STL model, 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 central midface deformities can benefit from virtual planning of the procedure and intraoperative navigation to ensure proper execution of the plan.

The clinical view shows a patient with granulomatosis with polyangiitis resulting in a severe defect of the central midface.

This case will be used to illustrate the principles of the procedure.

• 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 intact, fiducial markers can be fixed to dental splints.

Segmentation of anatomic regions (Images to left) of interest is performed using autosegmentation algorithms.The result can be adjusted by the surgeon.

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

After virtual reconstruction of the midface (right) the modified dataset serves as a virtual template that can be used for intraoperative navigation, intraoperative imaging and 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.

Patient specific implants are produced based on the stereolithographic model where the virtual reconstruction has been made.

In this case a sterile standardized phantom model was used intraoperatively to bend and size the standard titanium implants to fit the 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 slowly over the implant surface to correlate actual implant position with the contours of the virtual template via frameless stereotaxy.

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

A postoperative 3D-CT scan is obtained to evaluate the reconstruction.

Clinical view before and after computer assisted reconstruction of the left midface with titanium implants combined with free vascularized soft tissue graft.

Frontal view.

Lateral view.

Worm's-eye view two years postoperative.