Glenohumeral Contact Kinematics in Patients After Total Shoulder Arthroplasty

Daniel F. Massimini, MSc¹, Guoan Li, PhD¹ and Jon P. Warner, MD<sup>1

1</sup> Bioengineering Laboratory, GRJ-1215, Massachusetts General Hospital, 55 Fruit Street, Boston, MA 02114.

Investigation performed at Massachusetts General Hospital, Boston, Massachusetts

Disclosure The authors did not receive any outside funding or grants in support of their research for or preparation of this work. One or more of the authors, or a member of his or her immediate family, received, in any one year, payments or other benefits in excess of $10,000 or a commitment or agreement to provide such benefits from a commercial entity (Zimmer).

Background Knowledge of in vivo glenohumeral joint contact mechanics after total shoulder arthroplasty may provide insight for the improvement of patient function, implant longevity, and surgical technique. The objective of this study was to determine the in vivo glenohumeral joint contact locations in patients after total shoulder arthroplasty. We hypothesized that the glenohumeral joint articular contact would be centered on the glenoid surface because of the ball-in-socket geometric features of the implants.

Methods Dual-plane fluoroscopic images and computer-aided design models were used to quantify patient-specific glenohumeral articular contact in thirteen shoulders following total shoulder arthroplasty. The reconstructed shoulder was imaged at arm positions of 0°, 45°, and 90° of abduction (in the coronal plane) and neutral rotation and at 90° of abduction with maximum internal and external rotation. The patients were individually investigated, and their glenohumeral joint contact centroids were reported with use of contact frequency.

Results In all positions, the glenohumeral joint contact centroids were not found at the center of the glenoid surface but at an average distance (and standard deviation) of 11.0 ± 4.3 mm from the glenoid center. Forty (62%) of the sixty-five total contact occurrences were found on the superior-posterior quadrant of the glenoid surface. The position of 0° of abduction in neutral rotation exhibited the greatest variation of quadrant contact location; however, no contact was found on the superior-anterior quadrant of the glenoid surface in this position.

Conclusions In vivo, glenohumeral joint contact after total shoulder arthroplasty is not centered on the glenoid surface, suggesting that kinematics after shoulder arthroplasty may not be governed by ball-in-socket mechanics as traditionally thought. Although contact locations as a function of arm position vary among patients, the superior-posterior quadrant seems to experience the most articular contact in the shoulder positions tested.

Clinical Relevance The results from this study may serve as benchmark data for future studies aiming to determine optimum contact biomechanics of the glenohumeral joint  following primary total shoulder arthroplasty.