Knee Joint
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Direct answer
A hinge that locks, unlocks and rotates — the knee is the largest and most complex synovial joint, a modified hinge between the femoral and tibial condyles with the patella gliding in front. Inside a single cavity sit two menisci and the intracapsular cruciate ligaments, within the largest synovial membrane in the body; outside, strong collateral ligaments brace its sides. It flexes, extends and rotates slightly, and in full extension it screws home into a locked position that only popliteus can release.
What you must remember
- Type: bicondylar synovial modified hinge — two femorotibial articulations plus the patellofemoral joint, sharing one cavity.
- Menisci: medial is C-shaped, firmly attached to the capsule and tibial collateral ligament; lateral is nearly circular, mobile, separated from the fibular collateral ligament by the popliteus tendon; both are vascularised only at the periphery, so inner-zone tears heal poorly.
- Cruciates: anterior prevents anterior tibial displacement and hyperextension; posterior — the strongest ligament of the knee — prevents posterior displacement; both are intracapsular but extrasynovial.
- Collaterals: tibial (medial) — broad, fused with capsule and medial meniscus; fibular (lateral) — cord-like and free, from lateral epicondyle to fibular head.
- Locking and unlocking: full extension screws the joint home by medial rotation of the femur on the tibia, tightening cruciates and collaterals into the close-packed position; popliteus unlocks it by laterally rotating the femur on the planted tibia.
- Supply: five genicular branches of the popliteal artery form the genicular anastomosis with contributions from the femoral (descending genicular), lateral circumflex femoral and anterior tibial recurrent; Hilton's law brings femoral, obturator, tibial and common peroneal nerves.
- Bursae: suprapatellar communicates with the joint — effusions balloon it and the patellar tap turns positive; prepatellar bursitis is housemaid's knee, superficial infrapatellar is clergyman's knee, and the anserine bursa flares under the pes anserinus in osteoarthritis.
- Unhappy triad (O'Donoghue): anterior cruciate, tibial collateral and medial meniscus torn by valgus force with rotation on a flexed planted knee — the lateral meniscus is now recognised to be involved in many such injuries too.
Examining the injured knee, structure by structure
A footballer's flexed knee is struck on its lateral side with the foot planted. Deduce the order of damage. The valgus moment strains the tibial collateral ligament first — medial joint-line tenderness, laxity to valgus stress at 30 degrees of flexion (full extension masks it through the posterior capsule). Because the medial meniscus is pinioned to that ligament, the twisting component tears it next, classically a bucket-handle tear that intermittently blocks extension — true mechanical locking, unlike pseudolocking from pain and haemarthrosis. Finally the tibia shifts anteriorly with rotation and ruptures the anterior cruciate: the Lachman test at 30 degrees is the most sensitive, with the anterior drawer and pivot-shift tests added. A tense haemarthrosis within hours points to cruciate rupture, since the extrasynovial cruciates bleed freely; a meniscal tear alone gives a later, reactive effusion. Joint-line tenderness with the knee flexed remains the most useful single sign, and the explanation sentence the examiner wants is that the medial meniscus cannot escape the grinding condyle because it is tethered to the tibial collateral ligament.
Where students slip
The cruciates are labelled intrasynovial — they lie inside the capsule but outside the synovial membrane, covered behind by a synovial fold; that detail explains both the bleeding into the joint and the failure of central meniscal tears to heal. The collaterals are swapped: the medial is the broad sheet fused with the meniscus, tested in valgus; the lateral is a round cord to the fibula, tested in varus. "Locking" is given without its mechanism — say the close-packed terminal rotation of extension, name popliteus (tibial nerve, L4–S1) as the unlocker, and distinguish it from the mechanical blocking of a torn meniscus. Finally, quote the patellar reflex (L3–L4) and hamstring reflex (S1) — connecting anatomy to the neurological exam earns the extra mark.
Frequently asked questions
What is the unhappy triad?
Rupture of the anterior cruciate ligament, tibial collateral ligament and medial meniscus from a valgus force with rotation on a flexed, planted knee — the classic O'Donoghue triad.
Which bursae around the knee matter clinically?
Prepatellar bursitis is housemaid's knee; superficial infrapatellar bursitis is clergyman's knee; the anserine bursa flares under the pes anserinus in osteoarthritis.
What is locking of the knee, and which muscle unlocks it?
In full extension the femur medially rotates on the tibia, tightening the ligaments into the close-packed position; popliteus unlocks it by laterally rotating the femur on the fixed tibia, or medially rotating the tibia when the foot is free.
How is the blood supply of the knee arranged?
Five genicular branches of the popliteal artery plus the descending genicular branch of the femoral, the descending branch of the lateral circumflex femoral and the anterior tibial recurrent artery form the genicular anastomosis — the collateral pathway when the popliteal artery is injured. The suprapatellar bursa communicates with the joint, so an effusion balloons it and elicits the patellar tap.