Biomechanical Response of the Epiphyseal Vertebral Growth Plate under Static and Cyclic Compression: A Finite Element Study

Adolescent idiopathic scoliosis (AIS), a 3D deformity of the spine, affects 1-3% of adolescents, mainly females. Mechanical influences on spinal growth play an important role in AIS curve progression, mainly during the rapid growth periods such as adolescence. Fusionless corrective techniques of the spine, by means of local mechanical modulation of bone growth, have shown promising advances in the early treatment of moderate spinal deformities. Recent in vivo studies on rat growth plates using matched static and cyclic loadings in terms of average stress showed that cyclic loads were as efficient as static loads in terms of growth modulation but less detrimental to the growth plate integrity compared to static loads [1, 2]. However, it was shown that the combination of high frequency and oscillation amplitude resulted in infection in the rats. As a complementary approach to investigate spinal biomechanics from experiments, computational modeling provides a platform to extend our knowledge about spinal structures and their mechanical behavior [3]. Using computational modeling, one parameter can be changed easily to investigate its effect while the other parameters are kept constant. Finite element modeling is one of the widely used computational methods; it provides a fast numerical method for stress and strain analysis in complex problems, while avoiding limitations and difficulties associated with experimental studies. Although poroelastic models have been developed to investigate the time-dependent response of the spinal tissues, the comparative biomechanical behavior of cartilaginous tissues such as growth plates under static vs. cyclic loads has yet to be fully understood [3, 4] and could provide insights on understanding of the interaction of mechanical loading and tissue metabolism. The objective of this project was to study the biomechanical response of the growth plate to static and cyclic compressions using finite element models. To achieve this objective, an axisymmetric biphasic model of growth plate was first developed to investigate stress components and deformation within the model for different transversal permeabilities and peripheral pore pressures (part one). Then, a finite element model of a spinal functional unit was used to investigate the same parameters as well as fluid content using a more realistic model (part two). This thesis aimed at verifying the hypothesis that Cyclic and static compressive loads show the same total stress but different pore pressure (stress in fluid phase) and effective stress (stress in solid matrix) within growth plates.