Aim: The assessment of shoulder mobility is essential for clinical follow-up of shoulder treatment. Only a few high sophisticated instruments for objective measurements of shoulder mobility are available. The interobserver dependency of conventional goniometer measurements is high. In the 1990s an isokinetic measuring system of BIODEX Inc. was introduced, which is a very complex but valid instrument. Since 2008 a new user-friendly system (DynaPort ShoulderTest-System) is available. Aim of this study is the validation of this measuring instrument with the BIODEX-System.

Methods: The DynaPort ShoulderTest-System is a small, light-weighted three-dimensional gyroscope that is fixed on the distal upper patient arm, recording abduction, fiexion and rotation. For direct comparison we fixed the DynaPort on the lever arm of the BIODEX-system. The accuracy of measurement was determined at different positions, angles and distances from the center of rotation as well as different motion speeds in the radius between 0° – 180° in steps of 20°. All measurements were repeated ten times and observed with a digital water level. As satisfactory accuracy we defined a difference between both systems below 5°. The statistical analysis was performed with a linear regression model.

Results: The evaluation showed very high accuracy of measurements. The maximum average deviation was 0,5°. Below 60° the DynaPort was underestimating comparing the BIODEX system, whereas in higher positions higher data was measured. At higher angles the differences between both got higher. The distance to the center of rotation as well as the position of the Dyna-Port on the lever arm and different motion speeds infiuenced the results. The highest significant matches were measured at highest distance from the center of rotation (1,8° vs. 3,1°, p < 0,05) and a highest motion speed (2,1° at 60°/s, vs. 3,1° at 30°/s, p < 0,05).

Conclusion: In summary the results showed a high correlation and good reproducibility of measurements. All deviations were inside the tolerance interval of 5 °. These laboratory trials are promising for the validation of this system in shoulder patients. The challenge for both systems will be the changing of the center of rotation in the shoulder joint at elevations higher than 90°.