Viscoelastic Properties of the Hindfoot Bones

Abstract

Category: Hindfoot Introduction/Purpose: Obtaining and maintaining compression at an arthrodesis site is a key factor in achieving successful bony union. Bones, like other collagen containing tissues, are known to exhibit viscoelastic properties that may lead to stress relaxation at the arthrodesis site. The viscoelastic properties of the hindfoot bones when subjected to compression (as occurs during fusion surgery) are not known. The objective of this study was to quantify the viscoelastic properties of hindfoot bones under compression by measuring the time course of stress relaxation. Methods: 19 cadaveric human bone cubes 10 mm on each side consisting of trabecular and subchondral bone were cut from the hindfoot bones including the talus, calcaneus, and distal tibia. Each cube was scanned with micro computed tomography (µCT) to quantify bone volume/total volume ratio (BV/TV), trabecular thickness, trabecular separation, trabecular number, and connectivity density. Each specimen was then immersed in a saline bath and compressed 1 mm at a strain rate of 1 mm/s using an MTS machine (Fig 1). This compressed position was then held for 3 hours while the load was recorded. Following the compression test, each specimen was re-scanned with µCT. Results: Two distinct patterns of load relaxation were found. The first consisted of a uniform exponential decay. The second had a similar exponential decay but included a plateau occurring between 1-6 minutes. This second pattern was reflected in the average fractional load relaxation graph (Fig 2). The average peak load was 24.14 kg (SD ± 15.07 kg) and average end relaxation was 2.93 kg (SD ± 3.81 kg). The average time to achieve 95% decay in total load was 34.7 min (SD ± 29.1 min) although removing some outliers, it decreased to 24.9 min (SD ± 18.4 min) which is more representative of the overall data. Averages of BV/TV, trabecular thickness, and trabecular separation increased after stress relaxation while average connectivity density and trabecular number decreased. Conclusion: These data suggest that, due to the viscoelastic properties of bone, approximately 95% of an applied compressive load generated by a fixed displacement is lost within the first 30 minutes. Applied clinically, these findings may have a significant impact on the optimal surgical technique used for osteosynthesis and arthrodesis. Specifically, these data call into question whether the compression applied during surgery can be maintained throughout the healing phase without the application of continuous compression via an external fixator or internal continuous compression device. At minimum, these data suggest that lag or compression screws should be retightened prior to wound closure.