Bennett fracture is an intraarticular fracture involving the base of the thumb and leading to a subluxation of the carpometacarpal joint. It was described initially by Edward Hallaran Bennett in the late nineteenth century.
The fracture is inherently unstable and requires adequate reduction and immobilization or fixation.
Carpometacarpal joint is critical for pinching and opposition movements and this injury may lead to loss of function if not treated properly.
The thumb consists of two phalanges and a metacarpal. The metacarpal ends in the carpometacarpal joint.
The carpometacarpal joint is an articulation between the trapezium and the base of the first metacarpal.
It is a saddle-shaped joint.
It is stabilized by the anterior (volar) and posterior oblique ligaments, the anterior and posterior intermetacarpal ligaments, and the dorsal radial ligament. The anterior oblique ligament is the most important for stability in the carpometacarpal joint.
The dorsal ligament is not as strong as the volar ligament but is reinforced by the abductor pollicis longus (APL).
Pathophysiology and Causes
Thumb injuries are common.
A Bennett fracture results when an axial force is transmitted through a partially flexed thumb metacarpal. This is an intra-articular fracture of the base of thumb separating small part of the metacarpal onto where the volar oblique ligament inserts from rest of the thumb. The part where volar oblique ligament inserts remain in the anatomic position, and the remainder of the articular base subluxates.
The direction of the subluxation is dorsal, radial, and proximal because the fragment is pulled by the force of abductor pollicis longus.
Clinical Presentation of Bennett Fracture
There would be a history of trauma followed by immediate pain and swelling or ischemia. AS noted before axial compression when thumb in the flexion is the most common mechanism
On examination, there would be visible deformity and swelling.
Bruises over carpometacarpal joint area may be present. There would be tenderness to touch.
There is difficulty in moving the thumb and decreased pinch grasp and grip strength.
In suspected cases, stress to the joint may reveal the instability/subluxation.
Standard AP, lateral and oblique x-rays are ordered.
- True AP of thumb (Robert’s View)
- Arm in full pronation with dorsum of thumb on cassette
- True lateral of thumb
- hand pronated 30 degrees and beam angled 15 degrees distally
Most of the fractures and subluxations are clearly visible.
Subtle subluxations may be suggested by a broken V sign on the lateral x-ray. A normal V is formed by the radial aspect of the trapeziometacarpal articulation. Disruption of this may indicate undetected CMC joint subluxation.
CT could help in the identification of comminution and find any suspected impaction of the articular surface.
Treatment of Bennett Fracture
Small avulsion fractures with minimal articular incongruity and instability can be managed by thumb spica cast mobilization after closed reduction.
The patients are monitored with x-rays regularly. Greater than 1 mm of articular incongruity initially or at follow up after closed reduction is an indication for surgery.
Nonoperative treatment is not considered in
- Open fracture
- Unstable fracture
- Articular incongruity greater than 1 mm
- Joint subluxation
More than 1 mm of articular incongruity or persistent subluxation after closed reduction indicates the need for surgical treatment because articular incongruity is associated with an increased rate of articular degeneration.
Two 0.045-in. K-wires are used to fix the fracture. The wires are drilled through the dorsal radial thumb metacarpal base into the volar ulnar fragment after reduction.
A very small fragment would not be able to hold wires and in such cases, reduction may be maintained by placing the K-wire through the fragment into the trapezium or the index metacarpal
If the percutaneous technique does not result in acceptable reduction or fixation, open reduction and internal fixation is done.
Some authors recommend open reduction straightaway if displacement is more than 3 mm.
The fracture is then fixed by K-wires or mini-screws (2.0 mm).
After radiological evidence of union, immobilization is removed and the patient is put on joint mobilization, strengthening & flexibility exercises.
The patient may also require dexterity re-education.
- Degenerative arthritis
- Loss of motion
Articular incongruity leads to degeneration but in some cases, impact initial oseocartilaginous injury leads to arthritis.
Prolonged immobilization is also associated with loss of motion also occurs following prolonged immobilization
The energy associated with original injury is the most important prognostic factor in Bennett fracture.
Comminution, articular surface damage and extensive soft-tissue injury often are associated with poor outcomes.
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