Elbow and Basic science
Acromioclavicular joint ligamentous system contributing to clavicular strut function: a cadaveric study

https://doi.org/10.1016/j.jse.2013.01.004Get rights and content

Hypothesis

We hypothesized that the clavicle overrides the acromion during certain shoulder motions for individuals with acromioclavicular (AC) joint separation producing clinical symptoms. We measured 3-dimensional clavicular and scapular motions in AC joint separation models during humerothoracic motions, which should be impacted by the loss of AC joint continuity.

Materials and methods

Ten shoulders from 6 whole cadavers were used. The scapular and clavicular motions were measured in intact and AC joint separation models using an electromagnetic tracking device. The measurement was performed during shoulder abduction with humerothoracic neutral rotation. It was also measured during shoulder abduction with humerothoracic internal rotation, which could cause clavicular overriding. The kinematic changes caused by ligament sectioning were evaluated in these 2 arm motions.

Results

The clavicle completely overrode the acromion in all AC separation models during abduction with internal rotation, but not in any shoulders during abduction with neutral rotation. Upward clavicular rotation increased, posterior clavicular rotation decreased, and external scapular rotation decreased with ligament sectioning. These kinematic changes were common for both of the measured arm motions. Scapular upward rotation and posterior tilt did not change because of ligament sectioning during abduction with neutral rotation. However, these scapular rotations significantly decreased with ligament sectioning during shoulder abduction with internal rotation.

Conclusion

Scapular and clavicular kinematics were affected in AC separation models. Abduction with humeral internal rotation resulted in a decrease in scapular posterior tilt and upward rotation in AC separation models, and thereby could lead to AC joint articulation dysfunction.

Section snippets

Specimen

Six whole cadavers (mean age, 74 ± 7.7 years; range, 36-87 years; 2 females, 4 males) were used. Specimens with massive rotator cuff injury or degeneration of the glenohumeral or AC joint were excluded, so that a total of ten shoulders (4 right, 6 left) were included in this study.

Data collection device

An electromagnetic tracking device system (3 Space Fastrak; Polhemus, Colchester, VT, USA) consisting of a transmitter and sensors was used. The system measured 6 degrees of motion, with a root mean square accuracy of

Shoulder abduction motion with a neutral or internally rotated humerus

Maximum humerothoracic elevation was 142° ± 1.4° for abduction with neutral rotation, and 125° ± 1.3° for abduction with internal rotation. Reliability for the three trials is shown in Table I. During abduction with neutral rotation, the mean and standard error of humerothoracic internal rotation was −39° ± 3°, −31° ± 3°, and −25° ± 4° in intact models, −42° ± 4°, −36° ± 4°, and −29° ± 5° in AC separation models at 60°, 90°, and 110° of abduction, respectively. During abduction with internal

Discussion

The clavicle serves as a strut between the sternum and the scapula. Loss of clavicular function could cause weakness, discomfort, or dyskinesis in the shoulder girdle.7, 10, 13 Acromioclavicular joint discontinuity might produce these symptoms during overhead activities. A number of biomechanical studies have examined the AC joint.3, 6 However, most have evaluated clavicular stability against a rigidly fixed scapula by loading in one direction. It would be best to evaluate the entire shoulder

Conclusion

We revealed the mechanism of clavicular overriding in AC joint separation. Scapular anterior tilt and downward rotation associated with disruption of the AC and CC ligaments could lead to loss of clavicular strut function during shoulder joint abduction with internal humeral rotation.

Acknowledgment

The cadavers used in our study were donated to the Clinical Anatomy Laboratory, Keio University School of Medicine with the consent of the families. The authors are grateful to Sadakazu Aiso, MD, PhD, Nobuaki Imanishi, MD, PhD, and Naoto Hirakata, MD, PhD for approving the use of the laboratory.

Disclaimer

The authors, their immediate families, and any research foundation with which they are affiliated did not receive any financial payments or other benefits from any commercial entity related to the subject of this article.

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