Mechanobiology of bone regeneration: distraction osteogenesis and tissue engineering

Abstract

Bone regeneration is the natural process of bone reconstruction during the continuous adult remodeling, fracture healing, or complex clinical treatments of bone defects. In this biological problem, the role of engineering is essential for several aspects, including the control of the callus mechanical environment, predicting the complex mineralization phenomena, or designing materials and structures to optimize angiogenesis. The aim of this thesis is to find mechanical solutions to current challenging limitations in this field: the current mechanical incompatibility between brittle implants and external fixations in load-bearing models; the insufficient knowledge of the surrounding soft tissues’ mechanics during the elongation; the lack of understanding of the mechanical environment that promotes mineralization; or the need of quantitative tools for better clinical control of the processes. Distraction osteogenesis and tissue engineering experiments were carried out on merino sheep. A flexible Ilizarov-type external fixator was initially implanted in their right-back metatarsus. The distraction group underwent an osteotomy in a middle cross-section of the bone that, after a latency period of 7 days, was elongated to a final length of 15 mm (distraction rate of 1 mm/day). By cons, the tissue engineering group had 15 mm of bone directly replaced with a porous bioceramic scaffold biologically enhanced with cancellous bone autograft. Animals were slaughtered at different time-points of the consolidation phase. Before surgeries, an instrumented external fixator was devised to control bone formation through mechanical parameters indirectly. By working with a low-cost and size-optimized real-time wireless acquisition system, they reported promising results in estimating callus forces and stiffening. The resulting versatile system offers mechanobiological comparisons of the evolutions of different treatments. Concerning the scaffold design, a standardized optimization problem was defined and applied to optimize their inner architecture by covering biological parameters (porosity, pore size, and surface area) and mechanical constraints to ensure its structural integrity in vivo. During the distraction phase, higher viscoelastic forces with a similar relaxation rate were quantified compared to other distraction processes without limb elongation. After applying rheological models to the raw data, the mechano-structural changes suffered by this bone callus and its surrounding soft tissues were estimated. The contribution of these tissues to the distraction forces was limited to elongations above 4% of the original length when anatomical changes were evidenced. Likewise, the quantified elastic callus stiffening was mathematically modeled to elucidate the mechano-structural phenomena behind the callus accommodation-to-mineralization. It was based on ex vivo confocal imaging data applied to non-mineralized callus samples. The continuous collagen reorientation, densification and maturation seemed to control this problem. In the consolidation stage, several in vivo monitoring techniques were employed to build an exhaustive comparison between processes: gait analysis, internal force distribution, callus stiffening, x-rays, and CT imaging. All the parameters analyzed tended to healthy values in both groups but at different rates. In the short-term, the bone callus recovered its loading capacity and it exponentially increased its stiffness, being faster in the tissue engineering group. The healthy callus mineral density and limb bearing capacity were reached around 5-6 months after surgery. However, the recovery time of this last parameter seems to have been influenced by the walking conditions acquired by the animals due to pain and low confidence in the treated limb in the early stages. Finally, both the apparent geometry of the callus and its trabecular microstructure seemed to recover in the long remodeling phase.