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Biomechanics

Mechanical Evaluation of Cross‐Link Designs in Rigid Pedicle Screw Systems

Dick, Jeffrey C. MD*; Zdeblick, Thomas A. MD; Bartel, Brian D. BS; Kunz, David N. MS

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Abstract

Study Design. This study was designed to evaluate the biomechanical performance of 5 different cross‐link brands to determine which design characteristics are biomechanically desirable.

Methods. The Cotrel‐Dubousset, Isola, Puno Winter Byrd, Rogozinski, and Texas Scottish Rite Hospital systems were assembled to vertebral models according to the manufacturer's specifications. Three constructs were tested for each brand of instrumentation: without cross‐links, with one cross‐link, and with two cross‐links. Four modes of loading: axial, torsional, flexion‐extension, and lateral‐flexion were used. Load‐displacement curves were plotted. The stiffness was calculated from the slope of these curves.

Objectives. Five different rigid pedicle screw systems were tested to determine: 1) what are the characteristics of cross‐link design that are most effective in limiting torsional motion; 2) whether two cross‐links are more effective than one; and 3) whether cross‐linkage increases the construct stiffness in lateral bending.

Summary of Background Data. Cross‐linkage has been shown to increase the torsional stiffness of rod and screw constructs. Increased construct stiffness has been correlated with higher fusion rates.

Results. Increases in axial, flexion‐extension, or lateral‐flexion stiffness, with the addition of one or two cross‐links, were not statistically significant. In torsional loading, increases in stiffness within brands were statistically significant in every case. The average increase was 44% with one added cross‐link and 26% with two. The magnitude of the increase in torsional 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 stiffness in pedicle screw and hook constructs. This study 1) confirmed the effectiveness of cross‐linkage in limiting torsional motion and showed the superiority of two cross‐links to one cross‐link in limiting torsional motion, 2) showed that increase of torsional stiffness of a cross‐linked construct is proportional to the cross‐sectional area of the cross‐link, and 3) demonstrated that cross‐links do not increases stiffness in the lateral flexion mode.

© Lippincott-Raven Publishers.

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