Measurement of varus/valgus alignment in obese individuals with knee osteoarthritis

Frontal plane knee malalignment may increase progression of knee osteoarthritis (OA) and hasten functional decline. An accurate nonradiographic measure of knee alignment is necessary because the gold standard measure, the long-leg radiograph, is costly and often unavailable. Moreover, nonradiographic measures of knee alignment have not been validated in an obese population, where knee OA is common. The purpose of this study was to develop and assess the concurrent validity and reliability of a nonradiographic measure of frontal plane knee alignment and demonstrate the accuracy of the measure in an obese population.

Fifty-five subjects (41 women, 14 men; mean +/- SD age 62.9 +/- 10.3 years) with knee OA were examined. A nonradiographic measure (umbilical method) of frontal plane alignment, using the landmarks of the umbilicus, knee, and ankle, was compared with the radiograph gold standard. Statistical significance was accepted at P < 0.05.

Eighty-nine percent of the participants had a body mass index (BMI) placing them in the overweight or obese category (mean +/- SD BMI for all subjects 31.3 +/- 6.1 kg/m(2)). Radiographic measures of alignment ranged from 9.1 degrees valgus to 14.3 degrees varus (76% of the participants had varus alignment, 12% had valgus alignment, and 2% had neutral alignment). Umbilical measures ranged from 1 degrees valgus to 21 degrees varus. The umbilical measure was significantly correlated with the radiographic method (r = 0.75, P < 0.001). The error of the umbilical measure was not significantly correlated with the BMI (r = -0.21, P = 0.13).

The umbilical method of assessing frontal plane knee alignment is a valid surrogate for the radiographic gold standard and retains its accuracy in an obese population.