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Our goal is to design a new wheelchair for an eight-year-old boy with cerebral palsy. The family and teacher are concerned that the current wheelchair is allowing the child to slouch and shift his weight in his seat, causing problems such as a need for hip replacement, feet not staying in the footrest, and restraints causing skin irritation and discomfort. The headrest is fixed and rubs on the child’s head causing a bald spot, and the current removable activity tray is difficult to remove and attach.
Cerebral palsy can increase muscle tone and lead to spasms in the limbs. The child has generally increased tone in upper and lower extremities and fair head and trunk control. He is dependent in transfers and mobility and is incontinent. He also presents with a flexor synergy pattern and often struggles with independence. Due to this, our wheelchair will need special support to the torso, hips, and legs. For this project our team will be designing and developing a new wheelchair that will add support for muscle tone, minimize skin integrity issues, increase independence, and have a personal flair.
To help support muscle tone, the chair will need to prevent the child from being able to slide out of the proper 90 degree sitting position or unevenly distribute his weight. This positioning will be even more important after the child’s hip surgery in the coming year. Improper positioning can cause the hip to dislocate after surgery. To minimize skin integrity issues, a comfortable and easy to clean material will be needed and the seat restraints may need to be redesigned to reduce friction. We also have a goal to better facilitate his independence. We want to make it easier for him to participate in class and work on skills such as learning to feed himself.
Our family’s child has spastic cerebral palsy. We did some research on the symptoms and difficulties that come along with his condition so we could better understand the type of adjustments he may need for his wheelchair to make him more comfortable. We also talked to an Occupational Therapist (OT) who specializes in fitting children and their symptoms to wheelchairs and aftermarket products. We discussed the needs of the child and were recommended various strategies to think about when designing and building our own. She also suggested we talk to an Assistive Technology Practitioner (ATP) in our area once we have concept designs.
We also looked into wheelchairs and products currently on the market and determined that the wheelchair that meets all the current needs except the activity tray is the Zippy Tilt-in-Space wheelchair technology.
Below are local ATPs and their contact information:
Adjustable Tilt Wheelchair with Tray
Pros:
Cons:
Reconstructed Adjustable Tilt Wheelchair from Static Model
Pros:
Cons:
Fixed Tilt Wheelchair from Static Model
Pros:
Cons:
Concept 1 was the best based off the decision matrix; however it is outside of our expected budget and is therefore not feasible. Our next best option was Concept 3 with the second highest score on the decision matrix. We plan to add the adjustable head rest from Concept 2 to the design for Concept 3 to try to meet more of the goals requested by the family. Concept 3 was the best because Concept 2 was much more difficult to fabricate in our limited time this semester and the amount of failure points that would be introduced is too concerning to choose for a project that is only one semester long.
Update:
When looking for the base wheelchair that could be modified for Concept 3, we found a used tilt-in-space wheelchair within our budget. Our new plan is to make the modifications for Concept 1 as well as designing a full seat cushion spacer to account for the large size of the used wheelchair seat compared to what the child needs.
The design for our wheelchair takes an existing tilt-in-space wheelchair that is oversized for the small child that will use it. We are going to make a cushion seat spacer upholstered with Naugahyde and attach it with Velcro to the existing seat. We will also design a removable activity tray with four straps to attach to the existing armrests.
For the seat cushion, the ordered wheelchair is 14×14 in. seat with a 15 in. back, and the child has a hip to knee distance of 12.6 in., a hip width of 8.7 in., and a top of shoulder to seat distance of 13 in. This leaves a 2.65 in. gap between each side edge of the seat and the child’s hips, a 2 in. overshoot between the top of the chair and the top of the child’s shoulders, and a 1.4 in. gap between the backrest and the child’s back. These gaps and overshoots cause safety issues for the child, so they need to be minimized.
The seat cushion spacer will be designed to account for the oversized chair while giving the child adequate support. It will essentially be a foam chair of the right dimensions for the child that sits fixed inside the ordered wheelchair to ensure proper body position. The design includes raised edges on the seat cushion to minimize sliding left and right in the seat, a full seat base to place the shoulders in the ideal position, a full backrest to promote the 90-degree hip angle, and additional extrusions on the backrest for promoting proper torso alignment.
The cushion will be upholstered in Naugahyde for easy cleaning and sanitation, and the use of Velcro will allow for easier transportation and removal when outgrown.
The activity tray will be made of polycarbonate Lexan. Lexan has a smooth surface, is known for its impact resistance, and is able to be cut to size. The smooth surface will allow suction cups to be used for skill building activities and class participation. The tray is designed to lay over the wheelchair’s armrests and be fasted by 4 straps. There are 2 straps for each armrest and the fasten on both sides of the bar that hold the armrests to the chair. This and the compressibility of the armrest cushion allow the activity tray to stay on when various forces act on it.
For our forces analysis, our main goal is to find out how much force is experienced at the strap connection points when weight is applied to the tray. This helped us calculate the forces at each individual strap connection point and determine the type of material we should use. The tray and straps can be modeled as a simply supported beam.
This force analysis is for us to see how much force the straps will need to withstand while reacting to the force applied to the end of the tray. This will also help us determine the specific material and type of strap to use on our tray.
For our 3rd form of engineering analysis, we decided to use finite element analysis (FEA) to show the stresses in the tray when force is applied. We used Inventor for the stress analysis since the program has a built in FEA study. After developing the CAD model for the tray, the part file was put into the software. Constraints had to be set before running the analysis. The legs that attach to the arm rests were set as fixed ends, the part has been set to a material of Polycarbonate plastic, and a force of 100 lbs was applied to the end of the tray. The tray held up well, with bending stress at the connectors. We do not expect 100 lbs of force to be applied to the tray however, we want to make sure this tray will be built to last along with the rest of the wheelchair. The max force that can be applied to the tray before failure begins is 140lbs, which should be more than enough strength needed for the tray.
Before fabricating the tray, we mocked up the design with cardboard and brought it to the wheelchair fitting we did with the child. We found that our design addressed all of the concern of the family. These goals had been a tilted seat to prevent the child from pushing out of the proper position, an easily removable activity tray, and footrests that were in the proper position.
