Decompression of the foramen magnum in achondroplasia deserves specific mention as the surgical technique differs from foramen magnum decompression for Chiari malformation and requires familiarity with the surgical anatomy unique to this condition (2).
Anatomical features
- Small foramen: The foramen magnum is narrow, and the vertebral arteries are closer to the midline and are at risk during surgery.
- Thick rim with keel: The rim of the foramen magnum is substantially thickened, and commonly there is a deep midline bony keel that extends up from the foramen magnum in the dorsal midline of the posterior fossa.
Decompression technique
- Thin rim with drill: The bone should be removed gradually by drilling back to the inner cortex rather than attempting to use rongeurs. The procedure should be performed under the operating microscope.
- Remove final egg shell piecemeal: Once the cortex is thinned to an “egg shell,” the bone can be lifted out piecemeal, decompressing the dura.
- Cut fibrous band dorsal to C1: A thick ligamentous band lies at the level of the foramen magnum. This should be removed by sharp dissection.
- Dural opening not always needed: Formal dural opening is not required if intraoperative ultrasound demonstrates CSF flow in front of and behind the brainstem (3). Dural opening is associated with a high rate of CSF leakage (2).
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