The 4-year outcomes of a prospective study of patients undergoing Triathlon total knee arthroplasty (TKA) are presented. There were 2035 consecutive cemented TKAs performed on 1688 patients. Mean follow-up was 21 months, including 713 knees with a follow-up of > or =2 years. In a subgroup of 239 patients, the coverage of the patella cut surface by the patella component was analyzed. To our knowledge, this is the first article to study this issue. Mean Knee Society pain scores improved from 48 to 96, function improved from 63 to 85, and range of motion improved from a mean of -5 degrees to 104 degrees to a mean of -2 degrees to 126 degrees. Coverage of the patella by the prosthesis was full from medial to lateral in 88% and full from proximal to distal in 91% of knees. The implant system has performed well within the follow-up period, with no implant- or design-related failures.