Synovial fluid features and their relations to osteoarthritis severity: new findings from sequential studies

Osteoarthritis Cartilage. 2003 Jan;11(1):50-4. doi: 10.1053/joca.2002.0861.


Objective: Many factors are involved in the osteoarthritic process. It is not yet known which are initiators, promoters or simply results. Thus, we have evaluated some of those potentially important factors in osteoarthritis (OA) as observed sequentially for the first time in synovial fluids.

Design: Synovial fluids (SF) obtained between 1992-2002 were all routinely evaluated for gross appearance, leukocyte counts and microscopic examination of wet drop preparations. We used regular and polarized light and alizarin red s stains. We separated out all OA patients, then we looked for patients who had more than two synovial fluid analyses to get sequential information. Time between first and final aspiration ranged from 2 to 7 (3.6+/-1.6) years and number of analyses per patients from 3 to 6 (3.3+/-0.7). We related synovial fluid crystals, fibrils and white blood cell count (WBC) to age, sex, disease duration and radiographic assessment according to the Kellgren-Lawrence radiographic rating system.

Results: Of 4523 synovial fluid examinations, we found 855 in patients with knee OA; 330 patients with adequate clinical details for comparison were included in our study. Twenty-six patients (one woman and 25 men) had sequentially examined SF. We found that 52% of those OA patients with effusions studied had crystals identified in their synovial fluid. Twenty-one percent of all the patients had CPPD crystals, 47% had hydroxyapatite, also called basic calcium phosphate (BCP) crystals and 16% had both types of crystals. Microscopically identifiable fibrils were found in 60% of SF. In sequentially examined patients, CPPD crystals and apatite (BCP) were found in 19% and 23%, respectively, at the first aspiration and, in 34% and 58% at the final aspiration. Fibrils were seen in 54% at first examination and 85% later. Apatite and fibrils showed more significant correlation with time (r=0.51,r =0.92) than did CPPD (r=0.32). SF WBC correlated only with CPPD crystals and did not increase with OA duration or severity. CPPD, apatite and fibrils all correlated with higher radiographic grades of OA.

Conclusions: As noted before CPPD and apatite crystals were more common in patients with more severe OA. New findings are that our sequential cases showed that there were some patients with no crystals at onset but that crystals appeared with progression of the disease. Fibril presence in SF also correlated with progression of the disease.

MeSH terms

  • Aged
  • Apatites / analysis*
  • Calcium Pyrophosphate / analysis*
  • Crystallization
  • Durapatite / analysis
  • Female
  • Humans
  • Male
  • Middle Aged
  • Osteoarthritis, Knee / etiology*
  • Osteoarthritis, Knee / metabolism
  • Synovial Fluid / chemistry*


  • Apatites
  • Durapatite
  • Calcium Pyrophosphate