Target audience:
Clinical radiologists involved in the interpretation of knee MRI after meniscal surgery. Since meniscal surgery ranges in complexity from the very commonly performed partial meniscectomy to meniscal transplantation, this lecture will be relevant for all radiologists interpreting knee MR scans.

Learning objectives:
- To refresh the meniscal anatomy and to understand the important role the meniscus plays in the structure and function of the knee;
- To become familiar with the three surgical strategies for operative treatment of meniscal tears (resection, repair, and replacement) and to understand why meniscus-preserving surgery is increasingly advocated;
- To learn about MR protocol choices for postoperative assessment of the meniscus;
- To correctly interpret normal and abnormal MR imaging findings in the postoperative meniscus after each of the different surgical procedures.

Outline of lecture
Meniscus surgery is one of the most frequently performed orthopedic procedures in Western countries, and MR imaging is commonly indicated to investigate the cause of symptoms in patients after meniscus surgery.

This lecture will start with a brief review of the anatomy, structure, and function of the meniscus. The roughly C-shaped menisci lying between the femoral condyles and tibial plateau serve important biomechanical and other functions in the knee. The meniscal tissue deepens the contact between the femoral condyles and tibial plateau, conferring increased stability and congruity to the knee joint. Furthermore, the menisci fulfill a key role in load transmission from the upper to the lower leg by absorbing and distributing hoop forces applied to the knee evenly over the articular surface. Menisci also play a role in knee joint lubrication and cartilage nutrition. Nerve fibers located in the anterior and posterior parts of the menisci contribute to proprioception. Anatomically, the meniscus is usually divided into the anterior horn, the body, and the posterior horn. The anterior and posterior horns of the menisci are attached directly to the central tibial plateau by the meniscal roots, resisting outward displacement (extrusion) of the menisci during axial loading. Structurally, the main composites of the fibrocartilaginous menisci are collagen fibers, which are primarily oriented in a circumferential pattern and held together by radially oriented tie fibers. This arrangement provides the meniscus with tensile strengths and aids in the distribution of forces. In adults, the peripheral 20-30% of the meniscus is vascularized and referred to as the “red zone”, reflecting its appearance on arthroscopy. This red zone surrounds the inner two thirds that are relatively avascular and referred to as the “white zone”. Distinction between these differently vascularized zones is important from the perspective of surgery to treat meniscal tears.

Depending on the type of tear and clinical factors, meniscal tears may be managed non-operatively or operatively. There are three surgical strategies for operative treatment of meniscal tears, referred to as the 3Rs: resection (partial meniscectomy), repair (suturing), and replacement (transplantation). Several criteria determine the choice of surgical approach, most importantly the location and size of tear, stability of the tear, and integrity of the torn inner fragment. Because of important biomechanical properties of the menisci, increasing emphasis is placed on meniscal-preserving surgery in the event of a tear, which aims at preserving or restoring its normal function by leaving as much native meniscal tissue intact as possible.

Resection involves removal of damaged, instable meniscal tissue and contouring of the remaining meniscus. It has been clearly demonstrated in the literature that there is a markedly increased risk of developing early osteoarthritis in patients after meniscectomy, and that the amount of removed meniscal tissue correlates with rate of osteoarthritis onset. If a partial meniscectomy is needed, the outer third of the meniscus is usually preserved when possible, since it is the predominant load-bearing portion.

On meniscal repair surgery, the fragments are fixed using a variety of suture materials, bioabsorbable arrows, darts, and tacks. In general, tears limited to the peripheral 2 to 4 mm of the meniscus are considered good candidates for suturing, especially if their orientation is longitudinal and their size is greater than 1 cm. It takes approximately 4 months for the meniscus to heal, after which patients are usually asymptomatic.

Meniscal transplantation is an increasingly applied particularly in young patients with symptomatic irreparable tears with previous meniscectomy. Transplantation can be performed arthroscopically or via an open procedure either with collagen-based meniscal grafts or allografts that are attached to the tibial plateau and the joint capsule using bone plugs and sutures. Allograft transplants are most appropriate for extensive meniscus defects while partial meniscus implants can be used to fill partial meniscectomy defects. Complete meniscal implants are available in the context of clinical trials.

Postoperative imaging of the meniscus is aimed to assess the stability or recurrence of a tear on the meniscal remnant, identify tears in other areas of the meniscus, and identify other causes of postoperative knee pain. For correct interpretation, it is critical that the radiologist is informed about the site and duration of the current symptoms, the timing and type of surgical procedure, and preoperative imaging examination(s) for comparison.

Controversy exists as to whether to perform conventional MR imaging, indirect MR arthrography, or direct MR arthrography to evaluate the postoperative meniscus. One of the reasons for the enhanced conspicuity of direct MR arthrography compared to conventional MRI for the detection of recurrent meniscal tears is the increased intra-articular fluid pressure created by the contrast injection, which causes the joint capsule to distend and forcing fluid with diluted contrast agent to move into the meniscal tear. Conventional MR imaging, consisting of proton density and fat-suppressed T2 FSE sequences in 3 orthogonal planes may suffice in a majority of cases, while arthrography could be reserved for additional problem solving.

Each of the different surgical techniques results in a different MR imaging appearance of the postoperative meniscus. After meniscectomy, the remaining portion of the meniscus typically shows a blunted or cut-off appearance at the site of the resection. There should be no intrameniscal signal visibly communicating with the articular surface. As a rule of thumb, it has been shown that if less than 25% of the meniscus has been resected, the same MRI criteria as for nonoperated menisci apply to diagnose persistent or recurrent tears. Different criteria must be used when more than 25% of the meniscus has been resected. The main criterion for a recurrent tear is the presence of fluid signal on T2-weighted images communicating with the meniscal surface. Pitfalls include the presence of granulation tissue in the region of the resection, which may cause surfacing intermediate or high signal intensity exclusively on T1- or proton density weighted images. The conventional MRI appearance following direct meniscal repair is usually dominated by the presence of granulation tissue at the repair site. This granulation tissue may be visible for many years after surgery and usually exhibits intermediate to high signal intensity on T1-, proton density and T2-weighted images. After meniscus transplantation, complications assessable on MR imaging assessment include progressive high grade articular cartilage loss, allograft meniscus tears, allograft extrusion, allograft shrinkage, arthrofibrosis, and synovitis.