Biomechanics of Grade I degenerative lumbar spondylolisthesis. Part 1: In vitro model

Object. The authors sought to create and to evaluate an in vitro model of G rade I degenerative (closed-arch) spondylolisthesis. Methods. The model of spondylolisthesis was created by two primary procedur es: 1) resection of the disc; and 2) stripping of anterior and posterior lo ngitudinal ligaments away from the vertebral bodies (VBs). In 13 vertebral levels obtained from three cadaveric lumbar spines, the tissues were resect ed sequentially in alternating order to determine the relative contribution of each resection to spinal instability. The entire specimens were loaded with nonconstraining torques and then individual levels were loaded with an teroposterior shear forces. The motion values were measured optoelectronica lly for each specimen at individual levels. Conclusions. The integrity of the disc was more important than attachment o f the ligaments to the VB, but the resection of both structures was necessa ry to achieve substantial destabilization. The structures of the spine are highly resilient, and destabilization is difficult to achieve without perfo rming extensive resection. Using the techniques described in this paper to alter normal spines, a level of spinal instability (Grade I; 25% slippage) that may represent spondylolisthesis can be modeled in vitro.