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BACKGROUND: The role of arthroscopic debridement in the treatment of osteoarthritis of the knee remains to be defined, and few clinical and radiographic characteristics have been quantitatively associated with the outcome. The hypothesis of this study was that the outcome of arthroscopic debridement for osteoarthritis of the knee is associated with preoperative clinical and radiographic features and intraoperative characteristics and that there are subsets of patients who are more and less likely to respond favorably to the treatment. METHODS: We performed a cross-sectional study of a consecutive cohort of 122 patients who underwent arthroscopic debridement for the treatment of osteoarthritis of the knee that had been unresponsive to anti-inflammatory therapy. One hundred and ten patients were followed for a mean of thirty-four months. Pain was assessed with the pain domain of the Knee Society scoring system. Radiographs were scored with the Kellgren-Lawrence method, and limb alignment and the widths of the medial and lateral joint spaces were measured. The severity of cartilage lesions was scored intraoperatively with a modified Noyes grading system. Specific methods of data collection and analysis were incorporated to minimize bias. RESULTS: Fifty-two (90%) of fifty-eight knees with mild arthritis, normal alignment, and a joint space width of > or = 3 mm were improved after arthroscopic debridement. Conversely, only five (25%) of twenty knees with severe arthritis, limb malalignment, and a joint space width of < 2 mm had substantial relief of symptoms. Of seventy-two patients who had improvement, forty-four (61%) had it within six months after the arthroscopy. The severity of the lesion was highly predictive of the clinical outcome both in patients with mild arthritis and in those with severe arthritis. CONCLUSIONS: The severity of the arthritis, as assessed preoperatively with radiography and intraoperatively by rating the severity of cartilage lesions, influences the clinical outcome of arthroscopic debridement of an osteoarthritic knee. Knees with severe arthritis fare poorly, whereas those with mild arthritis fare well. We could not predict the outcome for knees with moderate arthritis. We believe that these observations are relevant for establishing indications for arthroscopy in patients with osteoarthritis of the knee and may be useful for designing studies with a more rigorous experimental design