Masseter muscle termination over the deep surface of the temporal fascia: look out the wrong path.

Purpose: The masticatory muscles have a common embryological origin. Despite numerous anatomical studies their close anatomical relationships are not always accurately described in anatomical treatises. An expansion of the masseter muscle inserted into the deep surface of the superficial temporal aponeurosis has been described. Despite the classical description of two transition bundles (Yoshikawa et al. in Kaibogaku Zasshi 37:206-217, 1962), the literature concerning these fibres has remained difficult to interpret. Given the clinical applications of the lengthening of the temporal muscle in myoplasty, it is important that we have accurate anatomical knowledge of it.

Methods: We dissected 14 embalmed or untreated anatomical cadaver heads with a sex ratio of 1 in order to analyse the aforementioned fibres and their variations. A radiologic study allowed us to understand the entanglement of the fibres. 10 craniofacial MRI were processed by 3D MPR reconstruction.

Results: The fibres were identified as an expansion of the posterior head of the masseter muscle and distinct from the two classically described transition bundles. These were present in all subjects of both sexes. We can systematically describe their origin, trajectory and termination.

Conclusion: Fibres belonging to masseter muscle pass up to the zygomatic arch and terminate on the deep surface of the temporal fascia. Awareness of the nature and trajectory of these muscular fibres allows us to avoid taking "wrong path" when approaching the temporal muscle tendon on the coronoid process via the temporal fossa.