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 Zygomatico-orbital complex correction can benefit from virtual planning of the procedure and intraoperative navigation to ensure proper execution of the plan.

The clinical view shows a posttraumatic defect of the left zygomatico-orbital complex. The left globe was previously removed and reconstructed with a prosthesis.

• Introduction to CAS: Asymmetry correction
• 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.

The preoperative 3D-CT scan shows a severe defect of the left midface.

Segmentation of anatomic regions is performed using autosegmentation algorithms.

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

Reconstruction of the orbital walls in unilateral trauma is achieved by mirroring the contralateral side.

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.

Stereolithographic models can be produced based on the original CT-scan data set and
based on 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 standard titanium implants have been bent 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 slowly over the implant surface to correlate actual zygoma position with the contours of the virtual template via frameless stereotaxy.

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

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

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

Intraoperative imaging is used to control the reduction result 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 in the 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.

Postoperative 3D-CT scan.

Clinical view after computer assisted reconstruction of the left midface with patient specific titanium implants.