Lateral-Grasp Prehension Orthosis with Solid Actuator Rod
JOHN HODGINS, CPO*West Orange, New Jersey
The three-point wrist-driven flexor-hinge orthosis as reported by Bisgrove1 has afforded the C-6 quadriplegic the opportunity of having a reasonably powerful three-jaw chuck grasp. Several types of flexor-hinge orthoses have evolved during the past two decades. The Rancho Los Amigos (Downey, California), University of Wisconsin (Madison, Wisconsin), Rusk Institute of Rehabilitation Medicine (New York, New York) and Engen (Houston, Texas) flexor-hinge orthoses all provide three-point grasp by a quadrilateral linkage system incorporating a solid actuator rod interconnected to the metacarpophalangeal (MCP) joint of the index finger and the radial side of the wrist joint. The Rehabilitation Institute of Chicago (Chicago, Illinois) wrist-driven prehension orthosis offers a three-point grasp by means of a flexible cable rather than a solid rod3.
At the workshop, "Current Status of Prosthetics and Orthotics and Trends for Future Research and Development," held at the University of Miami in 1977, participants identified the need to review prehension patterns suitable for quadriplegics4. Lehneis suggested that an orthosis be designed to provide lateral grasp2.
In response to this suggestion, the Burke Rehabilitation Hospital Center, White Plains, New York5, developed the first lateral-grasp prehension orthosis to be reported at a major scientific meeting. The Burke orthosis has three main components (thumb, hand-index finger and forearm) interlinked at the volar surface of the thumb carpometacarpal joint and radial side of the wrist joint by single-axis joints. A flexible Bowden cable connects the thumb component to the forearm segment. When the wearer extends the wrist, the motion pulls the cable to draw the thumb pad to the lateral surface of the index finger. A spring between the thumb and the hand-index finger extends the thumb when the tension on the cable is relaxed.
Several patients at Kessler Institute for Rehabilitation were fitted with the Burke orthosis. A basic problem was that the force generated by the spring to extend the thumb worked antagonistically to the radial wrist extensors. The difficulty was exacerbated in patients who displayed thumb-flexor tightness. Because thumb extension was not linked to wrist palmar flexion, pinch force was diminished as spring tension increased.
The Kessler orthotic department has developed a lateral-grasp prehension orthosis (Fig. 1 ) with a multiply pivoting, rigid actuator-rod linkage to replace the flexible cable and the thumb-extension spring. The new linkage derives from the linkage rod used successfully for more than 30 years in the wrist-driven orthoses that provide three-jawchuck prehension. The pivoting-rod attachments provide the synergistic linkage which coordinates wrist flexion-thumb extension and wrist extension-thumb flexion for lateral grasp. The thumb movement occurs in one plane and wrist movement in a separate plane. The orthosis has four pivot joints: a) between the hand and thumb components at the carpometacarpal joint, b) between the hand and forearm components at the wrist, c) between the distal end of the rod and the proximal end of the thumb component and d) between the proximal end of the rod and the midvolar point of the forearm. Both ends of the rod are connected to Delrin** multiple pivot posts by means of 4-40 machine screws. The distal rod connection is by means of a 8-32 screw threaded through the center of the distal post. The distal rod attachment remains stationary while rotation occurs between the center screw of the post and the proximal end of the thumb component as the thumb moves through its flexion-extension range. The mid-forearm connection allows the rod to pivot there during wrist flexion/ extension.
Use an indelible pencil to mark the thumb interphalangeal joint, volar aspect of the thumb, carpometracarpal joint and the radial side of the wrist. Measure the thumb-opening distance and the carpometarcarpal-to-wrist distance. Measure the thumb-opening length on the patient with the wrist in a relaxed palmar-flexed position. Measure the linear distance between the thumb pad and the radial aspect of the index finger where the thumb pad will contact during wrist extension. Measure thumb length either on the patient or on the plaster model; the length is the linear distance between the carpometacarpal joint and the center of the thumb pad. Take a two-stage plaster bandage mold. The first stage includes the hand (excluding fingers), thumb, and forearm. Maintain the wrist in 10 deg. of extension, permitting radial deviation. Position the thumb with the interphalangeal joint extended so the center of the thumb pad contacts the radial portion of the index finger at the distal interphalangeal joint. When the mold hardens sufficiently, begin the second stage by wrapping the index finger, making sure the bandage is securely anchored to the hand. Reposition the index finger to its original position approximating the thumb pad.
Prepare the plaster model from the patient's mold, reinforcing the index finger and thumb with flexible steel rods. Modify the model so that the transverse palmar arch is properly maintained. Incorporate plaster build-ups onto the model at the carpometacarpal and radial wrist joints. The build-ups should be 20 mm (3/4 in.) in diameter and 6 mm (1/4 in.) high. The surface of the carpometacarpal build-up should remain parallel to the long axis of the hand yet be slightly canted to the volar surface to accommodate the radial deviation that will occur during wrist extension.
Make paper templates to fit the plaster model. The templates are intended to correspond to the shape of the plastic components of the orthosis and the placement of the mechanical pivots. The angle between the proximal end of the thumb component and the wrist, DCE (Fig. 2 ), can be calculated from the following formula:
sin angle DCE = 1/2 thumb opening / thumb length
Determine the attachment point for the multiple pivot post at the proximal end of the thumb component by using the formula:
distance CE = (C2 - C1) / 2*sin angle DCE
Distance refers to the distance between the pivot at the carpometacarpal joint and the pivot at the proximal end of the thumb component. C1 is the linear distance from the wrist at the radial aspect to the carpometacarpal joint with the wrist palmar flexed. C2 is the distance from the same two anatomic points when the wrist is dorsiflexed (Fig. 3 ).
Transpose the templates onto sheets of Nyloplex*** 3-mm thick for the thumb component and 3.5-mm thick for the hand and forearm components.
Mold the hand component on the model first. Shape the ulnar hand portion to the dorsal surface while molding the finger portion into the thumb web space, over the radial metacarpophalangeal joint of the index finger, along the ulnar aspect of the finger and terminating at the proximal nail bed. Do not mold the distal finger flange to the model; mold it in a freehand manner off the cast to facilitate plastic removal from the model without breaking the model finger. Mark the center of the carpometacarpal and wrist joints on the plastic. Outline the circumference of the two joint build-ups, then remove the plastic section from the plaster model. Trim the plastic from the radial wrist to the circumference of the build-up. The plastic between the carpometacarpal and wrist joints should be trimmed to a width of 30 mm (1-1/8 in.). Trim the plastic around the carpometacarpal build-up circumference to maintain a width of 30 mm (1-1/8 in.). Place the hand section back on the model. Mold the thumb piece on the model, overlapping the hand section at the carpometacarpal joint. Mold the volar interphalangeal cross bar across the volar interphalangeal crease. Mold the distal dorsal interphalangeal cross bar between the interphalangeal and metacarpophalangeal joints. Allow the plastic to bridge the cast between the carpometacarpaljoint and proximal cross bar to ensure unrestricted movement of the thumb piece when fitted to the patient. The proximal flange of the thumb component should be parallel and on the same plane as the plastic over the carpometacarpal build-up.
Mold the forearm pattern to the model, overlapping the hand section of the wrist joint. The volar cross bar extends to the ulnar edge of the forearm, with its distal edge 30 mm (1-1/8 in.) proximal to the wrist. The dorsal cross bar terminates at the ulnar edge of the forearm. Mark the center of the carpometacarpal axis (point C) and the proximal end of the thumb component (point E) on the plastic thumb piece. Mark the center of the radial wrist axis and point C on the plastic forearm section. Remove all plastic components from the model.
Connect the thumb and hand sections at the carpometacarpal joint by means of plastic screws. Be sure the surface of the plastic is completely flat so that no binding occurs during mechanical joint movement. Connect the forearm and hand sections at the wrist in similar fashion. Trim the Nyloplex sufficiently about the wrist joint so that no restriction occurs in wrist palmar flexion or extension.
The distance between the proximal end of the thumb component (point E) and the center of the forearm component (point G) on the plaster model will be the length of the actuator rod. Connect the multiple pivot posts to the plastic at points E and G; then connect the actuator rod to the posts. Attach Velcro straps as shown in Figure 1 .
The lateral-grasp prehension orthosis provides C-6 quadriplegics with an efficient means of obtaining grasp of as much as 2 kg (4 1/2 lbs.). The orthosis has been fitted to more than 30 adolescents and young adults with traumatic spinal cord injury during the past four years. The longest wear period is approximately three years. Donning is no more difficult than applying the three-point prehension orthosis.
With the lateral-grasp orthosis, the wearer has an alternative mode of drinking (Fig. 4 ), rather than relying solely on a straw. Users prefer it for other daily activities including writing and applying urinary sheaths and condoms. It is suitable for vocational endeavors involving paint brushes, hammers and screwdrivers, and for leisure pursuits, such as constructing model trains.
The highly motivated C-6 quadriplegic would probably opt to use the lateral and three point orthoses and the simple cuff utensil holder interchangeably, depending on the task to be accomplished.
The lateral-grasp prehension orthosis with solid actuator-rod attachment has a multiply pivoting linkage that coordinates wrist flexion-thumb extension and wrist extension-thumb flexion which are the synergistic movements needed for lateral grasp.
Special thanks to William Lembeck, P.E., Research Scientist, Prosthetics and Orthotics, New York University Post-Graduate Medical School, for assistance in deriving the formulae noted in this article.
*Director of Prosthetics and Orthotics, Kessler Institute for Rehabilitation, 1199 Pleasant Valley Way, West Orange, NJ 07052
**Polyacetal resin. E. DuPont DeNemour, Wilmington, DE
***Nylon acrylic, Plexidur. Rohm and Haas, Darmstadt, Germany
- Bisgrove, J. G.: A New Functional Dynamic Wrist Extension-Finger Flexion Hand Splint: A Preliminary Report. Arch Phys Med and Rehabil 8:162-163, 1954.
- 2.Lehneis, H. R.: Upper-Limb Orthotics. Orthot and Prosthet 31:14-20, 1977.
- Malick, M. H., and C. M. H. Meyer: Dynamic Hand Orthoses: Manual on Management of the Quadriplegic Upper Extremity, pp 59-61. Pittsburgh, Harmarville Rehabilitation Center, 1978.
- McCollough, N. C., and R. Snell: Foreword. Orthot and Prosthet 31:1, 1977.
- Stern, P. H., and N. Griswold: Key-Grip Orthosis for Quadriplegic Patients. Proceedings, World Congress, International Society for Prosthetics and Orthotics, pp 75-76, New York, 1977.