Biomechanical study of lumbar pedicle screws in a corpectomy model assessing significance of screw height.
BACKGROUND
We tested the hypothesis that a pedicle screw construct's height is an important factor in strengthening a screw-rod system.


METHODS
Six corpectomy constructs were made, each using two ultra-high-molecular-weight polyethylene blocks, 6.5-mm pedicle screws, and two 6.35-mm rods. Pedicle screws were placed at +10-, +5-, 0-, and -5-mm depths in relation to the dorsal surface of the corpectomy model. Nondestructive testing was performed in flexion/extension and in torsion.


RESULTS
For all modes tested, the screw-rod constructs continued to increase in stiffness as the height of the construct was lowered, and this was statistically significant at all heights tested (P < 0.001). The stiffness increased 232% when comparing flexion at +10 and -5 mm and increased 231% in extension from +10 to -5 mm. The torsional stiffness increased 171% when comparing +10 and -5 mm.


CONCLUSIONS
Thus, lower-profile instrumentation systems should be used to take advantage of this by decreasing the size and bulkiness of the implants while increasing the strength of the construct.
Department of Orthopaedics, University of Michigan, Ann Arbor, MI 48109-0328, USA.
Mark R
MR
Mikles
Ferhan A
FA
Asghar
Elizabeth P
EP
Frankenburg
Derek S
DS
Scott
Greg P
GP
Graziano
D001696:Biomechanical Phenomena; D001863:Bone Screws; D006801:Humans; D066298:In Vitro Techniques; D008159:Lumbar Vertebrae / Q000502:physiology* / Q000601:surgery*; D008422:Materials Testing; D013123:Spinal Fusion / Q000295:instrumentation*; D014102:Torsion Abnormality