A Comprehensive Analysis of Bicortical and Unicortical Screw Fixation in a Dynamic Human Cadaveric Model With Implications to Defining Clinical Guidelines

A battery of tests were undertaken on twelve fresh human cadaveric cervical spines to compare the performance of unlocked unicortical and bicortical screws. Bone mineral densities of the spines were determined. The specimens were divided into two groups of six each. In each group, the load-displacement data in clinically relevant modes were sequentially obtained for the intact, injured-stabilized, and injured-stabilized-fatigued cases. The injured specimens were stabilized using Caspar plate systems with unlocked unicortical screws in one group and unlocked bicortical screws in the other group. Screw tightening torques were also quantified at various stages in addition to screw backout. The results indicate that bicortical fixation provides stability irrespective of cancellous bone quality. In the unicortical group, specimens with higher BMD exhibited performance similar to the bicortical group, however, those with lower BMD could not survive fatigue loading. It appears that there is a BMD threshold above which unicortical screws would provide sufficient stabilization, thereby reducing surgical time and the associated complications related to bicortical screw use. Based on the results of our work it may be possible to define quantitative standards which the surgeon would use to answer the question of whether biomechanically a unicortical screw could be used in place of a bicortical screw.