Shoulder Anatomy

The shoulder is one of the most complex joints in your body and allows your arm to rotate and move in many different directions. The shoulder joint is described as a “ball and socket” joint, however, the socket is very shallow and is similar to a golf ball resting on a tee. The shoulder is made up of three bones: the humerus (upper arm bone), the clavicle (collarbone), and the scapula (shoulder blade). The outer portion of the scapula bone is called the glenoid and is the golf tee where the ball of the humeral bone rests to make up the shoulder joint. The shoulder joint is also called the glenohumeral joint and you may see it referred as this in test results.

Due to the geometry of the shoulder joint, it is very flexible and allows a large range of motion, but this leads to an inherent risk of injury to the shoulder. Several soft tissue structures (non-bony structures) exist in the shoulder joint to improve the stability of the joint.

• Labrum: A rim of cartilage on the shoulder socket (glenoid) that helps stabilize the joint by providing a bumper for the ball around the socket.

• Capsule: Every joint has an envelope around it to contain the joint and the joint fluid.

• Ligaments: Several ligaments in the shoulder joint strengthen the capsule and increase stability of the shoulder joint

• Rotator cuff muscles: A group of 4 muscles that are an important contributor to shoulder stability and function. Each muscle attaches to the humeral head (ball of the shoulder joint) and work in a coordination fashion to keep the ball centered in the socket

  • The subscapularis is a large rotator cuff muscle that attaches to the front of the ball and allows you to rotate your arm inward

  • The supraspinatus and infraspinatus muscles attach at the top of the ball and allow you to raise your arm overhead

  • The teres minor attaches to the back of the ball and allows you to rotate your arm outward

Other soft tissue structures in the shoulder include:

• Bursa: These structures are located between bone and the surrounding soft tissue above the rotator cuff. They serve as a cushion between the rotator cuff and the bones of the shoulder

• Cartilage: This structure is present in all joints, where its called articular cartilage, and covers both ends of the bones in the joint. Cartilage allows the bones to glide easily as the joint is moved.

Two smaller joints complete the “shoulder girdle” and are the acromioclavicular joint (AC) and the sternoclavicular (SC) joints. The AC joint is the connection between the acromion (a bony protrusion from the shoulder blade) and the clavicle. The SC joint is the connection between the sternum of your chest and the clavicle.

Shoulder injuries can result from acute traumatic events, such as a fall, but more commonly result from repetitive overuse activities, such as overhead sports. Symptoms from an overuse injury can often be improved without surgery by reducing the inflammation in the joint and strengthening and retraining the muscles that support the shoulder’s movement.