Chapter 6B - Atlas of Limb Prosthetics: Surgical, Prosthetic, and Rehabilitation Principles
Upper-Limb Prosthetics: Harnessing and Controls for Body-Powered Devices
Charles M. Fryer, B.S., M.S.
In body-powered upper-limb prosthetic applications, the functions of control and suspension are closely interrelated. The prosthesis is suspended on the residual limb by the intimacy of the socket fit and by a system of Dacron straps collectively referred to as a "harness." In a well-designed harness the same straps are strategically positioned in relation to the shoulder girdle and/or thorax so that the amputee can control the prosthetic components with a minimum of exertion and body motion. To understand the two main functions of a prosthetic harness it is first necessary to examine the mechanical operating principles of prosthetic control systems.
MECHANICS OF THE BELOW-ELBOW (TRANSRADIAL) CONTROL SYSTEM
The transradial prosthetic control system is a one-cable or "single-control" system. A stainless steel control cable is firmly attached at its proximal end to one of the Dacron straps of the harness (Fig 6B-1.). Distally, the cable terminates at some type of prehension device (Fig 6B-2).
Prehension devices, usually referred to simply as "terminal devices," may be either prosthetic hands with one or more movable fingers or two-fingered devices with a hook-type configuration. With this type of terminal device the amputee uses shoulder motion on the amputated side to apply tension to the control cable (Fig 6B-2.). The cable tension is transmitted to the operating lever or "thumb" of the terminal device and causes one finger of the hook to move away from the other stationary finger (Fig 6B-2.,A). When cable tension is relaxed, the movable finger closes on the stationary finger (Fig 6B-2.,B). The force of prehension is, in this particular case, determined by the number of rubber bands located at the bases of the hook fingers. As a general rule each rubber band produces approximately 0.45 kg (1 lb) of prehensile force between the hook fingers.
For most of its length the control cable is encased in a flexible stainless steel housing (Fig 6B-3.). At its upper end, the housing through which the control cable passes is attached to the triceps pad of the prosthesis by a fixture called a "crossbar assembly." A base plate and retainer serve to anchor the distal end of the cable housing at approximately the midforearm level of the prosthesis.
The cable housing is an integral part of the transradial single-control system. In effect, the housing maintains a constant length of the control cable regardless of the angular attitude of the anatomic elbow joint. The amount of body motion used to operate the terminal device remains essentially the same with the elbow flexed to 135 degrees or with the elbow completely extended (Fig 6B-4.).
STANDARD TRANSRADIAL HARNESS
The standard harness for the unilateral adult transradial amputee is composed of 2.5-cm (l-in.)-wide Dacron webbing. The webbing is arranged to form a horizontally oriented figure of 8 (Fig 6B-5.).
The axilla loop serves as the primary anchor from which two other straps originate. As indicated by its name, the axilla loop encircles the shoulder girdle on the nonamputated side (Fig 6B-6.).
The second component of the transradial harness is the anterior support strap or, as it is sometimes called, "the inverted Y suspensor." The anterior support strap originates at the axilla loop, passes over the shoulder on the amputated side, and is attached to the anteroproxi-mal margins of the triceps pad of the prosthesis. The primary function of the anterior support strap is to resist displacement of the socket on the residual limb when the prosthesis is subjected to heavy loading (Fig 6B-7.).
The control attachment strap originates at the axilla loop and terminates at the proximal end of the prosthetic control cable (Fig 6B-8.). Anchored by the axilla loop, the control attachment strap acts, in effect, as an extension of the control cable. Located between the spine and inferior angle of the scapula, the control attachment strap permits the use of scapular abduction and shoulder flexion on the amputated side for operation of the terminal device.
The posterior junction of the axilla loop with the anterior support and control attachment straps-the cross point of the harness-may be either sewn together (Fig 6B-9.) or connected by a stainless steel ring (Fig 6B-10.). In the latter case, the harness is referred to as a "transradial, ring-type harness." (Because they are less restrictive, ring-type harnesses enjoy a high degree of acceptability by most transradial amputees.)
Whether the harness straps are sewn together or attached to the axilla loop by a steel ring, mechanical efficiency will be enhanced if the cross point is located below the spinous process of C7 and slightly toward the nonamputated side.
The primary body control motion for operating the terminal device of a transradial prosthesis is flexion of the glenohumeral joint (Fig 6B-11.). Glenohumeral flexion is excellent for the generation of force and provides more than enough cable travel for full terminal device operation. When terminal device operation close to the midline of the body is required, as when buttoning a shirt, the standard transradial harness permits the amputee to use biscapular abduction for terminal device operation (Fig 6B-12.).
HEAVY-DUTY TRANSRADIAL HARNESS
A major disadvantage of the standard figure-of-8 harness for transradial amputees relates to the axilla loop. The axillary portion of the loop should always be padded and worn on top of an undergarment. Even so, whenever significant tension is applied to the anterior support and control attachment straps, the tension drives the loop vertically upward into the axilla on the nonamputated side. Over a period of time, excessive pressure in the axillary area may cause skin irritation and, in extreme cases, produce neurotrophic changes from brachial plexus pressure. When it is anticipated that the transradial amputee will engage in very strenuous work activities, particularly the repeated lifting of heavy objects, it is recommended that a nonstandard transradial harness system be considered.
The nonstandard transradial harness is generally referred to as a "heavy-duty" or "shoulder-saddle" harness. With the heavy-duty harness, tension loading on the prosthesis is distributed over the shoulder on the amputated side rather than being transmitted to the axilla on the nonamputated side. This redistribution of loading is accomplished by fitting a fairly wide, leather shoulder saddle on the amputated side. Two support straps are extended from the posterior portion of the shoulder saddle through D-rings located on the medial and lateral surfaces of the triceps pad and terminate on the anterior surface of the saddle. The shoulder saddle is anchored in place by the use of a chest strap. Since the control attachment strap is located in essentially the same place as in the standard harness, the midscapular level, the amputee uses glenohumeral flexion and/or scapular abduction for terminal device operation (Fig 6B-13. and Fig 6B-14.).
BILATERAL TRANSRADIAL HARNESS
The harness pattern for the bilateral transradial amputee differs only slightly from the previously described standard transradial harness. Viewed from the rear, the control attachment strap for operation of the right terminal device extends obliquely upward across the back and terminates as the anterior support strap for the left prosthesis (Fig 6B-15.). Likewise, the control attachment strap for operation of the left terminal device becomes the anterior support strap for the left prosthesis. As in the case of the standard unilateral harness, the posterior cross point may be sewn together or connected by a stainless steel ring. The bilateral transradial amputee uses the same body control motions, glenohumeral flexion and/or biscapular abduction, for terminal device operation as does the unilateral transradial amputee.
TRANSRADIAL HARNESS MODIFICATIONS
Step-up hinges used with a split socket may be used for a short transradial stump to provide a 2:1 ratio of elbow flexion to stump motion but require the amputee to use approximately twice as much force to flex the prosthetic forearm. Since split sockets are used only at very short transradial levels of amputation, the extra force requirement may cause considerable discomfort on the volar or radial surfaces of the remaining portion of the amputee's forearm. In such instances, a relatively simple control system modification may be used to minimize discomfort and facilitate elbow flexion.
The modification consists of splitting the cable housing into proximal and distal segments similar to those used for the above-elbow (transhumeral) prosthesis. The proximal piece of housing is attached to the triceps pad and the distal piece to the prosthetic forearm. The control cable is now exposed as it passes anterior to the elbow joint. Tension applied to the control cable by glenohumeral flexion on the amputated side assists in elbow flexion (Fig 6B-16.).
In selected instances the unilateral transradial amputee can be fit with a socket, which obviates the need for the suspensory function of a harness. Such self-suspending prostheses are held on the residual limb by the intimacy of the socket fit proximal to the olecranon and humeral epicondyles and in the antecubital fossa. Since these fittings eliminate the need for a triceps pad and anterior support strap, the harness consists of a simple axilla loop around the shoulder on the nonamputated side. Extending obliquely downward across the amputee's back, the control attachment strap runs from the axilla loop to the terminal device control cable. As in the case of the standard harness, shoulder flexion and/or scapular abduction on the amputated side are the control motions for terminal device operation (Fig 6B-17.). The disadvantage of this type of harnessing is that long-sleeved clothing is difficult to wear.
MECHANICS OF THE TRANSHUMERAL CONTROL SYSTEM
Transhumeral prostheses are usually operated by two distinctly separate control cables (Fig 6B-18.). One cable serves both to flex the prosthetic elbow joint and to operate the terminal device. A second cable permits the amputee to lock and unlock the prosthetic elbow.
Elbow Flexion/Terminal Device Control Cable
The housing through which the elbow flexion/terminal device cable passes is split into two separate parts (Fig 6B-19.). The proximal portion of the split housing is attached to the posterior surface of the humeral section of the prosthesis. The distal portion of the split housing is fixed to the prosthetic forearm by a device called an "elbow flexion attachment."
The elbow flexion/terminal device control cable originates at the control attachment strap of the harness (Fig 6B-19, point C). Passing through the proximal portion of the split housing, the control cable is exposed anterior to the mechanical elbow axis (Fig 6B-19., point D). The elbow flexion/terminal device control cable continues through the distal portion of the split housing and terminates with its attachment at the terminal device (Fig 6B-19., point E). Since the housing is in two separate pieces and the control cable passes in front of the elbow axis, tension applied to the cable causes the prosthetic elbow to flex. The flexion is limited to the gap between the two cable housings.
The ease with which the amputee can operate the elbow unit and terminal device depends, to a considerable extent, on the location of the elbow flexion attachment. Greater force and less cable excursion are required where the elbow flexion attachment is closest to the elbow axis. Conversely, a more distal placement of the attachment requires less force but greater cable excursion.
Generally, the longer the residual limb, the further the elbow flexion attachment may be placed from the elbow axis. Higher transhumeral levels of amputation require a more proximal placement of the attachment to minimize the excursion required.
Although the initial placement of the elbow flexion attachment 3.1 cm (1.25 in.) distal to the elbow axis is usually satisfactory, in most instances, its precise location should be determined on an individual basis (Fig 6B-20.).
Elbow Lock Control Cable
The proximal end of the elbow lock control cable originates at the anterior suspension strap (Fig 6B-21.). Passing down the anteromedial surface of the humeral section of the prosthesis, the distal end of the cable engages the elbow locking mechanism. The elbow lock works on an alternator principle: pull and release to lock, pull and release to unlock. An excursion of 1.3 cm (½ in.) and a force of approximately 0.9 kg (2 lb) are necessary to cycle the elbow unit.
In summary, the operating sequence of the two cable systems used with most transhumeral prostheses is as follows: (1) tension applied to the elbow flexion/terminal device control cable causes the elbow to flex; (2) when the desired angle of elbow flexion is achieved, the rapid sequential application and release of tension on the elbow lock control cable locks the elbow; and (3) with the elbow locked, the reapplication of tension on the elbow flexion/terminal device control cable permits operation of the terminal device (Fig 6B-22.).
STANDARD TRANSHUMERAL HARNESS
Full operation of the terminal device of a transradial prosthesis requires only 5 cm (2 in.) of cable excursion. More than twice that amount of excursion is required for full elbow and terminal device operation of a trans-humeral prosthesis. Consequently, much greater attention must be paid to the details of fitting the trans-humeral harness. Precision in the location of the harness and control system components is essential for achieving satisfactory comfort and function.
Like the standard transradial harness, the transhumeral harness consists of a system of interconnected Dac-ron and elastic straps laid up in a figure of 8 (see Fig 6B-21). The common elements of the standard trans-humeral harness are the axilla loop, anterior support strap, lateral support strap, control attachment strap, and elbow lock control strap.
The axilla loop acts as the fixed anchor from which other harness components originate. Some of the straps originating at the axilla loop serve to suspend the prosthesis on the residual limb, while others provide the amputee with volitional control of the prosthetic components.
The anterior support strap, sometimes referred to as the elastic suspensor, originates at the axilla loop (see Fig 6B-21., left). Passing over the shoulder on the amputated side, the strap continues down the anteromedial surface of the humeral section of the prosthesis. The anterior support strap terminates with its attachment on the anterior surface of the prosthetic socket slightly proximal to the mechanical elbow joint (see Fig 6B-21, right). When viewed from the front, it should be noted that the distal two thirds of the anterior support strap consists of elastic rather than Dacron webbing (see Fig 6B-21, left).
The anterior support strap serves several functions in the transhumeral harness systems. Anchored to the axilla loop posteriorly and to the humeral section anteriorly, this strap helps to suspend the prosthesis against axial loading. However, since the anterodistal two thirds of the strap consists of elastic webbing, suspensory function is obviously limited.
A second function of the anterior support strap is to help prevent rotation of the prosthetic socket on the residual limb during prosthetic usage. The transhumeral amputee uses glenohumeral flexion on the amputated side to flex the prosthetic elbow and/or operate the terminal device. Since the proximal control cable housing is attached on the posterolateral surface of the humeral section of the prosthesis, glenohumeral flexion tends to cause the socket to externally rotate on the residual limb. The anterior support strap running downward mediolaterally resists external rotation of the socket.
As a key element of the entire harness, the axilla loop should encircle and fit the shoulder on the nonampu-tated side as securely as possible. A small, snug axilla loop, one that does not compromise amputee comfort to an excessive degree, provides the most positive prosthetic suspension and control. To maintain a fairly snug axilla loop, the posterior intersection of the harness straps should be positioned slightly toward the nonam-putated side of the body (Fig 6B-23.).
The lateral support strap is the primary suspensory element of the harness. Originating posteriorly from the upper portion of the axilla loop, the strap is directed horizontally and stitched to the anterior support strap at their intersection (Fig 6B-24.,A and B). The lateral end of the strap passes just anterior to the acromion and is attached close to the proximal trim line of the prosthetic socket (Fig 6B-24.,C). In addition to its suspensory function, the strap helps to prevent external rotation of the socket on the limb when tension is applied to the elbow flexion/terminal device control cable.
The control attachment strap originates at the posterior intersection of the axilla loop. Running obliquely downward across the amputee's back, the control attachment strap terminates with its direct attachment to the elbow flexion/terminal device control cable (Fig 6B-25.).
With the control attachment strap firmly fixed at its proximal end by the axilla loop, it is easy to visualize how shoulder flexion on the amputated side creates both the cable tension and cable excursion required for elbow flexion and terminal device operation.
The proper location of the control attachment strap as it passes from the axilla loop to the elbow flexion/terminal device control cable is important. If the control attachment strap lies too high on the amputee's back, shoulder flexion will not produce sufficient cable excursion for full operation of the mechanical elbow and terminal device. Too low a strap position requires the amputee to use unnecessarily forceful shoulder flexion for full operation. With the control attachment strap located at approximately the midscapular level, midway between the spine and inferior angle, the amputee will usually be able to achieve full operation of the components through the application of a moderate amount of force.
A cross-back strap is sometimes used as an adjunct to the standard transhumeral harness (Fig 6B-26.). Originating at the axilla loop close to the posterior axillary fold, the cross-back strap passes horizontally across the amputee's back and terminates at the distal end of the control attachment strap. Indications for the use of this strap relate primarily to amputee comfort and ease of prosthetic operation.
At midhumeral and higher levels of transhumeral amputation it becomes increasingly important that the harness be fitted as intimately as possible. Since a snug harness fit requires a relatively small axilla loop, the loop may tend to cause axillary discomfort on the non-amputated side. This discomfort is due, primarily, to vertical compression of the pectoral, teres major, and latissimus dorsi tendons by the axilla loop during strenuous prosthetic usage. The use of a cross-back strap in such instances helps to reduce the magnitude of the vertically directed force created by a snug axilla loop.
Another indication for the addition of a cross-back strap is when the posterior intersection of the harness rides too high on the amputee's back. With the posterior intersection of the harness on or superior to the spinous process of C7, the amputee is uncomfortable, and the work efficiency of the entire harness and control system is diminished. The cross-back strap helps to maintain the posterior intersection of the harness below the spine of C7.
As noted earlier in this chapter, the standard transhumeral prosthetic control system requires approximately 11.3 cm (4½ in.) of cable excursion for full elbow and terminal device operation. Whether or not the amputee is able to generate this much cable excursion depends to a great extent on the path of the control attachment strap as it crosses the amputee's back. Ideally, the path of the control attachment strap should run between the spine and inferior angle of the scapula. Cable excursion, normally produced by glenohumeral flexion on the amputated side, diminishes as the path of the control attachment strap moves closer to the shoulder joint. The addition of a cross-back strap helps to keep the path of the control attachment strap positioned lower on the back.
Cross-back straps may be made of either elastic or Dacron webbing. The nonelastic strap provides the amputee with more positive control of the prosthetic components and overall tautness of the harness. An elastic strap provides less positive control but greater degrees of comfort and mobility of the shoulder girdle.
The elbow lock control strap originates at the upper, nonelastic portion of the anterior support strap and is attached at its distal end to the elbow lock control cable (Fig 6B-27.).
To either lock or unlock the prosthetic elbow the amputee must first apply tension and then, in rapid sequence, relax tension on the elbow lock control cable. Although the cable excursion requirement for prosthetic elbow operation is small, approximately 1.3 cm (½ in.), the body motion is somewhat complex. The amputee applies tension to the elbow lock control strap and cable by slight extension and abduction of the gle-nohumeral joint combined with equally slight shoulder depression on the amputated side. This motion, in addition to exerting tension on the elbow lock control strap and cable, also stretches the elastic portion of the anterior support strap. With the rapid return of the prosthesis to the starting position, the elastic tension of the anterior support strap serves to complete the lock/unlock cycle.
The ring-type harness does not enjoy the same degree of acceptability in transhumeral harnessing as it does at the transradial level. At midhumeral and higher levels of amputation it becomes increasingly important that the harness fit be as snug as possible. Ring-type harnesses do not permit the same degree of tautness in the straps of the system as do stitched harnesses. Consequently, at the higher transhumeral levels the ring-type harness is found wanting in that it does not provide a very high degree of positive control of the prosthetic components, unless the straps are sewn in place after adjustment.
The standard figure-of-8 harness is suitable for and acceptable to the great majority of unilateral transhumeral amputees. However, the unilateral transhumeral amputee who, on a regular basis, engages in unusually strenuous physical activity may find the standard harness uncomfortable. During periods of heavy work, the relatively narrow straps of a standard transhumeral harness tend to subject the soft tissues over which they pass to inordinately high unit pressures. Particularly vulnerable are the skin, tendons, and neurovascular structures of the axilla on the nonamputated side. The problem is further compounded at the transhumeral level because maximal control of the components of the prosthesis requires the use of a small, snug axilla loop.
Alleviation of axillary discomfort for the transhumeral amputee who engages in unusually heavy work is best achieved through the use of a shoulder-saddle harness. The transhumeral shoulder harness distributes tension loading on the prosthesis to the shoulder on the amputated side. Since the control attachment and elbow lock control straps run along the same paths as they do in the standard harness, the body control motions for prosthetic operation remain essentially unchanged (Fig 6B-28.).
The harness for the bilateral transhumeral amputee consists essentially of two figure-of-8 harnesses without axilla loops (Fig 6B-29.). The control attachment strap for the right prosthesis is continued over the amputee's left shoulder and becomes the anterior support strap for the left prosthesis. Likewise, the left control attachment strap becomes the right anterior support strap. At their intersection in the midline of the amputee's back the two straps are sewn together. As in the unilateral harness system, the elbow lock control straps of the bilateral harness originate on the nonelastic portion of the anterior support strap. The lateral support straps consist of a continuous piece of Dacron webbing attached close to the proximal trim lines of both sockets and pass slightly anterior to the acromion processes. Posteriorly, the lateral support straps are stitched to the anterior support straps. Whereas a cross-back strap is considered optional in the standard unilateral transhumeral harness, it is an essential component in the bilateral harness. As seen in Figure 6B-29, the cross-back strap runs horizontally between the two control attachment straps. Two over-the-shoulder straps complete the bilateral figure-of-8 harness for the bilateral transhumeral amputee.
At their posterior origins the over-the-shoulder straps are sewn to the control attachment straps. Prior to passing over the amputee's shoulder, the straps are also stitched to the lateral support straps. The over-the-shoulder straps terminate anteriorly by attachment to the nonelastic portions of the anterior support straps (Fig 6B-30.).
The bilateral transhumeral harness permits the amputee to use glenohumeral flexion and/or scapular abduction for elbow flexion and terminal device operation. Elbow lock control is effected by slight glenohumeral extension and abduction combined with shoulder depression. Two major problems confront the bilateral amputee with the harness just described. First, there is some difficulty in operating both prostheses simultaneously. Tension applied to both elbow flexion/terminal device cables permits opening (or closing) of both terminal devices, but both terminal devices cannot be operated to effect simultaneous opening and closing on opposite sides without relaxing tension on one of the cables. Consequently, the possibility of active bimanual manipulation of objects is minimal. A second major deficiency of this harness system is that it does not permit the amputee to lift any significant amount of weight in the terminal device of either prosthesis.
SHOULDER DISARTICULATION HARNESS
At the shoulder disarticulation level of amputation the absence of glenohumeral flexion as a control source requires the use of other body motions for prosthetic operation. Biscapular abduction is, at least for most adult male amputees, a satisfactory body motion for generating sufficient cable tension to flex the elbow and operate the terminal device of the prosthesis.
The force generated by active biscapular abduction is best harnessed through use of a chest strap (Fig 6B-31., A). Composed of 3.8-cm (l½-in.)-wide nonelastic webbing, the chest strap originates by a buckle on the anterior surface of the shoulder cap of the socket. Running horizontally across the amputee's thorax, the strap passes immediately inferior to the axilla on the nonam-putated side. The chest strap terminates posteriorly with its attachment to the proximal end of the elbow flexion/terminal device control cable.
Vertical suspension of the chest strap and prosthetic socket is augmented by the use of an elastic suspensor strap. The anterior suspensor originates posteriorly on the chest strap (Fig 6B-31.,B). Passing over the shoulder on the amputated side along a diagonal path, the suspensor terminates with its attachment to the proximal surface of the shoulder cap. In addition to assisting with vertical support, the anterior suspensor helps to prevent external rotation of the socket on the shoulder during use of the prosthesis.
Biscapular abduction is usually strong enough to produce sufficient cable tension for fully operating the elbow and terminal device of a shoulder disarticulation prosthesis. Abduction of the scapulae is, however, a poor body motion for generating adequate cable excursion. Very few shoulder disarticulation amputees are capable, through biscapular abduction, of creating enough cable excursion to permit complete elbow and terminal device operation.
Since biscapular abduction is a good source for generating cable tension but a poor source of cable excursion, shoulder disarticulation harnesses frequently require the addition of an excursion amplifier (Fig 6B-32.). A simple excursion amplifier consists of a small pulley attached near the posterior end of the chest strap of the harness. The proximal end of the elbow flexion/terminal device cable passes through the pulley and is attached to the posterior surface of the prosthetic shoulder cap. With this type of amplifier each 2.5 cm (1 in.) of cable excursion generated by biscapular abduction causes the elbow flexion/terminal device control cable to move through an excursion of 5 cm (2 in.). Consequently 5.6 cm (2¼ in.) of chest expansion produces the 11.3 cm (4½ in.) of cable excursion required for full elbow and terminal device operation.
It should be noted that although the incorporation of a pulley in the harness system doubles the cable excursion, it also doubles the input force required for elbow flexion and/or terminal device operation. Since biscapular abduction is a good source of force generation, this increased force requirement does not generally pose a major problem for most adult shoulder disarticulation amputees. Nevertheless, an effort should be made to maximize the mechanical efficiency of the cable system by reducing friction to its lowest possible level.
Depending on factors such as body build, availability of adequate range of scapulothoracic motion, and the neuromuscular coordination of the amputee, locking and unlocking of the elbow unit of a shoulder disarticulation prosthesis can be effected in one of several different ways. The preferred method involves the incorporation of the elbow lock control strap as an anterior extension of the chest strap.
In this method the anterior attachment of the chest strap is bifurcated (Fig 6B-33.). The upper leg of the split strap consists of nonelastic webbing. The lower leg is nonelastic at its extremities-its origin on the chest strap and attachment on the socket-but has a segment of elastic webbing at its center. A nonelastic elbow lock control strap originates at the chest strap, passes laterally between the two legs of the split strap, and attaches directly to the proximal end of the elbow lock control cable. With this harness arrangement, cable tension for locking and unlocking the elbow is created by scapular adduction on the amputated side.
Incorporation of the elbow lock control strap with the chest strap makes it easier to don the prosthesis but requires a fairly high level of neuromuscular coordination for successful operation.
An alternative arrangement for elbow lock control requires the use of a waist belt (Fig 6B-34.). The waist belt serves to anchor the distal end of the elbow lock control strap. From its anchor on the waist strap the control strap runs obliquely upward where it is attached to the proximal end of the elbow lock control cable. With the waist belt system, the primary body control motion for cycling the elbow unit is shoulder elevation on the amputated side.
A third option for achieving elbow lock control requires the use of a nudge control mounted on the an-teroproximal surface of the prosthetic shoulder cap (Fig 6B-35.). The nudge control for locking and unlocking the elbow is operated by force exerted by the amputee's chin. Nudge control is usually reserved for severely disabled persons such as bilateral shoulder disarticulation amputees. Realistically, the functional expectations for persons with acquired bilateral shoulder disarticulation amputations are extremely limited. With the help of adaptive equipment, environmental modifications, modifications of clothing, and a unilateral prosthetic replacement, it may be possible to achieve a reasonable degree of partial independence in the basic functions of personal hygiene, dressing, and eating.
There is no such thing as a "standard" harness for bilateral shoulder disarticulation amputees. Although most authorities agree that fittings should be unilateral rather than bilateral, the specifics of the harness and control system are left to the experience and ingenuity of the prosthetist, therapist, physician, patient, and members of the patient's family.
The unilateral prosthesis should permit active operation and passive prepositioning of a lightweight terminal device, active or passive flexion of the wrist unit, active flexion and locking of the elbow unit, passive external and internal rotation of the humeral section, and passive prepositioning of the shoulder joint in flexion and abduction.
Absence of the humeral heads narrows the girth of the shoulder girdle and reduces the effectiveness of biscapular abduction as a work source. A small well-padded plastic cap covering the apex of the acromion on the side opposite the prosthesis enhances the available range of biscapular motion, thereby preserving this important control source (Fig 6B-36.).
Biscapular abduction and the use of an excursion amplifier should permit adequate cable excursion for producing a reasonable degree of elbow flexion and terminal device operation. Shoulder elevation on the amputated side may be used for elbow lock control.
Harnessing patterns for the forequarter amputation do not differ significantly from those used in the shoulder disarticulation, except that the efficiency of operation is less. Most persons with high-level amputations benefit from the use of externally powered (electronic) componentry, as discussed in Chapter 6C and Chapter 6D.
Chapter 6B - Atlas of Limb Prosthetics: Surgical, Prosthetic, and Rehabilitation Principles