What is the Structure of a Prosthesis?

Prosthetic structure:

• Endoskeletal structure has an internal framework or chassis

• Exoskeletal structure, where there is no inner chassis and all the rigidity is provided by the hollow outer structure. These are generally of laminated materials

• Conventional prostheses are aluminium alloy or wooden exoskeletal prostheses, which tend to be used by those who have been established amputees for a number of years

• Cosmetic prosthesis is used to describe soft foam prosthesis with little or no structural strength. Generally used for cosmetic purposes only

• Silicone prosthesis has a homogenous silicone structure

Endoskeletal prostheses

Endoskeletal means having an internal framework or chassis. A human is therefore endoskeletal as the skeleton is an internal framework. This is clearly a different situation from a crab which has an external framework – an exoskeletal construction.

Endoskeletal prostheses are generally modular. A custom made socket is attached to the more distal components which are selected to give the appropriate features and functions.

The modular structure is normally comprised of lightweight alloy or carbon fibre tubes which connect the other modules such as ankles, knees and hips and elbows. Modules can generally be easily replaced in the case of breakage or a change of specification. Alignment devices are normally incorporated into the structure. The cosmesis is made from a foam blank, machined to match the contralateral limb where possible.

In an endoskeletal prosthesis, lightweight tubes connect different components when there is sufficient distal clearance. Alignment devices are components which permit changes of angulations and translation. Lengths are adjusted by means of cutting the tubes to length. Components are selected on the basis of features such as weight, strength, longevity, dimensions and function.

Endoskeletal prostheses are never fully waterproof and are often not damage resistant enough for children. The modules are specified to suit different activity levels and amputee weights.

Exoskeletal prostheses

Exoskeletal means having an external framework. A crab is therefore exoskeletal. This is different from a human for example which, having an internal skeletal framework, is endoskeletal.

Modern exoskeletal prostheses are normally of a laminated construction where layers of fabric such as glass fibre and carbon fibre are incorporated into a resin under vacuum. Older designs of exoskeletal construction include shaped sheet aluminium alloy and wood. Some alloy prostheses still are made but wooden prostheses are now extremely rare in Ireland. Exoskeletal prosthesis are often prescribed for children because Endoskeletal prostheses are never fully waterproof and are often not sufficiently damage resistant.

Conventional prostheses.

This term refers to the old, traditional type of exoskeletal prostheses. Many of the designs hark back to the first World War and utilise materials such as leather, metal and wood. Most were/are referred to by a number code which was devised during WW1. This relates to amputation levels with No.1 being a hip disarticulation, No.2 a short transfemoral etc. This system was devised to enable doctors to swiftly prescribe prostheses and also related to the amputees’ pension in the case of British war pensioners.

Prostheses which are solely cosmetic in function are generally made from soft foam and are very light. They have almost no structural strength and therefore cannot even be used for transfers.

Essentially these are solely used to restore body profiles so that it is not obvious that the user is an amputee.

Silicone prostheses

• Are made completely or almost completely from medical grade silicone.
• In contrast to cosmetic prosthesis silicone prosthesis are functional.
• N.B. These should not be confused with silicone cosmetic covers, which can be fitted to many types of prostheses.