Summary

This study aimed to evaluate the effect of humeral canal fill ratios in a CT based computational analysis on the biomechanical stability of implant and cortical bone micromotion during cyclic compression testing of a short and standard stem implant.

Abstract

Introduction

Several studies have reported proximal bone resorption in press-fit short-and standard stem humeral implantation. The purpose of this study was to evaluate the effect of humeral canal fill ratios in a CT based computational analysis on the biomechanical stability of implant and cortical bone micromotion during cyclic compression testing of a short and standard stem implant. We hypothesized that increased canal fill ratios correlate with higher primary stability but reduced bone loading in the proximal humerus.

Methods

This study consisted of two parts (1) the retrospective analysis of the humeral canal in preoperative CT scans of a clinical RSA cohort (n=186) and (2) the canal fill calculations on cadaveric CT scans with subsequent biomechanical testing (n=40). The preoperative clinical CT data were used to segment the humeral canal. In addition to conventional two-dimensional measurements, volume dimensions were evaluated and applied in the cadaveric CT scans. Bone mineral densities were additionally analyzed by including density phantoms in these CT scans. Preoperative virtual implantation positioning (VIP, Arthrex) was compared to the position after the implantation using a registration tool to connect 2D x-rays and 3D models. Stem and cup size selection in short (n=20) and standard stem (n=20) implantation were virtually defined to cover a broad range of canal fill ratios (range 0.5-1). Biomechanical testing included cyclic loading with 220N, 520N, and 820N over 1000 cycles at 1.5Hz and was performed with a constant valley load of 25N. Optical recording allowed for spatial implant tracking and quantification of cortical bone deformations in the proximal humerus. Tests on equal variance and normal distribution were performed to select the respective analysis of variance (ANOVA) procedure and respective post-hoc comparative tests including Bonferroni corrections.

Results

The humeral canal volumes were normally distribution (Shapiro Wilk: p=.670) with a mean volume of 42993 +/- 12184 mm3 and significantly correlated with the two-dimensional measures (p=.034). Preoperative planning allowed for reproducible placement of the short and standard stem implant providing a significant correlation (r=0.810; p=0.001) between pre- and post-implantation calculated volumetric canal fills resulting in a broad range of canal fill ratios (0.52-0.96). Primary stability was significantly affected by the canal fill ratio in the 520 N (r=-0.752; p=0.008) and 820 N (r=-0.795; p=0.034) loading blocks. Short stem implants (0.669 +/- 0.519mm) subsided significantly more compared to standard stem implants (0.376 +/- 0.250 mm, p=0.027) in canal fills below 0.72.

Conclusion

High canal fill ratios are associated with increased medial calcar stress shielding and resulted in our study in significantly improved primary stability independent of the bone density. Implantation in low canal fills led to significantly increased implant micromotions but maintained more proximal bone loading compared to higher ratios. Preoperative analysis of 3D canal fills may help in planning stem sizes to achieve sufficient primary stability with maintained bone loading stimulus in the proximal humerus.