The supraodontoid space or "apical cave" at the craniocervical junction: a microdissection study

The extradural supraodontoid space lies anteriorly at the craniocervical junction (CCJ) between the alar ligaments and foramen magnum. It occupies the space between the tectorial and atlanto-occipital membranes. A variety of benign and traumatic lesions may result in neurological compression here with harmful effects. Decompression by the transoral surgical approach often provides relief from these effects. Knowledge of the detailed microanatomy of this space is fragmentary. The purpose of this study was to identify the boundaries and contents of this space by microdissection. Twenty-three en bloc preserved adult cadaveric specimens of the CCJ were dissected to identify the boundaries and contents of the supraodontoid space. The posterior bony elements of the CCJ were removed to enable microdissection (Zeiss DXE Microscope 4-40x) from the tectorial membrane (TM) forwards. The cave-like space faced posteriorly. It had a roof which extended into a wall (anterior atlanto-occipital membrane), a floor (superior surface of the alar ligament), and a mouth covered by the TM. The apical ligament and a thin lining membranous fatty layer divided the cave into a pair of symmetrical halves. The contents, from dorsal to ventral, lay deep to a thin subtectorial membrane. These were the superior fasciculus of the cruciate ligament, a fat-ensheathed knot of plexiform veins (which communicated with the surrounding CCJ vertebral venous plexuses), an arterial arcade between the veins, a pair of fat pads, and branches of the sinuvertebral nerves of the CCJ (lying on the floor). No synovial membrane was found. Knowledge of the anatomy of the apical cave may be of some assistance in transoral (extra- and transdural) surgical approaches to the anterior CCJ region.