Quantitative 3-dimensional CT analyses of intramedullary headless screw fixation for metacarpal neck fractures.

PURPOSE

Fixation countersunk beneath the articular surface is well accepted for periarticular fractures. Limited open intramedullary headless compression screw (HCS) fixation offers clinical advantages over Kirschner wire and open techniques. We used quantitative 3-dimensional computed tomography to assess the articular starting point, surface area, and subchondral volumes used during HCS fixation of metacarpal neck fractures.

METHODS

We simulated retrograde intramedullary insertion of 2.4- and 3.0-mm HCS and 1.1-mm Kirschner wires for metacarpal neck fracture fixation in 3-dimensional models from 16 adults. We used metacarpal head articular surface area (mm(2)) and subchondral volumes (mm(3)) and coronal and sagittal plane arcs of motion, during which we analyzed the center and rim of the articular base of the proximal phalanx engaging the countersunk entry site.

RESULTS

Mean metacarpal head surface area mated to the proximal phalangeal base in neutral position was 93 mm(2); through the coronal plane arc (45°) was 129 mm(2), and through the sagittal plane arc (120°) was 265 mm(2). The mean articular surface area used by countersunk HCS threads was 12%, 8%, and 4%, respectively, in each of these arcs. The 1.1-mm Kirschner wire occupied 1.2%, 0.9%, and 0.4%, respectively. Mean metacarpal head volume was 927 mm(3). Mean subchondral volume occupied by the countersunk portion was 4%. The phalangeal base did not overlap the dorsally located countersunk entry site through most of the sagittal plane arc. During coronal plane motion in neutral extension, the center of the base never engaged the dorsally located countersunk entry site.

CONCLUSIONS

Metacarpal head surface area and subchondral head volume occupied by HCS were minimal. Articular surface area violation was least during the more clinically relevant sagittal plane arc of motion.

CLINICAL RELEVANCE

The dorsal articular starting point was in line with the medullary canal and avoided engaging the center of the articular base through most of the sagittal plane arc. Three-dimensional computed tomography data support the use of an articular starting point for these extra-articular fractures.