Stability of lumbar spine with double pars fractures and after treatments

Complete pars fractures can lead to spondylolisthesis or degenerative disc disease (DDD). Double pars fractures (DPF) commonly occur at L4-L5 due to traumatic accidents in youth, while it occurs at L3-L4 from degenerative change in the elderly. No comprehensive biomechanical studies concerning DPF have been reported. Eighteen porcine L2-L6 spines were selected and assigned to intact, DPF, and DPF with DDD groups. The TPS group used transpedicular screw system to stabilize DPF defects. The D2TPS group used TPS system to stabilize the spine with DPF and DDD conditions. The D2TPSC group used TPS and interbody cages to stabilize the spine with DPF and DDD conditions. The biomechanical properties were estimated and compared among six groups. Motion segments were mounted and tested on a material testing machine. A series of loadings, including flexion, extension, lateral bending, torsion and compression, were applied, respectively. In flexion, DPF had a significantly smaller stiffness than the Intact spine. With TPS fixation, the stiffness was increased significantly. Double pars fracture significantly decreased stiffness of a lumbar spine in all directions of motion except lateral bending. TPS could increase the stiffness weakened by DPF. However, TPS could not significantly increase the stiffness of the construct with DPF and DDD except in extension. Addition of the interbody cage seemed to be an efficacious method to restore stiffness decreased by DPF with disc degenerative disease in flexion, lateral bending and compression. This study indicated that the transpedicular screw system was effective in stabilizing the motion segments with double pars fractures. The interbody cage can be added to strengthen the stiffness weakened by the degenerative disc disease.