This study aimed at evaluating the intervertebral disc implants for structural properties in comparison with the biological intervertebral disc. We tried to understand the load response of a cadaveric intervertebral disc structure under a physiologic complex loading compared to its replacement and also the response of the cadaveric disc structure to the current test standards for intervertebral implants.
Four cadaveric disc structures and four elastomeric intervertebral disc implants (E-d) (Theken Disc, Akron, OH) were tested under modified ISO testing specifications for replacements and also under single axis and coupled loads. The complex loading included a combination of flexion-extension (6°,-3°), left and right lateral bending (2°,-2°), axial rotation (2°,-2°) and axial compression (900-1700N). When tested under modified ISO loading, the flexion-extension stiffness and axial rotation stiffness values were found to be significantly different (p=0.0002 and p=0.0027, respectively) and no significant difference was found between lateral bending stiffness values (p=0.9304).
When the two groups were tested under single axis loading, there was a significant difference in the axial compression stiffness and axial rotation stiffness values (p= 0.0067 and p=0.0027, respectively) and no significant difference was seen in the flexion-extension stiffness and lateral bending stiffness values (p= 0.1092 and p=0.1348, respectively). Under coupled loading of flexion-extension and lateral bending there was a significant difference in the lateral bending stiffness values between the two groups (p=0.0485) but no significant difference was seen in the flexion-extension stiffness values (p=0.1197).
Fatigue characteristics of the cadaveric intervertebral disc structures, E-d and a pseudo Charite´ which was designed and fabricated similar to the Charite´ intervertebral disc (Depuy Spine, Inc), were determined and compared. All the discs were fatigued under modified ISO testing specifications. Stiffness values for single axis loadings and disc heights were used for comparison and failure was characterized by a decrease in disc height. The decrease in disc height at the given loading was considerably higher for the cadaveric specimens and all the cadaveric disc structures failed due to fractures in the vertebral bodies.
The study also aimed at evaluating the current testing standards for the intervertebral disc implants as we put forth the argument that the actual biological structure (intervertebral disc structure) itself would not survive the testing specifications which its replacement is supposed to bear without failure.