Fixation of surface microarchitecture for interbody fusion devices under spondylolisthesis loading conditions

Initial stability between a lumbar fusion cage and the vertebral endplates, particularly under the adverse loading conditions characteristic of spondylolisthesis, is vital for osseointegration and fusion. The aim of this study was to compare fixation strength and stability of surface microarchitecture designs of interbody fusion devices under shear loading in synthetic bone as a function of bone density and sagittal inclination.

Two surface design parameters were evaluated, serration height and pattern (1 mm-triangle 2 mm-triangle, and 2 mm-wedge serration patterns), under 30° and 45° of sagittal inclination. Each surface design and inclination combination was tested in three types of bone quality simulated using polyurethane foam with varying density and porosity.

Overall, sagittal migration and cyclic micromotion of the 2 mm wedge design were significantly larger than the other surface designs. Sagittal migration was 68 % to 95 % greater for the 2 mm-wedge design at similar forces, and 28 % to 63 % greater in cyclic micromotion. These differences were less pronounced when inclination was increased and/or bone density was decreased.

The results of this study indicated that surface serrations with tips closer aligned to the direction of shear force at the endplate, such as the wedge design, lead to greater migration and micromotion. Among the factors investigated, total sagittal migration was more heavily impacted by surface microarchitecture than micromotion and maximum force. Surface microarchitecture had a smaller effect on stability than bone density under higher inclination. Therefore, differences in bone quality and inclination are important considerations when selecting or designing interbody fusion devices.