Transtibial Diagnostic Prosthesis Fabrication

We have been provided with a modified cast, measurements and considerations required to proceed with fabricating a transtibial diagnostic prosthesis. We have reviewed the basic materials and techniques and we have fabricated a thermoplastic check socket. We are now prepared to identify and assemble the various components required to fabricate a diagnostic transtibial prosthesis.

Aside from the socket, most components can be reclaimed to use in the individual’s primary prosthesis to be fabricated once comfortable fit and function have been achieved.

Materials needed

While selecting materials, choose items that are appropriate for the design being implemented. With that in mind, component considerations for a transtibial diagnostic prostheses are as follows:

• synthetic and/or plaster cast socket (postoperative)
• thermoplastic or laminated check socket
• suspension, liner and socks
• socket-to-pylon adapter, pylon and pylon-to-foot
• appropriate foot; and
• cosmetic cover.

What’s next? All components of the transtibial diagnostic socket must meet specifications for weight and activity levels. All components should be weight-limit rated by the manufacturer. Activity levels are assigned a “K” rating known as a K level of 1 to 5 by the clinician – 1 is considered the lowest level, 5 is the highest. These designations are important in the design, material and component selection for any prosthesis. Your lab should consider establishing basic design, material and component criteria for each K level.

Check socket considerations

Sit down with your team and identify check socket designs and material requirements for each Level, K-1 to K-5. This will result in a choice of socket designs that can be modified as required. Each of these designs should have a follow-up, life expectancy and replacement identified. All components listed above could easily be worked into a custom spreadsheet used to clarify the design, material, component and follow-up requirements being implemented by your lab.

We would not likely fabricate a heavy-walled styrene-buta-1, 3-dieen copolymer (Orfitrans) thermoplastic check socket for a person who is postoperative and requires their prosthesis for standing and or transferring from position to position (K-1). For example, designate check socket design one as a postoperative synthetic cast socket for a new amputee. We might then designate check socket design two as a partial distal thin-walled (3 mm) PETG (Vivak). You will quickly determine which designs, materials and components you are familiar with and apply them to a spreadsheet to be used for fabrication consistency, scheduling, clinical, technical and follow-up reference.

Suspension choices

The prosthesis must be of the proper design to provide optimum suspension to the amputee. A wide variety of tissue, skin, bone spurs and medical conditions provide many suspension challenges. Accurate and intimate fit is critical to successful suspension. The following are some common methods of achieving optimum suspension for the individual’s particular conditions:

• Plug fit: A plug fit indicates a drop in and step out suspension. Strapping is often implemented.
• Supra-condylar: A “clipping” profile design that suspends the prosthesis by purchasing on the femoral condyles.
• Patella tendon: Indicates suspension through reduction in the cast at the patella tendon that is also used as a weight bearing point. This is most often formed into the socket.
• Inverted “Y” strap: Uses a circumferential strap above the femoral condyles to help achieve suspension.
• Corset/knee joints: Uses the same principle and also offers medial/lateral stability when required.
• Waist belt: Sometimes required for obese individuals.
• Suspension sleeve: Open- ended neoprene or gel sleeve that encapsulates the thigh and prosthesis.
• Liner/pin: Prefabricated or custom silicone liners with a pin for insertion into a “clutch lock” mechanism incorporated into the socket. The pin is removable by means of a simple release button.
• Distal liner/pin: This liner terminates within the socket.
• Suction assist: Simple expulsion valves are incorporated into the socket allowing the amputee to expel air captured within their socket. Introducing air by opening the valve assists with the removal of the prosthesis.
• Vacuum assist: Small mechanical or powered vacuum pumps are built into the prosthesis. This provides a larger range of vacuum for the amputee. Increased vacuum can increase suspension value.
• External ring liner: This liner has an accordion type of external ring which assists or provides suspension.
• Door: This suspension incorporates a bulbous appendage (if present) being purchased by a door and opening in the rigid socket. It is more commonly used in Symes amputations than transtibial amputations.
• Expandable wall: Custom design utilizing air, foam. Liquid or combinations of liquid are used to achieve suspension.

There are many methods of achieving optimum suspension. Again, identify the methods with which you have had success and implement them into your socket-design overview.

Liner considerations

The majority of the following liners could be used with a pin suspension that incorporates a pin at the distal aspect to hold the liner into a locking mechanism in the diagnostic prostheses:

• Hard socket: If you are using a hard socket no liner is required. Is it the optimum fit available? That depends. Sometimes it can be difficult to achieve.
• Leather socket: The use of a leather socket is uncommon. Additionally, this option is not as hygienic as some manmade materials.
• Kemblo liner: This older design is not commonly used. It is fabricated using a leather interface and applying bevel-edged, wedged strips of open cell rubber to form the liner profile.
• Pelite / EVA liner: This liner is commonly fabricated from sheet stock and cut to measurements and thermoformed to modified cast. Pre-fabricated cones are available to form to your modified cast.
• Urethane liner: Custom and pre-fabricated urethane liners are available. Its unique properties offer an even flow of cured material in pressure-sensitive areas. These can be more durable than most liners.
• Silicone liner: Custom and pre-fabricated versions of silicone liners are available. Pre-fabricated liners are commonly covered with a cloth exterior and offer excellent volume control, padding and suspension opportunities.
• Hot-melt vinyl liners: Sometimes misinterpreted as silicone or urethane, these are inexpensive to manufacture and do not have the same characteristics as urethane or silicone. The material is linear and is not as likely to adapt to radical residual limb profiles over time.
• Flexible thermoplastic liners: These liners are used in frame-design sockets for ease of donning and can allow muscles to fire which can reduce muscular atrophy. They are often used in a frame-style socket, which is not common in diagnostic transtibial prostheses. This is something to consider for final prosthesis design but must be accounted for in diagnostic socket design.

Prosthetic socks

Prosthetic socks provide comfort and allow for volume fluctuations commonly found in the stabilization of the residual limb. Interchangeable socks offer amputees a first line of defense for volume fluctuations.

Sheaths are available to contain tissue or stabilize volume when the prosthesis is not being worn.

Metal components

All components must be rated for the client’s weight and activity level. All set screws should be set to the manufacturer’s torque specifications using a certified torque wrench.

The pyramid design has been around a long time and is available from a multitude of manufacturers.

The following are the basic components for consideration when fabricating most transtibial prostheses:

• Wood block: This allows the technician to bond a four-hole socket connection that accepts a pyramid or rotational/static connection at the distal aspect of the socket. There must be enough space available to implement the design of block being used. Hyperbolic or foam blocks are also available.
• Four-hole pyramid: This allows the technician to attach a four-hole pyramid to the wooden block.
• Four-prong pyramid: This allows the technician to attach direct to socket providing a pyramid at the distal aspect of the socket.
• Four-hole rotator: Allows fixed rotation adjustment. Not as common to transtibial applications.
• Four-hole offset pyramid: Allows offset adjustments dependent on the component design.
• Pylon: This is typically a length of pipe with a pyramid connector attached to one end. Pylons are available in a variety of materials, lengths and basic diameters. The pylon should be cut to specific measurements and be fully seated in any connector used. Any defects, imperfections or damage to the connector or pylon can result in premature failure.
• Pylon- pyramid connector: This allows the technician to attach a pyramid connector to the end of the pylon that will connect to the foot plate adapter.
• Pylon-to-foot adapter: Most common is a plate design with a pyramid to which a foot bolt can attach the appropriate prosthetic foot.

Prosthetic foot options

These considerations require an in-depth review due to the vast designs available. The following designs cover the very basic feet available.

• Solid Ankle Cushion Heel (SACH): Your basic foot with a wooden or Delrin keel attached to foot strap and embedded in two-part urethane foam.
• Dynamic: Many designs are available. Your current practice will assist in determining with which prosthetic feet you and your patients are most comfortable.
• Passive: Two-part urethane foam shape with little structural value for the leverage required in an active transtibial prosthesis.

We have identified some basic components for fabricating a transtibial diagnostic prosthesis. In future articles we will identify the basic measurements, techniques and cosmetic considerations required to fabricate a basic transtibial diagnostic prosthesis.

Patrick J. Myrdal, RTPO(c), FCBC is the owner of Myrdal Orthopedic Technologies in Winnipeg, Manitoba and vice president of the Orthotic and Prosthetic Technological Association. He can be reached at patrick@myrdalorthopedics.com.