Practice Essentials
In 1882, Edward Hallaran Bennett, MD, described the fracture of the base of the first metacarpal that bears his name. [1] Bennett described the anatomic details of the fracture and suggested that early diagnosis and treatment are imperative to prevent loss of function of this highly mobile joint. [2, 3, 4, 5]
Unless properly recognized and treated, this intra-articular fracture subluxation may result in an unstable arthritic joint with secondary loss of motion and pain. Because the thumb carpometacarpal (CMC) joint is critical for pinch and opposition, this injury may severely affect function.
Closed reduction and thumb spica cast immobilization are effective in the treatment of Bennett fractures if the reduction can be maintained. The closed reduction technique consists of thumb traction combined with metacarpal extension, pronation, and abduction. Direct downward pressure is applied to the dorsal radial metacarpal base.
Generally, closed reduction utilizing the technique described above followed by percutaneous Kirschner wire (K-wire) fixation is successful. If adequate reduction cannot be achieved by means of this percutaneous technique, open reduction with internal fixation (ORIF) is performed.
Anatomy
The thumb affords prehensile abilities that were essential in human evolution. The bony anatomy of the thumb consists of two phalanges and a metacarpal, which articulates with the trapezium bone in the distal carpal row. The metacarpal is actually a primordial phalanx.
The CMC joint consists of an articulation between the trapezium and the metacarpal base composed of two reciprocally interlocking saddles with perpendicular longitudinal axes. Ligamentous stability at the trapeziometacarpal joint is maintained by the anterior (volar) and posterior oblique ligaments, the anterior and posterior intermetacarpal ligaments, and the dorsal radial ligament.
The anterior (volar) oblique ligament originates on the trapezium and inserts into the volar ulnar beak of the thumb metacarpal. This is the most important ligament in maintaining CMC stability. The dorsal ligament is not as strong as the volar ligament but is reinforced by the abductor pollicis longus (APL).
A cadaveric biomechanical study by Kang et al suggested that the importance of the ulnar collateral ligament in Bennett fractures may have been underestimated. [6]
Pathophysiology and Etiology
The thumb is a highly mobile border digit. For that reason, injury to this ray is common. Thumb CMC joint stability is maintained by five ligaments and the articular contours. The most critical of these stabilizers is the volar oblique ligament. This ligament courses from the volar lip of the trapezium to the volar ulnar corner of the thumb metacarpal base.
A Bennett fracture occurs when an axial force is transmitted through a partially flexed thumb metacarpal. [7] The portion of the metacarpal onto which the volar oblique ligament inserts remains in anatomic position, and the remainder of the articular base subluxates in a dorsal, radial, and proximal direction because of the pull of the APL.
Prognosis
The prognosis for Bennett fractures is most closely related to the amount of energy associated with the original injury. High-energy injuries produce comminution, articular surface damage, and extensive soft-tissue injury, leading to a poor outcome. With anatomic restoration of the joint surface and reestablishment of stability, the outcome is routinely good, especially in low-energy injuries with simple fracture patterns and limited soft-tissue involvement.
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Radiograph of a Bennett fracture.
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Percutaneous pinning of a Bennett fracture.
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Rolando fracture. This is differentiated from a Bennett fracture because of the presence of intra-articular comminution.
