Background/Purpose: Published studies indicate clinicians may miss the intrasynovial knee space during therapeutic injections in excess of 30% of attempts using landmark guided techniques due to small synovial fluid volumes or inexperience of the operator. The purpose of this study was to determine if the fluid volume in the knee available for aspiration or injection could be increased by pneumatic displacement into a target area confirmed using ultrasound imaging.

Methods: 40 knees from 37 consecutive patients from an academic rheumatology practice were studied. All of these patients previously had a medial (27) or lateral (13) knee aspiration or therapeutic injection using a pneumatic compression device. Ultrasound (US) examination was performed during inflation to 100 mmHg and then during deflation. The hook and loop secured device encircled the knee except for a 6 x 9 cm target area available for a US probe or needle placement and was inflated with a hand held manometer. Live images from a GE LOGIQ Book XP ultrasound machine using a 12L-RS probe were recorded onto a Surface Pro laptop computer and analyzed blinded. The fluid compartment (anechoic region) images were analyzed using a histogram based thresholding program which identified the area and linear distance of the anechoic fluid compartment during inflation and deflation.

Results: The study cohort included 28 female and 9 male patients with the following diagnoses: 25 RA, 8 OA, 2 Psoriatic arthritis, and 1 each with SLE and gout. The mean age was 58.6 years (range 23-86) and mean BMI was 26.9 kg/m²(range 19.1-35.6).  The mean increase during inflation vs deflation of the anechoic fluid area was 309% (range 14%-2,048%), p< 0.001, and the increase in linear distance during inflation was 182% (range 0%-552%), p<0.001 using paired T test analysis.  Since none of these patients required a joint aspiration or injection at the time of their study visits, the volume of fluid displaced during a clinically indicated joint aspiration or injection would have likely have been larger than reported in this study.

Conclusion: Quantitative digital image analysis of ultrasound recorded images demonstrate a significant increase in fluid displacement from the synovial space of the knee into a target area available for joint aspiration or injection during inflation to 100 mmHg of a pneumatic compression device. Pneumatic compression may facilitate more successful knee aspirations or injections by increasing the fluid volume in the medial or lateral knee joint space available for needle access and increasing the target area for needle placement.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/ultrasound-quantification-of-fluid-shifts-in-the-knees-of-arthritis-patients-before-and-after-inflation-of-a-pneumatic-compressive-device/