The Effect of Hydroxyapatite on Titanium Pedicle Screw Resistance: An Electrical Model.

Abstract Background Context: Intraoperative detection of a pedicle wall breach implicitly reduces surgical risk, but the reliability of intraoperative neuromonitoring has been contested. Hydroxyapatite has been promulgated to increase pedicle screw resistance and negatively influence the accuracy of electromyography. Purpose: The primary purpose of this experiment is to evaluate the effect of HA on pedicle screw electrical resistance using a controlled laboratory model. Study Design: Controlled Laboratory Study. Methods: Stimulation of pedicle screws was performed in normal saline (0.9% NaCl). The experimental group included eight HA coated (HAC) pedicle screws and matched manufacturer control pedicle screws without HAC (Ti6Al4V). All screws were stimulated at 5, 10-, 15-, 20-, and 25-mm submersion depths. Circuit current return was recorded, and pedicle screw electrical resistance was calculated according to Ohm's Law. Data were assessed for normality and variance. Mann-Whitney U and Kruskal-Wallis tests compared groups with Bonferroni correction for multiple testing. Effect size is reported with 95% confidence intervals (95CI). P-values Results: Current return was detected for all screws (N= 24) following subclinical 8.5 µA stimulation at 5, 10-, 15-, 20-, and 25-mm submersion depths (N=144). The effect estimate of HA on pedicle screw electrical resistance is -0.07 (-0.17-0.01 95CI). The estimated effect of HA on pedicle screw electrical resistance did not differ across manufacturers. Electrical resistance values were inversely related to submersion depth. Electrical resistance values were lower in the experimental group at 10mm (P=0.04), 15mm (P=0.04), and 25mm (P=0.02) submersion depths. The HA effect ranged from -0.03 to -0.08 as submersion depth varied. Conclusion: We found no evidence that HA increased pedicle screw electrical resistance in a matched manufacturer control laboratory model. Clinical Significance: Electrical stimulation of pedicle screws may be reliable for pedicle breach detection in the presence of HA. Future research should investigate if laboratory findings translate to clinical practice and confirm that electrical stimulation of pedicle screws is a reliable method to detect pedicle breach in the presence of HA.