Intraoperative navigation and imaging
1. Introduction
Indications
Secondary orbital correction can benefit from virtual planning of the procedure and intraoperative navigation to ensure proper execution of the plan.
Whenever an intraoperative CT scanner is available, an intraoperative scan should be obtained for intraoperative evaluation of the correction.
Any secondary orbital correction benefits from intraoperative CT scanning which can be fused to the virtual plan to determine the adequacy of the corrective procedure.
Introduction
The correction of secondary orbital deformities is more difficult than treating acute orbital defects. It therefore is helpful to use computer assisted techniques to plan and perform complex reconstructions.
The workflow is identical to the primary treatment of the same injury. For a detailed description please refer to the trauma section.
In secondary corrections it is recommended to perform an overcorrection of globe position resulting in approximately 2 mm of sagittal projection. This can be performed by placing additional material posterior to the equator of the globe at the lateral orbital wall.
Useful additional reading
2. Virtual planning
Ocular dystopia one year after conservative treatment of a blow-out fracture of the left orbit. The patient has double vision secondary to enophthalmos and hypophthalmos of the left eye.
Virtual reconstruction
Virtual reconstruction of orbital walls in unilateral trauma is achieved by first segmenting the unaffected orbit and mirroring it to the contralateral side.
After virtual reconstruction, the modified dataset serves as a virtual template that can be used for intraoperative navigation, intraoperative imaging and verification of correct reconstruction.
In this example, a complex posttraumatic deformity is visible. The defect extends from the anterior skull base along the medial wall to the lateral side of the orbital floor.
3. Orbital wall reconstruction using intraoperative navigation
Tracking of the patient's head is achieved with a dynamic reference frame (DRF) attached to the Mayfield clamp. Alternatively the DRF can be attached to cranial bone. A headband mounted DRF (headset) should be avoided due to soft tissue movement.
After reconstruction of the orbit is performed, the navigation pointer is placed on the surface of the orbital implant to correlate its position to the virtual reconstruction. The pointer is moved on the orbital implant surface and the different points are collected automatically by the navigation software.
This procedure has to be repeated after any changes in implant position.
4. Intraoperative assessment of reconstruction
Additionally an intraoperative cone beam CT scan may be performed using a 3D C-arm to verify that the orbit has been properly reconstructed.
Fusion of preoperative and intraoperative CT scans, and the virtual plan (green) allows verification of proper reconstruction. If software is not available for image fusion, the intraoperative CT scan can be qualitatively assessed to determine the adequacy of reconstruction.
In this case the correct anatomic shape of the titanium mesh used for orbital wall reconstruction could be verified in the intraoperative CT scan.
In secondary corrections it is recommended to perform an overcorrection of globe position resulting in approximately 2 mm of sagittal projection. This can be performed by placing additional material posterior to the equator of the globe at the lateral orbital wall.
Correlation of pre and intraoperative situation is performed in the coronal view...
...axial view...
...and sagittal view.
In a case where the orbital walls are not properly reconstructed, correction of shape and position of the implant is recommended followed by a second intraoperative CT-scan.
This technique can be used for all orbital wall reconstructions with radiopaque material.
