Prosthetic Knee Mechanisms: A Guide for the Prosthetist

Prosthetic Knee Mechanisms: A Guide for the Prosthetist

Bert Goralnik, CP 

Introduction

A function of the Veterans Administration Prosthetics Center (VAPC) is to assist VA Clinic Teams nationally in prescribing prosthetic devices, including, of course, prosthetic knees. Prescribing knee mechanisms, however, is a complex task because of the large variety available. Most often these devices differ not that much in function but in size, type of material used for the setup, and additional characteristics related more to assembly and installation processes than prescription rationales.

All too often clinicians prescribe either limited numbers or certain types of knee mechanisms found to be reliable in the past. Another inhibitor may be a lack of specific information on the full range and variety of all available systems. The clinician rarely has an opportunity to compare the relative merits of one knee with another.

In 1972, the Veterans Administration, through the Department of Medicine and Surgery, Washington, D.C., published a program Guide (M-2, part IX, G7) on "The Selection and Application of Prosthetic Knee Mechanisms." The guide was slightly modified and updated in 1976. A new Program Guide, reflecting developments of recent years and incorporating most commercially available knee mechanisms, will soon be published. This later Program Guide will provide a summary description of the various knee mechanisms thus far evaluated by the VAPC. It is intended to help maximize patient benefits.

Description of Program Guide

The Program Guide comprises six sections: Knee Function, Definitions, Classification, General Requirements, Prescription of Prosthetic Knee Mechanisms, and Catalog of Knee Mechanisms.

1. Knee Function: Here are described the normal function of the anatomical knee, specifically the relationships of its various parts during the gait cycle, and alignment stability as a key factor in prosthetic fitting. Discussion centers on the TKA line relative to the center of the knee in maintaining stability during the stance phase. Understanding these relationships and utilizing the special features of knee mechanisms for the patient's benefit is an asset for the prosthetist. The Clinic Team thereupon must strive to provide the patient with the specific knee mechanism whose features most closely match his individual needs.

2. Definitions: Reference terms are given to describe the variety of knee functions.

3. Classification: A chart classifying all types of commercially available knee mechanisms is provided. The chart shows functional criteria, specifically swing phase control and stance phase control. Additional topics in this section include extension aids, extension stops, mechanical locks, mechanical friction, and fluid resistance of hydraulic and pneumatic knees.

4. General Requirements: This section consists of a checklist on knee mechanism requirements.

5. Prescription: Prescription rationale is discussed, emphasizing the needs of the individual patient. Although the Program Guide concerns knee mechanisms, socket, shank, foot and suspension are also discussed to achieve the best type of prosthesis available. A chart shows the type of prosthesis best suited for different types of amputees. A classification chart of knee mechanisms is also included. To further assist the clinician, variations of basic prescriptions are given, i.e., for a short residual limb, a very long residual limb, and differences based on level of activity.

6. Catalogue of Knee Mechanisms: this section, the heart of the Program Guide, lists most commercially available knee mechanisms. Illustrations furnished by the manufacturers are included. A chart lists type of knee mechanisms, materials, exact dimensions, and types of control offered.

Conclusions

The new Program Guide on "The Selection and Application of Prosthetic Knee Mechanisms," will be available on or about June 1, 1980. It should prove to be of significance to all clinic teams. To obtain a copy of this publication, please write to the Veterans Administration Prosthetics Center, Attention: Mr. Bert Goralnik, 252 Seventh Avenue, New York, New York 10001.

I wish to thank Mr. Max Nacht, Technical Writer/ Editor, VA Prosthetics Center, for his aid in preparing this article.