Background The scapula has an important role for mobility and functional stability in the shoulder joint. The invasive methods of scapula tracking cannot be performed in clinical practice and the problem with non-invasive measurements is their accuracy and the skin deformation. The purpose of this study was to validate a new minimally invasive method of tracking internal and external rotation of the scapula using ultrasound imaging combined with the signal provided by a three-dimensional electromagnetic sensor.

Methods A descriptive test–retest design was employed for the reliability evaluation of this new tracking method. An in vitro phase was performed with an anatomic model of a thorax. The accuracy of the scapula rotation using the in vitro model was calculated by correlation between two image software measurements (MATLAB2014 and ImageJ1.48 v). Thirty subjects were recruited for in vivo analysis. The scapula rotation was measured by a two-dimensional cross-correlation algorithm using MATLAB2014. It was compared two different positions of the scapula. The two positions of the subjects were 0° of shoulder separation and 120° in the frontal plane. Sensor information was added to each rotation.

Results The correlation of rotations in the in vitro phase obtained by the two software was r=0.78 (p=0.009). A Student’s t-test compared the two methods and showed no significant difference (p=0.26, 95% CI=−0.98–0.30). No significant differences in any of the paired repetitions in in vivo measurements (p=0.67–0.34) were found by one-way ANOVA test. There was a correlation between paired repetitions between 0.96 and 0.86 (p<0.001).

Conclusions Ultrasound imaging combined with a motion sensor to track the scapula has been shown to be a reliable and valid method for measuring internal and external rotation during a separation of the upper limb. Future studies should continue to calculate the rotational levels of different types of people and describe ultrasound imaging’s clinical implications like a new opportunity to assess scapula diskinesia.