The shoulder is a synovial diarthrotic joint, which means it secretes synovial fluid and is freely moveable. There are 6 different types of synovial joints, two of which are found in the shoulder. The shoulder is made up of three bones and two joints, or three depending on whether or not you’re counting the scapulothoracic as a joint.
First let’s go over some terminology. All synovial joints have the following structures: articular cartilage, a joint capsule and a joint cavity. Most, but not all synovial joints also contain a structure called a bursa.
What is articular cartilage?
The ends of bones that articulate, or connect, with other bones are covered in a layer of connective tissue called hyaline cartilage. The hyaline cartilage creates ease of movement and protects the bones from wear and tear. The degeneration of this cartilage is known as osteoarthritis.
What is a joint capsule?
The joint capsule surrounds and encloses the entire joint, like an envelope. The outer layer is fibrous and dense, though it varies in thickness depending on the stresses placed on the joint. It also thickens in some places more than others to form capsular ligaments. The purpose of a joint capsule is to provide stability so these thickenings, such as the coracohumeral ligament that connects the coracoid process to the humerus, are meant to restrict movement in such a way that injury and harm can be prevented. The joint capsule weaves itself into the periosteum of the connecting bones, so in the case of the shoulder... the scapula, clavicle and humerus all articulate with each other inside of the joint capsule. Remember from a previous post that the periosteum is the layer of fascia that surrounds each bone.
The inner layer of the joint capsule is a synovial membrane. It secretes synovial fluid into the joint cavity which lubricates the joint, creates cushion for the bones and supplies the articular cartilage with nutrition.
What is a joint cavity?
A joint cavity is the space between connecting bones and is encapsulated by the articular cartilage and a synovial membrane. The synovial membrane secretes fluid into this space.
What is a bursa?
A bursa is a little fluid filled sac that lives between a bony projection (like the acromion) and surrounding tendons, muscles and ligaments. It’s role is to provide cushion and reduce friction between these structures as we move. You may have heard of bursitis? -itis means inflammation so bursitis means the bursa is inflamed. Bursitis is most commonly experienced at the shoulder, hip, elbow, heel and knee.
OTHER STRUCTURES OF THE SHOULDER
Subacromial bursa - sits under the acromion to reduce friction and prevent impingement. This bursa is actually not all that small, the lateral portion allows the acromion and deltoid to slide smoothly over the humeral head and rotator cuff tendons while the medial portion puts a cushion between the coracoacromial ligament and supraspinatus tendon.
Coracoacromial ligament - The coracoacromial ligament is unique in that it doesn’t connect one bone to another but rather a part of the scapula (the coracoid process) to another part of the scapula (the acromion). It forms an arch across the top of the shoulder that helps protect both the rotator cuff tendons and the subacromial bursa from injury by the acromion.
Glenohumeral joint - A ball-and-socket synovial joint, this is where the head of the humerus (the ball) articulates with the glenoid fossa (the socket). The socket in this case is pretty shallow, so we need the stability of the joint capsule and the strength of the rotator cuff to hold the head of the humerus in place. The rotator cuff muscles originate all over the scapula and insert on the humeral head but the just barely cross the joint, which is why rotator cuff injuries are pretty common.
Labrum - this is a fibrous, cartilaginous structure that's attached to the rim of the glenoid fossa. Its purpose is simply just to create a lip to deepen the shallow socket. It is important for the health of the labrum that we properly stabilize the whole shoulder before bearing weight, otherwise this structure can sustain wear and tear and painful injury.
Acromioclavicuar joint - this is the articulation between the acromion, which is the lateral end of the spine of the scapula, and the lateral or acromial end of the clavicle. The articulation surface is small, and it’s classified as a gliding joint as the surfaces glide past one another. It’s a small amount of movement, and injury can happen in this joint in the form of a separation. Ouch.
Scapulothoracic joint - this is where the scapula articulates with the rib cage, but even though it’s called one, it isn’t really a true joint.