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Background: Mobile-bearing unicompartmental knee arthroplasty (MB-UKA) is a proven implant that has reliably delivered excellent results for decades. Based on the constrained implant design in MB-UKA, the occasional occurrence of anterior impingement should be expected. However, surprisingly, there are no clinical reports. Methods: From 2016 to 2020, 14 patients with anterior medial knee pain were admitted to our arthroplasty center after MB-UKA implantation elsewhere. After taking the medical history and clinical examination, radiological imaging of the implant in at least 2 planes, including a whole-leg anteroposterior view, was performed. The “Knee Society Score (KSS)” and the “Knee Injury and Osteoarthritis Outcome Score (KOOS)” were recorded. Anterior impingement was diagnosed by reviewing the typical findings and specific exclusion of other diagnoses. Results: The 14 patients showed a KSS of 46.6 and a KOOS of 51.5. The average pain level on the “Visual Analog Scale” was 7.8. The positioning of the implants showed consistently noticeable deviations from the standard recommendations. All 14 patients were treated by removing the MB-UKA and changing to a complete TKA. At the 12-month follow-up, the average Visual Analog Scale score was 1.8, and KOOS and KSS were 86 and 82, respectively. Conclusions: The potential risk of anterior impingement in MB-UKA can be assumed. Diagnosis requires a detailed collection of medical history and clinical details combined with accurate radiological imaging. The cause of anterior impingement in MB-UKA is multifactorial and refers in our small group to the sum of minor deviations in implant positioning compared to the general recommendations.