Comparison of biomechanical function at ideal and varied surgical placement for two lumbar artificial disc implant designs: mobile-core versus fixed-core.

STUDY DESIGN

Finite element model.

OBJECTIVES

To estimate the effect of lumbar mobile-core and fixed-core artificial disc design and placement on the loading of the facet joints, and stresses on the polyethylene core.

SUMMARY OF BACKGROUND DATA

Although both mobile-core and fixed-core lumbar artificial disc designs have been used clinically, the effect of their design and the effect of placement within the disc space on the structural element loading, and in particular the facets and the implant itself, have not been investigated.

METHODS

A 3D nonlinear finite element model of an intact ligamentous L4-L5 motion segment was developed and validated in all 6 df based on previous experiments conducted on human cadavers. Facet loading of a mobile-core TDR and a fixed-core TDR were estimated with 4 different prosthesis placements for 3 different ranges of motion.

RESULTS

Placing the mobile-core TDR anywhere within the disc space reduced facet loading by more than 50%, while the fixed-core TDR increased facet loading by more than 10% when compared with the intact disc in axial rotation. For central (ideal) placement, the mobile- and fixed-core implants were subjected to compressive stresses on the order of 3 MPa and 24 MPa, respectively. The mobile-core stresses were not affected by implant placement, while the fixed-core stresses increased by up to 40%.

CONCLUSION

A mobile-core artificial disc design is less sensitive to placement, and unloads the facet joints, compared with a fixed-core design. The decreased core stress may result in a reduced potential for wear in a mobile-core prosthesis compared with a fixed-core prosthesis, which may increase the functional longevity of the device.