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P120: MICROSURGICAL PATHWAYS TO ANTEROLATERAL PONS AND UPPER CEREBELLOPONTINE CISTERN: UTILIZING SUPRATRIGEMINAL AND INFRATRIGEMINAL TRIANGLES
Kivanc Yangi, MD; Egemen Gok; Arturo Luna Arroyo, MD; Michael T Lawton, MD; Mark C Preul, MD; Barrow Neurological Institute

Introduction: Anatomical triangles can serve as useful landmarks guiding neurosurgeons to deep-seated targets. Among these, the supratrigeminal (STT) and infratrigeminal (ITT) triangles are yet to be explored in detail. As corridors to the anterolateral pons and upper cerebellopontine cistern, STT and ITT may provide direct access to peritrigeminal safe entry zones (SEZs). This study offers descriptive and quantitative analyses of STT and ITT, assessing their boundaries, expandability, and surgical exposure to optimize their use in microsurgical approaches.

Methods: Five formalin-fixed (10 sides), latex-injected cadaveric heads underwent dissection using extended retrosigmoid craniotomy and transcerebellopontine angle approach. Neuronavigation-based measurements were used to measure the triangle dimensions under standard and expanded exposures (i.e., on the brainstem surface). Statistical analysis, using R software (v.4.4.3), compared standard versus expanded brainstem exposures. Two additional cadaver heads were used to illustrate the pertinent brainstem anatomy. Ultrahigh-resolution 7-Tesla MRI and 3D-modeling were also employed to visualize the associated white matter tracts.

Results: STT is bounded medially by the cerebellar quadrangular lobule (QBL), superiorly by the tentorium, and inferiorly by the upper margin of the trigeminal nerve, with mean edge lengths of 14.3(4.7)mm, 11.1(1.6)mm, and 13.0(3.3)mm, respectively. Standard brainstem exposure through STT measured 64.6(27.6)mm². Retraction of individual borders significantly increased the area: tentorium to 108.4(33.1)mm² (p<0.001), QBL to 96.6(36.6) mm² (p<0.001), and CN5 to 92.3(37.8)mm² (p < 0.001). Combined retraction of all three edges yielded a maximal theoretical exposure of 168.1(48.9)mm² (p<0.001). Surgical freedom volume, referenced to the supratrigeminal SEZ, was 20.4(5.0)cm³. ITT, bounded posteriorly by QBL, superiorly by lower edge of CN5, inferiorly by superior vestibular nerve (SVN), and anteriorly by petrous bone, had mean edge lengths of 14.9(7.7)mm, 17.6(4.0) mm, 8.8(3.3)mm, and 6.8(4.0)mm, respectively. Standard brainstem exposure through ITT measured 73.9(15.2) mm² and increased significantly with border retraction, reaching 87.5(17.5) mm² after CN5 retraction (p<0.001), 113.9(22.4) mm² after SVN retraction (p<0.001), and 96.0(22.4)mm² after QBL retraction (p<0.001). Combined retraction of all borders yielded a maximal potential exposure of 149.6(30.9)mm² (p<0.001). Surgical freedom volume, referenced to infratrigeminal SEZ, was 25.9(8.4)cm³. STT offers a favorable trajectory for superiorly located lesions in the cerebellopontine cistern and provides access to Meckel’s cave, the superior petrosal sinus, Dandy’s vein, the cerebellomesencephalic fissure, and the S3 segment of the superior cerebellar artery, whereas the ITT grants access to the porus trigeminus and porus acousticus.

Conclusion: STT and ITT are consistent and expandable microsurgical anatomic guides to the upper cerebellopontine cistern. Quantitative assessment of these triangles supports their role in surgical planning for peritrigeminal cavernous malformations (CMs). Together, these two triangles indicate the largest corridors to the anterolateral pons and provide direct access to the supratrigeminal and infratrigeminal SEZs, which are particularly valuable for approaching peritrigeminal pontine CMs and mixed basilar–peritrigeminal lesions.

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