BackgroundMeasuring knee range of motion is important in examination and as a post-operative outcome. It is therefore important that measurements are accurate. Knee angles can be measured by traditional goniometers, smartphone apps are readily available and there are also purpose made digital devices. Establishing the minimum difference between methods is essential to monitor change. The purpose of this study was to assess reliability and minimum significant difference of visual estimation, short and long arm goniometers, a smartphone application and a digital inclinometer.MethodsKnee angles were assessed by 3 users: one consultant orthopaedic surgeon, one orthopaedic surgical trainee and an experienced physiotherapist. All 5 methods were used to assess 3 knee angles, plus full active flexion and extension, on 6 knees. The subjects had knee angles fixed using limb supports during measurement, whilst maintaining appropriate clearance to allow a reproduction of assessment in clinic. Users were then blinded to their results and the test was repeated. A total of 300 measurements were taken.ResultsInter-rater and intra-rater reliabilities were high for all methods (all > 0.99 and > 0.98 respectively). The digital inclinometer was the most accurate method of assessment (6° minimum significant difference). The long arm goniometer had a minimum significant different of 10°, smartphone app 12° and both visual estimation and short arm goniometry were found to be equally inaccurate (14° minimum significant difference).ConclusionThe digital inclinometer was the most accurate method of knee angle measurement, followed by the long arm goniometer. Visual estimation and short goniometers should not be used if an accurate assessment is required.