Determination of inter-individual differences in humeral head centre translations in subjects with healthy shoulders.

The aims of this study were to (i) quantify glenohumeral (GH) joint translations in healthy shoulders, (ii) assess subject-specific variability in GH translations, (iii) determine whether bony joint geometry (glenoid height, width, retroversion, inclination, GH asphericity and congruence), upright scapular posture, motion and handheld load affect translation magnitude and (iv) analyse the position of the humeral head centre (HHC) starting point relative to different glenoid centre definitions. We measured translations of the HHC relative to the glenoid axes in 19 healthy subjects during five arm motions. Mean 3D, superoinferior (SI), anteroposterior (AP), and mediolateral (ML) translation magnitudes were 14.7% of glenoid height, 9.5% of glenoid height, 15.4% of glenoid width, and 7.4% of glenoid height, respectively. Glenoid height affected 3D and SI translation magnitude inclination angle, glenoid asphericity and scapular tilt affected 3D and AP translation magnitudes and motion affected 3D, AP and SI translation magnitudes (p $\le$ 0.05, but effect sizes are small and large confidence intervals warrant further experiments). The starting point of the HHC was closer to the glenoid geometric mean than to the inferior glenoid circle center, which may be useful clinically when attempting to restore healthy HHC positioning.