Mechanical evaluation of cross-link designs in rigid pedicle screw systems

Study Design. This study was designed to evaluate the biomechanical perform ance of 5 different cross-link brands to determine which design characteris tics are biomechanically desirable. Methods. The Cotrel-Dubousset, Isola, Puno Winter Byrd, Rogozinski, and Tex as Scottish Rite Hospital systems were assembled to vertebral models accord ing to the manufacturer's specifications. Three constructs were tested for each brand of instrumentation: without crosslinks, with one cross-link, and with two cross-links. Four modes of loading: axial, torsional, flexion-ext ension, and lateral-flexion were used. Load-displacement curves were plotte d. The stiffness was calculated from the slope of these curves. Objectives. Five different rigid pedicle screw systems were tested to deter mine: 1) what are the characteristics of cross-link design that are most ef fective in limiting torsional motion; 2) whether two cross-links are more e ffective than one; and 3) whether cross-linkage increases the construct sti ffness in lateral bending. Summary of Background Data. Cross-linkage has been shown to increase the to rsional stiffness of rod and screw constructs. Increased construct stiffnes s has been correlated with higher fusion rates. Results. Increases in axial, flexion-extension, or lateral-flexion stiffnes s, with the addition of one or two cross-links, were not statistically sign ificant. In torsional loading, increases in stiffness within brands were st atistically significant in every case. The average increase was 44% with on e added cross-link and 26% with two. The magnitude of the increase in torsi onal stiffness was compared with the cross-sectional area of the respective cross-link. Greater stiffness correlated with larger cross-sectional area (r = 0.81 for one cross-link, and r = 0.60 for two). Conclusion. The use of cross-linkage in spinal fusion increases torsional s tiffness in pedicle screw and hook constructs. This study I)confirmed the e ffectiveness of cross-linkage in limiting torsional motion and showed the s uperiority of two cross-links to one cross-link in limiting torsional motio n, 2) showed that increase of torsional stiffness of a cross-linked constru ct is proportional to the cross-sectional area of the cross-link, and 3) de monstrated that cross-links do not increases stiffness in the lateral flexi on mode.