Biomechanical Evaluation of Interbody Devices by using Mechanical Compressive Test: PEEK Spacers versus PMMA Cement Spacers
Authors: Csakany Tibor , Kurutz Marta , Varga Peter 
Published: May 2015

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 end plates and vertebrae during or after the surgical stabilization. To avoid implant subsidence, a new technique is developed where PMMA bone cement is applied as interbody device providing better contact and even more load transfer along the vertebra–implant interface. In this study, the results of in vitro mechanical compression tests are presented comparing a traditional PEEK spacer and the new PMMA cement spacer.

Materials and Methods

Overall, 22 human cadaveric lumbar segments were prepared and included in the final analysis (group C: cement; N = 12, group S: spacer; N = 10). Preparation included the following: (1) isolation of a human cadaveric lumbar segment, (2) parallel embedding of cranial and caudal free end plates, (3) CT scanning 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 (7) CT scanning after the compression test. Measurement of geometrical parameters and bone mineral density were performed on CT images.

Results

Comparison of geometry and BMD of “C” and “S” groups showed no significant difference, specimens were selected for the two groups to be as similar as possible. Failure load was also similar (group C mean: 1,929 N, 95% CI: 1,732–2,126 N; group S mean: 1,914 N, 95% CI: 1,316–2,512 N), although standard deviation was higher in the PEEK spacer group (group C SD: 309 N; group “S” 836 N). Stiffness was significantly higher in the PMMA cement group (group C mean: 1,144 N/mm, 95% CI: 913–1,375 N/mm, SD: 363 N/mm; Group S mean: 525 N/mm, 95% CI: 396–654 N/mm, SD: 180 N/mm). Implant subsidence was significantly smaller in the PMMA cement group (Group C mean: 3,525 N/mm, 95% CI: 2.81–4.24 mm and group S mean: 5.64 N/mm, 95% CI: 4.89–6.39 mm). There was another important difference between groups in the relationship of failure load and BMD. In group S, the BMD correlated strongly with failure load ( R2 = 0.53), while in group C, there was no correlation ( R2 = 0.0019).

Conclusion

Analysis of mechanical test results showed that using PMMA cement spacer compared with PEEK spacer yields a significantly stiffer construct with smaller subsidence, although loads that cause irreversible mechanical failure of the segments are similar. In the case of PEEK spacer, the failure load depends greatly on bone quality, whereas in the case of PMMA cement spacer such correlation was not observed, as PMMA can penetrate into the irregular osteoporotic bone yielding a massive vertebra–implant interface. From these results, we can conclude that the use of PMMA cement spacer may be mechanically superior to traditional PEEK spacer in the elderly population.

Acknowledgment

The authors gratefully acknowledge the Hungarian Scientific Research Fund OTKA for providing financial support in the frame of the grant K-075018.