Biomechanical and microstructural evaluation of newly formed bone following static/dynamic bone growth modulation in an immature animal model.

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CONTEXT: Fusionless medical devices are one of the most promising treatments of pediatric spinal deformities due to their capacity to prevent spinal arthrodesis and repetitive/staged surgeries as in current instrumentation treatments. They are based on the mechanical modulation of bone growth, a process by which forces can modulate bone growth rate. To correct the deformities, these implants restrict unilateral bone growth on the convexity of the spinal curvature by locally increasing compressive stress over vertebral growth plates, where bone growth occurs. Up to now, experimental studies have mainly focused on the effects of static and/or dynamic compression on the growth plates, while the impact of growth modulation on the quality of « newly formed bone », emerging below the growth plate, has not been characterized neither in its microarchitecture nor its biomechanics. With rapidly increasing interest and developments in these fusionless implants, it becomes very important to identify which loading parameters transferred by these devices will impact on the quality of newly formed bone to further guide their design for the treatment of pediatric skeletal deformities. OBJECTIVES AND METHODOLOGY: This program comprises two main research objectives: (I) to characterize the effects of in vivo static vs dynamic growth modulation on the biology and microstructure of newly formed trabecular bone; (II) to evaluate the effects of in vivo static vs dynamic growth modulation on the biomechanical properties of newly formed trabecular bone. As a logical extension of previous work on growth plate mechanobiology, the effects of finely controlled static or cyclic loads on newly formed bone will be investigated using our rat tail model. Static or dynamic compression will be applied using our air-driven device implanted on the rat tail to load the 7th caudal vertebra. In vivo monitoring of newly formed bone microarchitecture will be done using micro-CT imaging to evaluate typical trabecular and cortical parameters. A finite element approach, based on extracted micro-CT geometrical and mechanical data, will be developed and used to non-destructively estimate the mechanical properties of the newly formed bone. The composition and biology of newly formed bone will also be characterized using immunohistochemistry/staining techniques for osteogenesis markers as well as labeling techniques to evaluate bone growth rates. Statistical analyses will allow comparing the impacts of static/dynamic growth modulation on the quality of newly formed bone in terms of microstructure, composition and biomechanics. NOVELTY AND EXPECTED SIGNIFICANCE: Current treatments of spinal deformities, such as scoliosis, involve invasive surgical instrumentation and vertebral fusion. Recent advances in fusionless techniques of the spine, by means of local mechanical modulation of bone growth, have shown great promise in the treatment of these progressive spinal deformities. They present less surgical risks and complications than the traditional surgery and preserve spinal growth, spinal motion and function. It is clear that a better understanding of the factors involved in bone growth modulation are essential to guide and improve the design of novel fusionless techniques, which could be designed more or less constrained to transmit either static or dynamic loads. However, it still is not clear at the moment what loading parameters would be efficient and non-damaging for bone growth modulation. The knowledge that will result from the proposed research program will be of great value for the design of novel fusionless implants for pediatric skeletal deformities, not only in the spine but also in the lower/upper limb deformities.