Home > Newsletters and Journals > ICIB 1973 Vol 12, Num 10 > pp. 7-12

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A Vacuum-Formed Plastic Insert Seat For Neurologically Handicapped Wheelchair Patients

Introduction

Alarge number of severely involved cerebral-palsied children and young adults, many with moderate to severe retardation, are confined to bed and to a wheelchair. The majority of these patients are permanently housed in public institutions although parents maintain some at home, often with great difficulty in physical management.

These patients fall into three categories. The most severely involved have never walked and may never have sat. The second group consists of those who were marginal sitters early in life, but became bedridden as contractures of the lower limbs and the spine increased in severity. The third group includes those cases who were poor walkers during early and mid-childhood. When a point was reached at which attendants and the child himself found that the energy expended was too great in relation to the benefits obtained, the wheelchair was chosen as a permanent solution.

In most public institutions, too few attendants are available for the ideal bed and chair positioning of these patients. Physical therapists to carry out joint-stretching exercises are even more scarce. Thus, insufficient time is devoted to prevention of contractures and pressure sores. In addition, these patients continue to grow, thus magnifying their problems.

The most severely involved patients characteristically have poor head and trunk control. In the ordinary wheelchair with a sling seat this lack of control may lead to a gradually increasing structural kyphosis. A combination of hip-adductor contracture on one side with hip-abductor contracture on the other (so-called "wind-blown hips") may produce a subluxated or dislocated hip on the ad-ducted side. Scoliosis, sometimes associated with fixed pelvic obliquity, may occur secondarily. A generalized extensor thrust is also quite common and may lead to the child sliding out of the wheelchair ( Fig. 1 ). Incontinence may lead to the absorption of offensive odors in seat cushions and cause them to disintegrate because their materials have poor resistance to chemical activity.

A well-molded wheelchair seat which holds the patient in a relaxed sitting posture with the spine straight and hips abducted may aid in the prevention of postural problems ( Fig. 2 ). Such a seat has been developed at the Alexander unit of the Arkansas Children's Colony and the Arkansas Cerebral Palsy Equipment Center during the past three years. The first 60 seats were made of fiberglass reinforced plastic. Since August 1972, 68 more have been fabricated using a much simpler process.

The A.B.S. Plastic Insert Seat

General Description
The seat is vacuum-formed of A.B.S. (Acrylonitrile Butadiene-Styrene),* a thermoplastic material well suited to this purpose because of its impact strength, resistance to abrasion and heat, and low absorption of moisture and odors.

This rigid seat conforms perfectly to the full sitting surface of the patient, thus distributing weight over the buttocks and posterior thighs as evenly as possible. An elevation between the distal thighs maintains hip abduction and keeps the patient from sliding forward off the seat ( Fig. 3 ). The seat may be removed so that the wheelchair can be folded.

The only modifications of the chair that may be needed are those related to the height of the seat which averages one inch at the ischial tuberosities and two and one-half inches at the distal thighs. The most common adjustment required is a raised foot support. The material is odor-resistant and easily kept clean.

Description of Manufacture
The first step in the manufacture of the A.B.S. seat is to obtain an accurate impression of the buttocks and posterior thighs with the patient seated on a flexible plastic cushion loosely filled with one-eighth inch polystyrene beads. The cushion is contained in a rigid box. The air is exhausted from the bead-filled bag using a vacuum pump. The patient is then lifted from the box leaving his body contours rigidly impressed on the upper surface of the bag ( Fig. 4 ). Molding plaster** is poured into the imprinted bag and allowed to harden, which takes approximately one hour ( Fig. 5 ).

The vacuum-forming process is done with commercially available equipment which combines an oven for heating the plastic with a vacuum former.* The inverted mold is placed in the machine over the vacuum chamber ( Fig. 6 ). A 26 in. by 38 in. sheet of A.B.S. is placed on a sliding frame which is then pushed into the oven to the rear of the machine. The sheet is heated simultaneously on both sides at a temperature of 400 deg. F. until it sags quite noticeably. The frame is then drawn forward over the mold and a vacuum force is applied drawing the plastic rigidly against the mold (Fig. 7 ). The plastic's high drawing ratio (4:1) with maintenance of a uniform wall thickness is of considerable advantage at this point. Upon cooling, the plastic becomes rigid, allowing separation from the mold. The seat is then trimmed of excess plastic and bolted to a three-quarter inch plywood board sized to fit the wheelchair's seating area. This unit is then simply clipped to the wheelchair frame. The entire process takes approximately four hours.

Special Points
To be properly positioned in the imprint box, the patient must be relaxed. In three cases, a second seat had to be made because the first impression was taken with the patient in an awkward, unrelaxed position.

Severe contractures may also prevent adequate positioning in the imprint box. Surgical release of contractures or femoral head excision in case of dislocation may be considered prior to seating.

Sitting tolerance in patients may vary considerably. The lowest tolerance is found in those cases with marked bony prominences, severe scoliosis, kyphosis, or severe contractures of the lower limbs?in short, the conditions we hope the seat will help prevent.

Summary

The manufacture of a vacuum-formed plastic insert seat for neurologically handicapped patients who are confined to wheelchairs has been described. By conforming to the patient's entire sitting surface the rigid seat helps distribute his weight evenly. The patient is held in a relaxed sitting posture with the spine straight and the hips abducted. It is hoped that this postural control will eliminate or reduce the incidence of scoliosis, kyphosis and hip contractures. The seats are being evaluated on a long-term basis.

Descriptors: Vacuum-forming; seat; wheelchair; cerebral palsy; A.B S (Acrylonitrile Butadiene-Styrene).

Associate Professor, Division of Orthopedic Surgery, University of Arkansas Medical Center, Little Rock, AR 72201

Manager, United Cerebral Palsy of Arkansas Equipment Center, 701 Scott St., Little Rock, AR 72201