Biomechanical Evaluation of Interbody Devices by using Mechanical Compressive Test: PEEK Spacers versus PMMA Cement Spacers

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,