What is a reliable landmark for tracing the facial nerve in parotidectomy?

Facial Nerve Landmarks in Parotidectomy

Primary Recommendation

The posterior belly of the digastric muscle (PBDM) is the most reliable and consistent landmark for identifying the facial nerve trunk during parotidectomy, with the nerve located approximately 15-20 mm superior and medial to the PBDM insertion. 1

Anatomical Landmarks for Facial Nerve Identification

Posterior Belly of Digastric Muscle (Primary Landmark)

• The PBDM provides the most consistent and reliable approach to the facial nerve trunk (FNT), with the nerve positioned 15-20 mm (mean 17 ± 0.87 mm) superior and medial to the muscle's insertion. 1
• This landmark demonstrated 100% success rate in FNT identification across multiple studies with no cases of facial nerve paresis when used properly. 1
• The PBDM is particularly advantageous because it remains consistently identifiable even in revision surgery where other anatomical planes may be distorted by scarring. 1

Supplementary Landmarks

Tragal Pointer:

• The FNT emerges from the stylomastoid foramen and can be located using the tragal pointer as a secondary reference point. 2
• This landmark is most useful when combined with the PBDM approach rather than used in isolation. 3

Parotid-Mastoid Fascia:

• The parotid fascia spans from the mastoid tip and tragal pointer to the parotid gland in an open-book fashion, with the FNT lying directly medial to this fascial layer. 4
• Once this fascia is removed, the facial nerve trunk is immediately identified beneath it. 4
• The fascia can be easily palpated intraoperatively and provides an additional depth landmark. 4

Borle's Triangle:

• This anatomical triangle is formed by the intersection of three imaginary lines along anatomical structures, with the FNT located approximately 12.18 ± 1.7 mm from one of the triangle's angles. 3
• Successfully identified the FNT in all cases studied, though it requires visualization of multiple anatomical structures simultaneously. 3

Posterior Auricular Nerve:

• Can be traced from the auricularis posterior muscle to the FNT with an average distance of 28 mm (±6.2 mm). 5
• The angle between the posterior auricular nerve and the vertical segment of the FNT is approximately 39.5° (±7.7°). 5
• This approach is particularly valuable in revision surgery where pre-auricular anatomy has been distorted by previous operations. 5
• Manipulation of this nerve carries minimal risk since the auricularis posterior muscle is vestigial. 5

Surgical Approach Based on Tumor Characteristics

Low-Grade, Early-Stage Tumors (T1-T2)

• Perform partial superficial parotidectomy with facial nerve preservation when preoperative nerve function is intact. 2
• Facial nerve preservation is strongly recommended when a dissection plane can be created between the tumor and nerve. 6

High-Grade or Advanced Tumors (T3-T4)

• Perform at least superficial parotidectomy with consideration of total/subtotal parotidectomy due to risk of intraparotid nodal metastases. 2
• Facial nerve branches should only be resected when they are encased or grossly involved by confirmed malignancy, or when preoperative facial nerve weakness is present. 6

Critical Pitfalls and Caveats

Vascular Variations:

• Be aware that the stylomastoid artery can rarely fenestrate the main trunk of the facial nerve, dividing it in two. 7
• This anatomic variant increases the difficulty of facial nerve dissection and requires careful identification before proceeding. 7
• When encountered, the stylomastoid artery should be divided to allow proper nerve dissection. 7

Avoid Isolated Landmark Use:

• Substantial variations exist in distances measured from anatomic landmarks to the FNT when used in isolation. 3
• Combining multiple landmarks (PBDM + tragal pointer + parotid-mastoid fascia) provides the safest approach to nerve identification. 1, 4

Quality of Life Considerations:

• Facial nerve sacrifice results in significant morbidity and should not be based on indeterminate preoperative or intraoperative diagnoses alone. 2
• Even in advanced tumors, facial nerve preservation when technically feasible does not compromise survival outcomes when combined with adjuvant radiation therapy. 6
• Intraoperative frozen section has 98.5% sensitivity and 99% specificity but should not be the sole basis for facial nerve sacrifice. 2