Finite element analysis of a ball-and-socket artificial disc design to suppress excessive loading on facet joints: A comparative study with ProDisc.

Facet arthrosis at surgical level was identified as major complication after total disc replacement (TDR). One of the reasons for facet arthrosis after TDR has been speculated to be the hypermobility of artificial discs. Accordingly, the artificial disc that can constrain the hypermobility of ball-and-socket type artificial discs and reduce loading on facet joints is demanded. The proposed artificial disc, which is named as NewPro, was constructed based on the FDA-approved ProDisc but contained an interlocking system consisting of additional bars and grooves to control the range of motion (ROM) of lumbar spine in all anatomical planes. The three-dimensional finite element model of L1 to L5 was developed first, and the biomechanical effects were compared between ProDisc and NewPro. The ROM and facet contact force of NewPro were significantly decreased by 42.7% and 14% in bending and by 45.6% and 34.4% in torsion, respectively, compared with the values of ProDisc, thanks to the interlocking system. In addition, the ROM and facet contact force could be selectively constrained by modifying the location of the bars. The proposed artificial disc with the interlocking system was able to constrain the intersegmental rotation effectively and reduce excessive loading on facet joints, although wear and strength tests would be needed prior to clinical applications.