| Summary: | Category: Hindfoot; Ankle; Ankle Arthritis Introduction/Purpose: Non-union and delayed union are common complications following hindfoot arthrodesis. The failure to obtain and maintain compression at the arthrodesis site may be a causative factor. Partially threaded cannulated ‘lag’ screws are commonly used for fixation in hindfoot arthrodesis; however, the ability of these screws to maintain compression in the hindfoot bones has not been well-characterized. The goals of this work were to: 1) quantify the stress relaxation response of hindfoot bones when compressed with a lag screw, particularly compression change upon screw retightening, and 2) compare the results to prior studies on stress relaxation in direct bone compression. Methods: 12 pairs of calcaneus, talus and tibia bone cylinders, 25mm in diameter, were cut with a keyhole saw from fresh thawed cadaveric feet. The bone cylinders were prepared for simulated arthrodesis by removing the cartilage and flattening the subchondral surface with an oscillating saw. A Futek LTH 300 donut load cell and two metal washers were sandwiched between the two bone cylinders. An 8.0mm partially threaded cannulated lag screw (Smith and Nephew) was placed from the posterior- inferior aspect of the calcaneus cylinder to the anterior-superior aspect of the talus cylinder to simulate subtalar arthrodesis, or from the superior aspect of the tibia cylinder to the inferior aspect of the talus cylinder. Compression was recorded continuously as the screws were tightened by three-quarters of a turn and left untouched for 3 minutes. Recording continued as the screws were retightened one quarter-turn and then left untouched for 30 additional minutes. Results: Both initial and subsequent compression with a lag screw demonstrated a stress relaxation pattern, with increased maximum compression and slowed decay upon retightening. Maximum compression after three quarter-turns and retightening averaged 284 N and 351 N, respectively (n=12; t=-2.55; p=0.0136). Time to decay to 80% of maximum compression after three quarter-turns and retightening averaged 34 and 528 (n=9; t=-2.59; p=0.0159) seconds, respectively, compared to 21 seconds for direct compression. Compression loss 30 minutes after retightening averaged 25.5% (SD=8.8%), compared to 34.3% for direct compression. Conclusion: The maximum compression attained following retightening was significantly greater than the maximum compression attained after initial tightening. The time required to reach 80% of maximum compression was significantly longer after subsequent compared to initial maximum compression. These findings suggest that screw retightening before surgical wound closure may slow compression loss, which could increase the likelihood of successful arthrodesis. Lag screw compression resulted in a lower percent compression loss and loss rate compared to direct compression, which was previously used to describe bone viscoelasticity.
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