Volume 8, Issue 10 (November 2011)
Primary Stability of a Dynamic Artificial Disc Tested in a Human Cadaveric Model
Sufficient anchorage of an interbody device will allow osseointegration and avoid implant migration. Shear forces in the lumbar spine are transferred across the bone/implant interface, resulting in relative motion between the implant and bone. Relative motion greater than 150 μm will have a detrimental effect on the osseointegration of titanium (Ti) implants. This study developed an in vitro, primary stability testing protocol to assess a total disc replacement (TDR). Testing was performed on seven human cadaveric L4 and L5 vertebrae. Cyclic shear loading of the prepared implant/bone interface was applied for 20 cycles in the anterior and posterior directions to ±350 N under an axial compressive load of 600 N. The relative motion was measured and the average of the last five cycles analysed. The shear force necessary to cause complete anterior migration of the TDR was also measured. The averaged cyclic motion for the last five cycles in both the anterior and posterior directions was below 85 μm for all specimens. The average yield force for complete anterior implant migration was 522.2 N (±105.9 N) and the failure force was 605.1 N (±118.4 N). A primary stability testing protocol was developed and employed to assess a TDR in human cadaveric bone. The measured relative motion for this TDR for each loading cycle was below 150 μm, which is favourable for osseointegration with the vertebral body endplates.