Hand injuries are common in amateur and professional boxers and result in time lost from training and competition.1-3 Injuries to the dorsal hood account for 16% of all hand and wrist injuries in boxers.1, 3 ‘Boxer's knuckle’ describes a closed injury to the metacarpophalangeal joint (MCPJ) of the hand and is used synonymously to describe tears of the sagittal bands of the dorsal hood and associated extensor tendon instability.4, 5 It can result from a direct blow to the flexed MCPJ, commonly from boxing or punching, or from relatively low-energy repetitive injuries.5 Patients typically present with a painful and swollen dorsal MCPJ, and the space between knuckles, with pain associated with forming a closed fist, loss of full extension and snapping of extensor tendons with MCPJ flexion.6, 7 Boxer's knuckle soft tissue injuries are less appreciated than boxer's fracture that typically involves a fracture of the fifth or fourth metacarpal neck with volar angulation and can occur from a similar mechanism of injury.8
Tears of the sagittal bands of the dorsal hood can be clinically overlooked or underappreciated, as the symptoms can often be non-specific, and the associated tendon subluxation or dislocation may not always be observed.9, 10 If not diagnosed and treated adequately and in a timely manner, sagittal band tears can result in long-term persistent pain at the MCPJ and hand function impairment.11-13 Diagnostic imaging, including sonography, can play an important role in directly imaging the soft tissue structures surrounding the MCPJ and diagnosing sagittal bands tears and tendon instability; however, an appreciation of the mechanism of injury, sonographic anatomy, sonographic technique, and normal and abnormal sonographic appearances is required.
The anatomy of the extensor (dorsal) mechanism of digits 2–5 of the hand is complex and often overwhelming. It combines an array of dorsal soft tissue structures including extensor tendons, the dorsal plate and the dorsal hood (extensor expansion).14 The dorsal hood is interrelated with intermetacarpal and palmar hand structures which aid in producing finger movement and MCPJ stability.15 The intermetacarpal structures include collateral ligaments, lumbrical and interosseous muscles and their associated tendons. Palmar structures of the hand around the MCPJ include the palmar plate, A1 pulley, flexor tendons, the deep transverse metacarpal ligament (DTMCL) and the associated neurovascular structures.15 The dorsal, intermetacarpal and palmar structures surrounding the MCPJ all need to be sonographically assessed in cases of suspected sagittal band tears.
Extension of the proximal interphalangeal joint (PIPJ) and distal in