A Digital Photographic Technique for Knee Range of Motion Measurement: Performance in a Total Knee Arthroplasty Clinical Population

Background: Knee range of motion is an essential outcome measure following total knee arthroplasty and it is important to measure this outcome with a simple and accurate tool. Measurement from digital photographs could provide a superior clinical alternative to the common goniometer as devices for capturing digital photographs are becoming more accessible, measurement accuracy is assisted with the associated software and images can be saved and stored for later reference. This ability to store images with measurements could be particularly useful in research involving knee range of motion. This study evaluated the validity and reliability of a simple photographic measurement technique to measure knee flexion and extension of patients following knee arthroplasty. Methods: Knee flexion and extension of 38 subjects who had undergone total knee arthroplasty were assessed at the one year post-operative review with the digital technique and a universal goniometer as the criterion reference. Digital measurements were repeated one month later by the first assessor (intra-tester reliability) and another independent assessor (inter-tester reliability). Results: Validity: the digital technique was found to have a high level of agreement with the universal goniometer measurements (all MAD < 2.35 ̊). Reliability: small differences were observed in intra and inter-tester measurements for both flexion and extension with almost perfect Intra-Class Correlations (Intra-tester; flexion 0.99, extension 0.96; inter-tester; flexion 0.97, extension 0.93). Conclusion: Measurement of knee range of motion from digital photographs can be used in routine clinical total knee arthroplasty follow-up instead of, or interchangeably with the universal goniometer.