COMPARING THE FIXATION OF A NOVEL HOLLOW SCREW AND CONVENTIONAL SOLID SCREW IN HUMAN SACRA UNDER CYCLIC LOADING

Marlis Sabo, M.D.
London, ON

S.D. McLachlin
R. Fernando
S.I. Bailey
K.R. Gurr
D.W. Holdsworth
C.E. Dunning
C.S. Bailey

Spinal fusions involving the sacrum, especially longer constructs, are associated with a clinically significant failure rate resulting, in part, from the length of the lever arm and the largely cancellous nature of sacral pedicle bone.

The purpose of this study was to determine if a novel hollow screw placed from a posterior approach into the S1 pedicles and loaded in cyclic flexion and extension was more resistant to failure than a conventional solid screw in the same circumstance.

One of each screw were placed into six previously fresh-frozen cadaveric human sacra.  Each screw was loaded cyclically in flexion and extension using an Instron servomechanical device in 1000 cycle steps of increasing loads (0.25 N·m increments) until the screws visibly failed.  Bone density was determined using quantitative computed tomography.

The solid screw withstood 11151 ± 4221 cycles while the hollow screw withstood 6301 ± 2131 cycles.  Bone density of the specimens and distance from the far cortex of the screw had no influence on the performance of the hollow screw.

The hollow screw was not more resistant to failure in cyclic flexion/extension loading than the conventional hollow screw when placed in human sacral pedicles.