Variations in human trigeminal and facial nerve branches and foramina identified by dissection and microcomputed tomography

The aim of this study was to identify branches of the trigeminal and facial nerves relevant to surgical incisions and injections and the scalp block techniques in the frontotemporal region, and to determine their relationships with superficial vascular structures and bony landmarks. Half-heads from consenting embalmed donors (6 male, 2 female, mean age at death 78.4 years) were used in this study. Detailed dissection was carried out to identify the position of the auriculotemporal nerve (ATN) relative to the superior temporal artery (STA) and the facial nerve (FN) in six subjects (5 male, 1 female). The results provide a minimum safe distance of 5 mm between the STA and the frontotemporal branches of the FN at the level of the low edge of zygoma and 8mm between the low edge of zygoma and the FN trunk, providing a pre-auricular triangle of safety for incisions and injections. Variability between subjects was up to 60%. Microcomputed tomography (microCT) scans were taken from all eight subjects and the three-dimensional reconstructions were used to identify the supraorbital notch (SON), the zygomaticotemporal foramen (ZTF) and the zygomaticofacial foramen (ZFF). The volume and relative locations of these foramina were calculated for 5-8 subjects. The closest distance between ZTF and the FZS ranged from 9 to 21mm (26% variation); 3 subjects had a single ZTF while 5 subjects had two ZTF. The angle at the centre of the orbit between ZFF and the FZS ranged from 156 to 166 degrees (2.5% variation). These findings demonstrate that both traditional cadaveric dissection methods as well as contemporary microCT methods can be used to investigate the relative locations of nerves or their foramina in the human head. The findings provide anatomical considerations for fronto-temporal incisions and local anaesthesia.