Inter-rater and intra-rater reliability of the fluid goniometer for measuring active knee flexion in painful knees; correlations do not mean agreement

[Purpose] The fluid goniometer is an instrument for measuring range of motion. Reliability of the fluid goniometer has not been established for subjects with painful knee joints. The purpose of this study was to determine the inter-rater and intra-rater reliability of the fluid goniometer in measuring active knee flexion of painful knees and to test its agreement with the gold standard ruler goniometer. [Subjects and Methods] Twenty-five individuals with either unilateral or bilateral painful knees participated in the study. Two raters each took three measurements with the same Baseline® fluid goniometer on 35 knees. [Results] Intraclass correlation coefficients (ICC) were 0.97 for both intra-rater and inter-rater measurements, denoting high relative reliability. The large standard error of measurement (SEM) value of 6.6 degrees, and the 95% limits of agreement, which revealed a potential difference of 18.4 degrees between raters of similar subjects, however, revealed poor absolute reliability. The smallest detectable difference (SDD) of 18 degrees was also large. [Conclusion] The results revealed excellent relative reliability, but a large amount of variability between the raters' measurements. The sensitivity of the fluid level of the goniometer to end range tremors of the lower leg flexed against gravity in the obligatory prone position may contribute significantly to the large variability in knee ROM values.