During development, the deltoid, teres major, ifraspinatus and supraspinatus muscles arise from a common premuscle mass continuous with the pectoral mass and the common arm sheath. In an 11 mm embryo the deltoid muscle has partially split off from the mass towards its origin from the acromion and clavicula. In embryos 14–16 mm in length it has much the adult form, with usually a distinct slip arising from the fascia over the infraspinatus muscle. In a 20 mm embryo it has practically the adult form and attachments. The development of the acromion from the cephalic border of the scapula partially separates the supraspinatus muscle from the infraspinatus in an 11 mm embryo. The infraspinatus and teres minor muscles are very closely associated from the outset and cover only a portion of the lateral surface of the scapula in an 11 mm embryo. In a 14 mm embryo the infraspinatus is quite distinct from the deltoid muscle, but does not cover the whole of the fossa infraspinata even in a 16 mm or 20 mm embryo [2–4].
Only a few variants of the infraspinatus muscle have been described previously. Macalister [15] described an infraspinatus muscle split into two laminae, which did not completely overlay each other [15]. The infraspinatus muscle fascia derived from the deltoid muscle to the infraspinatus muscle, and in the reverse direction from the infraspinatus to the deltoid [5, 15]. A case found by Ashaolu et al. [1] showed two infraspinatus muscles attached to the medial surface of the infraspinous fossa and the humeral greater tuberosity [1]. There was a case that described an infraspinatus accessory muscle [14]. This additional muscle derived from the medial scapular border, ran directly under to the scapular spine and ended on the greater tuberosity of the humerus [14].
The following two variations are most significant for this paper. The infraspinatus muscle can be fused with teres minor [5, 15, 18]. According to data completed by Mori [18], this fusion occurs in 10% of the Japanese population [18]. An additional muscle named the infraspinatus minor derives directly below the scapular spine and inserts into the greater tuberosity of the humerus. It can be observed not completely differentiated from the main muscle mass of the infraspinatus [7, 22]. An infraspinatus minor muscle with two bellies has not been mentioned in the previous literature [7, 22].
Kato et al. [13] redefined the structure of the infraspinatus muscle, dividing it into two parts, transverse and oblique. The oblique part has the shape of a fan. It originates from the infraspinous fossa and inserts into the greater tuberosity of the humerus. The transverse part originates from the inferior surface of the scapular spine and ends on the tendinous part of the oblique part of the infraspinatus muscle [13].
We identified the muscle masses located directly under the scapular spine not as the transverse part of the infraspinatus muscle, but as the infraspinatus minor muscle. We found that the muscle bundles were easy to separate from the main part of the infraspinatus.
Atrophy of the infraspinatus muscle has been observed by clinicians. The usual reason for this pathology is compression of the suprascapular nerve in the spinoglenoid notch [24]. Infraspinatus muscle atrophy commonly occurs in sports with overhead throwing motions such as tennis [8] and volleyball [9]. This pathology is not painful, but it limits the athlete’s achievements [8, 9]. Such atrophy can be treated with exercises that strengthen the external rotators of the glenohumeral joint. If the rehabilitation is not effective, this pathology can be treated surgically. Neurolysis of the compressed nerve succeeded by temporary immobilization and rehabilitation brings satisfactory results [9]. The infraspinatus muscle is important in the treatment of the Hill-Sachs lesion by remplissage; this lesion can occur secondary to an anterior glenohumeral joint dislocation. The manifestation of the Hill-Sachs lesion is a defect in the humeral head that leads to destabilization of the glenohumeral joint. One step in the remplissage method is translocation of the humeral attachment of the infraspinatus muscle [10, 11].
A double-headed infraspinatus minor muscle can provide additional strength and precision for movements in the shoulder girdle. It can be crucial for athletes practicing sports such as softball, tennis or volleyball [8, 9, 16]. Fusion between the infraspinatus and teres minor muscles with three additional muscle slips can support both muscles. The infraspinatus minor muscle can take a supportive role in remplissage.
There are situations in which a muscle is used as a graft. For example, the latissimus dorsi can be used for breast reconstruction after mastectomy [23]. Is there any possibility of using the double-headed infraspinatus minor muscle as a graft? In our opinion, surgical use of this additional muscle is doubtful. Harvesting it would be a problem as during dissection, we had to cut the deltoid muscle transversely. An endoscopic approach would also be problematic. Tight spaces and the presence of the suprascalupar nerve and artery could result in injury of those structures during graft preparation. Possible complications after such procedures would be serious.
Muscle variations are occasionally responsible for nerve and vessel compression. A good example of this problem is a variation of the lateral insertion of the latissimus dorsi muscle, the axillary arch. The muscle slip running from the latissimus dorsi to the pectoralis major muscle can compress the neurovascular bundle in the cervico-axillary region [6, 21]. In our case, there was little chance of nerve or artery compression. On the other hand, there is a chance that the suprascapular nerve and artery could run between the bellies of the infraspinatus minor muscle, so the double-headed infraspinatus minor muscle could cause neurovascular compression in that region.