The acromioclavicular (AC) joint contributes to shoulder elevation ____________________
a. between 30 ° and 90 ° of abduction when tension on the trapezoid component of the coracoclavicular ligament produces a posterior rotation of the clavicle
b. after 90 ° of shoulder abduction and until all scapulothoracic motion is taken up
c. between 30 ° and 90 ° of abduction when anterior rotation of the clavicle allows for 30 ° of scapular upward rotation
d. after 60 ° of abduction
ANS: B
The AC joint remains retrained well into the second phase of elevation due to the force of the coracoclavicular ligament. As the scapula continues to upwardly rotate, it places a great deal of tension on the conoid portion of the coracoclavicular ligament. During the second 30 ° of scapular upward rotation (after 90 ° osteokinematic motion), the conoid portion of the coracoclavicular ligament pulls the clavicle in a posteroinferior direction. At this point, the AC joint absorbs various amounts of force via anterior/posterior and medial/lateral scapular tipping.
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