HLTH1008 Chap.6 Knee Joint, Popliteal Fossa and Leg Compartments
Knee Joint, Popliteal Fossa and Leg Compartments
The knee is a hinge that locks by rotation. This chapter explains the screw-home mechanism and the role of popliteus in unlocking, sets out the attachments and roles of the cruciate and collateral ligaments and the structure, blood supply and injuries of the menisci, and reconstructs the unhappy triad from its mechanism.
It then divides the leg into four compartments with their muscles, nerves and arteries, describes the popliteal fossa, and finishes with the tibiofemoral angle, the Q angle and the pes anserinus. Worked mechanisms show how to move from the position of the knee and the direction of force to the structures that fail. The compartment table leads straight into the ankle and nerve chapters.
What this chapter covers
- 01
Tibiofemoral and patellofemoral articulations
- 02
Screw-home locking and unlocking
- 03
Cruciate and collateral ligaments
- 04
Medial and lateral menisci
- 05
The unhappy triad
- 06
Popliteal fossa and its contents
- 07
Four compartments of the leg
- 08
Genu valgum, genu varum and the Q angle
Worked example · free
Testing the knee ligaments
- +1Excessive forward movement of the tibia is a positive anterior drawer sign.
- +1The anterior cruciate ligament is the primary restraint to anterior tibial translation, so it is torn.
- +1A valgus force stretches the medial collateral ligament; opening on that side shows it is torn.
- +1With both injured, check the medial meniscus, which is attached to the medial collateral ligament.
Key terms
- Popliteus
- A muscle behind the knee that unlocks the extended joint by rotation.
- Medial meniscus
- The C-shaped meniscus attached to the capsule and the medial collateral ligament.
- Coronary ligaments
- Bands that attach the margins of the menisci to the tibial plateau.
- Q angle
- The angle between the line of quadriceps pull and the patellar ligament.
- Genu valgum
- Knock knees, a tibiofemoral angle above about 195°.
- Pes anserinus
- The joint insertion of sartorius, gracilis and semitendinosus on the medial tibia.
- Soleal venous plexus
- Veins within soleus that are a common site of deep vein thrombosis.
- Anterior cruciate ligament
- The intracapsular ligament that resists forward translation of the tibia.
- Tibiofemoral angle
- The angle between femur and tibia measured on the medial side.
- Lateral meniscus
- The more mobile meniscus, shaped like four-fifths of a circle.
Knee Joint, Popliteal Fossa and Leg Compartments FAQ
Why does the knee lock in full extension?
The lateral femoral condyle has a shorter articular surface than the medial one, so it finishes rolling and gliding first in the last 30° of extension. The final 5° then comes from lateral rotation of the tibia, which tightens the ligaments and seats the menisci in the close-packed position.
Why is the medial meniscus injured more often than the lateral?
It is attached to the joint capsule and the medial collateral ligament, so it cannot move out of the way when the knee is twisted. The lateral meniscus, about four-fifths of a circle, is more mobile, and pain on medial or lateral tibial rotation helps tell which one is torn.
Which nerve supplies the anterior compartment of the leg?
The deep fibular nerve, which runs with the anterior tibial artery between tibialis anterior and extensor digitorum longus. It supplies tibialis anterior, fibularis tertius, extensor digitorum longus and extensor hallucis longus, the dorsiflexors of the ankle.
What does popliteus do?
Popliteus runs from the lateral femoral condyle inside the capsule to the back of the tibia above the soleal line and is supplied by the tibial nerve. It unlocks the extended knee by rotating the femur laterally on the tibia, which is why it starts flexion from full extension.
What is the unhappy triad?
A combined injury of the anterior cruciate ligament, the medial collateral ligament and the medial meniscus, caused by a twist with a valgus force on a flexed knee with the foot fixed. The audible click is the cruciate rupturing, and rapid swelling reflects bleeding into the joint.
Exam move
Write the locking sequence as three short lines and practise it until you can say it in under twenty seconds. Draw the knee from above with both menisci and the cruciate attachments, then list each ligament with the test that checks it. For the leg, draw four boxes for the compartments and fill in the muscles, the nerve and the artery for each, then practise the unhappy triad from its mechanism.
Finish with alignment: write the normal tibiofemoral angle and the cut-offs for knock knees and bowlegs, then the two lines of the Q angle and the factors that increase it.
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