**3.2 US-guided injection technique**

All sonographic examinations and injections were performed by one expert physician with over 10 years of experience in the musculoskeletal US and managing knee OA. In the preparation, the subjects were placed in the supine position with the knee flexed 5-10 degrees. After marking the PAB area, the skin was prepped with the usual products. US assessment was performed on the medial part of the knee using a linear array probe with a 6–12-MHz frequency and penetration depths of 2.5–4 cm. The needle was inserted at the point where the PA crossed over the anterior fibers of the MCL. The transducer was positioned in a longitudinal orientation relative to the anterior fibers of the MCL, with an oblique transverse orientation relative to the PA. Under US guidance, a 25-gauge 38-mm needle was inserted through the skin proximal to the transducer in a longitudinal plane. Continuously advanced in the distal direction and introduced the needle tip into the tissue space between the PA and the MCL. When the needle tip was visualized between the middle of the PA and the MCL, the materials were injected under direct sonographic visualization (**Figures 2** and **3**).
