Mechanics and validation of an in vivo device to apply torsional loading to caudal vertebrae.

Axial loading of vertebral bodies has been shown to modulate growth. Longitudinal growth of the vertebral body is impaired by compressive forces while growth is stimulated by distraction. Investigations of torsional loading on the growth plate in the literature are few. The purposes of this study were two-fold: (1) to develop a torque device to apply torsional loads on caudal vertebrae and (2) investigate numerically and in vivo the feasibility of the application of the torque on the growth plate. A controllable torque device was developed and validated in the laboratory. A finite element study was implemented to examine mechanically the deformation of the growth plate and disk. A rat tail model was used with six 5-week-old male Sprague-Dawley rats. Three rats received a static torsional load, and three rats received no torque and served as sham control rats. A histological study was undertaken to investigate possible morphological changes in the growth plate, disk, and caudal bone. The device successfully applied a controlled torsional load to the caudal vertebrae. The limited study using finite element analysis (FEA) and histology demonstrated that applied torque increased lateral disk height and increased disk width. The study also found that the growth plate height increased, and the width decreased as well as a curved displacement of the growth plate. No significant changes were observed from the in vivo study in the bone. The torsional device does apply controlled torque and is well tolerated by the animal. This study with limited samples appears to result in morphological changes in the growth plate and disk. The use of this device to further investigate changes in the disk and growth plate is feasible.