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Objectives and questions: Joint replacement is a standard procedure in orthopedic surgery, but implant failure remains a challenge, often caused by aseptic loosening. Inflammatory reactions in aseptic loosening can result from wear particle release, making wear testing essential for new implants. Current wear testing standards use calf serum as a lubricant instead of synovial fluid (SF), yet calf serum is expensive and causes higher friction values. To develop a more suitable artificial SF, the understanding of the biochemical composition of native SF is crucial. However, SF composition varies considerably in the literature, particularly regarding changes following revision surgeries. This study analyzes SF from knee and hip joints, identifying differences between these joint types and changes associated with osteoarthritis (OA) and revision surgery.

Material and methods: SF samples from hip (n=30) and knee joints (n=33) were collected and analyzed. Patients’ age, implantation time, and OA severity (KL-Score) were recorded. The biochemical parameters examined included hyaluronic acid (HA), proteoglycan 4 (PRG4), albumin, total protein, and sulfated glycosaminoglycan (sGAG). Measurements were performed using ELISA for HA, Albumin, and PRG4, BCA for total protein, and Blyscan™ assays for sGAG quantification. Statistical analysis included one-way ANOVA with post-hoc tests for multiple comparisons.

Results: Significant differences in SF composition were found between knee and hip joints, as well as between OA and revision groups. Total protein content was significantly higher in hip OA (Mdn=45.9 g/L, 95%CI: 34.0–50.2) compared to knee OA (Mdn=30.0 g/L, 95%CI: 22.6–35.5,p=0.012). HA concentrations were higher in knee OA (mean ±SD: 1.9±0.6 g/L) than in hip OA (mean ±SD: 1.2±0.5 g/L,p=0.0289). Albumin levels were higher in hip OA (Mdn=28.9 g/L, 95%CI: 16.0–30.6) and hip revisions (Mdn=28.4 g/L, 95%CI: 22.1–31.8) compared to knee OA (Mdn=10.1 g/L, 95%CI: 1.2–16.4,p=0.0063) and knee revisions (Mdn=19.4 g/L, 95%CI: 6.7–23.5,p=0.002). PRG4 concentrations were higher in revision cases than in OA groups, though differences were not statistically significant (p>0.05). Furthermore, sGAG levels tended to be higher in hip groups than in knee groups, but also without statistical significance. Hip revision samples showed greater variability, whereas knee groups were more homogeneous.

Discussion and conclusions: The study confirms significant differences in SF composition between hip and knee joints. These findings suggest that results from knee samples cannot simply be transferred to hip joints. Future research should include healthy controls to establish baseline values and explore how these differences affect joint lubrication and implant performance.