23

The extracellular matrix is largely composed of collagen fibers. It is the special orientation of these fibers that provides strength to the meniscus. The majority of these fibers are circumferential in nature. Intermixed with these circumferential fibers are radial fibers that link and bond the circumferential fibers (Fig. 23–1). This unique configuration is the foundation of the strength of the meniscus and its ability to resist compressive and rotational stress forces. This natural-occurring architecture is the challenge to replicate in a synthetic meniscus. Although the majority of the functional component is type I collagen, types II, III, and V have also been identified. The vascular supply to the meniscus is via the geniculate system. The anterior portion of the meniscus and peripheral capsule are supplied by the medial and lateral branches of the inferior and superior geniculates. The posterior portion is supplied by the middle geniculates. The vessel branch ultimately forms a perimeniscal capillary complex that then supplies the peripheral aspect of the menisci (Fig. 23–1B and 23–2). On the medial side, the peripheral 10 to 30% of the meniscus is vascularized, whereas on the lateral aspect the peripheral 10 to 25% is vascularized. However, in children, up to 50% of the peripheral aspect of both menisci can be vascularized.

Grossly the menisci are semilunar in shape, being triangular in cross section with the thickened base oriented peripherally. The medial meniscus is longer in the anteroposterior direction (Fig. 23–3). An important factor when performing medial meniscal transplantation is that the anterior horn is in line with the medial tibial eminence, posterior to the fat pad and edge of the patellar tendon. The anterior fibers of the meniscus ultimately contribute to the formation of the transverse intermeniscal ligament. The attachment of the anterior horn may also go across the anterior slope of the tibia. The lateral meniscus is more circular in shape with a smaller anteroposterior length. The position of the anterior horn is anterior to the tibial spine and anterior cruciate ligament footprint, and must be replicated when performing a lateral meniscal transplant (Fig. 23–3). The lateral meniscal anatomy is further complicated by the presence of the popliteal hiatus and the meniscal femoral ligaments.

FIGURE 23–1 (A) The meniscus is composed of circumferential and radial fibers, which are held together by meshed network of fibers. (From Bullough PG, Munuera L, Murphy J, Weinstein AM. The strength of the menisci of the knee as it relates to their fine structure. J Bone Joint Surg Br 1970; 52:564–567, with permission.) (B) The circumferential fibers are in the periphery, whereas the radial fibers extend from the circumferential fibers toward the central portion of the meniscus.

FIGURE 23–2 The perimeniscal capillary complex supplies the peripheral third of the meniscus. Also known as the “redred” zone. (From Arnoczky SP, Warren RF. Microvasculature of the human meniscus. Am J Sports Med 1982;10:90–95, with permission.)

All these complexities allow the meniscus to achieve its function: load bearing, force distribution, joint stability, lubrication, and proprioception. The microstructure of the meniscus is an important component in meniscal function as the collagen forms circumferential fibers that are linked by regularly oriented collagen fibers, as previously noted. It is this network of fibers that helps to resist tension, compression, and shear and ultimately contributes to the ability of the meniscus to bear load and distribute forces. The circumferential fibers run from bony horn attachment to bony horn attachment and resist the axial load and compression loads, converting radial forces to tension forces, termed hoop stresses. As stresses can be defined as force per unit area, it is then apparent that the meniscus can reduce stress to the articular cartilage by both redirecting the forces through the hoop stress and increasing the surface contact area.

FIGURE 23–3 Topography of the proximal tibia showing the ligaments and menisci in their relevant relationships. (From Pagnani MJ, Warren RF, Arnoczky SP, Wickiewicz TL. Anatomy of the knee. In: Nicholas JA, Hershman EB, eds. The Lower Extremity and Spine in Sports Medicine, 2nd ed. St Louis:Mosby, 1995:581–614, with permission.)

Basic Science Considerations

The ability to utilize meniscal allograft as successful substitutes for the normal meniscus is founded in basic science considerations. The major factors that should be considered are immunology, chondroprotective potential, healing potential, and tissue preservation.

The relatively acellular nature of the meniscus allows it to be described as immunologically privileged.¹ Although the largest population of histocompatibility antigens are on the cellular components, the relatively low population and shielding by the extracellular matrix protects the tissue. Therefore, systemic rejection rarely occurs. However, it has been demonstrated that some form of immune response does occur.² The potential for a localized immune response exists: (1) even after deep freezing, histocompatibility antigens can be expressed on the cells of a meniscal allograft; (2) the bone and synovial attachments to the meniscal allograft will remain antigenic; and (3) the existence of immunoreactive cells in fresh frozen meniscal allograft has been demonstrated.³ This localized response may have an affect on graft revascularization, repopulation, incorporation, and overall healing response. However, it appears that clinical results have not been compromised by this localized immune response.

It is generally accepted, based on multiple studies, that the meniscal allograft will heal, gradually repopulate with host cells, and not be systemically rejected.⁴ One of the major concerns about meniscal transplantation is the chondroprotective potential, both short and long term. Several animal studies have demonstrated chondroprotective benefits of the meniscal allograft when compared with a total meniscectomy.⁵ However, the studies did not demonstrate results identical to a normal meniscus. Similar improvements in contact/stress in a cadaveric model have also been demonstrated. These protective benefits are short term, and animal studies demonstrating long-term chondral protection are lacking.

Allograft Selection

Meniscal allograft tissue should only be obtained from a tissue-banking facility that is in compliance with the criteria established by the American Association of Tissue Banks.

This standardization maximizes the quality of the tissue (no evidence of degenerative change) and minimizes the risk of disease transmission.

At the present time, meniscal allograft tissue is processed by one of four techniques: fresh, cryopreserved, fresh frozen, or freeze dried. Fresh allograft tissue offers advantages regarding cell viability, but is not practical. Freeze dried tissue is associated with cell death. However, the major disadvantage of this process is the ultimate affect it has on the structural properties of the meniscal allograft, including graft shrinkage and potential damage to the collagen fibers. The processes of cryopreservation and fresh frozen tissue avoid the structural damage seen with freeze drying. Cryopreservation involves a slow freezing rate of the allograft tissue by utilizing a chemical cryoprotectant such as dimethylsulfoxide (DMSO). This method allows for a slight increase in cell viability and preservation of some degree of cell membrane integrity. The processes of cryopreservation and fresh frozen tissue both allow for prolonged storage. These processes of prolonged storage facilitate not only complete serologic testing, but also creation of a tissue bank of various subtle sizes, potentially improving tissue matching. Meniscal allograft tissue that undergoes fresh frozen processes undergoes a more rapid freezing process that results in slightly increased cell death, but without adversely affecting the structural properties of the graft. Cell viability, however, may not be an important issue, as has been shown by experimental studies that have suggested that there are no significant outcome differences between cryopreserved and deep frozen grafts. Indeed it has been demonstrated that the repopulation of meniscal allograft tissue by host-derived cells is extremely common and occurs within the first year of implantation. Future research will provide insight into the enhancement of the process of repopulation and incorporation. At the present time the utilization of fresh frozen tissue, compared with either fresh or cryopreserved, is simpler, less costly, and still provides the essential biologic scaffolding for successful function and incorporation.

Patient Selection and Indication

The general indications for meniscal transplantation are a history of documented total or subtotal meniscectomy with pain and/or disability relating to the affected compartment of the knee. However, other significant variables need to be considered. The duration of time since meniscectomy is important, especially as it relates to articular cartilage wear and possible alignment issues.

Radiographic evaluation should include at least the following: anteroposterior (AP) standing x-rays of both knees in full extension; non—weight-bearing, 45-degree flexion laterals; 45-degree flexion weight-bearing [posteroanterior (PA)]; and an axial view of the patellofemoral joint. Although magnetic resonance imaging (MRI) is not an imperative radiologic test, additional important information can be obtained regarding the status of the articular cartilage and the biologic activity of the subchondral bone, especially when performed with gadolinium enhancement. The ideal indications for meniscal transplantation are a symptomatic patient with (1) documented previous subtotal meniscectomy or absence of the major weight-bearing portion of the meniscus, (2) pain in the involved knee compartment, (3) less than grade 3 or correctable articular cartilage damage, (4) ligamentous stability or correctable stability, and (5) normal or correctable alignment.6,7

However, these indications will continue to evolve as the concept of cartilage restoration of the knee advances. Although the chronologic age most often referred to as a limit is 50 years, age is in reality a relative consideration. It is important to assess the biologic age, taking into consideration the environment of the knee, activity level, and ability to be compliant with the postoperative course and rehabilitation.

There are several contraindications to meniscal transplantation: degenerative arthritis, inflammatory arthritis, crystal deposition disease, and active infection. It is important to determine whether the articular cartilage changes that exist are secondary to the previous meniscectomy and can be restored or whether this truly represents advanced osteoarthritic changes. Obesity has the potential to compromise a normal knee joint. Based on articular cartilage-related studies, a body mass index (BMI) of over 35 should be considered a relative contraindication.

The patient who has recently undergone a subtotal meniscectomy and has not yet developed symptoms poses a significant challenge. The treatment and timing of treatment is certainly debatable as well as controversial. The natural history of the meniscal deficient knee is well documented. Nevertheless, not every postmeniscectomy patient develops degenerative arthrosis. There are several factors that are important in this treatment algorithm. Certainly these patients must undergo rigorous education about the signs and symptoms associated with knee degeneration. They should also be followed closely with standing 45-degree PA radiographs. Currently the measurement of articular cartilage degradation products is changing from a laboratory setting to the clinical realm, and may play an added role in early detection of postmeniscectomy articular cartilage damage. However, is that enough? The breakdown of articular cartilage can progress very rapidly in the meniscal-deficient joint. The factors that need to be considered are activity level, physiologic age, rehabilitative potential, and goals and expectations. In general, in the select postmeniscectomy individuals who have higher demands on their knee, rehabilitation potential, and realistic goals, there is a role for early consideration of meniscal allograft transplantation. The potential for articular cartilage breakdown in the younger patient and creation of a negative knee environment is too great to ignore. By the time the patient is symptomatic and has positive x-rays or a positive bone scan, the articular cartilage has already been damaged.

Surgical Technique

When the patient has been selected for meniscal transplantation by meeting the previous described criteria, the meniscus must be obtained. Prior to ordering the meniscus, sizing studies must be performed. The most commonly accepted method utilizes standard AP and lateral radiographs with magnification markers as described by Pollard et al.⁸ It is important for both the surgeon and the tissue bank to measure these films and for the surgeon to comment on any special considerations that could alter accurate measurements, such as an osteophyte or previous trauma. It is also imperative to be certain of which side (right versus left) and which compartment (medial versus lateral) is being requested. This should be double-checked on the day of surgery prior to the patient’s being anesthetized and prior to defrosting of the tissue.

The evolution of meniscal transplantation has allowed the procedure to advance to an arthroscopically assisted procedure with a miniarthrotomy. It may also be performed as an open procedure, especially when done concomitantly with a major articular cartilage restoration. Several different techniques have been developed for implantation. These include the double bone plug technique and the bone bridge techniques—keyhole, slot, trough, and dovetail. Regardless of which technique is selected, the essential foundation of successful meniscal transplantation is anatomic reduction and stable fixation of the meniscal allograft.

The fixation of the meniscal allograft is accomplished by the bony attachments and stabilization of the peripheral rim. Most recently developed all-inside meniscal repair devices are not well adapted for meniscal transplantation fixation. The majority of these devices are designed for meniscus-to-meniscus repair, not for meniscus to capsular soft tissue. In the situation where an adequate meniscal rim remains, the use of these devices may be considered. The most reliable method of fixation is by inside-out suture fixation.

The double bone plug technique9,10 was developed for transplantation of the medial meniscus and represented the first bone-anchored procedure. The bone plug technique involved the creation of bone tunnels at the site of the anterior and posterior horn attachments of the meniscus (Fig. 23–4). As compared with the medial meniscus, the anterior and posterior horn of the lateral menisci are close enough that tibial tunnel creation and subsequent fixation could be compromised. Therefore, this technique is not indicated for lateral meniscal transplant. The medial meniscus is prepared by the creation of bone plugs at the anterior and posterior horn attachments of the allograft tissue. These bone plugs are then placed into the appropriately corresponding anterior and posterior tunnels. Placement of the posterior bone plug can often be difficult and can result in fracture of the bone plug, ultimately compromising fixation. As discussed earlier, it is imperative for anatomic reduction. Therefore, it must be remembered that the anterior horn attachment is usually on the anterior slope of the tibia. Therefore, creation of a true bony tunnel may not be possible and the anterior plug may be placed more as a press-fit. To compromise this anterior placement by simply placing the anterior horn where it lies best is certainly easier, but is unacceptable. This nonanatomic placement would compromise any chance of replicating normal biomechanical function¹¹ by creating abnormal stress and not establishing normal contact pressures. Although the use of this technique can be successful, careful attention must be given to maintaining the anatomic reduction to optimize function.

FIGURE 23–4 Bone plug technique. Note that although in this illustration, a bone plug is being used for a lateral meniscal allograft, this technique is recommended only for medial meniscal transplantation. See text for explanation.

The utilization of a bone bridge between the anterior and posterior horn allows for maintenance of the normal anatomic alignment of the attachment points and for superior bone fixation along the bridge.

Several different bone bridge techniques have been developed: keyhole, slot, trough, and dovetail.^10,12 The keyhole technique (Fig. 23–5) was developed for placement of the lateral meniscal allograft. This technique involves the creation of a keyhole in the tibial plateau with subsequent creation of a key bridge corresponding to this tunnel. The procedure can be very demanding to perform because it requires fine-tuning of the key bridge to fit the slope and shape of the keyhole. In addition, the creation of the key bridge may be compromised by the initial harvest and preparation of the meniscal allograft.

The dovetail (Fig. 23–6), slot (Fig. 23–7),⁷ and trough techniques (Fig. 23–8) have some basic similarities. They all utilize guide systems to create a trapezoidal or rectangular-shaped bone tunnel on the tibial plateau in an anteroposterior direction. This tunnel corresponds to the anatomic position of the anterior and posterior horn attachments. Both techniques can be used on either the medial or lateral side. The greatest difference in these two techniques is in the sizing of the bone bridge. For the trough technique, the bone bridge is created so that it is press-fit in the trough tunnel. The dovetail technique also creates a press-fit tunnel that allows for stabilization so that it will not ride up in the slot. The concept of press-fit is good only if there is extremely accurate sizing and placement. These two factors are extremely important for appropriate function. The actual placement of a press-fit plug can also be difficult, and if impaction is utilized it has the potential for fracture of the bone bridge or damage to the anterior horn attachment site. In addition, once engaged, if removal of the bone bridge were required this could be extremely complicated.

Initially weight bearing is limited to partial weight bearing for the first 4 weeks. After 4 weeks, weight bearing may be progressed to full weight bearing as tolerated. This progression corresponds to the increased metabolic activities of the meniscal cells and their ability to withstand stress. The requirement for loading and the positive influence and remodeling process then become a positive factor.

Although the basic concept of meniscal transplantation is similar to that originated in 1984, the basic and clinical science has undergone continuous improvement regarding understanding, application, and performance of the surgical procedure. When analyzing and comparing the results of any procedure, it is important to have minimal variables between the individual series. Unfortunately for meniscal transplantation, this is rarely the case. Since Milachowski et al’s13 first publication in 1989, multiple series have been reported. However, there are different variables in each study, such as patient population, knee arthrosis, graft preservation, surgical techniques, additional surgical procedures, rehabilitation, and parameters of follow-up. Therefore, it becomes challenging and difficult to make accurate conclusions about the role of meniscal transplantation. Though Milachowski et al were the first to perform a modern-type isolated meniscal transplant in 1984, their subsequent publication in 1989 on 22 patients with an average 14-month follow-up demonstrated healing of 15 to 18 (83%) grafts peripherally by arthroscopic evaluation. Milachowski et al had used both fresh frozen and freeze dried grafts, with the fresh frozen appearing more normal microscopically.

Garrett¹⁴ in 1993 reported on the 7-year follow-up of 43 patients. The patient population in this study was certainly complex (salvage-type cases). Only six patients had an isolated meniscal transplant, and 28 patients underwent second-look arthroscopies, with 20 patients (71%) demonstrating healing of the meniscal allograft. The eight failures were attributed to grade 4 chondromalacia. The remaining 15 patients were asymptomatic and considered to have a good outcome.

Noyes and Barber-Westin¹⁵ evaluated a large series of patients in 1995. This series reviewed 96 fresh frozen irradiated grafts in 82 patients. The results were extremely poor. Of the total 96 implants, 58% failed completely, 31% were partially healed, and 9% healed. Twenty-nine menisci had to be removed less than 2 years after surgery. The surgical technique involved attachment only of the posterior horn with bone plugs. No transplant was attached with bone both anteriorly and posteriorly. All grafts were irradiated. The presence of grade IV chondrosis was associated with 50% failure rate. Cameron and Seha¹⁶ reported their results on 67 irradiated menisci without bone anchors in patients with generally advanced unicompartmental arthritis. These patients also underwent an osteotomy to unload that compartment at the time of surgery. At an average of 31 months’ follow-up, 87% demonstrated a good or excellent result.

Van Arkel and deBoer¹⁷ reported on 23 patients with a minimum of 2-year follow-up who underwent isolated meniscal transplantation. There were three failures at less than 24 months with uncorrected alignment. The remaining 20 patients (87%) were felt to have a satisfactory result.

Stollsteimer et al¹⁸ reported a series of 22 patients with 23 cryopreserved allografts. Implantation was done arthroscopically assisted with bone plugs. The patients were followed for an average of 40 months. The most significant finding in this study was pain reduction in all patients. Although not associated with an adverse outcome, the allograft demonstrated a 37% shrinkage on MRI studies.

Carter¹⁹ reported on 46 transplants with a minimum 2-year follow-up. Thirty-eight of the transplants underwent arthroscopic second looks between 3 and 48 months, with most occurring at 6 months. At the time of arthroscopy, four patients were considered failures, four had visible shrinkage, and two showed signs of progression of arthritis. Thirty-two of the 38 patients (84%) reported relief of pain and improvement in activities.

In 1999, Cole and Harner²⁰ reported the results of 22 fresh frozen menisci, and 95.5% of the patients reported improvement of pain, with associated improvement in knee function in all but one case. Rodeo et al¹² represented a comprehensive review of the literature at a national orthopedic meeting. In their review, as of March 2001, a total of 1599 meniscal transplants were performed in 1551 patients. Of these, direct objective evaluation of the transplanted tissue was done in only 366 menisci in 338 patients. Ages ranged from 10 to 68 and averaged 33. The series included fresh, fresh frozen, cryopreserved, freeze dried, irradiated, and nonirradiated menisci. The meniscus was implanted both with and without bone plugs. Concomitant procedures were performed in all but 136 cases. The degree of arthrosis in the knee varied from grade I to grade IV, and there was no consistent rehabilitation.

Meniscal transplantation has evolved as a successful procedure for the reconstruction of the meniscal deficient knee. This success will be further enhanced and improved by collaboration with basic science research. Although early results were promising, with the development and understanding of the technique, more recent studies have demonstrated improved results. As meniscal repair and preservation techniques continue to advance, the meniscal deficient knee will become less common. However, the technology of meniscal transplant will continue to be an integral procedure for the preservation of knee function.