THE SIGNIFICANCE OF A SAGITTAL PLANE DEFORMITY IN THE RELATIONSHIP BETWEEN VALGUS DEFORMITY AND BODY WEIGHT IN OSTEOARTHRITIC KNEES.

Abstract

Previous studies of osteoarthritic knees have examined the relationship between the variables body mass index (BMI) and weight on the one hand and coronal plane deformity on the other. There is a consensus that weight and BMI are positively correlated to the degree and progression of a varus deformity. However, there does not appear to be a consensus on the effect of these variables on knees with a valgus deformity. Indeed, the view has been expressed that in knees with a severe deformity a relationship might not exist. A review of these studies reveals that in all cases, the alignment of the lower limb was obtained from a standing antero-posterior long leg radiograph. In no cases was the deformity in the sagittal plane measured. This study analyses the relationship between BMI, weight, deformity in the sagittal plane and valgus deformity.

The study group consisted of 73 patients with osteoarthritis and valgus knees. All of them had failed conservative treatment for their symptoms and were listed for navigated TKA. Their weight and height were measured two weeks preoperatively and the BMI calculated. At operation the coronal and sagittal deformities were measured using the Orthopilot^® navigation system (BBraun Aesculap, Tuttlingen). The results were analysed using SPSS 15.

Regression analysis showed a significant relationship (p< 0.05) with a negative correlation between valgus deformity and weight. the correlation coefficient for flexed knees (−0.59) showed a moderately strong relationship whereas that for extended knees (−0.38) showed a relatively weak relationship.

It is acknowledged that there is an increased force on the lateral compartment with increased valgus deformity. a larger deformity causes a larger moment arm about the centre of the knee. this study has shown that at the time of surgery, individuals with lower weights have larger valgus deformities. we postulate, therefore, that when the moment due to the weight of the individual and the length of the moment arm exceeds a certain value, a symptomatic threshold is crossed. in the presence of a fixed flexion deformity, the force on the patella-femoral joint is increased, contributing further to the onset of discomfort.

Further investigation into the subsets of valgus knees appears to be warranted.