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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 kill 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.
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 t h r o u g h the upper arm.
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 with 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 with 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.
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
