The Shoulder

Understanding the Shoulder

The shoulder joint is extremely complex, in fact it is the most complex joint in your body. Keeping this as simple as possible, the shoulder is a ball and socket type of joint. The ball part of the shoulder joint is made up of the head of the humerus bone and the socket is made from the glenoid of the scapula. The shoulder is known to be ‘sloppy’, this is because it is a loose joint that relies on the muscles and tendons to keep the ball and socket joint together.

The shoulder joint is similar to the hip joint however, it is much ‘looser’, much more flexible. This means it can be easier to damage.


The muscles of the shoulder bridge the movements from the body into the neck by the use of upper extremities of the arms. The shoulder is quite dexterous, due it contains large muscles that perform bulk movements through to finer stabilizer and fixator muscles.

The rotator cuff is key, most of the shoulder’s stability comes from the muscles of the rotator cuff. This cuff consists of four muscles: supraspinatus, infraspinatus, subscapularis, and teres minor. Each of these muscles has its origin on the scapula and inserts around the head of the ball (humerus). The rotator cuff keeps the head of the humerus centered in the socket whilst the shoulder is in motion.

The deltoid is next in line, these are rounded, triangular muscles located on the upper part of the arm and the top of the shoulder. They assist in rising and rotational movements.

Surrounding the rotator cuff and attaching to the deltoids, there are many other muscles that help perform bulk movements. These muscles connect the shoulder to your back and neck.


Tendons are elastic tissues that connect muscles to bones. The tendons main role is to stabilize the shoulder joint. Most commonly affected tendons in the shoulder are the four rotator cuff tendons and the biceps tendon. The rotator cuff is made up of four small muscles and their tendons that cover the head of your upper arm bone and keep it in the shoulder socket.  These are known as supraspinus, infraspinatus, teres minor and subscapularis

Notice the colour of the tendons, they are white. Whereas muscles are red. The reason is due to blood flow, muscles have good blood flow, whereas tendons do not. The rotator cuff tendons require careful observation, since many of the tendons attach in a similar location it may sustain damage easily.


Ligaments are designed to connect one bone to another. They are soft tissue structures that are strong, but not particularly flexible. Once stretched, they tend to stay stretched, if they stretch too far, they can snap.

The joint capsule in shoulder is formed by a group of ligaments that connect the ball and socket joint together. Ligaments provide the stability to the shoulder, there are 3 main ligaments directly around the ball and socket joint.

Superior glenohumeral ligaments
Middle glenohumeral ligaments
Inferior glenohumeral ligaments

These ligaments wrap around the shoulder joint and form the joint capsule. Ligaments act as passive stabilizers to stop the shoulder joint from dislocating.

There are many other ligaments in the shoulder joint as can be seen in the above picture.


The joint itself is cushioned by articular cartilage, very similar to the hip joint. This covers both the head of the ball (humerous) as well as the socket (glenoid). Cartilage also acts a shock absorber however, cartilage wears out and cannot repair itself.

Specifically, the glenoid labrum of the shoulder attaches to the rim of the glenoid cavity on the scapula (shoulder bone). This cartilage increases the depth of the glenoid socket by 50%. Its function is to increase the surface contact area for the ball on the humerus to create a better fit. The glenoid labrum acts a point of attachment for ligaments of the shoulder capsule as well as the biceps tendon.

Joint Capsule (lubrication membrane)

The capsule is a thick, fibrous structure that wraps around the shoulder socket. Inside the capsule is the synovial membrane which is lined by the synovium, a soft tissue that secretes synovial fluid and provides lubrication for the shoulder. The synovial fluid is both viscous and sticky lubricant. Synovial fluid is what allows us to flex our joints under great pressure without wear.

Joint Sacs

Joint sacs, known as bursae are fluid filled sacs which are lined with a synovial membrane. The bursae sacks are often found near joints, they are filled with synovial fluid and their function is to lessen the friction between soft and hard tissue interaction.

In the shoulder, bursa is typically found around what is referred to as the subacromial space which is located both above and below the rotator cuff. The image below shows the positioning of the bursa sacks of the shoulder.

Pathological Conditions of the Shoulder

Shoulder osteoarthritis:

Osteoarthritis is the most common form of arthritis, and often affects the hips. Caused by aging and wear and tear of cartilage, osteoarthritis symptoms may include hip pain, stiffness, and swelling. Osteoarthritis is not simple to understand, there are many links with the wear and tear of cartilage that also affects every other part of your joint. We discuss this in our treatment information pack.

Frozen shoulder:

Frozen shoulder is thought to happen when scar tissue forms in the shoulder. This causes the shoulder joint’s capsule to thicken and tighten, leaving less room for movement. Movement may become stiff and painful.The exact cause is not fully understood, and it cannot always be identified.

Septic arthritis:

An infection caused by bacteria, a virus, or fungus inside the hip can cause inflammation, pain, swelling, and difficulty moving the hip. Although uncommon, septic arthritis is a serious condition that usually gets worse quickly without treatment.

Rotator cuff tear:

A tear in one of the muscles or tendons surrounding the top of the humerus. A rotator cuff tear may be a sudden injury or result from steady overuse. It has a standalone name since there are 4 groups of muscles/tendons connecting at the one point.

Gout & pseudogout:

A form of arthritis caused by buildup of uric acid crystals in a joint. Though this is more common in the knee and feet, it is rare to occur in the shoulder joint.

Shoulder dislocation:

The humerus or one of the other bones in the shoulder slips out of position. Raising the arm causes pain and a “popping” sensation if the shoulder is dislocated.

Shoulder impingement:

The edge of the scapula presses on the rotator cuff as the arm is lifted. If the rotator cuff is previously injured or inflamed, this impingement can cause pain.

Issues that often occur with Osteoarthritis


Inflammation of the tendons that are connecting muscle to bone. As previously discussed tendonitis is very common. It can occur due to simple overuse, overstrain or even underuse. Tendons can become weak very quickly and they take a very long time to repair due to lack of blood flow.

Cartilage tear:

The labrum of your shoulder is the most common tear to occur. This is the ring of cartilage that follows the outside rim of the socket of your shoulder joint. Some of the symptoms noticed may be locking or catching sensation in your shoulder joint.

Ligament strain or tear:

The ligaments of your shoulder can strain when one of the muscles supporting the shoulder joint is stretched beyond its limit. In severe cases this can lead to a tear, not just a strain.  If the ligaments are degenerate due to low activity, this will be more likely to tear.


When the bursa sacks become inflamed, this will lead to pain, swelling, and warmth in any of the bursae of the shoulder. Bursitis often occurs from overuse or injury. Symptoms often associate with pain with overhead activities or pressure on the upper, outer arm.

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