RELATION OF VERTEBRAL BONE SCREW AXIAL PULLOUT STRENGTH TO QUANTITATIVE COMPUTED TOMOGRAPHIC TRABECULAR BONE-MINERAL CONTENT

Noninvasive prediction of the maximum axial load that a spinal bone sc rew will be able to withstand after anterior surgical placement would be highly useful. To investigate if this is feasible, we first perform ed preliminary experiments to distinguish the trabecular and cortical contributions to overall stiffness; the trabecular component was found to dominate. We then used a commercial computed tomography bone miner al package to determine the mineral density of the trabecular region o f 41 porcine vertebrae in terms of equivalent K2HPO4 concentration; va lues ranged from 104 to 343 mg/cm(3). A 6.5-mm diameter cancellous bon e screw was then inserted laterally in each vertebra, and the ultimate tensile strength (UTS) of the screw/bone interface was measured using a tensile testing machine. The UTS values ranged from 589 to 2,620 Ne wtons. A superlinear relation was found between UTS and the projected K2HPO4 concentration in the direction of the screw axis, expressed in units of mg/cm(2).