Knee joint
Articulatio genus
Also known as: knee, tibiofemoral joint, patellofemoral joint
The largest and most complex joint of the body — a modified (bicondylar) hinge synovial joint between the femoral condyles, the tibial plateau and the patella. Its stability depends on menisci, cruciate and collateral ligaments and the quadriceps.
Overview
The knee comprises three articulations in one capsule: medial and lateral tibiofemoral joints (condyles on the plateau, cushioned by the menisci) and the patellofemoral joint. Bony congruity is poor — the convex condyles sit on an almost flat tibial plateau — so stability comes from soft tissues: the menisci, the intracapsular but extrasynovial cruciate ligaments, the collateral ligaments, the capsule with its retinacula and posterior oblique and oblique popliteal ligaments, and the muscles — above all the quadriceps femoris.
The synovial cavity is the largest in the body: it extends ≈5–7 cm above the patella as the suprapatellar bursa beneath quadriceps (a site for aspiration and the reason effusions fill the area above the patella) and is invaginated posteriorly by the cruciate ligaments, which therefore lie outside the synovial membrane. The infrapatellar fat pad fills the space behind the patellar ligament.
Movements: flexion (≈135–140°) and extension, with some rotation when flexed. In the final ≈30° of extension with the foot on the ground, the femur rotates medially on the tibia (the larger medial condyle continues rolling) — the screw-home (locking) mechanism, which tightens the ligaments and makes the extended knee stable with minimal muscle effort. To flex the locked knee, popliteus "unlocks" it by laterally rotating the femur on the tibia.
- Location
Between the thigh and the leg.
Parts & subdivisions
- C-shaped fibrocartilage wedges deepening the plateau and absorbing load.
- Prevents anterior translation of the tibia on the femur.
- Prevents posterior translation of the tibia; stronger.
- Broad band; resists valgus; attached to the medial meniscus.
- Cord-like; resists varus; separate from the lateral meniscus.
- Continuation of the quadriceps tendon to the tibial tuberosity.
- Oblique popliteal ligamentExpansion of semimembranosus reinforcing the posterior capsule.
- Suprapatellar communicates with the joint; prepatellar, infrapatellar, pes anserinus, semimembranosus.
Anatomy
- Innervation
Femoral (via nerves to the vasti), obturator, tibial and common fibular nerves (genicular branches).
- Arterial supply
Genicular anastomosis: superior medial and lateral, middle, inferior medial and lateral genicular arteries (popliteal), descending genicular (femoral), descending branch of lateral circumflex femoral, anterior tibial recurrent artery.
- Articulations
Femoral condyles ↔ tibial condyles (via menisci); patella ↔ patellar surface of the femur.
- Ligaments
Patellar ligament, medial and lateral patellar retinacula, tibial and fibular collateral ligaments, anterior and posterior cruciate ligaments, oblique and arcuate popliteal ligaments, coronary and transverse ligaments (menisci), meniscofemoral ligaments.
Function
- Flexion and extension of the leg; rotation when flexed
- Supports body weight while allowing locomotion, squatting and jumping
- Locks in extension for energy-efficient standing
Clinical significance
Unhappy (O’Donoghue) triad
A blow to the lateral side of the knee with the foot planted (valgus + external rotation, e.g. a football tackle) tears the ACL, tibial collateral (MCL) ligament and medial meniscus, the meniscus being torn because it is attached to the MCL. (Modern MRI series show the lateral meniscus is actually injured more often in acute ACL tears, but the classic triad remains a high-yield exam fact.)
Knee examination tests
Lachman and anterior drawer (ACL), posterior drawer and posterior sag (PCL), valgus/varus stress at 30° flexion (MCL/LCL), McMurray, Apley and Thessaly tests (menisci); a rapid haemarthrosis within hours suggests ACL rupture or fracture.
Osteoarthritis
The knee is the commonest large joint affected; medial compartment involvement produces genu varum. Total knee replacement is performed for end-stage disease.
Mnemonics
Cruciate ligament attachments
"LAMP": Lateral femoral condyle → ACL; Medial femoral condyle → PCL (named by their tibial attachments: Anterior and Posterior intercondylar areas).
Test yourself
A footballer is struck on the lateral side of the knee while his foot is planted. Which combination makes up the classic "unhappy triad"?
- PCL, fibular collateral ligament and lateral meniscus
- ACL, PCL and patellar ligament
- Tibial collateral ligament, patella and lateral meniscus
- ACL, tibial collateral ligament and medial meniscus
Answer: ACL, tibial collateral ligament and medial meniscus
A valgus blow with rotation tears the tibial collateral (MCL) ligament, the ACL and the medial meniscus — the medial meniscus being attached to the deep MCL. (MRI studies show the lateral meniscus is often injured in acute ACL tears, but the classic triad remains a high-yield exam answer.)
Compiled from standard anatomy and physiology references — see the Skeletal System sources. For education only; not medical advice.