A ankle prosthesis is the endoprosthetic replacement of an injured or worn upper ankle joint. Nowadays the implantation of an ankle prosthesis is a good alternative to the so called stiffening surgery of the upper ankle. In Germany around 1,500 artificial ankle joints are inserted annually. The therapeutic goal is to free the upper ankle from pain when it is under pressure.
You should consider an artificial ankle implanted when all conservative and joint-preserving therapies do not promise a cure. Such a severe damage of the ankle joint surfaces may be caused by:
When earlier a so-called arthrodesis (ankle joint ossification) was used in such cases, nowadays an artificial ankle is inserted.
However, not all patient with wear, injuries or diseases of the upper ankle joint are equally suitable for an ankle prosthesis. If ankle bone quality is severely decreased, in particular the anklebone (talus), as well as in case of an extreme osteoporosis of the ankle the implantation of an artificial ankle joint is impossible. Among other possible contraindications for an artificial ankle are a lack of sufficient ligament stability of the ankle as well as severe vascular diseases.
An artificial ankle is tailored to suit the human anatomy and the movement of a normal upper ankle. It mimics its natural form, function and movement. With the aim of preserving as much bone material as possible, the joint surfaces of the worn and destroyed upper ankle, i.e. the surfaces of the anklebone (talus) and the shinbone (tibia), are coated with a surface replacement. If such an artificial ankle joint fails, it is always possible to perform a fusion operation (arthrodesis).
There are many different types of artificial ankle joints. These include:
- STAR® ankle prosthesis,
- Hintegra® ankle prosthesis,
- Salto® ankle prosthesis,
- Mobility® ankle prosthesis
- BOX® ankle prosthesis.
All these types of ankle prostheses consist of three components:
- a rounded cap for the talus head
- a plate for the shin component surface
- a freely movable polyethylene sliding core
Therefore, if a replacement operation is necessary, not the entire artificial ankle must be removed and replaced, but only the defective component.
Each of these ankle prostheses is available in different sizes and shapes. In this way, an artificial ankle joint is adapted to the anatomy of each individual patient. What particular artificial ankle will be implanted (its model and size) is decided by the treating surgeon based on various preliminary examinations. Among other things, the age, bone quality, weight and health of the patient play a decisive role, too.
The aim of an artificial ankle is to restore trouble-free and painless ankle movement and thus provide a long-term increase in the quality of life and mobility of the patient in everyday life, work and leisure. In order for an artificial ankle joint to satisfy these requirements, the materials used must meet different criteria. Since an artificial ankle joint stays in the body permanently, these include the greatest possible durability and a good body compatibility of the materials.
The exogenous material used for the components of a ankle prosthesis
- must not trigger rejection or intolerance reactions,
- must not wear out prematurely,
- must be corrosion-resistant and
- should be able to withstand the load of the body.
In addition, no wear may occur when the various components of an ankle prosthesis are rubbing on each other. Therefore, special metals and plastics are generally used for an artificial ankle joint. For example, the artificial joint surfaces of the anklebone and the shinbone are usually made of body-compatible metal alloys containing chromium and cobalt. On the other hand, the freely movable sliding core between the two joint surfaces is made of a high-quality special plastic called polyethylene.
Ankle prostheses have been used since 1969 and since have been continuously refined and improved in order to further optimize their mobility in all directions and their stability. The modern ankle prostheses implanted since the mid-1980s consist of three components that are modeled after the anatomical structure of a natural ankle:
- a rounded metal cap for the talus head,
- a metal plate for the shin component surface and
- a movable polyethylene sliding core.
The two metallic joint surfaces for the shin and the talus are implanted into the bone as a surface replacement, while a free-moving sliding core in between is made of high-quality polyethylene.
Metal cap for the talus head: The lower part of the ankle, i.e. the anklebone or talus head, is coated with a rounded cap made of a cobalt-chromium alloy. Using small pins and a special coating that sticks firmly to the bone and grows together with it over time, this metal cap is attached to the ankle bone. Its surface is designed in such a way that the individual components of the artificial ankle joint optimally interact during the joint movement.
Metal plate for the shin joint surface: The upper part of the artificial ankle consists of a straight plate made of a cobalt-chromium alloy, which is placed on the joint surface of the tibia and connected to the bone using a center-mounted dowel. The metal plate for the joint surface of the shinbone, just like the metal cap for the talus head, is also coated with a special material that with time fuses with the bones. In order to prevent the mobile bearing from slipping and hitting the inner bones, the metal plate also has a fin on the edge.
Freely movable polyethylene sliding core: The third component is a freely movable, mobile polyethylene sliding core that takes over the function of the natural joint cartilage. Accordingly, it is located between the ankle and the shin components of the artificial ankle. It is bent towards the anklebone, i.e. on the bottom, and sticks to the metal cap and it is flat on the top towards the shin and fits exactly to the metal plate. This sliding core enables a friction-free movement between the shinbone and ankle bone and minimizes the lifting and gravity forces between the two metallic components of the ankle prosthesis.
Nowadays, an artificial ankle is only anchored in a bone-sparing and cement-free way. Modern artificial ankle joints are thus implanted as ingrown prostheses without bone cement. This so-called cementless osseointegration significantly improves the durability and stability of an ankle prostheses and reduces the risk of its later loosening.
Cementless osseointegration of an artificial ankle using press fit method If an artificial ankle joint is attached to the bone without cement, the two prosthetic components on the anklebone (talus) and shinbone (tibia) are first mechanically connected to the bone using press fit method. A rough surfaces on the sides of the two prosthetic components facing the bone have a special coating, for example from titanium/potassium phosphate, that subsequently enables the gradual firm and stable fusion of the individual prosthetic components with the natural bone of the ankle.
Prerequisite for the cementless osseointegration of an artificial ankle joint To perform a cementless osseointegration of the prosthesis the precise preparation of the prosthesis stem is necessary. This is usually done in a bone-sparing manner. This means that only a couple o millimeters of the patient's anklebone (talus) and shinbone (tibia) sacrificed. On the other hand, in order to have a cementless osseointegration of an artificial ankle performed you need to have a good bone quality and bone substance, which enables the fusion of the body's own bone with the special coating of the prosthetic components.
The implantation of an artificial ankle is generally carried out under inpatient conditions, usually requiring a one to two-week hospital stay. The operation can be performed under general anesthesia as well as under regional anesthesia of the affected leg or the entire lower part of the body and lasts between one and two hours. In general, complications during or after implanting of an ankle prosthesis occur rather rarely. However, the operation, like any other surgical procedure, holds certain risks.
When the artificial ankle joint needs to be replaced (the procedure is called revision or replacement operation) either one or two of the three prosthetic components, or the entire previously implanted ankle prosthesis, are exchanged surgically.
The follow-up treatment after the implant of an ankle prosthesis lasts for about 6 to 12 weeks. The mobilization and rehabilitation of the artificial ankle starts in the hospital from the first day after the surgery. To do so physiotherapy and special exercises are used.