Bennett's fracture

Lore & Background

The Bennett fracture is named after Edward Hallaran Bennett, Professor of Surgery at Trinity College Dublin, who described it in 1882. Bennett noted that the fracture 'passed obliquely across the base of the bone, detaching the greater part of the articular surface, and the separated fragment was very large and the deformity that resulted there-from seemed more a dorsal subluxation of the first metacarpal.' He stressed the need for early diagnosis and treatment to prevent loss of function of the thumb CMC joint, which is critical to overall hand function.

Reader's Guide

Bennett's fracture is significant because it involves the carpometacarpal (CMC) joint of the thumb, which is essential for grasp, pinch, and opposition—functions that constitute about 50% of overall hand function. The fracture is characterized by a proximal fragment that remains correctly positioned due to its attachment to the anterior oblique ligament, while the distal fragment is displaced by tension from the abductor pollicis longus (APL) and adductor pollicis (ADP) muscles. This displacement can occur even if fragments are initially aligned, making intervention necessary. Treatment ranges from closed reduction and casting for minor displacement (<1 mm) to closed reduction with percutaneous pin fixation (CRPP) for 1–3 mm displacement, and open reduction with internal fixation (ORIF) for >3 mm displacement. Regardless of method, immobilization for four to six weeks is required. Long-term outcomes are similar between CRPP and ORIF, with post-traumatic osteoarthritis developing in almost all cases; the degree of weakness and arthritis correlates with the quality of reduction. Some authors have recently disputed the role of the APL tendon as a deforming force.

Did You Know?

• Bennett's fracture is classified as 'I1' under the Metacalamity classification.
• The proximal fragment remains in correct position because it is attached to the anterior oblique ligament, which is attached to the tubercle of the trapezium.
• Tension from the abductor pollicis longus (APL) subluxates the distal fragment dorsally, radially, and proximally, and rotates it into supination.
• The fracture is often caused by punching a hard object, a fall onto the thumb, or a car crash where the steering wheel hyperextends the thumb.

Anatomy and the Forces That Drive Displacement

Bennett's fracture is an oblique, intra-articular break at the base of the first metacarpal that extends into the carpometacarpal joint, earning it the Metacalamity designation I1. It stands as the most common fracture of the thumb and is almost invariably paired with some degree of joint subluxation or outright dislocation. The reason this injury resists simple healing lies in its biomechanics. The small proximal fragment stays tethered to the anterior oblique ligament, which anchors it to the tubercle of the trapezium, keeping that piece in its correct anatomical seat. The distal fragment, however, carries the bulk of the joint's articular surface and is subject to relentless muscular pull. The abductor pollicis longus yanks it dorsally, radially, and proximally while rotating it into supination, and the adductor pollicis drags the metacarpal head palmarly. Together these forces can displace the fragments even when they initially appear well aligned, which is why some form of intervention is nearly always required to restore and maintain proper position.

How the Injury Happens

The Bennett fracture results from an axial compressive force driven against a partially flexed first metacarpal, producing an oblique fracture-dislocation that crosses the joint line. In practice, this mechanism shows up in several recognizable scenarios. A person who punches a hard surface—whether a wall or an opponent's skull or shin—subjects the thumb's metacarpal to exactly this kind of concentrated load. Cyclists are another classic population: during a fall, the thumb is typically wrapped around the handlebar in an extended position, so the impact channels force straight down the metacarpal axis. Motor-vehicle collisions present a third common pathway. When a driver grips the steering wheel and the car strikes a fixed object, the hand travels forward while the wheel rim hyperextends the thumb, reproducing the same axial compression. Although some researchers have recently challenged the long-held view that the abductor pollicis longus tendon acts as a deforming force in this injury, the initial mechanism of injury remains consistent across all these settings.

A Graded Treatment Ladder

Since Edward Hallaran Bennett first described this fracture in 1882, the central clinical principle has been the same: diagnose early and treat promptly to protect the thumb's CMC joint. Modern management is stratified by the degree of displacement at the trapeziometacarpal joint. When a radiograph shows only a tiny avulsion fragment, minimal joint instability, and less than one millimetre of subluxation, a closed reduction followed by a thumb spica cast and serial X-rays can be sufficient. Displacements between one and three millimetres are typically addressed with closed reduction and percutaneous pin fixation, in which Kirschner wires anchor the first or second metacarpal to the trapezium rather than bridging the two fracture fragments. Beyond three millimetres, open reduction and internal fixation becomes the standard recommendation. Whichever route is chosen, the patient must wear a cast or thumb spica splint for four to six weeks. The critical caveat is that these fractures can look deceptively mild on plain films while harbouring the potential for severe, lasting hand dysfunction.

The Stakes: Function, Arthritis, and Long-Term Outcome

Thumb function accounts for roughly half of all hand capability, and that capability depends on a stable, mobile CMC joint that permits opposition, pinch, and grasp. When a Bennett fracture is missed or allowed to heal in a displaced position, the consequences are severe: a painful, unstable, arthritic joint with reduced range of motion, and a hand whose overall function is markedly diminished. Even after successful surgical correction—whether by percutaneous pinning or open internal fixation—patients typically experience some degree of permanent strength loss, and post-traumatic osteoarthritis of the thumb base develops in nearly every case. The encouraging nuance is that the severity of both the weakness and the arthritis tracks closely with how precisely the fracture was reduced. Long-term outcomes between the pinning and open approaches appear broadly comparable, which means the surgeon's primary objective is not the choice of technique but the quality of the anatomic reduction achieved. In short, the closer the joint surface is restored, the better the hand will function for years to come.

Gallery