An In vitro biomechanical load-to-failure test.Objectives.
To determine the comparative axial pullout strengths of pedicle screw versus transverse process screws in the upper thoracic spine (T1-T4), and to compare their failure loads with bone density as seen on computed tomography.Summary of the Background Data.
The morphology of the upper thoracic spine presents technical challenges for rigid segmental fixation. Though data are available for failure characteristics of cervical lateral mass screws, analagous data are wanting in regard to screw fixation of the upper thoracic spine.Methods.
Ten fresh-frozen human spines (T1-T4) were quantitatively scanned using computed tomography to determine trabecular bone density at each level. The vertebrae were drilled and tapped for the insertion of a 3.5-millimeter-diameter cortical bone screw in either the pedicle or the transverse process position. A uniaxial load to failure was applied.Results.
The mean ultimate load to failure for the pedicle screws (658 N) was statistically greater than that of the transverse process screws (361 N; P < 0.001). The T1 pedicle screw sustained the highest load to failure (775 N). No significant difference was found between load to failure for the pedicle and transverse process screws at T1. A trend toward decreasing load to failure was seen for both screw positions with descending thoracic level. Neither pedicle dimensions nor screw working length correlated with load to failure.Conclusions.
Upper thoracic pedicle screws have superior axial loading characteristics compared with bicortical transverse process screws, except at T1. Load behavior of either of these screws was not predictable based on anatomic parameters.