How does an artificial shoulder joint work?

A shoulder endoprosthesis is based on the human shoulder joint. Just like natural shoulder joints, it too comprises a glenoid cup and a humeral head which is mounted on a prosthesis stem.

The artificial prosthesis components are made of different materials:

Nowadays, both prosthesis components often use metal alloys, which have proved exceptionally suitable because of their good tolerance and durability. In addition, prosthesis components made of special plastic polymers are used which imitate the lubricative cartilage layer found in human joints.

The surgeon will decide which materials to use depending on the patient's anatomy.

Depending on the disease and patient, the following different types of prostheses can be used: partial endoprosthesis, total endoprosthesis or shoulder resurfacing.

Partial endoprosthesis: If the cartilage layer in the glenoid cavity is in good condition (for example after a humeral fracture), this does not need to be replaced. Then only the humeral head is replaced by an artificial humeral head fitted to a prosthesis stem. Following the operation, the artificial humeral head sits in the natural glenoid cavity. This is referred to as a partial endoprosthesis or hemi-endoprosthesis).

Total endoprosthesis: If the joint cartilage is damaged on both parts of the joint, both on the humeral head and in the glenoid cavity, all the components of the diseased shoulder joint are replaced: the damaged glenoid cavity is replaced by a prosthetic cup. The deformed humeral head is replaced by an artificial humeral head which is fitted to a prosthesis stem. This is referred to as a total endoprosthesis. A total endoprosthesis requires a functioning rotator cuff which provides the artificial joint with stability and mobility.

Inverted total endoprosthesis: If the rotator cuff is severely damaged, a special type of prosthesis is implanted for functional compensation, a so-called inverted total endoprosthesis: This consists of a spherical component, which is attached to the shoulder blade at the position of the natural glenoid cavity, and a cup-shaped part, which is mounted on a prosthesis stem and is inserted into the humerus. This design achieves a certain amount of stability. Mobility, however, depends on a properly functioning large shoulder muscle, the deltoid muscle. Since this type of prosthesis has not been in use for long, there are no long-term results available yet.

Shoulder resurfacing: With this method, only the damaged joint surface of the humeral head is replaced and lined with a metal spherical cup.

The prosthesis thus comprises a cap for the natural humeral head.

The advantage of this method is that the bone tissue of the humerus is preserved, the surgical intervention is smaller and the loss of blood remains minimal. However, the method is still very new so there are no long-term results available yet.