**5. Conclusions**

DISH was found in nearly 40% of patients with symptomatic cervical OPLL, about 60% of whom had a grade 2 DISH, using our classification system. Our DISH grade correlated with age and the indices of OSL in other areas at each high-prevalence level as well as the whole spine. Patients with cervical OPLL and severe DISH might also have a simultaneous severe OSL. In patients with symptomatic cervical OPLL, DISH extending to the cervical or lumbar spine is a radiographic sign suggesting a tendency toward diffuse ossification in the whole spine.

**Author Contributions:** Conceptualization, S.N., T.H., N.N. (Narihito Nagoshi), T.Y., K.M. (Kanji Mori), S.U., S.M., K.K., N.N. (Norihiro Nishida) and Y.K.; Methodology, S.N.; Software, S.N., T.H and N.N. (Narihito Nagoshi); Validation, S.N., T.H., N.N. (Narihito Nagoshi), T.Y., K.M. (Kanji Mori), S.M. and K.K. Formal Analysis, S.N. and N.N. (Narihito Nagoshi); Investigation, K.M. (Kanji Mori), T.Y., T.H., N.N. (Narihito Nagoshi), S.N., K.T. (Kazuhiro Takeuchi), S.M. and K.K.; Resources, M.Y., M.M. and A.O.; Data Curation, S.N., T.H., N.N. (Narihito Nagoshi), T.Y., K.M. (Kanji Mori), S.U., S.M., K.K., M.M., M.N., K.W. (Kota Watanabe) and Y.K. designed the study; S.N., T.H., N.N. (Narihito Nagoshi), T.Y., S.U., J.H., K.M. (Kanji Mori), S.M., K.K., K.T. (Katsushi Takeshita), T.F., K.W. (Kei Watanabe), K.W. (Kota Watanabe), T.K., S.K., K.N., M.K., H.N., S.I., Y.M. (Yuji Matsuoka), K.W. (Kanichiro Wada), A.K., T.O., H.K., H.O., K.M. (Kazuma Murata), Y.M. (Yu Matsukura), H.I. and Y.K.; Writing—Original Draft Preparation, S.N., T.H., N.N. (Narihito Nagoshi), H.H., Y.M. (Yukihiro Matsuyama), K.T. (Katsushi Takeshita), H.O., A.O. and Y.K.; Writing—Review and Editing, S.N., T.H., N.N. (Narihito Nagoshi), T.Y., S.U., K.M. (Kanji Mori), S.M., K.K., K.T. (Kazuhiro Takeuchi), T.F., K.W. (Kei Watanabe), N.N. (Norihiro Nishida), K.W. (Kota Watanabe), T.K., S.K., K.N., M.K., H.N., S.I., Y.M. (Yukihiro Matsuyama), K.W. (Kanichiro Wada), A.K., T.O., H.K., H.O. and Y.K.; Visualization, S.N., T.H., N.N. (Narihito Nagoshi), K.M. (Kanji Mori) and S.U.; Supervision, M.M., M.W., M.N., M.Y., A.O. and Y.K.; Project Administration, T.H. and Y.K. Funding Acquisition, M.M., M.Y. and A.O. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by a Health and Labour Science Research gran<sup>t</sup> (grant number 201610008B) and by the Japan Agency for Medical Research and Development (grant number 16ek0109136h0002).

**Institutional Review Board Statement:** The study was conducted according to the guidelines of the Declaration of Helsinki, and approved by the institutional review board of each participating institution (protocol code 28–34; 14 March 2017).

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.

**Acknowledgments:** We thank Nobuko Nakajima, Tomomi Kobayashi, Namiko Katayama and Yukiko Oya for data collection.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
