meniscus

**Anatomy** - Two C-shaped wedges of fibrocartilage (mostly type I collagen) sit between the femoral condyles and the tibial plateau. - **Medial meniscus:** larger and C-shaped. It is firmly attached to the deep MCL and the joint capsule, so it is less mobile and more prone to injury. - **Lateral meniscus:** smaller and more circular (O-shaped). It is more mobile and is not attached to the LCL. The popliteus tendon attaches to it. - **Blood supply:** the medial and lateral inferior and superior genicular arteries (branches of the popliteal artery) supply the periphery. - The outer 10–30% (red zone) is vascularized and can heal. - The inner (white) zone is avascular and heals poorly. The inner portion gets its nutrition from synovial fluid diffusion. - Innervation is also peripheral, so central tears may be less painful. - **Functions:** shock absorption, load transmission (they carry about 50% or more of the load in extension and up to about 85% in flexion), joint stability, proprioception, and lubrication. Meniscectomy accelerates osteoarthritis.

**Mechanism of injury** - A twisting or rotational force on a planted, weight-bearing flexed knee. This is common in athletes (cutting and pivoting sports). - Degenerative tears occur in older patients with minimal trauma, such as squatting or rising from a chair. - **O'Donoghue's "unhappy triad"** is ACL, MCL, and medial meniscus injury. It classically results from a lateral blow to the knee (valgus force). Lateral meniscus involvement is actually more common in acute ACL tears, and the medial meniscus is more often involved in chronic ACL tears.

**Clinical presentation** - Joint-line tenderness, effusion (often delayed, appearing hours to a day after injury), and "catching" or "popping." - **Locking** (mechanical block to full extension) from a displaced bucket-handle tear. - Pain with squatting or twisting, and a sense of the knee giving way.

**Exam** - **McMurray test:** the knee is flexed, then extended while the tibia is rotated. A palpable or audible click or pain suggests a tear. - External rotation with valgus stress tests the medial meniscus. - Internal rotation with varus stress tests the lateral meniscus. - **Thessaly test:** the patient does a weight-bearing, flexed-knee twist. It is more sensitive and specific than McMurray. - **Apley grind test:** pain with compression and rotation while the patient is prone.

**Diagnosis** - **MRI** is the imaging test of choice. Plain films rule out fracture and assess for osteoarthritis.

**Management** - Conservative treatment (RICE, NSAIDs, physical therapy) is used for stable, degenerative, or peripheral tears. - Arthroscopic repair is used for tears in the red zone, especially in younger patients, to preserve the meniscus. - Partial meniscectomy is used for irreparable tears (white zone) or persistent mechanical symptoms. - Surgery is urgent for a locked knee from a bucket-handle tear.

**Tear types:** bucket-handle, radial, horizontal, longitudinal, parrot-beak (oblique), and complex degenerative.

**Do not confuse with:** - **Fluid meniscus:** the concave or convex curve at the surface of a liquid (a general physics and lab term). - **Other menisci:** the TMJ articular disc and the sternoclavicular joint disc are fibrocartilaginous discs with similar functions.

**High-yield Step 1 and Step 2 points** - Medial meniscus tears are more common because of the MCL attachment. - The ACL prevents anterior tibial translation (anterior drawer and Lachman tests). Do not confuse these with meniscal tests. - Locked knee plus a history of twisting injury points to a bucket-handle meniscal tear. - A Baker (popliteal) cyst can accompany meniscal pathology and chronic effusion.