Oh interesting! That didn't come up when I searched for “frozen shoulder”, but yeah they're indeed both a thing: “四十肩” (40s shoulder) and “五十肩” (50s shoulder). Given that, tho, I'd be concerned with domesticating “四十肩” as “frozen shoulder” because if at any point a distinction is made with 50s shoulder, then what do you do? Tho, foreignizing as “40s shoulder” necessitates a translation note, whereas “frozen shoulder” doesn't, and if only 1 of the Japanese terms is ever used, then it's not an issue.