Facial Nerve Preservation for Supraorbital Approaches: Anatomical Mapping Based on Consistent Landmarks

Abstract

Background: The supraorbital keyhole approach (SOKA) provides a safe and advantageous access to the anterior cranial fossa. The implemented skin incision varies depending on surgeon's preferences and requirements. Facial nerve (FN) injury might appear in up to 5.6% of patients. There is a lack of validated tenets for avoiding FN injury.

Objective: To define a safety area for FN preservation during a SOKA.

Methods: Ten dried skulls and 5 injected cadaveric heads (10 sides) were used. A Cartesian frame was created with its horizontal axis at the level of the supraorbital notch and the vertical axis just lateral to the frontozygomatic junction (FZj). FNs were dissected and points along their course were registered and transferred to the Cartesian frame.

Results: Ten microscopic dissections of the FN were performed preserving all branches. A safety area could be defined 8 mm superior and 10 mm inferior to the FZj extending medially to the supraorbital notch and beyond. A 20 mm2 area superior and lateral to the FZj provided low probability (≤10%) of injuring the FN. Similarly, starting 4 mm inferior to the FZj, a lateral safety area was also found. A probabilistic colored heat map was built to represent the results.

Conclusion: We provide a "safety zone" for a SOKA incision in which the probability to encounter the FN is low. Clinical studies following our method may validate our findings and add evidence to the tenets for minimizing morbidity related to the SOKA incision.

Keywords: Facial nerve; Frontozygomatic junction; Keyhole; Landmark; Mapping; Safety Area; Supraorbital notch.

FIGURE 1.

A, A Cartesian frame was defined with an abscisae axis (X) just below the SOn and an ordinate axis (Y) just lateral to the FZj. B, Distances from the axis origin to the SOn (a) and to the FZj (b) were measured in 10 dried skulls and 5 specimens to establish an averaged Cartesian frame in which to transfer FN coordinates.

FIGURE 2.

A and B, FN cadaveric dissections. B, The “+” sign represents the entrance of the FN in the FO. The yellow “*” indicate the interconnections between the temporal and zygomatic branches of the FN. The dual innervation pattern of the OO is illustrated in this image: the black arrow points the entrance of one branch from the zygomatic division. Temporal branches piercing the OO are signaled with the empty arrows. FO: frontooccipitalis muscle; OO: orbicularis occuli muscle; t: temporal branch of the FN; z: zygomatic branch of the FN.

FIGURE 3.

The diagram represents the Cartesian frame in which the registered points of the FN were pooled. A, Registered points prior to the normalization process. B, Overlapping of the individual safety areas (amber) prior to the normalization process. C, Registered points after the normalization process. Each color indicates the specimen number and side.

FIGURE 4.

A, Colored heat map representing the probability to find the FN in a given area in 10% tiers. B, Artistic representation based on the interpretation of results.

FIGURE 5.

A, Colored heat map and safety area in indigo blue. B, Safety area overlayed onto a picure of one of FN dissections. C, Suggested incisions within the safety zone. The black dotted line represents a typical eyebrow incision. The amber dotted lines depict the possible extensions of the former incision that may be needed to widen the approach by exposing the pterion. D, Skin incision design within the suggested safety area in a patient in whom the SOn and FZj have been marked.