Abstract

Mechanical loading of bone defects through rehabilitation is a promising therapeutic approach to stimulate repair and reduce the risk of non-union; however, little is known about how therapeutic mechanical stimuli modulate early stages of repair before mineralized bone formation. In a previous study, we established an osteogenic mechanical loading protocol using early ambulatory rehabilitation and a compliant, load-sharing fixator in a rat model of BMP-2 mediated bone defect repair. The objective of this study was to investigate the early effects of osteogenic loading on cytokine expression, tissue composition, and angiogenesis during the first 3 weeks of repair in this model. Using a wireless implantable strain sensor for local measurements of mechanical boundary conditions, finite element simulations showed that osteogenic mechanical loading increased mean compressive strain in defect soft tissue during rehabilitative ambulation at 1 week (load-sharing: -1.54 {+/-} 0.17% vs. load-shielded: -0.76 {+/-} 0.06%), and that strain was amplified in remaining soft tissue regions at 3 weeks as mineralization progressed (load-sharing: -1.89 {+/-} 0.35% vs. load-shielded: -1.38 {+/-} 0.35%). Multivariate analysis of multiplex cytokine arrays revealed that loading significantly altered cytokine expression profiles in the defect tissue at 2 weeks compared to load-shielded defects. Specifically, loading reduced VEGF and increased CXCL5 (LIX) levels. Subsequently, vascular volume in loaded defects was reduced relative to load-shielded defects but similar to intact bone at 3 weeks. Endochondral bone repair was also observed histologically in loaded defects only at 3 weeks. Together, these results demonstrate that moderate ambulatory strains previously shown to stimulate functional bone regeneration significantly alter early angiogenic and cytokine signaling and may promote endochondral ossification in large segmental bone defects. Authors ContributionsB.S.K., N.J.W., and R.E.G. designed the research and performed surgeries; B.S.K., C.E.V., and J.K. performed experiments; B.S.K., C.E.V., J.K., and L.B.W., analyzed data; B.S.K., C.E.V., N.J.W., and R.E.G. wrote the manuscript; All authors interpreted data, critically edited, and have read and approved the final manuscript.