A biomechanical comparison of elliptical and spherical humeral head components in shoulder arthroplasty

Background: In the native shoulder, the humeral head is spherical in shape centrally with an elliptical peripheral contour. Elliptical humeral head implants were developed with the aim of more accurately restoring the native glenohumeral anatomy and biomechanics in shoulder arthroplasty. The purpose of this cadaveric biomechanical study is to compare differences in glenohumeral contact pressure, glenohumeral contact area, and humeral head translation during range of motion between a chamfer cut stemless elliptical stemless humeral head (EH) and stemless spherical stemless humeral head (SH), using the native humerus (NH) as a control. Materials and methods: Fifteen cadaveric specimen pairs were used. Specimens were dissected, preserving the deltoid, rotator cuff muscles, and coracoacromial arch. Specimens were first tested in the native state and then implanted with a chamfer cut stemless EH and retested. The implant was removed and replaced with a stemless SH. Specimens were cycled on a custom dynamic shoulder frame. Glenohumeral pressure maps were recorded with a Tekscan System (Norwood, MA, USA). Humeral head position and translation were recorded using a 3-dimensional (3D) digitizer with 0.3 mm accuracy (MicroScribe 3DLX; Immersion). Results: At 60° of abduction, the NH had a statistically greater contact area compared to the SH (265 mm ± 118 vs. 113 mm ± 63, P = .001), with no difference between NH and EH (265 mm ± 118 vs. 149 mm ± 76, P = .61). At 30° of abduction, peak pressure significantly differed between NH and EH (1826.8 kPa ± 800.8 kPa vs. 836.3 kPa ± 475.7, P = .004), in addition to NH and SH (1826.8 kPa ± 800.8 kPa vs. 700.1 kPa ± 439.2 kPa, P = .001) with higher pressures in the native shoulder. Peak pressure at 60° showed a significant difference between NH and SH (1121.5 kPa ± 460.2 mm vs. 581.7 ± 436.3 kPa, P = .006) and no difference between NH and EH (1121.5 kPa ± 460.2 mm vs. 790 ± 437.1 kPa, P = .08) or EH and SH (790 kPa ± 437.1 mm vs. 581.6 ± 436.3 kPa, P = .49). Conclusion: This biomechanical study suggests that an elliptical humeral head in anatomic shoulder arthroplasty more closely mimics the peak contact pressures and contact area of the glenohumeral joint compared to a spherical humeral head.