Subclavius Posticus ! - An accessory muscle of the Shoulder

SUBCLAVIUS POSTICUS

Subclavius posticus is an accessory muscle of the shoulder, originating from the superior surface of sternal end of 1st rib, coursing posterolaterally to attach along the superior surface of scapula, usually lateral to the inferior belly of omohyoid muscle. This muscle is usually supplied by the suprascapular nerve.

This accessory muscle courses superior to the Subclavian vessels and brachial plexus and is sited as a potential cause of effort thrombosis of Axillo-Subclavian veins (Paget Schrotter syndrome) and also Thoracic Outlet Syndrome (especially venous compression).

The muscle measured ~10.3cm in length in this patient. Appearance of subclavius posticus in our case was more or less supero-inferiorly flattened, measuring 16.3mm in width and 7.2mm in cranio-caudal thickness.

The coronal T2 images is shown below, which shows the relationship of the Subclavius posticus muscles with the supraspinatus, subscapularis muscles. The suprascapular nerve is seen coursing along the superolateral surface of the muscle, just before the suprascapular notch, with high likelihood of nerve compression.

Below image shows comparison of Sagittal T2 images of two different patients, on the left with Subclavius Posticus muscle and on the right without this accessory muscle.

Interestingly the normal Subclavius muscle was found absent in the present case. Below image, on the right shows the normal Subclavius muscle in another patient.

I couldn't find any reference on association of 'absence of normal Subclavius' in cases of Subclavius posticus.

I am ending this long post, with this T2 Axial image, which shows the relationship of Subclavian vessels and brachial plexus coursing inferior to the S.Posticus muscle.



REFERENCES:
1. SINGHAL, S.; RAO, V. V. & MANJUNATH, K. Y. Subclavius posticus muscle - A case report. Int. J. Morphol., 26(4):813-815, 2008.