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Biomechanics of the Anterior Longitudinal Ligament During 8 g Whiplash Simulation Following Single- and Contiguous Two-Level Fusion: A Finite Element Study

Study Design. A computational study of anterior longitudinal ligament (ALL) strain in the cervical spine following single- and 2-level fusion during simulated whiplash. Objective. To evaluate how cervical fusion alters the peak strain of the ALL in the adjacent motion segments. Summary of Background Data. Although an in vitro study of ALL strain during whiplash has been conducted in healthy cervical spines, no such study has been performed in a cervical spine with fused segments. It has been demonstrated that the loss of motion following fusion results in increased strain in the adjacent motion segments. However, the biomechanics of the adjacent motion segments during high energy acceleration-deceleration simulations have not been widely reported. Accordingly, we investigated the peak strain of the ALL following single- and 2-level fusion during simulated whiplash. Methods. A detailed finite element (FE) model of the human body in the driver-occupant position was used to investigate cervical hyperextension injury. The cervical spine was subjected to simulated whiplash at 8 g acceleration and peak ALL strains were computed. The results were validated against published experimental data. This validated FE model was then modified to simulate single- and 2-level fusion and tested under identical loading conditions. Results. The mean increase in peak ALL strain at the motion segment immediately adjacent to the level of fusion was 15.5% for single-level fusion when compared with 40.8% in 2-level contiguous fusion (P = 0.019). Conclusion. Cervical arthrodesis increases peak ALL strain in the adjacent motion segments. Two-level fusion increased ALL strain in the adjacent motion segments, on average, greater than single-level fusion did. Disc arthroplasty and other techniques that provide stability without loss of flexibility may be beneficial in patients undergoing multiple-level fusion. Detailed FE models such as ours can provide strong correlation with experimentally determined data.