Maximal scapular protraction increases the safety distance of the spinal accessory nerve during lower trapezius...

The lower trapezius (LT) transfer is a reliable option for treating irreparable posterosuperior rotator cuff tears. The procedure restores external rotation by reproducing the line of pull of the posterosuperior rotator cuff. Despite its proven clinical benefits, the spinal accessory nerve (SAN) remains at risk during LT harvest due to its close relationship with the medial border of the scapula. Moreover, accurate separation of the LT from the middle trapezius (MT) is crucial to obtain the full muscle excursion and optimal mechanical vector of the harvested portion of the muscle. The first aim of this study is to quantify the change in SAN-scapula distance between neutral and maximal scapular protraction positions, and the second aim is to identify a consistent anatomical landmark to differentiate the LT from the MT during tendon harvest.

Ten fresh-frozen human torsos (20 shoulders) were dissected. Each specimen was positioned in a beach-chair setup at 70°, and the LT and MT insertions were exposed through a posterior approach. The SAN was identified medial to the scapular border and marked. Distances between the SAN and the medial border of the scapula spine were measured in 2 standardized positions by 2 observers and compared: neutral (0° forward flexion, 20° abduction) and maximal protraction (90° forward flexion, 20° adduction). To identify a reproducible landmark distinguishing the LT from the MT, a straight line was drawn from the trapezius tubercle toward the vertebral spinous process, and its vertebral intersection was recorded. For comparison, another line following the axis of the scapular spine was traced to evaluate which orientation excluded proximal LT fibers.

The mean SAN-scapula distance measured 20.4 ± 2.3 mm in the neutral position and increased to 31.8 ± 2.8 mm in maximal protraction, demonstrating a mean increase of 11.4 mm (95% confidence interval, 11.0-11.9; P < .001). The SAN was located at a minimum of 17 mm from the medial border of the scapula in neutral position. The reference line from the trapezius tubercle to the vertebral column consistently intersected T3-T4, corresponding to the upper origin of the LT.

Maximal scapular protraction increases the distance between the SAN and the medial border of the scapula, providing a greater margin of safety during LT tendon harvest. In addition, a straight line from the trapezius tubercle to the vertebral spine facilitates clear differentiation of LT fibers from the MT.