Developing “New” Pediatric Prosthetic Components
Gerald Stark, BSME, CP, FAAOP Director of Product Development & Education

Fillauer and Hosmer wrestled with a variety of concerns while developing a new line of small endoskeletal componentry for pediatric or geriatric use. To answer these questions the design and marketing team first invited a group of pediatric prosthetists to give their input. The biggest desire was producing a system with a higher weight rating for the higher activity demands of a child. It was found that although children are well below the rating, their activity pushed the limits of existing componentry.

Another concern was the type of connections available. Children do not represent a large population, so componentry currently available was limited. The focus group agreed that they needed access to all the attachments and slide devices the adults have, to optimize fittings and give the clinician more choices.

After getting these specifications the Fillauer and Hosmer engineering team began to piece together the needs and compare them with existing componentry. The first design challenge they encountered was the inherent design weakness at the SACH foot plate where the foot bolt goes through the pyramid. The cross-section of the pyramid is limited as a result of the foot bolt. There were two existing sizes of pyramids at 35 kg and 45 kg. The larger size was used only at the ankle to increase strength. To get an even larger 60kg rating the larger pyramid size had to be chosen for all connections. The engineers found that a light pediatric polycentric knee was desired more than a single axis or stance control knee. The focus group expressed a desire for a knee that was easier to flex with a simple friction control. The adult spectrum knee had many of the same characteristics, so it was adapted for the small size.

Other choices had to be made as to which connectors would be available. A wide range of componentry was desired initially, but the project team had to limit the number of designs so that they could accomplish their timelines. The scope of designs was narrowed to a BK Adapter with Pyramid, AK adapter, 4-Hole Composite laminating plate, and 4-Hole rotatable adapters with thread male or female pyramid components. The design engineers were surprised to see how rough children are on their prostheses, so they concentrated on making attachments with a premium on ruggedness. They used Titanium posts and beefed up cross sections where possible to insure maximum strength. Cosmetic design was also important to create “fun” products that children would want to wear. Experience with adult component design also helped such as bigger tie off grooves, modularity, and ease of adjustment.

Standard 22mm aluminum pylons were not surviving testing at higher loads; even when the wall thicknesses were increased or different alloys were chosen. The decision was made to use a nickel-plated alloy steel which had superior strength and ductility in a thinner wall pylon. The other advantage was that it would bend under extreme load rather than snap like aluminum. There was only one major drawback. Since the pylon was harder, a hacksaw or hardened cutting wheel for tube cutters had to be used. Given the choice of easy-to-cut or higher weight rating the group overwhelmingly chose the higher weight rating.

Most of the connectors tested fine, but one that proved more difficult was the threaded pyramid receiver adapter. The external threads were natural stress risers. In addition, we had clearance issues with the pyramid inserting into it. After testing we increased wall thicknesses and closed the proximal opening for more rigidity. A titanium version was also developed will be available soon.

One additional issue was that children go to an adult size 5 shoe when they are only 9 or 10. The practitioners said they needed an adapter to use our smaller SACH foot adapter. This is also a problem when using the system for adults. Customers wanted to just use a spacer around the smaller foot bolt, but the team found that a 22mm pylon that accepted an adult pyramid would be better. This would allow the lighter weight components to be used in the rest of the prosthesis.

When introduced to the market, the response was overwhelming. The name “Mighty-Mite” rather than “pediatric” was chosen to indicate to the customer that they can use the product for a variety of uses including small adults. A favorable response about the new Mighty-Mite Knee was found as well. Customers have said emphatically that they like the greater range of motion and the ease of movement along with the impressive toe clearance.

Click here to view the MightyMite® Line

Fillauer LLC
Hosmer Dorrance Corporation
Motion Control
Center for Orthotics Design
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