DIFFERENT METHODS OF MEASUREMENT AND FACTORS INFLUENCING SEVERITY OF DEFORMITY IN KNEE OSTEOARTHRITIS

Abstract

The recognition of the correct pattern and severity of deformity in knee osteoarthritis has important implications in its surgical management. Our unit routinely uses standing long leg films and computer navigation. However, these modalities are not widely available and most surgeons rely on clinical assessment and short films. Our experience is that clinical assessment can give the opposite impression of the true deformity pattern particularly among obese patients and there is evidence that short knee films are not reliable. Our study aims to compare clinical, radiographic and computer measurements of knee deformity, assess the influence of Body Mass Index and asses the relationship between coronal and flexion deformity.

We measured 52 consecutive knees prior to arthroplasty using clinical, long leg radiographs and computer navigation methods. Systematic clinical measurement was done with patient standing. Standing radiographs stored in a Picture Archiving System were measured by two independent observers. The senior surgeon performed computer measurement while applying axial load to the foot to simulate weight bearing.

Using long leg films as baseline, clinical and X-ray measurement had a mean error of 0.8° (−12 to +12). Seven clinically valgus knees turned out varus on X-ray. Mean BMI for this group was the same as the rest. Using navigation as baseline, clinical and navigation coronal measurements had a mean error of 0.3° (+9 to −10.5). Four clinically valgus knees turned out varus with navigation. Mean BMI for this group was the same as the rest. Flexion deformity was similar between clinical and computer measurement. Three clinically normal knees showed significant varus in both X-ray and navigation. Compared directly, radiographic and navigation coronal deformity showed significant difference in the degree of deformity but not in the pattern of deformity. There was no correlation between BMI and both the error in clinical assessment of coronal deformity and navigation coronal alignment. If flexion deformity was > 5°, higher BMI indicates higher flexion deformity. There was a weak correlation between navigation coronal and flexion deformity.

Although error in clinical measurement did not reach statistical significance, based on our result, clinical assessment can give an incorrect pattern of deformity in up to 13% and hence should not be the sole basis of assessing deformity. Contrary to expectation, BMI did not influence error of clinical assessment or severity of coronal deformity. It however appeared to influence larger flexion deformities. The discrepancy between radiographic and navigation measurements reflects the absence of true weight bearing with navigation even though we tried to simulate this by applying axial load to the foot.