CAS: Prostodontic-driven mandibular reconstruction

Whatever reconstructive strategy is applied in the mandible, a prosthodontic driven backwards planned digital workflow should be advocated. This planning is performed prior to continuity resection of the mandible.

The ideal location of the dental implants are first planned virtually. This defines the required bone stock for the implants which in turn gives the necessary three-dimensional shape of the final reconstruction. Once the mandible is virtually reconstructed, an STL model allows for pre-contouring of a mandibular bridging plate. The plate defines the contour and position of the bone graft necessary for the planned dental implant position.

Please note that the ideal position of the bone graft may not match the outer contour of the mandible; especially in the molar region due to the limited size of the bone graft.

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).

The following clinical case demonstrates an example of this digital workflow step by step.

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

A 3D-scan already made during the preoperative staging process is used to diagnose the extension the tumor, to identify possible bony infiltrations and to decide on possible lymph node involvement. It also allows for a digital workflow in mandibular reconstruction.

The coronal section shows the extension of the tumor in the floor of the mouth. A resection of the right mandibular body is indicated.

Autosegmentation of patients individual anatomy is based on the 3D-CT scan of a normal adult skull without any artifacts.

This function is used to outline a virtually dentate upper jaw (blue) in specialized craniofacial reconstruction software. This is done without dental impression and without dental casts.

According to the dentate upper jaw, positioning of dental implants in the lower jaw is planned.

The yellow cylinders in the lower jaw demonstrate the required amount of vital bone around these dental implants (the vectors of the implants are demonstrated by the central pink lines).

The software allows matching the yellow cylinders together with an ideal virtually produced dentate mandible.

The blue sub-volume shows the adequate bony volume together with virtual teeth for a final reconstruction prior to ablative surgery.

The coronal section shows the amount of required bone around implants (yellow structures).

The blue contour shows the position of a bridging plate that would be contoured according to the outer contour of the pre-existing mandible. This plate would not meet the requirements for a later bone graft, because it is too far away from the digitally planned position of bone. If reconstructed without backwards planning, the result might be a buccal cross bite situation.

The red contour indicates the correct position of the reconstruction bridging plate.

A 3D overview shows the correct (red) and incorrect (blue) contour of an adequate patient-specific mandibular reconstruction plate to meet the requirements for bony reconstruction.

A reinforced STL model of the virtual reconstruction is prepared. This model is then used to preoperatively create a patient-specific mandibular reconstruction plate.

Note: Recent developments allow the fabrication of patient specific bone plates based on the virtual plan only. No 3D-models are required.

To facilitate the correct position of the patient specific reconstruction plate, temporarily-applied intermaxillary positioning plates help to secure the residual mandibular segments in their correct relative position.

Depending on the reconstructive strategy, the bridging plate can be:

1. Inserted alone together with an adequate soft tissue flap, followed by secondary bone grafting
2. Inserted together with a vascularized bone flap

The resection, bone harvest, and contouring of the bone graft can be performed according to procedures described in the section on postablative reconstruction.

Resection templates (cutting guides) and positioning devices can be prepared using computer assisted techniques ( link).

Recent developments allow for the incorporation of accurate reconstruction plate screw hole position in the cutting guides obviating the need of positioning devices.

In this case a microvascular fascio-cutaneous flap was applied together with the alloplastic bridging plate.

Secondary bony reconstruction will be performed after radiotherapy and one year of disease free survival. This is to avoid radiation of bone graft prior to dental implant placement.

Alternatively, primary reconstruction using a bone containing free flap can be performed at the discretion of the surgeon.

Following the healing of the reconstructed mandible, dental implants can be placed using the computer assisted plan describved in the dentoalveolar section.