Improved precision in quantifying knee alignment angle.

Knee alignment angle, a measure of mechanical axis, has been reported to correlate with severity and progression of knee osteoarthritis. However, quantification of knee alignment often results in unacceptable reproducibility because of poor definition of landmarks and imprecise measurement techniques. We describe and validate a reproducible, sensitive method. Fourteen patients (28 knees) with symptomatic knee osteoarthritis who had full-limb radiography were evaluated. Knee alignment angle, defined as the angle formed between the mechanical axis of the femur and that of the tibia, was determined using a conventional manual technique and compared with a novel digital method using freely available Image J software to designate landmarks digitally and to determine the angles formed by the femoral and tibial axes. We found agreement between the manual and digital methods. All assessments were performed in duplicate on separate days. Using the manual method, the limits of agreement were +1.65 degrees to -1.55 degrees, yielding a minimal detectable change of approximately 1.6 degrees. However, the digital method provided limits of agreement of +0.43 degrees to -0.37 degrees resulting in a minimally detectable change of 0.4 degrees. We describe a method to reliably determine mechanical axis with precision permitting detection of differences less than 1 degrees. [ABSTRACT FROM AUTHOR]