Orbital reconstruction, CAS: virtual planning and intraoperative navigation

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

When treating orbital fractures CAS allows intraoperative visualization of the reconstruction using intraoperative imaging combined with image fusion of preoperative and intraoperative CT scans.

Intraoperative imaging, combined with image fusion of preoperative and intraoperative CT scans, and the virtual plan, allow verification of proper reconstruction.

Intraoperative navigation is extremely useful for correctly reconstructing the orbit when dealing with a complex fracture. Rather than performing repeated intraoperative CT scans, radiation dose-free intraoperative navigation provides intraoperative guidance to the virtually planned reconstruction.

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

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

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

Some complex zygomatico-orbital defects may require patient specific implants for alloplastic reconstruction of the orbit. This can be performed by transferring virtual preoperative planning into CAD-CAM tools to create patient specific alloplastic implants or by pre-bending standard implants using stereolithographic models made from the patients' CT dataset or a virtually preplanned reconstruction dataset.

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

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

Fiducial markers must be included on the CT scan to facilitate intraoperative navigation. In a case where the maxilla is not fractured, fiducial markers can be fixed to dental splints. Alternatively, titanium bone screws can be inserted into the skull before CT scanning.

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

In the case shown here, fiducial markers attached to a dental splint were used.

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

For virtual orbital reconstruction, auto segmentation of the unaffected orbit is performed first and then mirrored to the affected side. Three-dimensional positioning of the mirrored segment allows for anatomically correct virtual orbital reconstruction.

In bilateral trauma, the contours of the auto-segmented orbits can be virtually modified to fit the patient’s anatomy (arrows).

Anatomically preformed titanium meshes help achieve the anatomically correct reconstruction of the medial wall and orbital floor fractures.

Virtual placement of the anatomically preformed titanium mesh allows preoperative selection of the correct implant size (large or small). The virtual orbital reconstruction (shown here by the green line) facilitates accurate three-dimensional positioning of the virtual implant.

After correct virtual positioning of the anatomically preformed orbital implant, areas to be trimmed are noted for later implant modification in the operating room.

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

Navigational guidance helps identify the proper anatomic structure, especially in the posterior orbit.

The screenshot on the left shows the pointer placed below the posterior ledge of the orbit, indicating a typical dissection mistake in the posterior orbit.

With navigational information, the surgeon can more readily identify the anatomy of the fracture.

The screenshot on the left shows the pointer placed on the posterior shelf of the orbit, indicating the correct position of the posterior end of the orbital floor reconstruction.

To verify satisfactory orbital wall reconstruction, the navigation pointer is placed on the surface of the orbital implant to correlate its position to the virtual reconstruction. The pointer is moved over the surface of the inserted implant, and the different points are collected automatically by the navigation software.

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

After detecting the surface of the inserted implant, the surgeon can check the position of the implant (visualized here by red spots) against the virtual orbital reconstruction (visualized here by a purple line). If malpositioned, the implant must be readjusted and its new position verified.

A CT scan is performed intraoperatively to verify that the orbit has been appropriately reconstructed.

The correct anatomic shape and position of the titanium mesh used for orbital wall reconstruction can be verified in the intraoperative CT scan. Due to the smaller radiation volumes used in intraoperative CT scans, the evaluation of symmetry is limited. Bigger scan volumes should be avoided due to higher radiation doses.

This case demonstrates preoperative 3D reconstruction, axial, coronal, sagittal CT scans views showing auto segmentation of the unaffected orbit mirrored to the affected side.

This allows for a correlation between the shape and position of the orbital implant and the ideal virtual orbital reconstruction (the virtual preoperative planning acts as a virtual template).

Note how the orbital implant perfectly fits the virtual plan (shown here in green).

This technique can be used for all orbital wall reconstructions with radiopaque implants.

If the orbital walls have not been adequately reconstructed, correction of shape and/or position of the implant is recommended, followed by navigational control or by a second intraoperative CT scan.

In the screenshot shown here on the left, the positioning of the titanium mesh does not match the contours of the virtual orbital reconstruction (shown by a green line). In this case the position of the implant was modified.

After the position of the implant was corrected, intraoperative imaging was used to verify anatomically correct orbital wall reconstruction.