‘FlexYonio’; a reliable instrument to support monitoring the length of the long finger flexors in Duchenne muscular dystrophy (DMD)

Background Shortening of the long finger flexors (FDP) results in extension limitation of both wrist and fingers and can hinder important activities of the upper extremities in Duchenne muscular dystrophy (DMD). Early detection of FDP shortening is important for timely interventions, but reliable measurement of FDP length is difficult. Aim To develop a new instrument to monitor FDP shortening more easily and precisely and to determine its intra -and interrater reliability. Materials and methods The new instrument, called the ‘FlexYonio’, was clinically developed according to biomechanical standards to be able to monitor FDP length, by measuring range of motion of wrist extension with extended fingers. A prospective reliability study was conducted during annual outpatient visits of people with DMD at the neuromuscular center of the Radboudumc. Repeated measures were conducted and two raters assessed the FDP length with the new instrument using a standardized measurement protocol. Reliability was measured using intra class correlation coefficient (ICC) calculation. Results The ‘FlexYonio’ was easy to use and became part of the standard daily clinical care in DMD in the Radboudumc. For reliability, 86 arm/hands were assessed; the intra- and interrater reliability were excellent with ICC > 0.99. The within-rater limits of agreement were −6 to 8 degrees and the between-raters −11 to 13 degrees. Conclusion The ‘FlexYonio’, in combination with the standardized measurement protocol, is a promising, easy to use, and reliable tool to support the monitoring of FDP shortening in people with DMD, who are able to extend their fingers.