Boxer's knuckle: Sonographic anatomy and assessment of sagittal band tears of the dorsal hood

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 interphalangeal joint (DIPJ) is achieved via a combination of extensor tendons and intrinsic muscles of the hand (lumbrical and interossei muscles). Extrinsic tendons at the MCPJ are formed by the extensor digitorum (ED) tendon to the fingers, the extensor indicis proprius (EIP) tendon to the second (index) finger and the extensor digiti minimi (EDM) tendon to the fifth (little) finger.¹⁶ These tendons arise from the muscles that originate from the lateral elbow (ED and EDM) and forearm (EIP) and pass through dorsal compartments 4 and 5 of the wrist to the hand (Figure 1).

The dorsal hood (also called the dorsal expansion or dorsal extensor mechanism) is a complex retinacular system over the dorsal or extensor aspect of the hand and fingers which acts to stabilise extensor tendons at the dorsal aspect of the MCPJ, PP and middle phalanx (MP).⁵ It is a coalescence of the all the extensor components and contributes to a broad, flat and thin aponeurotic expansion that covers 50% of the dorsal finger. It consists of three principle retinacular and stabilising bands from proximal to distal: sagittal, transverse and oblique bands^{5, 15} (Figure 5).

The sagittal bands are inter-related with palmar structures of the MCPJ, which include the palmar plates, the DTMCL, collateral ligaments and intrinsic muscles (lumbricals and interossei). As concurrent injuries may occur to these structures, they should also be sonographically assessed when sagittal band tears are suspected.

To sonographically assess the sagittal bands of the dorsal hood, a high-frequency (≥12 MHz) linear transducer is required. The dorsal aspect of the MCPJs and the sagittal bands should be assessed both with fingers extended, and with the hand in a fist formation with dynamic imaging during flexion and extension required. To allow scanning of the dorsal hand during flexion and extension of the MCJP, the hand can be placed over the edge of a foam pad, rolled up face washer or gel bottle. Due to the bony nature of the dorsal MCPJ, sufficient gel is required to ensure transducer contact is maintained during dynamic imaging with flexion and extension. A hockey stick transducer, with a small footprint can facilitate better transducer contact with dynamic imaging. Transducer pressure must also be light enough to allow extensor tendon subluxation or dislocation to be demonstrated in real time, as greater transducer pressure may prevent or obscure tendon movement.

Traumatic tears to the sagittal bands result from direct trauma to the dorsum of the MCPJ or resisted joint extension. Sagittal band tears tend to occur as longitudinal splits, extending in a proximal-to-distal orientation, and can result in extensor tendon instability and possibly impaired MCPJ extension.²² Tears can involve the proximal and/or distal component of the sagittal band, and the extent of the tear should be defined.⁵ Sagittal band tears usually involve the third or fourth MCPJ.¹⁹ The third (middle) finger is the most affected, followed in decreasing order by the fourth (ring), fifth and then second (index) fingers. The radial or ulnar portion of a sagittal band tends to be torn, rather than in the midline component and most often the superficial fibres are involved.⁵ Sagittal band tears can be defined as partial or complete.

Partial sagittal band tears sonographically demonstrate a focally thickened and hypoechoic sagittal band on either the radial or ulnar side.¹⁷ A partially torn sagittal band does not show a complete gap between band ends, and the extensor tendon/s remains encapsulated by the sagittal band when the MCPJ is flexed and extended. Partial sagittal band tears can result in extensor tendon subluxation. Partial tears through 50% of the depth of the proximal radial sagittal band have been demonstrated to be sufficient to cause extensor subluxation; however, partial tears of the distal sagittal band are most often not associated with extensor tendon subluxation.¹⁰ In digits 3 and 4, ED tendon subluxation at the MCPJ occurs when the ED tendon moves to either the ulnar or radial sides of the midline but remains in contact with the dorsal aspect of the MC head during MCPJ flexion. The tendon subluxates to the opposite side of the partial tear, due to force applied to the central tendon by the uninjured sagittal band.¹¹ Subluxation is most obvious with MCPJ flexion (forming a fist).²² For example, if there is a radial-sided sagittal band partial tear, the ED tendon will subluxate to the ulnar side (Figure 10).

Complete sagittal band tears (ruptures) demonstrate a gap between radial or ulnar aspects of a sagittal band with short-axis sonographic imaging. This results in lack of continuity of the sagittal band surrounding the ED tendon at the MCPJ level. Complete sagittal band tears can result in extensor tendon subluxation or dislocation, and the extent of tendon displacement and distinction between tendon subluxation and dislocation must be appreciated. The gap in the sagittal band and subsequent tendon instability may not be obvious with static imaging with the MCPJ in extension, so dynamic sonographic assessment with the MCPJ in multiple degrees of flexion is required.

Transient subluxation of the extensor tendon with flexion involves maintenance of contact of the tendon with the dorsal metacarpal condyle. Dislocation of extensor tendons involves displacement of the tendon into the groove between adjacent dorsal MC heads (valley between adjacent knuckles) and loss of contact with the dorsal aspect of the metacarpal head.^{16, 32} In complete sagittal band tears of the third and fourth MCPJs, ED tendon dislocation occurs when the tendon moves to the opposite side of the MC head relative to the side of the sagittal band tear. Subluxation or dislocation is best demonstrated when the finger of interest is flexed to touch the palm of the hand (Figure 11 and Video 1).

When complete sagittal band tears occur to the second and fifth MCPJs, due to the presence of multiple tendons, ruptures of the connections between these tendons have been identified to also occur and one of the extensor tendons may displace to the radial side and one to the ulnar side of the MC head relative to midline.¹⁹ The radial sagittal band is reported to be more susceptible to injury; this theory has been proposed as the radial sagittal band has been identified to be thinner and longer than the ulnar component on cadaveric studies.^{33, 34} Ulnar-sided sagittal band tears although not as common, are still encountered, and traumatic lacerations can be a cause.^{6, 11}

Radial subluxation of the ED tendon may occur following a traumatic laceration to the ulnar sagittal band.¹⁶ Complete tearing of the ulnar sagittal band doesn't contribute to the same degree of extensor instability with MCPJ flexion or extension as tears of the radial sagittal band, which has been attributed to the juncturae tendini.⁵ Sagittal bands may also become torn in repetitive injuries and conditions such as rheumatoid arthritis where it is associated with chronic synovitis.¹⁶ In arthritic patients, the superficial layer of the sagittal bands has been reported to rupture spontaneously from light, normal daily activity such as snapping, crossing a finger or crumpling paper.¹⁴

The extensor tendons may be concurrently partially torn in association with a sagittal band tear. Partially torn extensor tendons may sonographically appear increased in thickness and decreased in echogenicity in comparison with the contralateral asymptomatic limb. In addition, disrupttion to the fibrillar echotexture will be identified. Trauma to the sagittal bands may also result in a concurrent structural injury involving the MCPJ capsule, juncturae tendinum, palmar plate, intrinsic muscles of the intermetacarpal spaces and osteochondral fractures.⁵ The dorsum of the MCPJ can also be infected via a puncture wound that occurs when the blow occurs to the open mouth with a clenched fist.¹³ This is known as ‘fight bite’. The bite can cause tears to the sagittal bands and extensor tendons, and the wound can cause infection that can extend deeper to involve the MCPJ and bones.²²

Plain hand radiographs following trauma to the dorsum of the hand are required to exclude or identify any fractures. Magnetic resonance imaging (MRI) can be utilised to image structures of the hand including the collateral ligaments of the MCPJs of digits 2–5 which, due to their position between the MC heads can be better imaged with MRI. The extensor hood of the hand may require MRI sequences to be obtained with the MCPJ in the maximum flexion and extension to demonstrate any dislocation or subluxation of the extensor tendons. Ultrasound imaging has the advantage of being dynamic and quick and can be performed in an emergency setting. Direct transducer pressure over the torn sagittal bands in the acute setting, however, can cause some patient discomfort. As ultrasound is an operator-dependent imaging modality, knowledge of the anatomy, mechanisms of injury and sonographic technique is also required to allow the structures of the dorsal hand to be optimally imaged.

Different methods of treatment of sagittal band tears include conservative management or surgical repair, and optimal management of sagittal band tears remain undefined.⁵ The main aim is to prevent the re-dislocation of extensor tendons and maintain the MCPJ motion. Conservative management involves the use of extension splinting.⁵ Numerous surgical techniques have been described but mostly involve relocation of the central tendon, and direct repair of the sagittal band defect with sutures.⁵

The dorsal hood is a complex retinacular system of the hand. Injuries to the sagittal bands of the dorsal hood should be considered following blunt trauma to the dorsal hand such as boxing or punching, with subsequent pain and swelling to the dorsal knuckles and space between knuckles. The sagittal bands are the most important stabilising component of the extensor tendons and partial or complete sagittal band tears and can result in extensor tendon subluxation or dislocation, which may be clinically underappreciated. Sagittal band tears and the degree of associated tendon instability can be efficiently and effectively imaged with ultrasound; however, familiarity with the detailed relative anatomy, sonographic technique, and normal and abnormal sonographic appearances is essential to allow a timely diagnosis to optimally guide patient management.

No conflicts of interest to declare.