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Dynamic Alignment Options for the Flex-Foot(TM)

C. Michael Schuch, C.P.O.

The alignment devices provided by Flex-Foot Incorporated⁴ for alignment of a prosthesis with the Flex-Foot™ component allow for the alignment adjustments normally required: angular, linear, and rotary. However, even after considerable experience with the Flex-Foot™, it has been found difficult to make alignment changes with the devices furnished. Furthermore, the devices do not create confidence in the prosthetist or patient with regards to patient safety, especially when the patient actively tests the alignment under dynamic conditions. Several alternative methods have been devised and used to overcome these deficiencies.

Review of Flex-Foot™ Alignment Devices

The original Flex-Foot™ alignment tool (part #0700) consists of four major parts: the mounting plate, the alignment pad, the alignment ring, and the pylon clamping spheres. As a unit, they allow for all the adjustments needed in the connection between the socket and pylon. The fixture can be adjusted in the medial-lateral plane, as well as for internal-external rotation by loosening the anterior and posterior bolts and sliding and/or rotating the alignment ring. The socket can be adjusted in the anterior-posterior position by loosening the medial and lateral set screws and sliding the mounting plate. Flexion-extension and abduction-adduction angles are adjusted by using the pylon clamping spheres and anterior bolts.

In the most recent modification of the original alignment device, the manufacturer deleted the alignment ring and clamping spheres, and added an aluminum "T" adaptor (part #0760). This "T" adaptor allows the angular adjustments of flexion-extension and abduction-adduction. The other adjustments, anterior-posterior and medial-lateral linear motion and internal-external rotation are made in the same manner as with the original alignment tool.

Flex-Foot™ Pylon Connector Angles

In an attempt to "modularize" the Flex-Foot™ and avoid having to laminate the socket directly to the pylon, prosthetists at the Division of Prosthetics and Orthotics of the University of Virginia Medical Center began making and using adaptors of steel angle material that could be bolted to the proximal end of the Flex-Foot™ pylon. The flat top surface of these adaptors provides a platform for a four-hole arrangement that is compatible with Otto Bock¹ modular components, as well as other systems. These adaptors are the key component to a series of alignment component options available for use with the FlexFoot™. As this system of using connector angles with Flex-Foot™ fittings routinely came into use at the University of Virginia, Flex-Foot Incorporated independently reached a similar conclusion and began making available a very light-weight version (part #0775 or 0780, depending on patient weight) that is also compatible with components of Otto Bock and other manufacturers (Figure 1) .

Alignment Component Options

The alignment component options found to be useful are based upon Otto Bock modular components and Proteor⁷ alignment devices. Proteor makes available, through Durr-Fillauer³ and Daw Industries,² several alignment devices specifically designed for use with below-knee prostheses. These devices use sliding actions to effect alignment changes, and are thus quite simple to use. The Proteor units are designed to be shaped and incorporated into the finished prosthesis. Proper reinforcement is recommended when using this finishing technique. The Proteor units may also be transferred out of the finished prosthesis and replaced with an Otto Bock socket attachment block (5RI = 2 or 5R1 = 3).

First Option

The first and simplest option is to bolt the Proteor alignment device (PAD- lKlO) directly to the Flex-Foot™ plyon using the connector angles (Figure 2) . The socket is mounted on the Proteor alignment device in normal bench alignment. This particular Proteor alignment device was developed for use with below-knee prostheses and allows both angular and linear changes in alignment. Adjustment is made possible by a series of dovetailed plates that slide relative to each other. Two of the dovetailed joints are curved with the radii of the curves occurring on arcs about the knee joint (of an average sized below-knee amputee). Thus, changes in angular alignment do not effect changes in linear alignment. The only deficiency of this alignment option is that internalexternal rotation is not available within the system, and can only be accomplished by removing and remounting the socket in the desired rotation.

Second Option

This option might be the most practical, as it permits adjustment in all three dimensions. This system consists of the Otto Bock rotatable socket adaptor with adjustment screws (4R5 1 titanium, 4R37 stainless steel) mounted to the FlexFoot™ connector angles. The Proteor sliding device (PSD-lKl 1) is connected to the rotatable socket adaptor via the Otto Bock socket adaptor with pyramid (4R54 titanium, 4R23 stainless steel)(Figure 3) . Angular and rotary adjustments are thus possible with the Otto Bock components. Linear adjustments are provided by the Proteor sliding device. With this system, it is practical to incorporate the Proteor alignment unit into the finished prosthesis with no transfers or remounting of the socket being necessary.

Additional Options

There are additional options worthy of mention. For those active patients who would benefit from torsion absorption, such as golfers and tennis players, the Otto Bock torsion adaptor with attachment plate and adjustment screws (4R40) can be used in place of the rotatable socket adaptor with adjustment screws as described in the second option (Figure 3) . In this system, simple rotary alignment adjustments are sacrificed for the gain in torque absorption. As mentioned in the first option, the socket can be removed and subsequently remounted to correct rotary alignment deficiencies.

For an ultralight system, the second option can be used with a test socket, and after establishing desired dynamic alignment (using Hosmer⁵ VFJ-l00), the Proteor sliding device as well as all Otto Bock components are removed and a thermoplastic socket system with appropriate adaptor plate in the distal end, as described by Schuch and Wilson⁸ is used. The adaptor plate will allow socket connection to the Flex-Foot™ pylon via the connector angles (Figure 4) .

Summary

Key components for allowing Flex-Foot™ alignment options have been identified and described. Some of these options allow for finishing the prosthetic system without the need for alignment transfer. The use of synthetic casting tape, such as Scotch® cast,⁶ to reinforce the Proteor device itself, as well as the socket connection, allows for safe and extended dynamic testing of the alignment. All systems allow for a finished "modular" system which does not require a permanent attachment of the Flex-Foot™ pylon. Besides simplifying the finishing of the prosthetic system, this modularity allows for future socket changes and reuse of the Flex-Foot™ pylon. All systems described allow prosthetists to offer more options to their patients.