Background Calcium pyrophosphate deposition disease (CPPD) and Osteoarthritis (OA) are frequently associated and CPPD with OA is recognized as a clinical subtype . However, the differences in pathogenetic, microscopic and clinical aspects between the two diseases are not clear and how CPPD and OA could affect each other is still a matter of debate.
Objectives To assess the differences between CPPD and OA in terms of anatomic alterations of the joint, evaluated with US, and characteristics of the synovial fluid (SF) of knees affected by CPPD and/or OA.
Methods consecutive patients reaching the outpatient clinic for the presence of knee pain and with any amount of joint effusion were eligible for the study. Patients with diagnosis or suspicion of chronic inflammatory rheumatic conditions were excluded. All enrolled patients underwent US of the knee for the assessment of joint effusion (JE), synovial hypertrophy (SH), synovial power Doppler (PD), femuro-tibial osteophytes (FTO) and alterations of the femoral hyaline cartilage (FHC), using a semiquantitative score (0: normal to 3: severe alteration) and finally a US guided aspiration of the SF. SF was examined by optical and polarized light microscopy to determine total and differential white blood cell (WBC) counts, and for crystal identification. The concentration of the main inorganic ions involved in CPP crystal formation (P2O74–, PO43–, Ca2+ and Mg2+) was assessed by fluorometric and colorimetric assays. CPP crystal detection was also used for the classification of patients. Further, in CPPD patients, a count of the CPP crystals detectable in a single slide was carried out. Depending on the variables, chi-square, Mann Whitney and Spearman Ro tests were used for statistical analysis.
Results 49 patients (28 women), mean age 70.29 yo (SD±10.93) were enrolled in the study; 23 subjects presented OA and 26 CPPD (23.07% acute arthritis, 77.6% CPPD with OA). At US, a statistically significant difference between CPPD and OA was found only for the grade of effusion, being more abundant in CPPD patients. On the contrary, no differences were found regarding SH, PD, FTO, FHC. SF analysis showed that CPPD patients presented a higher volume of SF, a higher total WBC count with a higher polymorphonuclear (PMN) cells percentage and lower monocytes percentage than patients with OA. Further, both total cell count and PMN percentage were positively correlated with the number of crystals in the SF. On the other hand, no statistically significant differences were found in the content of inorganic ions between the two groups.
Conclusions According to these results, patients with CPPD and OA present some distinct features, mainly regarding the characteristics of the SF, compared to patients with OA alone. These differences may reflect different underlying pathogenetic pathways for the two diseases. Surprisingly, the concentration of inorganic ions in the two populations was similar.Further studies are necessary in order to better understand the link between CPPD and OA and the role of ions concentration in the SF for the formation of crystals.
Zhang W et al. European League Against Rheumatism reccomandations for calcium pyrophosphate deposition. Pat I: terminology and diagnosis. Ann Rheum Dis 2011; 70: 563–570.
Disclosure of Interest None declared