An Update on Meniscal Allograft Transplantation
This review article examines the nuances of meniscal allograft transplantation, identifying which patients would benefit from the procedure, describing an ideal surgical strategy, and outlining expected outcomes.
Utkarsh Anil, BA; Ryan Roach, MD; and Eric J. Strauss, MD
The medial and lateral menisci are critical to normal knee kinematics and load distribution. Meniscal deficiency is strongly associated with articular cartilage damage and knee instability, and it can lead to premature, progressive osteoarthritis with resultant joint space narrowing in the affected compartment. 
Improved understanding of the chondroprotective effects of the meniscus has resulted in a trend toward an increased number of isolated meniscal repairs being performed without a concomitant increase in meniscectomies over the same time frame.  Meniscectomy is often unavoidable, however, due to complex or recurrent tears, failed meniscal repairs, or iatrogenic injuries. 
Options for managing these patients are often limited. They may benefit from meniscal allograft transplantation (MAT), which has been shown to restore more normal knee kinematics and load distribution in cadaveric studies and to be an effective treatment option for patients with symptomatic meniscal deficiency (Figure 1). [4,5]
Figure 1. Medial meniscal allograft transplantation fixed peripherally with inside-out sutures.
The purpose of this review is to examine the nuances of the procedure, identifying which patients would benefit from the procedure, describing an ideal surgical strategy, and outlining expected outcomes to help educate patients and establish reasonable expectations.
Indications and Contraindications
Multiple factors are considered when evaluating a patient for MAT: Identifying appropriate patients is critical to ensuring good postoperative outcomes. Ideal patients for MAT are under age 50 and have: [3,6-9]
- Symptomatic meniscal insufficiency that limits activities of daily living and is unresponsive to conservative treatment
- Minimal evidence of degenerative changes in the affected compartment
- No ligamentous instability
- Normal coronal and sagittal alignment
- Full knee range of motion
The biggest contraindication to MAT is advanced diffuse arthritic changes, especially with radiographic evidence of osteophyte formation, joint space narrowing, or tibial or femoral condyle flattening.  As a result, age is a generalized contraindication, with the underlying assumption that most older patients would exhibit advanced arthritic changes.
Relative contraindications include obesity, skeletal immaturity, inflammatory arthritis, history of septic arthritis, and synovial disease.  Malalignment and ligamentous instability are not absolute contraindications but should be surgically addressed prior to or at the time of MAT with concomitant osteotomy or ligament reconstruction, respectively. [12,13]
A small but growing body of evidence suggests that early-stage arthrosis may not be the strong contraindication it was previously thought to be, with some studies showing similar therapeutic benefits in patients with chondral damage as in the traditional, ideal patient population. 
Surgical Planning and Technique
The first and arguably most important technical aspect of an MAT procedure is ensuring an accurate size match. Dienst et al  showed that the size of the graft should be within 10% of the native meniscus to recreate biomechanics similar to a healthy knee. Oversized grafts lead to greater contact stresses on the articular surface, while undersized grafts result in favorable contact mechanics but increased material strain on the graft. 
Pollard et al  developed the classic sizing method for meniscal allografts using plain radiographs, with medial meniscal length equaling 80% and lateral meniscal length equaling 70% of the measured sagittal length of the tibial plateau. In more recent years, graft selection techniques using high-resolution MRI to account for 3-dimensional structures have gained popularity and are becoming the standard. [18,19]
There are 4 possible methods of allograft preservation:
- Fresh. Fresh allografts often need to be harvested within 12 hours of cold ischemic time and can only be stored for a few days, which is often not enough time for bacteriologic and virologic screening of the donor and prevents long-term storage in tissue banks. 
- Freeze-dried. The freeze-drying technique has largely been abandoned as it has been shown to change the ultrastructure of the meniscal collagen net, making the allograft prone to graft shrinkage. 
- Cryopreserved. Cryopreservation does not alter the ultrastructure of the allograft, but it does induce significant apoptosis in the meniscal tissue. The percent cell survival ranges from 4% to 54%. 
- Fresh-frozen. Fresh-frozen grafts are generally the most available, cost-effective, and biomechanically stable. Thus, they are the graft of choice.
Various techniques have historically been used for graft fixation in MAT surgery, including soft tissue fixation, bone plug technique and bony bridge-in-slot techniques. However, it has been shown that anatomic fixation of both meniscal horns is essential to mimic the function of a native meniscus; without bony fixation, the biomechanical advantage of meniscal transplant is lost. [22,23]
As a result, anatomic rigid bony fixation has become the gold standard for graft fixation, with the bridge-in-slot technique becoming the preferred surgical technique for cases of lateral meniscus or isolated medial meniscus transplantation (Figure 2).
Figure 2. Bridge-in-slot meniscal allograft for isolated lateral meniscal allograft transplantation.
In certain cases, such as combined medial meniscal transplant with a concomitant ACL reconstruction, the bridge-in-slot technique is not feasible as it interferes with ACL tibial tunnel placement. In these cases, bony fixation using bone plugs has been successfully employed, although this method is technically demanding and outcomes may depend on ideal tunnel placement (Figure 3). [12,24]
Figure 3. Double bone plug technique for medial meniscal allograft transplantation performed in conjunction with an ACL reconstruction.
The first MAT was performed by Milachowski in 1984. Although multiple studies assessing clinical outcomes from this procedure have been published, certain factors have prevented affirmative conclusions.
- Many studies include patients who underwent MAT with concomitant procedures such as ligament reconstruction, osteotomy, or osteochondral allograft implantation.
- As discussed earlier, a variety of graft fixation techniques and allograft preservation techniques have been used over time, with varying levels of success.
- Studies comparing surgical intervention with a control group of conservatively treated symptomatic post-meniscectomy patients are lacking.
Hence, it becomes very difficult to apply the results of published studies to a specific patient.
A recent meta-analysis of 696 meniscal allografts across 9 studies compared survival of lateral meniscal allografts with that of medial meniscal allografts. Survival time was defined as time to conversion to knee arthroplasty and/or subtotal resection of the allograft.
The study authors found that 86% of medial and 89% of lateral meniscal allografts survived at medium-term (5 to 10 years) follow-up.  However, the survival rate dropped to 53% and 57% for medial and lateral allografts, respectively, at long-term follow-up ( more than 10 years). 
Pain and Patient-Reported Functional Outcomes
A large meta-analysis of 44 clinical trials representing 1136 grafts in 1068 patients found that regardless of follow-up period and scoring system used, patients continuously showed clinical improvement and reported continuously satisfactory outcome, with restoration of working capacity in active patients.  A prospective study of meniscal transplants in symptomatic patients younger than age 50 showed encouraging results in reducing knee pain and increasing daily function. 
Similarly, a multicenter, prospective, long-term (minimum 7-year follow-up) evaluation of patients undergoing MAT showed statistically significant improvement in every Knee Injury and Osteoarthritis Outcome Score (KOOS) subscore (pain, symptom, activities of daily living, sports, and quality of life), SF-12 physical and mental component summary, Lysholm score, and International Knee Documentation Committee (IKDC) form score.  The authors concluded that MAT should be considered a safe and reliable treatment for refractory post-meniscectomy symptoms in selected patients.
Return to Sport
Outcomes of MAT in athletes are not well understood due to the low incidence of the procedure in this specific patient population. However, some smaller studies examining outcomes in athletes show promising results. In a retrospective case series from Spain, 86% of young soccer players with a history of meniscectomy returned to play following meniscal transplants, with significant improvements in their Lysholm, subjective IKDC form, and VAS for pain scores. 
Another study of young athletes by Cole et al  showed 77% return to sporting activity, with significant improvements in patient-reported outcome measures such as KOOS, IKDC, and Lysholm score. A recently published systematic review of the literature concluded that MAT allows return to the same level of competition in 75% to 85% of patients at short- to medium-term follow-up. 
Prevention of Osteoarthritis
Meniscal allograft transplantation is not the ideal procedure for preventing progression to osteoarthritis. The authors of a retrospective study of 329 MAT procedures in 313 patients concluded that MAT did not delay or prevent tibiofemoral osteoarthritis progression, with 1 in 5 patients being converted to knee arthroplasty at a mean follow-up of 10 years.  A long-term prospective study evaluating MAT with concurrent articular cartilage repair from Stone et al  showed a 20% conversion to total knee arthroplasty at medium-term follow-up, with a Kaplan-Meier estimated survival until knee replacement of 9.9 years.
The authors of a case series evaluating long-term function and survival rates in 72 consecutive meniscus transplants concluded that most patients experience short- and long-term benefits of reduced symptoms and improved function. However, these transplants eventually undergo a degenerative remodeling process and eventually fail to prevent progression to osteoarthritis, necessitating additional surgery. 
Although most evidence seems to show that MAT lacks utility in preventing osteoarthritis, there have been some positive results. An outcome study from The Netherlands with a mean follow-up of nearly 14 years showed that MAT is a salvage option for symptomatic meniscus-deficient knees and may be have some success in postponing total knee arthroplasty in younger patients. 
Meniscus preservation during surgery is vital to good long-term outcomes. In patients with meniscus deficiency who are symptomatic, MAT is a good surgical option as it provides reliable pain relief, increased quality of life, return to working capacity, and a relatively high rate of return to sport. A large fraction of meniscal transplants, however, undergo degenerative changes over the long term and have, to date, not been shown to effectively prevent the progression of degenerative arthritis.
Utkarsh Anil, BA; Ryan Roach, MD; and Eric J. Strauss, MD, are from the Division of Sports Medicine, Department of Orthopaedic Surgery, NYU Langone Health, New York, New York.
Mr. Anil and Dr. Roach have no disclosures relevant to this article. Dr. Strauss has disclosed that he is a paid consultant of Arthrex and Smith & Nephew.
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