Background: Traumatic disruption of the acromioclavicular joint capsule is
associated with pain and instability after the injury and may lead to degen
erative joint disease. The objective of this study was to quantify the effe
ct of transection of the acromioclavicular joint capsule on the kinematics
and the in situ forces in the coracoclavicular ligaments in response to ext
ernal loading conditions.
Methods: Eleven fresh-frozen human cadaveric shoulders were tested with use
of a robotic/universal force-moment sensor testing system. The shoulders w
ere subjected to three loading conditions (an anterior, posterior, and supe
rior load of 70 N) in their intact state and after transection of the acrom
ioclavicular joint capsule.
Results: Transection of the capsule resulted in a significant (p < 0.05) in
crease 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 f
orce (and standard deviation) in the trapezoid increased from 14 <plus/minu
s> 14 N to 25 +/- 19 N, while the mean force in the conoid increased from 1
5 +/- 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 t
o 38 +/- 23 N, or 66% (p < 0.05), while the mean force in the conoid increa
sed only 9%.
Conclusions and Clinical Relevance: The large differences in the change of
force in the conoid and trapezoid ligaments suggest that these ligaments sh
ould not be considered as one structure when surgical treatment is consider
ed. Furthermore, transection of the capsule resulted in a shift of load to
the coracoclavicular ligaments, which may render the intact coracoclavicula
r ligaments more likely to fail with anterior or posterior loading, The res
ults of the present study also suggest that the intact coracoclavicular lig
aments cannot compensate for the loss of capsular function during anterior-
posterior loading as occurs in type-II acromioclavicular joint injuries.