Stiffness between different directions of transpedicular screws and vertebra

OBJECTIVE: This study was undertaken to evaluate the variation in bone density within the vertebral body and to determine the biomechanical stiffness of the screw-bone interface for different superior-inferior transpedicular screw orientations in the vertebral body. DESIGN: The stiffness of three directions of screw placement (upper, middle, and lower) were measured in two modes of loading (flexion and torsion). All screws were inserted to 80% of the distance along the path of the screw from the point of insertion to the anterior vertebral cortex. BACKGROUND: The placement of transpedicular screws within the vertebral body is a fundamental determinant the stiffness and strength of the bone-screw interface and consequently the stability of an implant construct. The bone stiffness within the pedicle and vertebra must be sufficient to resist spine forces and moments. METHODS: The stiffness of pedicle-screw fixation was tested for three different screw orientations based on the position of the screw tip in the vertebral body (upper, middle, and lower). Fixation rigidity was measured for two modes of loading (flexion and torsion). In all, eighteen individual vertebrae (L(2)-L(4)) from 6 cadaveric spine segments were examined. All the screws were inserted to 80% of the depth of the posterior to anterior vertebral cortex dimension. Quantitative bone density measurements were obtained from vertebral mid-sagittal cross sections of the additional vertebral bodies using an image analysis system to determine the distribution of bone density in the mid-sagittal plane. RESULTS: Based on area fraction measurements of cancellous bone in seventeen L1 mid-sagittal regions, highest densities were observed adjacent to the endplates (area fractions > 0.29). Regions of lowest density were found in the central portions of the vertebral bodies, above the mid-line of the body (area fractions < 0.20). In flexion, the stiffness of all three groups of screws were similar, ranging from 4.0-4.2 Nm deg(-1). In torsion the upper and middle directions were significantly more rigid than lower position (p < 0.04). CONCLUSION: Pedicle screw insertion aimed toward the superior-anterior aspect of the vertebral body, adjacent to the superior endplate provides the best overall rigidity considering both directions of loading. Screws placed in the mid-portion of the vertebral body may provide good lateral torsion rigidity, but would not be optimal for flexion stability if bone density is reduced in that region.