Determining the resting position of the glenohumeral joint: A cadaver study

Study Design: Single-session repeated-measures design. Objectives: To define the resting position of the glenohumeral joint by investigating the magnitude of the anterior and posterior displacements of the humeral head and medial and lateral rotation ranges of motion (ROMs) of the glenohumeral joint at different abduction angles in cadaver specimens. Background and Purpose: The resting position of a joint is the position in the joint's ROM at which the joint capsule has its greatest laxity. It is frequently chosen as the position for assessing and treating joints with dysfunction. However, no study has been conducted to determine the resting position of the glenohumeral joint. Methods: Seven freshly frozen cadaver shoulder specimens (age at time of death [mean ± SD] was 66.9 ± 2.5 years) were studied. Specimens were mounted on a system that uses computer-controlled hydraulics and motors to induce and monitor translation and rotation movements of the glenohumeral joint. The magnitudes of total displacement (D_Total) of the head of the humerus and total ROM (R_Total) of the glenohumeral joint were measured in the plane of the scapula at 0° (neutral), 30°, 40°, 50°, 60°, and the end range of glenohumeral joint abduction. The resting position was determined as the midpoint of the shared range of the 95% to 99.9% confidence intervals of the predicted abduction position where the peaks of displacement and rotation occurred. Results: The D_Total measurements (mean ± SD) at 0°, 30°, 40°, 50°, 60°, and the end range of glenohumeral joint abduction were 30.53 ± 9.35, 44.87 ± 7.34, 45.35 ± 8.53, 43.99 ± 10.02, 39.63 ± 9.85, and 23.80 ± 10.42 mm, respectively. The R_Total measurements (mean ± SD) for the same positions were 67.15° ± 15.87°, 95.64° ± 24.26°, 98.88° ± 29.56°, 97.08° ± 30.17°, 90.91° ± 28.73°, and 63.48° ± 25.93°, respectively. The resting position was located at 39.33° ± 4.37° of glenohumeral abduction (45.13% ± 7.58% of the available abduction ROM). The resting position (Y) varied linearly with the maximum available abduction ROM (X) (Y = 0.607X - 13.120, R² = 0.679, F = 10.61, P = 0.023). There was a main effect of joint position on both displacement (P < 0.001) and rotation ROM (P < 0.001). Conclusion: In the plane of the scapula, the resting position of the glenohumeral joint (angle measured between the scapula and humerus) occurred at 39° of abduction (45% of the maximum available abduction ROM) and varied linearly with the amount of available abduction ROM. This finding suggests that in patients with glenohumeral joint hypomobility the resting position is located closer to neutral and that evaluation and treatment should be initiated accordingly at a smaller angle of abduction than the traditional resting position. Our data were derived from cadaver specimens, therefore, caution should be taken when generalizing the results of the present study to a patient population.