Most of us know about the pec muscle(s), those of us with awareness of anatomy may be familiar with the upper (clavicular head), middle and lower portions (sternal attachments) of pectoralis major. “Pec major” attaches to the upper arm and is forceful adductor and internal rotator of the arm, paramount in throwing activities, push ups and related activities.
In addition, there is general awareness of the pectoralis minor muscle underneath the main pec muscle. By attaching to the shoulder blade to the ribacage (3rd, 4th and 5th ribs) “pec minor” has a different role to major and can have significant impact on shoulder blade positioning, including postural faults such as protracted or forwardly rotated shoulders (aka “round shoulders” which in turn can have implications on thoracic spine kyphosis and further kinetic chain affects). We tend to rely on these standard anatomical descriptions.
However a recent case report (1) on a 48 year male female dissection has revealed an unusual PECTORALIS INTERMEDIUS described as originating at the 5th and 6th ribs and aponeurosis of external oblique, deep to pec major and blending with pec minor. The majority of fibres inserted into the capsule of the shoulder joint (glenohumeral joint [GH]) with some fibres on the coracoid process (same insertion as pec minor) and some fibres fusing with the fascia over the biceps (short head) and coracobrachialis muscles. Interestingly the opposite side showed the same muscle but with slightly different anchor points, in addition pec minor on this dissection originated on ribs 2-4 on one side and ribs 2-5 on the other! The paper also quoted other case studies on the presence of other accessory pec muscle such as PEC MINIMUS, PEC TERTIUS and PEC QUARTUS!!!!
This is all goes to show how we may not all have “text book” anatomy. In addition to muscle variations my discussions with shoulder surgeons have included them telling me about the regular variations they see under arthroscopy with other tissues, for example patient may be missing shoulder ligaments on one side or another. Such anatomical variations may or may not have implications for injury predisposition and management, for example could the fibre attachment, mentioned above, on the GH capsule affect joint mobility and the likelihood of developing adhesive capsulitis (frozen shoulder)?
(1) Unsual concurrence of intermediate pectoralis muscle and variant insertion with it’s clinical aspects. Vani PC, Anbalagan J, Rajasekar SS. International Journal of Anatomical Variations 2018;11(3)91-83