CAS: virtual planning and intraoperative navigation (ORIF with orbital reconstruction)

When using computer assisted surgery (CAS), reduction (and fixation if necessary) is performed according to standard procedures described in the AO Surgery Reference. CAS should be considered an adjunct to surgical treatment.

When treating fractures CAS allows virtual preoperative planning of the desired reconstruction using preoperative CT scans and appropriate software.

Intraoperative imaging combined with image fusion of preoperative and intraoperative CT scans and the virtual plan allows verification of proper reduction.

When dealing with complex fractures, intraoperative navigation is extremely useful in reducing the zygoma and correctly reconstructing the orbital walls. Intraoperative corrections of fragment reduction and orbital wall reconstruction are often necessary. Rather than performing repeated intraoperative CT scans, radiation dose-free intraoperative navigation provides intraoperative guidance for the virtually planned reconstruction.

In complex orbital floor fractures, radiopaque material for orbital wall reconstruction can easily be visualized with imaging, allowing intraoperative or postoperative verification of proper reconstruction.

With this technique, insufficient fracture reduction can be identified and corrected, eliminating the need for secondary procedures that may be necessary if only postoperative imaging is performed.

Intraoperative imaging requires an additional 10–15 minutes.

Reconstruction of complex orbital wall defects may benefit from preoperative virtual insertion of anatomically preformed orbital implants.

Some complex zygomatico-orbital defects may require patient-specific implants for alloplastic reconstruction of orbital walls. This can be achieved by transferring virtual preoperative planning into CAD-CAM tools to create patient-specific alloplastic implants, or pre-bending standard implants using stereolithographic models either from the patients' anatomy or from the virtually preplanned reconstruction.

• An introduction to computer assisted surgery
• Intraoperative navigation: operating room setup
• Intraoperative navigation: cone beam vs fan beam
• Preparation for intraoperative navigation

The preoperative CT scan shows a severely displaced fracture of the entire midface. Complex fractures of both orbits are also present.

Proper preoperative examination of CT scans requires multiplanar views consisting of axial, coronal, sagittal, and 3D-reconstruction.

Titanium screws (used as fiducial markers) must be inserted into the skull before CT scanning to facilitate intraoperative navigation. In a case where the maxilla is not fractured, fiducial markers can be fixed to dental splints.

Skin fiducials or laser surface scanning are not applicable in craniofacial trauma due to soft-tissue changes (swelling).

The first step in preoperative planning is the anatomically correct orientation of the data set according to the patient's anatomy.

The second step for virtual zygomatico-orbital reconstruction is auto segmentation of temporal bones, zygoma, and orbits.

This procedure is performed by the software using auto segmentation algorithms. The result can be manually changed to correspond with the patient’s anatomy.

Virtual simulation of zygoma reduction and orbital wall reconstruction is performed by mirroring the unaffected contralateral zygoma and orbit after auto segmentation or by manually moving the affected segments.

As shown here, virtual bending and shaping of the segmented areas allows virtual orbital reconstruction in bilateral cases.

After virtual reconstruction, the modified dataset serves as a virtual template that can be used for intraoperative navigation, intraoperative imaging, and postoperative verification of the reduction/reconstruction.

The patient's head is tracked with a dynamic reference frame (DRF) attached to the cranial bone. Alternatively, the DRF can be attached to a Mayfield clamp. Headsets should be avoided due to soft-tissue movement.

Registration of the DRF to the fiducial markers in the preoperative CT scan is performed using the navigation software.

Pointer-based infrared navigation provides radiation-free verification of the reduction of the displaced zygoma according to the virtual template created during the planning.

After the reduction of the zygoma (using standard techniques), the tip of the pointer is placed on the malar prominence to correlate the actual zygoma position with the contours of the virtual template via frameless stereotaxy.

This procedure must be repeated after any changes in the zygoma position.

Stable fixation with miniplates follows the successful reduction of the zygoma.

The navigation pointer is placed on the surface of the orbital implant to correlate its position with the virtual reconstruction and verify satisfactory orbital wall reconstruction.

This procedure must be repeated after any changes of implant position.

After fusing the preoperative CT scan (gold) with the virtual template (purple and green) and intra- or postoperative CT scan (blue), the quality of orbital wall reconstruction and zygoma reduction can be assessed.

Note that the post-reconstruction left zygoma and orbit image fit perfectly to the plan (purple).