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The reported incidence of HO varies hugely. This study aimed to determine prevalence of HO, progression, predisposing factors to occurrence and to progression, and potential effect on clinical outcomes.<\/jats:p>\n              <\/jats:sec><jats:sec>\n                <jats:title>Methods<\/jats:title>\n                <jats:p>Eighty-three patients treated with ADRs for cervical radiculopathy at one or two segments were extracted from a previously published RCT. Of the 83 patients, 59 had remaining ADR (79 implants) and sufficiently high-quality X-rays at 5\u00a0years of follow-up to allow analysis. HO was graded on plain films according to Mehren\/Suchomel. Prevalence, progression, predisposing factors and effect on clinical outcome (Neck Disability Index\u2014NDI) were analyzed.<\/jats:p>\n              <\/jats:sec><jats:sec>\n                <jats:title>Results<\/jats:title>\n                <jats:p>At 2\u00a0years, HO was seen in 46\/55 ADR implants (84%), severe enough to affect mobility in 27\/55 (49%). At 5\u00a0years, HO was seen in 92% of 79 implants, severe in 71%. Male sex was a predisposing factor to HO. No predisposing factors to progression were identified. NDI was not affected by the severity of HO.<\/jats:p>\n              <\/jats:sec><jats:sec>\n                <jats:title>Conclusion<\/jats:title>\n                <jats:p>Almost all ADR implants in our study have HO at 5\u00a0years of follow-up. Male sex is a risk factor. Severe HO did not affect clinical outcome.<\/jats:p>\n              <\/jats:sec><jats:sec>\n                <jats:title>Trial registration<\/jats:title>\n                <jats:p>Study registered at ISRCTN (registration number: 44347115).<\/jats:p>\n              <\/jats:sec>","DOI":"10.1007\/s00586-021-06722-6","type":"journal-article","created":{"date-parts":[[2021,1,20]],"date-time":"2021-01-20T15:02:56Z","timestamp":1611154976000},"page":"1662-1669","update-policy":"https:\/\/doi.org\/10.1007\/springer_crossmark_policy","source":"Crossref","is-referenced-by-count":5,"title":["Unintended fusion in cervical artificial disk replacement: a prospective study on heterotopic ossification, progression, and clinical outcome, with 5-year follow-up"],"prefix":"10.1007","volume":"30","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-5530-2909","authenticated-orcid":false,"given":"Catarina","family":"Marques","sequence":"first","affiliation":[],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-9231-7608","authenticated-orcid":false,"given":"Anna","family":"MacDowall","sequence":"additional","affiliation":[],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-2228-2478","authenticated-orcid":false,"given":"Martin","family":"Skeppholm","sequence":"additional","affiliation":[],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-5503-1793","authenticated-orcid":false,"given":"Nuno","family":"Canto Moreira","sequence":"additional","affiliation":[],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-2111-6868","authenticated-orcid":false,"given":"Claes","family":"Olerud","sequence":"additional","affiliation":[],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"297","published-online":{"date-parts":[[2021,1,20]]},"reference":[{"key":"6722_CR1","doi-asserted-by":"publisher","first-page":"E194","DOI":"10.1227\/01.neu.0000163419.59635.78","volume":"57","author":"RH Bartels","year":"2005","unstructured":"Bartels RH, Donk R (2005) Fusion around cervical disc prosthesis: case report. 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Study registered at ISRCTN (registration number: 44347115).","order":2,"name":"Ethics","group":{"name":"EthicsHeading","label":"Ethics approval"}},{"value":"CM, AM, MS, NCM: None. CO: Research grants: CSRS-E, DePuy Synthes; Speaker\u2019s bureau: AO Spine, DePuy Synthes, Medtronic; Board: CSRS-E.","order":3,"name":"Ethics","group":{"name":"EthicsHeading","label":"Conflict of interest"}},{"value":"All participants provided oral and written informed consent.","order":4,"name":"Ethics","group":{"name":"EthicsHeading","label":"Consent to participate and consent for publication"}},{"value":"Data and material are available if necessary.","order":5,"name":"Ethics","group":{"name":"EthicsHeading","label":"Availability of data and material"}}]}}