Spondylolysis Fracture Angle in Children and Adolescents on CT Indicates the Facture Producing Force Vector: A Biomechanical Rationale

The fracture angle of spondylolysis with respect to the frontal plane on CT scan shows variations individually. We hypothesized that the angle may indicate the major force vector responsible for the pars stress-fracture. To prove this hypothesis, the highest stress lines at the pars interarticularis were analyzed using a 3-D finite element model (FEM) of a lumbar motion segment (L3 to S1) during various modes, and compared to the clinical data of fracture angles on CT scan of 32 patients with spondylolysis. The location/direction of high stresses as predicted by the FEM indicated that the extension loading may cause spondylolysis at coronal orientation, and rotation loading, may cause spondylolysis at sagittal orientations. The CT findings revealed that the fracture angle in patients, who were active in sports requiring frequent twisting motion of the trunk, were more sagittal in orientation as compared to sport activities that required less twisting. From the spondylolysis fracture angle on CT, the type of loading (i.e.; extension, rotation) causing stress fracture at pars interarticularis may be determined and the information may be used to prescribe the right type of braces for a particular patient.