Background and purpose Impingement syndrome is probably the most common cause of shoulder pain. Abnormal abduction and proximal humeral translation are associated with this condition. We evaluated whether the relative distribution between glenohumeral and scapular-trunk motions (the scapulohumeral rhythm) and the speed of motion of the arm differed between patients with impingement and a control group without shoulder symptoms.
Patients and methods 30 patients with shoulder impingement (Neer stage 2) and 11 controls were studied during active abduction and 21 patients and 9 controls were studied during passive abduction. Dynamic RSA at a speed of 2 simultaneous exposures per second was used to record the shoulder motions for 5–6 seconds.
Results Within the interval statistically evaluated (observations between 20–55° of relative active abduction in the glenohumeral joint), the patient group showed more scapular and trunk motions (p = 0.04), especially at up to 40°. The pattern of motion at passive abduction was somewhat similar to that in the controls.
Both controls and patients showed an increasing absolute (i.e. global) proximal displacement of the center of the humeral head with increasing active and passive abduction of the glenohumeral joint and humerus, without any certain difference between the groups. The mean maximum absolute proximal displacement in the patient and control groups amounted to about 30 mm and 20 mm, respectively. The corresponding relative displacement (with fixed scapula) was only 2.0 and 0.5 mm.
Active abduction was initiated with angular velocity of about 50 and 80 degrees per second, respectively, in the patients and the controls. In both groups it decreased with progressing abduction down to about 20 degrees per second (controls) after 3 seconds without there being any statistically significant difference. The angular velocities at passive abduction showed a similar pattern, still without any difference.
In both groups, the speed of proximal translation during active abduction peaked 0.5–1 second later than the speed of rotation and remained relatively even for about 1 second, followed by a deceleration.
Interpretation We found that the glenohumeral-thoracoscapular ratio during abduction of the arm in our study, measured as the distribution of motion between the glenohumeral joint and the trunk in both controls and patients with impingement, was less than or equal to 1:1. This finding differs from earlier results, probably due to the use of a method with high resolution and small influence of motions out of the frontal plane. The reason for reduced glenohumeral motions in the early phase of active abduction in the patient group is uncertain, but pain or avoidance of pain elicited by the motion was probably of importance.