Evaluation of the fracture site mechanical properties in vivo by monitoring the motion of a dynamic pin clamp during simulated walking

Objective. To monitor pin clamp motion using a displacement sensor over time during weight bearing and to investigate whether this method could contribute to quantitative assessment of the mechanical properties of fracture healing. Methods. Patients with a shaft fracture of the tibia were monitored, all of whom were in the stage of dynamization. The hifixator equipped with a ball bearing mechanism was used as a dynamic external fixator. The displacement was recorded versus time during stamping with weight bearing of the patients. The displacement curve had an oscillatory component synchronized with touch down and lift up of the foot, a time-dependent component expressed by shifting of the baseline, and an irreversible component during a non-weight bearing period after stamping. The three components were analyzed with a simple Voigt model. Results. In all patients, both the amplitude of the oscillatory component and the time-dependent component expressed as retardation time decreased as fracture healing proceeded, and by the time of fixator removal the irreversible component had disappeared. Conclusions. The monitoring method could quantitatively evaluate viscoelastoplasticity of the healing fracture site.