Prosthetic components Prosthetic Components A limb prosthesis has 3 main parts: Interface Structural components Cover The prosthesis attaches to the body by direct skin contact or by an interface made of various thin viscoelastic cushion... read more include the socket, suspension and control system(s), joints, and appendage. There are many different options for prostheses, but all options aim to achieve a stable, comfortable fit. The prosthetist helps patients choose the type of prosthesis and options they need to accomplish their goals. For example, prostheses can be designed for general daily mobility, for specific activities such as swimming, or for high-impact and competitive sports such as running. The patient's physical and cognitive abilities and gadget tolerance are important in the initial selection of prosthetic components.
Limb prostheses are exoskeletal or endoskeletal.
Exoskeletal prostheses have a rigid external plastic structure in the shape of a limb. They are permanently fixed and not adjustable. Exoskeletal prostheses are more durable and are preferred primarily when the prosthesis may be exposed to harsh environmental conditions, such as impact damage during physical labor or a caustic environment.
Endoskeletal prostheses have a central inner skeletal structure that includes modular components and couplings that provide angular adjustably in all three planes and facilitate removal of damaged components. The endoskeletal system is often covered with a soft material in the shape of a limb and a synthetic skin applied over the anatomic shape.
(See also Overview of Limb Prosthetics Overview of Limb Prosthetics A limb prosthesis is an artificial limb that replaces a missing body part. The main causes of limb amputation are Vascular disease (particularly from diabetes and peripheral arterial disease)... read more )
Upper Extremity Prostheses
The hand has significant psychosocial meaning. Amputation can affect a person's self-perception and/or identity and have a significant impact on relationships and career. Psychologic counseling should be routinely offered.
The human hand is complex, and often two different prostheses are necessary to provide optimal function for general daily activities and for specific activities.
General types of upper extremity prosthesis
There are 5 general types of upper extremity prosthesis:
Externally powered myoelectric prostheses
Passive prostheses assist in balance, stabilization of objects (such as paper when writing), or recreational/vocational activities. They look like a natural limb, are the lightest and cheapest, but they provide no active hand prehension.
Body-powered prostheses are the most often prescribed because they tend to be less expensive, more durable, and require less maintenance. A harness-cable system suspends the prosthesis and captures scapular and humeral motion to operate the hook, hand, or elbow joint. Some systems use the opposing arm to trigger one particular function; one end of a strap encircles the opposite arm at the axilla, and the other end connects to a cable that controls the terminal device (hook, hand, or specialty device for particular function). People involved in physical labor typically favor this type.
Externally powered myoelectric prostheses provide active hand and joint movement without the need of scapular, humeral, or trunk motion. Sensors and other inputs detect muscle movement of the residual limb or upper body and control powered actuators that provide greater grasp force than body-powered prostheses.
Hybrid prostheses are typically prescribed for higher level upper-limb amputations. They combine specific features of body power and myoelectric power, for example a body-powered elbow might be combined with an externally powered hand or terminal device.
Activity-specific prostheses are designed to allow participation in activities that would otherwise damage the patient's residual limb or everyday prosthesis, or when the everyday prosthesis would not function effectively. These prostheses often include a specialty design interface, socket, suspension system, and terminal device. Activity-specific terminal devices can allow the patient to grasp a hammer and other tools, a golf club, or baseball bat, or hold a baseball glove. Others aid in various specific activities (eg, swimming, fishing). These devices can be passive or controlled by the amputee.
Partial-hand amputations range from single or multiple digits to carpal-metacarpal; wrist flexion and extension is usually preserved. Functional prosthetic restoration is possible for complete hand or one or more missing fingers using mechanical or external power. Grasp and pinch can often be achieved by the opposition of some combination of natural and prosthetic digits.
Wrist disarticulation prostheses
Wrist disarticulation amputation removes all carpal bones, leaving no ability to flex or extend the wrist. Pronation and supination are mostly retained. A prosthetic hand, hook, or special activity terminal device can be used. Passive, body-powered, or externally powered (myoelectric) devices can be used.
Transradial/ulnar amputations can be long (2/3 or more of original radius bone length), medium (1/3 to 2/3 of original length), or short (≤ 1/3 of original length). Long and medium level amputations can retain some pronation and supination. Passive, body-powered, or externally powered devices can be used.
Elbow disarticulation and above-elbow prostheses
Elbow disarticulation and above-elbow prostheses both require a mechanical elbow. Elbow disarticulation prostheses typically employ body power to flex the elbow (gravity extends the elbow) and myoelectric control of the terminal device. Two external elbow hinges are attached to the outside of the plastic socket. There are many combinations of elbow and control systems.
Shoulder disarticulation and interscapular/forequarter prostheses
In shoulder disarticulation and interscapular prostheses, heat dissipation, weight distribution, and comfort are of paramount concern. The contact surface can be rigid or flexible plastic, or a gel cushion material such as silicone. The most functional prostheses for these levels of amputation typically include myoelectric control for one or more joints and hand functioning.
Lower Extremity Prostheses
There are many variables and options for lower limb prostheses—there are 350 different foot/ankle systems and 200 different knees. The amputee and prosthetist evaluate different joint and foot components to determine which provides optimal balance, safety, function, and gait efficiency. Selections may change during the fitting process when biokinematic evaluation determines optimal gait efficiency.
Most lower extremity prostheses are endoskeletal because they provide continual adjustment of biomechanical alignment. This allows the prosthetist to refine kinematics of prosthetic foot, ankle, and knee components under the center of gravity minimizing energy expenditure of walking.
Prosthetic ankle and foot systems may include hydraulic systems that dampen impact forces; some automatically adjust to cadence changes. Microprocessor-controlled ankle/foot systems regulate function in real time based on user input and/or environmental conditions. Some are passive mechanisms; others provide active propulsion which greatly reduces energy requirements for walking. Axial or horizontal rotation lost from amputations above the ankle can be replaced with endoskeletal torsion units; this feature is especially helpful for golfers. Patients who have shoes with different heel heights (eg, cowboy boots, high heels) can choose a prosthetic ankle that adjusts to different heights; however, prosthetic feet with adjustable heel height may not provide sufficient dynamic functioning.
Prosthetic knee systems can be passive body-powered pneumatic or hydraulic systems with a single- or multi-axis joint. Microprocessor-controlled knee systems are available.
Sport-specific prosthetic foot and knee systems help amputees achieve the highest level of physical performance. Some systems are effective for multiple sport and recreational activities. Others are designed for specific events (eg, sprinting, long-distance running, skiing, swimming). Running is more challenging for above-knee amputees than below-knee. Socket and suspension are more critical for athletes. Muscle atrophy and volume fluctuation are more common in athletes and require more frequent socket adjustments.
Transmetatarsal, Lisfranc, Chopart, and Boyd partial-foot amputations retain normal limb length, and the residual foot provides a sensate natural load-bearing surface; many patients can stand and walk short distances without a prosthesis. Patients with partial-foot amputation expend less energy walking than those with higher amputation levels.
A silicone slipper-type prosthesis allows some ankle motion and simple ambulation at slow and medium speeds. For more vigorous activities (eg, fast walking, running, climbing stairs and ramps), patients can use a semi-rigid plastic socket that encapsulates the remaining foot and ankle and extends up to the inferior border of the patella.
Syme ankle disarticulation prostheses
Syme ankle disarticulation amputations retain a thick heel tissue pad, which provides weight-bearing capability. Although limb length is shortened by 7 to 9 cm, patients can usually stand and walk short distances without a prosthesis (eg, transferring in and out of bed or chair, walk to adjacent room). Several prosthetic foot/ankle systems are available.
A modified Syme amputation reduces the typical bulbous end by shaving the prominences of the tibial and fibular malleolus. This modification simplifies prosthesis fitting and results in a less bulky ankle appearance.
Transtibial (below-knee) prostheses
Patients with a properly fitted and aligned below-knee prosthesis may function at a high level, often without visible gait abnormality. Walking ability and function are limited mainly by the patient's preoperative status and postoperative comorbid conditions. Amputation that maintains residual-limb length > 9 cm (just below the tibial tubercle) maintains the quadriceps insertion, which provides better function than through-knee or above-knee amputation.
A total-surface-bearing socket with a primary flexible socket and semi-rigid retaining socket with vacuum volume management and suspension improves comfort, connectivity, and gait symmetry. Vacuum is established via mechanical or electric pump. Other methods of suspension are available and can be employed effectively. The prosthetist and patient evaluate different foot/ankle components to identify components that provide optimal balance, stability, and gait fluidity.
Knee disarticulation and transfemoral (above-knee) prostheses
Knee disarticulation amputation has advantages and disadvantages compared to transfemoral-level amputation. Knee disarticulation retains the femoral condyles and improves distal load bearing, which reduces pressure and shear forces on the residual limb and improves proprioception. A disadvantage is that the center of rotation of the prosthesis will not match the rotational center of the contralateral knee, which affects function and appearance—when sitting, the knee extends beyond the other knee. An alternative is to use a prosthesis with medial and lateral knee hinges attached to the outside of the socket, which can establish a better center of rotation but increases width of the prosthesis and makes it difficult to fit clothing.
Socket designs for transfemoral and knee disarticulation can be rigid, rigid with flexible panels to allow muscle expansion, or adjustable to accommodate normal diurnal volume fluctuations. The suspension system can include a mechanical strap, integrated suspension pin, suction, or vacuum. Patients and prosthetists should evaluate different prosthetic knee and foot/ankle systems to achieve optimal balance, stability, and mobility.
Hip disarticulation and hemipelvectomy prostheses
Two percent (2%) of all amputations are at these levels, so few prosthetists and physical therapists have experience with these patients. The weight of the prosthesis is considerably greater than any other and also requires greater attention to socket fit and suspension. Even with an optimal prosthesis, walking speed is half that of able-bodied individuals and energy expenditure is 80% higher. Because of the challenges, long-term acceptance of prostheses for amputation at this level ranges between 35 and 45%. Unreasonable expectations increase rejection rate. Successful outcomes depend on patients' realistic understanding of the challenges and limitations; their degree of motivation, core strength, balance, and coordination; and their ability to support 100% of bodyweight on the residual limb remaining from amputation.