Cytology of polychrome-stained equine synovial fluid smears. Comparison with clinical findings, histologic specimens, Wright-Giemsa-stained smears and outcome.

Polychrome-stained equine synovial fluid specimens from 34 normal joints and 129 joints with clinical abnormalities were examined cytologically. The smears from joints with abnormalities were categorized as within normal limits (4.7%), slight abnormality (27.9%), proliferative synovitis (21.7%), neutrophilic pattern (20.2%), elongated cell pattern (10.1%), other moderate to marked abnormality (11.6%) and unsatisfactory (3.9%). Cytologic abnormalities that were not restricted to a single category included spindle cells, crystals, stellate cells and cartilage fragments. Multinucleate cells and mononucleate cells with dense cytoplasm and a delicate periphery were seen in smears from cases with clinical diagnoses of osteochondrosis or fracture; interpretation of these cells as osteoclasts and their mononucleate precursors was supported by positive staining with tartrate-resistant acid phosphatase. Smears within the same cytologic category were not found to correspond with a single clinical diagnosis. The identification of several cytologic patterns in cases with the same clinical diagnosis suggests that multiple stages of disease were sampled. Except in cases with the cytologic neutrophilic pattern, there was not a consistent relationship between the histologic features in synovial biopsy specimens and the cytologic findings; the morphologic variation within synovial membrane sections and between sections from different locations was sometimes marked. When compared with air-dried, Wright-Giemsa-stained smears, the polychrome-stained smears were more sensitive in the detection of cytologic abnormalities and were less often falsely negative or unsatisfactory. Following surgery, cases with clinical diagnoses of osteochondrosis (29 cases) and fracture (25 cases) were analyzed according to clinical outcome and cytologic category. While 80% of the horses with proliferative synovitis in cytologic specimens were sound, only 67% of those with the elongated cell pattern, 50% of those with slight abnormality and 33% of those with other moderate to marked abnormality were sound. A statistically significant relationship (P less than .02) was found in cases with a diagnosis of osteochondrosis: animals with a proliferative synovitis pattern were almost three times as likely to be sound as compared to those with slight abnormality. These findings indicate that polychrome-stained equine synovial fluid smears (1) provide information that is different from that found in corresponding histologic sections and (2) are superior to air-dried, Wright-Giemsa-stained smears for cytologic examination. The polychrome-stained equine synovial fluid smears were found to provide information supportive of clinical, radiographic and prognostic data.