Effects of Charite artificial disc on the implanted and adjacent spinal segments mechanics using a hybrid testing protocol

Study Design. Finite element model of L3 - S1 segment and confirmatory cadaveric testing were used to investigate the biomechanical effects of a mobile core type artificial disc (Charite artificial disc; DePuy Spine, Raynham, MA) on the lumbar spine. Objective. To determine the effects of the Charite artificial disc across the implanted and adjacent segments. Summary of Background Data. Biomechanical studies of artificial discs that quantify parameters, like the load sharing and stresses, are sparse in the literature, especially for mobile-type core artificial disc designs. In addition, there is no standard protocol for studying the adjacent segmental effects of such implants. Methods. Human osteo-ligamentous spines (L1 - S1) were tested before and after L5 - S1 Charite 'artificial disc placement. The data were used to validate further an intact 3-dimensional (3-D) nonlinear L3 - S1 finite element model. The model was subjected to 400-N axial compression and 10.6 Nm of flexion/extension pure moments ( load control) or pure moments that produced the overall rotation of the L3 - S1 Charite model equal to the intact case ( hybrid approach). Resultant motion, load, and stress parameters were analyzed at the experimental and adjacent levels. Results. Finite element model validation was achieved only with the load-controlled experiments. The hybrid approach, believed to be more clinically relevant, revealed that Charite 'artificial disc leads to motion increases in flexion (19%) and extension (44%) at the L5 - S1 level. At the instrumented level, the decrease in the facet loads was less than at the adjacent levels; the corresponding decrease being 26% at L3 - L4, 25% at L4 - L5, and 13.4% at L5 - S1 when compared to the intact. Intradiscal pressure changes in the L4 - L5 and L3 - L4 segments were minimal. Shear stresses at the Charite 'artificial disc-L5 endplate interface were higher than those at S1 interface. However, in the load control mode, the increase in facet loads in extension was approximately 14%, as compared to the intact case. Conclusions. The hybrid testing protocol is advocated because it better reproduces clinical observations in terms of motion following surgery, using pure moments. Using this approach, we found that the Charite artificial disc placement slightly increases motion at the implanted level, with a resultant increase in facet loading when compared to the adjacent segments, while the motions and loads decrease at the adjacent levels. However, in the load control mode that we believe is not that clinically relevant, there was a large increase in motion and a corresponding increase in facet loads, as compared to the intact.