Dorsoscapularis triangularis: embryological and phylogenetic characterization of a rare variation of trapezius

Abstract

The muscle trapezius shows considerable morphological diversity. Variations include an anomalous origin and complete or partial absence of the muscle. The present study reported, a hitherto undocumented complete bilateral absence of the cervical part of trapezius. Based on its peculiar origin and insertion, it was named dorsoscapularis triangularis. The embryological, phylogenetic and molecular basis of the anomaly was elucidated. Failure of cranial migration of the trapezius component of the branchial musculature anlage to gain attachment on the occipital bone, cervical spinous processes, ligamentum nuchae between 11 mm and 16 mm stage of the embryo, resulted in this anomaly. A surgeon operating on the head and neck region or a radiologist analyzing a magnetic resonance imaging of the cervical region would find the knowledge of this morphological variation of trapezius useful in making clinical decisions.

Keywords: HOX D4; Molecular; Morphology; Trapezius; Variation.

Fig. 1. Dissection of the back and neck showing absence of cervical part of trapezius. Dotted black line depicting the normal extent of the cervical part of trapezius. SC, splenius capitis; Tz, trapezius.

Fig. 2. Muscles undercover of trapezius seen after reflecting it laterally. LS, levator scapulae; Rh, rhomboides; Tz, trapezius.

Fig. 3. Diagrammatic representation of a 7 mm embryo showing the Anlage for sternocleidomastoid and trapezius lying ventral to the cranial myotomes. Modified from Keibel and Mall (1910) [8]. An, branchial muscle anlage for sternocleidomastoid and trapezius.

Fig. 4. Diagrammatic representation of an 11 mm embryo showing the splitting of the anlage into ventral sternocleidomastoid and dorsal trapezius. Modified from Keibel and Mall (1910) [8]. Sc, scapula; SCM, sternocleidomastoid; Tz, trapezius.

Fig. 5. Diagrammatic representation of a 16 mm embryo showing failure of cranial migration of the trapezial component to gain attachment on the cervical spinous processes, ligamentum nuchae and the occipital bone. Modified from Keibel and Mall (1910) [8]. De, deltoid; SC, splenius capitis; SCM, sternocleidomastoid; Tz, trapezius.