Intramedullary Screw Fixation of Distal Metacarpal Fractures: A Biomechanical Study.

Abstract

Intramedullary headless compression screw (IHCS) fixation of metacarpal (MC) fractures is popular. A concern for distal fractures is the potential loss of fragment/head fixation and with intra-articular incursion of hardware. We evaluated the biomechanical stability of IHCS fixation of progressively distal MC osteotomies using a cadaveric simulated active motion model. We included 12 matched-pair fresh-frozen cadaveric hands (median age, 73.5 years; 75% men), excluding matched-pair fingers with deformities/severe arthritis at the metacarpophalangeal joint or proximal interphalangeal joint. Bone cuts were made in each MC creating a gap simulating comminuted unstable fractures. The distal edge of each gap was 15, 10, and 5 mm proximal to the dorsal edge of the MC head cartilage. Matched pairs of fingers were used for each of these osteotomy levels, with the 15 mm versus 5 mm pairs as our primary comparison group. Each MC osteotomy was stabilized with a retrograde IHCS, excluding matched pairs with any inadequate MC isthmus fit. Each hand was loaded into a simulated active finger motion model, with finger flexion and extension driven by a computer-controlled linear actuator. Each finger underwent 2,000 cycles of full extension to flexion movement to simulate 6 weeks of immediate postoperative full active range of motion. Displacement at the osteotomy sites was measured with a differential variable reluctance transducer with defined failure as >1 mm of displacement. There were significant differences in group-level failure rates and average maximum distal fragment displacement between the 15 and 5 mm osteotomy level matched pairs and the 10 and 5 mm matched pairs. Intramedullary headless compression screw fixation of distal MC gapped osteotomies was significantly less stable at 5 mm proximal to the MC head dorsal articular margin. Providers should consider these findings when advising patients on immediate full active range of motion following IHCS fixation of comminuted, unstable distal MC fractures.