Can the sheep model fully represent the human model for the functional evaluation of cervical interbody fusion cages?

Sheep model is the most favourable choice for animal study for functional evaluation of the cervical fusion prostheses before clinical application; however, significantly large differences between sheep and human existed in terms of morphological characteristics and daily-activity motions. Questions should be raised as whether the differences between the two species have influence on the reliability of sheep model. Finite element models (FEM) of the cervical spinal system were built to characterize the differences between the two species with respect to the range of motion (ROM) and biomechanical behaviour, and experimental cadaver tests on both species were employed for validation purposes. Results indicate that sheep model represents the worst-case scenario of the human model with exaggerated stresses (up to 3 times more) and ROM (up to 10 times more). Moreover, sheep model is very sensitive to the variation of prostheses design, whilst human model does not, which denotes that the sheep model provides a rather amplified effect of a certain design for its biomechanical performance. Therefore, caution needs to be taken when sheep models were used as the animal model for functional evaluation over various design, and the FEM built in this study can be employed as an effective methodology for performance evaluation of cage prostheses of cervical spine.