Introduction: Trigeminal neuralgia is a serious condition resulting in lancinating, episodic facial pain. Percutaneous stereotactic radiofrequency rhizotomy is frequently used to treat trigeminal neuralgia. A thorough understanding of the microsurgical anatomy of the foramen ovale and surrounding structures is required for efficient, effective and safe use of this technique. This study was undertaken to define the anatomical variations of the foramen ovale (FO) and to assess cannulation difficulty, as well as the potential risk of injury to surrounding structures.

Methods: Bilateral foramina from 174 adult human dry skulls (348 foramina) were analyzed in both an anatomic and surgical orientation using photographs obtained from standardized projections. Measurements were obtained of multiple variables including shape, size, presence of adjacent structures, and the morphometric variability effect on cannulation.

Results: From the anatomic exocranial view, 6 distinct anatomical shapes of the foramen ovale were identified as well as 5 anomalous variants. From the surgical view, 6 distinct procedural shapes were identified. When the surface area (SA) of the FO was measured in the surgical view, there was a significant loss of SA compared to the anatomic exocranial view. On average, the SA lost was 18.5 5.7%. Morphometrically, we found significant variability in foramen size, and obstruction of the foramen in up to 7.8% from a calcified pterygoalar ligament. Importantly, we found that 8% of foramina were very difficult to cannulate and the risk of inadvertent cannulation of the foramen lacerum was 12%.

Conclusions: We determined that there is significant variability in the shape and size of the FO, which we believe impacts the ability to safely and effectively cannulate this structure. Preoperative imaging with a 3D head CT may be of value in predicting difficulty of cannulation and guide treatment decisions when considering the use of a percutaneous approach over microvascular decompression or radiosurgery.

Patient Care: Our research helps improve patient care by highlighting the need to study pre-operative anatomy for each individual patient and consider neuronavigation in patients with variable skull base anatomy in order to prevent complications and improve efficacy of treatment when performing a percutaneous stereotactic radiofrequency or balloon rhizotomy.

Learning Objectives: By the conclusion of this session, participants should be able to: 1) Recognize that there are variables sizes and shapes of the foramen ovale that must be recognized as not all patients may be good candidates for PSR based on variable anatomy and 2) Understand that there are multiple danger/high risk zones of injury during PSR or balloon rhizotomy and that anatomic variations of surrounding neurovascular structures are important to recognize when cannulating the foramen ovale to prevent complications

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