The Safety of Volar to Dorsal Percutaneous Screw Fixation of Bennett Fracture-Dislocation - A Cadaveric Study.

Bennett fractures are traditionally fixed with percutaneous K-wires from dorsal to volar, or with a volar to dorsal screw via a volar open approach. While volar to dorsal screw fixation is biomechanically advantageous, an open approach requires extensive soft tissue dissection, thus increasing morbidity. This study aims to investigate the practicality and safety of Bennett fracture fixation using a percutaneous, volar to dorsal screw, particularly with regard to the median nerve and its motor branch during wire and screw insertion. Fifteen fresh frozen forearm and hand specimens were obtained from the University of Auckland human cadaver laboratory. A guidewire is placed under image intensifier from volar to dorsal with the thumb held in traction, abduction and pronation. The wire is passed through the skin volarly under image intensifier, then the median nerve is dissected from the carpal tunnel and the motor branch of the median nerve (MBMN) is dissected from its origin to where it supplies the thenar musculature. The distance between the K-wire to the MBMN is measured. In 14 of 15 specimens, the wire was superficial and radial to the carpal tunnel. The mean distance to the origin of the MBMN is 6.2 mm (95% CI 4.1-8.3) with the closest specimen 1 mm away. The mean closest distance the wire gets to any part of the MBMN is 3.7 mm (95% CI 1.6-5.8); in two specimens, the wire was through the MBMN. Wire placement, although done under image intensifier, is subject to significant variation in exiting location. While research has shown the thenar portal in arthroscopic thumb surgery is safe, our guidewire needs to exit further ulnar to capture the Bennett fracture fragment, placing the MBMN at risk. This cadaveric study has demonstrated the proposed technique is unsafe for use.