Deviation analysis in custom-made mandibular reconstruction: how to evaluate results

Background

The purpose of this case report is to present a deviation analysis study made to evaluate the accuracy and reproducibility of virtual surgical planning—computer-aided design—computer-aided manufacturing (VSP-CAD/CAM) technology.

The authors report a case of a mandibular reconstruction in a 18-year-old man, in which a double-barrel fibular-free flap (DBVFG) was performed to treat an ameloblastoma.

CAD/CAM together with VSP technology were used to achieve the greatest outcome. Additionally, a deviation analysis study was performed to assess the precision of the mandibular reconstruction with fibular-free flap based on the work planned and the accuracy of CAD/CAM technology.

Case presentation

In March 2020, an 18-year-old man with a previous histological examination of mandibular ameloblastoma was referred to Maxillo-facial surgery Department in Siena (Policlinico Le Scotte) with a swelling on the right lower third of the face.

During the oral examination, an ulcerated lesion in the area of the dental elements 4.4 and 4.5 was observed.

The CT scan reported the presence of a 27 mm lesion of cystic appearance with irregular margins in the right side of mandible (figures 1 and 2), no laterocervical lymphadenopathies were detected and CT angiography did not reveal any anomalies in vascularisation of the limbs.

A segmental right mandibulectomy and, at the same time, a reconstruction with a DBVFG were scheduled, aiming to have enough vertical tolerance for dental rehabilitation and a good aesthetical result.

To maximise precision and accuracy of this complex reconstruction, the authors decided to use the virtual surgical planning technology in order to reach the best outcomes.

Investigations

The development of the virtual planning began with the acquisitions of the craniofacial CT scan and angiographic CT scan of the legs. Biomedical engineers processed the DICOM-format data, using the 3matic software by Materialise.

Moreover, a 3D stereomodel 1:1 of the mandible was requested by the surgeon and printed thanks to STL file obtained from CAD/CAM planning.

Virtual meetings between surgical team and the biomedicals engineers were mandatory to agree on the best decision concerning different points: the amount of mandible bone resection to allow an adequate oncological margin; the limb to choose as donor site; the distance between the malleolus and the first osteotomy to preserve the stability of the ankle joint; the number, the direction and the position of the osteotomy sites of the fibula to preserve the branches of the peroneal artery; the number of barrel to re-establish alveolar height; the direction of the pedicle to promote anastomosis with receiving vessels (figure 3). The goal of the treatment was to achieve a good functional result performing the reconstruction following all such points.

A second virtual meeting was necessary to observe and approve the mandibular and the fibular cutting guides; the position, the dimension and the number of screw holes; the custom plate.

Relying on surgeon’s directions, biocompatible resin cutting template and the reconstructive titanium plate that fit the patient’s anatomy were produced. To obtain the customised manufactured components, 15 working days were necessary since the last meeting and about 7–10 days from planning approval.

Treatment

A single team performed both the demolitive and the reconstructive surgeries.

The mandibulectomy, the harvesting of the fibular-free flap and the insetting of the flap were performed according to the virtual preoperative plan.

First, segmental mandibulectomy with a trans-oral approach using mandibular cutting guide was performed. The authors did not report any complications.

During the flap harvesting, only some difficulties in positioning the cutting guide on the fibula had been detected (figure 4). Prior to cut the vascular pedicle and the flap detached from the donor site, the fibula segments were fixed to the customised reconstructive titanium plate with a 2.0 locking system.

Both fibular and mandibular osteotomies were performed using a piezoelectric saw (Piezosurgery Mectron s.p.a. Italy).

Since the holes of the fixing screws of the cutting guides and the ones on the reconstructive plate were the same, inserting the flap with the reconstructive plate in the planned position did not present any challenge.

Bone reshaping was not necessary. Also, no major or minor intraoperative complications occurred. The operative time was around 6 hours and the patient hospitalisation lasted for about 11 days.

Outcome and follow-up

Three days after the surgery a control CT scan was performed; it showed a visual good result and postoperative DICOM files were elaborated to assess the precision of the reconstruction (figure 5).

The authors, in accordance with biomedical engineers, decided to generate a 3D analysis that could spotlight the best and synthetic information regarding the accuracy of the surgical procedure. Thanks to a specific software (GOM Inspect V.8), biomedical engineers used the DICOM files to create high-resolution 3D measure data.

The software generated a 3D representation of the preoperative planned reconstruction and a 3D representation of the postoperative result. These 3D data were superimposed in order to obtain an evaluation of the millimetric deviation of the bone position against what was programmed. Frontal, posterior, superior, inferior and both right and left side deviation analysis views were obtained.

A 3D report page with a coloured accuracy mandible mapper in three axial planes (X; Y; Z) was generated; the deviation of the postoperative result from what was planned was indicated with different colour, allowing the authors to visually render the differences between preoperative planning and surgical outcome.

The superior and inferior accuracy cut-off was set at 2 mm and in such range, variation within 0.5 mm was considered.

On the map, parts indicated with green colour are the one in which deviation was contained between 0.5 mm; red and blue-coloured parts are those where deviation observed was between 1.5 mm and 2 mm.

Yellow, orange and light blue parts of the diagram were the ones in which deviation felt mid-range, in between 0.5 mm and 2 mm (figure 6).

The patient presented and reported good functional and aesthetical results. He started the ambulation with the aid of crutches after 15 days and did not show instability of the ankle. No major or minor complications occurred in the postoperative period.

After 1 year of clinical and radiological follow-up, no recurrence was notified. Furthermore, the patient has undergone dental rehabilitation on the fibular flap.