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

• Published in: Journal of shoulder and elbow surgery 2023-02, Vol.32 (2), p.232-239
• Main Authors: Spangenberg, Gregory W., Faber, Kenneth J., Langohr, G. Daniel G., Reeves, Jacob M.
• Format: Article
• Language: English
• Subjects: Arthroplasty, Replacement - methods; Arthroplasty, Replacement, Shoulder; circular vs. elliptical humeral head implants; Humans; Humeral Head - surgery; humeral head implant; humeral head resection shape; implant sizing; Joint Prosthesis; Shoulder Joint - surgery; spherical vs. ellipsoid humeral head implants; Total shoulder arthroplasty
• ISSN: 1058-2746; 1532-6500
• DOI: 10.1016/j.jse.2022.07.014

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