Does improved resection plane coverage during shoulder arthroplasty influence proximal humeral bone stress? A comparison of circular vs. elliptical humeral heads.

Background: Stress shielding remains a concern following total shoulder arthroplasty using press-fit short humeral stems. While the effect of alterations in implant geometry, positioning, and sizing on stress shielding have been investigated, the effects of coverage of the cortical boundary of the resection plane have not yet been fully explored. The purpose of this study was to quantify the effect of improved cortical coverage using elliptical vs. circular humeral heads on changes in bone stress and resorbing potential. We hypothesized that better cortical coverage would reduce stress shielding potential.

Methods: Finite element models of 8 cadaveric humeri were virtually reconstructed with a short stem implant and an optimally fitted circular or elliptical humeral head. Trabecular bone material properties were assigned based on computed tomography attenuation and cortical bone was assigned uniform properties. Loads were applied to mimic 45° and 75° of abduction, and the resulting changes in bone stress were compared to the intact state and the expected time-zero bone resorbing potential were ascertained.

Results: The elliptical humeral heads significantly improved cortical coverage and load transfer in the medial and lateral quadrant resulting in less alteration in cortical bone stress compared to intact and significantly less cortical bone with resorbing potential. However, this came at the cost of significant but comparatively lower increases in cortical resorbing potential in the anterior and lateral quadrants. No significant effects were detected for trabecular bone in any quadrant.

Discussion: The results of this work show that improvements in cortical coverage have a protective effect resulting in less bone volume with resorbing potential. However, in the case of spherical vs. elliptical heads these improvements in the medial and lateral cortex came with tradeoffs in the anterior and posterior cortex because of reduced load transfer in these regions.