Unexpected Intramedullary Canal Fill During Ultrasound-Guided Subchondral Intraosseous Platelet-Rich Plasma Injection: A Case Report.

Ultrasound (US)-guided subchondral intraosseous (IO) injection is increasingly utilized for treating knee osteoarthritis (OA) associated with bone marrow lesions (BMLs). However, accidental intramedullary misplacement ("canal fill") during this procedure is a rarely documented complication. We report an 83-year-old female patient with painful knee OA and medial compartment BML who underwent US-guided subchondral IO platelet-rich plasma (PRP) injection. Due to osteoporosis, the initial needle insertion inadvertently penetrated deeper into the intramedullary canal of the tibia. Fluoroscopy revealed intramedullary contrast medium spread, correlating with a compact, ball-shaped Doppler hotspot under US Power Doppler Imaging (PDI). Needle repositioning towards the subchondral bone resulted in a typical thin, sheet-like Doppler blush pattern beneath the cortical bone. Subsequent PRP IO injections provided significant symptomatic relief after one month, reducing pain from a visual analog scale (VAS) score of 7 to 2. Recognizing US features indicative of intramedullary IO misplacement is crucial. Real-time US imaging, specifically observing Doppler distribution patterns, can prevent inadvertent intramedullary injections, ensuring precise subchondral delivery and optimal therapeutic outcomes.