Avoiding Fifth Metatarsal Intramedullary Screw Head Cuboid Impingement: A Weightbearing Computed Tomography Anatomic Study.

BACKGROUND: Multiple case reports of fifth metatarsal (MT) intramedullary fixation highlight symptomatic hardware with screw head impingement on the cuboid. We developed a fifth MT intramedullary screw trajectory model using weightbearing computed tomography data. The goal was to assess for cuboid impingement with simulated intramedullary screw position.

METHODS: For 20 weightbearing foot computed tomographs (CTs), an automated tool was used to simulate fifth MT screw fixation in the ideal trajectory down the shaft and with a 7-mm screw head. (1) The closest distance from the simulated ideal trajectory to the cuboid in 3 dimensions was measured. A measurement less than 3.5 mm (the radius of the screw head) indicated screw head impingement on the cuboid if not countersunk into the metatarsal. (2) In 3 dimensions, a simulated screw head was then advanced from the proximal tip of the metatarsal distally into the metatarsal until it was entirely avoiding the cuboid.

RESULTS: In this model, 95% (19/20) of the patients would have cuboid impingement if the screw was not countersunk. The average ideal pin start distance was 0.15 mm (SD 2.4 mm) inside the cuboid. In this cohort, the screw head would have to be countersunk an average of 8.1 mm (SD 2.7 mm) relative to the proximal tip of the metatarsal to avoid cuboid impingement. For all cases, the simulated fluoroscopic oblique view was a reliable indicator of cuboid impingement, demonstrating visible overlapping of the screw with the cuboid. The overlap resolved on the oblique foot view once the screw was sufficiently countersunk, confirmed on 3-dimensional imaging.

CONCLUSION: The ideal guidewire placement for fifth MT intramedullary fixation is directly against the cuboid. Approximately 95% of patients would have cuboid impingement if the screw is not countersunk. The oblique fluoroscopic view of the foot is a reliable assessment of screw head impingement on the cuboid.

LEVEL OF EVIDENCE: Level III, retrospective study.