The acromion is a bony process that juts out from the lateral end of the scapular spine. It is continuous with the blade and the spinous process. The process is rectangular, and carries a facet for the clavicle. Inferiorly is sited the subacromial bursa. Inferior encroachment or displacement of the acromion can result in impingement.

The aim of this osteological study was to assess the presence of acromial displacement and variations predisposing to compaction of the subacromial space. Using the method described by Morrison and Bigliana, we assessed the scapulae of 128 men and women ranging from 35 to 92 years of age. We found a flat acromion in 30%, no hook in 48%, a small hook in 18% and a large hook in 4%. The presence of a hook was associated with a subacromial facet and a large hook with glenoid erosion.

This study confirms the presence of four types of acromion.

The rotator cuff is sited on the anatomical neck of the humerus and is formed by the insertion of the supraspinatus (SP), infraspinatus (IS), teres minor (TM) and subscapularis. All play a vital role in the movement of the glenohumeral joint, and the anatomy is of critical importance in arthroscopic rotator cuff repair. We undertook an osteological and gross anatomical dissection study of the insertion mechanism of these tendons, in particular the SP .

The SP inserts by a triple or quadruple mechanism. The ‘heel’ (medial) and capsule fuse, inserting into the anatomical neck proximal to the anterior facet of the greater humeral tubercle. The ‘foot arch’ inserts as a strong, flat, fibrous tendon into the facet. This area is cuboidal, rectangular, or ellipsoid, and measures 36 mm² to 64 mm². In about 5%, the insertion is fleshy (pitted), rendering it weaker than a tendinous attachment. The ‘toe’ lips over the edge of the facet laterally and fuses with the periosteum, fibres of the inter-transverse ligament and the IS. A proximal ‘hood’ of about 4 mm stretches down inferiorly and fuses with the periosteum of the humeral shaft. The subacromial or subdeltoid synovial bursa are sited laterally.

The IS and TM insert into the middle and posterior facets (225 mm and 36 mm²) at respective angles of 80° and 115°. The inferior portion of the TM facet is not fused with the shoulder capsule. The subscapularis inserts broadly into the lesser tubercle, and the superior fibres fuse with the shoulder capsule and intertransverse ligament. The insertion of the subscapularis does not contribute directly to the formation of the ‘hood’, which belongs exclusively to the SP, IP and TM.

This study confirms the complexity of the SP insertion and suggests that an unfavourable attachment or biomechanical anatomical malalignment may lead to eventual tendon/cuff degeneration.

The supraspinatus tendon (SP) often ruptures. Gray established that the tendinous insertion always attaches to the highest facet of the greater tubercle of the humerus. Our osteological study of 124 shoulders in men and women between the ages of 35 and 94 years refocuses on the humeral insertion of the SP in relation to infraspinatus (IS) and teres minor (TM).

We found type-I SFs (cubic) in 53 shoulders (43%) and type-II SFs (rectangular or oblong) in 21 (17%). Type-III (ellipsoid) SFs were present in 20 shoulders (16%) and type-IV (angulated or sloping) in 11 (9%). SFs were type V (with tuberosity) in 12 shoulders (10%) and type VI (pitted) in three (2%). The facet area of the SP, IP and TM varied from 49 mm, 225 mm and 36mm2. Of the three muscles, the IS facet was consistently the largest (p < 0.05) and shaped rectangularly.

The SP inserted in a cubic or rectangular facet format in 75% of people. SP facet-size may relate to tendon strength, degeneration and rupture. This information may contribute to the understanding of tears of the rotator cuff.