December 15, 2005
Anatomy of the Shoulder
by Anne Kossacheff, YKHC Senior Physical Therapist
Your shoulder girdle is made up of three bones: the clavicle (collar bone), scapula (shoulder blade) and the humerus.
Together with ligaments, these bones make up the sterno-clavicular joint, the acromio-clavicular joint (aka the AC joint) and the gleno-humeral joint of the shoulder girdle. Can you find the joints in the picture? There are 6 ligaments holding the shoulder joints together: the sterno-clavicular, acromio-clavicular, gleno-humeral and the coraco-acromial ligaments (there are three separate parts of the coraco-acromial ligament). These ligaments, along with muscles and tendons, keep your arm connected to your body. This allows your shoulder to move in a wide range of motion, but makes it inherently unstable, and prone to injury.
Major muscles that provide stability and mobility to the shoulder girdle include the pectorals, biceps, triceps, trapezius, rhomboids and the rotator cuff. You can palpate the pectorals on the front of your chest. They are the muscle tissue you feel along the front of your ribcage, and into the front of your shoulder. The pectoralis major (the larger muscle) and minor (the smaller muscle, underneath pectoralis major) internally rotate your upper arm, and provide a lot of stability. This proximal (close to the center) stability allows for more distal (far from the center) mobility.
Your biceps and triceps are more minor shoulder stabilizers directly attached to the shoulder joint. These two muscles provide more mobility than stability at the shoulder, and also move the elbow joint. There are two parts of the trapezius muscle --the upper and lower trapezius. The upper trapezius runs from the cervical spine (remember where that is?) to the scapula, or shoulder blade. The lower trapezius runs from the thoracic spine to the scapula. Running along the upper half of the inner edge (or superior medical border) of your scapula is the rhomboid muscle. Together, the rhomboid and trapezius muscles provide stability of the scapula and the shoulder joint.
The powerhouse of the shoulder joint is the rotator cuff: the supraspinatus, infraspinatus, teres major and subscapularis. These four little muscles hold your humerus (the upper arm bone) on your scapula and allow you to lift your arm over head, throw a ball, cast a fishing line, drive a snow machine, and do just about everything you do with your arm. If your rotator cuff muscles are injured or irritated, the stability of the gleno-humeral joint is poor, and any movement of the arm away from your side can cause impingement or pinching of the soft tissues in the shoulder. If not addressed early on, this can lead to a number of chronic problems, including tendonitis, bursitis, and degeneration of the AC joint.Other important muscles in the shoulder girdle are the deltoid and serratus muscles. The deltoid muscle moves your arm out to the side, and also helps stabilize the shoulder for overhead work. Your serratus muscle stabilizes the scapula, and moves the scapula forward, allowing more range of motion at the gleno-humeral joint.
Any impact injury can damage any one or more of the ligaments and muscles described above. Problems in the shoulder are not always visible, as swelling can hide in small spaces in the shoulder region, and it is often difficult to palpate the muscles deep in the joints. The brachial plexus (the nerve bundle that runs down your arm from your neck) runs along the front of the shoulder, under the pectoralis muscles. Any chronic irritation or swelling can irritate the nerves, and cause symptoms in the arm. In addition, any tightness in the pectoral muscles, or an imbalance in the tension of the shoulder girdle muscles can cause nerve irritation.
Check next month's column to learn easy exercises to strengthen and stretch the muscles of your shoulder girdle.