Posterior fossa decompression for the Chiari I malformation includes removal of bone from the subocciput, the area of skull at, and above, the foramenmagnum. Traditionally, reconstruction of the skull (cranioplasty) has not been performed as part of the decompression surgery. While this has worked well for most patients, reconstructing the area of bony removal may provide two benefits.
One is cosmetic. Without skull reconstruction, many patients have a sunken defect in the suboccipital area. A second possible, and more important advantage, is the ability to attach the suboccipital muscles to the plate. Without reconstruction, these muscles scar to the dura and adjacent musculature reducing their effectiveness. The need for post-operative physical therapy is more likely. The long term effect of de-attachment on the cervical spine, while likely limited, is unknown.
From my evaluation of patients with previous posterior fossa decompressions, I have encountered a few who have undergone reconstruction of the bony defect. The materials used have varied from mesh prepared at the operating table to an unsuccessful attempt at using a bone filler.
In December 2004, I developed a craniectomy plate for posterior fossa reconstruction in collaboration with the W. Lorenz Company (now part of Biomet Microfixation ). This curved titanium plate is designed to cover a 3 cm wide cranial defect. Recently, a plate for 4 cm craniectomies has been developed.
Plate Application
Once the wound is opened and the subocciput and arch of the C1 are exposed, a craniectomy based at the foramen magnum is marked on the skull. Using a high-speed drill with a 5-round burr, the bone is slowly thinned to a residual shell. Recently, as the incision has become smaller, I have used the microscope for this step. The bone is usually thickest at the foramen and in the midline. Drilling is performed carefully to avoid injuring the dura.
It is important to create a 180-degree dorsal bony decompression at the foramen magnum. This requires special attention to the dorso-lateral bone. A small match-stick type bit is often helpful in thinning this bone.
The residual bony shell is then elevated with bone ronguers. The resulting craniectomy and C1 laminectomy are illustrated below.
A duraplasty is then performed as described in a previous post. The appropriate sized plate is selected and bent by hand into a gently curved arch. The plate is attached with 5 to 6 microscrews, 4 to 5 mm in length. The longer screws are used at the cephalad end where the bone is thickest.
During wound closure, the suboccipital muscles are attached to the plate with 2-0 absorbable suture.
The image above is a post-operative 3D-CT showing the craniectomy plate in place. (The row of staples used in skin closure is seen at the left.)
Results
I have implanted 82 plates as part of a posterior fossa decompression for the Chiari I malformation. No plate dislodgements have been identified on follow-up MRI. One patient was struck in the occiput by the edge of car door. CT showed no plate dislodgement.
Overall, use of the plate has been straightforward. While the benefit of reattachment of the suboccipital muscles has not been proven, it makes sense.
Comments welcome.
Posted by
John Oro’, MD
Posterior fossa decompression for the Chiari I malformation includes removal of bone from the subocciput, the area of skull at, and above, the foramenmagnum. Traditionally, reconstruction of the skull (cranioplasty) has not been performed as part of the decompression surgery. While this has worked well for most patients, reconstructing the area of bony removal may provide two benefits.
One is cosmetic. Without skull reconstruction, many patients have a sunken defect in the suboccipital area. A second possible, and more important advantage, is the ability to attach the suboccipital muscles to the plate. Without reconstruction, these muscles scar to the dura and adjacent musculature reducing their effectiveness. The need for post-operative physical therapy is more likely. The long term effect of de-attachment on the cervical spine, while likely limited, is unknown.
From my evaluation of patients with previous posterior fossa decompressions, I have encountered a few who have undergone reconstruction of the bony defect. The materials used have varied from mesh prepared at the operating table to an unsuccessful attempt at using a bone filler.
In December 2004, I developed a craniectomy plate for posterior fossa reconstruction in collaboration with the W. Lorenz Company (now part of Biomet Microfixation ). This curved titanium plate is designed to cover a 3 cm wide cranial defect. Recently, a plate for 4 cm craniectomies has been developed.
Plate Application
Once the wound is opened and the subocciput and arch of the C1 are exposed, a craniectomy based at the foramen magnum is marked on the skull. Using a high-speed drill with a 5-round burr, the bone is slowly thinned to a residual shell. Recently, as the incision has become smaller, I have used the microscope for this step. The bone is usually thickest at the foramen and in the midline. Drilling is performed carefully to avoid injuring the dura.
It is important to create a 180-degree dorsal bony decompression at the foramen magnum. This requires special attention to the dorso-lateral bone. A small match-stick type bit is often helpful in thinning this bone.
The residual bony shell is then elevated with bone ronguers. The resulting craniectomy and C1 laminectomy are illustrated below.
A duraplasty is then performed as described in a previous post. The appropriate sized plate is selected and bent by hand into a gently curved arch. The plate is attached with 5 to 6 microscrews, 4 to 5 mm in length. The longer screws are used at the cephalad end where the bone is thickest.
During wound closure, the suboccipital muscles are attached to the plate with 2-0 absorbable suture.
The image above is a post-operative 3D-CT showing the craniectomy plate in place. (The row of staples used in skin closure is seen at the left.)
Results
I have implanted 82 plates as part of a posterior fossa decompression for the Chiari I malformation. No plate dislodgements have been identified on follow-up MRI. One patient was struck in the occiput by the edge of car door. CT showed no plate dislodgement.
Overall, use of the plate has been straightforward. While the benefit of reattachment of the suboccipital muscles has not been proven, it makes sense.
Comments welcome.
Posted by John Oro’, MD