6 - Anatomical Pattern Identification of the Temporal Branches of CNVII Using Cadaveric Dissection

Poster Board Number: 6
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Abstract Body : A novel, less invasive approach to temporomandibular joint replacement (TJR) surgery aims to reduce adverse outcomes by making only one incision and avoiding the transection of the masseter and medial pterygoid muscles. However, the results of a 2021 pilot study of this single endaural incision approach (SEIA) included additional adverse impacts that are novel to this procedure. Each patient experienced temporary upper facial paralysis due to damage to the temporal branch of the facial nerve. The topology and distribution of the branches of the facial nerve near the area where the new operation was performed is obscured by superficial tissues. Unintended impacts to the facial nerve might be prevented if reliable surface landmarks were available during the planning phase of the procedure.

The objective of this study is to identify patterns in the distribution of the CN VII branches near the TMJ that can be inferred from palpable landmarks. In particular, the most posterior temporal branch of the facial nerve was the focus of this study. The temporal branch(es) crosses the zygomatic arch anterior to the external auditory meatus in variably close proximity to the TMJ and inadvertent impacts to these nerves during a surgical procedure may well produce the post-operative functional deficits reported in the SEIA pilot study.

To identify the distribution of the facial nerve branches, cadaveric dissection was performed on 20 human donors at the Central Michigan University College of Medicine. The extracranial origin and circum-parotid distribution of peripheral branches of the nerve was identified during dissections and landmarked with pins, then documented with photographs that included a scale bar for subsequent photogrammetric analysis. Palpable bony landmarks near the TMJ and the described surgical field were identified and used as a framework to map the patterns in nerve branch distribution. In particular, the superior apex of the external auditory meatus (EAM) and the inferior protuberance of the zygomaticotemporal suture (ZTS) were found to be the most useful. The distance between these landmarks was segmented into five equally sized divisions in each donor, whereby each division represented one-fifth of the total length between these landmarks.

The most posterior intersection of a temporal branch and the zygomatic arch is critical and can be used to estimate the area of the nerve-free space anterior of the EAM. A Chi-squared test was used to confirm that the frequency of the location of this intersection across the predefined five divisions was not uniform (X2 = 46; p< 0.001), and the distance between the EAM and the point of this intersection was found to correlate with the length of the arch (r2 = 0.22; p< 0.001). The distance between the EAM and the most posterior intersection of a temporal nerve branch in any sampled donors was >20% of the total length of the zygomatic process, i.e., we did not observe a branch in the most posterior fifth of the length between these two palpable landmarks EAM and ZTS.

These results suggest that the palpable landmarks on the auditory and zygomatic components of the temporal bone can be used to predict the horizontal breadth of the space near the TMJ where no branches of the facial nerve occur. While a larger sample would add value to this dataset, the findings of this cadaveric study identified the “first-fifth” division along the zygomatic arch to be free of branches of the facial nerve. These results stand to benefit evolving surgical approaches that aim to access the TMJ region while mitigating the risk to the muscles of facial expression.