The sizing and suitability of nonspherical ellipsoid humeral heads for total shoulder arthroplasty.

Abstract

Total shoulder arthroplasty (TSA) implants have evolved to include more anatomically shaped components that better replicate the native state. The geometry of the humeral head is nonspherical, with the frontal diameter of the base of the head being up to 6% larger than the sagittal diameter. Despite this, most TSA humeral head implants are spherical, meaning that the diameter must be oversized to achieve complete coverage, resulting in articular overhang, or portions of the resection plane will remain uncovered. It is suggested that implant-bone load transfer between the backside of the humeral head and the cortex on the resection plane may yield better load-transfer characteristics if resection coverage were properly matched without overhang, thereby mitigating proximal stress shielding. Eight paired cadaveric humeri were prepared for TSA by an orthopedic surgeon who selected and prepared the anatomic humeral resection plane using a cutting guide and a reciprocating sagittal saw. The humeral head was resected, and the resulting cortical boundary of the resection plane was digitized using a stylus and an optical tracking system. To simulate optimal sizing of both circular and elliptical humeral heads, both circles and ellipses were fit to the traces. Two extreme scenarios were also investigated: upsizing until 100% total coverage and downsizing until 0% overhang. By switching from a spherical (circular) to an ellipsoid (elliptical) humeral head, a small, 2.3%±0.3% increase in total coverage occurred (P<.001), which led to a large, 19.5%±1.3% increase in cortical coverage (P<.001). Using a circular head resulted in 2.0%±0.1% greater overhang (P<.001), defined as a percentage of the total enclosed area that exceeded the bounds of the humeral resection. As a result of increasing the head size until 100% resection coverage occurred, the ellipse produced 5.4%±3.5% less overhang than the circle (P<.001). When the head size was decreased until 0% overhang occurred, total coverage was 7.5%±2.8% greater for the ellipse (P<.001) and cortical coverage was 7.9%±8.2% greater for the ellipse (P=.01). Cortical coverage was greater for circular heads when the head size was shrunk below -2.25% of the optimal fitted size. Reconstruction with ellipsoid humeral heads can provide greater total resection and cortical coverage than spherical humeral heads while avoiding excessive overhang; however, cortical coverage can be inferior when undersized. These initial findings suggest that resection-matched humeral heads may yield benefits worth pursuing in the next generation of TSA implant design.