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Anatomy and Physiology

Understanding the anatomy of the superficial temporal fascia, superficial temporal artery, temporal branch of the facial nerve, and the safe and dangerous zones occupied by these structures is crucial to perform a temporal artery biopsy safely.

Superficial Temporal Artery (Figures 1 & 2)

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The superficial temporal artery, one of the terminal branches of the external carotid artery, arises within the parotid gland. The superficial temporal artery courses superiorly in front of the tragus, immediately giving off an anterior branch, the middle temporal artery. The anterior auricular branch courses posteriorly and arises at the superior root of the helix; it then divides into the frontal and parietal branches, each of which has multiple secondary branches. The bifurcation of the anterior auricular artery into the frontal and parietal branches is anomalous; it is high in 73% of patients, intermediate or just below the apex of the helix in 20%, and low or just above the tragus in another 7%. This anomaly is of clinical importance as the risk of injury to the temporal branch of the facial nerve is higher in patients with a low bifurcation; the artery is closer to the nerve.

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Relationship of the Superficial Temporal Artery to the Temporal Facial Nerve (Figures 3 & 4)

The superficial temporal fascia, also known as the temporoparietal fascia, lies just deep to the subcutaneous adipose tissue of the scalp. This fascia is contiguous with the superficial musculoaponeurotic system (SMAS) of the face inferiorly and the occipitofrontalis-galea aponeurotica complex superiorly. Coursing through the superficial temporal fascia, the superficial temporal artery travels cephalad within the preauricular cheek before bifurcating into the frontal and parietal branches. The temporal branch of the facial nerve exits the parotid gland, travels over the middle third of the zygoma, then within the superficial temporal fascia in the temple region to reach the frontalis, corrugator supercilii, procerus, and orbicularis oculi muscles, which it innervates. Its course is approximated by the Pitanguy line, which runs from 0.5 cm inferior to the tragus to 1.5 cm superior to the lateral brow.[5] The distal rami of the temporal branch of the facial nerve always travel deep to the muscles they innervate. The superficial temporal artery lies within the superficial temporal fascia, while the temporal branch of the facial nerve runs slightly deeper but also within or on the deep surface of this tissue.

Safe Zone (Video 1)

The anterior temporal hairline is lateral to the frontalis muscle; therefore, dissecting superior or posterior to the anterior hairline during a temporal artery biopsy is unlikely to injure the temporal branch of the facial nerve. However, because there are no muscles of facial expression in the temple region, only the thin superficial temporal fascia, the temporal branch of the facial nerve is particularly susceptible to injury over the zygoma and within the temple region.

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Some authors define the danger zone for the temporal branch of the facial nerve by outlining a region bounded inferiorly by a line from the earlobe to the lateral eyebrow and superiorly by a line from the intertragal notch or earlobe to the lateral edge of the highest forehead crease. Other authors define up to 1.5 cm posterior to the lateral orbital rim and up to 1 cm anterior to the attachment of the helix along the level of the zygoma as safe zones to avoid nerve injury.[6][7][8][9] See the video for an example of how to mark the safe zone preoperatively, based on cadaver studies by Shin et al.[10]

Biopsy Site

Some authors recommend obtaining the biopsy specimen from the trunk of the superficial temporal artery, proximal to the division into frontal and parietal branches.[11] Others feel that this technique sacrifices an unacceptably large arterial zone. Branches of the superficial temporal artery are useful for perfusing local and regional craniofacial reconstruction flaps and transferring free flaps to repair large facial defects. For this reason, it may be more prudent to limit the extent of superficial temporal artery sacrifice to an anterior branch.

Other surgeons have suggested performing the biopsy on the parietal branch of the superficial temporal artery to eliminate the chance of injury to the temporal branch of the facial nerve.[12] The parietal branch of the artery travels subperiosteally after separating from the main trunk of the superficial temporal artery. The sensitivity and specificity of parietal branch specimens are not known. However, the parietal branch of the superficial temporal artery provides collateral circulation to the brain, which may warrant preservation.

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