Injury and loss of the meniscus has been shown to alter load transmission in the knee by decreasing contact area and increasing contact pressure and shear stress across the articular surface. Over time, this can lead to progressive degenerative changes and osteoarthritis. Meniscal allograft transplantation (MAT) offers a biologic solution for managing patients with meniscal deficiency. The following chapter outlines meniscal function; biomechanical changes after meniscectomy; and the indications, techniques, and outcomes associated with MAT.

Once considered a vestigial component of the knee, the meniscus is responsible for many essential functions that influence long-term knee health and stability. The knee contains medial and lateral menisci whose collagen architecture, biochemical fluid composition, and proteoglycan-collagen meshwork allow them to function in shock absorption and load transmission between the femur and tibia.¹,²,³ The menisci also aid in knee stability, knee joint proprioception, joint lubrication and cartilage nutrition, and prevention of osteoarthrosis.¹,³,⁴,⁵,⁶,⁷,⁸ As load transmitters, the menisci distribute stress across the tibiofemoral joint by increasing the contact surface area of the joint and improving the congruence of the articulating surfaces.³ The improved tibiofemoral congruency and increased joint contact area allows the menisci to function in load sharing.⁹ This aids in the knee’s ability to withstand the load forces associated with walking, running, and standing while preventing damage to the articular cartilage and the subchondral bone of the femur and tibia.

The composition of the meniscus allows it to withstand high compressive forces. The meniscus is composed of 70% water, and type I collagen makes up 60% to 70% of its dry weight. Proteoglycans and water provide a swelling pressure that decreases the peak stress across the articulating surfaces of the femur and tibia. This prevents hyaline cartilage from progressive degeneration. The meniscus may also support articular cartilage nutrition by compressing synovial fluid into articular cartilage and maintaining a lubricating film over the articular surface.⁷,⁸

During weight bearing, the meniscus experiences tensile or “hoop” stress in the circumferential collagen fibers as it withstands radially extrusive force. The percentage of joint load that occurs across the knee depends on knee flexion angle along with femoral translation and rotation and differs between the medial and lateral menisci. The menisci transmit 50% of the joint load in the extended knee and 90% of the load in the flexed knee.³,¹⁰ The medial meniscus transmits 40% of the joint load in its compartment, whereas the lateral meniscus transmits 70% of joint load in the lateral compartment.³,⁶,¹¹ Total or partial meniscectomy can increase the peak contact stresses on the articular cartilage between 40% and 700%.¹¹ These high contact stresses may result in hyaline articular cartilage degeneration with time.

The menisci also play a role in knee joint stability. The medial meniscus is a secondary stabilizer to anterior tibial translation and thus a significant restraint to anterior translation in the anterior cruciate ligament (ACL)-deficient knee.¹² In the setting of a medial meniscal-deficient knee, the forces on the ACL graft are increased.¹³ Additionally, the medial meniscus plays a role in the varus-valgus stability of the knee, as biomechanical studies have shown increased varus-valgus rotation in the ACL insufficient and medial meniscal-deficient knee.¹⁴

The lateral meniscus plays a key role in the stability of the knee during a pivoting maneuver.¹⁵,¹⁶ Lateral meniscal loss may contribute to instability as well as rapid progressive joint degeneration because of the convexity of the articulating surfaces in the lateral compartment. The presence of nerve endings and mechanoreceptors near the anterior and posterior horn insertions, as well as the capsular attachments of the menisci, may also contribute to joint proprioception.⁸

It is clear that the meniscus plays several important roles in knee function. The rationale behind MAT is to prevent articular cartilage degeneration and its associated clinical symptoms by restoring the important functions provided by healthy meniscal tissue.

Clinical outcome studies, in addition to radiographic imaging and biomechanical assessments, have been consistent in identifying the consequences of meniscectomy on the knee. The protective properties and load transmission capabilities have been demonstrated in comparisons of knee health with an intact meniscus versus partial and total meniscectomy. Outcomes are dependent on the presence of articular cartilage defects or degenerative changes, the amount and location of meniscal tissue removed, ligament instability, and malalignment. Secondary
factors that also play a role are patient sport and activity level, which are of increased importance and consideration in adolescent patients.

Biomechanical studies following meniscectomy have shown that meniscal loss can lead to a decrease in contact area of approximately 75% and an increase in peak contact stresses of approximately 235%.¹⁷ Increased contact stress and decreased contact area are thought to overload the articular cartilage, resulting in damage ranging from surface fibrillation to full-thickness cartilage loss.¹⁸,¹⁹

Such biomechanical changes have been verified with clinical and radiographic outcome studies following meniscectomy. Fairbank²⁰ reported radiographic hallmarks of osteoarthritis following meniscectomy as the formation of a ridge on the margin of the condyle, joint space narrowing, and flattening of the condyle. The progression of arthritis following meniscectomy has been documented in many studies. A study following patients 21 years after meniscectomy found that the relative risk for advanced radiographic changes representing definitive osteoarthritis following meniscectomy was 14.0.²¹

The severity of each of these factors has been outlined via comparisons between operative and nonoperative knees. In an 11.5-year follow-up study of medial meniscectomy surgeries, Chatain and colleagues²² reported that radiographic imaging revealed a 22.4% greater prevalence of joint space narrowing in the operative knee when compared to their contralateral, nonoperative knee. Similarly, in a 17-year follow-up study conducted by Johnson et al.,²³ the patients’ operative knee was diagnosed with degenerative joint disease 40% of the time as compared to 6% in the nonoperative knee. In the longest follow-up study to date, Pengas et al.²⁴ reported that at mean follow-up of 40 years in adolescents who underwent open total meniscectomy, all patients were symptomatic; a greater than 4-fold relative risk increase for osteoarthritis existed as well as a 132-fold increase in the rate of total knee replacements.

Partial meniscectomies tend to have more positive outcomes than total meniscectomies. A direct comparison of radiographic results by McGinity and colleagues²⁵ found that there were radiographic changes in 60% of those who received total meniscectomies compared to 30% for those who had partial meniscectomies over the span of 5.5 years. Schimmer et al.²⁶ reported excellent to good results for 91.7% of patients at 4 years following partial meniscectomy and 78.1% at 12 years postoperatively. Partial meniscectomies offer a quicker return to function, faster rehabilitation, and lower morbidity rates following surgery, as the amount of meniscal tissue resected appears to be inversely related to knee function.⁶ This timeline for recovery may be more appealing to patients, particularly in adolescents.

It must be noted that the long-term follow-up of partial meniscectomy is not overwhelmingly advantageous for patients with meniscal injuries; partial meniscectomies in ACL-deficient knees may result in more than twice the incidence of Fairbank’s changes when compared to meniscal repairs, stable tears left untreated, and normal menisci.²⁷ It is important to consider the degree to which circumferential collagen fibers are disrupted and whether it affects the ability of the meniscus to functionally distribute forces across the knee. In the case of significant disruption, a partial meniscectomy may be equivalent to a total meniscectomy.⁶ Damage to articular cartilage plays a key role in patient outcomes as well. Schimmer et al.²⁶ reported that following partial meniscectomy, 62% of patients with additional cartilage damage reported good to excellent results as compared to the 94.8% of patients with isolated meniscal tears.

As there are differences in the geometry of the articular surfaces and meniscal shapes in the medial and lateral compartments, there are also differences in the outcomes following functional meniscal loss in each compartment. The literature suggests that patients who undergo medial meniscectomy tend to achieve better outcomes than those who undergo lateral meniscectomy.⁶,²³ A 30-year longitudinal study by McNicholas and colleagues²⁸ reported good to excellent results for 80% of patients with medial meniscectomy compared to 47% of patients with lateral meniscectomy. This is primarily due to a slower progression of degenerative changes for those undergoing medial compared to lateral meniscectomy.²⁹ The anatomic differences between the medial and lateral compartments of the knee account for this variability: The convexity of the medial femoral condyle aligns with the concavity of the medial tibial plateau, whereas the convexity of the lateral femoral condyle is mirrored by the convexity of the lateral tibial plateau. In the case of a meniscectomy, the distribution of force within the medial and lateral compartments of the knee joint will differ, as peak stresses in the lateral compartment are significantly greater.

The cause of meniscal tears and functional meniscal deficiency may also play a role in the outcomes and possible indications for meniscal transplantation. McBride et al.³⁰ reported satisfactory results in 96% of patients following partial meniscectomy for nondegenerative tears versus 65% satisfactory outcomes for degenerative tears. The best results were in those patients who had mechanical symptoms. Schimmer et al.²⁶ reported 95% satisfaction following partial meniscectomy with no degenerative joint disease versus 62% with degenerative joint disease. Burks and colleagues³¹ reported 88% satisfactory results 15 years following partial meniscectomy in ACL-intact knees versus only 48% satisfactory results in ACL-deficient knees.

There are many variables to consider when making the decision to perform a meniscectomy. Although the risk factors for degenerative change and knee instability are significant, patients may benefit from a partial or total meniscectomy based on preoperative symptoms, goals, and lifestyle. This is an especially important discussion to have with adolescent patients and their parents. Patients with functional meniscal deficiency may be candidates for MAT when certain indications are met.

The goal of MAT is to replace the biomechanical function of the meniscus. MAT provides increased joint surface area and decreased joint contact stress compared to a meniscectomy; however, it does not fully reproduce the properties of an intact meniscus. For example, Paletta et al.³² reported that allograft replacement of the lateral meniscus decreases peak contact pressures in the knee joint by 55% to 65% when compared to the 235% to 335% increases in peak pressures after total meniscectomy. They also found that lateral meniscectomy resulted in a 45% to 50% decrease in total contact area compared to the native knee versus allograft transplantation with bone plug fixation which increased the contact area by 42% to 65% compared to the meniscectomy condition. These stress distributions with
allograft transplantation only occurred with bone plug fixation of the anterior and posterior meniscal horn attachments. Paletta et al.³² concluded that MAT can distribute joint stresses more effectively than a meniscal deficient knee but not as well as the native meniscus. Cummins and colleagues³³ found that articular cartilage showed histologic evidence of fewer degenerative changes with meniscal allografts compared to meniscectomy. Kelly et al.³⁴ demonstrated in a sheep model that meniscal kinematics and cartilage contact mechanics were closer to normal following MAT compared with the meniscectomized knee.

The biomechanical performance of a meniscal allograft in the human knee depends on the fixation method, material properties of the graft, and the placement and surgical technique. Alhalki et al.³⁵ reported that medial meniscal transplantation requires anatomic fixation of bone plugs attached to meniscal horns in order to most closely replicate normal tibiofemoral contact mechanics. Bone block meniscal transplantation is significantly stronger and stiffer than suture-only fixation. Chen and colleagues³⁶ reported that a joint with an intact meniscus provides the largest contact area and smallest peak contact pressure, whereas a total meniscectomy gives the smallest contact area and largest peak contact pressure. A MAT that either has a tibial bony bridge or both horns securely attached provides similar results to an intact joint. If only one horn is secured, results lie between an intact joint and meniscectomized joint. If neither horn is secured, contact mechanics are similar to a meniscectomized joint.

Although meniscal transplantations may increase the longevity of the knee joint, the long-term function of the transplant is variable. Patients who receive a transplant are typically allowed to return to daily function and low-impact sports but are advised to avoid high-impact, cutting, and pivoting activities.

The clinical indications for meniscal transplantation have expanded over recent years due to more comprehensive outcome assessment in specific patient populations. Traditionally, MAT was reserved for patients younger than 50 years old with pain localized to a meniscal-deficient compartment. In order to maximize success and predictability of the procedure, the knee had to have minimal arthrosis with no chondral lesions greater than grade II, normal axial alignment, and a stable ligamentous exam. Current practice has now broadened to consider concomitant procedures such as ACL reconstruction, cartilage restoration/resurfacing, and osteotomy.