Correlation of ProDisc-C failure strength with cervical bone mineral content and endplate strength

Abstract

Study design: This study was designed to evaluate the effectiveness of preoperative computed tomography (CT) scan in predicting endplate strength.

Objective: To demonstrate a correlation between the cervical trabecular bone density and the failure strength of the endplate.

Summary of background data: Cervical total disc arthroplasty devices have to transmit the force to the endplate for the remainder of the patients' life. One potential complication at this interface is endplate fracture and implant subsidence, which usually occurs early postoperatively and may be related to weakness of the boney endplate.

Methods: Six fresh human cadaver cervical spines were harvested and scanned for the determination of trabecular density using a peripheral quantitative CT (QCT) scanner. The specimens were then disarticulated and the inferior endplate of each vertebral segment was biomechanically tested using a 2-mm indentation probe to determine average endplate strength. The superior endplate of each vertebral body was then fitted with the appropriately sized ProDisc-C endplate and biomechanically tested until failure. Regression analyses were used to compare the interface failure stress of the implant with the bone mineral content and the average endplate stress as measured with the indentation probe.

Results: The average bone mineral content of the specimens was 322+/-57 mg/cm. The average endplate strength and stress measured by the indentation test was 176+/-129 N and 56+/-34 N/mm, respectively. The average ProDisc-C/endplate failure load and failure stress were 1875+/-1023 N and 10.2+/-4.1 N/mm, respectively. There was a direct correlation between the ProDisc-C/endplate failure stress and the bone mineral content measured by peripheral QCT (R=0.48, P<0.01). There was also a significant correlation between ProDisc-C/endplate failure stress and the endplate indentation stress.

Conclusions: This study demonstrates the utility of a preoperative QCT scan in predicting the failure stress of the cervical endplate before total disc replacement. This information may potentially decrease early complications of device subsidence or endplate fracture.