Technical Notes: Wedge Adaptors For Alignment Of Lower-Limb Prostheses
During the past three years I have used wedge adaptors in the alignment procedure of lower-limb prostheses in two ways: by themselves in determining alignment, and as a means of duplicating alignment that has been determined with the Winnipeg wedge-disc alignment unit  .
I have found that, when using the Winnipeg wedge disc units, once a satisfactory alignment has been achieved no changes are required later in the majority of cases. Therefore, I designed a series of adaptors (Fig. 1 ) to aid in duplication of the alignment determined with the Winnipeg wedge discs so that the unit could be used over and over, and also to lessen the problems of loosening and the consequent slippage that had been experienced with the multiple wedge discs.
The wedge adaptors have seen greatest use at the foot connection and under the socket in the below-knee (BK) prosthesis (Fig. 2 ). They are also used at the foot in above-knee, knee-disarticulation, and hip-disarticulation prostheses. A prefabricated rubber cosmetic cover or a custom-made plastic-laminate cover is used.
By duplicating the Winnipeg wedge discs with the wedge adaptors, the problem of disc slippage has been eliminated. When an adaptor replaces a wedge disc unit under the socket and over the foot a total weight reduction of approximately 4 ounces is realized.
Wedge Adaptors
Five configurations (Fig. 3 ) with angulation of 0,2,4,6, and 8 degrees, respectively, have proven to be adequate in meeting the alignment needs of our patients. Each adaptor is made from 2024ST4 aluminum round stock and weighs approximately 2 1/2 ounces. (A Winnipeg wedge disc unit weighs approximately A 1/4 ounces.)
A moveable wedge nut (Fig. 3-B) within the adaptor is used to connect the adaptor to the foot with the standard bolts, except for the 0° adaptor (Fig. 3-A) which obviously does not require a wedge nut.
Alignment Duplication By Adaptors
A short review of the Winnipeg wedge disc unit is presented here for those not familiar with this alignment device. Each unit consists of two wedge discs, each having an angulation of 6° (Fig. 4 ), to make possible a total angulation in one plane of 12° (Fig. 5) and 6° each in two planes. When both discs are moved synchronously, angulation occurs in one plane. To provide angulation in two planes the discs are moved independently of each other.
A detailed description of the principle and use of the Winnipeg wedge disc unit is given in Fig. 6, Fig. 7, Fig. 8, Fig. 9, Fig. 10 and Fig. 11.
In duplicating the alignment, the angulation produced by the wedge discs in one or both planes is determined and the required wedge adaptor is substituted, using the following rules:
- Determine which quadrant or quadrants the thin edges of both wedge discs are acting upon (Fig. 6 and Fig. 7 ).
- The maximum number of degrees a wedge disc has is 6, and this must be shared by the direction in the quadrant or quadrants it is acting upon (Fig. 8 and Fig. 9 ).
- Add the effects of the wedge discs (Fig. 10 and Fig. 11 ).
Duplication can be performed by using the Hosmer Vertical Fabrication Jig or drawing reference lines on the prosthesis for duplication by eye.
When the wedge disc units are replaced by the wedge adaptors, the pylon tubing must be lengthened to accommodate for the shortening that occurs inherently.
Alignment Procedure Using Adaptors
Bench alignment of the BK prosthesis is performed in the usual manner, appropriate adaptors being placed beneath the socket and over the foot. During dynamic alignment, adaptors are changed as required until the alignment is satisfactory.
Carrying out alignment changes in one or two planes is a simple matter with these adaptors. For example, an 8° adaptor can produce 6° in an interior direction and 2° in a lateral direction (Fig. 12 ). If a reduction from 6° to 4° is desired in the anterior direction while maintaining the 2° in the lateral direction, a 6° adaptor will perform the adjustment (Fig. 13 ).
Discussion
In using wedge adaptors for determining alignment the prosthetist is concerned only with one quadrant of angulation and two directions (Fig. 7 , Fig. 12 , and Fig. 13 ). When using the Winnipeg wedge discs he often is concerned with two quadrants and three or four directions (Fig. 10 and Fig. 11 ). I have found the wedge adaptors easier to use than the Winnipeg wedges in determining alignment.
Conclusion
Wedge adaptors have been found useful as an alignment tool and as a duplication method in a modular type of prosthesis. Ease of adjustment has made changing alignment a simple matter.
James W. Breakey, B.Sc.
References:
- Foort, J., and D.A. Mobson, A pylon prosthesis system for shank (B/K) amputees, Report No. 6, Prosthetics and Orthotics Research and Development Unit, Manitoba Rehabilitation Hospital, Winnipeg, Canada, November 1965
- Prosthetics and Orthotics Research and Development Unit, The wedge disc alignment unit, Report No. 3, Manitoba Rehabilitation Hospital, Winnipeg, Canada, December 1964
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