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O&P Library > Atlas of Limb Prosthetics > Chapter 6A

Reproduced with permission from Bowker HK, Michael JW (eds): Atlas of Limb Prosthetics: Surgical, Prosthetic, and Rehabilitation Principles. Rosemont, IL, American Academy of Orthopedic Surgeons, edition 2, 1992, reprinted 2002.

Much of the material in this text has been updated and published in Atlas of Amputations and Limb Deficiencies: Surgical, Prosthetic, and Rehabilitation Principles (retitled third edition of Atlas of Limb Deficiencies), ©American Academy or Orthopedic Surgeons. Click for more information about this text.


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Chapter 6A - Atlas of Limb Prosthetics: Surgical, Prosthetic, and Rehabilitation Principles

Upper-Limb Prosthetics: Body-Powered Components

Charles M. Fryer, B.S., M.S.
John W. Michael, M.Ed., C.P.O.

OVERVIEW

Body-powered components have been used in upper-limb prostheses for centuries and are still commonly prescribed today. The term body powered acknowledges that the force to operate such components comes from mechanical transmission of muscular effort generated elsewhere in the body, remote from the amputation site.

When body power is insufficient or undesirable, externally powered components may be utilized. "External power" comes from a source outside the body; contemporary versions are battery-powered electronic devices, although pneumatic, hydraulic, and other power sources have been utilized in the past.

As a group, body-powered devices enjoy the triple advantages of low cost, light weight, and high reliability due to mechanical simplicity. Their widespread application today throughout the world verifies the practical advantages offered by such components.

They also share significant disadvantages, however. The harness required to transmit muscle forces inevitably restricts the amputee's work envelope and often encumbers the noninvolved side. The amputee must exert effort to generate sufficient force and excursion to operate the component. Some find this objectionable; the higher-level amputee may find it impossible to generate sufficient motion or strength due to the very limited leverage offered by short bony remnants (see Chapter 6B). Finally, the robotlike appearance of some body-powered components can be disconcerting to the general public as well as to the amputee.

TERMINAL DEVICES

The most distal component of an upper-limb prosthesis is termed the terminal device and subdivided into two functional classes: passive and prehensile devices. Since passive devices have no moving parts and require no cables or batteries for operation, they are typically extremely lightweight and reliable.

Passive Terminal Devices

The most commonly prescribed passive terminal device is the passive hand (Fig 6A-1.). Chapter 7C discusses the custom-sculpted hand in more detail and emphasizes the functions of static grasp and social acceptance offered by these devices. A much less expensive production hand is also available. The production passive hand is created from a donor mold that is similar to (but not identical to) the missing appendage and offers acceptable cosmesis to some patients.

Another category of passive terminal devices resembles children's mittens, and hence they are called "mitts." The passive mitt is usually a soft, flexible humanoid shape similar to the cupped human hand. They are often recommended for infants and for sports activities. Some have specialized shapes to facilitate particular activities (Fig 6A-2.). The prosthetist may also design specialized passive terminal devices as is illustrated in Chapter 12C, which discusses sports and recreational devices in more detail.

Prehensors

Prehensors offer active grasp and may be classified according to their mode of operation. "Voluntary-opening" devices are normally held closed by a spring or rubber band mechanism but open when the control cable is pulled. "Voluntary-closing" devices operate in a converse manner. Prehensors may also be subdivided into handlike and utilitarian shapes. The traditional utilitarian shape is the split hook.

Voluntary-Opening Hook Terminal Devices

Hosmer-Dorrance is the name associated with a broad range of body-powered, voluntary-opening hook terminal devices. Many have a similar characteristic shape and differ principally in size and materials. Originally designed by an upper-limb amputee in 1912, the versatility and reliability of the voluntary-opening hook with canted fingers made it the most commonly prescribed terminal device in North America (Fig 6A-3.).

The series 5 hooks are intended for adults and were originally manufactured only in stainless steel. Steel remains available but is usually reserved for the heavy-duty, transradial (below-elbow) user. The letter "X" indicates the addition of neoprene rubber finger linings to improve friction and grasp. The letter "A" indicates aluminum alloy and reduces weight about 50% over the steel versions. The alloy hooks are satisfactory for all but the most rugged users.

The series 8 hooks are slightly smaller and intended for females but offer similar options in materials and finger linings. The series 9 hooks are for adolescents, series 10 is for children, and series 12 is an infant's hook. Addition of the letter "P" indicates that it has been coated with "plastisol," a soft rubber material available in both Negroid and Caucasion tones.

A second characteristic shape is the "work hook," identified by the large opening between the two fingers that is designed to grasp shovel handles and similar objects (Fig 6A-4.). This is a heavy-duty, stainless steel device reserved almost exclusively for adult male amputees. The specialized fingers also have a number of subtle contours that facilitate holding, grasping, and carrying such items as buckets, chisels, knives, nails, and carpentry tools. It is sometimes referred to as a "farmer's hook" but has value for anyone engaged in manual tasks including workshop activities. Variations add a larger opening or a locking mechanism to the basic hook.

The term canted refers to the slanted configuration of the hook fingertips, which facilitates visual inspection during fine motor tasks. Since no prehensor yet offers sensation, the amputee must rely on vision to confirm that grasp has been successful.

Some hook fingers offer a more symmetrical shape that grasps cylindrical objects such as bottles more readily than the canted approach (Fig 6A-5.). The "two-load" hook has "lyre-shaped" fingers for this reason. As its name suggests, a small switch at the base of the thumb permits the amputee to engage either one spring (1.6 kg, 3 lb) or two springs (3.2 kg, 7 lb) to vary the pinch force. Because the fingers are hollow alloy, it is not suitable for heavy-duty use. The Dorrance "555" series has more rugged solid fingers in the same "lyre" shape and is available in steel or aluminum alloy (Fig 6A-6.).

The "contour" hook is a recent addition that uses two "C"-shaped fingers to facilitate cylindrical grasp. Since most amputees find the canted approach satisfactory, the specialized shapes tend to be more commonly prescribed for the bilateral upper-limb amputee, but on one side only. The combination of one canted hook and one straight hook offers two different grasp patterns.

A few other manufacturers offer voluntary-opening hooks. The United States Manufacturing Company (USMC) hook is a steel design similar to the series 5 type. It has a small triangular opening in the stationary finger that can latch onto a serrated attachment. Hand tools and sports equipment can be modified by mounting serrated attachments that allow the amputee to lock the implements securely onto the hook. In some cases, the movable hook finger can then be used to pull the trigger on a drill, pistol, or similar object (Fig 6A-7.).

The CAPP terminal device (originally developed at the Child Amputee Prosthetics Project at UCLA) offers a voluntary-opening utilitarian shape that is not a hook. Clever use of contours and rubber materials provides a reasonably secure grasp despite a limited pinch force.This device is most popular for small children, whose ability to generate body-powered force is limited, but it is also available in adult size (Fig 6A-8.).

European manufacturers, most notably Otto Bock and Hugh Steeper, have recently made a number of terminal devices available to the American market. Many are really "tools" that interchange for specific tasks rather than multipurpose devices. Thus far, they have not developed widespread popularity in the United States (Fig 6A-9.,A). The USMC has recently announced a series of adapters that allow direct attachment of standard mechanics' tools to the prosthetic wrist unit (Fig 6A-9.,B).

Voluntary-Closing Hooks

The APRL hook was developed by the Army Prosthetics Research Laboratory after World War II. It differs from all hooks previously discussed in several major respects:

  1. The APRL hook is a voluntary-closing device.
  2. The fingers automatically lock in any position once grasp is accomplished.
  3. A selector switch permits the amputee to choose either
    1. A large finger opening (7.5 cm, 3 in.).
    2. A small finger opening (3.4 cm, l 3/8 in.).
    3. "Freewheeling"-a small opening with the locking function eliminated.
  4. The operating lever or thumb is located on the ulnar side of the device (Fig 6A-10.).

This device was originally developed to use biceps cineplasty as a source for body power. The voluntary-closing mode provides graded prehension: the pinch force is as gentle or strong as the force generated by the amputee. Particularly with a cineplasty, this can improve proprioception.

Unfortunately, the mechanical complexity of this device makes it both expensive and prone to breakdown. The hollow aluminum lyre-shaped fingers it shares with the "two-load" hook make it somewhat fragile. Combined with the waning popularity of cineplasties, these factors limit its prescription primarily to previous wearers. The graded prehension may also be of value to selected bilateral upper-limb amputees.

Bob Radocy, a recreational therapist and transradial amputee, has introduced a series of voluntary-closing utilitarian devices. They are available in both aluminum and steel versions as well as plastic-coated styles for children. Patient acceptance has been good, particularly for children and sports-minded adults (see Chapter 12C).

None have a locking mechanism, which means that the amputee must maintain continuous force to grasp an object. Although this is physiologically normal, some find it objectionable. Acceptance has been greatest for unilateral transradial amputees, particularly those with long residual limbs (Fig 6A-11.).

Voluntary-Closing Hands

Although voluntary-closing hands theoretically offer the same advantage of graded prehension as do hook devices, the frictional losses in the mechanism are much greater. The rubber cosmetic glove that covers the hand further impedes motion, and the contours often block visual inspection of the fingertips. For all these reasons, voluntary-closing hands have never enjoyed widespread popularity.

The APRL hand, available in an adult male size only, has similar features to the APRL hook:

  1. Automatic locking when grasp is accomplished
  2. Small opening (with the thumb in the standard position)
  3. Large opening (with the thumb in the second position) (Fig 6A-12.)

Otto Bock of Germany exports a lightweight and inexpensive voluntary-closing hand in several sizes. It uses many of the same internal components as their electronic hand and has an identical external appearance and cosmetic glove. It is also available in a voluntary-opening configuration (Fig 6A-13.).

Voluntary-Opening Hands

Although a number of voluntary-opening hands are available, few if any are used as active terminal devices. In addition to the problems of frictional loss, glove restriction of motion, and contours that block visual inspection, all voluntary-opening devices offer only limited pinch force.

Many new amputees desire an interchangeable hand for social occasions in addition to a hook device for manual work. This is the most common indication for body-powered hands. As a result of their extremely limited functional capabilities, they are rarely appropriate for bilateral upper-limb applications. As will be discussed in Chapter 6C, externally powered hands offer far greater pinch force and function and are therefore often preferable to body-powered hands.

Becker Plylite Hand.-The Becker Plylite hand is a simple, lightweight, voluntary-opening hand that is available in sizes 6 to 10 in 1.3-cm (-in.) increments (e.g., 6, 6, 7, 7, 8) (Fig 6A-14.). The only moving component is the thumb. The larger models permit sufficient thumb movement to grasp objects of up to 7.5 cm (3 in.) in thickness. An optional locking mechanism that locks the thumb in the closed position is available.

Becker Lock-Grip and Imperial Hands.-The Becker Lock-Grip and Imperial hands are voluntary-opening hands with control cable tension that causes all five fingers to open (Fig 6A-15.,A and B). The Lock-Grip model contains a mechanism that locks the fingers in the closed position. Finger opening from the fully closed position can be effected only by control cable tension. Lock-Grip hands are available in 1.3-cm (-in.) increments from size 6 to size 10. The Imperial model, available in size 8 only, permits easy adjustment of finger prehension force with the use of a screwdriver.

Robin-Aids Mechanical Hand.-The Robin-Aids mechanical hand is a voluntary-opening hand with control cable tension that causes digits 2, 3, 4, and 5 to move away from a stationary thumb (Fig 6A-16.). The thumb can be manually prepositioned for normal or large opening prehension. The force of prehension is generated by springs and may easily be increased or decreased by the prosthetist. This is the only commercially available hand with an adjustable length feature that permits its use with very long transradial and wrist disarticulation amputation levels.

Robin-Aids Soft Mechanical Hand.-The Robin-Aids soft mechanical hand is a voluntary-opening hand (Fig 6A-17.). Tension on the central cable causes the thumb and first two fingers to open. The endoskeletal frame is encased in plastisol and covered with a urethane foam of low density that provides "softness." Both of the Robin-Aids hands are available in sizes 7, 7, 8, 8, and 9.

Sierra Voluntary-Opening Hand.-The Sierra voluntary-opening hand, like the APRL hand, has a two-position stationary thumb (Fig 6A-18.). From the fully closed position, control cable tension causes the first two fingers to move away from the thumb. As tension on the control cable is relaxed, springs cause the fingers to move close toward the thumb. A "Bac Loc" feature operates in all finger positions and permits the amputee to hold heavy objects securely. Finger opening and release of the Bac Loc mechanism are operated simultaneously through a single control cable. The Sierra voluntary opening hand is available in size 8 only.

Hosmer-Dorrance Functional Hands.-Hosmer-Dorrance functional voluntary-opening hands permit the prosthetist to adjust finger prehension by the installation of different tension springs (Fig 6A-19.). The hands are available in four sizes: 8, 7, 6, and 5.

COSMETIC GLOVES

A cosmetic glove is the rubberized covering that determines the external appearance of the prosthesis. It is applied over the shell of a passive hand or over the mechanism of an active prehensor and can be replaced when it deteriorates from use.

Three levels of cosmetic restoration are possible. A stock glove is the most common covering and is ordered by the prosthetist on the basis of hand size and skin tone. Most come in generic male and female, adolescent and child's contours in a few shades of Caucasion and Negroid plastic. Many amputees find the manni-kin-like appearance quite acceptable. The appearance can be improved by subtle painting of the veins and other structural details; fingernail polish can be applied and removed by the amputee.

A custom production glove is manufactured from a donor mold of a hand similar in shape to the amputee's. The prosthetist sends a precise mold of the remaining hand to the factory so that the best match can be selected. A wider selection of skin tones are available than in the stock glove; artistic painting and fingernail polish can add realism.

The custom-sculpted glove offers the greatest cosme-sis: it is hand-made from a sculptured reverse copy of the remaining hand. Such artistic restorations are usually made of a special silicone rubber that is more durable than the polyvinylchloride (PVC) plastic commonly used for the less expensive gloves (see Chapter 7C). Some prosthetists refer the amputee, with the completed prosthesis, to a cosmetic restorationist who creates the custom-sculpted glove to match the amputee and to fit over the prosthetic mechanism (Fig 6A-20.). Even a myoelectric hand can be covered with a sculpted glove.

WRIST UNITS

Prosthetic wrist units are designed to serve two basic functions: to attach a terminal device to the forearm of the prosthesis and to permit the amputee to preposition the terminal device prior to operation. The need for the first function is obvious. To the uninitiated, the importance of the second function of wrist units may be less clear.

The above-elbow (transhumeral) amputee has lost all ability to supinate and pronate the prosthetic forearm. The transradial amputee with a short residual forearm (50% or less than the length of the nonamputated forearm) no longer retains active transmissible supination and pronation. Even at the very long transradial levels of amputation, the motions of supination and pronation are severely restricted. Consequently, the upper-limb amputee must be provided with a device that permits some form of substitution for active forearm rotation.

Friction Wrist Units

Commercially available wrist units permit the amputee to substitute for supination and pronation by manually rotating the terminal device with the remaining normal hand (Fig 6A-21.). Bilateral amputees usually preposition the terminal devices for use by striking one device against the other, thereby rotating it to the desired position of function.

Friction wrist units are available in aluminum or stainless steel in the adult size (5-cm [2-in.] diameter) and medium size (4.4-cm [l-in.] diameter).

Oval-shaped friction wrist units are available in adult and medium sizes (Fig 6A-22.). The oval configuration provides better cosmesis in cases of long transradial levels of amputation. Also, since most prosthetic hands have an oval base, the oval-shaped wrist unit provides for a smoother transition from the prosthetic hand to the prosthetic forearm. This wrist unit does not provide constant friction.

Friction wrist units designed specifically for wrist disarticulation levels of amputation are made as thin as possible to conserve the length of the prosthetic forearm (Fig 6A-23.). These wrists do not provide constant friction and function in the same manner as previously described units. The units are available in two sizes: adult (5-cm [2-in.] diameter) and medium (3.4-cm [1 3/8-in.] diameter).

The foregoing wrist units do not provide constant friction. As the terminal device stud is screwed into the wrist unit, a rubber washer is compressed to create friction. As the terminal device is unscrewed, friction is reduced. It is highly desirable that wrist units provide constant friction. Modern units permit the amputee to rotate the terminal device through 360 degrees of motion without a change in the effective friction.

Constant-Friction Wrist Units

Constant-friction wrist units are designed to provide constant friction throughout the range of rotation of the terminal device. Most units of this type employ a nylon-threaded insert with steel lead threads (Fig 6A-24.). Turning a small set screw in the body of the wrist causes the nylon thread to be deformed against the stud of the terminal device, thus creating constant friction. Damage to the insert threads may be repaired by simply removing and replacing the entire insert.

Constant-friction wrist units are available in both the round and oval configurations (Fig 6A-25.). In the round configuration, four sizes are available: infant (3.1 cm, l in.), child (3.8 cm, 1 in.), medium (4.4 cm, 1 in.), and adult (5 cm, 2 in.). In the oval configuration two sizes are available: adult and medium.

Quick-Change Wrist Units

Quick-change wrist units are designed to facilitate rapid interchange of different terminal devices, usually a hook and a hand (Fig 6A-26.). All commercially available quick-change units permit the amputee to do the following:

  1. Remove the terminal device from the wrist unit
  2. Replace the terminal device with a different terminal device
  3. Manually position the terminal device in supination or pronation
  4. Lock the terminal device in the desired attitude of supination or pronation

Most quick-change units employ an adapter, which is screwed tightly on the studs of the two (or more) terminal devices to be interchanged. In these units light downward pressure on the activating lever by the amputee unlocks the terminal device but does not cause its ejection. With the terminal device unlocked, the amputee manually rotates the hook or hand to the desired attitude of pronation or supination. Next, the application of a proximally directed axial force with the sound hand causes the terminal device to be locked in the new position. Heavy downward pressure on the activating lever causes ejection of the adapter and attached terminal device.

Quick-change units are available from the Hosmer-Dorrance Corporation in the adult size and round configuration only (Fig 6A-27.).

Wrist Flexion

Wrist flexion is particularly useful for activities at the midline: toileting, eating, shaving, dressing, et cetera. Such activities are performed more easily with the remaining hand than with a prosthesis. For this reason, prosthetic wrist flexion is seldom necessary for the unilateral amputee unless there is a restricted range of motion in the more proximal joints.

However, it is of crucial importance for the bilateral upper-limb amputee who must perform all daily functions with prostheses. Because the mechanism adds weight near the termination of the prosthesis, it is sometimes prescribed only for the dominant side. Two types of mechanism can provide wrist flexion.

The "Flexion Wrist" replaces the common constant-friction wrist and allows manual prepositioning of the hook in neutral, 30 degrees of volar flexion, or 50 degrees of volar flexion (Fig 6A-28.). The hook can also rotate about its mounting stud in any of the positions.

The "Sierra Wrist Flexion Unit" is used in addition to the friction wrist (Fig 6A-29.). This dome-shaped device also has three locking positions at zero, 30, and 50 degrees of volar flexion. Because the entire unit can rotate where it mounts to the wrist, the terminal device covers a much wider arc than with the first alternative. This can be advantageous for the bilateral amputee struggling to perform midline activities. On the other hand, this unit is significantly heavier than the Flexion Wrist.

Rotational Wrists

Previously discussed friction wrist units may present difficulties for those amputees who engage in work or avocational activities that exert high rotational loads on the terminal device. Friction and constant-friction wrist units tend to permit unwanted rotation when subjected to very high torsional loading.

Rotational wrist units are cable-controlled, positive-locking mechanisms (Fig 6A-30.). In the unlocked mode, these units permit manual prepositioning of the terminal device in almost any attitude of supination or pronation through a 360-degree range. Once locked in position, these units provide much greater resistance to rotation than do friction units.

The bilateral amputee may find that rotational wrist units facilitate prepositioning of the terminal devices. With the wrist unit unlocked and the terminal devices fully supinated or pronated, tension on the terminal device control cable causes the terminal device to rotate back to the "neutral" position.

Ball-and-Socket Wrist

A ball-and-socket wrist unit is also available (Fig 6A-31.). The unit permits universal prepositioning of the terminal device with constant friction. The magnitude of the friction loading can be easily adjusted by the amputee.

ELBOW UNITS

Elbow Units for the Transradial Amputee

With amputation through the distal third of the forearm, the amputee retains a limited amount of active supination and pronation. Flexible hinges facilitate the transmissions of this residual forearm rotation to the terminal device, thereby minimizing the requirement for manual prepositioning by the amputee.

Flexible Hinges

Flexible hinges of metal or leather are commercially available. Dacron webbing may also be used. Attached proximally to the triceps pad and distally to the prosthetic forearm, these hinges permit the transmission of approximately 50% of the residual forearm rotation to the terminal device (Fig 6A-32.).

Rigid Hinges

For all practical purposes, amputations at or above the midforearm level obviate the possibility of transmitting active supination or pronation to the terminal device. At these levels of amputation the amputee must resort to manual prepositioning of the terminal device.

Single-Axis Hinges.-Single-axis hinges are designed to provide axial (rotational) stability between the prosthetic socket and residual forearm during active prosthetic use (Fig 6A-32.). Correctly aligned single-axis hinges should not restrict the normal flexion-extension range of motion of the anatomic elbow joint. Single-axis hinges are available in both adult and child sizes.

Polycentric Hinges.-Short transradial levels of amputation require that the anteroproximal trim line of the prosthetic socket be close to the elbow joint. With a high anterior socket wall, complete elbow flexion tends to be restricted by the bunching of soft tissues in the antecubital region. Polycentric hinges help to increase elbow flexion by reducing the tendency for bunching of the soft tissues (Fig 6A-34.). Polycentric hinges are available in adult, medium, and child sizes.

Step-Up Hinges.-Amputations immediately distal to the elbow joint require a prosthetic socket with extremely high trim lines. Consequently, flexion of the anatomic elbow joint is often restricted to 90 degrees or less. In those situations in which a full range of elbow flexion is essential, step-up hinges may be employed.

The use of step-up hinges requires that the prosthetic forearm and socket be separated (Fig 6A-35.). Consequently, protheses employing step-up hinges are frequently referred to as split-socket prostheses. Step-up hinges amplify the excursion of anatomic elbow joint motion by a ratio of approximately 2:1. Sixty degrees of flexion of the anatomic elbow joint causes the prosthetic forearm (and terminal device) to move through a range of approximately 120 degrees of motion. The increased range of motion requires that the amputee exert twice as much force to flex the step-up hinge. Step-up hinges are available in adult, medium, and child sizes.

Stump-Activated Locking Hinge.-Amputees with very high transradial levels of amputation are often unable to operate a conventional transradial prosthesis for the following reasons:

  1. Inadequate strength of the elbow flexors
  2. Inadequate range of elbow flexion
  3. Inability to tolerate the high unit pressure on the volar surface of the forearm when step-up hinges are used

With stump-activated locking hinges, the transradial prosthesis is controlled in much the same manner as a transhumeral prosthesis (Fig 6A-36.). As in the case of step-up hinges, a split-socket prosthesis is used. Shoulder flexion on the amputated side flexes the mechanical elbow joint. The residual limb is used only for locking and unlocking the mechanical joint. Stump-activated locking hinges are available in two sizes, adult and small.

Elbow Units for Elbow, Transhumeral, and Below-Shoulder Disarticulation Amputees

Loss of function of the anatomic elbow joint requires a mechanical substitute that permits controlled flexion and extension through a range of approximately 135 degrees. In addition, the unit must permit the amputee to lock and unlock the elbow at numerous points throughout the 135-degree range of motion.

Outside-Locking Hinges

Elbow disarticulation and transcondylar levels of amputation usually require the use of a specially designed elbow unit. The length of the residual humerus preeludes, for both aesthetic and functional reasons, the use of standard prosthetic elbow units.

Outside-locking hinges are available in standard and heavy-duty models (Fig 6A-37.). The standard units provide seven different locking positions throughout the range of flexion and come in adult, medium, and child sizes. The heavy-duty model provides five locking positions and comes in the adult size only.

Inside-Locking Elbow Units

Amputations through the humerus approximately 5 cm (2 in.) proximal to the elbow joint provide adequate space to accommodate inside-locking elbow mechanisms. Inside-locking units permit the amputee to lock the elbow in any of 11 positions of flexion (Fig 6A-38.). In addition, inside-locking units incorporate a friction-held turntable. The turntable permits manual preposi-tioning of the prosthetic forearm as a substitute for external and internal rotation of the humerus.

Flail Arm Hinges

Flail arm hinges contain an oversized clock spring mechanism to partially counterbalance the weight of the forearm. They may be used singly or in pairs depending upon the degree of counterbalance desired. They may also be combined with a single free joint or a single locking joint, as necessary.

Friction Units

Friction elbows require passive positioning of the forearm but are very lightweight and simple to operate. For this reason, they are often appropriate for cosmetic restorations, pediatric applications, congenital anomalies, and instances when brachial plexus injury or other factors preclude active elbow function.

Spring Lift Assist

The spring lift assist is a clock spring unit, similar in function to the flail arm hinge, that can be added to any mechanical elbow. The function is to counterbalance the prosthetic forearm and reduce the force necessary for elbow flexion. Reduced force requirements may permit subtle harnessing adjustments that require less excursion from the amputee. Although optional, the spring lift assist is commonly prescribed, particularly for use with heavier steel terminal devices or hand prehensors.

SHOULDER UNITS

Shoulder mechanisms vary according to the degree of motion allowed. The simplest design is termed a bulkhead when the humeral segment is directly connected to the socket and no motion can occur. Many unilateral amputees find this acceptable and appreciate the weight savings from omitting the joint.

Passively movable friction-loaded shoulder joints are available and provide some assistance with dressing and desktop activities. Single-axis units permit only abduction, double-axis units (Fig 6A-39.,A) allow abduction and flexion, and triple-axis (Fig 6A-39.,B) and ball-and-socket configurations permit universal passive motion. Most are available in small, medium, and large sizes.

As noted in Chapter 10B, prosthetists sometimes must custom-build shoulder joints if locking functions are desired. Fig 6A-40. illustrates one such design commercially available on a limited basis.

NUDGE CONTROL UNIT

The nudge control unit is a paddle-shaped lever that can be pushed by the chin or phocomelic digit or against environmental objects to provide a small amount of cable excursion. It is usually prescribed when other body motions are not available. Although originally designed to provide elbow locking and unlocking, it can also be adapted to operate other body-powered components, including flexion and rotation wrist units.

ENDOSKELETAL UPPER-LIMB PROSTHESES

Two endoskeletal upper-limb prosthetic systems are currently available in the United States. They are composed of tubular humeral and forearm elements, and the components allow for encasement in cosmetic foam covers. After final shaping and covering with a skin-colored stockinette, the completed prosthesis affords a high degree of cosmetic acceptability (Fig 6A-41.). In addition to improved cosmesis and softness, modular prostheses are lighter in weight than conventional artificial limbs.

The Otto Bock Pylon Arm system for transhumeral and shoulder disarticulation amputees permits passive or cable-operated elbow flexion with manual locking (Fig 6A-42.). Passive prepositioning of the humeral segment in internal or external rotation and the forearm in supination or pronation is achieved by the use of rotation adaptors.

The system hands (Fig 6A-43.) provide a wide variety of terminal device options: cable-controlled, voluntary-opening or -closing units and a passive hand unit with a spring-activated thumb and fingers. For the shoulder disarticulation level, the Otto Bock system offers two friction-loaded, passively positionable shoulder units: a ball-and-socket joint and a flexion-extension, abduction-adduction hinge (Fig 6A-44.).

The endoskeletal system of the Hosmer-Dorrance Corporation includes components for transradial, trans-humeral, and shoulder disarticulation levels of amputation (Fig 6A-45.). All terminal devices with the standard W-20 thread can be used with the Hosmer-Dorrance system. Socket attachment turntables permit passive rotation of the humeral and forearm segments. A separate wrist unit allows for manual prepositioning of the terminal device in flexion.

Three elbow units are available for either cable-controlled or manual operation: a constant-friction elbow, an elbow with a manual lock, and an elbow joint with a cable-controlled locking mechanism. For the shoulder disarticulation level, a manually positionable flexion-extension, abduction-adduction hinge is available.

Chapter 6A - Atlas of Limb Prosthetics: Surgical, Prosthetic, and Rehabilitation Principles

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