Mechanical Factors and the Threshold of Trauma of the Intervertebral Joint: A Finite Element Analysis with Experimental Validation

Experimental and analytical finite element analyses were conducted to determine the effects of external load resulting in spinal injury. Axial compressive loading on human thoracolumbar functional units exhibits initial ambient, physiologic, traumatic and post traumatic loading phases. Initiation of trauma was defined as the point where the instantaneous tangent stiffness of the structure begins to decrease for the first time. For both normal and degenerate segments this microfailure load was 80% of the conventional traumatic failure load. The more flexible degenerate spines resulted in a lower value for these loads. Hypotheses regarding these microfailures which were not obvious on radiographs, were confirmed by morphologic cryosections. The energy absorbed by normal discs between microfailure and traumatic failure was 43% of total energy required for traumatic failure. In degenerate discs this was 28%. This may be an important index in determining the reserve strength after the initiation of injury. A materially and geometrically non-linear axisymmetric finite element model was constructed using geometry from experimental records. The purpose was to estimate the load dependent material constants and to explain the mechanics of injury based on regional strain energy density distribution. Experimental load-deflection data was used for validation. A critical evaluation of the inverse finite element method typically used for such estimations revealed the importance of obtaining the deformation data from a large number of experimental measurements. Results indicated the Young\u27s modulus value of the annulus fibrosus to be the highest in the physiologic loading phase. It reduced in the traumatic phase after the initiation of trauma. Stress-deformation analysis included the strain energy, annulus and endplate stresses, and nucleus pressure distributions, from the ambient to traumatic phases. The structure exhibited a concentration of strain energy density at the annulus-endplate junction near the outer disc circumference. A gradual yielding of the junction results in a settlement of the supports of the endplate at its outer edges. Due to external load and internal nucleus pressure, this support yielding phenomenon adds to its instability producing microfailures. This study has provided experimental evidence and a theoretical basis for the initiation of injury to a functional unit. Microfailures occur at a lower physiological loading than the conventional traumatic failure of the structure