Validity and reliability of using photography for measuring knee range of motion: a methodological study

Design: Methodology study assessing the validity and reliability of one method ('Marker Method') which uses a skin marker over the greater trochanter and another method ('Line of Femur Method') which requires estimation of the line of femur. Setting: Radiology and orthopaedic departments of two teaching hospitals. Participants: 31 volunteers (13 arthritic and 18 healthy subjects). Knee range of motion was measured radiographically and photographically using a goniometer. Three assessors were assessed for reliability and validity. Main outcomes: Agreement between methods and within raters was assessed using concordance correlation coefficient (CCCs). Agreement between raters was assessed using intra-class correlation coefficients (ICCs). 95% limits of agreement for the mean difference for all paired comparisons were computed.Validity (referenced to radiographs): Each method for all 3 raters yielded very high CCCs for flexion (0.975 to 0.988), and moderate to substantial CCCs for extension angles (0.478 to 0.678). The mean differences and 95% limits of agreement were narrower for flexion than they were for extension. Intra-rater reliability: For flexion and extension, very high CCCs were attained for all 3 raters for both methods with slightly greater CCCs seen for flexion (CCCs varied from 0.981 to 0.998). Inter-rater reliability: For both methods, very high ICCs (min to max: 0.891 to 0.995) were obtained for flexion and extension. Slightly higher coefficients were obtained for flexion compared to extension, and with the Marker compared to the Line of Femur Method. For intra- and inter-rater reliability, the mean differences (within 2 degrees) and 95% limits of agreement (within 5 degrees) were generally clinically acceptable for both methods.Photography potentially offers a superior method of measurement over standard goniometry as visualising the centre of the knee is simplified in a two-dimensional plane and the permanent record provides greater assessor transpare