14 Transhumeral Suspension and Control
In transhumeral limb loss the functions of the hand, wrist, and elbow are absent, but those of the shoulder and residual limb remain. The transhumeral prosthesis must provide substitutes for the missing functions. To accomplish this, the transhumeral harness must transmit power to flex the prosthetic forearm, to lock and unlock the elbow unit, and to operate the terminal device.
The patient uses two separate control cables to operate the prosthesis
| Cable | Fairlead cable | Elbow Lock Cable |
| Component operation | TD operation and elbow flexion | Elbow lock/unlock |
| Body movement | Arm flexion and scapular abduction | Arm extension, shoulder depression and arm abduction |
When the transhumeral fairlead (aka dual-control) system is used, the control cable arranged so that glenohumeral flexion can be used to operate two functions: forearm flexion and TD operation. When the forearm is extended and the elbow unlocked, flexing the ipsilateral shoulder transmits force to the lever loop, flexing the forearm. The force is also transmitted to the TD operating lever, but the latter does not operate, since the amount of force required to operate the VO TD is greater than that needed to flex the forearm. Live lift is the ability to flex the forearm while holding something in the TD.
- When the patient flexes the forearm to the level at which he/she wants to use the TD, he/she locks the elbow by simultaneously extending and abducting the arm so that it moves obliquely to the side. The point of the shoulder must be stabilized or perhaps moved forward slightly, and the trapezius muscle is bulged by downward rotation of the scapula. This small but complex movement increases the distance between the elbow locking lever and the point at which its cable is anchored at the superior end of the deltopectoral interval. The relative motion thus produced between the locking lever and its control cable anchor point trips the lock. This elbow lock control movement is scarcely perceptible when performed by a well-trained patient. However, it is probably the most difficult control to master. [1]
Usefulness of a body-powered or hybrid prosthesis is dependent on an individual’s ability to capture excursion, generate force to operate the components and pre-position the TD in space.
| Operation | Force (lbs) | Displacement (inches) |
| Forearm flexion (no load on hook) | 9 | 2-2.5 |
| Prehension, VC hook | 9-35 | 1.5 |
| Prehension, VO hook | 10-20 | 1.5 |
| Elbow lock | 2-4 | 0.6 |
| Upper Limb Prosthetics, Prosthetist’s Supplement, New York University Medical Center |
| Control Motion | Force (lbs) | Excursion (inches) |
| Glenohumeral flexion | 56 lbs | 5.3” |
| Biscapular abduction | 49 lbs | 1.6” |
| Shoulder depression | 44 lbs | 1.1” |
Triple control harness variation: Scapular abduction is used with the triple control system, to operate the TD through a Bowden cable control. Arm flexion is used only to flex the forearm. Each function is separately harnessed and considerable skill is required to manage the operations independently. The separation is accomplished by slitting the harness in back, attaching the cable to one side of it, and the cable housing to the other. When the patient abducts the scapulae, the two halves of the harness are pulled apart at the break, thus pulling on the cable and operating the TD.[2]
The Elbow Flexion/TD Control Cable is one cable that operates the TD and elbow flexion. This cable can be called the fair-lead cable, split cable or dual control cable.
the ability to flex the forearm while holding something in the VO TD