Effect of capsular injury on acromioclavicular joint mechanics.

BACKGROUND: Traumatic disruption of the acromioclavicular joint capsule is associated with pain and instability after the injury and may lead to degenerative joint disease. The objective of this study was to quantify the effect of transection of the acromioclavicular joint capsule on the kinematics and the in situ forces in the coracoclavicular ligaments in response to external loading conditions.
METHODS: Eleven fresh-frozen human cadaveric shoulders were tested with use of a robotic/universal force-moment sensor testing system. The shoulders were subjected to three loading conditions (an anterior, posterior, and superior load of 70 N) in their intact state and after transection of the acromioclavicular joint capsule.
RESULTS: Transection of the capsule resulted in a significant (p < 0.05) increase in anterior translation (6.4 mm) and posterior translation (3.6 mm) but not in superior translation (1.6 mm). The effect of capsule transection on the forces in the coracoclavicular ligaments was also significant (p < 0.05) in response to anterior and posterior loading but not in response to superior loading. However, differences were found between the forces in the trapezoid and conoid ligaments. Under an anterior load, the mean in situ force (and standard deviation) in the trapezoid increased from 14 +/- 14 N to 25 +/- 19 N, while the mean force in the conoid increased from 15 +/- 14 N to 49 +/- 23 N, or 227%. In contrast, in response to a posterior load, the mean in situ force in the trapezoid increased from 23 +/- 15 N to 38 +/- 23 N, or 66% (p < 0.05), while the mean force in the conoid increased only 9%. CONCLUSIONS AND CLINICAL RELEVANCE: The large differences in the change of force in the conoid and trapezoid ligaments suggest that these ligaments should not be considered as one structure when surgical treatment is considered. Furthermore, transection of the capsule resulted in a shift of load to the coracoclavicular ligaments, which may render the intact coracoclavicular ligaments more likely to fail with anterior or posterior loading. The results of the present study also suggest that the intact coracoclavicular ligaments cannot compensate for the loss of capsular function during anterior-posterior loading as occurs in type-II acromioclavicular joint injuries.