A novel method for the remote measurement of trismus in clinical trials using mobile phone cameras

Objective To evaluate the reliability and validity of a novel method for remotely measuring trismus. Materials and methods We recruited 60 volunteers who took three types of photographs at a fixed restricted jaw position mimicking limited mouth opening, including one selfie and one portrait with or without a reference frame. Additionally, the interincisal distance and the width of the upper central incisors were measured with a ruler, as per common practice. Measurements of trismus were made using image analysis software comparing different types of photos and calibration methods. Intraclass correlation coefficient (ICC) and 95% limits of agreement (LoA) with 95% confidence interval were calculated to evaluate reliability and validity. Results The proposed method demonstrated high reliability (ICC 0.998; 95% CI 0.997, 0.999). Calibration of photographs using at least a baseline photograph with an external reference frame yielded unbiased measurements and minimised variability. The use of selfies compared to portrait photos also increased variability. Conclusion The measurement of trismus can be performed using images taken remotely by patients using their mobile phone cameras. The proposed method is highly accurate, with best results obtained by using a reference frame for calibration of portrait photographs. Clinical relevance We propose an easy, cheap, and accurate method that allows for remote and frequent monitoring of trismus in clinical studies using patients’ mobile phones.