The frontal branch of the facial nerve: can we define a safety zone?

PURPOSE: The temporal branch of the facial nerve, a particularly important branch in facial expression, is commonly exposed to surgical trauma. The frontal branch is the most important branch of the temporal branch in the clinical point of view. However, it does not really define in the international nomenclature. The objective of this study was to clearly identify this branch, to perform a cartography of the crossing areas of this branch; and therefore to define statistically a zone of safety within the fronto-temporal region.
METHOD: We used 12 fresh cadavers to perform 24 facial nerve dissections. After the identification of the facial nerve, the branches of the temporofacial trunk were identified, dissected and followed till their penetration. We measured the relationship of the frontal branch with the zygomatic arch, temporal vessels and lateral border of the orbit. We conducted a statistical study to assess the risk of injury of this branch within the temporal region.
RESULTS: We observed an important variability in the distribution of this branch in the temporal region. We defined three zones of decreasing safety at the level of three interest landmarks: at the level of the inferior part of the zygomatic arch, we estimated an elevated risk of nerve injury (>85%) from 22.6 to 26.06 mm in front of the tragus; at the level of the superior part of the zygomatic arch, we estimated an elevated risk of nerve injury (>85%) from 27.46 to 30.43 mm in front of the tragus; at the level of the lateral border of the orbit, we estimated an elevated risk of nerve injury (>85%) from 16.20 to 19.17 mm behind this landmark.
CONCLUSIONS: There exists no real area of anatomical safety in the temporal region. It seems, however, possible to define areas of relative safety that would be of great help for the surgeon or the morphologist wishing to approach pathologies of this region.