Salvage of C2 pedicle and pars screws using the intralaminar technique - A biomechanical analysis

Study Design. Human cadaveric biomechanical analysis. Objective. The purpose of this study is to evaluate the ability of using 1 of the remaining 2 methods of instrumenting C2, should the initial method fail. Summary of Background Data. Although 3 different methods of C2 fixation ( pedicle, pars, and laminar screws) are possible, occasionally an attempt at screw insertion fails. In such cases, the surgeon needs a viable alternative to salvage/obtain fixation to obviate the need to instrument an additional motion segment. Methods. Eleven fresh-frozen cadaveric specimens (Occ-C4) were DEXA scanned for bone mineral density. On the left side, pedicle screws were first inserted, then pulled out. Then, salvage pars screws were inserted, then pulled-out, followed by laminar screws. On the right, a similar sequence was repeated, except that a pars screw was followed by a pedicle screw, then a laminar screw. All screws were placed by experienced cervical spine surgeons. Insertional torque ( IT) was measured in Newton-meters (Nm). Tensile loading to failure was performed in-line with the screw axis at a rate of 0.25 mm/s using a MTS 858 MiniBionix II System with data recorded as peak pull-out strength (POS) in newtons N. Results. Pedicle screws generated statistically greater IT and POS than other techniques as the initial fixation type (P < 0.0001). Similar trends were observed with transpedicular fixation as a salvage procedure ( P > 0.05). Laminar screws yielded consistently higher POS values than pars fixation when applied in a salvage scenario ( POS range: LS = 146-707 N; PrS = 8-548 N); however, high standard deviation precluded statistical significance ( P > 0.05). Significant predictive relationship was established between IT and POS for all screws using Pearson correlation coefficient and bivariate linear regression analysis (r = 0.75 and r(2) = 0.511, respectively; P < 0.01). Conclusion. Our results suggest that pedicle screws provide the strongest fixation for both initial and salvage applications. If they should fail, lamina screws appear to provide stronger and more reproducible fixation than pars screws.