15 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 fully body-powered transhumeral prosthesis operates through the use of a dual-cable harness. The two separate cables are the fairlead cable and the elbow lock cable.

The elbow-lock cable is a bowden cable which extends from the elbow lock control strap to the locking mechanism in the elbow joint. When pulled and released, it operates one thing, the elbow lock.

The fairlead cable extends from the control strap to the TD. When pulled, it operates two things (i.e., dual-control): elbow flexion and TD opening.

The fairlead cable is guided by two separate lengths of housing (i.e., split-housing). The lengths of housing are fastened to the prosthesis with two fairleads at points where the cable must be supported or operated through an angle. When force is transmitted through the housing and cable, side forces are exerted on the fairleads where the cable bends through an angle. The fact that these forces are exerted where the cable operated through an angle is used as an advantage in the transhumeral dual-control system. With the forearm adjusted in 10 degrees of initial elbow flexion (i.e., it’s maximum extension is 10 degrees short of full extension), the control cable causes a force to be exerted on the lever loop to flex the forearm.

Usefulness of a body-powered is dependent on an individual’s ability to capture excursion, generate force to operate the components and pre-position the TD in space.

Fairlead cable operation:

• Glenohumeral flexion operates two functions: elbow flexion and TD operation
• When the forearm is extended and the elbow unlocked, flexing the ipsilateral shoulder transmits force to the lift tab, flexing the elbow unit. 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.

Elbow lock cable:

• 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.

Fairlead cable operation:

• Once the elbow is locked, force is again transmitted through the fairlead cable but now the force and excursion pull on the TD operating lever.

“Live-lift”

• The dual-control cable must be aligned accurately to capture livelift.
  • The amount of force required to operate the VO TD must be greater than that needed to flex the forearm.
  • F_{flex elbow} <<< F_{closeVO TD}
• The dual-control cable setup relies on livelift. This setup limits the maximum force and therefore ability to handle many objects. In most cases the person flexes the forearm to the position desired, locks the elbow, then grasps the object to be lifted.^[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]

Body-powered Harness Questions

1. The control attachment strap ideally lies over the lower third of the scapula in order to capture as much excursion as possible. What happens when the based plate is moved proximally with a shorter residual limb?
2. With the fairlead cable, you need 2-2.5″ of excursion to fully flex the elbow unit and about 1.5″ to open the VO TD. With short transhumeral residual limbs, patients have difficulty generating excursion. What do you need to do to increase the excursion to enhance operation of the cabling system?
3. The fairlead cable system is dependent on livelift. If the TD is opening before the elbow is flexing, what can you do to improve live lift?
   • F_{flex elbow} <<< F_{closeVO TD}
   • What can you do to increase the F to open the TD
   • What can you do to decrease the F to flex the elbow?
4. The images below illustrate the location and path of the cable. The left images shows the path between the scapula and humerus (glenohumeral motion) and the right image shows the path between the left and right sides of the spine (protraction motion).

• Which of the images show the path to capture more excursion?
• Which of the images show the path to capture more force?