Synovial fluid (SF) may contain a number of crystals that optical microscopy is unable to identify with certainty. Scanning electron microscopy (SEM) was utilized in this study to characterize SF crystals in the context of knee osteoarthritis (OA).
SF was collected from the knees of 25 patients with OA and examined under optical light microscopy. Calcium pyrophosphate dihydrate (CPPD) crystals were assessed by means of compensated polarized light microscopy, while alizarin red S staining was performed to identify apatite (BCP) crystals. All the specimens were also analyzed by SEM and x-ray diffractometry, as gold standards.
CPPD crystals were found in 32% and BCP in 24% of the SF examined by SEM. The degree of concordance between polarized light microscopy and SEM was 0.83 for CPPD and 0.46 for BCP (kappa statistic). The secondary and backscatter electron SEM observations allowed identification of silicon dioxide (SiO2) crystals in 8 out of 10 patients in whom polarized light microscopy revealed irregular and polymorph crystals.
SiO2 crystals cannot be readily identified by their morphology or polarization properties under optical microscopy. Their presence, nevertheless, did not lead to misclassification.