Lower limb anatomy
Lower limb on imaging: bones, joints, muscles, arteries and veins
Lower limb imaging ranges from hip ultrasound in babies to radiographs of fractures, knee MRI in athletes and angiography of the leg arteries. CPSP asks which lines on a hip film reveal disease, how the femoral head and talus lose their blood supply, the MRI patterns of knee ligament injuries, the named foot and ankle fractures, and where the arteries change their names.
One of the most tested topics in past papers
Hip
- Shenton line: smooth arc from the inferior border of the femoral neck to the upper border of the obturator foramen; broken by a neck fracture, dislocation or dysplasia.
- Blood supply of the femoral head: retinacular branches of the medial circumflex femoral artery run up the neck under the capsule; the artery of the ligamentum teres (obturator) adds a little in children. Intracapsular fractures tear the retinacular vessels, so avascular necrosis follows; extracapsular ones rarely do this.
- Avascular necrosis (steroids, alcohol, sickle cell): on MRI a low-T1 band outlines a wedge of the anterosuperior weight-bearing head, before collapse is seen on radiographs.
- Posterior dislocation (knee against the dashboard): leg shortened, flexed, adducted, internally rotated; look for a posterior acetabular wall fracture and sciatic nerve injury.
- A septic hip is held flexed, abducted and externally rotated, the position of greatest capsular volume.
| Child's hip | Imaging |
|---|---|
| Developmental dysplasia (first-born girl, breech, left side) | Ultrasound under about 4–6 months because the head is cartilage; the alpha angle measures the bony acetabular roof (60° or more normal) |
| Perthes disease (boy about 4–8 years) | Femoral epiphysis small and dense, later fragmented |
| Slipped capital femoral epiphysis (overweight adolescent, knee pain) | On the frog-leg view a line along the upper femoral neck (Klein line) does not cut the epiphysis |
- Hip disease is often felt in the knee because the obturator and femoral nerves supply both joints.
- Cam deformity: bony bump at the anterosuperior head–neck junction causing femoroacetabular impingement.
Knee
- A horizontal-beam lateral film showing a straight fat–fluid level (lipohaemarthrosis) means a fracture into the joint.
| MRI or radiograph finding | Injury |
|---|---|
| Bone bruises of lateral femoral condyle and posterolateral tibial plateau | ACL tear (pivot shift) |
| Small fragment off the lateral tibial rim (Segond fracture) | Anterolateral capsular ligament avulsion; ACL tear likely |
| Low-signal band in the notch in front of the PCL ("double PCL") | Bucket-handle meniscal tear (locking) |
| Bruises of medial patellar facet and lateral femoral condyle | Transient lateral patellar dislocation, torn medial patellofemoral ligament |
| Curved calcification beside the medial femoral condyle | Old medial collateral ligament injury |
| High-riding patella after a jump | Patellar tendon rupture |
- Medial meniscus tears more often: it is fixed to the medial collateral ligament. Only its outer third has a blood supply, so tears there can be repaired. Acute haemarthrosis after an ACL tear comes from the middle genicular artery.
- Normal variants: bipartite patella (smooth gap at the superolateral corner); fabella (sesamoid in the lateral head of gastrocnemius).
- Baker cyst: semimembranosus–gastrocnemius bursa; rupture mimics calf DVT.
Ankle and foot
- Weber classification of lateral malleolus fractures by level of the tibiofibular syndesmosis: A below, B at the level, C above (syndesmosis torn). Maisonneuve: medial clear space widening with a proximal fibular fracture.
- Achilles tendon ruptures 2–6 cm above its insertion (poorly vascular zone). The anterior talofibular ligament is torn in most inversion sprains.
- Talar neck fracture risks avascular necrosis of the body, which receives its blood from the neck. Calcaneal fracture from a fall onto the heels flattens Böhler angle (normal about 20–40°); look for thoracolumbar spine fractures too.
- Lisfranc injury: widening between the bases of the first and second metatarsals.
| Fifth metatarsal and forefoot | Feature |
|---|---|
| Avulsion of the tuberosity | Pulled by peroneus brevis (inversion) |
| Jones fracture (metaphyseal–diaphyseal junction) | Vascular watershed, slow healing |
| March (stress) fracture | Second metatarsal; radiograph normal at first, fluffy callus later |
| Morton neuroma | Hypoechoic mass in the third intermetatarsal space |
- Tarsal coalition in a rigid flat foot: elongated anterior calcaneal process reaching the navicular (calcaneonavicular bar).
- Os trigonum behind the talus causes posterior impingement in dancers. The two sesamoids under the first metatarsal head lie in the tendons of flexor hallucis brevis.
- Gout: punched-out erosions with overhanging edges, joint space kept. Non-ossifying fibroma: eccentric cortical lucency in a child's metaphysis. Osteochondroma with a cartilage cap over 2 cm in an adult suggests malignant change.
- Behind the medial malleolus, front to back: tibialis posterior, flexor digitorum longus, posterior tibial artery and veins, tibial nerve, flexor hallucis longus (which then runs under the sustentaculum tali). Peroneus longus grooves the cuboid. The spring ligament supports the talar head; its failure with tibialis posterior dysfunction flattens the arch.
Arteries
- Common femoral puncture is made over the medial third of the femoral head, below the inguinal ligament.
- Profunda femoris arises about 4 cm below the ligament from the posterolateral side of the common femoral. The superficial femoral becomes popliteal at the adductor hiatus.
- Popliteal artery is the deepest structure in the fossa and is tethered above and below, so knee dislocation tears it. Popliteal entrapment: artery deviated around the medial head of gastrocnemius; pulses vanish on plantarflexion.
- It divides at the lower border of popliteus into the anterior tibial artery and the tibioperoneal trunk; the peroneal artery runs behind and medial to the fibula.
- Dorsalis pedis is felt just lateral to the extensor hallucis longus tendon; its deep plantar branch dives between the first and second metatarsals to the plantar arch.
Veins
- Great saphenous vein: in front of the medial malleolus (cut-down site), joins the femoral vein at the saphenous opening about 3 cm below and lateral to the pubic tubercle (saphena varix). Stripping injures the saphenous nerve.
- Small saphenous vein: behind the lateral malleolus, up the middle of the calf to join the popliteal vein; the lateral heel drains with it to popliteal nodes.
Remember Klein line for a slipped epiphysis, Shenton line for a hip fracture or dislocation.
Exam trap A Segond fracture looks trivial but signals an ACL tear.
Exam trap An isolated medial ankle injury with a widened clear space needs a film of the whole fibula.
Test yourself on this topic
5 of the 142 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
After an injury to the front of his thigh, a high-jumper cannot strongly flex his hip and extend his knee at the same time, as in a kick. Which muscle has been injured?
Answer: Rectus femoris
Rectus femoris is the only part of quadriceps that crosses the hip. It arises from the anterior inferior iliac spine (straight head) and the area above the acetabulum (reflected head) and joins the quadriceps tendon to the patella and tibial tuberosity. It therefore flexes the hip and extends the knee together, the combination used in kicking and in the take-off of a jump. It is supplied by the femoral nerve. The vasti extend only the knee, and iliopsoas flexes only the hip.
Rectus femoris
- From the AIIS and above the acetabulum
- Only quadriceps head crossing the hip
- Flexes hip + extends knee
- Nerve: femoral
Remember Kicking uses rectus femoris.
Exam trap In adolescents a forceful kick can avulse the anterior inferior iliac spine rather than tear the muscle.
Why the other options are wrong
- Sartorius Flexes both the hip and the knee.
- Vastus lateralis Extends the knee but does not cross the hip.
- Iliopsoas Flexes the hip but does not act on the knee.
- Pectineus Flexes and adducts the hip only.
A 28-year-old man's knee locks after a twisting injury, and he cannot fully straighten it. At arthroscopy, a long vertical tear has separated the inner part of the medial meniscus, which has flipped into the intercondylar notch while staying attached at both horns. What is the diagnosis?
Answer: Bucket-handle meniscal tear
In a bucket-handle tear, a long vertical (longitudinal) tear separates the inner part of the meniscus, which flips into the intercondylar notch while staying attached at both horns, like the handle of a bucket. The displaced fragment blocks full extension, causing locking. The medial meniscus is affected more often because it is less mobile, being firmly attached to the capsule and the medial collateral ligament. The inner part of the meniscus is avascular, so such tears often need repair or partial removal.
Bucket-handle tear
- Long vertical tear
- Fragment flips into the notch
- Usually medial meniscus
- Causes locking
Remember A locked knee after a twist: think bucket-handle tear.
Exam trap Only the outer, red zone of the meniscus has a good blood supply, so inner tears heal poorly.
Why the other options are wrong
- Torn and retracted ACL Would cause instability rather than a meniscal fragment in the notch.
- PCL tear with a stump Would cause posterior instability, not a displaced meniscal fragment.
- Discoid lateral meniscus A congenitally thick, disc-shaped lateral meniscus, not a tear.
- Meniscal cyst A fluid-filled cyst beside the meniscus, not a displaced fragment.
A 38-year-old man on long-term steroids for nephrotic syndrome has groin pain from avascular necrosis of the femoral head. In which part of the head does this process usually begin?
Answer: Anterosuperior weight-bearing part
Avascular necrosis of the femoral head usually begins in the anterosuperior, weight-bearing segment, the end territory of the lateral epiphyseal (retinacular) branches of the medial circumflex femoral artery. The dead marrow and trabeculae are surrounded by a reactive zone of hyperaemia and repair. Under load, the dead subchondral bone fractures (seen as a crescent) and the head collapses, leading to secondary arthritis. Steroids, alcohol, sickle cell disease, caisson disease and trauma are common causes.
Femoral head AVN
- Starts anterosuperior (weight-bearing)
- End territory of medial circumflex femoral
- Subchondral fracture: crescent
- Then collapse and arthritis
Remember AVN of the hip starts where the weight falls.
Exam trap Radiographs are normal early; MRI is needed in a high-risk patient with hip pain.
Why the other options are wrong
- Inferomedial part near the fovea Partly supplied by the artery of the ligament of the head; not the usual site.
- Posterior non-weight-bearing part Not the usual starting site.
- Femoral neck below the head The neck is not the site of AVN of the head.
- Lesser trochanter region Far from the femoral head.
A 42-year-old squash player feels he has been kicked in the back of the heel and now has a palpable gap in his Achilles tendon. At what distance above its calcaneal insertion does the Achilles tendon most often rupture?
Answer: 2–6 cm
The Achilles tendon most often ruptures about 2 to 6 cm above its insertion into the calcaneus. This segment has the poorest blood supply, a watershed between vessels from the muscle above and from the bone below, so degeneration builds up there and the tendon fails under sudden load. Squeezing the calf then fails to plantarflex the foot (Simmonds–Thompson test). Ruptures at the insertion or at the musculotendinous junction are less common.
Achilles rupture
- Site: 2–6 cm above insertion
- Watershed of blood supply
- Feels like a kick to the heel
- Positive calf squeeze test
Remember The Achilles tears where its blood is poorest, a few cm above the heel.
Exam trap Plantarflexion may still be possible through the long flexors, so a normal-looking action does not exclude rupture.
Why the other options are wrong
- At the insertion itself Insertional ruptures occur but are less common.
- 8–10 cm Higher than the usual watershed segment.
- At the musculotendinous junction Gives a calf muscle tear, as in 'tennis leg' with medial gastrocnemius.
- 12–15 cm Far above the tendon, in the muscle bellies.
A 24-year-old athlete has calf claudication after running, and his ankle pulses disappear with active plantarflexion. At surgery the popliteal artery runs medial to, and then beneath, an abnormally attached muscle head in the popliteal fossa. Which muscle is it?
Answer: Medial head of gastrocnemius
In popliteal artery entrapment syndrome the artery takes an abnormal course in relation to the muscles of the popliteal fossa, most often looping medial to and under the medial head of gastrocnemius, or the medial head arises abnormally far laterally on the femur. Contraction of the muscle during plantarflexion compresses the artery, causing claudication in a young person without atherosclerosis. Repeated compression can lead to stenosis, thrombosis or aneurysm. Normally the artery runs between the two heads of gastrocnemius.
Popliteal entrapment
- Young athlete with claudication
- Artery loops medial to medial gastrocnemius
- Pulses lost on plantarflexion
- Risk of thrombosis, aneurysm
Remember Young claudicant, no atheroma: think medial gastrocnemius entrapment.
Exam trap Pulses may be normal at rest; test them with active plantarflexion.
Why the other options are wrong
- Lateral head of gastrocnemius Rarely involved; the usual cause is the medial head.
- Popliteus Can rarely be involved, but it lies deep and is not the classic cause.
- Semimembranosus Lies medially but is not the usual entrapping muscle.
- Plantaris Rarely implicated.
Where this sits in the CPSP syllabus
- Diagnostic Radiology, Paper 2: Limbs, breast and lymphatics, Lower limb: bones, joints, muscles, arteries and veins