Analysis of healthy glenohumeral arthrokinematics using four-dimensional computed tomography throughout internal rotation and forward elevation.

Background: The glenohumeral (GH) joint is the most mobile joint in the human body and can translate, as well as rotate in its socket. Currently, it is not well established in literature how much the healthy humeral head translates, and how that changes overtime as people age. The objective of this study is to quantify GH joint proximity and translation in healthy participants and determine if there are any age-related, position-related, or direction-related differences. Methods: Thirty-one participants were recruited for this study and split into 2 cohorts: young (aged ≤ 37 years) and old (aged ≥ 45 years). Four-dimensional computed tomography scans were taken as these participants completed internal rotation (IR) to the back and forward elevation. Three-dimensional bone models of the humerus and scapula were created using 3D Slicer. An interbone distance algorithm and an iterative closest point algorithm were used to determine GH joint proximity and translation, respectively. Results: This study found that older participants displayed significantly closer joint proximity (63% of glenoid surface was within 4 mm of humeral head) during the middle of IR, compared to younger participants (52% of glenoid surface within 4 mm of humeral head). Additionally, younger participants had significantly more translation in the superior/inferior direction (16% of glenoid height) compared to the anterior/posterior direction (10% of glenoid width) throughout IR. Conclusion: This study demonstrates the significance of translational movements within the GH joint throughout IR and forward elevation, which will aid implant manufacturers in designing implants that will allow for more normalized GH translations.