A finite element investigation of upper cervical instrumentation

Study Design. The finite element technique was used to predict changes in b iomechanics that accompany the application of a novel instrumentation syste m designed for use in the upper cervical spine. Objective. To determine alterations in joint loading, kinematics, and instr umentation stresses in the craniovertebral junction after application of a novel instrumentation system. Specifically, this design was used to assess the-changes in these parameters brought about by two different cervical anc hor types: C2 pedicle versus C2-C1 transarticular screws, and unilateral ve rsus bilateral instrumentation. Summary of Background Data. Arthrodesis procedures can be difficult to obta in in the highly mobile craniovertebral junction. Solid fusion is most like ly achieved when motion is eliminated. Biomechanical studies have shown tha t C1-C2 transarticular screws provide good stability in craniovertebral con structs; however, implantation of these screws is accompanied by risk of ve rtebral artery injury. A novel instrumentation system that can be used with transarticular screws or with C2 pedicle screws has been developed. This d esign also allows for unilateral or bilateral implantation. However, the au thors are unaware of any reports to date on the changes in joint loading or instrumentation stresses that are associated with the choice of C2 anchor or unilateral/bilateral use. Methods. A ligamentous, nonlinear, sliding contact, three-dimensional finit e element model of the C0-C1-C2 duplex and a novel instrumentation system w as developed. Validation of the model has been previously reported. Finite element models representing combinations of cervical anchor type (C1-C2 tra nsarticular screws vs. C2 pedicle screws) and unilateral versus bilateral i nstrumentation were evaluated. All models were subjected to compression wit h pure moments in either flexion, extension, lateral bending. Kinematic red uctions with respect to the intact (uninjured and without instrumentation) case caused by instrumentation use were reported. Changes in loading profil es through the right and left C0-C1 and C1-C2 facets, transverse ligament-d ens, and dens-anterior ring of C1 articulations were calculated by the fini te element model. Maximum von Mises stresses within the instrumentation wer e predicted for each model variant and loading scenario. Results. Bilateral instrumentation provided greater motion reductions than the unilateral instrumentation, When used bilaterally, C2 pedicle screws ap proximate the kinematic reductions and instrumentation stresses (except in lateral bending) that are seen wit C1-C2 transarticular screws. The finite element modal predicted that the maximum stress was always in the region in which the plate transformed into the rod. Conclusions. To the best of the authors' knowledge, this is the first repor t of predicting changes in loading in the upper cervical spine caused by in strumentation. The most significant conclusion that can be drawn from the f inite element model predictions is that C2 pedicle screw fixation provides the same relative stability and instrumentation stresses as C1-C2 transarti cular screw use. C2 pedicle screws can be a good alternative to C2-C1 trans articular screws when bilateral instrumentation is applied.