Knowledge about scapular movement under the influence of glenohumeral motion aids the understanding of the mechanics and pathology of the shoulder joint. Cathcart
 was the first to recognize the contribution made by the scapulothoracic joint to normal shoulder complex kinematics. Codman
 termed this synchronous motion as scapulohumeral rhythm (SHR). Inman et al.
 reported a SHR of 2:1 in healthy subjects. Since, much research on shoulder kinematics has been directed toward the study of SHR
[2, 14, 15]. Braman et al.
 reported that shoulder motion differed between patients with advanced glenohumeral osteoarthritis and healthy individuals, and that total shoulder arthroplasty, restored SHRs to normal values.
In the present study, SHR was 2.4:1 on operated shoulders and 4.1:1 on contralateral shoulders at 120 degrees of abduction. This means that there was less glenohumeral motion and more scapular motion in operated shoulders compared to nonoperated, contralateral shoulders. We consider this postoperative change a useful adaptation of scapular mechanics to maintain the tension of the deltoid muscle to generate the forces necessary for shoulder motion, as was mentioned by Mell et al.
In particular, because the function of deltoid muscle is important and the rotator cuff has no function in patients with an implanted reverse total shoulder system, this scapular motion represents a highly meaningful change. However, we carefully propose the hypothesis that that increased scapular motion after RTSR might be able to lead more stretching and fatigue of periscapular muscles at long term follow up after surgery. The specific rehabilitation programs might be needed to be designed to prevent periscapular muscle fatigue after RTSA.
In the present study, during 0 to 30 degrees of abduction, distance between the vertical vertebral line and inferior pole of the scapula was decreased on operated shoulders, which means downward rotation of the scapula on initial abduction; described as motion of the glenohumeral type by Yano et al.
. Furthermore, the angle between the vertical vertebral line and scapular medial border increased less than in contralateral shoulders on initial abduction. However, beyond 30 degrees, changes in distance and angle were higher than in contralateral shoulders. Therefore, more scapular upward rotation occurred during middle and late abduction.
Yano et al.
 proposed two types of upward rotation during the initial phase of elevation. For the glenohumeral type (much glenohumeral motion), the scapula slightly rotates downward and then progresses upward, and for the scapulothoracic type (much scapular motion), the scapula directly rotates upward. Inman et al.
 used the term ‘the setting phase’ to describe the early phase of shoulder motion during the first 60 degrees of abduction, indicating preparatory stabilization of the scapula to permit controlled humeral motion. Yano et al.
 reported that SHR was generally greatest (less scapular motion) during the setting phase and that it then decreased beyond 60 degrees of abduction (more scapular motion). Furthermore, the SHR increased again below 60 degrees of abduction. They mentioned that muscular stabilization of the scapula increases while raising the arms, and thus, that less scapular motion is seen during the setting phase.
The present study has some limitations. The most obvious of which is the small number of cases enrolled. This investigation is a pilot study of our early experience with this prosthesis. Based on our inclusion criteria and the fact that it is difficult to have patients return for follow-up, we were only able to include 7 patients at this time. In addition, we only considered upward rotation of the scapula, and not the other planes. Nevertheless, the study has relevance in the context of evaluating changes of scapular upward rotation after reverse total shoulder arthroplasty. Because it is the main movement of the scapula, upward rotation is frequently addressed during treatment and research
[17–19]. This preliminary work is a proof of concept study. Future work will include more patients and may be able to use of CT to measure scapular position.