A prosthetic is an appliance designed to replace, to the greatest possible extent, the work and the look of a missing limb or part of it. There are conditions to fulfill for an accepted prosthetic; it should be comfortable, easy to wear, and easy to remove. It must be well functioning with the least maintenance. Finally, it has to combine an accepted cosmetic appearance and reasonable functioning. Prosthetics can be either body-powered (cable controlled) or externally powered (Esquenazi and others, 1989).
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Different methods of upper limb suspension
The role of a suspension system is not only to fix the prosthetic to the residual stump safely and securely. It also has to bear weight and deliver the forces resulting from lifting or manipulating weight as well as the weight of the prosthetic equally and uniformly. Thus, it should enable the patient to get the best possible performance of the prosthetic. Methods of suspension are many, starting from basic leather straps to complex suction sockets. Many factors settle which type of suspension to use. The most important factors to consider are; weight-bearing, the activity of the person to use, the structure of the prosthesis and its units, individual patient considerations and preferences for using the prosthetic, and the cost of the prosthetic. There are three types of upper limb prosthetics suspension in common use, harnessed (straps fitted) based, self-suspending sockets, and suction sockets (Quigley, 1992).
Considerations at different surgical levels
Residual limb length is important to settle what prosthetic units fit the stump. The longer the remaining part, the easier the patient uses a body-powered or an externally powered prosthetic. However, with excessively long residual limb parts, the patient’s prosthetic choices can be limited. The transradial level is most suitable for the patient, besides being the commonest level a prosthetist meets. The patient has the benefit of a suitable remaining limb length that delivers weight forces uniformly during most of the patient’s daily activities. In this situation, the patient has the choice to use either a body-powered or electrically powered prosthetic with the conventional units as a well-fitting five functions wrist (Lake and Dodson, 2006).
Harnessed (strap fitted) based suspension systems
A Harnessed based system is a set of straps fitted to a patient to fix the prosthetic in place. They are widely used with different types of prosthetics, with the 8-shaped harness is the commonest. To provide counterforce for suspension, a harness winds around the axilla of the healthy side; this should stabilize the harness. On the type of prosthetic, there are two straps, an anterior and posterior strap. The anterior one attaches to the socket in transhumeral or transradial prosthetics, either directly or indirectly, through a Y-shaped strap and a triceps cuff. The posterior strap attaches to control the cable. Thus an 8-shaped harness is formed. The anterior strap carries most of the suspending forces to the prosthetic. In case more weight is desired to lift, and the prosthetic is a transradial one, a chest strap is used for suspension instead of the previous 8-shaped harness. In case the prosthetic is transhumeral, the patient can use a chest piece alone (Gitter and Bosker, 2005).
For heavier lifting or as an alternative to the figure 8-shaped harness, a shoulder saddle with a chest strap suspension can be used with transradial prosthetics. For cases of long transradial stumps or cases with wrist disarticulation, the use of a 9-shaped harness provides the control cable with the needed attachment point and opposing force. However, the figure 9-shaped harness provides the least suspension and needs, in this case, a self-suspending socket (Gitter and Bosker, 2005).
Bilateral transradial harness
The model of harness for patients with bilateral transradial amputations is slightly different from the standard strapping patterns. The reason is these patients still use the elbow flexion movement as a control for the terminal device used, as do patients with unilateral trans-radial amputation (Freyer, 1992).
Shoulder disarticulation harness (Fryer, 1992)
The characteristic of shoulder disarticulation is the absence of flexion at the elbow joint (absent glenohumeral flexion). Thus, the patient has to utilize another body movement for working the prosthetic. This force-producing movement is bis-capsular abduction, and a chest harness best straps this force. Bis-capsular abduction is good for producing force (in terms of cable tension) but is not enough for alternating movements (excursion). Therefore, a small pulley block attached near the posterior end of the chest strap of the harness is used for movement augmentation (excursion amplifier) (Freyer, 1992).
To achieve locking or unlocking of the elbow unit of this prosthetic, an elbow-lock control strap is included as an anterior extension to the chest strap. Alternatively, a waist belt can fix the distal end of the elbow-lock control strap. However, this alternative may not be preferred in patients with full waists. Generally, a well-designed harness is the one where straps are placed to provide the patient with better control of the prosthetic parts using the least possible force (Fryer, 2006).
A word about patients with bilateral shoulder disarticulations
There is no standard harness for patients with bilateral shoulder disarticulations. The specifications of a control system and harness are up to the experience and ingenuity of the managing team of doctors, physiotherapists, and the prosthetic, all in the light of the patient’s desired activity and comfort (Freyer, 1992).
These sockets can provide acceptable prosthetic suspension with or without a harness. The commonest use of self-suspending sockets is in cases of wrist or elbow disarticulation or in cases of trans-radial amputation (Gitter and Bosker, 2005).
Design of self-suspending sockets ( Brenner, 1992)
Socket design variations for transradial amputations are:
- Supracondylar lips (rims) that include the humeral epicondyles and the posterior olecranon. There are four types according to the stump length. Miinster sockets for short transradial stumps. The Northwestern supracondylar socket for medium-length stumps. The adjusted supracondylar brim with an olecranon cut out and the floating brim suspension for long transradial stumps (Brenner, 1992 and Miguelez and others, 2003).
- Sleeve suspensions: Use atmospheric pressure or skin traction for upholding the suspension. The technique includes; latex rubber sleeves, neoprene sleeves, and elastic sleeves (Brenner, 1992 and Miguelez and others 2003).
- Suprastyloid suspension for cases of wrist disarticulation with prominent styloid processes: This category includes; Silicone bladder suspension, window-door suspension, and soft removable slit (Brenner, 1992 and Miguelez and others 2003).
The transradial anatomically contoured (TRAC) coupling influences prosthetic control in many ways (Miguelez and others, 2003). The proper design positively affects the stability and range of motion of the prosthetic as well as the comfort of the patient. Its muscle contouring has the potential to improve the electromyographic, hydrostatic, and cellular health of the remaining part of the limb (Miguelez and others, 2003).
To affirm the upper limb prosthetic on the remaining part, a differential pressure is used to create a vacuum. The choices with upper limb suction sockets are similar to those of the lower limb. Suction sockets use one of three mechanisms (Mather and Otchin, 2002):
- An external elastic suspension sleeve.
- A one-way air valve.
- Roll-on gel suspension liner with a pin locking mechanism.
Suction sockets for upper limb prosthetics need the following (Mather and Otchin, 2002):
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- The demands of the stump are; a perfect stump with no scarring, no skin folds or ingrowths, and a residual limb with a stable volume.
- The socket design should provide complete contact between the prosthetic and the residual limb.
· The commonest uses of suction sockets are (Mather and Otchin, 2002):
- In cases of transhumeral amputations with good soft tissue cover: The type commonly used is suction socket with air valve. It has the advantage of securing suspension of the prosthetic without suspension straps. However, it needs a stable residual volume harder to put on than other suspension systems.
- In cases of transradial amputations or transhumeral with scarring and non-perfect skin integrity: The type commonly used is gel sleeve with locking pin. This suction socket has the advantage of adapting the residual limb volume with decreased skin shear.
The method of suspension of a prosthetic should provide the best way of holding it and enabling the prosthetic to perform the needed work. It varies from one patient to another depending on stump and body contours, the climate, which activity desired and patient preferences. The main role of the prosthetist is to evaluate the pros and cons of each method of suspension tailored for a particular patient and to recommend which is the best. This needs spending time in fitting trials and dynamic alignment for choosing the most suitable method of suspension to complete a prosthetic.
Quigley, M.J (1992). Prosthetic Management: Overview, methods and materials.. In Atlas of limb prosthetics: surgical, prosthetic and rehabilitation principles (Chapter 4, pp.61-81). St. Louis, MO: Mosby-Year Book, Inc.
Esquenazi, A, Leonard, J.A Jr. , Meier, R.H et al (1989). Prosthetics, Orthotics and assistive devices. Arch. Phys. Med. Rehabil, 70 (5-S0, S-206:209.
Lake, C, and Dodsn, R (2006). Progressive upper limb prosthetics. Phys Med Rehabil Clin N Am, 17, 49-72.
Gitter, A. and Bosker, G. Upper and lower extremity prosthesis. In De Lisa, J.A (Ed.). (2005). Rehabilitation Medicine: Principles and Practice 4th. edition. Philadelphia: Lippincott Williams and Wilkins. Pp. 1325-1354
Fryer, C.M. (1992).Upper limb prosthetics: Harnessing and control of body-powered devices. In Atlas of limb prosthetics: surgical, prosthetic and rehabilitation principles (Chapter 6B, pp.133-150). St. Louis, MO: Mosby-Year Book, Inc.
Brenner, C.D. (1992). Prosthetic principles. In Atlas of limb prosthetics: surgical, prosthetic and rehabilitation principles (Chapter 8B, pp.241-250). St. Louis, MO: Mosby-Year Book, Inc.
Miguelez, J.M, Lake, C, Conyers, D, and Zenie, J (2003). The transradial anatomically contoured (TRAC) interface: Design, Principles, and Methodology. JPO, 15 (4), 148-157.
Department of Veterans Affairs. (2002). Traumatic amputation and prosthetics. Mather, S. H, and Otchin, N.S (Editors): Veterans Health Initiative, Employee Education System.