Pedicle screw salvage: the effect of depth and diameter on pull-out strength: a biomechanical study

Abstract

Objectives: To investigate the effects of change in length, diameter, length and diameter on axial pull-out strengths of revisional pedicle screws and determine the best method of salvage for failed pedicle screws. Methods: We used pedilen rigid foam (equivalent to the average bone mineral density of lumbar vertebrae in cadavers as measured by quantitative computerized tomography) and Synergy pedicle screws with 1.8 mm threads. All variables but the length and diameter were kept constant during insertion of the pedicle screws. Ten different pull-out tests were performed in each of the five groups, using an Instron Model 1195 universal test machine. In the first group, screws 6.5 mm x 40 mm in size were used to draw reference values. In the remaining groups, insertion and removal of 6.5 mm x 40 mm screws was succeeded by insertion of pedicle screws of (i) the same size (6.5 mm x 40 mm), (ii) increased length (6.5 mm x 45 mm), (iii) increased diameter (7.0 mm x 40 mm), and (iv) increased length and diameter (7.0 mm x 45 mm), respectively. Pull-out strengths and load versus time curves were recorded. Results were evaluated using SPSS program and Student’s t-test. Results: The average pull-out strength decreased by 26% after substitution of the pedicle screw for one of the same size. Conversely, pull-out strengths increased by 15%, 33%, and 49% as the length, the diameter, and the length and diameter of the salvage screws increased, respectively. Differences in pull-out strengths between groups were highly significant (p<0.0001). Conclusion: Substitution of the previous pedicle screw for one of the same size would be a major mistake. Pedicle salvage using screws with increased length and diameter seems to be the most appropriate solution in restoring the strength of mechanical fixation.