Anatomy and biomechanics of the acromioclavicular and sternoclavicular joints

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Developmental anatomy/embryology

The clavicle is classified mainly as a membranous bone, and is the first to ossify at around intrauterine week 5 [1]. The acromion develops later in the second month of gestation and remains primarily as cartilage until birth [2], [3]. The primary centers of ossification for the coracoid appear around the time of birth. Two secondary epiphyseal centers appear for the acromion around age 8 to 10, and coalesce with the rest of the bony scapula between the ages of 20 and 25 [2]. DePalma viewed 114

Mechanical properties of the supporting ligaments

Strength-to-failure testing in the Biomechanics Laboratory at the Mayo Clinic in Rochester, Minnesota, demonstrated the AC capsule/ligament complex to be the strongest and stiffest, followed by the conoid ligament, and then, last, by the trapezoid ligament. The most common site of breakage was the midsubstance region, followed by rupture about the origin for all three ligaments. Isolated, or pure, ruptures from the insertion of origin were unusual, and when seen, were usually associated as

Deltoid and trapezius

The fibers of the deltoid and trapezius muscle also blend with the fibers of the superior AC ligament. Undoubtedly, they strengthen the AC ligaments and add stability to the AC joint, although their exact contribution to dynamic stabilization of the distal clavicle remains unknown [38]. This is a major limitation of many of the biomechanical studies using cadaveric specimens, and may explain why symptoms are often absent clinically after even type III injuries. The forced direction of the

Developmental anatomy/embryology

The epiphysis at the medial end of clavicle is the last to appear (at age 18–20) and to close (at age 23–25). One postmortem evaluation of the medial clavicular physis revealed that a complete closure of the medical physis may not occur until 31 years of age [64]. As a consequence, many “dislocations” in teenage and adolescent patients, are actually physeal injuries.

Osseous anatomy/joint morphology

The sternoclavicular joint is a diarthrodial, saddle-type joint (concave in the antero-posterior direction, and convex

Summary

The acromioclavicular and sternoclavicular joints have important soft-tissue static constraints that, based on biomechanical studies, imply a great deal of stability. The infrequency of significant symptoms following dislocations of these joints certainly highlights the fact that the dynamic muscle support is also very important. In performing resections of these joints for degenerative disease, our goal should be to preserve these important ligamentous supports by minimizing the amount of bone

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