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Animal models are essential for making the transition from scientific concepts to clinical application. Such models have proven valuable for spinal research. The cervical spine of sheep is often used because there is similar geometry between sheep and human. Although anatomical similarities are important, biomechanical correspondence is imperative to understand the effects of disorders, surgical techniques, and implant designs. Therefore, the purpose of this study was to conduct a comprehensive study of the sheep cervical spine biomechanics, including experimental and finite element analysis. To determine the flexibility of the multilevel spine, ten adult Suffolk sheep C2-C7 spines were tested, undergoing flexion-extension, lateral bending, and axial rotation. In addition to intact multilevel testing, the roles of the stabilizing structures were studied by sequentially destabilizing function spinal units. The sheep spine is highly flexible, especially in lateral bending (±65˚); motion increases with caudal progression. The sheep spine also has a large neutral zone accounting for 50-75% of the total motion. The facets and capsular ligaments play a key role in stabilization, providing the most stability at the C2-C3 level. In addition to flexibility testing, the sheep spinal ligaments underwent tensile testing until failure to determine the material properties. The ligamentum flavum has the largest failure stress and the capsular ligaments have the largest mean failure force. The longitudinal ligaments have the largest failure strain and the lowest failure force. Overall, the C2-C3 ligaments had the highest failure forces as compared to the ligament type at different levels. This corresponds to the stability the ligaments have at the C2-C3 level during flexibility testing. Moreover, a finite element model of the C2-C7 sheep cervical spine was developed and validated to provide additional insight in the sheep biomechanics. The model compared favorably with experimental testing for all loading cases except extension. In general, the model matched the experimental results within one standard deviation for the multilevel motion as well as the motion at each level. Since the sheep is highly flexible and there is a large neutral zone it was difficult to capture the nonlinearity in all loading directions. The model was used to study the effects of fusion at the C3-C4 level. As expected the motion at the fusion was less than one degree, with the non-fused levels accommodating the loss in motion. The motion increased 15-27%, with the largest increase at C6-C7. To obtain the same rotation as the intact model (±2.5 Nm), larger moments were required, increasing to over 5 Nm for flexion and lateral bending and over 3 Nm for extension and axial rotation.
The study provides insight into the sheep cervical spine biomechanics. Researchers and scientists should consider the high flexibility and large neutral zone when designing a study that is to correlate to human spines. The model provides additional details such as stresses in the bone and intervertebral disc that can help researchers determine the effects of different surgical techniques and implant designs. Overall, this study provides valuable biomechanical data that can aid designing preclinical animal studies of the sheep.