gms | German Medical Science

Questionnaire: Synovial fluid (SF) contains hyaluronan (HA), lubricin and surface-active phospholipids (SAPLs), which might contribute either independently or additively to boundary lubrication. Already several studies reported that the boundary lubrication system in osteoarthritic (OA) SF is impaired since decreased concentrations of HA and lubricin were found in OA SF with a reduced average molecular weight of HA.

In this line, we hypothesize that OA SF contains altered amounts of individual SAPL species which might contribute to cartilage destruction during OA. Thus, the aim of our present study is to determine whether the concentration and relative distribution of individual SAPL species is changed in OA SF compared to RA SF and whether these alterations are dependent on the stage of disease.

Methods: Before starting the experiments, approval by the ethical board of our university and the written informed consent of the patients were obtained. SF of OA and RA patients was aspirated from the knee joints of 57 OA and 19 RA patients during arthroscopy or knee replacement surgery. OA patients were classified into subgroups according to Kellgren/Lawrence (K/L) grades of radiographic severity (range 0-4) and to macroscopical appearance of cartilage surface according to Outerbridge. Lipids were extracted from cell- and microparticle-free SF using the method described by Bligh and Dyer (Bligh and Dyer, 1959). Known amounts of internal phospholipid standards were added. Phospholipids (PL) classes and their individual species were quantified using electrospray ionization tandem mass spectrometry (ESI-MS/MS). Urea concentrations in serum and SF were used to correct data obtained from ESI-MS/MS for dilution of SF due to possible effusion according to the method described by Kraus et. al. in 2002. Differences in PL concentrations were analyzed using two-way ANOVA followed by Bonferroni post-hoc test. A p-value of less than 0.05 was considered statistically significant. Statistical analyses were performed using the Prism 5.0 software (GraphPad Inc., USA).

Results and Conclusions: SF of OA and RA patients contain 11 different PL classes with phosphatidylcholine, phosphatidylinositol and sphingomyelin being the major PL present. From 339 individual PL species being analyzed, 199 different PL could be qualitatively and quantitatively determined in OA and RA SF with 24 PL species being altered. Less concentrations of 23 individual species were found in OA SF compared to RA SF. Only one specie from the phosphatidylserine class (PS 40:7) was found to be present in higher concentrations in OA SF compared to RA SF.

Our data show for the first time that there are significant differences in concentrations of individual PL species between OA and RA SF. Furthermore, our results indicate that the PL biosynthesis and release is dependent on the stage of the disease. Our research now focuses on those cells being responsible for PL production and its biosynthetic control.