Virtual planning and intraoperative navigation

When a STL model is useful to plan surgery and/or pre-adapt plates, virtual planning is necessary.

Secondary correction of facial deformities due to ballistic injuries can benefit from virtual planning of the procedure and intraoperative navigation to ensure proper execution of the plan.

General considerations about variety and special treatment guides for gunshot injuries are not explained in this section.

In the following we focus on the use of computer assisted surgery for secondary correction of gunshot injuries of the facial skeleton.

Gunshot injuries of the facial skeleton often involve multiple functional areas. In the case used to illustrate the principles of the computer assisted surgery, the gunshot caused a complex panfacial fracture with loss of the right globe and a bony defect of the posterior right mandible and the centro lateral right midface.

Previous reconstruction of the mandible and the right zygomatico orbital complex of unknown details had been performed three months previously.

• Introduction to CAS
• OR setup
• 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.

To facilitate intraoperative navigation, titanium screws have to be inserted as fiducial markers into the skull prior to CT scanning. In a case where the maxilla is not fractured, fiducial markers can be fixed to dental splints (arrows).

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

The first step in preoperative planning is anatomically correct orientation of the data set according to patients individual anatomy.

The preoperative 3D-CT scan shows a severe defect of the centro lateral right midface.

The second step for virtual reconstruction is autosegmentation of zygoma, orbits, nasal bone and zygoma .

Using autosegmentation algorithms, this procedure is performed by the software. The result can be virtually adjusted by the surgeon.

Virtual simulation of the midface reconstruction is performed by mirroring the unaffected right side after auto segmentation or by repositioning the segmented affected bones virtually.

Reconstruction of orbital walls in unilateral trauma is achieved by mirroring the contralateral side, virtually bending and shaping of the segmented areas allows virtual orbital reconstruction.

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.

Anatomic 3D models can be produced based on the original CT-scan data set and the modified dataset with virtual reconstruction as shown here.

Patient specific implants are produced based on the 3D model where the virtual reconstruction has been made. In this case standard titanium implants have been contoured and sized to fit the specific reconstruction.

Tracking of patients head is achieved with a dynamic reference frame (DRF) attached to the cranial bone. Alternatively the DRF can be attached to a Mayfield clamp.

Pointer based infrared navigation provides radiation dose-free placement of the titanium implants according to the virtual template created during the planning.

After positioning of the titanium implants, the tip of the pointer is moved over the implant surface to correlate actual zygoma position with the contours of the virtual template.

This procedure has to be repeated after any changes of the implant position.

The monitor screen shows the implant position (green) in correlation to the virtual reconstruction (orange).

A slight overcorrection after inserting the implants in secondary reconstructions is recommended.

Intraoperative imaging is used to control the reduction with a 3-D C-arm. No sterile draping of the device is needed. The time necessary for setup and scanning is approximately 10-15 minutes.

The correct anatomic shape of the titanium implants used for midface reconstruction can be verified with an intraoperative CT scan.

In case the implants are not properly positioned, correction of shape and position of the implant is recommended followed by a second intraoperative CT-scan.

This technique can be used for all reconstructions with radio opaque material.

In the postoperative 3D-CT scan, implant position for reconstruction of the zygomatico orbital complex can be assessed.