Injury to the Base of the Thumb

DESCRIPTION

A 29-year-old right hand dominant man sustained injury to the base of the thumb during a physical altercation. His symptoms included numbness and tingling across the dorsal aspect of the thumb base and in the first web space.

QUESTIONS

1. What is the diagnosis?

2. What are the other types of fractures found at the base of the first metacarpal joint?

3. What is the mechanism of injury?

4. What are the presenting signs and symptoms?

5. What are the treatment options?

Figure 1. Classification of thumb metacarpal base fractures.

The radiograph shown in the figure demonstrates a minimally displaced intra-articular oblique fracture at the base of the thumb metacarpal with diffuse, local soft tissue swelling consistent with a Bennett's fracture.

A Bennett's fracture is a noncomminuted, intra-articular fracture through the volar-ulnar aspect of the thumb metacarpal base with subsequent dislocation of the thumb carpometacarpal (CMC) joint. Dr Edward H. Bennett first described this eponymous fracture in 1882 during a series of cadaver dissections.

Nearly 80% of all thumb metacarpal fractures involve the base and can be further subdivided into 4 distinct groups on the basis of the level and pattern of fracture: basal extra-articular, Bennett type (2-part fracture), Rolando type Y and T (3-part fracture), and comminuted (4- or more-part fracture).

These types of thumb metacarpal base fractures often share a common injury pattern. The mechanism of injury to the patient typically involves a direct axial blow against a partially flexed, extended, or abducted thumb, as can happen during a fist fight or falling on an outstretched hand. The resulting anatomic derangement of a Bennett's fracture can be explained by examining the tough capsular ligaments of the thumb CMC joint.

Figure 2. Drawing of bennett's fracture (Dial et al, 1972). The tendon of the abductor pollicis longus (a), the thumb metacarpal (b), and the trapezium (c) are depicted.

Four ligaments have classically been described as part of this joint: (1) the posterior oblique ligament, which extends from the dorsoulnar tubercle of the trapezium to its palmar-ulnar tubercle at the base of the thumb metacarpal, (2) the intermetacarpal ligament which connects the bases of the thumb and index finger metacarpals, (3) the dorsoradial ligament which originates from the dorsoulnar tubercle of the trapezium and inserts into the dorsal rim of the thumb metacarpal base, and finally (4) the anterior oblique ligament, which extends from the palmar tubercle of the trapezium to the ulnopalmar aspect of the thumb metacarpal base. In addition, an ulnar collateral ligament can sometimes be recognized as part of the thumb CMC capsular ligament structure. However, the most critical ligament involved in a Bennett's fracture is the anterior oblique ligament, also known as the “volar oblique ligament,” which has been described as opposing the action of the abductor pollicis longus tendon, which also inserts at the base of the thumb metacarpal. In a Bennett's fracture, the ulnopalmar aspect of the thumb metacarpal base (where the volar oblique ligament inserts) remains in anatomic position, while the larger metacarpal fragment is displaced proximally and radially because it remains attached to the abductor pollicis longus tendon.

Patients usually present following a fist fight, with pain and swelling at the thumb base. On examination, there is limited motion at the CMC joint, and mild stress here reveals disproportionate instability. Standard posteroanterior, lateral, and oblique radiographs will confirm the diagnosis. If needed, CT or traction radiography can be used to assess the presence or extent of comminution.

Several methods to treat this particular type of fracture have been described; however, the most appropriate treatment for Bennett's fractures is still debatable. Options include closed reduction with percutaneous pinning and open reduction.

Closed reduction with percutaneous pinning can be used if a 1-mm or less articular step-off between the metacarpal shaft and the Bennett fragment can be accomplished. In this technique, the fracture is reduced by pronating the hand, applying longitudinal traction, and also applying pressure to the thumb metacarpal base. A Kirschner wire (K-wire) is then anchored transversely across the fracture fragments and an additional wire through the trapeziometacarpal joint for additional stabilization may be used. If the fracture is stable with minimal displacement, often the patients can be treated conservatively with closed reduction and casting only.

If indicated, the patient can also be treated with open reduction and internal fixation. In this case, an L-shaped incision is made over the metacarpal base and extended radially with reflection of the thenar musculature for joint exposure. K-wire, plates, and 2 mm cortical lag screws can then be utilized for fixation. Patients are then casted in a thumb spica splint for 4 to 6 weeks followed by thermoplastic casting until fully healed. The most common long-term complications resulting from this injury include premature joint arthritis and limited joint mobility.