CAS: virtual planning and intraoperative navigation - Gunshots

Complex facial fractures, especially those that result from ballistic injuries, benefit from virtual surgical planning of the reconstructive procedure and intraoperative navigation.

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

Here, we focus on using computer assisted surgery to treat the facial skeleton's gunshot injuries.

Gunshot injuries of the facial skeleton often involve multiple functional areas. In the case used to illustrate computer assisted surgery principles, a self-inflicted gunshot caused a complex panfacial fracture with loss of the left globe. The projectile entered submentally and stopped subcutaneously over the left frontal bone, as seen in the 3D CT scan. In the clinical picture, an instrument is inserted into the bullet channel.

• An introduction to computer assisted surgery (CAS)
• 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.

If intraoperative navigation is needed, titanium screws should be inserted as fiducial markers into the skull bone or fixed to dental splints. Skin fiducials or laser surface scanning in craniofacial trauma are not applicable due to soft-tissue changes.

The first step in preoperative planning is the correct orientation of the data set to the natural head position.

The second step is the segmentation of anatomic regions of interest. This procedure is performed by the software using auto-segmentation algorithms.

Virtual simulation of frontal bone reconstruction and zygoma reduction is performed by mirroring the unaffected 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. In bilateral cases, virtually bending and shaping the segmented areas allows virtual orbital reconstruction.

After virtual reconstruction, the modified dataset serves as a virtual template that can be used for rapid prototyping/3D printing of stereolithographic models, intraoperative navigation, intraoperative imaging, and postoperative quality control.

Because the entire mandibular body is exposed, reduction and reconstruction with locking plates are performed as in a standard trauma case without intraoperative navigation. The reconstructed mandible is then used as a guide for the maxillary reconstruction by establishing the correct occlusion.

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

After the repositioning of bony segments (using standard techniques described in the trauma section), the tip of the pointer is placed on the surface of each segment to correlate its actual position with the contours of the virtual template.

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

Stable internal fixation is applied following correct positioning as described in the trauma section.

When the buttresses of the midface have been restored, the maxilla is reconstructed guided by the mandibular occlusion.

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.

The pointer is placed on the surface of the titanium mesh at the medial wall of the left orbit to correlate mesh position and virtual reconstruction with infrared navigation.

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

After confirmation of correct positioning, the mesh is fixed as described in the trauma section.

Reconstruction of the supraorbital rim and the frontal bone is performed with a 3D-titanium mesh. The bony fragments of the anterior wall of the frontal sinus are fixed to the titanium mesh with mini-screws. Positioning and shape are guided by intraoperative navigation as described above for orbital wall reconstruction. Medial canthopexy is performed using a barb wire fixed to the titanium mesh, as described in the trauma section.

After fusion of the virtual plan and postoperative CT scan, the quality of reconstruction can be assessed.

In this postoperative quality control, anatomically-correct shaping and positioning of the titanium mesh used to reconstruct the orbital floor and medial wall can be seen in the left orbit. Similarly, the reconstruction of the frontal bone and the supraorbital rim can be assessed.

The comparison of pre- and postoperative 3D reconstruction of the postoperative CT scan gives an overview of the panfacial reduction and reconstruction. The symmetry of the facial buttresses and outer contours can be verified in the frontal view.

The symmetry of the facial buttresses and outer contours can also be verified in the lateral view.

These clinical pictures show the patient one year after primary panfacial reconstruction and prosthetic reconstruction of the left globe.

A submental vertex (worm’s eye) view of the same patient showing good projection of the midface.