Anatomy of the Shoulder
Bones of the shoulder
The shoulder floats in place supported by soft tissues and a small connection to the breastbone, or sternum, via the clavicle bone. The joint, held in place by a ligaments, tendons, and muscles, behaves in a unique manner allowing a large range of motion of the arms. The shoulder bones in the joint can become damaged and developing shoulder arthritis.
Scapula (shoulder blade) – A tough and hard to break triangular shaped bone rests within the muscles of the back providing flexible support as it connects the upper arm bone, called the humerus, to the clavicle.
Clavicle (collar bone) – The clavicle, also called the collar bone or the beauty bone, acts as a ridged intermediary in connecting the sternum of the chest to the shoulder blade. People have two clavicles, one on each side, and the clavicle plays a large role in prominently defining the chest and neck. Technically a long bone, the clavicle lays horizontally unlike every other long bone in the body.
Humerus (arm bone) – The proximal humerus plays a role in the shoulder joint and the distal end in the elbow joint. If you hit your “funny bone” you have hit your humerus despite you not finding it humorous. The humeral head lays inside the joint capsule and rotates freely in the joint as you move your arm. When damaged it is possible to resurface the ball shaped head so the shoulder can continue to function without total shoulder replacement.
Held together by soft tissues such as ligaments, tendons, muscles, and a joint capsule, the bones form a platform for the arm to function with a large range of motion.
Joints of the shoulder
The shoulder has one articulation, a pseudo-joint like relationship between the scapula (shoulder blade) and the chest wall; the articulation of the shoulder blade and chest wall lacks ligaments. The main joint of the shoulder is the glenohumeral joint. This joint comprises a ball (the humeral head) on a golf-tee-shaped joint (the glenoid of the scapula). When arthritis effects the joint, a surgeon performs a total shoulder replacement or shoulder resurfacing.
Glenohumeral joint – The main joint of the shoulder the glenohumeral joint connects the ball shaped humeral head and the partial socket of the Scapula forming the portion of the shoulder joint surrounded by a joint capsule. The joint capsule can swell, often in patients older that 40, with inactivity causing a frozen shoulder.
Acromioclavicular joint – The Acromioclavicular joint connects the clavicle to the scapula with ligament allowing the shoulder to perform abduction and flexion, away from and towards the centerline of the body, movements
Sternoclavicular joint – The sternum connects to both clavicles and the clavicles to each other via the sternoclavicular joint. This joint allows for large amounts of motion while allowing the clavicle to also provide structural support to the shoulder and arms.
Muscles of the Shoulder
The muscles of the back and the chest play a role in the support of the shoulder, but the muscles that play the largest role in the clinical presentation of shoulder pain include the muscles that compose the rotator cuff. The composition of the rotator cuff includes the subscapularis, supraspinatus, infraspinatus and teres minor. These muscles have tendon anchors that connect the rotator cuff complex to the humerus and the scapula and can become compressed along with the bursa causing shoulder impingement.
Deltoid muscle, a muscle just beneath the skin, which gives the shoulder a rounded appearance. The deltoid muscle helps to bring the arm overhead. Directly beneath the deltoid muscle lays the sub-deltoid bursa, a fluid-filled sac, analogous to a water balloon.
Tendons of the shoulder
The tendons of the rotator cuff are prone to injury such as tears that can cause a loose shoulder. Each of the four muscles the subscapularis, supraspinatus, infraspinatus and teres minor, have a tendon on each side of the muscle to anchor them to either the humerus or the scapula.
Biceps tendons – at the shoulder the biceps tendon has two attachment points. The long head biceps tendon travels through the shoulder joint making it more prone to injury such as a partial tear, rupture, or tendonitis. The short head tendon does not share the same exposure and acts as a redundancy to keep the bicep working if the long head is torn.