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Loss of meniscal tissue leads to increased pain and decreased clinical function and activity levels. We hypothesized that patients receiving a collagen meniscus implant would have better clinical outcomes than patients treated with partial medial meniscectomy alone.Three hundred and eleven patients with an irreparable injury of the medial meniscus or a previous partial medial meniscectomy, treated by a total of twenty-six surgeon-investigators at sixteen sites, were enrolled in the study. There were two study arms, one consisting of 157 patients who had had no prior surgery on the involved meniscus (the “acute” arm of the study) and one consisting of 154 patients who had had one, two, or three prior meniscal surgical procedures (the “chronic” arm). Patients were randomized either to receive the collagen meniscus implant or to serve as a control subject treated with a partial meniscectomy only. Patients underwent frequent clinical follow-up examinations over two years and completed validated outcomes questionnaires over seven years. The patients who had received a collagen meniscus implant were required by protocol to have second-look arthroscopy at one year to determine the amount of new tissue growth and to perform a biopsy to assess tissue quality. Reoperation and survival rates were determined.In the acute group, seventy-five patients received a collagen meniscus implant and eighty-two were controls. In the chronic group, eighty-five patients received the implant and sixty-nine were controls. The mean duration of follow-up was fifty-nine months (range, sixteen to ninety-two months). The 141 repeat arthroscopies done at one year showed that the collagen meniscus implants had resulted in significantly (p = 0.001) increased meniscal tissue compared with that seen after the original index partial meniscectomy. The implant supported meniscus-like matrix production and integration as it was assimilated and resorbed. In the chronic group, the patients who had received an implant regained significantly more of their lost activity than did the controls (p = 0.02) and they underwent significantly fewer non-protocol reoperations (p = 0.04). No differences were detected between the two treatment groups in the acute arm of the study.New biomechanically competent meniscus-like tissue forms after placement of a collagen meniscus implant, and use of the implant appears safe. The collagen meniscus implant supports new tissue ingrowth that appears to be adequate to enhance meniscal function as evidenced by improved clinical outcomes in patients with a chronic meniscal injury. The collagen meniscus implant has the utility to be used to replace irreparable or lost meniscal tissue in patients with a chronic meniscal injury. The implant was not found to have any benefit for patients with an acute injury.Therapeutic Level I. See Instructions to Authors for a complete description of levels of evidence.