ASSOCIATION BETWEEN SYNOVIAL-FLUID LEVELS OF INORGANIC PYROPHOSPHATE AND SHORT-TERM RADIOGRAPHIC OUTCOME OF KNEE OSTEOARTHRITIS

Objective-To test the hypothesis that high concentrations of extracell ular inorganic pyrophosphate (PPi), which associate with increased cel l synthesis and turnover in cartilage, may act as a marker for structu ral outcome in knee osteoarthritis (OA). Method-One hundred and thirty five consecutive patients referred to hospital with knee OA (59 men, 76 women; mean age 71 years, range 41-88) were followed prospectively for a median of 2.5 years (interquartile range 1.75-3.0). Synovial flu id (SF) aspirated at presentation (202 OA knees: 68 bilateral, 66 unil ateral) was assessed for PPi content by radiometric assay. Knee radiog raphs at presentation and at final review were assessed for change in global (Kellgren) and individual features (narrowing, osteophyte, scle rosis, cyst, attrition) of OA. Results-The median SF PPi level was 10. 5 mu mol (range 0.07-72.4). At baseline, high PPi was significantly as sociated with presence of calcium pyrophosphate crystals, chondrocalci nosis, and bone attrition. Radiographic change was observed in 164 kne es. High PPi levels were negatively associated with change in Kellgren and Lawrence grade, further narrowing, and increase in osteophyte, bu t positively associated with development of attrition. In the 68 patie nts from whom bilateral data were obtained, there was correlation betw een right and left knees for PPi levels, all baseline radiographic sco res, and changes in radiographic features. Multiple logistic regressio n analysis for PPi as a continuous variable (age, gender, and patient number included in model) showed a negative correlation with change in global Kellgren and Lawrence grade (odds ratio (OR) 0.97, 95% confide nce interval (CI) 0.95 to 0.99) and a positive correlation with attrit ion (OR 1.04, 95% CI 1.02 to 1.07). Conclusion-High SF levels of PPi a re associated with favourable radiographic outcome in terms of progres sive change in Kellgren grade. Such elevated PPi levels, however, may inhibit new bone formation and remodelling in knee OA.