Noyes and S. D. Barber-Westin, unpublished data, 1995; S. Rodeo et al., unpublished data, 1998). Only long-term follow-up, which is not yet available, will determine whether a meniscal transplant can delay or prevent the progression of degenerative changes.
Objective evaluations have demonstrated that the meniscus heals well to the joint capsule in over 90% of patients, and the bone plugs heal reliably to the recipient tibia (F. R. Noyes and S. D. Barber-Westin, unpublished data, 1995; S. Rodeo et al., unpublished data, 1998). Arthroscopic inspection reveals that, in knees with minimal articular cartilage degeneration, the meniscus has a normal appearance, consistency, and position in the joint (Fig. 7). Degeneration of the meniscus and, sometimes, small tears have been observed, most commonly in the posterior horn. Shrinkage of the meniscus is reported for freeze-dried menisci, but this is uncommon with fresh or fresh-frozen transplants (Ref. 33; F. R. Noyes and S. D. Barber-Westin, unpublished data, 1995; S. Rodeo et al., unpublished data, 1998). There is frequently abnormal intrameniscal signal in the posterior horn on MRI (Fig. 8) (F. R. Noyes and S. D. Barber-Westin, unpublished data, 1995; S. Rodeo et al., unpublished data, 1998). In knees with more advanced degenerative changes and with flattening of the femoral condyle, some degree of extrusion into the medial or lateral gutter is sometimes observed. Extrusion is usually associated with breakdown of the meniscus adjacent to the posterior horn attachment (Fig. 9). Potter et al.[40] demonstrated that allograft degeneration, indicated by fragmentation or an increase in signal intensity on MRI, is associated with articular cartilage degeneration. In that study, frank extrusion of the transplant was seen in association with full-thickness chondral degeneration, condylar flattening, and posterior horn meniscal degeneration.
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The outcome of meniscal transplantation may be best evaluated by analysis of isolated meniscal transplants, but there are very few follow-up reports on such patients. A comprehensive review of the results published in the literature as well as reports from national meetings reveals only 136 cases in which it is definitely stated that no other concomitant procedures were performed (Refs. 6, 9, 19, 51, 53; W. Del Pizzo, unpublished data, 1996; F. R. Noyes and S. D. Barber-Westin, unpublished data, 1995; E. Rath et al., unpublished data, 2000; S. Rodeo et al., unpublished data, 1998; R. Ryu, unpublished data, 1998; E. A. Yoldas et al., unpublished data, 1998). Undoubtedly, there are many more cases in the reported series, but this important factor is often not stated. These 136 cases include fresh, fresh-frozen, cryopreserved, irradiated, and nonirradiated menisci, making it even more difficult to draw conclusions. Furthermore, the reported series do not distinguish the results of isolated transplants from those that were done with other concomitant procedures.
The largest series of isolated transplants appears to be reported by van Arkel and de Boer.[51] These authors reported clinical follow-up at a minimum of 2 years for 23 patients with cryopreserved transplants and performed arthroscopic evaluation in 12 of the transplants. Partial detachment was found in five menisci, and three of these were eventually removed. There were signs of degeneration in five transplants. There was no change in the articular surfaces. Standing radiographs made from hip to ankle for assessment of mechanical axis demonstrated no change in 18 patients and improvement in 5 patients. Rodeo et al. (unpublished data, 1998) reported three failures from seven isolated lateral meniscal transplants. All of the patients in this series had grade IV articular degeneration. Cameron and Saha[6] reported good-to-excellent results in 19 of 21 knees (90%) that underwent isolated meniscal transplantation, but the majority of these transplants were not evaluated objectively. Verdonk[53] performed 24 isolated transplants in a series of 40 fresh meniscal transplants, but the results of these 24 patients are not distinguished from the overall group.
A review of several of the larger series demonstrates important information on the results of this procedure that will aid in refining the indications for meniscal transplantation. Milachowski et al.[33] were the first authors to report on the outcome of meniscal transplantation. These authors reported on 22 transplants (16 freeze-dried, 6 flesh frozen) at an average follow-up of 14 months. Evaluation by arthroscopy or arthrography, or both, demonstrated three (14%) failures. Shrinkage was noted in the freeze-dried transplants. The fresh transplants were noted to appear more normal than the freeze-dried menisci. Updated long-term results (14-year follow-up) in this same group of patients were recently reported (G. Peters et al., unpublished data, 2000). Patients with deep-frozen transplants continued to have better results than patients with lyophilized transplants. Magnetic resonance imaging demonstrated good preservation of the deep-frozen transplants even after 14 years. There was a slight decline in the Lysholm scores from 82 [+ or -] 15 points at 3 years to 74 [+ or -] 23 points at 14 years, and radiographs demonstrated an increase in degenerative changes of one grade according to Fairbank’s criteria.
The largest and most carefully studied group of patients was reported by Noyes and Barber-Westin (unpublished data, 1995), who performed 96 fresh-frozen, irradiated transplantations in 82 patients. All patients had objective evaluation of the transplant with either MRI or arthroscopy. Twenty-nine transplants (in 28 patients) failed before 2 years and were removed. Of the total 96 transplants, 56 (58%) failed, 30 (31%) were partially healed, 9 (9%) healed, and 1 (1%) was unknown. Notably, improvements in pain and function scores were not different between patients with healed or failed transplants. Many of the patients underwent concomitant ACL reconstruction. The high failure rate appears to be due to the presence of advanced arthrosis at the time of transplantation, as well as possible irradiation-induced weakening of the meniscus.[56] Failure was found to be related to joint degeneration as graded by MRI. In 18 patients with essentially normal articular surfaces, there were only 2 failures. In contrast, in 15 knees with advanced arthrosis there were 12 failures.
Goble and Nelson (unpublished data, 1998) reported that 13 of 18 transplants (72%) that had arthroscopic inspection were intact. These authors reported 11 failures of a total of 69 transplants (16%) at a minimum 2-year follow-up. Failures were associated with more advanced joint degeneration. Garrett[19] evaluated 28 transplants with arthroscopy and found 8 failures (29%). Failures were related to the degree of arthrosis: in the overall group of 43 patients, there were only 2 failures of 32 transplants in patients with grade III chondral changes, while 6 of 11 transplants failed in patients with grade IV changes. Garrett[20] also reported significantly improved KT-1000 arthrometer (MEDmetric San Diego, California) measurements in patients who underwent ACL reconstruction in conjunction with medial meniscal transplantation compared with patients who underwent isolated ACL reconstruction and who had persistent medial meniscal deficiency. Del Pizzo (unpublished data, 1996) reported on 19 cryopreserved allografts at a minimum 2-year follow-up. Arthroscopic inspection of 17 transplants demonstrated 1 tear. Pain was improved in all patients and all returned to their previous activity level. Stollsteimer et al.[47] reported on 20 cryopreserved allografts. Arthroscopic inspection in seven patients demonstrated 10% to 15% shrinkage in the transplants and one torn lateral meniscal transplant. There were three failures (15%).
Rodeo et al. (unpublished data, 1998) reported an objective evaluation of 33 fresh-frozen, nonirradiated meniscal transplantations performed at The Hospital for Special Surgery at a minimum 2-year follow-up. The patients were evaluated with MRI or arthroscopic inspection, or both. Preoperatively, 18 patients had grade IV and 6 patients had grade III articular degeneration in the involved compartment. Based on the objective evaluation of the 33 total meniscal transplants, there were 8 good, 14 moderate, 4 poor, and 7 failed meniscal transplants. There was no significant change in joint degeneration at this follow-up interval. Magnetic resonance imaging and arthroscopic inspections of the meniscal transplants demonstrated consistent healing of the meniscal transplant to the capsule and at the bone plug attachment sites. Magnetic resonance imaging showed that there was frequently some degree of extrusion of the transplant from the tibiofemoral compartment. The degree of extrusion was greatest in knees with more advanced articular degeneration (Fig. 9). Magnetic resonance imaging also demonstrated variable amounts of increased intrameniscal signal within the substance of the meniscus, which is indicative of ongoing remodeling of the transplant or degeneration, or both. Increased signal was most frequently observed in the posterior horn of the meniscus, in areas where the overlying articular cartilage was thinned or absent.
There were significant improvements in the final Lysholm rating and the final International Knee Documentation Committee (IKDC) rating. There were significant improvements (P [is less than] 0.05) in the Lysholm scores for locking, pain, swelling, and instability, and there was a significant improvement in the visual analog scale scores for both pain and function. The results were significantly better for menisci that were transplanted with attached bone plugs as compared with those implanted without bone plugs. Fourteen of 16 (88%) of the menisci implanted with bone plugs were rated as good or moderate, while 8 of 17 (47%) of the menisci implanted without bone plugs were rated as good or moderate (P = 0.03). There was no correlation between the degree of arthrosis at the time of meniscal implantation and the final status of the meniscus at follow-up, most likely because of the presence of advanced arthrosis in all of the patients in this group, making it impossible to demonstrate a difference.
Rodeo et al. (unpublished data, 1998) found that the failed transplants all occurred in male patients who were somewhat older (average age, 39). One patient had grade III articular changes, while all others had grade IV articular changes. Of the seven failed transplants, six were lateral menisci. Two patients in the failure group had received an isolated meniscal allograft. Only three of the seven failed allografts had been implanted with attached bone plugs.
Yoldas et al. (unpublished data, 1998) reported on the first 31 consecutive fresh-frozen, nonirradiated transplants from the University of Pittsburgh. There were 11 isolated transplants (9 lateral, 2 medial) and 20 transplants performed with concomitant ACL reconstruction. All patients returned for follow-up at an average of 36 months (range, 24 to 72). The patients had an average 2.4 previous surgical procedures (range, 1 to 4), and articular cartilage changes ranged from grade I to grade III. The menisci were transplanted with bone plugs medially and a bone bridge laterally. All patients were evaluated using physical examination, functional testing, flexion weightbearing radiographs, and standardized outcome scales (Knee Outcome Survey, SF-36, Lysholm, and IKDC). Twenty-two patients reported that they were greatly improved, eight were somewhat improved, and one was unchanged. The knees of all but one patient were normal or nearly normal based on the IKDC scores, and the average Lysholm score was 84 (SD, 14). Functional testing using vertical jump and hop tests demonstrated scores of 85% and 84%, respectively, compared with the contralateral knee. Radiographs demonstrated no significant changes compared with the preoperative radiographs. The authors concluded that meniscal transplantation is a viable treatment option for carefully selected patients who have undergone previous meniscectomy and continue to have pain and relatively intact articular cartilage.
Carter[9] reported on 46 transplants at a minimum 2-year follow-up. Arthroscopic inspection of 38 transplants demonstrated 4 failures (11%), with shrinkage noted in 4 others. The second-look arthroscopies in this series were done at a range of 3 to 48 months after surgery, with most performed at 6 months. Eleven patients had arthroscopy after 1 year. Chondral degeneration had progressed in two knees, both of which had varus malalignment and flattening of the femoral condyle. Plain radiographs demonstrated progressive degeneration in two patients. All but one patient reported improvements in pain and activity level on the IKDC form. Cryolife, Inc. (Marietta, Georgia) has compiled the results of 1023 cryopreserved meniscal transplants performed by 166 different surgeons (D. D. Mack et al., unpublished data, 1997). Seventy-three percent (747) of the transplants were medial and 27% (276) were lateral. No objective data were presented, but the overall failure rates were determined based on whether the graft required removal. Because of the lack of objective data, this information should be considered preliminary until more rigorous evaluation of these patients is completed. Of 728 grafts with attached bone plugs for fixation, 662 (91%) were intact, 29 (4%) were totally removed, and 36 (5%) were either partially removed or believed to be nonfunctional. The principal causes of graft failure were graft tears, progressive degenerative joint disease, and graft loosening. Cox Life-Table calculations showed a 76% [+ or -] 5.8% survival rate for medial menisci at 48 months and a 73% [+ or -] 8.5% survival rate for lateral menisci at 24 months. There were no differences in graft survival between medial and lateral grafts, or between male and female patients.
The importance of appropriate axial alignment is demonstrated by several series in the literature, van Arkel and de Boer[51] reported on 23 cryopreserved transplants at a minimum 2-year follow-up. There were three failures (transplant removed) and these failures were found to be related to malalignment. The scores on standardized knee scales (Lysholm and Knee Assessment Scoring System) were higher for patients with neutral alignment. Cameron and Saha[6] reported on 34 knees that received a meniscal allograft in combination with osteotomy (valgus high tibial, varus high tibial, or varus distal femoral). Good or excellent results were found in 29 (85%) of these patients. These authors acknowledged the difficulty in determining which part of the procedure was most important in providing clinical improvement.
Fresh menisci have been used in an effort to transplant tissue with viable cells. These tissues are maintained in tissue culture medium at either 4 [degrees] C or 37 [degrees] C and transplanted within the 1st week. Verdonk[53] and Verdonk et al.[54] transplanted 40 fresh menisci in 36 patients and found intact grafts using MRI in all patients and arthroscopic inspection in 12 patients. Analysis of DNA from cells cultured from a small biopsy of the transplanted meniscus demonstrated host DNA in some (indicative of cellular repopulation from the host) and donor DNA in others (indicating survival of donor cells). However, it must be noted that animal studies have not definitively established that viable donor cells survive after transplantation. As previously mentioned, Jackson et al.[23] used DNA probe analysis in a goat model to demonstrate that donor cells in a fresh meniscal transplant were rapidly replaced by host cells. It is possible that an immune response against the graft may have been at least partially responsible for this finding, although there was no rejection of skin grafts in these animals, suggesting that the meniscal cells died before an immunologic reaction.
Zukor et al.[58] transplanted 28 fresh menisci as part of a tibial plateau osteochondral allograft. Objective evaluation with arthroscopy or arthrotomy was performed for 14 transplants. The transplants were found to be structurally intact and well attached. There were degenerative changes and small tears seen in some, but none required removal. Garrett[19] transplanted 16 fresh menisci and 27 cryopreserved menisci and found no significant difference between the two types of transplants. No significant immune rejection has been noted in these series, although there are isolated patients with persistent effusions and synovitis after transplantation reported in these and other series.
It is difficult to draw definitive conclusions about the effect of graft processing on the clinical outcome based on the results reported to date. The arthroscopic appearance and clinical results associated with freeze-dried grafts appear to be inferior to other graft types. Milachowski et al.[33] and Peters et al. (unpublished data, 2000) compared the results in 16 freeze-dried transplants with 6 fresh-frozen transplants and found that the fresh transplants appeared more normal. There was significant shrinkage in all of the freeze-dried grafts. Essentially comparable results have been reported for fresh, fresh-frozen, and cryopreserved transplants. However, there are no trials that have carefully compared different graft types in matched patients. Garrett[19] found no difference between fresh and cryopreserved menisci. Similarly, the effect of irradiation is unclear. Noyes and Barber-Westin (unpublished data, 1995) reported a high failure rate using fresh-frozen grafts that had received 2.5 mrad of irradiation. The poor results with irradiated menisci may be secondary to weakening of the tissue by the irradiation. Yahia and Zukor[57] reported a significant reduction in compliance to long-term creep in frozen, irradiated meniscal transplants in rabbits, compared with nonirradiated fresh or frozen transplants. Long-term follow-up will be important for determining the fate of different types of transplants.
Another important factor may be whether the graft is transplanted with attached bone plugs at the anterior and posterior horns. The only study that has compared the outcome between grafts transplanted with or without bone plugs found improved healing rates in menisci transplanted with attached bone plugs, suggesting that healing of bone plugs in a bone tunnel is more secure than healing of the meniscus to bone (S. Rodeo et al., unpublished data, 1998). There is very little information available on the healing of meniscus to bone, and no studies have compared healing of bone plugs to healing of meniscal tissue in a bone tunnel. Gao et al.[18] recently reported that the tensile strength of a healed meniscal attachment after detachment and repair to bone in a rabbit model approached only 20% of the strength of the normal meniscal horn attachment. Clearly, firm anchorage of the allograft is critical for initial healing, remodeling of the allograft, and long-term function. Our findings also support cadaveric models that have demonstrated superior load transmission with meniscal horn bone plug fixation compared with no bone plugs.[10,37]
HISTOLOGIC ANALYSIS OF HUMAN MENISCAL ALLOGRAFTS
There is scant information available on the histologic characteristics of human meniscal transplants. Most authors have only examined biopsy specimens of failed transplants. Noyes and Barber-Westin (unpublished data, 1995) examined 29 failed frozen, irradiated transplants and found incomplete cellular repopulation with fibroblastic cells rather than fibrochondrocytic cells. Rodeo et al.[41] used immunohistochemistry and routine histology to examine biopsy tissue from meniscus and synovium of patients with both intact and failed transplants. There was incomplete cellular repopulation, with more cells at the periphery (Fig. 10). The central area often remained hypocellular or acellular. The repopulating cells had several phenotypes: mononuclear/synovial cells, fibroblasts, and fibrochondrocytes. There was active matrix remodeling by the repopulating cells, de Boer and Koudstaal[11] examined three failed cryopreserved transplants using histochemical techniques and found absence of cellular proliferation.
[ILLUSTRATION OMITTED]
Histologic analysis also demonstrates the possibility of a microscopic immune response against the transplant. The cells in frozen, unimplanted menisci stained positively for class I and class II human leukocyte histocompatibility antigens, indicating immunogenicity at the time of transplantation.[27,41] Rodeo et al.[41] found that the majority of the specimens contained a small number of immune-reactive cells (B-lymphocytes or T-cytotoxic cells) in the meniscus or synovium, or both. Although there was no evidence of frank immune rejection, the presence of these cells suggests the possibility of a subtle immune reaction against the transplant. Such an immune reaction may modulate graft healing, incorporation, and graft revascularization. The clinical outcome (successful versus failed transplants) was not related to the presence of an immune response in the meniscus or synovial biopsy. Previous studies also support the possibility of an immune response to meniscal transplants.[12,21,42,48] Hamlet et al.[21] reported a case of presumed acute rejection of a cryopreserved, nontissue-antigen-matched meniscal allograft, and van Arkel et al.[52] demonstrated sensitization to HLA class I and class II antigens in recipients of cryopreserved, nontissue-antigen-matched meniscal allografts. There was no clinical evidence of rejection in these patients.
MECHANISMS OF FAILURE OF MENISCAL TRANSPLANTS
It is likely that both biomechanical and biological factors combine to result in failure of meniscal transplants. The principal factor involved in failure appears to be advanced articular cartilage degeneration. The presence of osseous remodeling of the tibiofemoral compartment, with flattening of the femoral condyle, is associated with degeneration of the meniscus. This mechanism is supported by Noyes and Barber-Westin (unpublished data, 1995), who found that failure of meniscal transplants correlated with flattening of the femoral condyles as noted on MRI. Degeneration and tears most commonly occur adjacent to the posterior horn attachment, where the contact stresses on the meniscus are highest. The use of bone plugs attached to the anterior and posterior horns may provide more secure fixation of the horn attachments.
It is also likely that both surgical technique and graft sizing play important roles in outcome. Proper placement of the anterior and posterior horn fixation sites will affect the ability of the meniscus to transmit hoop stresses and to ultimately function in load transmission across the knee. Any disruption of meniscus morphology because of improper placement of the horn fixation sites is likely to adversely affect the biomechanical function of the meniscus. Cadaveric studies demonstrate that secure, anatomic fixation of bone plugs attached to the anterior and posterior horns is required to best restore normal contact mechanics for both medial[2] and lateral transplants.[10] Likewise, proper graft sizing is critical for optimizing conformity between the meniscus and the femoral condyle. An undersized graft would likely be exposed to excessive loads because of poor congruity with the femoral condyle, while an oversized graft is likely to extrude from the compartment and thus fail to transmit compressive loads across the knee. There are currently no data available as to the knee’s tolerance for graft size mismatch.
Biologic factors are also likely to play a role in the failure of meniscal transplants. Histologic studies in animal models as well as human biopsies demonstrate significant structural remodeling of the matrix associated with the process of graft revascularization and cellular repopulation. This remodeling may weaken the tissue and predispose it to tears and graft failure. This possibility is supported by Bylski-Austrow et al.,[5] who demonstrated in a goat model of meniscal transplantation that grafts with the greatest degree of cellular repopulation were actually the least effective in load distribution. It would appear that the presence of viable cells is critical for the long-term maintenance and repair of the tissue. Thus, it is possible that results may be improved by transplantation of meniscus with viable cells. However, it has not been definitively established that viable donor cells in the meniscus at the time of transplantation (using either fresh or cryopreserved tissue) survive after transplantation.[11,23]
Although frank immune rejection does not appear to occur, there is microscopic evidence of an immune response against the transplant.[41] It is possible that such a subclinical immune response may contribute, to some degree, to the graft shrinkage and persistent effusions that have been reported, and may modulate allograft revascularization and cellular repopulation.
REHABILITATION
There is very little information available in the literature to guide rehabilitation after meniscal transplantation. The principles used for rehabilitation after meniscal repair can provide some guidelines for determining the ideal program after meniscal transplantation. The loads placed on the healing meniscal allograft during rehabilitation activities are unknown. Although in vitro studies have provided some information about the load-to-failure of meniscal sutures, the failure strength of meniscal horn fixation in a drill tunnel in the tibia and of meniscal sutures after repetitive cyclic loading is unknown. Because meniscal transplants are thought to be under higher stresses in a joint with early degenerative changes, a more conservative protocol is recommended.
The author’s current postoperative protocol involves use of a standard double-upright, hinged knee brace for the first 6 weeks. Only toe-touch weightbearing with the knee in full extension is allowed for the first 4 weeks, with gradual progression to full weightbearing by 6 weeks postoperatively. The need to protect the healing meniscal transplant is supported by a rabbit study of meniscal transplantation in which alterations in stiffness and viscoelasticity were found in the early posttransplant period, with gradual recovery over time.[28] Further studies are required to determine the loads that are borne by the meniscus and the fixation sites during rehabilitation activities, and to define the magnitude and types of stress (shear versus compression) that may impair meniscal healing.[4,14]
Early range of motion exercise is begun immediately, including full extension. Flexion is limited to 90 [degrees] during the first 6 weeks since progressive knee flexion subjects the meniscus to greater stress.[55] This is supported by the observation by Morgan et al.[34] that extension appears to reduce the meniscus to the capsule, while flexion causes posterior horn tears to displace from the capsule. Also, Thompson et al.[49] demonstrated that the menisci translate posteriorly with flexion; however, meniscal movement was minimal below 60 [degrees] of flexion. No significant flexion limitations have occurred using this protocol.
Range of motion is progressed after 6 weeks. Closed kinetic chain strengthening exercises within the flexion limits are begun in the 3rd week and then progressed. Fritz et al.[17] suggest avoidance of early open chain knee flexion exercises because of the attachment of the semi-membranosus muscle on the medial meniscus and the popliteus muscle on the lateral meniscus. Gentle sport-specific activities are initiated after 4 months for further development of strength and proprioception. Running is not recommended before 6 months. Squatting and hyperflexion are discouraged for 6 months after meniscal transplantation. Return to high-load activities involving cutting, jumping, and pivoting are not currently recommended after meniscal transplantation. If there is concomitant ACL reconstruction, the usual ACL rehabilitation protocol is modified as previously described. Early full extension is emphasized. Concomitant cartilage resurfacing procedures may also require modifications in the postoperative program.
SUMMARY
Meniscal transplantation has emerged as a useful treatment option for selected patients. Predictable improvements in pain, swelling, and knee function have been demonstrated in early reports; however, no long-term results are currently available. Results are poor in patients with advanced articular cartilage degeneration, and this procedure should not be performed in such patients. Results are likely to be improved by earlier transplantation; the current challenge is to detect early cartilage breakdown before development of advanced degeneration. In the future, advanced imaging techniques or synovial fluid analysis of cartilage degradation products may allow earlier identification of early cartilage breakdown.
Further understanding of the biology of the transplanted meniscus will also refine the use of this technique. In particular, we need to understand the process of cell migration into the meniscus during cellular repopulation, the resultant phenotype of these repopulating cells, and the effect of an immune response on graft remodeling. Improved understanding of meniscal allograft biology will improve the feasibility of using fresh transplants with viable cells, which may lead to improved clinical results. Finally, the experience gained from meniscal transplantation will be useful in the future as other meniscal replacement options become available, such as bioactive scaffolds and tissue-engineered menisci.
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From the Sports Medicine and Shoulder Service, The Hospital for Special Surgery, New York, New York
Scott A. Rodeo, address correspondence and reprint requests to Scott A. Rodeo, MD, Hospital for Special Surgery, 535 East 70th Street, New York, NY 10021.
Neither the author nor the related institution has received any financial benefit from research in this study.
COPYRIGHT 2001 American Orthopaedic Society for Sports Medicine
COPYRIGHT 2001 Gale Group
