Prosthetic Control Biomechanics and Voluntary Closing Prehensors: Technical Note
BOB RADOCY, MS
Ten years' experience with modern voluntary closing prehensors (terminal devices)1,2 including the recently reported successful clinical research studies with children,3-7indicates that prosthetic controls are prehensor specific, as well as patient and prosthesis specific. Modifications in cable routing and harnessing are necessary to achieve optimal biomechanical efficiency with voluntary closing devices. Elbow flexion, humeral flexion, bilateral scapular abduction and internal shoulder rotation are all critical in providing the belowelbow handless person with sufficient biomechanical force to create prehension with a voluntary closing device.
Conversely, shoulder retraction and shoulder elevation controls allow the patient to relax prehension and "release" effectively, especially close to the body.
During the prosthetic checkout, each control motion described should be evaluated individually to assess its effect on prehensor function. Failure of any isolated control motion to produce the proper prehensor action indicates that the cable/harness system needs improvement.
Proper cabling and harnessing of the voluntary closing prehensor will enable the patient to manipulate objects with dexterity in most body zones, using subtle control motions. Figures 1 and 2 illustrate two traditional body powered systems which have been modified to improve biomechanical efficiency with voluntary closing prehensors. In both systems, the cable is routed laterally on the prosthesis and then runs superior to the axis of the elbow to make use of the elbow flexion control movement.
Biomechanically efficient prosthetic controls conserve the patient's energy and provide for prehension output which is equivalent to or greater than the corresponding anatomical hand.
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- Radocy R, Dick RE: "A Terminal Question," Orthotics and Prosthetics 35:1-6, 1981.
- Radocy B: "Voluntary Closing Control: A Successful New Design Approach to an Old Concept," Clinical Prosthetics and Orthotics 10:82-86, 1986.
- Crandall RC, Hansen D: "Clinical Evaluation of a Voluntary-Closing Terminal Device," Journal of the Association of Children's Prosthetic-Orthotic Clinics 24:36, 1989.
- Kaniewski B: "Voluntary Closing Terminal Device," Journal of the Association of Children's Prosthetic-Orthotic Clinics 24:35, 1989.
- Kuchler-O'Shea R, Baldassare B, Tollefson T: "Rehabilitation Training of a Quadrimembral Amputee," Journal of the Association of Children's Prosthetic-Orthotic Clinics 24:35, 1989.
- Libby A, Greenberg S, Dudgeon B, Jaffe K: "Transition from a Voluntary-Opening to a Voluntary Closing Terminal Device," Journal of the Association of Children's Prosthetic-Orthotic Clinics 24:35, 1989.
- Shapiro S, Locast M: "Comparison of ADEPT Terminal Device to Hook Terminal Device for Children with Below-Elbow Amputations," Journal of the Association of Children's Prosthetic-Orthotic Clinics 24:34, 1989.