BackgroundNon-spherical humeral head implants more closely resemble native humeral anatomy than spherical components and may better replicate native shoulder range of motion and kinematics. The purpose of this study was to compare shoulder range of motion and kinematics of a commercially available non-spherical humeral head implant with the native humeral head and a height matched, custom manufactured spherical implant. MethodsSix fresh frozen cadaveric shoulder specimens were used with a custom shoulder testing system. The native shoulder was tested in multiple positions under anatomic muscle loading. Each specimen was tested for range of motion and glenohumeral joint kinematics by measuring the humeral head apex (HHA) and humeral head center (HHC) translation per degree of rotation using a MicroScribe digitizer. Measurements were then repeated after implantation of a spherical and, subsequently, a non-spherical humeral head prothesis. ResultsThe non-spherical implant had significantly more internal rotation compared to the spherical implant at 0° abduction (10.6 ± 6.2° more internal rotation, p=0.004, 95% CI:-13.3, 34.5), 30° abduction (5.7 ± 2.8°, p=0.009, 95% CI:-12.6, 24.0) and 60° abduction (6.8 ± 2.7°, p=0.002, 95% CI:-8.3, 22.1) in the scapular plane, and 60° abduction (6.9 ± 2.0°, p=0.031, 95% CI:-12, 25.6) in the coronal plane. The non-spherical implant had more internal rotation than the native head at 60° of abduction in the scapular plane (7.0 ± 2.2° p=0.002, 95% CI:-10.3, 24.3). The spherical head had less internal rotation than the native head at 0° abduction (7.2 ± 4.8°, p=0.031, 95% CI:32.5, 18.1). There were no differences in humeral head apex (HHA) translation per degree of rotation noted between the spherical implant or non-spherical implant and the native shoulder. The non-spherical head had less HHC translation than the native shoulder at 30° abduction in the forward flexion plane (p=0.007); otherwise, there were no statistically significant differences in HHC translation between the native shoulder, the spherical head, and the non-spherical head. There was no significant difference observed between the average difference in AP and SI ROC of the non-spherical implants (2.0 ± 0.7 mm) and the native humeral heads (1.9 ± 1.3 mm) [p = 0.926]. ConclusionThe results of this biomechanical study suggest that the commercially available non-spherical humeral head has improved internal rotation when compared to a custom, height controlled spherical implant and offers range of motion and kinematics similar to the native humeral head in a cadaveric model.