Mechanobiology of distraction osteogenesis

Abstract

Distraction osteogenesis (DO) is a widely used treatment for debilitating skeletal pathologies requiring the formation of a large volume of new bone. The results can be truly life-changing, but a high complication rate of approximately 10-33% persists. In order to reduce the prevalence of severe complications, a better understanding of the mechanobiology is critical. Predictive numerical models based on quantitative, mechanobiological tissue differentiation theories could allow optimization of treatment protocols and fixation devices. A robust numerical simulation must reproduce the viscoelastic response and mechanical evolution of the callus tissue during distraction to appropriately implement a mechanoregulatory hypothesis. Furthermore, histological response to strictly isolated cyclic stimulation must be examined for the various deformation modes present during ambulation in the maturation phase. Such data is invaluable for the validation of the simulation and further development of tissue differentiation hypotheses. To meet these goals, two electromechanical, external fixators were developed to perform distraction and cyclic stimulation in an in vivo, ovine, lateral DO model applied to the tibia. The surgical procedure maintains the mechanical competence of the bone, eliminating the unquantified mechanical stimulation of physiological loading present in the healing region of osteotomy models. Two compressive magnitudes were chosen to simulate stable and unstable fixation. One magnitude was chosen for tension and for shear to create deviatoric strain in the tissue approximately equivalent to the large magnitude compressive stimulation. During the distraction phase, only fibrous connective tissue was present in the distraction gap. The intrinsic resistance in the tissue, quantified by the average instantaneous relaxation modulus, increases from approximately 2 kPa to 1100 kPa while the equilibrium modulus increases from approximately 0 kPa to 200 kPa. A Prony series was used to model the relaxation process and tissue evolution. Subsequent histological studies were performed to quantify the influence of isolated cyclic stimulation on tissue differentiation and revascularization during the maturation phase of DO. The analysis revealed that only intramembranous bone formation could be seen in the healing zone independent of the magnitude of tissue strain or direction of IFM. Our extremely stable distraction procedure suppressed chondrocyte differentiation during stimulation with magnitudes that typically lead to endochondral ossification in the distraction and maturation phases of osteotomy-based callus distraction studies. Mesenchymal stem cells show up-regulation of osteogenic genes under tensile strain whereas compression leads to expression of genes contributing to cartilaginous differentiation. We hypothesize that pure tensile strain in the callus tissue during very stable distraction induces a strong osteogenic differentiation of mesenchymal stem cells. We have shown that a high, approximately equal vessel density results from both small and large amplitude cyclic compression, while tension and shear result in significantly lower vessel density. Additionally, the larger compressive stimulation led to significantly more bone formation than the small compressive stimulation. In comparison to the small amplitude compressive stimulation, the larger purely tensile and shearing movement also increased the bone formation during the maturation phase. However, bone formation was approximately twice as high for large compression as tension or shear. These results suggest that the new bone formation under cyclic deformation may be driven primarily by the magnitude of tissue strain when sufficient perfusion is available. The reduced bone formation in tension and shear may be due to the reduced vessel formation resulting from the specific mode of IFM. These fixator systems allowed us to investigate the time-dependent mechanical response of the callus tissue during distraction as well as characterize the temporal evolution of the intrinsic material properties. This is a critical step towards calibrating Ulm’s advanced simulation of distraction osteogenesis. Furthermore, these are the first experiments to apply truly isolated modes of mechanical stimulation directly to the distraction callus. The data generated can be used to inform clinical device design and treatment protocols. Our results indicate it is the stability of the distraction phase that critically determines the progression of healing. Based on our results, the best bone healing for distraction procedures can be expected when a very stable distraction phase applying purely uniaxial tensile strain is followed by moderate cyclic axial compression during maturation due to weight bearing activity.