Biomechanical strength comparison of pedicle screw augmentation using poly-dicalcium phosphate dihydrate (P-DCPD) and polymethylmethacrylate (PMMA) cements.

Abstract

A basic science, hypothesis-driven experimental study of the biomechanics of two bone cements in their ability to augment pedicle screws in bone foam. The purpose of our study was to compare the pullout and torque resistance of conventional pedicle screws (CPS) augmented with either polymethylmethacrylate (PMMA) or poly-dicalcium phosphate dihydrate (P-DCPD) cement in polyurethane foam blocks mimicking osteoporotic bone. P-DCPD cement has attractive safety characteristics such as non-exothermic curing and drug-eluting capacity. PMMA cement lacks these safety features yet is the current standard in pedicle screw augmentation. Standardized low-density polyurethane open-cell foam blocks were instrumented with conventional pedicle screws and categorized into three groups of six each. Group 1 was the control group and no cement was used. Groups 2 and 3 were augmented with PMMA and P-DCPD, respectively. An Instron machine applied an axial load to failure at a rate of 2 mm/min for 3 min and a torsional load to failure at a rate of 1°/s. Failure was defined by an evident drop in the load after maximum value. Maximal pullout load for PMMA and P-DCPD was significantly greater than control (p < 0.0001). Interestingly, there was no significant difference in the pullout load to failure for the PMMA and P-DCPD groups. Analysis showed significant difference in torsional resistance between PMMA and P-DCPD, with PMMA having greater resistance (p = 0.00436). No difference was observed between PMMA and P-DCPD in pullout load to failure conducted in low-density open-cell, rigid foam blocks. Although a significant difference did exist in our torque analysis, the clinical significance of such a load on a native spine is questionable. Further investigation is warranted for this promising compound that seems to be comparable in pullout resistance to PMMA and offers attractive safety features. Basic science, not applicable.