A prosthesis in medicine is an artificial device used as a replacement for a missing limb or other body part. It is one of the primary goals in medicine to develop prostheses that supply a level of functionality that can be compared with that of the organ or body part being replaced. To the general public, a prosthesis is mostly seen as a substitute for a lost limb. The Paralympics, for example, demonstrate just how far the research and development of prostheses has advanced in enabling virtually perfect replacements for limbs. They allow sports people to achieve a high level of performance despite being users of, for instance, a prosthetic leg or other artificial limb. The field of prosthetics is diverse and concerned with more than just limb replacements. Prostheses are frequently encountered in other fields of medicine, such as dentistry, where are they are also of great importance.

Distinguishnig between different types of prosthesis

A primary distinction is made between prostheses that are employed outside the body and those that are completely enclosed by body tissue. Another type of prosthesis, known as an epithesis, serves aesthetic requirements, and is frequently used as a visual compensator. Familiar examples of this type of prosthesis are imitation ears, eyes and noses. An externally worn prosthesis is known as an exoprosthesis. This group includes prosthetic arms, hands and legs. An enclosed prosthesis, which is fully surrounded by body tissue, is referred to as an endoprosthesis. One example of this is the artificial hip. In addition to enclosed prostheses, so-called open prostheses are also sometimes encountered. An open prosthesis is anchored in a bone and a defined portion of it protrudes from the body tissue. An example of this is a dental prosthesis or implant.

Overview of common types of prosthesis

  • Exoprosthesis (outside the body)
  • Endoprosthesis (fully surrounded by body tissue)
  • Epithesis (a prosthesis that serves aesthetic requirements)

Series HFUC-2A component sets as knee-joint prostheses

The field of bionic prosthetics employs highly modern technologies and technically sophisticated components to enable people to preserve their quality of life after losing a body part such as a leg. A particularly good example of this is the artificial knee joint, which is based on a combination of Artificial Intelligence, sensors, and actuators. It must be constantly ensured that the kinetic and kinematic movement sequences of the prosthesis wearer are precisely reproduced. This requires the use of highly integrated reduction gearing in conjunction with a compact servo motor. Together with sensors and microprocessors, these provide the basis of a proactive leg replacement which, equipped with an artificial muscle, is able to perform everyday movements. The lifelong absence of mechanical play, low-noise operation, and weight optimisation are just some of the requirements fulfilled by the HFUC-2A series of component sets from Harmonic Drive AG.

Functional advantages of modern prostheses

The first prostheses suffered from a lack of functionality, for which reason, limb replacements concentrated only on visual factors. This was the case with both artificial limbs and specific prostheses. The fields of prosthetics and medical engineering are meanwhile far more advanced than they used to be and now permit more sophisticated prostheses. Modern arm and leg replacements are controlled by microprocessors and are able to perform complex sequences of motion. Furthermore, modern prosthetic arms and legs enable users to take part in sports activities. Artificial arms used to be produced either as passive prostheses (for aesthetic purposes) with no further function or they were equipped with a hook function. Modern exoprostheses can be equipped with a grip function, with movements that simulate those of a human hand. However,  a prosthesis with such functionality is still very expensive.

The skin of a modern exoprosthesis is commonly made of PVC. As a polymer, PVC has proven itself to be very robust, with a good resemblance to human skin. However, a disadvantage of PVC is that unlike leather-steel materials and wood, it tends to soil. Moreover, the artificial material tends to discolour and needs be regularly replaced. As an alternative to PVC, so-called silicone cosmetic gloves can also be used for the external skin. However, silicone is not as robust as PVC, as it tears more easily and is more susceptible to abrasion. It is also more expensive to purchase. But silicone does have one essential advantage: it is not subject to appreciable soiling. Prosthetic legs are generally made of cosmetic foam that is shaped to resemble the missing body part. A cosmetic stocking which forms the replacement skin is then applied.

Categorisation of exoprostheses

There are several categories of prostheses that can be used for extremities. Extremities are subdivided into prosthetic feet, lower legs and upper legs. A prosthetic foot is required following an amputation below the ankle joint. Lower leg replacements are fitted for amputations below the knee. In the case of artificial upper legs, a distinction is made between those used above the knee following an amputation and those used following disarticulation of the knee. If a hip amputation has been performed, a whole leg prosthesis is required. The upper extremities are subdivided into lower arm and upper arm prostheses. Further subdivision is made on the basis of the prosthetic function.

State of development of prostheses

The development of prostheses of all types is of great importance from a general social point of view. As a subdivision of medicine, the field of prosthetics serves to secure a high quality of life for many people. Modern exoprostheses are already fitted with microprocessors that enable the generation of complex movements. Prosthetic hands can perform gripping movements, while a modern prosthetic leg restores a walking ability that also aids the wearer’s stability and generates a sequence of movements that is as natural as possible. Prostheses are also available today that enable wearers to take part in sports activities. Prosthetics also make use of innovations and findings from related technologies and disciplines. In the field of tissue engineering, for example, research is being made into the cultivation of tissue that can be used for heart valves. The aim is to create an appropriate and functional replacement from the patient’s own tissue.