This had led to some controversy regarding the evolutionary origin of turtles, because in both pareiasaurs and non-mammalian therapsids the acromion is located at the dorsal tip of the scapula. In this highly specialized endoskeletal structure, the scapula is a dorsal (directed upwards) process attached to the first rib the coracoid is a posteroventral (directed backward and down) process and the acromion is a medioventral (directed inwards and down) process (also known as the prescapular process) located at the base of the scapula. Turtles have an acromion that forms the anterior part of the triradiate pectoral girdle (together with the coracoid and scapula). The acromion process of bats ( Mammalia: Chiroptera) is particularly elongated compared to that of humans. Radiograph of the shoulder showing an os acromiale In people with symptoms of os acromiale, dynamic ultrasound sometimes shows hypermobility in the area during shoulder movement, or graded compression with the probe. This feature was common in skeletons recovered from the Mary Rose shipwreck: it is thought that in those men, much archery practice from childhood on with the mediaeval war bow (which needs a pull three times as strong as the modern standard Olympic bow) pulled at the acromion so much that it prevented bony fusion of the acromion with the scapula.Īlthough historically regarded as an incidental finding, the os acromiale may occasionally produce symptoms from subacromial impingement or instability at the site of non-union. A non-union between the pre- and meso-acromia between the pre- and meso-acromia and between the meta- and basi-acromia.A non-union between the pre- and meso-acromia and between the meso- and meta-acromia, atypical.A non-union between the pre- and meso-acromia.A non-union between the meso- and meta-acromia, the most common or typical os acromiale.įour types of os acromiale can be distinguished: Earlier estimates of its prevalence were as low as 1.4%, and this higher estimate was made by Sammarco in the year 2000, based on radiographic and anatomical studies. This condition is referred to as os acromiale, but rarely causes pain. However, in between 1% and 15% of cases, this osseous union fails and the acromion remains separate as an accessory bone. In most cases, the first three fuse at 15–18 years, whereas the base part fuses to the scapular spine at 12 years. The acromion has four ossification centers called (from tip to base) pre-acromion, meso-acromion, meta-acromion, and basi-acromion. There are three morphologically distinct types of acromia and a correlation between these morphologies and rotator cuff tear: Its medial border, shorter than the lateral, is concave, gives attachment to a portion of the trapezius, and presents about its center a small oval surface for articulation with the acromial end of the clavicle. Its lateral border is thick and irregular, and presents three or four tubercles for the tendinous origins of the deltoid. Its inferior surface is smooth and concave. Its superior surface, directed upward, backward, and lateralward, is convex, rough, and gives attachment to some fibers of the deltoideus, and in the rest of its extent is subcutaneous. The acromion forms the summit of the shoulder, and is a large, somewhat triangular or oblong process, flattened from behind forward, projecting at first lateralward, and then curving forward and upward, so as to overhang the glenoid cavity.

Type 3 acromion