I’ve noticed that I may not be paying as much attention to the acromioclavicular joint, also known as the AC joint, when treating patients recovering from shoulder pain and dysfunction. This joint is an important component in achieving proper kinematics of the shoulder and improving motor control. I wanted to touch upon the AC joint and its normal motion during shoulder movements.
I came across an article that measured the motion of the AC joint as well as the sternoclavicular joint (SC joint) and the scapulothoracic junction. The researchers measured shoulder abduction to 90 degrees in cadavers and healthy living subjects. They found the AC joint increased internal rotation motion of about 4.3 degrees, upward rotation of 14.6 degrees, and posterior tilting of about 6.7 degrees in living and cadaver subjects. (They mentioned that excessive AC joint internal rotation can cause excessive anterior tilting of the scapula leading to impingement of the subacromial tissues during shoulder elevation.) They talked about the importance of the coupling motion of the clavicle during posterior rotation which combined with greater scapular upward rotation than posterior tilting at a ratio of 3:1. They mentioned that if the clavicle only elevates and does not rotate, the coupling motion involves greater anterior tilt of the scapula than upward rotation. They also reported motions measured at the SC joint and found about 6 degrees of retraction, elevation of 6 degrees, and posterior rotation of 10 degrees during healthy shoulder abduction.
Knowing these values and the coupling motions may assist a practitioner with improvement of shoulder dysfunction and prevention of future repetitive shoulder injuries.
(Click the link below for access to the article)
Purpose: To examine 3-D movement at the AC joint during both active elevation in asymptomatic subjects and passive elevation in cadaver specimens with no signs of shoulder injury.
Methods: The dominant arm of 30 healthy living subjects were studied who did not have a history of shoulder pathology and were also cleared using a clinical screening exam. There were 16 males and 14 females studied. The researchers also studied 8 fresh frozen cadaver specimens without shoulder joint degeneration and no known history of shoulder problems.
They used electromagnetic tracking with motion monitor software.
The subjects completed 3 repetitions of shoulder abduction at a speed of 3 seconds up and 3 seconds down while in an upright position. The cadavers were placed in an upright position as well and were passively ranged in abduction. All motion was measured from 0 degrees to 90 degrees shoulder abduction.
Results: The researchers found that the AC joint demonstrated increased internal rotation of about 4.3 degrees, increased upward rotation of 14.6 degrees, and increased posterior tilting of approximately 6.7 degrees. All these values were significant. They found similar values in the cadaver measurements.
Limitations: The researchers mentioned 4 primary limitations. The first was that there was potential error in AC joint position due to sensor placement and movement of the sensor on the skin over the bone surface. The second is that they did not have direct validation of the AC joint. The third limitation is that they only studied subjects to 90 degrees of shoulder abduction and not greater due to concerns about the tracking mechanism. Finally, they stated that the results cannot be generalized due to the study examining the specific subject population age, studying individuals without impairments, and only studying dominant shoulders.