The largest joint in the body, a modified hinge that also allows a little rotation when bent
Knee joint — flexion, extension and slight rotation.
Anatomy · Joints & cartilage
Knee joint
The largest joint in the body
Lower limb · thigh to leg
The knee is the largest and one of the most complex joints in the body: a modified hinge between the femur and tibia, with the patella gliding in front on the femur. It has two menisci between the bones, two cruciate ligaments inside the capsule and collateral ligaments at either side, and because it is a weight-bearing joint between two long levers it is easily injured.
How to spot them
- ▸Femoral condyles rest on the flattened tibial plateau, cushioned by C-shaped menisci
- ▸The patella lies in the quadriceps tendon, continuing below as the patellar ligament to the tibial tuberosity
- ▸Two cruciate ligaments cross inside the joint; the medial and lateral collateral ligaments lie outside
- ▸The fibula does not take part in the joint
What to know
Two crescent-shaped fibrocartilage pads cushion it, and two crossing ligaments inside the capsule keep the femur and tibia in line
Knee joint — menisci and cruciate ligaments.
A sesamoid bone in the quadriceps tendon glides across its front, and a bursa in front of that bone gives 'housemaid's' trouble
Knee joint — the patella and the prepatellar bursa.
A blow to the outside of the extended limb can tear, in sequence, the medial collateral ligament, the medial meniscus and the anterior cruciate ligament
Knee joint — the 'unhappy triad' of O'Donoghue.
A locking mechanism rotates the femur on the tibia at the end of extension, and popliteus must unlock it to start bending
Knee joint — the screw-home mechanism.
Key ideas
- Bone shape gives almost no stability, so the knee depends on ligaments and menisci; the fibula plays no part.
- Locking at full extension lets you stand with almost no muscle effort; the popliteus 'unlocks' it.
Quick-fire facts
- •It is the largest joint in the body, and its synovial cavity is the largest of any joint.
- •OpenStax: the knee functions primarily as a hinge joint, allowing flexion and extension; the tibiofemoral joint is supplemented by the patellofemoral joint.
- •The patella is the largest sesamoid bone in the body.
- •The 'unhappy triad' (terrible triad) is tearing of the medial collateral ligament, medial meniscus and anterior cruciate ligament, from a blow to the lateral side of the knee.
- •The medial collateral ligament (tibial collateral ligament) is firmly attached to the medial meniscus, so the two are often injured together.
- •Housemaid's knee is prepatellar bursitis, from kneeling; clergyman's knee involves the infrapatellar bursa.
- •The knee jerk reflex is tested by tapping the patellar ligament.
- •Total knee replacement is one of the most common orthopaedic operations.
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Sources
- OpenStax — Anatomy of selected synovial joints (Anatomy and Physiology 2e)
- TeachMeAnatomy — Knee joint
- NHS — Knee pain