Abstract
Introduction
Degenerative spinal changes are often accompanied by osteoporosis in elderly patients. In these cases traditional interbody devices can strongly subside into the irregular deformed endplates and vertebrae during or after the surgical stabilization. To avoid implant subsidence, a new technique is developed where PMMA bone cement is applied as a custom-made interbody device providing better contact and more even load transfer along the vertebra-implant interface. The aim of this study was to compare the biomechanical properties of the two different spacer types. In this presentation, we focus on the connection between the bone mineral density and geometrical parameters of vertebrae and the compression failure of the spinal segment.
Materials and Methods
22 monosegmental human cadaveric lumbar specimens were included in the analysis (Group “C” - cement: N = 12, Group “S” - spacer: N = 10). There were 8 steps to prepare the specimens: 1, isolation of a human cadaveric lumbar segment; 2, parallel embedding of cranial and caudal free endplates; 3, qCT scanning and vBMD measurement before applying interbody device; 4, introducing either a D-shaped PEEK spacer (Sanatmetal) or a custom made PMMA (Cemex) spacer as interbody device; 5, CT scanning after applying interbody device; 6, performing uniaxial compression tests (Instron 8872) and analyzing results (failure load and displacement, stiffness, elastic limit load and displacement); 7, CT scanning after the compression test; 8, finite element model was built to analyze the failure process using CT data.
Results
Comparison of initial geometrical data (vertebral cross sectional area, height, volume) and vBMD of “C” and “S” groups showed no significant difference. Failure load was similar in both groups. However, in the “S” group, there was a significant correlation between vBMD and failure load (R=0.73, p < 0.05), while such correlation was not observed (R = −0.04, p < 0.90) in the “C” group. The same association was observed for vertebral cross sectional areas (“S”: R=0.78, p < 0.01; “C”: R = −0.47, p < 0.20), vertebral height (“S”: R=0.94, p < 0.00; “C”: R = −0.27, p < 0,50), volume (“S”: R=0.85, p < 0.00; “C”: R = −0.49,p < 0.20) and failure load.
Conclusion
Analysis of mechanical test results showed that application of a PMMA cement spacer yields a significantly stiffer construct with smaller risk for subsidence compared with a PEEK spacer. However, load, that caused irreversible mechanical failure of the segments, were similar in both groups. In case of the PEEK spacer, the failure load does not depend on bone quality only, but on vertebral size parameters as well. In case of the PMMA cement spacer, such correlations were not observed. Finite element models showed completely different load transfer and failure process in the two groups. This difference is in line with our results above. In case of a relatively small vertebral size and poor bone quality, a prefabricated PEEK intervertebral cage may be associated with higher risk of mechanical failure then the use of PMMA as an interbody device.