In vivo study of human mandibular distraction osteogenesis. Part II: Determination of callus mechanical properties.

Distraction Osteogenesis (DO) is a surgical technique used to reconstruct bone defects. To improve the current treatment protocols, the knowledge of the mechanical properties of the bone regenerate is of major interest. The aim of this study, constituting the second part of our paper previously published in Acta of Bioengineering and Biomechanics, was to identify the elastic and viscous properties of bone callus. This is done in the case of a mandibular DO by analyzing the experimental measurements of the forces imposed on bone regenerate by a distraction device. The bone transport forces were evaluated thanks to strain gauges glued on the distraction device. A rheological model describing the callus constitutive behavior was developed and the material constants involved were identified. The time-dependent character of the bone regenerate mechanical behavior was confirmed. The viscous response of the mesenchymal tissue was described by two characteristic times. The first one describing the viscoelastic callus behavior was estimated to be 140 seconds and the second one representing the permanent bone callus lengthening was evaluated to be 5646 seconds. An average value of 0.35 MPa for the regenerate Young's modulus was deduced. The elastic properties of mesenchymal tissue found are in agreement with the rare data available in the literature.

In vivo study of human mandibular distraction osteogenesis. Part I: bone transport force determination.

Distraction Osteogenesis (DO) is a surgical technique used to reconstruct bone defects. The evolution of forces acting during DO is known to be strongly influencing the clinical issue of the treatment. The aim of this study was to determine experimentally the time-dependent forces imposed on bone regenerate by a distraction device in the case of a mandibular DO consecutive to a gunshot wound. To evaluate the bone transport forces, some fixing pins of the distraction device were equipped with strain gauges. Measurements were done during the first weeks of the treatment. An equilibrium analysis was achieved to determine the forces acting in bone regenerate from strains in the pins. Those quantities evolved during the records approximately from 5 N to 3-4 N and 2 N to 0-0.3 N for the tension and shear forces, respectively, depending on the record duration. For the longest record, the callus lengthening reached 0.17 mm during 75 minutes. This decrease of force and simultaneous callus extension can be attributed to the viscosity of regenerate and the elastic energy release of the device. Essential data were obtained concerning forces, extension and their evolution during mandibular DO. The low force level obtained was attributed to the absence of resistance of the soft tissues in the case of ballistic trauma restoration.

Modeling of bone adaptative behavior based on cells activities.

Bone remodeling occurs in an adult's skeleton to adapt its architecture to external loadings. This involves bone resorption by osteoclasts cells followed by formation of new bone by osteoblasts cells. During bone remodeling, osteoclasts and osteoblasts interact with each other by expressing autocrine and paracrine factors that regulate cells' population. Therefore, changes in bone density depend on the amount of each acting cell population. The aim of this paper is to propose a model for the bone remodeling process, which takes into account the opposite activity of both types of cells. For this purpose, a system of differential equations, proposed by Komarova et al. (Bone 33:206-215, 2003), is introduced to describe bone cell interactions using parameters which characterize the autocrine and paracrine factors. Such equations allow us to determine how the autocrine and paracrine factors vary in response to an external stimulus. It is assumed that an equilibrium state can be obtained for values of stimulus near to some reference quantity. Far from this value, unbalanced activity of osteoblasts and osteoclasts is observed, which leads to bone apposition or resorption. The proposed model has been implemented into the finite element software ABAQUS to analyze the qualitative response of a bone structure when subjected to certain mechanical loadings. Obtained results are satisfactory and in accordance with the expected bone remodeling behavior.