Reverse total shoulder arthroplasty (RSA) was first performed by Grammont in 1985 to treat painful arthritis of the rotator cuff-deficient shoulder [1]. RSA reduces pain and increases the range of motion (ROM); therefore, excellent clinical results have been reported [[2], [3], [4]]. Due to its stable long-term results, it has been used as the standard treatment method for painful arthritis of the rotator cuff-deficient shoulder in Europe for a long time [2].

The usefulness of RSA has been recognized in Japan, and it became available for use in April 2014. Until 2014, there were limited options to effectively treat irreparable massive rotator cuff tears (MRCT) and cuff tear arthropathy (CTA). Recently, the introduction of RSA has expanded the treatment options for these conditions. However, RSA has a high complication rate, even for short-term outcomes [5,6]. Due to RSA's abnormal structure, specific complications such as dislocation [7], scapular notching [8,9], and other consequences, can occur [10]. In view of specific RSA complications, strict eligibility requirements have been applied in Japan: patients with pseudoparalysis must be at least 70 years old at the time of surgery and must have a preoperative anterior elevation (AE) of <90°. Another concern is that Asians, including the Japanese, are smaller in size compared to Westerners. Mizuno et al. reported differences in glenoid morphology between the Japanese and French populations and that the glenoid width and height were predominantly smaller in the Japanese [11]. Moreover, Matsumura et al. reported that the size of the Japanese glenohumeral joint was smaller than that reported in Caucasian populations [12]. Ji et al. further reported that the glenoid was occasionally smaller than the RSA baseplate in Korean females with particularly small bodies and that an appropriately sized glenoid component was necessary [13]. It is unclear whether an implant designed for the French population will work well for an Asian population with a smaller body, such as the Japanese population, even under the strict age restrictions and limited preoperative ROM.

Therefore, this study aimed to compare the ROM, functional outcomes, and scapular notching rates in patients who underwent standard Grammont-style RSA between the French and Japanese populations and investigate the racial differences in RSA performed at approximately the same time in France and in Japan with a minimum follow-up of 2 years. We hypothesized that RSA may not provide as good ROM and functional results in the Japanese population, at the final follow-up, as the French population.