Transhumeral Harness Variations
There are multiple variations of harness designs individually designed to maximize the efficiency of the control system, the suspension of the prosthesis and ideally fit comfortably on the person. Variations on harness designs may include any of the elements from the following: Hessing pad, Shoulder Saddle and Chest Strap, Triple Control Harness, Bilateral harness and Shoulder Disarticulation Harness designs.
In addition, material selection is important. Most harnesses are fabricated with polyproylene webbing, but other options like climbing webbing may be used if increased tension stregnth is necessary. Finally, buckles might be used to ease one-handed application/removal such as fidlock, click medical or others from WBC medical.
Ultimately, the key points on harnessing must be adhered to:
- The harness suspends the prosthesis by distributing the load and maintains prosthetic stability through various ROM.
- The control system allows for the transmission of power from the individuals body motion to the prosthesis.
- Harness and cable system together should be operable with relatively inconspicuous body motions.
- The harness must be easy for the person to don, doff, launder; and it must be comfortable and durable.
Fitting Questions
- How does the location of the path of the control attachment strap affect the efficiency of force and excursion?
- How does the force on the strap and thus the cable affect the socket fit on the residual limb?
Refer to Alternative Harnessing Techniques by Gerald Stark AAOP J of Proceedings, 1999. http://www.oandp.org/default.asp?page=JOP_1999
Variations
- Hessing Pap
- Shoulder Saddle and Chest Strap
- Triple Control Harness
- Bilateral Transhumeral Harness
Hessing Pad
The Hessing pad can replace axilla loop: Fits under scapula on contralateral axilla
Purpose: to distribute and offload weight/pressure from the contralateral axilla
Located laterally on chest wall under the scapula from about T7-T11 ribs. Notice that it does not extend very far medially.
Materials: Leather, Plastic, Spenco
Reference: Fillauer
Shoulder Saddle and Chest Strap
The greatest disadvantages of the chest strap harness are difficulty in applying and removing, and a tendency to rotate about the chest because of lack of a stable anchoring point. In addition, many amputees, particularly women, cannot tolerate the chest strap.
In general, the use of the chest strap harness is confined to those patients who must lift heavy loads, or who cannot tolerate the axilla loop, even with a pad, or who have habitually worn the chest strap harness and do not wish to change. (NYU 1979). The control attachment strap is usually connected to the lateral corner of the posterior portion of the shoulder saddle.
- Think about reaction points!
- Compare this design with the transradial shoulder saddle harness.
- Note that only one lateral suspensor loop is used, connected to the socket by a D-ring, through which it is free to slide.
Triple Control Harness
The NYU Triple Control Harness consists of 3 control elements:
- Scapular abduction to operate the TD
- Arm flexion to flex the forearm
- Arm extension to lock and unlock the elbow unit
Consider challenges with separation of control for the patient.
Bilateral Transhumeral Harness
The bilateral harness is a combination of two above-elbow Figure-8 harnesses; each axilla loop is replaced by an attachment to the opposite prosthesis. A cross-back strap and two over-the-shoulder straps may be added to improve harness stability. The dual control system used on each prosthesis is identical with that used on a unilateral above-elbow prosthesis.
- 2 figure-of-eight harness without axilla loops
- Control attachment strap is continuous with the anterior support strap
- Intersection is at midline of back
- Lateral support straps consist of a continuous piece of nylon webbing passing over acromion and attaching to the lateral proximal aspect of socket just anterior to acromion processes. Posteriorly, stitch to anterior support straps.
- Cross-back strap runs horizontally between the control attachment straps
- Over-the-shoulder straps connect the control attachment strap to the lateral support strap and the anterior support strap.
The fairlead control cable system is used in the above-elbow prosthesis to transmit force for two functions: elbow flexion and terminal device operation. A second cable controls elbow locking and unlocking: the elbow-lock cable. When the elbow is extended and unlocked, flexing the shoulder transmits force to the forearm lever loop, raising the forearm to the desired level. If the user wishes to use the TD at this point, he/she locks the elbow and then operates the TD by continuation of the control motion, i.e. shoulder flexion.
Control motions:
- Humeral flexion and/or scapular abduction operate elbow flexion and TD
- Humeral extension, abduction and shoulder depression operate the elbow lock
- Difficulty operating both prostheses simultaneously
- Limits weight limit of either prosthesis
