Bennett Fracture Deforming Forces
The abductor pollicis longus (APL) muscle has traditionally been cited as the primary deforming force in Bennett fractures, pulling the metacarpal shaft proximally, radially, and into supination, making reduction challenging—however, this is a fundamental misconception, as the APL is not actually a deforming force in Bennett fractures. 1
The True Biomechanical Challenge
The difficulty in maintaining reduction of Bennett fractures stems from the inherent instability created by:
- Severe deforming forces that act to displace the fracture into a varus and shortened position, regardless of the ease of initial reduction 2
- The thumb trapeziometacarpal joint's exposure to high compressive forces during prehensile hand function, which continuously stress the fracture site 2
- Loss of the joint's prime stabilizers: the volar beak of the thumb metacarpal and the dorsal radial ligament complex, which normally provide stability during power pinch and power grasp 1
Common Misconceptions to Avoid
Critical pitfall: The medical literature perpetuates the myth that the APL is a deforming force in Bennett fractures—this is incorrect and should not guide your treatment approach 1. The surgeon must understand that:
- The volar beak ligament alone is not the prime stabilizer (contrary to traditional teaching) 1
- The dorsal ligament complex plays a significant role in TMC joint function and must be assessed 1
- Stability depends on both the volar beak and dorsal radial ligament complex working together 1
Clinical Implications for Reduction
The challenge in maintaining reduction requires:
- Anatomic reduction with less than 1 mm of articular step-off to minimize long-term posttraumatic arthritis risk 2
- Recognition that even 1 mm of fracture dislocation warrants surgical treatment at the earliest possible time 3
- Understanding that good functional results can occur even with residual deformity, though anatomic reduction remains the goal 2
The screw-home-torque technique can be used to anatomically reduce acute Bennett fractures when treating closed with percutaneous fixation 1. This technique utilizes the final phase of opposition to achieve and maintain reduction 1.