Maximizing safety in screw placement for posterior facet fixation in calcaneus fractures: a cadaveric radio-anatomical study.

BACKGROUND: Successful screw fixation of reduced posterior facet fragments to the unexposed, nondisplaced sustentaculum tali avoids breaching the subtalar joint or disrupting surrounding soft tissue structures. Safe passage for screw fixation through this narrow bony corridor has not been rigorously defined.

METHODS: Computed tomography scans of 8 cadaveric feet were digitally reconstructed in 3-D; 3.5-mm-diameter screws were simulated, aiming at the center of the sustentaculum tali from 5 locations (0%, 25%, 50%, 75%, and 100%) along the posterolateral facet joint. The range of entry points, screw paths trajectories, and screw lengths that did not breach the subtalar joint or the medial calcaneal cortex were evaluated.

RESULTS: To prevent violation of the subtalar joint or the medial calcaneal cortex while reaching the center of the sustentaculum tali, screws must be inserted at least 5 mm below the joint line. Screw placement 15 ± 1 mm below the posterior facet measured perpendicular to the joint line provided the widest safe corridor with the trajectory of the ranges from 6 to 36 degrees parallel to the joint depending on the location along the posterior facet and 20 ± 2 degrees perpendicular to the joint at all locations. The average maximal length of screws placed at the ideal entry points ranged from 44 to 46 mm, longest at the 100% location and shortest at the 25% location.

CONCLUSIONS: Operative guidelines facilitating instrumentation into the sustentaculum tali have been defined applying to most calcanei, assuming the fractures are well reduced: screws, approximately 40 mm in length, should be started 15 mm below the posterior facet measured perpendicular to the joint line and aimed 20 degrees perpendicular to the joint line toward the joint and 6 to 36 degrees anteversion parallel to the joint line increasing at each position from anterior to posterior.

CLINICAL RELEVANCE: The operative guidelines described in this study may assist surgeons in the placement of screws for the fixation of posterior facet fragments to the sustentaculum tali.