Analysis of in Situ Posterior Spinal Fusion with Pedicle Screws and Rods in Grade I and Grade Ii Isthmic

Spondylolysis has never been found in a newborn. Repetitive cyclic loading ultimately results in a stress fracture. Impingement between the inferior articular process of the cephalad verbetra and the superior articular process of the caudal vertebra creates pending moment that must be resisted by the pars. The hard cortical bone of the pars predisposes it to fatigue fracture, as well as nonunion, decreasing the likelihood of spontaneous healing. If healing occurs, the pars often heal in an elongated position. Either non-union or healing with elongation permits vertebral subluxation. This fundamental change in bony anatomy exposes the disc to increased shear load, even though the axial load remains unchanged. When the stiffness of the cortical bone is reduced by an underlying condition (congenital dysplasia), smaller amounts of tensile forces are required to produce a fatigue fracture. Another possible mechanism is an extension torque which is developed by the erector spinae acting through its attachment to the spinous process, resulting in a stress concentration at the pars. The aim of the study is to analyze the post-operative outcomes in, in-situ posterior spinal fusion with pedicle screws and rods in Grade I and Grade II isthmic and degenerative spondylolisthesis.