Design and use of a new socket-adjustment tool: Residency Project

By Chad McCracken, CPO
Summer 2007

Objective 

The purpose of this project is to research and develop solutions related to the practice of adjusting thermoplastic prosthetic sockets for improved fit and function. Traditionally, prosthetists adjust thermoplastic sockets by a means of heating and remolding the socket while in a viscoelastic state. This adjustment employs subjective measures by the prosthetist as to the correct temperature of the thermoplastic and the change in shape required. The current method is not measurable or consistent among practitioners. To date, there is no tool or device readily used by prosthetist to make consistent, quantifiable adjustments. This project aims to define the most common socket adjustments and design a tool for practitioners to use in making adjustments to existing sockets for the goal of improved fit and function. The design of this socket adjustment tool will consider:

• Local and global socket adjustments
• Measurement of adjustments
• Custom shaped adjustments
• Portability
• Ease and efficiency of use
• Reliability

The final outcome of this project was to develop a specialized tool that would allow socket adjustments to be made in a consistent, reliable and measurable fashion. The approach taken is to consider current tools that may be adapted to best suit the needs of the prosthetist. Designing a device for purposes of patent and sale are not mentioned as design criteria.

What adjustments to make 

Prosthetists make several types of adjustments to sockets during initial fitting and subsequent follow up care. Through an informal survey and observation of multiple practitioners, the most common socket adjustments include:

• Increases in fibula relief
• Increases in distal tibia relief
• Increases in proximal medial and posterior flares
• Changes in proximal trim lines
• Tighten socket in pressure tolerant areas

These adjustments can be separated into three categories: increasing relief and flares, decreasing volume and grinding trim lines. Most prosthetists observed address the problem of trim line adjustment by simply grinding material away with a traditional Trautman-style router. Most adjustments to reduce volume in pressure-tolerant areas include the addition of material strategically place inside the socket. The most common strategy observed is to tighten the overall fit of the socket by placing material below the posterior trim line to press more firmly against the gastrocnemius area. The primary materials of choice include leather and firm pelite skived to create a smooth transition and glued in place. Ultimately, if neither trimming nor tightening resolve fitting problems, then fabrication of a new socket may be required.

Traditionally, issues related to creating local relief and flares are by means of heating and remolding the socket while in a viscoelastic state. Prosthetists heat an area of the socket with a heat gun and stretch the problem area with a long handle or makeshift tool long enough to reach the required depth inside the socket and push outward stretching the plastic. The socket will then cool in this new position. This adjustment employs subjective measures by the prosthetist as to the correct temperature of the thermoplastic and the change in shape required. The current method is not reproducible or consistent among practitioners. More importantly, there is no way to accurately quantify these changes. It is this last type of adjustment that will be addressed in this project. To date, there is no tool or device readily used by prosthetists to make consistent, quantifiable reliefs and flares.

Researching design 

There were several criteria considered when designing this device. The underlying design criteria are measurable changes, reliability and simplicity of use. Other important design considerations include portability and customization of shape. In an effort to keep development simple, designs of other tools were considered. Included in these tools were various shoe stretchers, clamp systems, small jacks and industrial gear mechanisms. The final two design concepts were based on shoe stretching mechanisms and clamp mechanisms. An attempt to modify or create shoe stretchers that could be adjusted at a distance four to seven inches from a shaping tool to be infeasible. The final design was created by modifying a bar clamp tool, incorporating aluminum arms extending into a socket.

Tool design and fabrication 

The bar clamp tool was purchased at a local hardware store. It was selected due to its dual function design. In addition to its ability to clamp material together, it also has "spreading" and "free-sliding" features activated simply by turning a switch. The spreading feature is most important when using this device to adjust sockets. In addition to the stock clamp arms, seven inch aluminum arms were attached to each of the original clamp arms. This transfers the clamping / spreading force seven inches away from the actual bar. The aluminum arms were made from 3/16" x 5/8" aluminum bar stock like that used to fabricate conventional ankle foot orthoses. Threaded holes, ¼" in diameter, were added to the distal ends of the aluminum arms. These threaded holes allow the integration of custom shaped attachments. These attachments are the contact points with the inside walls of the prosthetic socket. There are two arms that reach into the socket, one to create the desired changes and one to stabilize the device inside the socket providing an opposing force. A stabilizing plate attachment was made with 1 ½" aluminum band stock and firm crepe. All attachments incorporated a ¼" X 28 aluminum threaded rod that screws into the hole at the end of the aluminum arms. The arms reaching into the socket are attached in a way that allows them to flex slightly, creating a spring tension. This tension creates a spring force pushing outward on the socket walls. The benefit of the spring force is discussed later.

Attachments were made by creating negative impressions in foam impression boxes. These negative impressions were sealed with lacquer and filled with poly-acrylic resin. A ¼" x 28 aluminum threaded rod was suspended in the negative impression. Once the resin set, the shapes were removed and sanded smooth. Various shapes were created, the most commonly used including half-spheres and teardrops of differing sizes. A picture of this device is displayed in on the next page.

Instructions for use ▲

1. Select and attach the desired head shape into the distal end of the spreading arm. The stabilizing plate should be threaded in the opposite spreading arm.
2. Place the distal end of the spreading arms to the desired depth inside the socket and move the spreading arms to a width that allows contact on opposing walls. To slide the arms freely, switch the mechanism into the "sliding" position. Once the desired width is reached, switch the mechanism to the "spreading" position.
3. Note the distance measurement on the bar clamp and calculate the desired amount of change required.
4. At this time, squeeze the trigger slightly until the measurement on the bar reads the desired value. The arms will deflect slightly pushing outward on the socket walls; however, there will not be any change of socket shape at this time.
5. Using a heat gun, apply heat directly to the area you are adjusting. Keeping the heat gun 2" - 4" from the outside wall of the socket, move in a pattern that outlines the desired shape. As the heated area of the socket becomes more pliable, the spring effect of spreading arm will push the socket outward until the arm is straight. At this point, the desired change in shape has been reached.
6. Keep the tool engaged until the socket has cooled such that is can tolerably be touched by the bare hand.
7. Disengage the spreading mechanism and removed the tool from inside the socket.

Discussion 

The goal of this project was to understand common socket adjustments and design a tool that could assist the practitioner with these adjustments. The most common adjustments identified included creating local reliefs and flares, altering trimlines and tightening socket pressures in tolerable areas. The focus of the project narrowed to the adjustments of creating reliefs and flares in a quantifiable and consistent manner. A tool was designed, incorporating an existing bar clamp design, and customized to allow spreading forces to be applied at a depth of seven inches. Custom-shaped heads allowed the creation of relief areas inside a prosthetic socket similar to those made for distal tibia and fibular head relief. One of the major design benefits was the spring-effect of the spreading arms. With spreading tension applied, once the minimum viscoelasitic temperature is achieved in the plastic, the spring-effect of the spreading arms automatically creates the desired relief. This reduces the guesswork out of heating material based on variable material properties and thickness and heat-gun capacity.

The majority of the initially stated goals and design criteria were achieved. These included:

• Creating local relief adjustments inside of prosthetics sockets
• Allowing customization of adjustments by incorporating custom shapes when necessary
• Quantifying relief adjustments
• Easy to use and replicate
• Efficient use
• Portability

Upon extended use of this device, certain deficits to the original design presented themselves. First, the aluminum bar stock exhibited permanent deflection when high forces were applied. This was addressed by changing the aluminum bars to stainless steel bars of similar dimensions. A second failure occurred when the measuring tape secured to the bar clamp became bound by the clamp that slides on the bar. Alternative ways to delineate measurements are currently being explored. Thirdly, the current design is not effective in providing consistent flare adjustments to sockets. This issue is being addressed by attempting to design custom-shaped heads that will effectively create flares specifically to proximal trimlines. These improvements demonstrate the evolutionary nature that is inherent in creating a new tool, especially one as specialized as the one discussed in this report. Along with the evolutionary nature of creating a new tool is the idea of how the tool can be incorporated in other applications. Already, this socket adjustment tool has been used in ways not originally conceptualized. This tool has been used in clinical practice as a shoe stretcher providing a solution for a difficult to stretch shoe attached to a conventional AFO that would otherwise have to be disassembled in order to achieve the desired result. Although this represents a slight variation from the original use, it provided an optimal solution that saved the orthotist time in his clinical practice and allowed him to focus on providing patient care. Due to its ease of portability, this design may prove to be a good solution for practitioners who travel to clinics with limited tools in-tote.

The development of the described tool allows for measurable and reproducible adjustments made by prosthetists. Although not a "cure-all" tool, this socket adjustment tool provides a means of making common adjustments and a foundation for tools designed specifically at improving the fit of existing prosthetic sockets. It is the hope of this author that improvements to the current tool design and future tools will assist prosthetists in providing well fitting and functioning prosthetic devices.