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.
Mikles
Mark R
MR
Department of Orthopaedics, University of Michigan, Ann Arbor, MI 48109-0328, USA.
Asghar
Ferhan A
FA
Frankenburg
Elizabeth P
EP
Scott
Derek S
DS
Graziano
Greg P
GP
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