Objective: The posterolateral extradural suboccipital approach can be used to reach
the anterior epidural space and the retro-odontoid regions. The extent of necessary
bone removal of the atlas vertebra (C1) has not yet been defined. We studied the changes
in the size of the horizontal and vertical surgical windows using stepwise bone removal
of C1. A representative case is shown. Methods: The anatomical study was performed
bilaterally on five Thiel-fixed human cadavers (mean age, 83.7 years). The surgical
window (horizontal x vertical) required to access the retro-odontoid region via a
posterolateral approach was measured for an intact C1 posterior arch, after a semicircular
inferior partial resection of the C1 arch, after resection of the unilateral hemiarch
of C1, and finally after drilling approximately 3 mm from the medial aspect of the
lateral mass of C1. Results: The intact C1 resulted in a very narrow surgical window
of 6.3 mm x 9.7 mm (horizontal x vertical). The vertical window increased to a 13
mm after the semicircular inferior partial resection of the C1 arch and to 17.3 nun
in the case of removal of the ipsilateral C1 posterior arch. The bone removal from
the medial aspect of the C1 lateral mass resulted in a widening of the horizontal
surgical window to 10.3 mm. The final size of the surgical window was 10.3 mm x 17.3
mm. The patient with severe kyphoscoliosis of the craniocervical spine was successfully
operated on using odontoid and C1-2 facet osteotomies. Conclusion: If only the anterior
epidural space or the base of the odontoid needs to be reached, the semicircular inferior
partial resection of the C1 arch allows for an adequate surgical window. The tip of
the odontoid could only be reached if the ipsilateral posterior arch is resected.
