FINITE ELEMENT ANALYSIS OF THE SPACE CREATED BY SPLIT SPINOUS PROCESSES IN DOUBLE-DOOR LAMINOPLASTY TO OPTIMIZE SHAPE OF AN ARTIFICIAL SPACER

In double-door laminoplasty, several types of artificial spinous process spacers have been used instead of grafted bone from the iliac crest. However, inadequate contact between the spacer and the spinous process has recently been reported. From the observation during operation, we suspect that the main cause of the inadequate contact is the difference in shape between the spacer and the widened space created by the split spinous processes. The purpose of this study was to investigate the shape of the widened space by means of a finite element analysis in order to confirm our observation objectively and to provide a shape design of a spacer adapting to the space. Half-sectioned finite element models of the second cervical (C2) vertebra and the C6 vertebra were made from both the computed tomography (CT) of a clinical case and a plastic model of a cervical spine. The finite element model was designed to have almost the same size and shape as those of the genuine vertebra in the clinical case. Since cancellous bone and soft tissues were thought not to meaningfully influence the rigidity of the model, the model was made of only cortical bone with a thickness of 1.5 mm. The x-axis was defined as the lateral direction of the vertebral body, the y-axis as the anteroposterior direction of the vertebral body and the z-axis as the craniocaudal direction along the posterior margin of the vertebral body. The boundary conditions were fixed at the inner surface of the half-sectioned vertebral body. A force of 100 N was applied to the inner surface of the half-sectioned spinous process (to the cranial side and the caudal side, 50 N each) in the direction of the x-axis. The lateral deviation of each split spinous process was defined as the degree of deviation in the x-axis direction. The degree of lateral deviation of each split spinous process was analyzed in two types of models with and without making a lateral gutter 4 mm wide along the z-axis direction. The lateral deviation at the cranial side was larger than that at the caudal side in both the C2 and C6 vertebrae. The difference between the lateral deviation at the cranial side and the caudal side of each vertebra was larger in the type of model with the lateral gutter than in the type of model without it. It was confirmed that the shape of the widened space is trapezoidal in not only the axial but also frontal sections. In conclusion, the optimal shape of a spacer adapting to the widened space in double-door laminoplasty is trapezoidal in not only the axial but also frontal sections.