Background/Purpose: Calcium pyrophosphate (CPP) and basic calcium phosphate (BCP) crystals are frequent features of synovial fluid (SF) in patients with osteoarthritis (OA). Although these crystals have shown pro-inflammatory properties in vitro, their pathogenetic role is not completely clear. Our previous studies demonstrated that the presence of CPP in SF is associated with a mild inflammatory status and more advanced stages of the disease (Oliviero F, 2023; Frallonardo P, 2018). With this study we aimed to investigate SF cytokines and clinical indices of disease in a cohort of OA patients with or without CPP and BCP in their SF.

Methods: SF samples were collected from 40 patients with OA attending our outpatient clinic and analyzed as part of routine procedures. Patients were asked to complete the WOMAC and Lequesne questionnaires and a VAS scale for pain. OA severity was classified according to the Kellgren and Lawrence system. Total leucocytes, differential cell count and crystal search were performed in all the samples. CPP crystals were identified by polarized light microscopy (PLM) while BCP were assessed by Environmental Scanning Electron Microscopy (ESEM) and energy-dispersive X-ray (EDX) spectra, after samples centrifugation. Discarded SFs were stored for cytokine (IL-1, IL-18, IL-8, CCL23) determinations.

Results: ESEM analysis confirmed the presence of CPP in all SF positive at the PLM (n. 14) and in no additional samples. BCP have been found in 13 specimens, 7 of which were also positive to CPP. Given the low number of BCP+ SFs, we firstly compared the CPP+ and the CPP- groups. Patients with CPP showed higher WOMAC (p=0.04) and VAS pain (0.039). Although non-significant, IL-8 and CCL23 were higher in CPP+ samples. Examining data according to ESEM results, we found a significant distribution of CPP in the group of patients with a KL=3-4 and BCP in the group KL=1-2 (p=0.025) (Fig.1). Patients positive for both CPP and BCP had the worse clinical indices (WOMAC and pain 0.04). The levels of CLL23, which we demonstrated to be highly induced in vitro by CPP crystals (Baggio C, 2025), are similar in the groups CPP+ and CPP+BCP+, and decrease in the group BCP+, CPP-BCP-. IL-8 was higher in the group positive to both crystals but lower in the BCP+ group.

Conclusion: This study supports the concept that calcium crystals are not mere bystanders in OA, but rather play a pathogenetic role. CCL23 emerges as a new chemokine that warrants further exploration in OA research.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/the-pathogenetic-role-of-calcium-pyrophosphate-and-basic-calcium-phosphate-crystals-in-osteoarthritis-associations-with-synovial-fluid-cytokines-and-clinical-indices/