Artificial Shoulder Joint
How Does an Artificial Shoulder Joint Work?
The artificial prosthesis components are made of different materials:
- metal alloys: (titanium, cobalt-chromium, stainless steel)
- special plastic (polyethylene) which replicate the joint cartilage
The surgeon decides which materials to use based on the anatomy, bone condition and age of the patient.
Types of Prosthesis
As the shoulder is a primarily muscle-guided joint, it is important that the surgery is performed in such a way as to protect the soft tissues, muscles, tendons and ligaments as far as possible.
Depending on the condition, age and bone condition of the patient, the following types of prosthesis can be used: partial replacement, total replacement and reverse and cap prosthesis or resurfacing.
Shoulder Resurfacing (Cap Prosthesis)
With this method, only the damaged joint surface at the head of humerus is replaced, with the addition of a metallic crown. This is referred to as shoulder cap prosthesis.
The prosthesis therefore consists of a cap for the natural humeral head, making this the most minimal form of shoulder joint replacement.
The advantage of this method is that it preserves the bone material of the humeral head, the surgical intervention is relatively small, and blood loss is kept to a minimum. The cap prosthesis does, however, require a sufficiently large contact area, as well as a stable bone condition. This means it is not suitable for osteoporosis patients, for example.
Partial Replacement
Partial replacement is also referred to as hemiarthroplasty or humeral head prosthesis. If the cartilage layer in the glenoid cavity is still in good condition (for example after a humeral fracture), it does not need to be replaced. If this is the case, only the existing humeral head is replaced by an artificial humeral head, which is fitted to a prosthesis stem. After surgery, the artificial humeral head sits inside the natural glenoid cavity.
Total Replacement
If the layer of joint cartilage is damaged on both joint components, i.e. both on the humeral head and in the glenoid cavity, the entire shoulder joint is replaced. The damaged glenoid cavity is replaced by a prosthetic cup and the deformed humeral head is replaced by an artificial humeral head, which is fitted to a prosthesis stem. Total joint replacement requires a functioning rotator cuff, which provides the artificial joint with stability and mobility.
Inverted Total Replacement
If the rotator cuff is severely damaged, a special type of prosthesis is implanted for functional compensation; this is known as reverse total endoprosthesis. This shoulder prosthesis is intended to stabilise the joint as the muscles are no longer capable of doing so. The reverse shoulder prosthesis consists of two parts:
- the ball of the artificial joint sits in the glenoid cavity
- the artificial glenoid cavity sits on the upper arm stem
This design provides a certain level of stability. However, the mobility of the joint is dependent on a properly functioning large shoulder muscle, the deltoid muscle.