Anatomical Study of the Biceps Sheath for Safe and Accurate Unguided Anterior Shoulder Injections

The tendon of the long head of the biceps brachii (LHBT) contributes to shoulder joint stability, but can become a source of shoulder pain because of trauma or degeneration. Injection of local anesthetics into the biceps sheath (BS) is an effective treatment for managing anterior shoulder pain and is widely performed clinically; however, the accuracy of unguided injections remains low. This study aimed to clarify the anatomical characteristics of the BS and identify the optimal injection site to improve the accuracy and safety of unguided anterior shoulder injections. Eight 5% formalin-fixed cadavers (16 shoulders) and 1 Thiel-embalmed cadaver were examined at Kurume University School of Medicine. Morphological measurements and histological evaluations were performed on cadaveric shoulders. In addition, ultrasound examinations were performed on 28 shoulders from 14 healthy adults. Vascular distribution around the BS was evaluated using latex injection in the Thiel-embalmed cadaver. The mean BS length in cadavers was 36.2 ± 8.4 mm, and the mean width was 9.4 ± 1.2 mm; these were not significantly different than those in live subjects. Macroscopic observations confirmed continuity between the BS and the shoulder joint capsule. The anterior circumflex humeral artery ran along the lateral aspect of the LHBT, while the transverse humeral ligament was located approximately 23 mm proximal to the BS. The BS is continuous with the joint capsule, and injections into the BS allow intra-articular delivery of medication. The optimal injection site is approximately 1.5 transverse fingerbreadths distal to the superomedial edge of the greater tubercle and medial to the LHBT. This location minimizes the risk of injuring the anterior circumflex humeral artery and the extended tendon of the subscapularis muscle while improving the accuracy of needle placement.