Popliteal fossa and knee joint

Popliteal fossa

A diamond-shaped space behind the knee.

• Contents from superficial to deep: tibial nerve, popliteal vein, popliteal artery (deepest, lying on bone and capsule). The common peroneal nerve follows the medial border of biceps to the fibular neck.
• Also: the end of the small saphenous vein, the popliteal lymph nodes, genicular branches of the artery and nerves.
• Popliteal lymph nodes receive lymph that runs with the small saphenous vein, from the lateral side of the foot, the heel and the back of the calf. An infected lesion there can enlarge them; the rest of the limb drains to the superficial inguinal nodes.

Popliteal artery problems

• Why it is vulnerable in knee dislocation: the artery is tethered above at the adductor hiatus and below by the fibrous arch of soleus, and lies directly on the back of the tibia and capsule, so it cannot move away when the tibia displaces. A pale, pulseless foot after dislocation needs urgent vascular assessment; even a palpable pulse does not exclude an intimal tear, so measure the ankle–brachial pressure index.
• Supracondylar femoral fracture: gastrocnemius tilts the distal fragment backwards onto the artery.
• Popliteal aneurysm: the commonest peripheral aneurysm, often bilateral and frequently associated with an abdominal aortic aneurysm, so the next step after finding one is ultrasound of the abdominal aorta (and the other knee). Its main danger is thrombosis or distal embolism, not rupture. A pulsatile lump behind the knee must never be aspirated as a cyst.
• Baker's cyst: distension of the semimembranosus–gastrocnemius bursa, which communicates with the joint, in knee arthritis; rupture into the calf mimics DVT.

Knee joint

A compound synovial joint: two condylar (modified hinge) joints between the femur and tibia, plus a saddle-like joint between the femur and patella. The fibula is not part of it. The nerves are branches of the femoral, obturator, tibial and common peroneal nerves; the knee jerk is L3–4 (femoral nerve, quadriceps).

• The cruciates lie inside the capsule but outside the synovial cavity. Both are supplied by the middle genicular artery, which pierces the posterior capsule; its tearing with the ACL explains the rapid haemarthrosis within 1–2 hours. Fat droplets in the aspirate suggest an intra-articular fracture.
• Unhappy triad: ACL, MCL and medial meniscus torn together after a lateral blow to a flexed, weight-bearing knee.

Remember Bleeding within hours points to the ACL; swelling the next day points to a meniscus.

Menisci

• Crescentic fibrocartilage that deepens the tibial surface, spreads load and aids lubrication.
• Outer third vascular (red zone, can heal after repair); inner part avascular (white zone, does not heal).
• Tears follow rotation on a flexed, weight-bearing knee. A torn fragment wedges between the bones, causing locking (inability to fully extend), giving way and joint-line tenderness; swelling develops slowly over hours because the inner meniscus is avascular. McMurray test, MRI and arthroscopy.

Locking and unlocking

• In the last 30° of extension, the femur rotates medially on the fixed tibia (the screw-home movement, driven by ligament tension and the shape of the condyles), so the knee locks and we can stand with little muscle work.
• Popliteus (tibial nerve) unlocks the knee by laterally rotating the femur on the tibia to begin flexion. Its tendon is intracapsular and attaches to the lateral meniscus, pulling it back out of harm. If popliteus fails, the knee stays extended and locked at the start of flexion.

Extensor mechanism and patella

• Quadriceps, quadriceps tendon, patella, patellar ligament and tibial tuberosity; the medial and lateral patellar retinacula are expansions of the vasti beside the patella.
• An undisplaced patellar fracture with intact retinacula still allows active straight leg raising, so it can be treated without surgery. Inability to raise the straight leg means the extensor mechanism is broken. A bipartite patella (usually superolateral, often bilateral) can mimic a fracture.
• The patella tends to dislocate laterally because the Q angle makes quadriceps pull laterally; wide hips and knock knees increase it.

Bursae and lateral or medial pain

Tenderness below the joint line is not meniscal pain; joint-line tenderness with effusion suggests a meniscal tear.

• Skin around the knee: front by the femoral cutaneous nerves, medial side by the saphenous nerve (continuing along the medial leg to the medial foot), lateral side by the lateral cutaneous nerve of the thigh and the lateral sural cutaneous nerve.

Exam trap The popliteal artery is the deepest structure in the fossa; the tibial nerve is the most superficial.

Exam trap The medial meniscus tears more often because it is fixed to the MCL and capsule.

How FCPS asks about this

• Contents of the popliteal fossa superficial to deep: tibial nerve, popliteal vein, popliteal artery.
• Twisting injury with rapid haemarthrosis and positive Lachman: ACL tear; blood from the middle genicular artery.
• Pale pulseless foot after knee dislocation: popliteal artery tethered above and below the fossa.
• Popliteal aneurysm found: scan the abdominal aorta next; aneurysms are often multiple.
• Lateral blow to the knee: MCL (valgus), with ACL and medial meniscus in the unhappy triad.
• Muscle that unlocks the knee: popliteus, laterally rotating the femur (tibial nerve).
• Patellar fracture but can still lift a straight leg: patellar retinacula intact. Upper medial tibial bursitis lies under sartorius, gracilis and semitendinosus.