Chapter 10B - Atlas of Limb Prosthetics: Surgical, Prosthetic, and Rehabilitation Principles
Shoulder Disarticulation and Forequarter Amputation: Prosthetic Principles
Robin Cooper, B.A., C.P., F.B.I.S.T.
It is important in any description of the through-shoulder level of upper-limb absence to distinguish between the appearance of an acquired amputation site and congenital absence of the upper limb. Each of these groups present a separate and quite distinct clinical picture that affects prosthetic management.
When considering the congenital group (Fig 10B-1.), it should be noted that the clavicle and scapula are often misshapen, may be fused, and are usually foreshortened with the lateral aspects swept upward. These features create a prominent and usually very mobile bony spur. The remainder of the shoulder area is often fleshy and has the potential for weight support, but the lack of bony structures tends to make stability a problem. The shoulder profile drops away quite sharply from the bony point of the glenoid area, and the incorporation of a prosthetic shoulder joint presents no great cosmetic or technical difficulty.
In the acquired group (Fig 10B-2.), the amputation site is likely to be rather elevated laterally, with surfaces that are capable of bearing some weight unless they are scarred by the trauma. It is unusual for the remnant shoulder musculature to be atrophied, although some muscle bulk may have been lost during the trauma or surgery. The major prosthetic problems are therefore prosthesis stability and cosmetic appearance, in particular, retention of a natural shoulder profile. In addition, when trying to provide for a functional device, it is sometimes difficult to decide whether or not a prosthetic shoulder joint should be included and, if so, what type of component is appropriate.
In addition, the two groups can be further subdivided to include cases in which other factors must be considered. For instance, in the acquired group one might wish to include those individuals who have received surgery for ablation of a neoplasm or discuss the treatment of the fragile stump in instances where the surface has been burned or grafted. In the congenital group one might wish to include individuals with digits at the shoulder or with similar longitudinal defi-cits.
In clinical practice two types of sockets are commonly fitted at this level. These can be classified as those that enclose the shoulder and are formed to its contours (Fig 10B-3.) and those incorporating some type of perimeter frame (Fig 10B-4.) that encompasses the shoulder and provides structural mounting points for the prosthesis and location and reference points for a variety of controls.
When providing an enclosed socket, two factors must be taken into account: first, it should be remembered that external forces that are transmitted to the shoulder from the hand will often be large because of the long moment arm involved. It is therefore important that the edges and surfaces of the socket that interface with the amputee be rounded and relieved very carefully. For example, it is good practice to make an allowance for relief over all bony prominences and provide additional flaring where the edge of the socket crosses protuberant bone. Alternatively, it is possible to make the whole socket interface from a soft material, supported by stiffening where appropriate. A range of materials may be employed for this purpose.
With few exceptions, the socket is generally made of plastic. This may be a laminate using silicone or poly-urethane resins and a woven reinforcement, or it may be one of a number of soft thermoplastics such as certain forms of polyethylene that can be drape-formed. A second point to note is that with such a high-level loss the effects of perspiration may cause difficulties. Although an intimate fit is necessary to provide optimal stability, it may be desirable to provide some form of ventilation or moisture-permeable surface next to the skin. If this is necessary, the prosthetist must take care to make, an appropriate allowance on the positive model in addition to relief for bony structures.
Where a user with a congenital deficiency is to be fitted, it is often convenient to manufacture a frame socket. This approach uses suitably padded metal strips brazed together to form a frame. Alternatively, a high-stiffness laminate (e.g., carbon fiber reinforcement) can be made. The frame covers very nearly half the torso, extends down to about the fifth rib to within an inch or so of the anterior and posterior center lines, and passes over the shoulder near the neck. In the case of a bilateral fitting, a hinge is provided to link the posterior frames, and a mouth-positioned Velcro fastener is provided at the front.
As with all prosthetic procedures the manufacturing process is built upon the foundation of accurate measurement, information recording, casting, and modeling. This process starts by a determination of the dimensions of the prosthesis based either on the remaining limb or, in the case of a bilateral loss, on estimates of these dimensions related to general physique. Care must be taken to estimate the amount of shoulder elevation remaining once the weight of the prosthesis is being carried. One must not be misled by the position of the axilla because surgery or scarring may distort the tissue in the axillary area.
It is useful to record both the amputation site and the contralateral contours so that a faithful copy of the shoulder profile can be created. This may be done by extending the normal plaster cast over both shoulders, high up onto the neck, and over the upper part of the chest and back by using plaster of paris slabs. A second cast of just the amputation site is then obtained, with care taken to mark the likely trim line of the socket and accentuate the bony contours. The shoulder should be depressed slightly to allow for the weight of the prosthesis. A photographic record may also be helpful.
In preparing the positive cast, after the outlined contours are smoothed and rectified, they should be built up by using plaster or the appropriate thickness of closed-cell foam stapled to the cast. This technique may also be used to provide additional flare to areas where the edge of the socket and bony contours coincide and to delineate the boundary of the socket. The membrane that is applied to seal the cast during laminating will crush the edges of the foam as the vacuum is applied and will thus form a prerounded border to the socket. It will usually be necessary to remove material from the anterior and posterior aspects of the cast to eliminate gaping in these areas; it may also be necessary to remove plaster from the subglenoid lateral aspect to accommodate any shoulder mechanism that is to be attached. The area of the socket superior to the scapuloclavicular joint may have to be removed to allow the prosthetist to achieve a reasonable profile, and this should be taken into account when carrying out these procedures. If a frame socket is to be constructed, the foam technique is a useful way of making channels in which to lay up Kevlar or carbon fiber.
The hardware fitted from the humeral rotator downward can be selected from any of the major manufacturers. It is obviously important to provide a relevant prescription that will suit the planned life-style of the individual. Although it is commonly believed that prostheses fitted at this level are likely to be nonfunctional, this does not need to be the case. Active functions can be provided with the judicious addition of appropriate components. While an endoskeletal, lightweight passive arm (Fig 10B-5.) is frequently supplied to individuals who have a sedentary life-style, a more robust prosthesis can be supplied for manual workers following a traumatic loss. In these cases it may be necessary to custom-make special shoulder units with locking functions that will enable tools to be positioned and operated in working environments. For instance, the author has provided prostheses that allowed one amputee to work as a welder under tanker trailers and another to work as a public parks employee who was required to shovel soil onto trucks and to plant and handle saplings (Fig 10B-6.). There are no commercially available units that will survive such rigors.
Harnessing and cabling present a difficult challenge in such cases, and this makes one or more powered units a good option. For instance, a Steeper switch-controlled electric lock can be provided at the elbow (Liberty Mutual Research Center, Hopkins, Mass) and modified with an interlock to allow single cable control of both elbow flexion and a Servo Electric hand. A shoulder unit with variable friction in two planes such as the Hosmer Child Amputee Prosthetics Program (CAPP) device (Hosmer Dorrance Corp, Campbell, Calif) is a good addition to this prescription. For some individuals, provision of a powered elbow, a powered wrist, and a powered hand is appropriate. This will provide function without exertion, but the cost must be assessed for the individual concerned, not only in financial terms but also with regard to the weight penalty and the likely difficulties in learning efficient control strategies. If the user finds operation difficult or robotlike, the prosthesis may represent over gadgetization, and it is likely that even with the best technical advice and training the device will be rejected.
When a very lightweight limb is required, the whole shoulder area may be shaped from Plastazote fitted at the transhumeral level into a lightweight socket with a manually controlled endoskeletal system attached. However, this type of prosthesis may also be rejected as "useless" since it is purely passive. Externally powered components may be either switch controlled or myoelectric. It is important to place the battery holder as high on the prosthesis as is possible, consistent with good cosmesis and practical fabrication for best results.
The provision of harnessing for the through-shoulder prosthesis (Fig 10B-7.) has two objectives. First, it is designed to hold the prosthesis in place, minimize slip and movement on the stump, and spread the weight of the prosthesis across the body. Second, by utilizing differential body motion, the harness can provide control inputs with force, speed, and displacement components. The control element also provides some sensory feedback if resistance to the motion is sufficiently large.
To meet the first objective, the harness must provide a medial force at two points. These forces counterbalance the effects of gravity, dynamic forces occurring during operation and the forces generated by external loading. The first force is applied just inferior to the point at which the socket edge crosses the clavicle, and the second is applied to the posterior surface, inferior to the spine of the scapula.
The simplest harness is a padded strap that passes under the contralateral axilla and connects these two points, possibly with the addition of an elastic element. In some cases, more complex solutions are required. It may be desirable, on occasion, to eliminate the chest strap and replace it with a figure-of-8 harness around the contralateral shoulder. For very lightweight systems fitted to women, all that is required are anterior and posterior ribbons fitted with clips that allow attachment to a brassiere. Activation of switch-controlled components is simply accomplished with a posterior ribbon fastened through a safety pin mounted vertically in the brassiere under the contralateral axilla.
The switch control elements of the harness are usually linked to the suspension components. This provides fixed points against which force or displacement can be generated. The establishment of the connection points depends on the signal required. Usually, force/ power signals are taken from straps across the back, which takes advantage of biscapular abduction, while control signals are linked to the position of the shoulder or some other small but independent motion.
A frame socket is also used when it is desirable to leave the shoulders completely free. This provides complete freedom for the thoracic girdle, either to maximize the ability to operate controls or to minimize discomfort caused by perspiration. With such prostheses, the shoulder can be used to activate simple push switches, which should be suitably perspiration proof, or alternately, the frame can be used as a fixed point against which force and displacement transducers may be operated to control actuators.
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Chapter 10B - Atlas of Limb Prosthetics: Surgical, Prosthetic, and Rehabilitation Principles