| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | |
Commentary & Perspective | ||||||||
Commentary on The acromioclavicular (AC) joint is a diarthrodial joint ostensibly connecting the acromion and the distal clavicle, but in reality suspending the entire arm, via the clavicle and sternoclavicular joint, from the axial skeleton. Because of the small size of the AC joint in relation to the large loads transmitted (e.g., the force of the pectoralis major), the stresses on the AC joint and its stabilizing structures may be quite high. Codman noted that very little motion occurred at the AC joint due to the ligaments which were "cunningly arranged."1 The acromioclavicular capsule and ligaments and the coracoclavicular ligaments (conoid and trapezoid) contribute to joint stability. In 1917, Cadenat noted that incomplete acromioclavicular dislocations involved rupture of the capsule alone, while complete dislocations resulted from rupture of the coracoclavicular ligaments2. Urist disagreed with this view, reporting that transection of the acromioclavicular capsule in cadavers could yield a full dislocation when the clavicle was pulled "upward and posteriorly."3 However, he further noted that "when it was pulled straight upward, only incomplete disarticulation was produced."3 Fukuda et al., using sequential sectioning of the ligament during load-displacement testing4, found that the relative contributions of the various ligaments, in regard to constraint, changed according to the degree of displacement. At small displacements, the most restraint was provided by the acromioclavicular capsule and ligaments, while at larger displacements the coracoclavicular ligaments became important in resisting superior displacement. Posterior translation, however, continued to depend on the intact acromioclavicular capsule and ligaments at all translations studied. The change in forces can be inferred to be due entirely to the missing ligament only if it is assumed (as in this type of testing) that the ligaments return to precisely the same position after the selective cutting. In this issue of The Journal, Debski et al. report use of a robotic manipulator and a universal force-moment sensor to achieve precise reproduction of positions allowing them to study the AC joint with great sophistication. Subluxation forces were applied to cadaveric acromioclavicular joints, and their resulting positions were recorded. Then the ligamentous constraints were sequentially sectioned; after each section, the previously recorded paths of motion for the intact joint were reproduced by the robot, and the forces and moments for each loading position were measured. After each ligament transection, the reduction in the applied subluxation force required to produce the previously recorded subluxation represented the force in that ligament before it was sectioned. The subluxation forces were again applied after the acromioclavicular capsules had been entirely sectioned, and the resulting positions were reproduced after coracoclavicular ligament sectioning in a separate test, which allowed assessment of the distribution of load to the trapezoid and conoid ligaments when the acromioclavicular capsule is violated. This is a superb and thorough investigation using state-of-the-art testing technology. My one minor concern is that the choice of scapular axes, rather than clavicular axes, for loading may result in oblique loading of the acromioclavicular joint, recruiting articular contact as a stabilizing mechanism. For example, in the transected-capsule shoulders (Table 1), the trapezoid and conoid ligaments contributed a mean of 45% and 23%, respectively, of the resistance to a posteriorly directed load. Since these contributions total 68%, the only structures remaining (after all the soft tissues have been cut), the articular surfaces, by inference, account for the remaining 32%. On the other hand, acromioclavicular injuries are messy events, as evidenced by the not infrequent associated injuries such as articular fractures and posttraumatic osteolysis of the distal clavicle, which is likely related to compressive failure of the articulation. Also, acromioclavicular injuries usually result from indirect loading, especially a direct blow to the scapula. Thus Debski et al. may have chosen loading states which reflect the clinical situation. This elegant and carefully designed study offers many lessons for the clinician. The acromioclavicular joint capsule emerges as an important stabilizer against anterior and posterior translation, though less so against superior translation. This assessment agrees with results of other studies5 and with clinical findings. Early experience with arthroscopic distal clavicle resection demonstrated that preservation of the acromioclavicular capsule might reduce load failure resulting from the posterior translation of the clavicle stump6. Furthermore, the importance in acromioclavicular dislocations of posterior translation, which is easily missed on anteroposterior radiographs, has been emphasized by Rockwood7. The finding that simple separation of the superior and inferior acromioclavicular joint capsule can reduce stability, albeit to a small degree, emphasizes that ligaments in the shoulder are often linked and do not operate in isolation (e.g., the glenohumeral ligaments which function as a complex restraint system within the glenohumeral capsule). Finally, the coracoclavicular ligaments, the conoid and trapezoid, demonstrate different mechanics. Procedures which reconstruct each ligament (i.e., coracoclavicular fixation to reconstruct the conoid ligament and coracoacromial ligament transfer to reconstruct the trapezoid) may thus restore complex acromioclavicular joint mechanics better than procedures that treat the coracoclavicular ligaments as a single structure.
References 1. Codman EA. The Shoulder. Boston, MA: Thomas Todd; 1934. | ||||||||
| ||||||||