Microanatomical Study On The Retrosigmoid Keyhole Approach

Objective:A microanatomical study of the retrosigmoid keyhole approach was performed to observe the related anatomical structure, exposure and operating spaces through this approach, in order to provide anatomical reference for the clinical application of the approach. For further study, use the anatomical research to discuss the indication and clinical value of the approach.Methods:The main arteries and veins of 15 cases of the Chinese adult cadaver's heads were perfused with lactoprene and fixed on the headstock. Simulating operation technique, a retrosigmoid keyhole craniotomy was made and the bone window was controlled within 2.5×2.0 cm. Twenty-four sides of 15 cadaver heads were dissected under 4 to 24 magnification via operating microscope. The cerebellopontine cistern was entered to expose the area and the neurovascular structures. Using microscope with varying angles of view, observe and analyze the related anatomical variation, and then measure and record related parameters. Combine with clinical operations and other approach the clinical application of the retrosigmoid keyhole approach was assessed.Results:The retrosigmoid keyhole approach exposes nearly the same anatomic region as that of conventional approach. It can expose the area superior to the anteriolateral margin of the tentorium, inferior to the foramen Magnum and the jugular tubercle, medial to the anteriolateral of pons and medulla. Using microscope with varying angles of view, retrosigmoid keyhole approach can get a clear visualization of petrosal vein, superior cerebellar artery(SCA)or its branches, the anteroinferior cerebellar artery(AICA)and its branches, the posteroinferior cerebellar artery(PICA), trochlear, trigeminal nerve, abducens, facial, and vestibuloco-chlear nerves, the glossopharyngeal, vagus, accessory, and hypoglossal nerves. SCA of 11 specimens(37%) contacted with trigeminal nerve. Of the 30 SPSs examined in 15 cadaver heads, the SPV emptied into the sinus as a single venous stem in 24 sides and as two stems in the remaining 6.22.2% of SPV emptied into the SPS above or lateral to the internal acoustic meatus (IAM),63.8% of the SPV emptied into the SPS between the lateral limit of the trigeminal nerve and the medial limit of the IAM.13.9 %the SPV emptied into the SPS at a point medial to the lateral limit of the trigeminal nerve at its site of entry into the Meckel cave. AICAs were the compressing vessel involved in 23 (77%) of 30 cases, in 14 cases, the AICAs pass between the facial and vestibulocochlear nerves. Retrosigmoid keyhole approach can provide ample exposure of the posterior fossa cranial nerves and vascular structures, but was also influenced by some bone structures. The internal auditory meatal and jugular foramen also can be clearly explored. The suprameatal tubercle was developed variably in modality, but in all specimens it obstructed exposure of Meckel's cave under microscopic, also the jugular tubercle restricted visualization of the anterior foramen magnum. The basilar artery was not observed well in most of the specimens using the approach. With higher magnification the endoscope can achieve a clear visualization of bloodvessels and nerves, it also have the ability to look around corners and behind anatomic structures, which is not possible with a microscope.Conclusion:Retrosigmoid keyhole approach is the classical keyhole operation technique. The most accurate location and size of bone flap is much fit to the need of operation, which reduces the unnecessary cut of the scalp and muscle, reduces the unnecessary craniectomy and exploration of brain tissue, The keyhole approach through making the best of intracranial nature spaces, not only minimize the brain trauma and operation complication, but also have the advantage of less operational time, less wound response, more quick recover, better tissue and appearance restoration. Through choosing individualized bone-window location, the retrosigmoid keyhole approach can exposure the cranial nerves and vascular structures of relatived area of the posterior fossa, have wide application in the operation of the extramedullary lesions involved cerebellopontine angle, upper petroclivus, middle petroclivus, lower clivus, such as trigeminal neurinomas, hemifacial spasm, cholesteatoma, coustic neuroma, meningeoma. The retrosigmoid keyhole approach quest for minimally invading, it is in conformance with the contemporary concept of minimally invasive surgery. Clinical practice proves that it is a safe and effective technique, is a alternative replacement for the traditional retrosigmoid approach.