5 Suspension and 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.

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
  • Excursion captured by the prosthesis
  • Excursion required to control the components (see table below)

The force and excursion required to operate typical body powered prostheses are approximately as stated in this table:

Operation Force Excursion
Prehension, voluntary closing hook 9-35 lbs. 1½-2 inches
Prehension, voluntary opening hook 10-20 lbs. 1½-2 inches
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]

Parts of the transradial body powered figure-8 harness

  • Axillary loop
    • Captures the motion of the scapula through the soft tissue.
  • Control attachment strap
    • Originates at the cross point, passes over the distal 1/3 of the scapula to the metal hanger on the control cable, then looped back through a 4-bar buckle for adjustability (largest space goes toward the ring)
    • Direct pull on the cable for control operation
    • Hanger should not be on scapula.
  • Anterior Suspension Strap
    • Prevents distal migration, suspension of the prosthesis!
    • Originates at the cross point, passes over the affected shoulder and the deltopectoral groove.
    • Attaches to the buckles at the anterior proximal portion of the triceps pad in the form of an inverted “Y”.
  • Humeral component
    • Force transmission to the harness from the socket with a flexible triceps pad
    • Force transmission to the harness from the humeral cuff if full cuff
    • Location of the crossbar assembly varies on the length of the lever arm and arc of cable travel.
    • Prevents cable from hitting arm and transmits force.
  • Crosspoint
    • Optimal position is about 1” (2.5cm) below C7 and 1” (2.5cm) toward the contralateral side on the back.
  • Crossback strap (optional)
    • Placed between the control attachment straps of the bilateral harness or the axilla loop and the control attachment strap of a unilateral harness system.
    • Often necessary to prevent the harness from rising up the back to increase efficiency of the harness to capture excursion.

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Upper Limb Prosthetics and Orthotics: Techniques Copyright © by Sue Spaulding. All Rights Reserved.

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