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Aim: Complete arthrocentesis of the effusive knee ameliorates patient pain, reduces intra‐articular and intraosseous pressure, removes inflammatory cytokines, and has been shown to substantially improve the therapeutic outcomes of intra‐articular injections. However, conventional arthrocentesis incompletely decompresses the knee, leaving considerable residual synovial fluid in the intra‐articular space. The present study determined whether external pneumatic circumferential compression of the effusive knee permitted more successful arthrocentesis and complete joint decompression. Methods: Using a paired sample design, 50 consecutive effusive knees underwent conventional arthrocentesis and then arthrocentesis with pneumatic compression. Pneumatic compression was applied to the superior knee using a conventional thigh blood pressure cuff inflated to 100 mm Hg which compressed the suprapatellar bursa and patellofemoral joint, forcing fluid from the superior knee to the anterolateral portal where the fluid could be accessed. Arthrocentesis success and fluid yield in mL before and after pneumatic compression were determined. Results: Successful diagnostic arthrocentesis (≥3 mL) of the effusive knee was 82% (41/50) with conventional arthrocentesis and increased to 100% (50/50) with pneumatic compression (P =.001). Synovial fluid yields increased by 144% (19.8 ± 17.1 mL) with pneumatic compression (conventional arthrocentesis; 13.7 ± 16.4 mL, pneumatic compression: 33.4 ± 26.5 mL; 95% CI: 10.9 < 19.7 < 28.9 mL, P <.0001). Conclusions: Conventional arthrocentesis routinely does not fully decompress the effusive knee. External circumferential pneumatic compression markedly improves arthrocentesis success and fluid yield, and permits complete decompression of the effusive knee. Pneumatic compression of the effusive knee with a thigh blood pressure cuff is an inexpensive and widely available technique to improve arthrocentesis outcomes. [ABSTRACT FROM AUTHOR]

Introduction/objectives: We hypothesized that mechanical compression of the knee in rheumatoid arthritis (RA) would mobilize occult extractable fluid and improve arthrocentesis success. Methods: Sixty-seven consecutive knees with RA and 186 knees with OA and were included. Conventional arthrocentesis was performed and success and volume (milliliters) determined; the needle was left intraarticularly, and mechanical compression was applied with an elastomeric knee brace. Arthrocentesis was then resumed until fluid return ceased. Fluid was characterized as to volume and cell counts. Results: In the RA, knee mechanical compression decreased failed diagnostic arthrocentesis from 56.7% (38/67) to 26.9% (18/67) (− 47.4%, p = 0.003) and increased absolute arthrocentesis yield from 4.7 ± 10.3 ml to 9.8 ± 9.8 ml (108% increase, 95% CI − 8.5 < − 5.1 < − 1.7 p = 0.0038). Total extractable fluid yield was 96% greater in RA (9.8 ± 9.8 ml) than OA (5.0 ± 9.4 ml, p = 0.0008), and occult extractable fluid was 77% greater in RA than OA (RA 5.3 ± 8.7 ml, OA 3.0 ± 5.5 ml, p = 0.046). Large effusions versus small effusions in RA demonstrated increased neutrophils in synovial fluid (p = 0.04) but no difference in radiologic arthritis grade (p = 0.87). In contrast, large effusions versus small effusions in OA demonstrated no difference in neutrophils in synovial fluid (p = 0.87) but significant different radiologic arthritis grade (p = 0.04). Conclusion: Mechanical compression improves the success of diagnostic and therapeutic knee arthrocentesis in both RA and OA. Large effusions in RA are associated with increased neutrophil counts but not arthritis grade; in contrast, large effusions in OA are associated with more severe arthritis grades but not increased neutrophil counts. Key points • Mechanical compression of the painful knee improves arthrocentesis success and fluid yield in both rheumatoid arthritis and osteoarthritis. • The painful rheumatoid knee contains approximately 100% more fluid than the osteoarthritic knee. • Large effusions in the osteoarthritic knee are characterized by higher grades of mechanical destruction but not increased neutrophil counts. • In contrast, large effusions in the rheumatoid knee are characterized by higher synovial fluid neutrophil counts but not the grade of mechanical destruction, indicating different mechanisms of effusion formation in rheumatoid arthritis versus osteoarthritis. [ABSTRACT FROM AUTHOR]