Bennett Fractures: A Biomechanical Model and Relevant Ligamentous Anatomy.

PURPOSE: This study examined a palmar beak fracture model to determine which thumb carpometacarpal (CMC) joint ligament is the primary ligament relevant to the pattern of injury.

METHODS: Six fresh-frozen cadaveric wrists were used. The radius, ulna, and first metacarpal were secured and tested with a materials testing system, holding the wrist in 20° extension, 20° ulnar deviation, and 30° palmar abduction of the first metacarpal. Testing consisted of preconditioning cycles followed by compressive loading at 100 mm/s. We confirmed fractures with fluoroscopy and dissected the specimens to examine the CMC joint ligaments. The metacarpal was stressed through a range of motion to determine which maneuvers reduced or displaced the fractures.

RESULTS: Our model successfully created palmar beak fractures in all cadaveric specimens. All fractures were displaced and intra-articular. The anterior oblique ligament (AOL) was thin and partially attached to the palmar beak fracture fragment. The ulnar collateral ligament was attached in its entirety to the fracture fragment and represented a thicker, more robust ligament compared with the AOL. Radial abduction and pronation of the metacarpal reduced fracture displacement. Extension of the CMC joint or tensioning the AOL did not decrease fracture displacement.

CONCLUSIONS: This model successfully created a reproducible and clinically relevant palmar beak fracture in a biomechanical setting. The primary ligament attached to the palmar beak fracture fragment was the ulnar collateral ligament, and not the AOL as previously described. These findings suggest that the AOL may not be a substantial contributor to palmar beak fracture morphology.

CLINICAL RELEVANCE: A refined description of the ligamentous anatomy of the palmar break fracture enhances opportunities for improved reduction and treatment of this common hand injury.