The muscles

Tibialis anterior tendon The tibialis anterior muscle arising in the anterior compartment of the calf gives rise to its tendon at the level of the tibiotalar articulation, which passes to the foot deep to the extensor retinaculum before inserting on the medial cuneiform and the medial base of the first metatarsal. As such the tibialis anterior is a dominant flexor and inverter of the foot and injury is therefore induced acutely by forced extension and eversion. In most cases rupture is the sequel of chronic trauma, w i th repeated encroachment of the tendon by the extensor retinaculum over time. Os trigonum syndrome The os trigonum represents a congenital non-union of the lateral tubercle of the posterior process of the talus, to which it remains intimately related in adulthood. During flexion extension, synovial tissue may interdigitate between it and the posterior tibia, resulting in capsular entrapment, local pain and inflammation. Inflammatory changes are usually clearly visualized in the sagittal plane, intimately related to the posterior tibiotalar joint recess. The relationship between the inflammatory process and the medially located flexor hallucis tendon sheath is best visualized in the axial plane.

The subtalar joint

This synovial joint is responsible for inversion and eversion of the ankle. It has two parts, each w i th its own capsule and synovial cavity as follows: • The talocalcaneal joint between the posterior facet on the inferior surface of the talus and the corresponding facet on the calcaneus; • The talocalcaneonavicular joint has four contiguous parts (see also the inferior surface of the talus): — between the inferior surface of the talus and the sustentaculum tali of the calcaneus — the plantar calcaneonavicular (spring) ligament and the anterior talocalcaneal facet; and — between the head of the talus and the navicular bone.

Radiological features of the subtalar joint

Plain radiography In radiography of the subtalar joint oblique views of the ankle w i th the foot internally rotated 45° are used. Films are exposed w i th tube angulations of 10°, 20°, 30° and 40° for complete visualization of the complex articular surfaces.

Computed tomography The three components of the talocalcaneal joint, the posterior, the sustentaculum tali and the anterior components, can be seen by coronal imaging w i th CT. Magnetic resonance imaging In addition to bones, MRI can identify muscles - peroneal muscles - that contribute to the stability of the joint as well as ligaments, such as: • The interosseus talocalcaneal ligament; • The plantar calcaneonavicular 'spring' ligament from the sustentaculum tali to the navicular; • The short plantar ligament from the calcaneus to the cuboid; and • The long plantar ligament from the calcaneus to the base of the metacarpals.

Other joints of the foot

The intertarsal, tarsometatarsal, metatarsophalangeal and interphalangeal joints of the foot are synovial joints, each w i th its own capsule and synovial cavity.

THE MUSCLES OF THE LOWER LIMB

A knowledge of the origin, course and insertion of muscles is less important in radiology than an understanding of the relative position of muscle groups as seen on cross-sectional imaging modalities such as CT and MRI (Figs 8.16 and 8.17). A brief description of the cross-sectional layout of muscles is therefore appropriate.

In the upper thigh (Fig. 8.18) muscles that flex the hip and extend the knee are found anteriorly, that is, the quadriceps femoris muscle, made up of the rectus femoris and vastus lateralis, medialis and intermedius muscles. Sartorius, a thin flat strap muscle w i th a similar action, is also found superficially in the anterior compartment.

The adductor muscles are found medially. These are, from anterior to posterior, the adductor longus, brevis and magnus muscles, w i th the adductor gracilis lying superficially.

Posteriorly lie the extensors of the hip, the glutei and the flexors of the knee - the hamstrings, that is the biceps femoris, semimembranosus and semitendinosus muscles.

The femoral artery, vein and nerve lie superficially between the anterior and adductor compartments, w i th the profunda femoris vessels lying deeply close to the femur in the same plane.

At midthigh level (Fig. 8.19) the shaft of the femur is surrounded by the rectus femoris and vastii muscles medially, anteriorly and laterally. The shaft of the femur is surrounded posteromedially by the adductors and posteriolaterally by the biceps, semitendinosus and semimembranosus.

The femoral vessels lie in the adductor canal deep to the sartorius muscle and between the anterior and adductor compartments. The profunda vessels lie almost posterior to the femur, also between these compartments. The sciatic nerve lies between the muscles of the adductor compartment and the hamstring muscles.

Fig. 8.16 Axial proton density weighted MRI scan of thigh.

1. Medullary cavity of femur 2. Cortex of femur 3. Rectus femoris muscle 4. Vastus lateralis muscle 5. Vastus intermedius muscle 6. Vastus medialis muscle 7. Sartorius muscle 8. Superficial femoral vessels in adductor canal 9. Adductor magnus 10. Biceps femoris muscle (long head) 11. Semitendinosus muscle 12. Semimembranosus muscle 13. Gracilis muscle 14. Position of sciatic nerve 15. Deep femoral vessels

Fig. 8.17 Axial proton density weighted MRI scan of calf.

1. Medulla of tibia 2. Cortex of tibia 3. Medulla of fibula 4. Cortex of fibula 5. Muscles of anterior tibial compartment 6. Tibialis posterior muscle 7. Anterior tibial artery, vein and nerve 8. Posterior tibial artery, vein and nerve 9. Medial head of gastrocnemius muscle 10. Lateral head of gastrocnemius muscle 11. Soleus muscle 12. Flexor digitorum longus muscle 13. Peroneus longus 14. Popliteus muscle

Fig. 8.18 Axial section of upper thigh.

Fig. 8.19 Axial section of midthigh.