The effect of bone quality on pedicle screw loading in axial instability. A synthetic model.

STUDY
DESIGN: In this biomechanical analysis of pedicle screw bending moments, custom-fabricated vertebral analogues were loaded in axial compression to produce sagittal bending forces. Moments were measured directly from internally instrumented pedicle screws.
OBJECTIVES: To establish the role of cancellous vertebral modulus on pedicle screw bending moments within the vertebral body and the vertebral pedicle. SUMMARY OF BACKGROUND DATA: Pedicle screws are often used to manage axial instability of the spine. Clinical studies report a high incidence of screw bending failure, resulting in kyphosis and pain in some patients. Factors predisposing to bending failure are not well understood, although recent studies have shown that vertebral morphometry is important.
METHODS: Axially canullated 7.0-mm pedicle screws, internally instrumented with paired strain gauges, were inserted into analogue vertebrae of uniform dimension. Cancellous modulus was varied from 25-100 MPa. Screws were rigidly mounted to a vertical testing frame, and axial loads were applied to the superior vertebral endplate, producing sagittal bending moments. Moments were recorded from gauges applied in the intrapedicular and intravertebral portions of the screw. Mean moments were compared using a Student's t test, with significance defined as P < 0.05.
RESULTS: Cancellous modulus did not affect bending moments experienced in either the intrapedicular or intravertebral portions of the pedicle screws. Gauge accuracy was excellent, and with no gauge drift.
CONCLUSIONS: Although small changes in pedicle morphometry can alter screw bending moments significantly, changes in cancellous modulus had no measurable impact on bending moments at these same loads. Bone density is likely to play a limited role in screw bending failure.