Background: The orientation and the position of glenoid are two factors which have been pointed out as a cause of notch in case of reverse shoulder arthroplasty (RSA). Our hypothesis is that the notch is mostly depending on the relationship between the humerus and the glenoid.

Material and method: The inclusion criteria of this prospective study were: RSA with a 2 years minimum follow up, pre and post op x-rays of good quality to allow accurate measurements, a minimum of 3 x-rays regularly separated in the first 2 years. On these x-rays, done at rest, we have analyzed: the vertical orientation of the glenoid, the degree of abduction of the humerus, the gleno-metaphyseal angle (GMA) which represents the relationship between glenoid and humerus. Sixty one shoulders in 60 patients (57 females, 7 males) have been included. The mean age was 74,6 y.o (56–82) and the mean body mass index (BMI) was 25,4 (16–36).

Results: The patients with a notch had a significant correlation with a lower BMI (p< 0,001), a more upward pre op orientation of glenoid (p< 0,01), a less downward post op orientation of the glenoid, a lower degree of abduction of the humerus (p< 0,01), a lower GMA (p< 0,001) that means either that the humerus is less abducted or that the glenoid is orientated more upward or both. In addition we found a strongly correlation between the BMI and the degree of abduction of humerus which is lower if the BMI is low (p< 0,001). We studied the evolution of the angles and we noted that there was no difference during the first 3 months. After 3 months, the GMA stayed stable in patients without notch but was decreasing during the first year in those with a notch.

Discussion: The occurrence of a notch is strongly increased in patients with a lower BMI because of a less degree of abduction of the humerus, mostly if the glenoid is orientated upward. So the relationship between humerus and glenoid is more important to analyse than position of glenoid alone. The BMI is another important factor to take into account in case of indication of RSA because the risk of occurrence of a notch is quite higher.