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Because individual case reports indicate a high morbidity rate, the aim of this retrospective study was a detailed analysis of this rare anatomical variant.<\/jats:p><\/jats:sec><jats:sec><jats:title>Methods<\/jats:title><jats:p>Our systematic imaging-based search for SKs, conducted in the period from 2000 and to 2017, yielded 9 cases in total (5 men, 4 women; mean age: 51.8\u2009\u00b1\u200922.8\u2009years).<\/jats:p><\/jats:sec><jats:sec><jats:title>Results<\/jats:title><jats:p>The SKs were observed on the right in six and on the left side in three cases. In six subjects (66%) they were located caudal and in three cases (33%) cranial to the ipsilateral kidney. Calculi were found in three (33%) of the renal collecting systems. Five (56%) of the SKs had hydronephrosis grade IV and one SK had recurrent pyelonephritis (11%). Two of the ureters opened into the ipsilateral seminal vesicle (22%). Two (22%) SKs were functional but atrophic. Clinically relevant findings were made in 33% of the IKs: atrophy (<jats:italic>n<\/jats:italic>\u00a0=\u20092), calculi (<jats:italic>n<\/jats:italic>\u00a0=\u20091), and reflux with recurrent pyelonephritis (<jats:italic>n<\/jats:italic>\u2009=\u20091); another 33% had anatomical anomalies without functional impairment. The correct diagnosis of a SK is possible using CT imaging in all subjects. The prevalence of SK based on CT imaging can be estimated to be 1:26750.<\/jats:p><\/jats:sec><jats:sec><jats:title>Conclusions<\/jats:title><jats:p>CT is the method of choice for visualizing SKs. The correct diagnosis is crucial in preventing dispensable surgical procedures and for providing optimal patient treatment and outcome.<\/jats:p><\/jats:sec>","DOI":"10.1186\/s12894-019-0522-0","type":"journal-article","created":{"date-parts":[[2019,10,17]],"date-time":"2019-10-17T13:10:08Z","timestamp":1571317808000},"update-policy":"https:\/\/doi.org\/10.1007\/springer_crossmark_policy","source":"Crossref","is-referenced-by-count":18,"title":["Supernumerary kidneys: a clinical and radiological analysis of nine cases"],"prefix":"10.1186","volume":"19","author":[{"given":"Peter","family":"Rehder","sequence":"first","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Rafael","family":"Rehwald","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Julia M.","family":"B\u00f6hm","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Astrid E.","family":"Grams","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Alexander","family":"Loizides","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Marco","family":"Pedrini","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Jannik","family":"St\u00fchmeier","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Bernhard","family":"Glodny","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"297","published-online":{"date-parts":[[2019,10,17]]},"reference":[{"issue":"4","key":"522_CR1","doi-asserted-by":"publisher","first-page":"331","DOI":"10.1016\/S0022-5347(17)71961-1","volume":"38","author":"JF Geisinger","year":"1939","unstructured":"Geisinger JF. 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Due to the retrospective design of this study, individual patient consent to be included in this investigation was waived by the institutional ethics committee.","order":1,"name":"Ethics","group":{"name":"EthicsHeading","label":"Ethics approval and consent to participate"}},{"value":"Due to the retrospective design of this study, no individual patient consent was required and all data used has been fully anonymized.","order":2,"name":"Ethics","group":{"name":"EthicsHeading","label":"Consent for publication"}},{"value":"The authors declare that they have no competing interests to declare with regard to this study.","order":3,"name":"Ethics","group":{"name":"EthicsHeading","label":"Competing interests"}}],"article-number":"93"}}