Anatomic variations of the deltoid muscle insertion: a cadaveric study

The deltoid is a trisegmented muscle with anterior, middle, and posterior components. While the clinical relevance of the presence of anatomic variations of the deltoid origin and insertion continues to be debated, the architecture of the deltoid muscle is more complex than initially believed. This study aimed to evaluate the gross anatomy of the deltoid muscle insertion by qualitatively and quantitatively characterizing the insertion and location of the deltoid muscle's anterior, middle, and posterior components. This information is valuable to surgeons as it raises awareness of potential variants that could be encountered during surgery, promotes mindfulness of neurovascular proximities, and reduces the likelihood of confusion between adjacent muscle fibers.

Eight nonpaired, fresh-frozen clavicle-to-fingertip cadaveric shoulders were acquired for the study (6 left, 2 right). The average age of the cadavers was 79.5 years (range: 64-92). The standard deltopectoral approach was carried out on all specimens. The planes dividing the anterior, middle, and posterior deltoid were identified and marked. Once complete exposure had been achieved, digital calipers were used to record the size of the deltoid insertion. The specimens were qualitatively assessed to characterize the style of insertion they demonstrated.

The average length of the deltoid insertion was 39.45 ± 9.33 mm (n = 8). Six of the eight shoulders demonstrated an insertion style previously characterized in the literature. The remaining two shoulders highlighted an insertion pattern not previously described.

The current study demonstrates a novel insertion pattern for the deltoid muscle that has not been previously characterized. This "step-off" insertion pattern shows that the anterior, middle, and posterior tendons are inserted superior-medial, directly on, and inferior-lateral to the deltoid tuberosity and was found in 2/8 of our cadaveric specimens.