Pedicle and transverse process screws of the upper thoracic spine. Biomechanical comparison of loads to failure.
METHODS
An In vitro biomechanical load-to-failure test.


OBJECTIVE
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.


BACKGROUND
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, analogous 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-mill meter-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.
Heller
J G
JG
Emory Spine Center, Department of Orthopaedic Surgery, Emory University School of Medicine, Atlanta, Georgia, USA. john_heller@emory,org
Shuster
J K
JK
Hutton
W C
WC
D000368:Aged; D001696:Biomechanical Phenomena; D015519:Bone Density; D001863:Bone Screws; D002102:Cadaver; D005260:Female; D006801:Humans; D066298:In Vitro Techniques; D008297:Male; D013123:Spinal Fusion / Q000295:instrumentation*; D013314:Stress, Mechanical; D013904:Thoracic Vertebrae / Q000000981:diagnostic imaging / Q000502:physiology / Q000601:surgery*; D014057:Tomography, X-Ray Computed; D016474:Weight-Bearing