Measurement of the knee flexion angle with a Smartphone-application is precise and accurate.

We hypothesized that the measurement of the knee flexion angle measured with a specific Smartphone application was different from the reference measurement with a navigation system designed for total knee arthroplasty (TKA). Ten consecutive patients operated on for navigation assisted TKA were selected. Six navigated and 6 Smartphone measurements of knee flexion angle were obtained for each patient. The paired difference between measurements and their correlation were analyzed. The mean paired difference between navigated and Smartphone measurements was -1.1° ± 6.8° (n.s.). There was a significant correlation between both measurements. The coherence between both measurements was good. The intra-observer and inter-observer reproducibility were good. The Smartphone application used may be considered as precise and accurate. The accuracy may be higher than other conventional measurement techniques.