SURGICAL ANATOMY OF THE INFRATEMPORAL FOSSA - THE STYLOID DIAPHRAGM REVISITED

INTRODUCTION: The infratemporal fossa (ITF) gives passage to most majo r cerebral vessels and cranial nerves. Dissection of the ITF is essent ial in many of the lateral cranial base approaches and in exposure of the high cervical internal carotid artery (ICA). We reviewed the surgi cal anatomy of this region. METHODS: Direct foraminal measurements wer e made in seven dry skulls (14 sides), and the relationship of these f oramina to each other and various landmarks were determined. Ten ITF d issections were performed using a preauricular subtemporal-infratempor al approach. Preliminary dissections of the extracranial great vessels and structures larger than 1 cm were performed using standard macrosc opic surgical techniques. Dissection of all structures less than 1 cm was conducted using microsurgical techniques and instruments, includin g the operating microscope. The anatomic relationships of the muscles, nerves, arteries, and veins were carefully recorded, with special emp hasis regarding the relationship of these structures to the styloid di aphragm. The dissection was purely extradural. RESULTS: The styloid di aphragm was identified in all specimens. It divides the ITF into the p restyloid region and the retrostyloid region. The prestyloid region co ntains the parotid gland and associated structures, including the faci al nerve and external carotid artery. The retrostyloid region contains major vascular structures (ICA, internal jugular vein) and the initia l exocranial portion of the lower Cranial Nerves IX through XII. Landm arks were identified for the different cranial nerves. The bifurcation of the main trunk of the facial nerve was an average of 21 mm medial to the cartilaginous pointer and an average of 31 mm medial to the tra gus of the ear. The glossopharyngeal nerve was found posterior and lat eral to stylopharyngeus muscle in nine cases and medial in only one. T he vagus nerve was consistently found in the angle formed posteriorly by the ICA and the internal jugular vein. The spinal accessory nerve c rossed anterior to the internal jugular vein in five cases and posteri or in another five cases. It could be located as it entered the medial surface of the sternocleidomastoid muscle 28 mm (mean) below the mast oid tip. The hypoglossal nerve was most consistently identified as it crossed under the sternocleidomastoid branch of the occipital artery 2 5 mm posterior to the angle of the mandible and 52 mm anterior and inf erior to the mastoid tip. CONCLUSION: The styloid diaphragm divides th e ITF into prestyloid and retrostyloid regions and covers the high cer vical ICA. Using landmarks for the exocranial portion of the lower cra nial nerves is useful in identifying them and avoiding injury during a pproaches to the high cervical ICA, the upper cervical spine, and the ITF.