Objective: To evaluate the clinical significance of the newly proposed Japan Otological Society (JOS) classification for mastoid extension (M subdivision: M0, M1, M2) in patients with middle ear cholesteatoma.
Methods: Patients with newly diagnosed middle ear cholesteatoma who underwent primary surgery at participating institutions between January 2021 and December 2022 were included. The extent of disease was classified according to the 2015 JOS staging system, with mastoid extension further subdivided into M0 (no mastoid extension), M1 (limited to the antrum), and M2 (extending beyond the antrum or the lateral SCC).
Results: A total of 527 patients from 11 institutions were registered. Among them, 332 ears (63%) had pars flaccida cholesteatoma, 59 (11%) had pars tensa cholesteatoma, 69 (13%) had congenital cholesteatoma, 40 (8%) had cholesteatoma secondary to pars tensa perforation, and 27 (5%) were unclassifiable. Staging distribution included 112 ears (22%) classified as Stage I, 350 (66%) as Stage II, and 65 (12%) as Stage III; none were Stage IV. Mastoid extension was classified as M0 in 245 ears (46.2%), M1 in 123 (23.8%), and M2 in 159 (30.0%). Mastoid involvement was significantly more common in pars flaccida cholesteatoma, and higher M subdivision correlated with higher JOS stage. The transcanal atticotomy was most often used for M0 cases, whereas canal wall up and canal wall down tympanoplasty predominated in M1 and M2 groups. The use of transcanal endoscopic ear surgery decreased significantly with greater mastoid extension. No significant differences in recurrence rates or postoperative hearing outcomes were observed among the M subdivisions.
Conclusion: The JOS classification of mastoid extension provides a practical and meaningful framework for assessing middle ear cholesteatoma. Although it did not affect short-term outcomes, the M subdivision offers useful guidance for surgical planning and reflects disease extent more precisely.

