AssessmentsAnatomy of the pectoral and scapular regions
USMLE® Step 1 style questions USMLE
A 60-year-old man is found to have thickening of the supraspinatus muscle on a shoulder MRI. Which of the following best describes the function of the supraspinatus muscle?
Content Reviewers:Scott Caterine, BSc (Hons.), MSc, MB, BCh, BAO (Hons.), Viviana Popa, MD
Contributors:Patricia Nguyen, MScBMC, Elizabeth Nixon-Shapiro, MSMI, CMI, Sam Gillespie, BSc, Evode Iradufasha, MD
The pectoral and scapular regions consist of various structures that include muscles, blood vessels, and nerves; which all act together to make our upper limbs functional. The muscles of the pectoral region are divided into groups based on their locations.
First, let’s take a look at the axio-appendicular muscles, which are divided into two large groups: the anterior and posterior groups of muscles.
The pectoralis major is a relatively flat, fan shaped muscle, which covers the upper half of the thorax.
These muscles don’t come completely together, thus creating a narrow gap between them that’s called the deltopectoral groove, which is where the cephalic vein passes.
The lateral pectoral nerve receives fibers from the anterior rami of C5, C6, and C7; and the medial pectoral nerve receives fibers from the anterior rami of C8 and T1, and they are also connected by a communicating branch.
Alright, so when these two heads of the pectoralis major muscle contract simultaneously, they produce adduction, and medial rotation of the arm. Additionally, each muscle head can also contract independently.
You can think of these two muscles as cousins, where the major covers and gives comfort to the minor one.
The fibers of pectoralis minor converge towards its apex superolaterally into a tendon that attaches to the coracoid process of the scapula. The pectoralis minor is innervated by the medial pectoral nerve.
It also helps during deep inhalation because this muscle can elevate the ribs as it contracts, thus increasing the volume of the thoracic cavity, so more air can get into the lungs.
Next in the anterior axio-appendicular group is the subclavius, which is this small round muscle lying horizontally inferior to the clavicle and superior to the first rib. It’s best seen when the arm is in the anatomical position.
Proximally, the subclavius attaches to the first rib and its costal cartilage, while distally, it attaches to the inferior surface of the middle third of the clavicle. This is where this muscle gets its name.
The subclavius helps to stabilize the shoulder by anchoring and depressing the clavicle and also adding some strength to stabilize the acromioclavicular joint when you hit those heavy weights at the gym.
Last but not least, there’s the serratus anterior muscle, which forms the medial wall of the axilla and lies over the lateral part of the thorax.
This is a fitting name for the muscle because “Serratus” means saw and the way this muscle’s fibers are arranged on its anterior border gives it a saw-like appearance.
The muscle originates on the external surfaces of the lateral parts of the upper eight ribs and then prolong laterally and posteriorly to insert onto the whole length of the anterior aspect of the medial border of the scapula, along with the inferior angle of the scapula.
This happens when you are trying to reach for something anteriorly or when making a punching motion, which is why this muscle is also referred to as the boxer’s muscle.
OK, so now let’s pause and reflect a bit on what we've seen so far. In the meantime, can you recall the muscles of the anterior axio-appendicular group and their innervations?
After branching off from the axillary artery, the thoracoacromial artery curls around the superomedial border of the pectoralis minor, and then pierces the costocoracoid membrane of the clavipectoral fascia.
After it emerges on the other side of the fascia, that’s when it divides into four branches in which the first is the pectoral artery.
The pectoral branch passes between the pectoralis major and minor where it provides branches that supply these two muscles.
The thoracoacromial artery also gives the acromial branch, which runs superior to the coracoid process, towards the acromion.
Finally, towards the lateral side, the thoracoacromial artery gives the deltoid artery, which then crosses over the pectoralis minor muscle, and then passes alongside the cephalic vein in the deltopectoral groove between the pectoralis major and deltoid muscle.
At this level, the deltoid artery provides branches that supply both of these muscles. Now, let’s move to the muscles of the posterior axio-appendicular group.
The trapezius is this large triangular muscle extending from the posterior aspect of the neck to the superior half of the back.
There’re two trapezius muscles in the back, which when seen together, look like a trapezium.
Based on the orientation of its fibers, the trapezius is divided into the ascending part, the middle part, and the descending part.
Contraction of the descending part results in the elevation of the scapula, like when shrugging the shoulders.
It’s also a powerful adductor and is the muscle used when pulling yourself up during rock climbing, chin ups, or movements such as swimming or chopping wood.
The second subgroup of the posterior axio-appendicular muscle group consists of two deep muscles, which are the levator scapulae and the rhomboids.
Proximally, it attaches to the posterior tubercles of the transverse processes of C1 to C4 vertebrae and then descends to distally attach to the medial border of the scapula, superior to the root of the scapular spine.
The levator scapulae is innervated by the dorsal scapular nerve which arises from the anterior rami of C4 and C5, as well as by direct branches from the anterior rami of the C3 and C4 cervical nerves.
It also fixes the scapula and holds it in place to resist downward forces like when carrying a heavy piece of luggage up the stairs. It also rotates the glenoid cavity inferiorly by rotating the scapula downwards.
Now, let’s have a look at the rhomboids, the muscles that are immediately located deep to the trapezius.
Both rhomboids are innervated by the dorsal scapular nerve, which receives fibers from the anterior rami of C4 and C5.
These movements enable us to forcibly lower the arm, like when driving a stake with a sledgehammer.