Object. The effect of craniovertebral decompression surgery on CSF flow dynamics in Chiari I patients has been incompletely characterized. We used computational fluid dynamics to calculate the effect of decompression surgery on CSF flow dynamics in the posterior fossa and upper cervical spinal canal.

Methods. Oscillatory flow was simulated in idealized 3D models of the normal adult and the Chiari I subarachnoid spaces (both previously described) and in three models of the Chiari malformation post cranio-vertebral decompressions. The three postoperative models were created from the Chiari model by virtually modifying the Chiari model subarachnoid space to simulate surgical decompressions of different magnitudes. Velocities and pressures were computed with the Navier-Stokes equations in Star-CD for multiple cycles of CSF flow oscillating at 80 cycles per minute. Pressure gradients and velocities were compared for 8 levels extending from the posterior fossa to the C3/4 level. Relative pressures and peak velocities were plotted by level from the posterior fossa to C3/4. The heterogeneity of flow velocity distribution around the spinal cord was compared between models.

Results. Peak systolic velocities were generally lower in the post-operative models, than in the pre-operative Chiari model. The two larger surgical defects had peak systolic velocities that were closer to normal model velocities than the smallest surgical defect and equal to those in the normal model at the C3 and C4 levels. For the smallest defect, peak velocities were decreased, but not to levels in the normal model. In the postoperative models, heterogeneity in flow velocity distribution around the spinal cord increased from normal model levels as the degree of decompression increased. Pressures in the 5 models differed in magnitude and in pattern. Pressures gradients along the spinal canal in the normal and Chiari models were non-linear with steeper gradients below C3/4 than above. The Chiari model had a steeper pressure gradient than the normal model above C3/4 and the same gradient below. The postoperative models had lower pressure gradients than the Chiari model above C2/3. The most conservative decompression had smaller than normal model pressure gradients above C2/3 The two larger decompression defects had CSF pressure gradients below those in the normal model above C2/3. These two models had a less steep gradient above C3 and a steeper gradient below.

Conclusions. In computer simulations, cranio-vertebral surgical defects generally diminished CSF velocities and CSF pressures.