MEASURING MALALIGNMENT IN KNEE OA

Abstract

Purpose: To compare a computer-based program for measuring frontal plane leg alignment with a hand-measuring system.

Methods: Five patterns of frontal limb alignment, simulating full-length radiographs of healthy and malaligned limbs, were drawn in AutoCAD and exported as digital images. The patterns included variations in varus and valgus alignment and joint space slope. Copies of each pattern were given to 7 trained readers for analysis. The readers used a ruler and protractor to measure predefined angles and lengths following an established method which defines limb and joint margin landmarks to derive mechanical and anatomic (shaft) axes. Custom software was used to define the same landmarks on the digital copies of the patterns; the angles and lengths were calculated by the software, replicating the method above. A simple fixed calibration factor was used to convert the digital linear dimensions to their paper equivalent. The order of hand and computer analysis of the 5 patterns was randomized and each was repeated daily for 3 days. Measured angles included hip-knee-ankle (HKA), femoral condylar tangent-hip (CH), tibial plateau tangent-ankle (PA), condylar-plateau (joint orientation-CP) and angles between the femoral mechanical axis (FM), femoral shaft axis (FS), tibial mechanical axis (TM)and tibial shaft axis (TS). Repeated measures analysis of variance was performed to compare the methods, with a significance level set at p< 0.05.

Results: The analysis revealed differences between hand and computer measures in femoral lengths and apparent leg lengths (although these differences were less than 1 mm). Differences between methods were also obtained for CP and FM-FS angles (the differences in angles were less than 1 degree). The variance of the computer measures was the same as (19/55 measures) or significantly less than (35/55 measures) the hand measures, with the exception of angle CH on a single pattern. For all limb length measures and the angles HKA, FM-FS, FM-TS and FS-TS there were differences between the readers. There was no difference across days for any measure.

Conclusions: The analysis revealed similar results for limb alignment angles and for limb lengths between hand and computer methods. The computer application improved on the variance obtained with the hand measuring method, indicating a more precise system. Significant differences were found between readers, suggesting the need for standardization of methods for measuring alignment.