Stress analysis of the interface between cervical vertebrae end plates and the Bryan, Prestige LP, and ProDisc-C cervical disc prostheses: an in vivo image-based finite element study.

STUDY DESIGN: Segmental motion and bone-implant interface stresses were analyzed at C5-C6 levels with Bryan, Prestige LP, and ProDisc-C cervical disc prostheses using an image-based finite element modeling technique.

OBJECTIVE: To predict stress patterns at the interface between prosthesis and lower vertebral end plate to better understand the underlying mechanisms of subsidence and how the load transfer pattern of each disc design affects segmental motion.

SUMMARY OF BACKGROUND DATA: Subsidence is one of the most commonly reported device-related complications in intervertebral disc arthroplasty. Although clinical outcomes have been reported regarding many types of cervical prostheses, few reports have analyzed the effects of stress from cervical artificial discs to the vertebral end plate.

METHODS: Three-dimensional voxel finite elements were built for C5-C6 spine unit based on computed tomography images acquired from a patient with indication for cervical disc arthroplasty. Models of facet joints and uncovertebral joints were added and artificial disc designs were placed in the intervertebral disc space. Static analyses were conducted under normal physiologic loads in flexion, extension, and lateral bending with precompression.

RESULTS: Bryan disc recovered highest range of motion (4.75 degrees ) due to the high elastic nucleus, and therefore imposed the lowest stresses superior to C6. The ProDisc-C and Prestige LP discs caused high stress concentrations around their central fins or teeth, and may initiate bone absorption. Analysis of Prestige LP disc may indicate possible subsidence posteriorly caused by the rear-positioned metal-to-metal joint.

CONCLUSION: Rigidity of the cores ("nuclei") in Prestige LP and ProDisc-C prostheses guarantee initial maintenance of disc height, but high contact stress takes place at the bone-end plate interface if they are improperly placed or undersized. Anchorage designs add an additional factor that may increase propensity of subsidence, indicated by the high contact stress occurring at the end plate flanges of Prestige LP, and at midline keel fixation on the end plate of ProDisc-C. Although Bryan disc differs in these 2 concerns, it also creates much larger displacement during motion with more variation in disc height that may theoretically increase the load sharing of facet and/or uncovertebral joints compared to more rigid artificial discs.