The majority of bone fractures consolidate via secondary bone healing with callus formation. It is widely accepted that the formation of fracture callus is promoted by mechanical stimulation via induced micromotions in the fracture gap. Still, experimental data describing the callus response to strain stimuli over shorter timeframes is hardly available. Therefore, we developed a system for continuous measurement of callus stiffness to explore hourly, daily, and weekly variations in the bone healing progression and to analyze the short-term response of the repair tissue to mechanical stimulation.
The system was implemented on a double osteotomy model instrumented with an active fixator . A force sensor was integrated in the fixator to continuously measure callus stiffness. A dedicated control unit was developed that enables autonomous execution of the stimulation protocol and acquisition of the experimental data. The system was implanted in four Swiss White Alpine Sheep. From the first day after surgery, the animals were subjected to a daily stimulation protocol consisting of 1000 stimulation cycles equally distributed over 12 hours from 9:00 to 21:00.
One animal was excluded due to some inconsistencies in the force sensor data. The callus stiffness started to increase after day 10 post operation. However, on a daily basis, the stiffness was not steadily increasing, but instead, an abrupt drop was observed in the beginning of the daily stimulations. Following this initial drop, the stiffness increased until the last stimulation cycle of the day. This fairly consistent pattern over 12 hours suggests that the applied mechanical stimulation firstly disturbed the healing tissue that was formed over the resting time and only later allowed for increase of tissue stiffness.
The continuous measurements of callus stiffness enabled detection of the tissue stiffening onset and resolved the short-term response of fracture callus to mechanical stimulation. The collected data describing hourly, daily, and weekly variations in callus stiffness can serve to optimize clinical rehabilitation protocols for fracture patients.
 Tufekci et al, J Orthop Res, 36:1790-1796, 2018