The muscles

branch and a deep motor unit, such that fracture may disrupt either or both branches by direct contusion or persistent compression. Anatomy of the thumb and associated injury Gamekeeper's thumb derives its name from an injury to the metacarpophalangeal articulation acquired by Scottish gamekeepers attempting to k i ll rabbits by strangulation. A contemporary term, skier's thumb, is now more frequently employed as it is in this group that the injury is now more frequently recognized.

The injury is characterized by disruption of the ulnar collateral ligament at the base of the thumb, integrity of which dictates the ability to successfully appose the thumb and digits. Two forms of injury are recognized, one in which a small ossific fragment is avulsed at the insertion of the ligament (type 1), readily identified on radiographs, and the second, which is radiographically occult, is characterized by intrasubstance rupture without avulsion of bone (type 2).

Similar collateral ligaments are responsible for radial and ulnar stability in all the interphalangeal joints and, using MRI, may be imaged following suspected trauma. The volar plate The volar plate represents the ligamentous thickening of the volar capsule that bridges and stabilizes the volar aspect of the interphalangeal and metacarpophalangeal joints. The volar plate is primarily responsible for passive resistance to hyperextension at these articulations, active resistance being provided by contraction of flexor muscle groups.

Volar pain on induced hyperextension w i th local tenderness suggests the injury, which can be clearly visualized in both passive and stressed sagittal views of the digits using MRI. Extensor tendon injury - mallet finger The term mallet finger is used to describe the flexion deformity of the DIP joint resulting from loss of extensor tendon continuity to the distal phalanx. The term 'mallet finger of bony origin' is used to describe the same deformity occurring secondary to intra-articular fracture of the dorsal lip of the distal phalanx. Three patterns of tendon-related mallet finger are recognized, and include type 1 injury resulting from stretching of the ligament, type 2 injury characterized by rupture of the tendon at its insertion, and type 3 injury characterized by a subtle avulsion at the site of tendon insertion.

Extensor tendon injury: boutonniere deformity

Boutonniere or buttonhole deformity is caused by disruption of the central slip of the extensor tendon combined w i th tearing of the triangular ligament on the dorsum of the middle phalanx, allowing the lateral bands of the extensor tendon to slip below the axis of the PIP articulation.

Although clinically apparent, MR imaging allows detailed evaluation of tendon position and integrity, triaging patients into surgical and non-surgical groups.

Flexor digitorum profundus tendon injury - jersey finger

Avulsion of the flexor digitorum profundus tendon from its insertion into the base of the distal phalanx is a relatively uncommon injury, usually occurring during active sports, typically when a football or rugby player attempts to tackle the opposition but ends up grabbing a handful of jersey, hence the term 'jersey finger'.

THE MUSCLES OF THE UPPER LIMB

Modalities such as CT (Figs 7.12 and 7.13) and especially MRI have made imaging of the muscles of the limbs possible. A knowledge of the origin, course and insertion of these muscles is less important in radiology than an

Fig. 7.12 Axial proton density weighted MRI section through the upper arm.

1. Biceps muscle - short head (flexor of the elbow) 2. Biceps muscle - long head (flexor of the elbow) 3. Brachialis muscle (flexor of the elbow) 4. Triceps muscle medial head (extensor of the elbow) 5. Triceps muscle lateral head (extensor of the elbow) 6. Triceps muscle long head (extensor of the elbow) 7. Basilic vein (neurovascular bundle) 8. Brachial artery (neurovascular bundle) 9. Median nerve (neurovascular bundle) 10. Cortex of humeral shaft 11. Medullary cavity 12. Subcutaneous fat in anterior aspect of the upper arm

understanding of their positions relative to one another in cross-section. A brief description of the cross-sectional layout of muscle compartments is therefore appropriate. Muscles related to the shoulder joint are described in that section (see p. 256).

In the upper arm (Fig. 7.14) the deltoid muscle covers the upper lateral aspect of the humerus. The flexors of the shoulder - the coracobrachialis and biceps muscles - lie anterior to the humerus in its upper third, and the flexors of the elbow - brachialis and biceps muscles - lie anterior to the lower part of the humerus. Extensors of the elbow joint - the triceps muscle w i th its long, lateral and medial heads - lie posterior to the humerus.

The neurovascular bundle of brachial artery, basilic vein and median nerve lies superficially, medial to the humerus. The radial nerve and profunda brachii artery lie deeply close to the humerus, at first medially then passing posteriorly close to the lateral side.

Fig. 7.13 Axial proton density weighted MRI scan through the forearm. The scan was obtained with the palm facing up. Extensive fat allows easier visualization of the muscle groups. The flexors are anterior and the extensors posterior.

1. Radius 2. Ulna 3. Extensor carpi radialis muscle (longus and brevis) 4. Extensor carpi ulnaris muscle 5. Extensor digitorum muscle 6. Extensor pollicis longus muscle 7. Abductor pollicis longus muscle 8. Flexor carpi ulnaris muscle 9. Flexor carpi radialis muscle 10. Flexor digitorum superficialis muscle 11. Flexor digitorum profundus muscle 12. Brachioradialis muscle 13. Position of radial artery and vein 14. Cephalic vein