What Affects AC Joint Stability?
Abstract & Commentary
Synopsis: The AC joint capsule was most important for AP translation, while the coracoclavicular ligament’s main restraint was to superior migration of the clavicle.
Source: Debski RE, et al. Effect of capsular injury on acromioclavicular joint mechanics. J Bone Joint Surg Am. 2001;83-A(9):1344-1351.
Acromioclavicular (AC) joint injuries represent almost 45% of all shoulder injuries sustained by athletes and are commonly seen by all sports medicine physicians. The typical treatment is nonoperative. Most of these involve a partial injury such as Grade I or Grade II, with sprain of the AC joint capsule and preservation of the coracoclavicular (C-C) ligaments. What has been less well known is how the AC joint capsular injury affects mobility of the clavicle, and the specific role of each ligamentous restraint about the clavicle.
The team at the University of Pittsburgh used their robotic/universal force-moment sensor testing system with 11 fresh, frozen, human cadaveric shoulders to address these questions. Three loading conditions were performed: anterior, posterior, and superior, each with a load of 70N. This was done both for intact shoulders and after selective sectioning of the inferior AC joint capsule, the superior AC joint capsule, and the trapezoid and the conoid components of the coracoclavicular ligaments. They found that the AC joint capsule was the main restraint to anterior and posterior loads. The C-C ligaments were the main restraint to superior forces. When the AC joint capsule had been transected, the C-C ligaments did not provide much restraint to anterior and posterior loads. That is, more than 6 mm of anterior translation and nearly 4 mm of posterior translation were possible with intact C-C ligaments and a transected AC joint capsule. Furthermore, with a transected AC joint capsule, much increased loads were placed on the remaining C-C ligaments, potentially leading to late failure.
Comment by David R. Diduch, MS, MD
The findings of this well-constructed cadaveric study confirm the generally accepted principle that horizontal or anterior and posterior stability is mainly the function of the AC ligaments and capsule, while vertical stability is mainly the role of the C-C ligaments. What this study has shown, however, is that sectioning the AC joint capsule as would occur with a grade I or grade II AC joint sprain, puts much increased forces on the remaining C-C ligaments. This was as much as greater than a 200% increase. This makes sense given that the vertical orientation of the C-C ligaments would not be an ideal restraint to anterior and posterior forces.
They also found that the 2 components of the C-C ligaments do not function as a single entity. Instead, they function differently depending on the direction of the applied load. The conoid served as the primary restraint against anterior and superior loading. The trapezoid functioned as the primary restraint against posterior loading. This is perhaps due to the relative orientations of these 2 ligaments. These functional differences may have implications for surgical reconstruction. Replacing the torn C-C ligaments with a single-bundle construct may provide suboptimal restraints to anterior and posterior forces. This is especially true without something done to reconstruct the AC joint capsule and ligaments. It may be that a primary repair that preserves the distal clavicle and allows the AC joint capsule to heal would be ideal for type 4, 5, and 6 dislocations. Resecting the distal clavicle and associated AC joint capsule would leave the reconstruction dependent upon only the C-C ligaments. In this case, excessive forces would be placed on the reconstruction, as excess anterior and posterior motion would be permitted. This may be a cause for late failure that can occur after this operation. At a minimum, it should cause us to consider preserving or trying to repair the AC joint capsule if at all possible at the time of the Weaver Dunn type reconstruction of the C-C ligaments. These results also may explain why some patients may have persistent pain after the operation, perhaps due to excessive anterior and posterior motion of the distal clavicle.