Anatomical Variations of the Pectoralis Major Muscle: Notes on Their Impact on Pectoral Nerve Innervation Patterns and Discussion on Their Clinical Relevance

Abstract

Background. The presented study attempts to classify individual anatomical variants of the pectoralis major muscle (PM), including rare and unusual findings. Rare cases of muscular anomalies involving the PM or its tendon have been presented. An attempt has also been made to determine whether anatomical variations of the PM may affect the innervation pattern of the lateral and medial pectoral nerves. Material and Methods. The research was carried out on 40 cadavers of both sexes (22 males, 18 females), owing to which 80 PM specimens were examined. Results. Typical PM structure was observed in 63.75% of specimens. The most frequently observed variation was a separate clavicular portion of the PM. In one female cadaver (2.5% of specimens) the hypotrophy of the clavicular portion of the PM was noticed. In two male cadavers (5% of specimens) the fusion between the clavicular portion of the PM and the deltoid muscle was observed. In one of those cadavers, small sub-branches of the lateral pectoral nerve bilaterally joined the clavicular portion of the deltoid muscle. The detailed intramuscular distribution of certain nerve sub-branches was visualized by Sihler’s stain. PM is mainly innervated by the lateral pectoral nerve. In all specimens stained by Sihler’s technique, the contribution of the intercostal nerves in PM innervation was confirmed. Conclusions. Surgeons should be aware of anatomic variations of the PM both in planning and in conducting surgeries of the pectoral region.

An unusual finding of the pectoralis major muscle: decussation of sternal fibers across the midline

“…Plentiful examples of additional muscles can be found in the literature, for example, an accessory head of the pectoralis major, one or more sternalis muscles, various slips of muscle originating or inserting at unusual locations, and even a double pectoralis major [ 1 - 3 , 6 - 8 ]. Separated clavicular heads are the most common variation in the grouping of the pectoralis major and may or may not be fused with the deltoid muscle [ 3 - 5 , 9 ].…”

“…While the separated clavicular heads seen in this particular case are not unheard of, decussation of the sternal fibers is rare. A study by Haladaj et al [ 4 ] of 80 cadaveric specimens found that separation of the clavicular head from the sternocostal head occurred in 27.5% of cases. However, aside from a brief mention of decussating clavicular fibers by Bergman et al, other reports of decussation of the sternocostal fibers is difficult to find in the literature [ 2 ].…”

“…Unsurprisingly, with a wide range of morphological variation comes a wide range of clinical presentations. Patients with atypical pectoralis major musculature usually do not notice any functional deficits [ 3 - 5 ]. A study by Lee and Chun [ 5 ] in 1991 found some decrease in peak torque and horizontal adduction power in the setting of congenital absence of the pectoralis major, but aside from difficulty with specific actions like throwing and climbing the patient was not significantly affected.…”

“…Dissections by Soni et al [ 10 ] and Venieratos et al [ 3 ] demonstrated that fusion of the clavicular head with the deltoid may displace the cephalic vein superficially or abort it altogether, presumably without significant symptoms during the patient’s life. In cases of atypical musculature, patterns of innervation are generally conserved, although of course independently formed variations of innervation have been documented [ 4 , 11 ]. In addition to their local associations, pectoralis major variations are correlated with greater incidence of skull anomalies, adrenal anomalies, and anencephaly [ 12 ].…”

“…Variation in the degree of separation between the left and right sternocostal heads as well as interdigitation of the fibers has been described, but complete decussation remains rare [ 1 , 2 ]. Many muscles surrounding the pectoralis major are capable of compensation, so anatomical variations usually produce only minor functional deficits, if they manifest clinically at all [ 3 , 4 ]. However, awareness of potential variation has important surgical implications for flap reconstructions, breast surgeries, and other chest wall operations.…”