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Background Ultrasound-guided intra-articular injection of the shoulder via the rotator cuff interval (RCI) is increasingly used for adhesive capsulitis, but conventional techniques may be technically demanding and potentially risk iatrogenic rotator cuff tendon injury.Objective To describe and validate, in cadaveric specimens, a novel long-axis RCI injection technique aligned with the intra-articular portion of the long head of the biceps tendon.Methods Six shoulders from three cryopreserved cadavers were evaluated. Under ultrasound guidance, a 22-gauge needle was advanced in-plane along the long axis of the biceps tendon, targeting the interval between the coracohumeral ligament and intra-articular biceps tendon. Injectate spread was confirmed by dissection.Results All six shoulders demonstrated accurate intra-articular injectate spread without intratendinous injection or unintended bursal accumulation.Conclusion The long-axis RCI approach provides reliable intra-articular delivery with improved visualization and fewer traversed structures. Clinical studies are needed to establish its safety, reproducibility, and therapeutic outcomes.