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Radial Deviation Modification to the Wrist Driven Flexor Hinge Orthosis

Harvey Johnson, C.O.

Introduction

The wrist driven flexor hinge orthosis (WDFHO) is an important device for the quadriplegic patient with wrist extensor function. Active wrist extension in the quadriplegic is often accompanied by radial deviation. Resistance to radial deviation/wrist extension is found in WDFHOs that utilize joints that permit motion in only one plane; these orthoses compromise prehension. This article describes a modification that allows up to 35 degrees of radial deviation without compromising pinch force.

Radial deviation is a result of the angular pull of the extensor carpi brevis and longus. The extensor carpi ulnaris often lacks innervation. If it were innervated, it would assist in extending the wrist in a neutral plane. Clinical analysis of the quadriplegic patient often reveals a hand that radially deviates first and then extends. The clinician, by blocking the quadriplegic hand from radially deviating, can prevent wrist extension. This mimics a three-point pressure system. Acting as a three-point pressure system, WDFHOs that permit motion in only one place also resist radial deviation and wrist extension: resulting in reduced prehension. Power loss results from having the wrist travel an oblique path while the orthosis must be aligned longitudinally to match the axis of wrist and metacarpophalangeal joints.1 Grasping small objects becomes particularly difficult because it requires a greater amount of radial deviation/wrist extension. Since the WDFHO's introduction by Bisgrove in 1954,2 it has had few major modifications.3 Although different designs have evolved, all WDFHOs, custom and those available in kits, use single axis wrist joints. The TIRR WDFHO design attempts to solve the radial deviation problem with the use of a flexible steel leaf spring.4 The relatively short leaf spring allows only a limited amount of radial deviation. Additionally, the leaf spring resists bending. This translates into more work for the patient. The custom fabricated Warm Springs WDFHO5 design uses a nickel alloy leaf spring for its side bar; although flexible, it too resists bending and lacks sufficient radial excursion. Lamb, et al. reported in 1974 on a single axis modification to allow radial deviation. The parts for this modification were custom made and to date have not been commercially available.

The Modified WDFHO

The single axis wrist joint and the actuating rod joint (wrist end) are replaced with rod end bearings (Figure 1) . The rod end bearing is a tri-planar joint; as used in a WDFHO, it allows 360 degrees of flexion/extension, 35 degrees of ulnar/radial deviation and approximately 20 degrees of pronation/supination. It is maintenance free and regardless of its position, the joint has no play; i.e., unlike a universal ball and socket joint, no range of motion is lost while the joint loads up.

Also, the radial deviation occurs at the wrist joint closely mimicking the anatomical wrist joint. The two rod end bearings should be aligned with each other to maximize available range of motion. But unlike the single axis joint, the rod end bearings do not have to be aligned with the MCP joints. They function smoothly and freely, independent of radial deviation.

The bearings at the wrist joint and actuating rod joint (wrist) are different sizes. The wrist joint bearing head is bored out to accept a 3/16" machine screw and it adapts perfectly to the Rancho kit. To adapt the bearings to the WDFHO, six modifications are made (Figure 2) :

  1. Cut off the wrist joint from the palmar piece and make a washer from this piece the same size as the Teflon washers.

  2. The brass bushing in the actuating level wrist joint is removed. A new bushing about 3/8" is silver soldered to the actuating level. The new bushing protrudes from both sides of the actuating lever. On the inside the bushing extends just enough to reassemble the original wrist joint (now a metal washer) and the two Teflon washers. This allows the actuating lever to fit correctly to the gear box mechanism. On the outside of the actuating llever, the bushing extends approximately 3/16". This protrusion holds the rod end bearing away from the face of the actuating lever; therefore, allowing the bearing housing adequate space for radial deviation.

  3. The old wrist joint machine screw is replaced with one that is 3/4" long. The head of the new screw is ground down to almost 3/16". This prevents the head of the screw from binding on the bearing head as it rotates within the race, therefore allowing maximum radial deviation.

  4. The rod end bearing housing has a 1032 threaded female inlet. A 3/16" stainless steel rod is threaded about 5/8" with a 10-32 die. The rod is offset to keep the proper longitudinal alignment between the radius and the third metacarpal. Cut off the remainder of the wrist joint extension at the opponens bar. Rivet the 3/16" threaded offset rod to the palmar piece.

  5. The actuating rod and lever arm are now modified to accept the rod end bearing. The head of the smaller bearing is bored to accept a 5-40 screw. Tap the actuating rod lever for a 5-40 thread. A small bushing is needed to hold the bearing away from the surface of the actuating rod lever. Use a 3/16" GD. aluminum or steel bushing with a 1/8" I.D. The bushing needs to be 3/16" long.

  6. The actuating rod end bearing housing has 6-32 thread female inlet. Cut off the appropriate amount of the actuating rod and silver solder a 6-32 machine screw to the actuating rod.

Fitting and Alignment Notes

  1. A strap is added at the distal forearm. Otherwise, the orthosis will bowstring and pull away from the dorsum of the forearm.

  2. Taper the Plastazote® liner at the wrist so it does not restrict the proximal hand as it deviates.

  3. Because the bearing housings are threaded, the orthotist can make linear adjustments to the distance between the wrist joint and MCP joint. Note, lock nuts are added to lock the threaded rods to the bearing housings.

  4. If the patient has more radial deviation than 25 degrees, the orthosis will need more offset than normal. Normal longitudinal alignment puts the shaft of the radius in line with the shaft of the third metacarpal. To increase available radial deviation, make a larger offset in 3/ 16" palmar rod and actuating rod. This will bias the bearings. The bearings will be deviated when the hand is in neutral; therefore, allowing a greater range of motion into radial deviation.

  5. Many different rod end bearings are available. Bearings with a stainless steel head and brass race were chosen in this instance. If the bearing head does not work freely within the race, place the housing over a vise and lightly tap the bearing head to slightly expand the race.

Case One

A 25-year-old complete C-6 quadriplegic (Figure 3) had been fitted with a standard Rancho WDFHO design one month before receiving the rod end bearing modification. The patient's hand radially deviated 35 degrees without the orthosis. Pinch strength was measured with a digital dynamometer before and after the WDFHO was modified. Because the dynamometer did not have a decimal function, precise measurements could not be taken. Measurements were taken in each of the five gear settings. All five settings had an increase of at least one pound of force. In the neutral position and in the position just above neutral, pinch force increased from one pound to three pounds.

Case Two

A 21-year-old incomplete C-7 quadriplegic was fitted with the modified WDFHO. The patient had complete wrist function. Intrinsics and the flexor digitorum superficialis and profundus were absent. Sensation was intact. Radial deviation was not a problem. In fact he could selectively control radial and unlar deviation. The modified WDFHO was used because it allowed him near normal wrist function; i.e., the patient could selectively circumduct the WDFHO during functional activities.

Summary

Radial deviation in the upper extremity quadriplegic wrist has been a difficult problem to accommodate. The rod end bearing modification provides a functional solution to that problem. It is felt that using the rod end bearing modification would benefit most WDFHO users, whether it be as a better fitting orthosis or to allow increased pinch.


Harvey Johnson, B.S., C.O., is with Duke University Medical Center Department of Prosthetics and Orthotics.

References:

  1. Perry, J., M.D., "Prescription Principles," Atlas of Orthotics, 1975, p. 116.
  2. Bisgrove, J.G., "New Functional Dynamic Wrist Extension - Finger Flexion Hand Splint - A Preliminary Report," Journal of Association of Physical and Mental Rehabilitation, Sept.-Oct. 1954, pp. 162-163.
  3. McCluer, S. and J.E. Conry. "Modifications of the Wrist-Driven Flexor Hinge Splint," Archives of Physical Medicine and Rehabilitation, May 1971, pp. 233-235.
  4. Malick, M.H. and C.M.H. Meyer, "Orthoses for Quadriplegics," Manual on Management of the Quadriplegic Upper Extremity, 1978, p. 57.
  5. Long, C. and A. Schutt, "Upper Limb Orthotics," Orthotics Etcetera, J.B. Redford, Editor, 1986, p. 222.
  6. Lamb C.R., M.D., A.J. Booth and M.E. Godfrey, O.T.R., "Flexor Hinge Splint: Modification to Allow Radial Deviation," Archives of Physical Medicine and Rehabilitation, July 1974, pp. 322-323.