New technology and materials have advanced prosthetic designs to enable people who rely on artificial limbs to achieve feats never dreamed before. However, the latest and the greatest technology is not appropriate for everyone. The aim of this article is to present contemporary options that are available for people who rely on artificial limbs to enhance their quality of life for mobility and independence.
Key points
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Provision of a prosthesis is only a component of prosthetic rehabilitation. It takes a coordinated team to optimize outcomes and functional independence.
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The most sophisticated components are not the most appropriate for everyone. All options must be weighed with consideration of the person who will be wearing the prosthesis, their environment, and realistic goals.
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Suspension is integral in most socket designs and must be optimized in order to prevent rubbing, slipping and fitting complications.
Introduction
Prosthesis ( noun ): a single artificial limb
Prostheses ( plural noun ): more than one artificial limb
Prosthetic ( adjective ): of or relating to artificial limbs (ie, prosthetic leg)
Prosthetics ( noun ): the profession or field of study related to artificial limbs
The explosion of options that modern technology has afforded individuals who sustain amputations or who are born with congenital limb deficiencies can be overwhelming for health care practitioners and people who rely on prosthetic technology alike. Powered, microprocessor-equipped components offer enhanced control and sophistication. Material and technology advances, improved socket designs, surgical techniques, and prosthetic rehabilitation have empowered prosthetists (and the health care team) with the ability to truly deliver the most advanced prostheses ever invented. This trend will continue in perpetuity.
Yet the most sophisticated device is not the most appropriate for everyone. The excitement provided through the media often gives people unrealistic expectations of the capacities that can be attained through the utilization of the latest and greatest. And for people who are not candidates for the best currently available prostheses, for what type of prostheses are they candidates? Strong scientific evidence dictating choices regarding specific components, suspension systems, and/or socket designs does not (yet) exist. How do you choose?
The aim of this article is to present the options that are available for people who rely on artificial limbs to enhance their quality of life for mobility and independence. Components by name, manufacturer, or coding category have been bypassed in lieu of a focus on features and considerations that must be made in order to make informed decisions; however, specific examples are included. Sockets, liners, and suspension systems for all levels of amputation or limb deficiency are presented first, followed by sections about feet, ankles, knees, and hip joints (for lower limb prostheses) and then sections on terminal devices, wrists, elbows, and shoulder joints (for upper limb prostheses). Although funding sources play a significant and often primary role in decisions regarding access to prosthetic rehabilitation services, the impact of funding limitations on one’s choices related to prosthetic rehabilitation services are not considerations of this article.
Introduction
Prosthesis ( noun ): a single artificial limb
Prostheses ( plural noun ): more than one artificial limb
Prosthetic ( adjective ): of or relating to artificial limbs (ie, prosthetic leg)
Prosthetics ( noun ): the profession or field of study related to artificial limbs
The explosion of options that modern technology has afforded individuals who sustain amputations or who are born with congenital limb deficiencies can be overwhelming for health care practitioners and people who rely on prosthetic technology alike. Powered, microprocessor-equipped components offer enhanced control and sophistication. Material and technology advances, improved socket designs, surgical techniques, and prosthetic rehabilitation have empowered prosthetists (and the health care team) with the ability to truly deliver the most advanced prostheses ever invented. This trend will continue in perpetuity.
Yet the most sophisticated device is not the most appropriate for everyone. The excitement provided through the media often gives people unrealistic expectations of the capacities that can be attained through the utilization of the latest and greatest. And for people who are not candidates for the best currently available prostheses, for what type of prostheses are they candidates? Strong scientific evidence dictating choices regarding specific components, suspension systems, and/or socket designs does not (yet) exist. How do you choose?
The aim of this article is to present the options that are available for people who rely on artificial limbs to enhance their quality of life for mobility and independence. Components by name, manufacturer, or coding category have been bypassed in lieu of a focus on features and considerations that must be made in order to make informed decisions; however, specific examples are included. Sockets, liners, and suspension systems for all levels of amputation or limb deficiency are presented first, followed by sections about feet, ankles, knees, and hip joints (for lower limb prostheses) and then sections on terminal devices, wrists, elbows, and shoulder joints (for upper limb prostheses). Although funding sources play a significant and often primary role in decisions regarding access to prosthetic rehabilitation services, the impact of funding limitations on one’s choices related to prosthetic rehabilitation services are not considerations of this article.
Sockets, liners, and suspension systems
If the interface between the wearer and the device is intolerable, nothing else matters.
Contemporary socket designs include lightweight carbon outer frames wrapped thoughtfully around advanced thermoplastic, anatomically optimized shells. Socket liner technology has evolved from wool socks and polyethylene foam to liners made from urethane, silicones, or thermoplastic elastomers. Grouped generically as gel liners, they are the most commonly used prosthetic interface in North America. All gel-type liners must be rolled onto a residual limb with careful attention to avoid air between the skin and the liner; hence, dexterity is required for independent donning. Liners also require daily washing, require regular replacement (6–12 months), may be hot, and perceived as bulky. Different types of liners are integral to different suspension systems.
Cushion liners consist of different thicknesses of gel with or without a fabric covering and are rarely sufficient to provide suspension alone ( Fig. 1 ). In transtibial applications, they are combined with a knee sleeve, which seals the residual limb/socket chamber, and an expulsion valve creating a suction socket. The chamber is created between the inner surface of the prosthetic socket and the outer surface of the liner, not between the liner and the skin. Negative pressure (subatmospheric) is created when the residual limb is pressed into the socket on loading and subsequently extracted during unloading. Variations of this sealed suction system have been used on transfemoral, hip disarticulation, and upper limb prostheses.
Elevated vacuum (also known as vacuum assisted) suspension takes the same system described earlier; however, the negative pressure in the chamber is increased by the addition of an electric or mechanical vacuum pump. Elevated vacuum suspension reduces residual limb pistoning, reduces residual limb volume loss, and has demonstrated value in residual limb wound healing. In addition to added weight and the need for daily charging (for electric pump systems), the knee sleeve may restrict the knee range of motion in transtibial elevated vacuum wearers. Liners for an elevated vacuum are most often made of urethane or silicone. If the knee sleeve is punctured, the suspension is compromised.
To provide the benefits of a sealed chamber suspension while eliminating the proximal sleeve, some cushion liners have one or more sealing rings on their outer surface that press against the socket inner wall to create the atmospheric chamber ( Fig. 2 ). Liners with sealing rings can be used with expulsion valves or elevated vacuum suspension.
Locking liners are cushion liners with an umbrella-shaped threaded nut at the distal end into which pins, straps, or lanyards are installed. These distal attachments, in turn, are secured to the exterior of the socket (using straps and lanyards) or to distal shuttle locks (pins) to provide a mechanically locked attachment between the liner and the prosthesis ( Fig. 3 ) As locking liner suspensions concentrate the suspension force at the distal end of the socket, they may create a milking effect on the distal residual limb in people with lower limb amputations.
Atmospheric Suspension, Anatomic Suspension, and Osseointegration
Movement between the residual limb and the prosthesis, or the lack thereof, is determined by the prosthetic suspension system. Any suspension system that relies on an air chamber being created to prevent or minimize prosthetic slippage can be categorized as atmospheric suspension. Skin fit suction suspension, knee sleeves, liner-assisted pin and lanyard systems, and elevated vacuum are all types of atmospheric suspension systems. An alternative to atmospheric suspension systems is anatomic suspension.
Anatomic suspension consists of hanging a prosthesis over a boney prominence. This technique can be achieved by having wedges or bladders built into a prosthetic socket or by using strapping and belts. When atmospheric suspensions are not feasible, anatomic suspension systems often provide a solution. It is not uncommon for a single prosthesis to have more than one type of suspension mechanism. For people that have serious socket-fitting and/or suspension challenges, osseointegration presents as another promising solution.
Available in Europe, osseointegration is the ultimate prosthetic suspension system because it eliminates the need for a socket altogether by attaching the prosthetic components directly to a skeletally integrated implant. Surgical procedures and healing time are necessary for the implant to become integrated before prosthetic fitting can commence. Ease of donning, decreased energy expenditure, and improved hip range of motion have been reported in people with osseointegrated transfemoral amputations. Transfemoral is the most common level for osseointegration; however, it has also been successfully applied to transtibial, digital, transradial, and transhumeral level amputations. The incidence of infection is one of the reasons osseointegration has not received approval by the Food and Drug Administration in the United States; however, human trials are undergoing.
Lower limb prosthetics
Prosthetic Foot and Ankle Mechanisms
Prosthetic feet are often grouped according to historical categories because these categories evolved to become descriptors for reimbursement codes. Examples of common foot categories include solid ankle cushion heel (SACH) feet, flexible keel feet, dynamic response feet, and single axis and multi axis feet; however, it has been acknowledged that the codes (categories) into which feet have been assigned lack a standardized methodology to test mechanical abilities. Scientific evidence to recommend specific prosthetic foot-ankle mechanisms is lacking. As early as 1975, Daher demonstrated that even feet within the same category do not perform the same in mechanical testing. Rather than using historical categories, the following sections focus on primary prosthetic foot selection considerations, the prosthetic foot keel, the prosthetic foot heel, as well as prosthetic foot aesthetics and shoes.
Primary Foot Selection Considerations
Two foot-choice considerations are inviolate: weight rating and build height. All feet are designed to support a certain amount of load. A foot’s weight rating must be considered with the wearer’s body weight plus any loads they may eventually carry to be structurally safe. Prosthetic foot build height is the amount of space required to install a prosthetic foot such that it will fit between the ground and the distal end of the residual limb. One should not install a prosthetic foot on someone who weighs more than the foot is designed to withstand. One cannot install a prosthetic foot that requires clearance space that does not exist ( Fig. 4 ).
Taken from the surgical perspective, the amount of limb amputated results in clearance space for the available prosthetic components. If one asks what the ideal amputation length for a transtibial amputation is, the answer depends on the type of foot/ankle mechanism for which the individual is indicated and how much they weigh. Table 1 presents build heights and weight ratings for different prosthetic feet (based on representative examples).
| Foot/Ankle Type | Build Height/Space Needed (cm) | Weight Rating (lb) |
|---|---|---|
| Chopart foot | .6 | 324 |
| Syme level foot | 6 | 365 |
| Intrinsic keel carbon foot | 10–12 | 500 |
| Extrinsic keel carbon foot | Up to 35 | 440 |
| CF running blades | 19–42 | 365 |
| CF with hydraulic ankle | 10–12 | 220–275 |
| CF with hydraulic ankle + microprocessor control | 10–18 | 220–275 |
| CF with hydraulic ankle + microprocessor control + active power generation | 22 | 250 |
The extent to which any prosthesis (prostheses) can meet the functional demands and aesthetic preferences of the wearer delineates the boundaries of prosthetic acceptance and rejection.
The Prosthetic Foot Keel
With the exception of peg legs, the most critical component of any prosthetic foot is the keel. The keel is the part of the prosthetic foot that simulates the human musculoskeletal anatomic structures responsible for structural stability and mobility during loading and movement. For some people with lower limb amputations, stability is paramount; for others, mobility is the primary objective. Prosthetic foot keels, historically made of hardwood, are now made using a variety of materials, such as Delrin or Kevlar (DuPont, Wilmington, DE); urethane; and, most commonly, carbon fiber composites. Some keels are split to facilitate the simulation of inversion/eversion of the prosthetic foot ( Fig. 5 ).
Before the introduction of the carbon keel becoming extrinsic to the foot, prosthetic feet had keels that were intrinsic to the foot. Adding carbon and other dynamic materials to foot keels facilitated feet that first could accommodate terrain and absorb shock (ie, flexible keel and multiaxial feet) followed by feet that could store and return energy (also known as dynamic response or energy storage and return feet) and then ultimately to keels that combined features (multiaxial, dynamic response feet) and feet designed for running.
The Prosthetic Foot Heel
At the initial contact/loading response phase of the gait cycle, a prosthetic foot must simultaneously absorb shock and maintain forward progression. Human feet accomplish this through the evolved capacities of the fat pad of the heel, the bones and ligaments of the foot ankle complex, as well as eccentric contraction of the pretibial muscles of the shank. Prosthetic feet accomplish shock absorption and maintenance of forward momentum in several ways, the simplest is the cushioned foam heels or interchangeable foam heel plugs. Single and multiaxial feet use elastic bumpers that prosthetists tune to the wearer’s gait. In some instances, the heel durometer can be adjustable by the wearer.
Having a prosthetic foot that plantarflexes, either through articulation (single and multiaxial feet) or simulation (cushioned heels or keel deformation), reduces the knee flexion moment for the wearer in the early stance, which ultimately increases knee stability. This stability is vital for people with transtibial amputations with compromised knee extensors and for people with transfemoral amputations who have a compromised ability to control the prosthetic knee.
Aesthetics of Prosthetic Feet and Shoes
Historically, foot appearance and even color were considered unimportant because feet were always meant to be worn in a shoe. As the foot-manufacturing processes improved, feet of varying skin shades and sculpted features became available. With the expansion of carbon fiber feet came the introduction of the foot shell, which not only mimics human foot appearance but it also protects the carbon composite keel. Foot shells and prosthetic feet with intrinsic keels come in different skin tones and levels of detail. A split toe option between the great and second toes facilitates wearing sandals and flip-flops. Sculpted toes and toenails allow for polish to be added if desired.
Apart from prosthetic foot-ankle mechanisms with microprocessors that automatically adjust to the heel height of the shoe, prosthetic feet are designed to be worn with shoes that have a specific heel height. Wearing shoes with heel heights that differ from the shoes with which the prosthesis was initially aligned without accommodating for the heel height differences will adversely affect the performance of the foot, potentially causing injury to the residual limb and/or increasing risk for falls. To facilitate wearing shoes with different heel heights, feet with user-adjustable heel heights are available.
Prosthetic Ankles and Shanks
In general, motions attributable to human ankles are incorporated into prosthetic foot designs; therefore, separate ankle components are often unnecessary. Exceptions are when vertical loading and/or torsional demands go beyond the capabilities of the prosthetic foot (for example, with golf or basketball). To meet these demands, vertical shock pylons and torque absorbers are available.
Although the application of hydraulics to dampen ankle motion is not new, several manufacturers have recently reintroduced hydraulically controlled ankle mechanisms as a means to passively adapt to different terrains. The addition of microprocessors to prosthetic foot/ankle mechanisms to control ankle motion allows prosthetic feet to behave similarly to human feet. For example, the prosthetic foot/ankle mechanism can dorsiflex during the swing phase to facilitate toe clearance and automatically adapt to different heel heights between shoes. The recent introduction of an actively powered prosthetic foot/ankle mechanism (BiOM Ankle System, iWalk Inc, Bedford, MA) now allows for gait performance of people with lower limb amputations to not be statistically significantly different from that of people without amputations across different gait velocities. The downside to the addition of these sophisticated components is the added weight, bulk, maintenance, and financial cost.
Prosthetic Knees
Prosthetic knees and the ability to control them have a drastic effect on the quality of life for individuals with knee disarticulations or more proximal lower limb amputation levels. People with amputations who lack the ability to maintain the prosthetic knee in extension during the stance phase or insufficient flexion during the swing phase for toe clearance are at risk for falls. Active community ambulators who vary their cadence and terrain require a knee that can accommodate ever-changing speeds and conditions. Before the advent of microprocessor control of prosthetic knees, wearers would often comment that they had to think about every step in order to prevent inadvertent knee collapse and falls. When compared with walking with non–microprocessor-controlled knees, people report that walking requires less attention when wearing a microprocessor-controlled knee (MPKs).
The impact that microprocessor control has had on prosthetic knees is so significant that knees are now categorized as either exclusive mechanical control (non-MPKs) or MPKs. By way of introduction to knee designs and features, the following sections on knee axis configurations, fluid dampening versus constant friction, and 4 additional knee features are specific to non-MPKs. The subsequent section addresses MPKs.
Knee Axis Configurations
Historically, prosthetic knees were categorized according to their axis design (single axis vs polycentric knees) because different axis designs were indicated based on the wearer’s residual limb length, functional demands, and their ability to control the knee. For non-MPKs, these indications still apply. As the name implies, single-axis knees rotate about one axis and are mechanically simpler and lighter than their polycentric counterparts.
Polycentric knees (also known as 4-bar knees, although some designs have greater than 4 linkages) actually rotate around an infinite number of axes as an instantaneous center of rotation changes as the knee flexes or extends ( Fig. 6 ). The advantages of polycentric knees include inherent stance stability, shortening in the swing phase, and a low build height (critical for long transfemoral and knee disarticulation amputations when sitting).
