For the purposes of classification, it should be specified whether osteoarthritis (OA) of the knee is of unknown origin (idiopathic, primary) or is related to a known medical condition or event (secondary). Clinical criteria for the classification of idiopathic OA of the knee were developed through a multicenter study group. ____and radiographs were used to develop sets of criteria that serve different investigative purposes. In contrast to prior criteria, these proposed criteria utilize classification trees, or algorithms.

Clinical criteria for the classification of patients with hip pain associated with osteoarthritis (OA) were developed through a multicenter study. Data from 201 patients who had experienced hip pain for most days of the prior month were analyzed. The comparison group of patients had other causes of hip pain, such as rheumatoid arthritis or spondylarthropathy. Variables from the medical history, physical examination, laboratory tests, and radiographs were used to develop different sets of criteria to serve different investigative purposes. Multivariate methods included the traditional “number of criteria present” format and “classification tree” techniques. Clinical criteria: A classification tree was developed, without radiographs, for clinical and laboratory criteria or for clinical criteria alone. A patient was classified as having hip OA if pain was present in combination with either 1) hip internal rotation ≥15º, pain present on internal rotation of the hip, morning stiffness of the hip for ≤60 minutes, and age >50 years, or 2) hip internal rotation <15º and an erythrocyte sedimentation rate (ESR) ≤45 mm/hour; if no ESR was obtained, hip flexion ≤115º was substituted (sensitivity 86%; specificity 75%). Clinical plus radiographic criteria: The traditional format combined pain with at least 2 of the following 3 criteria: osteophytes (femoral or acetabular), joint space narrowing (superior, axial, and/or medial), and ESR <20 mm/hour (sensitivity 89%; specificity 91%). The radiographic presence of osteophytes best separated OA patients and controls by the classification tree method (sensitivity 89%; specificity 91%). The “number of criteria present” format yielded criteria and levels of sensitivity and specificity similar to those of the classification tree for the combined clinical and radiographic criteria set. For the clinical criteria set, the classification tree provided much greater specificity. The value of the radiographic presence of an ostophyte in separating patients with OA of the hip from those with hip pain of other causes is emphasized.

Clinical criteria for the classification of symptomatic idiopathic (primary) osteoarthritis (OA) of the hands were developed from data collected in a multi- center study. Patients with OA were compared with a group of patients who had hand symptoms from other causes, such as rheumatoid arthritis and the spondylarthropathies. Variables from the medical history, physical examination, laboratory tests, and radiographs were analyzed. All patients had pain, aching, or stiffness in the hands. Patients were classified as having clinical OA if on examination there was hard tissue enlargement involving at least 2 of 10 selected joints, swelling of fewer than 3 metacarpophalangeal joints, and hard tissue enlargement of at least 2 distal interphalangeal (DIP) joints. If the patient had fewer than 2 enlarged DIP joints, then deformity of at least 1 of the 10 selected joints was necessary in order to classify the symptoms as being due to OA. The 10 selected joints were the second and third DIP, the second and third proximal interphalangeal, and the trapeziometacarpal (base of the thumb) joints of both hands. Criteria derived using the "classification tree" method were 92% sensitive and 98% specific. The "traditional format" classification method required that at least 3 of these 4 criteria be present to classify a patient as having OA of the hand. The latter sensitivity was 94% and the specificity was 87%. Radiography was of less value than clinical examination in the classification of symptomatic OA of the hands.To promote uniformity in the reporting of the rheumatic diseases, the Diagnostic and Therapeutic Criteria Committee of the American College of Rheumatology established subcommittees to develop classification criteria. During the last decade, criteria for systemic sclerosis, Reiter's syndrome, systemic lupus er ythematosus, rheumatoid arthritis (RA), osteoart hri-

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