Dissection to Expose the Trochlear Nerve. A, Superior view. The uncus has been retracted laterally to expose the tentorial edge and the trochlear nerve. The trochlear nerve runs anteriorly along the tentorial edge to pierce the dura at the level of the rostrolateral free edge of the tentorium posterior to the posterolateral margin of the oculomotor triangle. Two margins of the oculomotor triangle are formed by the anterior and posterior petroclinoidal dural folds that extend, respectively, from the anterior and posterior clinoid processes to the petrous apex. B, The tentorial edge has been retracted laterally to expose the trochlear nerve and its groove on the tentorium. The trochlear nerve entrenches itself progressively in a groove on the lower surface of the free tentorial edge. The groove is a smooth transition from the free cisternal segment to the definitive fixed dural entry point. C, Enlarged view of B. The dura overlying the oculomotor and trochlear nerves has been incised and retracted laterally to expose the oculomotor and trochlear cisterns. The oculomotor nerve has an impression at the dural penetration site. D, The right combined presigmoid approach to expose the upper cerebellopontine angle and petrous apex. The presigmoid dura is opened from the anterior jugular bulb toward the superior petrosal sinus with a cut made superior to the transverse sinus toward the superior petrosal sinus, which can then be ligated and cut. The tentorium is cut posterior to the superior petrosal sinus toward the tentorial edge, taking care to avoid the trochlear nerve. Retractors are then placed using the tentorium between the retractor blade and the temporal lobe to protect the underlying temporal lobe cortex. The trochlear nerve courses under the tentorium behind the superior petrosal sinus. (Images courtesy of AL Rhoton, Jr.)