A fractal anterior digastric: a case report with surgical implications

Introduction The digastric muscle normally consists of a single posterior belly and single anterior belly joined by an intermediate tendon. The anterior belly originates from the digastric fossa of the mandible and inserts at the intermediate tendon to the body and greater horn of the hyoid, the same insertion as the posterior belly; however, the posterior belly originates at the mastoid notch of the temporal bone. Bilaterally, the digastric muscles function synergistically with the infrahyoid musculature to accomplish depression of the mandible against resistance as well as elevation and stabilization of the hyoid during deglutition and phonation [1]. Each belly of the muscle develops from different embryonic tissues; the first pharyngeal arch gives rise to the anterior belly while the second pharyngeal arch gives rise to the posterior belly [2]. Likewise, the anterior belly is innervated by the nerve to mylohyoid, a branch of the inferior alveolar nerve from the mandibular division of the trigeminal nerve (CN V3), while the posterior belly is innervated by the digastric (preparotid) branch of the main branch of the facial nerve (CN VII) [1]. Anatomical variations of the anterior belly have been demonstrated to occur unilaterally, bilaterally, symmetrically, and asymmetrically [3, 4]. Prior reports differ dramatically with regard to the frequency of variation among the anterior belly ranging from an incidence of 2.7% to 69.6% [5–8]. Case Report While attempting to remove the tongue per suprahyoid musculature in a 75-year-old Caucasian male cadaver, a bilateral asymmetrical variation of the anterior bellies of the digastric muscles was observed. On the right side, the anterior belly bifurcated as it arose from the intermediate tendon to produce an oblique accessory belly partially continuous with the posterior muscle fibers of the anterior belly. The majority of the oblique accessory belly was found to insert into the mylohyoid raphe and the investing fascia of the mylohyoid itself; however, a small portion of superior fibers gave rise to a sagittal accessory belly which joined the anterior belly at the ipsilateral digastric fossa of the mandible. The sagittal accessory belly was also composed of fibers originating from the mylohyoid raphe (Figure 1a). Upon reflection of the anterior, oblique, and sagittal muscle bellies, the posteroinferior part of sagittal belly was observed to give rise to a small muscular loop which had a medial part that coursed superolaterally and reflected to become the lateral part which traveled inferomedially, encircled posterior fibers of the oblique belly, and joined the muscular fibers of the mylohyoid. The reflection of the muscular loop was supported by a fascial sling arising from the junction of the anterior and sagittal bellies (Figure 1b). It appeared that the digastric muscle gave rise to a smaller “digastric” muscle which then gave rise Matthew J. ZDILLA [1] Hannah J. SOLONINKA [1] H. Wayne LAMBERT [2]