Biomechanical characterization of an annulus-sparing spinal disc prosthesis

Background contextCurrent spine arthroplasty devices require disruption of the annulus fibrosus for implantation. Preliminary studies of a unique annulus-sparing intervertebral prosthetic disc (IPD) found that preservation of the annulus resulted in load sharing of the annulus with the prosthesis.PurposeDetermine flexibility of the IPD versus fusion constructs in normal and degenerated human spines.Study design/settingBiomechanical comparison of motion segments in the intact, fusion and mechanical nucleus replacement states for normal and degenerated states.Patient settingThirty lumbar motion segments.Outcomes measuresIntervertebral height; motion segment range of motion, neutral zone, stiffness.MethodsMotion segments had multidirectional flexibility testing to 7.5 Nm for intact discs, discs reconstructed using the IPD (n=12), or after anterior/posterior fusions (n=18). Interbody height and axial compression stiffness changes were determined for the reconstructed discs by applying axial compression to 1,500 N. Analysis included stratifying results to normal mobile versus rigid degenerated intact motion segments.ResultsThe mean interbody height increase was 1.5 mm for IPD reconstructed discs versus 3.0 mm for fused segments. Axial compression stiffness was 3.0±0.9 kN/mm for intact compared with 1.2±0.4 kN/mm for IPD reconstructed segments. Reconstructed disc ROM was 9.0°±3.7° in flexion extension, 10.6°±3.4° in lateral bending, and 2.8°±1.4° in axial torsion that was similar to intact values and significantly greater than respective fusion values (p<.001). Mobile intact segments exhibited significantly greater rotation after fusion versus their more rigid counterparts (p<.05); however, intact motion was not related to motion after IPD reconstruction. The NZ and rotational stiffness followed similar trends. Differences in NZ between mobile and rigid intact specimens tended to decrease in the IPD reconstructed state.ConclusionThe annulus-sparing IPD generally reproduced the intact segment biomechanics in terms of ROM, NZ, and stiffness. Furthermore, the IPD reconstructed discs imparted stability by maintaining a small neutral zone. The IPD reconstructed discs were significantly less rigid than the fusion constructs and may be an attractive alternative for the treatment of degenerative disc disease.