Comparison of a novel mineral-organic bone adhesive to polymethylmethacrylate for augmentation of pedicle screw pull-out resistance in an osteoporotic human cadaveric spine model

Introduction: Polymethylmethacrylate (PMMA) augmentation of pedicle screws is a well-described technique for improving pull-out resistance in osteoporotic bone; however, its use poses risk due to thermal necrosis of bone, lack of biodegradability, and a non-physiologic modulus of elasticity. We evaluated the efficacy of a novel mineral-organic bone adhesive composed of tetracalcium phosphate and phosphoserine (TTCP-PS) for augmentation of pedicle screws and compared its pull-out resistance to PMMA-augmented and non-augmented instrumentation in osteoporotic cadaveric spines. In animal studies, TTCP-PS has been shown to be osteoconductive, less exothermic than PMMA, and to be resorbed over 6-12 months and replaced by bone.

Methods: Three cadaveric human lumbar spines with donor histories of osteoporosis were mounted to a table. The L1-L5 levels (30 pedicles) were randomized to be injected with TTCP-PS, PMMA, or no augmentation (control). The pedicles were cannulated and tapped to 45 mm depth with a 6.5mm diameter tap. After tapping, pedicles randomized to augmentation were injected with 3 cc of either PMMA or TTCP-PS. 6.5 mm diameter, 45 mm length screws were then inserted into the pedicles. Repeat CT scans were obtained to assess screw placement. Subsequently, the individual vertebral bodies were disarticulated and potted in cuboidal resin blocks. Peak failure pull-out force was measured with an integrated load cell-hydraulic jack attached to each screw head, with ANOVA testing performed to assess for statistical differences between augmented vs. non-augmented conditions.

Results: Post-procedure CT scans showed that all pedicle screws were accurately placed. 7 pedicles augmented with TTCP-PS, 8 with PMMA, and 8 control pedicles were available for analysis. The mean failure pull-out forces were 3160.0N, 3227.2N, and 677.2N, respectively. TTCP-PS augmented pedicle screws had a significantly higher pull-out resistance compared to controls and did not significantly differ from PMMA-augmented screws (p=0.001 and p=0.724, respectively).

Conclusion : Augmentation of lumbar pedicle screws with TTCP-PS or PMMA resulted in statistically equivalent increases in pull-out resistance in an osteoporotic human cadaver spine model. Both materials yielded statistically significant increases in screw pull-out resistance compared to no augmentation (~4.6X). Given the favorable physical properties of TTCP-PS compared to PMMA, it may prove to be a superior material for screw augmentation.