Analysis of screw pullout strength: a function of screw orientation in subtalar joint arthrodesis

The purpose of this cadaveric study was to compare the pullout strength and failure load of 2 different screw orientations for subtalar arthrodesis. Twenty-six specimens from 13 cadaver donors (1 left and 1 right each) were used. A 7.3- x 65-mm long-thread profile (32-mm length) cannulated screw inserted into the calcaneus from dorsal to plantar (calcaneal specimens) was compared with a 7.3- x 65-mm short-thread profile (16-mm length) cannulated screw inserted into the talus from plantar to dorsal (talar specimens). A torque screwdriver with a calibrated electronic vernier scale measured the torque of screw insertion. Screw pullout strength and load failure were measured by using a servohydraulic materials testing machine. Distraction was applied along the axis of the screw at a displacement rate of 25 mm/min. The peak torque of insertion in all calcaneal specimens was reached on initial insertion through the dorsal subchondral bone plate of the calcaneus; in talar specimens, it was reached as the screw threads were completely buried into the talus. A significant difference (P = .00647) was found between the mean torque of insertion for the calcaneal (1.50 Nm) and talar specimens (1.30 Nm). A comparison of pullout forces showed a significantly stronger mean failure load for calcaneal specimens (P = .000085). The mean failure load for paired calcaneal specimens was 1782 N compared with a mean 1245 N for talar specimens. Although the pullout force was clearly greater in the dorsal-to-plantar screw application, the pullout force in the plantar-to-dorsal orientation was also considerable.