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A 27-year-old female with Down's presents with neck pain, progressive gait instability, and loss of fine motor dexterity in her hands. Flexion and extension radiographs are shown in Figure A and B and demonstrate occipitocervical instability. When performing an occipitocervical fusion, what location in Figure C is most appropriate for placement of an 8mm unicortical screw?
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The safe zone for screw placement in the occiput for occipitocervical fusion is in a triangular region created by connecting 2 dots 2cm lateral to the EOP and a point 2 cm inferior to the EOP. Point B in Figure C falls within this safe zone, as is the most appropriate place for placement of an 8mm unicortical screw.
The thickest region of the occiput is at a point 5 cm lateral to the EOP. However, this point is too far lateral to be useful for occipital-cervical fusion, and is instead where you place the posterior inferior pin of a Mayfield holder.
Ebraheim et al measured the thickness and quality of occipital bone regions to determine screw placement during occipitocervical fusion. They concluded that 8-mm screws could be safely inserted in the region extending 2 cm laterally from the center of the external occipital protuberance, 1 cm from the midline at a level 1 cm inferior to the external occipital protuberance, and 0.5 cm from the midline at a level 2 cm inferior to the external occipital protuberance. The authors also demonstrated that the major dural venous sinuses are situated immediately beneath the this region of the occiput and are at risk of penetrative injury. Therefore, screws should be unicortical in nature.
Nadim et al examined the safe zone for placement of occipital screws without endangering the sinuses. They used angiography in cadaveric skulls, and found the transverse sinus usually rest at the level of the EOP. They therefore recommend against bicortical screws in this region due to risk of injury to the transverse sinus.
Illustration A shows the safe zone in relation to the Figure C in the question stem. Illustration B shows the safe zone as defined by Ebraheim et al. Illustration C shows the safe zone in relationship to skull diameter determined by Heyworth et al. Illustration D shows the relationship of the transverse sinus to the safe zone from an intra-cranial view.
Nadim Y, Lu J, Sabry FF, Ebraheim N
Orthopedics. 2000 Jul;23(7):717-9. PMID: 10917248 (Link to Abstract)
Nadim, ORTHO 2000
Ebraheim NA, Lu J, Biyani A, Brown JA, Yeasting RA
Spine. 1996 Aug;21(15):1725-9; discussion 1729-30. PMID: 8855456 (Link to Abstract)
Ebraheim, SPINE 1996
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Odontoid and Cranio-Cervical Controversies was presented by Rick C. Sasso, M.D....