Articular surfaces and ligaments
Three articulations share one synovial cavity: the medial and lateral tibiofemoral compartments and the patellofemoral compartment. The femoral condyles are rounded in both planes and the tibial plateaus are almost flat, so the bones alone give little stability.
| Ligament | Attachments | Main role |
|---|---|---|
| Anterior cruciate (ACL) | Anterior intercondylar area of the tibia, up and back to the inner (medial) wall of the lateral femoral condyle | Stops the tibia sliding forwards; resists rotation and hyperextension |
| Posterior cruciate (PCL) | Posterior intercondylar area of the tibia, up and forwards to the outer (lateral) wall of the medial femoral condyle | Stops the tibia sliding backwards; the stronger cruciate |
| Medial collateral (MCL) | Medial epicondyle to the medial tibia well below the joint line; deep fibres attach to the medial meniscus | Resists valgus stress |
| Lateral collateral (LCL) | Lateral epicondyle to the head of the fibula; a free cord outside the capsule | Resists varus stress |
| Meniscofemoral ligaments | Posterior horn of the lateral meniscus to the medial femoral condyle, in front of (anterior) and behind (posterior) the PCL | Steady the lateral meniscus |
The cruciates lie inside the fibrous capsule but outside the synovial membrane, which is reflected around them from the front. At the back, the oblique popliteal ligament, an expansion of semimembranosus, reinforces the capsule.
Menisci
The menisci are C-shaped wedges of fibrocartilage that deepen the plateaus and spread load. The medial meniscus is a wider C and is anchored to the capsule and the deep MCL, so it moves little and tears more often. The lateral meniscus is almost circular, is separated from the LCL by the popliteus tendon, and is more mobile. Only the peripheral rim has a blood supply, so only peripheral tears heal after repair.
Locking and unlocking
In the final degrees of extension with the foot on the ground, the femur rotates medially on the tibia because the medial condyle's articular surface is longer. This 'screw-home' movement tightens the ligaments and locks the knee. Popliteus unlocks it by rotating the femur laterally at the start of flexion.
Relations
The knee is surrounded by tendons in front and at the sides and by the popliteal fossa behind. In front lie the quadriceps tendon, patella and patellar ligament. Medially, sartorius, gracilis and semitendinosus insert together as the pes anserinus, over the tibial attachment of the MCL. Laterally lie the iliotibial tract, going to Gerdy's tubercle, and the biceps femoris tendon, going to the fibular head around the LCL. Behind, the popliteal artery lies directly on the capsule, with the popliteal vein and tibial nerve superficial to it.
Several bursae surround the joint. The suprapatellar bursa is continuous with the joint cavity, so an effusion fills it. The prepatellar and superficial infrapatellar bursae lie under the skin in front. The semimembranosus bursa behind can swell as a popliteal (Baker's) cyst.
The proximal tibiofibular joint is a separate synovial plane joint between the fibular head and the underside of the lateral tibial condyle.
Blood supply and innervation
The knee is supplied by the genicular anastomosis. The popliteal artery gives superior medial, superior lateral, middle, inferior medial and inferior lateral genicular arteries; the middle genicular pierces the capsule to supply the cruciates. The descending genicular branch of the femoral artery, the descending branch of the lateral circumflex femoral artery and the anterior tibial recurrent artery join the network.
By Hilton's law, the nerves to the muscles crossing the knee also supply it: branches of the femoral nerve, the obturator nerve, and genicular branches of the tibial and common fibular nerves. Hip disease can therefore present as knee pain through the femoral and obturator nerves.
Clinical relevance
Knee injuries follow predictable mechanisms, and each ligament or meniscus fails in a recognisable way.
- ACL rupture: a non-contact twist on a planted foot, a pop and a rapid haemarthrosis. The Lachman test is the most sensitive examination; the pivot shift confirms rotational instability. A Segond fracture, a small avulsion from the lateral tibial rim, strongly suggests an ACL tear.
- PCL rupture: a blow to the front of the flexed knee, as on a car dashboard. The tibia sags backwards when the knee is flexed to 90 degrees.
- MCL injury: a valgus blow; tested by valgus stress at about 30 degrees of flexion.
- Meniscal tear: a twist of the loaded, flexed knee, giving joint line tenderness, delayed swelling and sometimes locking.
- Tibial spine avulsion: in children the ACL pulls off its bony attachment rather than tearing.
- Knee dislocation: the popliteal artery is tethered above and below the joint and is at risk; pulses must be documented before and after reduction.
On the specimen
The two stations show an opened knee from the front and the bones with their ligaments, so the markers fall on bone, ligament and meniscus. Orient first by the fibular head, which is always lateral and below the joint.
To tell the cruciates apart, look at the tibia: the ACL leaves the front of the intercondylar area and the PCL the back. On the femur, the ACL reaches the lateral condyle and the PCL the medial. A thin band running behind the PCL from the lateral meniscus is the posterior meniscofemoral ligament. The tibial spines are the two tubercles of the intercondylar eminence between the plateaus; the intercondylar notch is the gap between the femoral condyles that houses the cruciates. The LCL is a round cord to the fibular head, with the popliteus tendon passing deep to it into the joint.