5 Suspension and Control
The suspension system of a prosthesis ensures that the socket is held securely on the residual limb. Good suspension is necessary to limit pistoning of the residual limb within the prosthesis, which provides a secure connection between the residual limb and the socket, reduces shear forces, and enhances control.
In transradial limb loss, the functions of the hand and wrist are absent but those of the elbow, arm and shoulder remain. The transradial prosthesis provides substitutes for the functions of hand prehension, wrist rotation and flexion. Body-powered prosthetic control is generated by force and excursion. Work energy is transmitted from the shoulders through a cable to the prosthetic components.
Force is a push or pull upon an object that causes motion or change.
Excursion is the displacement of the cable as the patient moves their joint (e.g., glenohumeral joint). The amount of cable excursion is related to the amount of joint rotation, the amount of soft tissue on the body, the location of the attachment points on the prosthesis, and the harness setup.
We can describe excursion in three ways:
- Excursion generated by the body: capacity of body to generate movement. (see figure below)
Excursion generated by the body: ipsilateral glenohumeral flexion and biscapular protraction[footnote]Images from the UCLA manual, Prosthetic Checkout Chapter 11, William R. Santschi, Lester Carlyle and Norman Berger[/footnote] - Excursion captured by the prosthesis through the harness.
- Excursion required to control the components (see table below)
| Table. Approximate force and excursion required to operate typical body powered hooks | ||
| Operation | Force | Excursion |
| Prehension, voluntary closing hook | 9-35 lbs. | 1½-2 inches |
| Prehension, voluntary opening hook | 10-20 lbs. | 1½-2 inches |
* Excursion capacity depends ON the person, but excursion is measured on the device.
* The suspension force vector needs to be different than the force vector needed for control/operation of the component.
| Technical criteria | How will you evaluate the technical criteria? |
| Prosthesis is securely suspended | While the patient wears the prosthesis, apply an axial load and observe the tension in the inverted-y and anterior suspension strap. Ensure the strap remains in the deltopectoral groove and is comfortable.
Check comfort of suspension forces through axilla loop or chest strap. |
| Efficient transmission of body movements to terminal device |
Position control strap over lower 1/3 of scapula.
Measure the control system efficiency = (F at TD)/(F at harness) If system is not efficient, reduce curves/bends at reaction points (e.g., baseplate) and consider use of teflon lining to reduce friction.
|
| Patient must be able to generate force and excursion to operate components | Initial evaluation
Fitting evaluation
|
| Comfort | Ensure all straps lay flat on the patients back.
Ensure cable system is not too tight or too loose. Position harness crosspoint so the user Provide break-in schedule to develop tolerance for the axilla strap (or consider harness revision). Provide padding and care instructions on cleaning axilla loop. Provide instructions on stretching tendons in axilla. |