Robotic simulation of disc arthroplasty surgery: Influence of surgical placement on motion segment dynamics.

摘要

A variety of total disc replacement (TDR) designs exist for the treatment of disc pathologies. A key design parameter for a constrained ball and socket device is the location of the fixed center of rotation (COR). A previous study demonstrated that intact motion segment unit (MSU) mechanics and range of motion (ROM) were sensitive to the location of a prescribed sagittal plane rotational axis. Mal-alignment between the implant COR and the COR of the MSU may lead to an overloaded or over constrained condition.Two paradigms exist for the placement of a fixed COR TDR device relative to MSU anatomy: positioning the implant midline or posterior to midline. Presently, there are no data to indicate which paradigm may lead to better biomechanical/clinical outcome. This research attempts to evaluate changes in MSU mechanics and ROM as a result of variations in the size and placement of a simulated ball and socket TDR, like the ProDisc-L lumbar disc prosthesis.Six human cadaveric lumbar MSUs, L4-L5, were tested in flexion/extension using the Spine Robot to an end load limit of 8Nm. A fixed axis protocol was used to impose a pure rotation about a desired anatomical location. The Spine Robot was programmed to rotate the MSU about the COR of the implant. Subsequently, with the MSU held rigid, the implant was removed and rotation about the implant's COR was repeated. Thereafter, simulated CORs were tested in different anatomical locations as defined by a customized grid pattern. The grid pattern consisted of 8 CORs which simulated the placement of a medium and large size constrained ball and socket device. Measurements of shear forces along the disc plane, axial force normal to the disc plane, segmental bending moment, and segmental ROM were analyzed at each grid point.Analysis of MSU mechanics and ROM for the ProDisc-L and Simulated Implant cases revealed that the two conditions were not comparable. Transfer of tissue pretension from the implant to the Spine Robot on removal of the implant, and dynamic contact forces at the implant surfaces were the contributing factors to the differences observed.Simulated COR testing demonstrated that the posterior tissue response was sensitive to varying placements of the simulated implant. For both implant sizes, posterior positioning of the COR required distraction of the disc space. During flexion, posterior positioning resulted in significantly higher shear and axial forces as well as a trend for reduced ROM. ROM in flexion may have been influenced by different starting positions within the neutral zone due to disc space distraction. During extension, the posterior placement of the COR reduced loading and increased rotation suggesting better alignment with, or separation of the facet joints.This novel study was able to delineate significant differences in spinal tissue response to varying simulated sizes and placements of an ideal fixed COR TDR device. The results of this study suggested that with both implant sizes the posterior placement of the COR will tend to distract the disc space and provide significantly increased ROM in extension at the expense of increased loads on posterior ligaments in flexion.