Knee and Popliteal Fossa
Type: Largest and most complex synovial joint in the body; functionally a modified hinge (compound, bicondylar), permitting flexion/extension plus a rotatory component when flexed.
- Articular surfaces: Femoral condyles with tibial condyles (tibiofemoral joint, two articulations, medial and lateral), and patella with the femoral patellar surface (patellofemoral joint), all sharing a single synovial cavity.
- Menisci:
- Medial meniscus: C-shaped, larger radius, firmly attached to the medial collateral ligament and joint capsule, relatively immobile, which makes it more vulnerable to tearing with rotational/valgus stress.
- Lateral meniscus: more O-shaped (nearly a complete ring), smaller, not attached to the lateral collateral ligament (separated by the popliteus tendon passing between them), more mobile, less commonly injured.
- Function: deepen the tibial articular surfaces, improve congruency with the femoral condyles, distribute load, absorb shock, and assist lubrication.
- Peripheral third is vascularised ("red zone," better healing potential); inner two-thirds is avascular ("white zone," relies on synovial fluid diffusion, poor healing potential).
- Capsule: Extensive, attaches near the articular margins of the femur, tibia, and patella; reinforced by numerous ligamentous expansions; posteriorly reinforced by the oblique popliteal ligament and arcuate popliteal ligament.
- Anterior cruciate ligament (ACL):
- Origin: anterior intercondylar area of tibia; insertion, posteromedial aspect of the lateral femoral condyle.
- Prevents anterior translation of the tibia relative to the femur (or posterior translation of the femur on a fixed tibia); also resists excessive internal rotation and hyperextension.
- Intracapsular but extrasynovial (covered by its own synovial fold).
- Posterior cruciate ligament (PCL):
- Origin: posterior intercondylar area of tibia; insertion, anterolateral aspect of the medial femoral condyle.
- Prevents posterior translation of the tibia relative to the femur; the primary stabiliser against posterior tibial sag, particularly important on stairs/descending slopes.
- Stronger and shorter than the ACL.
- Medial collateral ligament (MCL, tibial collateral ligament):
- Broad, flat band from the medial femoral epicondyle to the medial tibial condyle/shaft; firmly attached to the medial meniscus.
- Resists valgus stress; taut in extension, relatively lax in flexion.
- Lateral collateral ligament (LCL, fibular collateral ligament):
- Cord-like, from the lateral femoral epicondyle to the head of fibula; not attached to the lateral meniscus (separated by the popliteus tendon).
- Resists varus stress; taut in extension, relatively lax in flexion.
- Patellar ligament: Continuation of the quadriceps tendon distal to the patella, from the patellar apex to the tibial tuberosity, the functional final common pathway of the extensor mechanism.
- Oblique popliteal ligament: Posterior capsular reinforcement, formed by a recurrent expansion of the semimembranosus tendon, running superolaterally across the posterior capsule.
- Arcuate popliteal ligament: Posterior capsular reinforcement, arching over the popliteus tendon as it emerges from beneath the lateral meniscus/LCL, from the fibular head to the posterior joint capsule.
- Bursae:
- Suprapatellar bursa: communicates with the joint cavity, extends deep to quadriceps tendon.
- Prepatellar bursa: anterior to the patella, subcutaneous; inflammation from repeated kneeling ("housemaid's knee").
- Infrapatellar bursae (superficial and deep): superficial to and deep to the patellar ligament respectively; deep bursa does not communicate with the joint.
- Pes anserine bursa: between the pes anserinus tendons (sartorius, gracilis, semitendinosus) and the tibia/MCL.
- Semimembranosus bursa: posteromedial, may communicate with the joint and enlarge as a Baker (popliteal) cyst.
- Blood supply: Genicular anastomosis; formed by superior medial/lateral, inferior medial/lateral, and middle genicular arteries (from the popliteal artery), plus descending genicular (from femoral), circumflex fibular, and anterior/posterior tibial recurrent arteries.
- Innervation: Femoral, obturator, tibial, and common fibular nerves all contribute articular branches (Hilton's law); the obturator nerve's dual hip/knee innervation explains referred hip pathology presenting as knee pain, particularly in children (e.g., slipped upper femoral epiphysis presenting with knee pain).
- Movements: Flexion/extension (primary); internal/external rotation of the tibia on the femur, but only available when the knee is flexed (rotation is mechanically locked in full extension by the screw-home mechanism and taut collateral ligaments).
- Locking mechanism ("screw-home mechanism"):
- During the final ~20–30° of extension, the larger medial femoral condyle continues to roll and glide after the smaller lateral condyle has stopped, producing a relative external rotation of the tibia (or internal rotation of the femur on a fixed tibia), "locking" the knee into a stable, fully extended position.
- This locked position allows standing with minimal quadriceps activity, since the joint is mechanically stable without continuous muscular effort.
- Unlocking by popliteus:
- To initiate flexion from the fully extended, locked position, popliteus contracts to internally rotate the femur on the tibia (or externally rotate the tibia on a fixed femur), "unlocking" the joint before flexion can proceed.
- Popliteus is therefore functionally the "key that unlocks the knee."
- Stability: Depends on a combination of bony congruency (moderate, condyles are not deeply socketed), the menisci (improve congruency), the four major ligaments (ACL/PCL for anteroposterior stability, MCL/LCL for varus/valgus stability), the joint capsule, and substantial dynamic muscular support (quadriceps, hamstrings, iliotibial tract, pes anserinus group), overall a joint that trades some inherent bony stability for mobility, relying heavily on soft tissue and muscular stabilisation.
