114. Dual thread screw design provides equivalent fixation to upsized screw diameter in revision pedicle screw instrumentation

Abstract

BACKGROUND CONTEXT There are a number of clinical situations that require the replacement of pedicle screw instrumentation. In adjacent segment degeneration and other revision situations, pedicle screw instrumentation is often exchanged in order to accommodate a different sized rod or different pedicle screw system. Although the screws in these revision situations are not overtly loose, the screws have been physiologically loaded for a period of time. Traditionally, pedicle screws are salvaged by up-sizing the core diameter until an interference fit is obtained in the pedicular isthmus. PURPOSE To determine if dual thread screws allow for increased fixation when revising previous pedicle screw instrumentation. STUDY DESIGN/SETTING Cadaveric biomechanical study. PATIENT SAMPLE Not applicable. OUTCOME MEASURES Not applicable. METHODS Pedicle screws were tested in the lumbar vertebrae from ten cadavers. Every pedicle was first tested with a 5.5 mm diameter standard screw using break-in loading. Screws were removed and replaced with one of the following four screw types: 5.5 mm Standard Thread, 5.5 mm Dual Thread, 6.0 mm Standard Thread, or 6.0 mm Dual Thread. The sizes were varied in an S curve fashion over all 10 spines to distribute the screw types over the five lumbar levels (L1 through L5). Failure testing was done using a stepwise increasing cyclic loading protocol for 100 cycles. RESULTS The average pedicle screw cut out load (N) was 133.71 +/- 30.50 and 158.53 +/- 55.28 for standard and dual thread screws, respectively. There was significantly increased pullout strength with dual thread screws compared to the standard thread design (p = 0.001). When comparing 5.5 mm dual thread screws to 6.0 mm standard thread screws, there was no difference in ultimate strength (137.3 +/- 32.47 vs 142.86 +/- 56.50, respectively). CONCLUSIONS In cases requiring replacement of pedicle screws, both upsizing core diameter and utilizing dual thread design may increase bony purchase. The was no difference in upsizing core diameter by 0.5 mm compared with changing screw pitch to a dual thread design. FDA DEVICE/DRUG STATUS This abstract does not discuss or include any applicable devices or drugs.