Ten sets of 25-mm-diameter bone cylinders were cut from the talus and calcaneus in fresh-thawed cadaveric feet. A load cell was compressed between cylinders with an 8.0-mm partially threaded cannulated lag screw simulating arthrodesis. For 7 sets, screws were tightened by 3 quarter-turns, rested for 3 minutes, retightened 1 quarter-turn, and rested for 30 minutes. Three sets served as controls in which screws were not retightened.

Maximum compression after initial screw tightening and retightening averaged 275 and 337 N (P 1⁄4 .07), respectively. Compression 3 minutes after initial screw tightening and retightening averaged 199 and 278 N (P 1⁄4 .027), respectively. 

The compression recorded 3 minutes after screw retightening was an average of 40% higher than that recorded 3 minutes after initial tightening. The average compression 30 minutes after screw retightening was 255 N, a compression loss of 25% from the average maximum compression after retightening. Eighty percent of this compression loss happened in an average of 5.5 minutes.

Hindfoot bones exhibit compression loss over time during simulated arthrodesis. Retightening lag screws before wound closure may improve compression at the arthrodesis site and thereby decrease the chance of nonunion. Compression maintenance in bone is improved with screw retightening. 

Further work is needed to understand the mechanism of action and determine optimum time for recompression.