Endplate weakening during cage bed preparation significantly reduces endplate load capacity.

ABSTRACT

PURPOSE: To analyze the effect of endplate weakness prior to PLIF or TLIF cage implantation and compare it to the opposite intact endplate of the same vertebral body. In addition, the influence of bone quality on endplate resistance was investigated. METHODS: Twenty-two human lumbar vertebrae were tested in a ramp-to-failure test. One endplate of each vertebral body was tested intact and the other after weakening with a rasp (over an area of 200 mm2). Either a TLIF or PLIF cage was then placed and the compression load was applied across the cage until failure of the endplate. Failure was defined as the first local maximum of the force measurement. Bone quality was assessed by determining the Hounsfield units (HU) on CT images. RESULTS: With an intact endplate and a TLIF cage, the median force to failure was 1276.3N (693.1-1980.6N). Endplate weakening reduced axial endplate resistance to failure by 15% (0-23%). With an intact endplate and a PLIF cage, the median force to failure was 1057.2N (701.2-1735.5N). Endplate weakening reduced axial endplate resistance to failure by 36.6% (7-47.9%). Bone quality correlated linearly with the force at which endplate failure occurred. Intact and weakened endplates showed a strong positive correlation intact-TLIF r = 0.964, slope of the regression line (slope) = 11.8, p < 0.001; intact-PLIF r = 0.909, slope = 11.2, p = 5.5E-05; weakened-TLIF r = 0.973, slope = 12.5, p < 0.001; weakened-PLIF r = 0.836, slope = 6, p = 0.003. CONCLUSION: Weakening of the endplate during cage bed preparation significantly reduces the resistance of the endplate to subsidence to failure endplate load capacity is reduced by 15% with TLIF and 37% with PLIF. Bone quality correlates with the force at which endplate failure occurs.