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Evidence-based physiotherapy and rehabilitation resources
Editorial & review policyHuman Anatomy · Lower limb
The knee carries more load than any other joint and has almost no bony stability to do it with. A large round femur sits on a nearly flat tibia, and everything holding that arrangement together is soft tissue. It is the most commonly injured joint in the body, and this chapter explains why.
Look at the bones alone and the knee should not work. Two rounded femoral condyles rest on a tibial plateau that is almost flat. There is no socket, no wall, nothing to stop the femur rolling off the back.
What makes it function is entirely soft tissue: two menisci deepening the surface, four main ligaments checking movement in each direction, and a set of muscles crossing it. Remove any one and the joint is measurably less stable — which is exactly what a knee injury is.
Figure 1 · What holds the knee together
| Articulation | Between | Note |
|---|---|---|
| Medial tibiofemoral | Medial femoral condyle and tibial plateau | Carries more load than the lateral side, and wears first in most people. |
| Lateral tibiofemoral | Lateral condyle and plateau | More mobile than the medial side. |
| Patellofemoral | Patella and the front of the femur | Load here rises steeply with knee flexion, which is why stairs and squatting provoke anterior knee pain. |
The superior tibiofibular joint sits just below and lateral, and is not part of the knee. The fibula takes no part in it.
Two crescents of fibrocartilage sitting on the tibial plateau, thicker at their outer edge so they form shallow cups for the femoral condyles. They deepen the surface, spread load over a wider area, and help lubricate the joint.
Why the medial meniscus tears more
The medial meniscus is attached to the medial collateral ligament and to the capsule, so it cannot move out of the way as the joint rotates. The lateral is free of its collateral ligament and far more mobile. Fixed structures tear; mobile ones escape. This also explains why a blow to the outside of the knee can injure the medial ligament and the medial meniscus at the same time.
Remember from the cartilage chapter that only the outer third has a blood supply. That is why a peripheral tear may be repaired and a central one is usually trimmed.
The cruciates are named for where they attach to the tibia, not the femur, and they cross each other inside the joint — which is what "cruciate" means.
| Ligament | Runs | Prevents | Typically injured by |
|---|---|---|---|
| Anterior cruciate | From the front of the tibia, upwards and backwards to the lateral condyle | The tibia sliding forwards on the femur; also checks rotation | A twisting injury on a planted foot, often with no contact at all. Frequently a pop, then rapid swelling. |
| Posterior cruciate | From the back of the tibia, upwards and forwards to the medial condyle | The tibia sliding backwards | A blow to the front of the flexed tibia — the classic dashboard injury. |
| Medial collateral | Medial epicondyle to the tibia, blended with capsule and meniscus | Valgus force, opening the joint on the inside | A blow to the outside of the knee. |
| Lateral collateral | Lateral epicondyle to the head of the fibula, separate from the capsule | Varus force | A blow to the inside, which is far less common. |
Figure 3 · The knee joint
The knee is described as a hinge, and it is not quite one. In the last part of extension the tibia rotates laterally on the femur, because the medial femoral condyle is longer than the lateral and the tibia has to keep travelling on that side after the lateral side has run out.
The effect is to lock the joint in full extension, so standing costs almost no quadriceps effort. To unlock it, popliteus rotates the tibia medially at the start of flexion. That is the whole job of a small muscle you would otherwise never think about.
Quadriceps, the patella, and the patellar tendon act as one unit. The Q angle — the angle between the pull of quadriceps and the line of the patellar tendon — means the resultant force tends to pull the patella laterally.
Three things resist that. The lateral lip of the femoral groove is higher than the medial. Vastus medialis, particularly its lowest oblique fibres, pulls the patella medially. And the medial retinaculum restrains it passively.
Why this matters so much in practice
Anterior knee pain is one of the commonest complaints you will meet. The mechanics above explain a good deal of it. It is more common in people with a wider pelvis, and worse on stairs and squatting where patellofemoral load rises. It also explains why so much treatment is directed at hip control — a femur that rotates inwards increases the lateral pull just as effectively as a weak vastus medialis does.
A diamond-shaped hollow behind the knee, bounded above by the hamstrings and below by the two heads of gastrocnemius. Its contents matter because they are all vulnerable together.
| Contents, superficial to deep | Note |
|---|---|
| Tibial nerve | Most superficial, running straight through the middle |
| Popliteal vein | Between nerve and artery |
| Popliteal artery | Deepest, lying against the joint capsule — which is why a posterior knee dislocation threatens it directly |
| Common fibular nerve | Leaves along the lateral border, heading for the fibular neck |
Figure 2 · Compartments of the leg
Three compartments again, and here the nerve supply is unusually elegant: the common fibular nerve divides at the fibular neck into a deep branch for the anterior compartment and a superficial branch for the lateral. Damage at that one point takes out both.
| Structure | Worth knowing |
|---|---|
| Gastrocnemius | Crosses the knee as well as the ankle, so its length depends on knee position. Test calf length with the knee straight and again bent. |
| Soleus | Crosses only the ankle. Deep to gastrocnemius, rich in type I fibres, and the main muscle holding you upright. |
| Calcaneal tendon | The strongest tendon in the body, formed by both. Its blood supply is poorest a few centimetres above its insertion, which is where it ruptures. |
| Tibialis posterior | Deep, and the main dynamic support of the medial arch of the foot. |
| Fibularis longus | Crosses under the sole to the opposite side, supporting the transverse arch. |
Figure 4 · Cross-section of the leg and the popliteal fossa
Illustration to be added
Two panels. Panel one: a cross-section through the mid-leg viewed from above, showing the tibia and fibula joined by the interosseous membrane, with the anterior, lateral, superficial posterior and deep posterior compartments clearly separated by fascia and shaded differently. Name the muscles in each and place the anterior tibial vessels with the deep fibular nerve, the superficial fibular nerve in the lateral compartment, and the posterior tibial vessels with the tibial nerve in the deep posterior compartment. Panel two: the popliteal fossa from behind, with the diamond boundaries formed by the hamstrings above and the two heads of gastrocnemius below, and the contents layered superficial to deep - tibial nerve, popliteal vein, popliteal artery against the capsule - with the common fibular nerve leaving along the lateral border towards the fibular neck. Muscle brick, bone ivory, nerves gold, artery red, vein blue.
| Problem | Anatomy behind it | What you find |
|---|---|---|
| Anterior cruciate rupture | Checks anterior translation and rotation, in a joint with no bony stability | Twisting injury, often a pop, swelling within hours, and a knee that gives way. |
| Medial meniscal tear | Fixed to the capsule and medial ligament, so it cannot escape rotation | Joint line tenderness, catching or locking, slower swelling than a cruciate. |
| Patellofemoral pain | Lateral resultant pull, and load rising steeply with flexion | Pain on stairs, squatting and prolonged sitting. Look at hip control, not just the knee. |
| Calcaneal tendon rupture | Poor blood supply a few centimetres above the insertion | Sudden pain, a palpable gap, and no plantarflexion when the calf is squeezed. |
| Compartment syndrome of the leg | Tough deep fascia and rigid septa around the anterior compartment | Pain out of proportion, much worse on passive stretch. Pulses may be present. |
| Foot drop | Common fibular nerve at the fibular neck, superficial and on bone | Cannot lift the foot; a high-stepping gait; numbness over the dorsum. |
It rotates, particularly near extension. Without that the cruciates and popliteus make no sense.
They are named for their tibial attachment. Anterior cruciate stops the tibia moving forwards.
The superior tibiofibular joint is separate. The fibula carries no weight.
A femur that rotates inwards increases the lateral pull on the patella. Hip control is part of the treatment.
Gastrocnemius crosses the knee, soleus does not. Test both ways or you cannot say which is short.
Rapid swelling within hours suggests bleeding — cruciate or fracture. Slower swelling over a day suggests a meniscal or capsular injury. The timing is a clue.
Ten questions on this chapter. Tap one to see the answer and the reasoning.
Answer: (B) There is no socket. Stability comes almost entirely from menisci, ligaments and muscle, which is why the knee is so commonly injured.
Answer: (B) The cruciates are named from their tibial attachments. The anterior cruciate prevents the tibia sliding forwards on the femur.
Answer: (C) Being fixed, it cannot move out of the way during rotation. The lateral meniscus is free of its collateral ligament and far more mobile.
Answer: (B) The posterior cruciate prevents backward translation of the tibia, so a blow to the front of the flexed tibia is its classic mechanism.
Answer: (B) The tibia keeps travelling on the medial side after the lateral side has run out, which locks the joint. Popliteus unlocks it by rotating the tibia medially.
Answer: (B) Popliteus rotates the tibia medially, reversing the screw-home mechanism. It is a small muscle with one important job.
Answer: (C) Vastus medialis, especially its lower oblique fibres, is the active restraint. The higher lateral lip and the medial retinaculum are passive.
Answer: (C) The artery lies deepest, against the joint capsule. This is why a posterior knee dislocation threatens it directly.
Answer: (B) The deep branch supplies the anterior compartment and the superficial branch the lateral. Damage at that one point takes out both, producing foot drop and loss of eversion.
Answer: (B) Gastrocnemius crosses knee and ankle; soleus crosses only the ankle. Testing both positions is the only way to say which is short.
Everything on this page, in one screen
| Book | What it adds here |
|---|---|
| Anatomy and Human Movement Palastanga, Field and Soames |
The best treatment of knee mechanics, including the screw-home mechanism. |
| B D Human Anatomy, Volume 2 Chaurasia |
Regional detail and the popliteal fossa at examination level. |
| Clinical Anatomy by Regions Snell |
The clinical consequences of injury here. |
| Gray's Atlas of Anatomy Drake, Vogl and Mitchell |
Keep the knee plates open alongside. |
Reviewed by the Physiotherapist India Team. · Human Anatomy contents