Pullout strength of pedicle screws augmented with particulate calcium phosphate: an experimental study.

BACKGROUND CONTEXT

Pressure-injected and in situ curing bone cements have been studied as alternatives in augmenting lumbar pedicle screw fixation but are frequently found to leak outside the confines of the target vertebra.

PURPOSE

The objective is set to determine the mechanical efficacy of a porous granular/particulate calcium phosphate (CP) bone augmentation product (Skelite) applied manually without pressurized injection in this application.

STUDY DESIGN/SETTING

The biomechanical analysis compared the axial pullout strength and insertional torque of augmented and nonaugmented pedicle screws in cellular polyurethane foams.

METHODS

The insertion torque and pullout strength of 6.5-mm pedicle screws inserted (via 3.5-mm pilot holes) into polyurethane blocks mimicking the porosity of cancellous bone were measured. New pilot holes were then packed with granular particles of Skelite and retested. Last, those blocks initially tested to failure without augmentation were packed with Skelite and retested. Measurements were performed for polyurethane densities of 0.16 and 0.32 g/cc (corresponding to the porosity of osteoporotic and normal bone) and strain rates of 0.5 and 5mm/min.

RESULTS

Peak pullout force averaged 2132.5+/-119.3 N and 1840.1+/-216.7 N in high density samples without and with augmentation and 688.2+/-91.4 N and 861.6+/-74.5 N in low density samples without and with augmentation. After failure, approximately 50% and 77% of the peak pullout force of original high and low density samples was regained by augmentation. Statistical analysis revealed significant (p<.0001) correlation between the addition of CP, peak pullout resistance, and insertion torque.

CONCLUSION

Granular CP augmentation improved the pullout strength in both failed (pulledout) samples and low density (porosity of osteoporotic cancellous bone) polyurethane blocks.