Accelerometer- and photographic-based smartphone applications for measuring joint angle: are they reliable?

We read the article by Jenny [1] with great interest and agree that computer assisted measurement of joint angles using digital imaging, even though shown to be valid, could be too time consuming to be used on a routine basis (e.g. due to the necessity to transfer data from camera to personal computer). This disadvantage can be overcome by smartphones that are all-in-one instruments having computer-like functionality and an integrated digital camera as well as sensors as accelerometers. To date there are two kinds of software available for smartphones to measure joint angles: accelerometerand photographic-based applications. The first kind uses the built-in accelerometers of the mobile phone to measure the inclination of the device and thereby calculate angles. The second kind (photographic-based) does the measurement by positioning a virtual goniometer, visible on the smartphone screen, on a photograph obtained via the smartphone camera (Fig. 1). The article written by Jenny [1] was aimed to assess the agreement between a free accelerometer-based application – Angle, Smudge Apps – and a navigation system designed for total knee arthroplasty, in knee joint measurement. Observing the figure 5 of the paper, the twomethods seem to show a range of the limits of agreement (i.e. the interval of two standard deviations of the measurement differences either side of the mean difference) of 27.4° (−15.8°/+11.6°). Similarly, Hambly et al [2] and Ockendon and Gilbert [3] assessed the level of agreement between another accelerometer-based smartphone application – Knee Goniometer, Ockendon.net – and a long arm goniometer. In the measurement of the maximum active knee flexion (120–145°) the range was 7.01° (−2.13°/+4.88°) [2], while at knee angles between 5 and 45° of flexion the range was 15.2° (−8°/+7.2°) [3]. Recently, our group published two papers on the reliability of a photographic-based application –DrGoniometer, CDM S.r.L. – in the angle measurement of elbow [4] and knee [5]. The range of the limits The Journal of Arthroplasty 29 (2014) 448–451