Reproducibility of a new method for measuring lowering and medialisation of the humerus after reverse shoulder arthroplasty.

BACKGROUND: Reverse shoulder arthroplasty (RSA) requires lowering the humerus and medialisation of the centre of rotation. Several techniques based on standard X-rays are available in the literature. The aim of this study was to expose and validate a new technique for measuring these parameters.

MATERIALS AND METHODS: Radiographic data of 20 patients were included. Three observers examined each X-rays three times to establish the inter- and intra-reproducibility of each technique (Jobin, Renaud and our method).

RESULTS: Intra-observer reproducibility was between 0.10 and 0.94. The best intra-observer intraclass correlation coefficient (ICC) was obtained using Renaud's and our techniques. The inter-observer reproducibility was between 0.37 and 0.74. The best inter-observer ICC was obtained with our technique.

DISCUSSION: We describe a reliable and reproducible method to evaluate lowering and medialisation of the humerus after RSA. This technique may allow determine the optimal threshold of lengthening after RSA.