A Method of Below-Knee Socket Fitting Using Transparent Prefabricated Sockets

JAMES W. BREAKEY, B.Sc.


Prefabricated below-knee (BK) sockets can be used as check sockets to determine the desired shape and fit of the custom-made definitive item. These sockets were designed primarily to serve as an intermediate step between plaster-temporary and custom-fabricated sockets1. The combination of socket transparency with predetermined socket shapes provides another valuable assistive procedure for securing optimal fittings of BK amputees.

Design of Prefabricated Sockets

Based on anthropometric data taken from BK stumps1, seven right and seven left prefabricated socket sizes were designed. Through the courtesy of the Ontario Crippled Children's Centre, Toronto, Canada, transparent polycarbonate (Lexan) sockets were prepared from plaster models appropriate to these shapes and sizes. These 14 transparent sockets are kept on the shelf. They are hard, open-ended, patellar-tendon-bearing, with supracondylar suprapatellar suspension. They are fitted in conjunction with a modular type of BK pylon and SACH foot.

Also kept on hand are 14 correspondingly sized laminated plastic shell forms split posteriorly. Whenever a new plaster model of a desired size is needed, it is prepared by using the appropriate plastic shell, which can then be separated from the mold.

Fitting Approach

Thirteen measurements of the BK stump are taken1. Allowing for the thickness of a wool stump sock, the stump measurements are related to the 13 socket measurements of the different prefabricated socket sizes. A prefabricated transparent socket approximating the desired stump size is chosen. To insure total contact, polyethylene film or some other filler can be placed in the bottom of the socket. The patient stands and walks in the socket ( Fig. 1 ).

From these initial trials the following information is obtained:

  1. the relationship between stump and socket measurement;
  2. a visible stump-socket relationship under both static and dynamic conditions; and
  3. verbal feedback from the patient concerning pressures and comfort.

Utilizing this information, a plaster male model corresponding to the size of the prefabricated socket is modified, i.e., the shape and dimensions of the prefabricated socket's plaster model are altered to meet the needs of the stump. Once these modifications have been completed, any type of socket fabrication is possible, e.g., plastic lamination ( Fig. 2 ).

Upon completion of socket fabrication, the prosthesis is fitted ( Fig. 3 ) and, when optimal alignment has been achieved, the limb is cosmetically finished ( Fig. 4 ). For this purpose either a prefabricated cover (Hosmer/Dorrance of Campbell, Calif., or Manitoba Rehabilitation Hospital of Winnipeg, Canada) is used, or a hollow plastic shell is fabricated. To build this custom cover the pylon is covered with beeswax, the wax is shaped, and the shell is laminated over the wax and socket. The wax is then melted out during curing in the oven. Usually a PVA bag is pulled over the waxed shape and socket in order to obtain a plastic cover that can be removed from the prosthesis in a manner similar to that used with a prefabricated rubber cover.

Clinical Experience

Six amputees have been fitted satisfactorily by this method. Three had been previous prosthesis wearers and three were new amputees whose stumps were maturing. Five of the six wore the prefabricated sockets for several weeks before the model size was modified to obtain a more intimately fitting socket. In five of the cases the sockets were shortened and their bases were reshaped to allow for easier cosmetic restoration.

Discussion

The approach described eliminates cast taking and major plaster model modifications. Thus the error factor introduced in cast taking and the guesswork in model modification are eliminated. From the information gained by using the transparent socket as a fitting tool it should be possible to provide the patient with a custom-made socket that requires little or no modifications at the time of delivery.

Summary

Transparent prefabricated sockets offer the prosthetist a valuable fitting tool for use with BK amputees. This method should reduce the time and errors involved in the custom fabrication of BK sockets.

Acknowledgment

The assistance of Mr. Alex Artamonov of the Ontario Crippled Children's Centre in Toronto for fabricating the transparent polycarbonate sockets from the plaster models of the prefabricated shapes is gratefully acknowledged.

Descriptors: Prefabricated sockets: below-knee: transparent; polycarbonate; amputation.

Director of the Prosthetic-Orthotic Unit at the Eastern Ontario Rehabilitation Center, and Clinical Assistant in Rehabilitation Medicine, Queens University, Kingston, Ont., Canada.

References:
1. Breakey, James W., Prefabricated below-knee sockets for the maturing stump. Bull. Pros. Res. BPR 10-19 Spring 1973.