CAS: Midface reconstruction

In cases of coexisting maxillary defect with an intraoral opening, reconstruction has to meet the requirements outlined in the section on postablative reconstruction.

This section will focus on the computer assisted reconstruction of the midface with or without involvement of the orbit.

There are different options for reconstruction (see section of postablative reconstruction) which can be applied primarily, delayed primarily (ie. within weeks) or secondarily (ie. after 6-12 months eg. following radiotherapy).

Whatever reconstructive strategy is applied, a computer assisted surgery might serve different purposes:

1. Trajectory guided biopsy
2. Volumetric assessment of pre-treatment tumor size
3. Computer assisted and navigated tumor removal including intraoperative mapping of frozen sections and biopsies.
4. Preoperative planning of immediate or delayed hard tissue reconstruction including fabrication of a STL model with or without preparation of patient-specific implants via rapid prototyping procedures.
5. Postoperative assessment of and comparison of the intended virtual reconstruction with the postoperatively achieved reconstruction.
6. Image fusion of different data sets during postoperative follow up as a restaging procedure.

The following clinical cases demonstrate the use of computer assisted planning and surgery in midfacial tumors.

• Introduction to CAS: Asymmetry correction
• OR setup
• Cone beam vs. fan beam
• Preparation for intraoperative navigation

A 3D-CT scan is made preoperatively to diagnose the extent of the already biopsied ossifying fibroma in a young male adult. The software platform allows quantification of the lesion. The multiplanar 2D views show the tumor and the resulting right midfacial deformity.

By using the autosegmentation algorithm of the software, the left midface is automatically segmented (pink).

The unaffected sub volume is mirrored onto the deformed right midface. The sagittal and axial views show the discrepancy between the mirrored template and the deformed bony structures.

A midfacial stereolithographic model is fabricated after mirroring of the unaffected side onto the affected side. A patient-specific implant is prepared for the medial orbital wall and the orbital floor. Additionally, a mesh implant is adapted to the pyriform aperture and the area between the malar prominence, infraorbital rim and the alveolar process.

Alternatively a sterile standardized artificial skull could be used for anatomically pre-bending the reconstruction implants as shown to the left.

Via a transoral approach, the midfacial skeleton is exposed and osteotomies are performed to remove the lesion.

The bony lesion is removed en bloc.

In this case the orbital implant is positioned via a retroseptal transconjunctival approach without lateral canthotomy.

The implant position in all three dimensions is verified using navigation. This procedure is described in detail in the section on orbital reconstruction.

In cases where a sterile standardized skull is used for the pre-bending of the implants, the correct shape of the implant is also verified by navigation.

The outer midfacial implant is placed through the transoral approach.

Each implant has to be adequately stabilized (minimum one screw per implant).

Postoperative views in the cone beam CT scan show the correct recontouring following ablative midfacial surgery.

Preoperative worm's view of the midface deformity.

3 Year postoperative worm's view.

To illustrate the use of computer assisted planning and surgery in midface malignancies, this case of a young female adult is demonstrated. The patient presented without double vision although she shows significant hyper globus on the right side. Her right nasal airway is obstructed. Biopsy proved a sinonasal carcinoma.

The preoperative coronal CT-scan shows the tumor extension from the right maxillary sinus and ethmoids into the orbit.

The CT and the MRI datasets of the patient have been fused.

The unaffected left facial skeleton is mirrored onto the deformed right midface. All three 2D views show the discrepancy between the mirrored template and the deformed bony structures.

A midfacial STL model after mirroring of the unaffected side onto the affected side is fabricated preoperatively. The model is used to define the geometry of the patient-specific implants for recontouring the orbit and the midface.

Via a transfacial approach, the midfacial tumor is resected and removed en-block. Frozen sections of the sinus membrane in the alveolar recess proved to be free of disease; that is why the alveolus was spared.

This illustration shows the defect after tumor removal. The blue suture marks the detached medial canthal ligament. After resection of the medial orbital wall, the orbital floor and the infraorbital rim, the remaining orbital content is displaced into the defect.

The outer midfacial implant is first placed through the transfacial approach.

The orbital implant is also positioned via the transfacial approach.

The position of the both implants are verified using navigation.

Each implant has to be adequately stabilized (minimum one screw per implant).

Image fusion of the preoperative CT scan with the postoperative cone beam CT scan show the correct recontouring following ablative midfacial surgery.

In this case computer assisted surgery helped to safely resect the tumor. The midfacial hard tissues could be anatomically reconstructed. The globe and ocular muscles were spared and the patient has no diplopia.

This picture shows a fusion of the preoperative virtual reconstruction plan (pink) together with the final result.

Note: Whatever hard tissue reconstruction is applied, adequate vascularized soft tissue has to envelope the construct. For this purpose microvascular free flaps or pedicled flaps, (eg. temporalis flap) might be chosen.

In this case a pedicled temporalis muscle flap was used to envelope the patient specific midfacial implant. The postoperative 3D-CT scan on the left shows an additional 3D-titanium mesh (0.6 mm thick), which camouflages the temporal donor defect (arrow).

The clinical lateral view one day after the surgery shows:

• The staples are in place
• The patients head is not shaved
• No temporal hollowing

Bird's view 1.5 years after surgery and adjuvant radiotherapy.

Note: In this case there was no need to remove the maxillary alveolus.

If that would have been the case, the oral cavity should be sealed by the use of a vascularized soft tissue flap vs. a composite soft and hard tissue flap. Typically latissimus dorsi, anterior serratus muscle flap, lateral upper arm free flap, radial forearm flap, rectus abdominous flap, DCIA flap, fibular flap, scapular flap, anterolateral thigh flap, might serve this purpose. Depending on whether or not the patient wishes dental restoration, simultaneous or secondary bone grafting has to be performed (see the section on maxillary reconstruction with dental restoration).

For the bone grafting procedure, computer-assisted planning including fabrication of a drill guide is strongly recommended.