medial meniscus

ANATOMY: C-shaped (semilunar) fibrocartilage wedge (triangular in cross-section) on the medial tibial plateau. The posterior horn is larger than the anterior horn. The meniscus is composed mainly of type I collagen, circumferentially arranged to resist hoop stresses. Vascular supply comes from the medial and lateral genicular arteries (branches of the popliteal) and is limited to the peripheral third (the "red-red" zone, which can heal). The inner two-thirds ("white-white" zone) is avascular and receives nutrition by diffusion from synovial fluid, so it heals poorly. Innervation is mainly in the peripheral portions (pain and proprioception).

ATTACHMENTS: The anterior horn attaches to the tibial intercondylar area anterior to the ACL. The posterior horn attaches to the intercondylar area anterior to the PCL insertion and posterior to the lateral meniscus attachment. The meniscus is attached peripherally to the joint capsule via coronary ligaments, and to the deep fibers of the MCL. The medial meniscus is therefore less mobile than the lateral meniscus, which is more mobile and not attached to the LCL (the popliteus tendon separates them). This difference explains the higher injury rate of the medial meniscus.

FUNCTION: (1) Load transmission and shock absorption (the menisci transmit about 50% of load in extension and about 85% in 90° of flexion). (2) Increases congruity and contact area between the femoral condyles and the flat tibial plateau. (3) Secondary stabilizer, particularly against anterior tibial translation (the posterior horn of the medial meniscus acts as a secondary restraint when the ACL is deficient). (4) Joint lubrication and proprioception.

PATHOLOGY / CLINICAL: - Mechanism: twisting or rotational injury on a planted foot with a flexed knee (athletes), or degenerative tears in older adults with minimal trauma (squatting, getting up from a chair). - Unhappy (terrible) triad: ACL + MCL + medial meniscus tear, from a lateral blow to the knee causing valgus stress. Classically described with the medial meniscus, although lateral meniscus tears are actually more common with acute ACL tears in modern series. - Symptoms: joint line pain, swelling (often delayed, over 24 hours, compared with the rapid hemarthrosis of an ACL tear), catching, locking (a displaced bucket-handle tear blocks full extension), giving way. - Exam: medial joint line tenderness; McMurray test (palpable or audible click or pain with flexion, tibial external rotation, and varus stress to the knee, then extension, which suggests a medial meniscus tear); Apley grind test; Thessaly test. - Imaging: MRI is the best test (the "bow-tie" sign on sagittal images is a normal meniscus; disruption suggests a tear). Plain films are used to rule out fracture or osteoarthritis. - Tear types: bucket-handle, radial, longitudinal, horizontal, flap, complex/degenerative. - Management: conservative treatment (RICE, NSAIDs, physical therapy) for stable or degenerative tears. Arthroscopic repair for peripheral (vascular zone) tears in younger patients, or partial meniscectomy for non-repairable symptomatic tears or mechanical symptoms (locking). Meniscal loss increases the risk of early osteoarthritis. - Associated: a Baker (popliteal) cyst may form from chronic effusion and often accompanies a medial meniscus tear. Pes anserine bursitis can mimic medial knee pain (but lies distal to the joint line, over the anteromedial proximal tibia).

HIGH-YIELD TAKEAWAYS: Medial meniscus is injured more often than lateral because it is less mobile (attached to MCL and capsule). Only the outer third is vascularized. McMurray test and MRI are key. Locked knee suggests a bucket-handle tear. Unhappy triad is a favorite test item with a valgus force from a lateral hit.