AN ANALYSIS OF KNEE KINEMATICS WITH COMPUTER NAVIGATION: A CLASSIFICATION OF KINEMATICS AND NOMENCLATURE FOR ARTHRITIS OF KNEE

Abstract

Arthritic knees, for the purpose of surgical correction during arthroplasty, are generally thought to be either varus knees or valgus knees and soft tissue releases are done in accordance with the same concept. This view is dependent on the clinical deformity in extended knee and the plain AP radiograph of the extended knee. This concept is now challenged by the observations from our study of the arthritic knee kinematics using computer aided navigation when performing total knee replacement arthroplasty. We performed 283 total knee replacements with computer aided navigation. Imageless navigation was used with Stryker and Orthopilot systems. Bone trackers were fixed to the bones and through real time infrared communication the data was collected. The knee kinematics were recorded before and at the end of surgery. This included measurement of biomechanical axis with the knee extended and then gradually flexed. The effect of flexion on the coronal alignment was recorded real time on the computer. The results were then analysed and compared with plain radiographic deformity on long leg films.

Majority of the knees did not behave in a true varus or valgus fashion. We classified the deformity into different groups depending on the behavior of the knee in coronal plane as it moves from extension to flexion. 2 degree was taken as minimum deviation to signify change, as the knee bends from full extension to flexion. The classification system is as follows

Neutral

Deformity - Varus/Valgus to start with in extension

Gp1

1. Deformity remains the same as the knee flexes

2. Increasing deformity as the knee flexes

Gp2

1. Decreasing deformity but does not reach neutral in flexion

2. Decreasing deformity reaches neutral in flexion

Gp3

Decreasing deformity and crosses to opposite (Varus to valgus or valgus to varus) deformity in flexion

Gp4

1. Deformity first increases and then decreases but does not reach neutral

2. Deformity first increases and then decreases to neutral

3. Deformity first increases and then decreases to cross over to opposite deformity in flexion

Traditional releases of medial or lateral structures without realising the true picture of what happens when the knee is flexed, may not be correct. From our study it is clear that not all arthritic varus or valgus knees behave in the same way. Some of the releases we perform conventionally may not be required or need to be modified depending on the knee kinematics.