When they measured the amount of contact pressure as the scapula was elevated, the pressure “decrease linearly with anterior tilt,” and the difference between that and neutral scapular position was 20 degrees. Peak contact pressure did not change much during scapular internal rotation and downward rotation. Thus, this study — among several previous similar studies and reviews — does not support the belief that shoulders need to be “adjusted” to treat SIS since shoulder elevation and rounded shoulders do not necessarily cause painful symptoms.
The causes of SIS is multifactorial, not just one cause as some manual therapists and surgeons believe. “Several factors including glenohumeral joint position, capsule tightness, subacromial spur, rotator cuff dysfunction, and scapular position increase the subacromial contact pressure. These pathologic factors can lead to the damage on the rotator cuff and surrounding tissues, resulting in shoulder pain,” Muraki added. “However, an increase of subacromial contact pressure does not always provoke shoulder pain. In many cases, existence of inflamed tissue with low pain-threshold is necessary. Pain should be caused when contact pressure on the inflamed tissue increases in the subacromial space.”
Dr. Kai-Nan An, Professor Emeritus of the Mayo Clinic College of Medicine, who coauthored the research paper, emphasized that contact pressure does not always equate to pain unless the amount of pressure exceeds the patient's pain threshold.” Inflammation will reduce that threshold, so the likely of pain increased in inflamed tissue,” Dr. An mentioned. “On the other hand, the contact pressure along with gliding friction could cause micro-damage of the tissue or adaption and remodeling of the tissue, which ultimately leads to degeneration process.”
So what does this mean for manual therapists?
“Clinicians have believed that scapular posterior tilting lead to widening the subacromial space, then results in decrease of subacromial contact. This belief is one of the reasons why scapular posterior tilting is recommended to improve shoulder pain in patients with SIS who often show scapular anterior tilting during arm elevation,” Dr. Muraki explained. “However, our findings suggest that posterior tilting from anteriorly tilted position increase subacromial contact pressure and that exercise to tilt the scapular posteriorly is not recommended if subacromial contact pressure is causing pain.”
“The clinical significance of our finding is that anterior tilting may reduce the peak contact pressure. So it should be considered in the control of scapular motion in treating SIS,” Dr. An added.
Like many studies, this one isn't without a few limitations. The authors mentioned that the scapular and humeral rotation in the study may likely be different in a living person. However, they cited a study by Ludewig et al. that demonstrated “in vivo glenohumeral joint was almost constant during thoracohumeral elevation in the scapular plane,” which is similar to what this study found. (2) Another limitation is that the cadaver study may not apply much to living patients with altered scapular motion.
While there are many factors that influence pain, this study — among many — stacks on top of others that indicate biomechanics factors alone do not cause pain. A recent systematic review published in Manual Therapy did not find a strong association between the curvature of the thoracic spine and shoulder pain, including SIS. (3) (We have covered this topic here.) An earlier systematic review published in the British Journal of Sports Medicine did not find a scapular position and movement to be reliable indicators of pain. Ratcliffe et al. stated, “deviation from a ‘normal’ scapular position may not be contributory to SIS but part of normal variations.” (4)
1. Muraki T, Yamamoto N, Sperling JW, Steinmann SP, Cofield RH, An KN. The effect of scapular position on subacromial contact behavior: a cadaver study. J Shoulder Elbow Surg. 2017 Jan 11. pii: S1058-2746(16)30540-7. doi: 10.1016/j.jse.2016.10.009.
2. Ludewig PM, Phadke V, Braman JP, Hassett DR, Cieminski CJ, LaPrade RF. Motion of the shoulder complex during multiplanar humeral elevation. J Bone Joint Surg Am. 2009;91:378-89.
3. Barrett E, O'Keeffe M, O'Sullivan K, Lewis J, McCreesh K. Is thoracic spine posture associated with shoulder pain, range of motion and function? A systematic review. Man Ther. 2016 Jul 21;26:38-46. doi: 10.1016/j.math.2016.07.008.
4. Ratcliffe E et al. Is there a relationship between subacromial impingement syndrome and scapular orientation? A systematic review. British Journal of Sports Medicine. 2014 Aug;48 (16):1251-6. doi:10.1136/bjsports-2013-092389.