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6b. Assembly of the Bowden Control Cable

Purpose of Bowden Cable

The Bowden cable is a single control system, it operates one component.

In the transradial prosthesis, the Bowden cable operates the TD. The linear movement of the inner cable transmits a pull force. Other applications include clutch cables, throttle cables, brake cable, valve control cable, handbrake cable, bicycle brake cable. In each application, the cable has one purpose.

Single Control System:  Bowden Control 

Bowden cable control system

Bowden control consists of a continuous length of stainless-steel flexible housing, with a retainer at each end, through which the control cable slides. The retainers on the housing fasten it to the prosthesis and serve as reaction points when force is applied to the cable. The Bowden cable can be shaped to follow a straight or curved path with intermediate retainers, and can be moved through various angles and curves while being operated, without these movements affecting the over-all length relationship between the operating ends of the cable.

A typical Bowden cable control system is shown with a transradial single control system. The Bowden is also used on the transhumeral Triple control system and may be used in conjunction with a linear transducer to operate an electrically powered component.

Bowden cable setup for transradial prosthesis

The control system assembly for a transradial body powered prosthesis consists of the cable, housing, housing cover, ferrules, hanger, retainer, baseplate and rubber disc, crossbar, crossbar assembly and ball terminal. The control system may be constructed with standard or heavy-duty parts.

 

The instructions below describe two processes: 1) assembly of the Bowden cable and 2) control system bench test.

 

Process to Assemble the Bowden Cable

1) Gather supplies and consider how all components will fit together, e.g., check how cable length may differ if using two different types of TDs.

  • Cable (forearm + 3/4 humeral length)
  • Triple swivel
  • Hook to hand adaptor (if using)
  • Housing
  • Housing cover (plastic)
  • Teflon lining (if using)
  • Ferrules
  • Baseplate, rubber disk and small screws
  • Retainer
  • Crossbar
  • Crossbar assembly
  • Hanger
2) Attach the ball terminal to the cable and to the terminal device.

  • If necessary, assemble the hook-to-cable adaptor.

Hook to cable adaptor: When using both a hook and a hand, notice that the location of the attachment for the ball terminal may be longer or shorter. The difference in length requires a hook-to-cable adaptor.
3) Attach the baseplate to socket with rubber disc in place.

  • The retainer should be placed 2.5cm distal to proximal trimline or as far proximal as possible minimizing cable angles and friction.
  • The location from medial to lateral depends on the type of TD. Assess the alignment of how the cable exits the TD to determine the appropriate location for the baseplate and retainer.  In general:
    • VO hooks – place along lateral-anterior side
    • VC hooks – place along lateral-posterior side
    • VC hand – place along lateral-posterior side
4) Attach the crossbar assembly to the cuff.

  • Hinged socket design – located at the center of the cuff or triceps pad (approximately mid-length of humerus).
  • Self-suspension prosthetic socket – located at the posterior edge of the socket
5) Set up the housing

  1. Place the prosthesis on the table with the elbow extended (or imagine the elbow extended).
  2. Rotate the TD so the thumb is the closest distance to the retainer. Open the TD and note the most proximal position of the thumb by placing a mark on the forearm. This mark is likely 3.5 cm proximal to thumb of the closed TD.
  3. Measure the housing so that it extends from the forearm mark to 2.5cm proximal to mid-length of humerus.
    1. The finished length of the housing should allow full opening of the TD and extend at least 2.5cm proximal to crossbar.
  4. Use the bicycle cable cutter to cut the housing to length. File or grind the housing ends to remove projecting end of cut wire. (In class, please leave the housing long so we can continue to reuse the parts. Thanks.)
  5. Screw the retainer over the housing until it is positioned over the baseplate.
  6. Screw the crossbar onto the proximal aspect of the housing and fit it into the crossbar assembly allowing full elbow extension.
6) Slide the housing over the cable.

  • If adding a nylon liner, cut the nylon liner 1cm longer than the housing and flare the ends with a heated flaring tool. Or the liner may be cut to the same length and held in place with ferrules.
7) Check the locations of the retainer, crossbar, and overall length of the housing.

  • A = Proximal housing extends 2.5 cm beyond proximal edge of crossbar to avoid sharp bends in cable. B = Distal housing allows 2-3 mm clearance to allow full opening of TD at shortest distance to baseplate.

    Retainer: Distal housing length (B image)

    • Align the TD so that the length between the thumb to the baseplate is at the shortest distance. With the retainer in the baseplate, check the opening of the TD. Adjust the location of the retainer on the housing to allow clearance (2-3mm) from the triple swivel. The housing should not impede the TD operation (B).
  • Length between retainer and crossbar
    • Flex and extend the elbow – the housing should allow the cable to travel unimpeded between the retainer and the crossbar assembly yet not allow too much extra cabling between the two attachments.
  • Crossbar: Proximal housing length (A)
    • The length of housing should extend at least 2.5cm beyond the crossbar assembly to avoid sharp bends as the cable exits the housing.
    • The length of housing may be longer if the crossbar assembly is attached directly to the posterior brim of the socket to protect the skin at the posterior aspect of the arm (e.g., self-suspension socket design).
  • Hanger
    • Align the TD in full pronation and elbow in extension. Then attach the hanger close to the proximal end of housing (2-3mm clearance). The hanger should never hit the proximal end of the housing.

Bench Test the Control System

Do this BEFORE the patient comes in for fitting.

  1. Check the housing length and position:
    • Full operation of TD in pronation and supination
    • No stretching of the housing when the elbow is in full extension between baseplate and crossbar
    • 2.5cm of housing proximal to the crossbar to prevent strain
    • NO sharp bends
  2. Check the cable:
    • Long enough to allow full ROM of pronation and supination without activating the TD
    • Fittings crimped or soldered without concentrating bending forces at transition
  3. Measure the control system efficiency: The efficiency of a control system is influenced by the friction between the cable and its housing. This efficiency refers to the system’s ability to transmit the same amount of force at the output as is applied at the input.
    •  Control system efficiency  = Force at TD/Force at harness
        • The control system efficiency should be 80% or greater for the single control harness
    • Force applied at TD =
    • Force applied at Harness =
Force to operate TD, at TD
Force to operate TD, at harness

Historical Knowledge

Biceps Cineplasty is a surgical procedure performed in the 1940s and 50s intended to allow operation of the prosthesis without a cable. The biceps muscle was divided: one belly continued to flex the elbow as normal while the other muscle belly was used to pull on the cable. To do this, a skin tunnel was constructed from the 2nd belly and a transverse rod was placed through the skin tunnel. This 2nd muscle belly no longer acted on the skeleton. This operation is no longer performed because the training to operate two different muscle bellies was too complicated to learn and the introduction of myoelectric control provided a more useful method to operate the prosthesis without a harness.

The image below illustrates the bench test for someone who has a biceps cineplasty.

 

License

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

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