Background: The deltoid muscle (DM) plays an essential role in retaining the stability and correct function of the upper limb. The aims of the study were to perform a detailed morphological analysis of the DM including its innervation, structure, attachments and relationship with adjacent structures.

Materials and methods: The study was carried out on 17 formalin-fixed cadavericupper limbs. After dissection of the shoulders, the DM was visualised and analysed.The following measurements of the muscle were performed for all cases: width of attachments (acromial, clavicular, spinal), entire width of origin, length of the component parts (acromial, clavicular, and spinal) and length of the arm.

Results: In all specimens, a characteristic ‘segmented’ innervation scheme of the DM was observed. The axillary nerve (AN) was always divided into an anterior branch (abAN) and a posterior branch (pbAN). Two variations of the DM innervation were distinguished: variation I, where the clavicular and the acromial parts were innervated by the abAN, while the spinal part was supplied both by abAN (anterior fibres) and by pbAN (posterior fibres), and variation II, in which the spinal part did not have double innervation — the abAN innervation area covered only the acromial and clavicular parts, and the entire spinal part was supplied by pbAN. Both variations had a segmented arrangement of sub-branches reaching individual parts of the DM, which was particularly distinct in the clavicular and acromial parts. Correlations were found between the entire width of the DM originand the length of the arm (p = 0.001), between the length of the acromial part of the DM and the length of the arm (p = 0.003), between the width of the spinal attachment and the length of the spinal part (p = 0.002), and between the width of the spinal attachment and the length of the arm (p = 0.0008).

Conclusions: The study confirmed the existence of a characteristic segmented innervation scheme of the DM which corresponds with the segmented morphology of its individual parts. An analysis of the internal structure of the muscle specific architectonics based on the tendon system was also presented.