Endoscopic Anatomic Study Via Drilling Petrous Ridge To The Middle Fossa In The Retromastoid Keyhole Approach

Objective: This study aimed to observe the anatomical structures in the middle fossa which can be exposed after the suprameatal tubercle and petrous apex removal in the Retromastoid keyhole approach by using neuroendoscope and measure the distance between important anatomical structures in this approach. We also investigate the range of exposure, indications and feasibility of this approach and provide anatomical basis for clinical application.Method: Simulated surgery via etromastoid keyhole approach with assist from rigid neuroendoscope(4mm rod, from Rohtec,China) was performed on eight adult cadaver heads(Sixteen sides) fixed by formalin. A vertical skin incision of approximately 4cm was created, which located 2 cm posterior to the mastoidale. The first hole should be drilled at the position of asterion. The keyhole is 2.5 cm in diameter and the upper edge of the keyhole should reach the transverse sinus. Then expose the intersection of the sigmoid sinus and the transverse sinus by drilling part of mastoid base. The cerebral dura mater was opened through Semi-circular shape and suspended to the edge of bone window. The endoscope, through the subdural space, was used to observe the anatomic marks of the cerebello pontine angle(CPA) after pushing the Cerebellum back gently. A incision on the tentorium cerebelli, 0.5~1.0 cm away from petrous ridge, was performed to expose oculomotor nerve, posterior cerebral artery(PCA). Drill the suprameatal tubercle and petrous apex between the facial nerve and the trigeminal nerve to measure and analysise the important anatomical structures in the middle fossa through this approach.Result: 1. The vital anatomic landmarks which have anatomic location in this approach, including asterion, intersection of the sigmoid sinus and the transverse sinus, facial nerve, trigeminal nerve, trochlear, suprameatal tubercle, petrous ridge and Meckel’s cave. By using endoscope, the exposure range of this approach includes CPA, brainstem ventrolateral, incisure of tentorium of cerebellum, cavemous sinus lateral wall and petroclival region. lateral cerebellum, cavemous sinus lateral wall and petroclival region. 2. The exposed anatomical structures in the middle fossa include: trigeminal ganglion, cavernous segment of trochlear and abducent nerve, petroclinoid segment oculomotor, posterior curve segment of internal carotid artery(ICA) in cavernous sinus, communicating segment of ICA, posterior communicating artery(PCo A). 3.Measurement results: ①The distances from the posterior border of junction of lateral sinus and sigmoid sinus to Meckel sack opening, Dorello canal, internal auditory pore, foramen auditoria intema, glossopharyngeal nerve, hypoglossal nerve are(48.03±2.34)mm 、(55.41±2.76)mm 、(36.90±2.10)mm、(37.39±3.02)mm、(47.71±3.08)mm. ②The distances from the midpoint of posterior border of sigmoid sinust to above points are:(45.35±2.35)mm 、(53.42±2.98)mm、(32.49±2.55)mm、(34.59±2.39)mm、(41.25±2.80)mm. ③The increase length of trigeminal nerve after suprameatal tubercle and petrous apex removal is(8.13±1.51)mm. The length of all sides of the bone window connecting the posterior and middle fossae are:(6.91±1.34)mm、(15.49±2.24)mm、(13.44±2.91)mm、(21.24±2.22)mm. The distances from the midpoint of posterior border of sigmoid sinust to suprameatal tubercle, trigeminal ganglion, posterior curve segment of ICA are:(34.42±2.14)mm,(54.52±2.87)mm,(65.15±3.13)mm. ④ The length of all sides of Parkinson triangle are:(18.97±2.93)mm,(16.23±2.02)mm,(8.04±2.34)mm..Conclusions: A wide surgical area including the middle cranial fossa, lateral wall of the cavernous sinus, suprasellar region, brainstem ventrolateral, incisure of tentorium of cerebellum, the middle and inferior petroclival region, was exposed via the retromastoid keyhole approach by using neuroendoscope. Through the natural gap and grinding the suprameatal tubercle and petrous apex, it is able to treat lesions in these areas.