In Vivo Mechanical Characterization of the Distraction Callus During Bone Consolidation

Abstract

Understanding the evolution of callus mechanical properties over time provides insights in the mechanobiology of fracture healing and tissue differentiation, can be used to validate numerical models, and informs clinical practice. Bone transport experiments were performed in sheep, in which a distractor type Ilizarov was implanted. The forces through the fixator evolution were measured and the callus stiffness was estimated from these forces. Computerized tomography images were taken and bone volume of the callus at different stages was obtained. The results showed that the maximum bone tissue production rate (0.146 cm³/day) was achieved 20 days after the end of the distraction phase. 50 days after the end of the distraction phase, the callus was ossified completely and had its maximum volume, 6-10 cm³ In addition, 80-90% of the load sustained by the operated limb was recovered and the callus stiffness increased exponentially until 5.4-11.4 kN/mm, still below 10% of the healthy level of callus stiffness. The effects of the bony bridging of the callus and the time of the fixator removal on callus force, stiffness and volume were analyzed. These outcomes allowed relating quantifiable biological aspects (callus volume and tissue production rate) with mechanical parameters (callus force and stiffness) using data from the same experiment.

Comparison of methods for assigning the material properties of the distraction callus in computational models

“…For example, Figure A shows that the stiffness of the callus at 137, 205, and 311 days from surgery (time points from the same animal) is approximately three times the callus stiffness at 277, 379, and 471 days from surgery (time points from a different animal). This finding is due to the difference in volume of the callus of both animals that can be observed in 3D reconstructions (Figure ) . Figure also shows that inter‐individual differences are mainly due to callus geometry differences since the level of scatter is lower in the evolution of the mean elastic modulus compared with the callus stiffness, which depends on geometry.…”

“…A, Predicted callus stiffness until 80 days after surgery for the methods used (mechanical properties assignment according to simple manual segmentation (MA), manual segmentation‐based experiments (MAE), and according to the level of Hounsfield units (HU) versus the measured callus stiffness in vivo (Mora‐Macías et al). The line represents the ideal position of the points represented if the predictions of the callus stiffness for each time point would coincide with the value of the callus stiffness measured in vivo.…”

“…An experiment on bone transport was conducted in the metatarsus of 11 female merino sheep that were 3 to 5 years of age . The bone transport process consists of different phases.…”

“…The values were taken from those measured experimentally in vivo for each day of the distraction process according to Mora‐Macías et al (Figure B). These loading and boundary conditions simulate the mechanical conditions in the callus stiffness that cause compression of the metatarsus during gait . For each time point, the model was simulated with the corresponding load, and the displacements were calculated.…”

“…The aim of this work is to provide and compare different estimations of the distraction callus stiffness during the process of bone transport by means of CT image‐based finite element analysis. It was possible to estimate the callus stiffness until 69 weeks after surgery, increasing the time range of the in vivo measurements reported in the literature . Finite element models were obtained from CT images that were obtained during an experiment in sheep .…”