Pathomechanisms of failures of the odontoid

Study Design. A finite element investigation to deter-Imine the causal mech anisms that lead to odontoid fracture. Objectives. To elucidate which loading scenarios, including rotational mome nts, compression-tension, and lateral and anteroposterior shear, can result in Type I, Type II, and Type III odontoid failures. Summary of Background Data. There is considerable controversy about the maj or loading path that causes odontoid fractures. A review of the clinical an d laboratory research literature did not provide a consensus on this issue, Methods, A three-dimensional, nonlinear finite element model of the occipit o-atlantoaxial (C0-C1-C2) complex was generated from human cadaveric data. Force loads were applied at the posterior margin of the occiput a-nd were a pplied as lone entities or after the model was prepositioned in flexion, ex tension, or lateral-bending moments through applied rotation moments, Intra osseous stresses were reported to characterize the probability Of fracture due to the applied loadings. Results, The data indicate that hyperextension can lead to failure of the o dontoid at its superior tip (Type I). Finite element model predictions also demonstrated the propensity of loads that induce axial rotation to create relatively high maximum von Mises stress in the Type II fracture region. Fl exion prepositioning reduced the stress response of the odontoid. Conclusions. Force loading that puts the head in extension coupled with lat eral shear or compression leads to Type I fractures, whereas axial rotation and lateral shear can produce Type II fractures. The model failed to eluci date causal mechanisms for Type III fractures. Flexion seems to provide a p rotective mechanism against force application that would otherwise cause a higher risk of odontoid failure.