Summary

This biomechanical study aimed to evaluate the implant micromotion of differently augmented glenoid baseplate fixations in variable preoperatively evaluable bone densities.

Abstract

Introduction

Currently there are no primary stability data of augmented baseplates in reverse shoulder arthroplasty (RSA), particularly with the use of different augmentation types and amount of lateralization. The purpose of this biomechanical study was to evaluate the implant micromotion of differently augmented glenoid baseplate fixations in variable preoperatively evaluable bone densities. It was hypothesized that metal augmented baseplate stability in poor bone densities and reduced lateralization improves reproducibility compared to bony augmentation in these cases.

Methods

This biomechanical study included a computational analysis of the glenoid bone densities in CT scans and two biomechanical testing phases. Thirty cadaveric shoulder specimens were scanned in a clinical CT scanner. Bone model development was performed based on CT voxel data imported as a 4-dimensional point cloud (i.e. [x, y, z, mgHA/cm3]) allowing for three-dimensional segmentation of the bone density regions of interest. Augments were planned in a virtual implant positioning software and allowed for assignment of the specimen in groups of n=10, respectively: Treatment: Metal augment vs. bone augment (BIO RSA) vs. Reference group without augmentation and the degree of augmentation 20° vs. 10° vs. no augmentation. Biomechanical testing included pre and post cyclic micromotion measurement in a rocking horse micromotion setup, according to ASTM. Cyclic loading was performed in an articulating setup. A constant load of 450N was applied while the polyethylene component cyclically articulated around the glenosphere from 45° to 93° abduction. Optical recording allowed for spatial implant tracking at the implant-bone interface. Pearson correlation analysis was performed for biomechanical and bone density parameters. Tests on equal variance and normal distribution were performed to select the respective analysis of variance (ANOVA) procedure and respective post-hoc tests including Bonferroni corrections.

Results

In the pre-to-post cyclic micromotion comparison, the BIO RSA group showed significantly higher variability (P=.013) compared to the metal and reference group and a significantly increased micromotion compared to the reference group. The bone density variables of the higher and lower degree augmentations (None vs. 10° vs. 20°) did not differ significantly (P>.126). Lateralization with a 20° augment resulted in significantly higher variability in pre- (P=.026) and post-cyclic (P=.017) micromotion compared to lower degree (10°) and no augmentation group with significantly increased micromotion compared to the group with no augment used. Precyclic implant micromotion significantly interacted with the covariate of the amount of lateralization (P = .005), but not with the bone density variables (P>.130). The treatment groups did not differ significantly including the bone density and lateralization covariates (P=.135). In the groupwise correlation analysis, the pre- to post cyclic micromotion significantly correlated with the subchondral cylindrical BV/TV (r=-0.63, P=.036) for the BIO group. The micromotions in the MIO (P>.178) and reference (P>.117) group did not correlate significantly with the bone density variables.

Conclusion

Biomechanical time-zero implant micromotions showed higher variability and dependency to the bone density for the BIO RSA procedure compared to a metal augmentation. However, the higher degree augmentation showed reduced stability regardless of the bone density and augment used.