Analysis of simulated mandibular reconstruction using a segmental mirroring technique

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Abstract

Purpose

When deforming pathology limits intraoperative plating of the mandible, three-dimensional (3D) models can be generated by digitally replacing the deformed segment of bone with an inverted segment from the contralateral unaffected mandible to adapt a reconstruction plate. The purpose of this study was to use 3D conformance analysis to evaluate the degree of accuracy of this “segmental mirroring” technique.

Methods

Using a pre-existing melanoma database (January 1, 2005–September 20, 2015), high-resolution computed tomography (CT) scans of the head and neck were obtained from patients without evidence of bony disease or defects involving the mandible. Using 3D software (Mimics, Materialise, Leuven, Belgium), each mandible was segmented based on four defect classes (Ic, II, IIc and III) of the Brown et al. (2016) classification system. An inverted, or “mirrored”, image of each segment was digitally created and manually co-registered with the corresponding contralateral segment of the mandible. Conformance analysis was performed by calculating the root-mean-square (RMS) conformance distance and through evaluating 3D generated conformance maps. The primary outcome was degree of conformance. Data were analyzed using descriptive statistics and tests of statistical significance. The significance level was set at a p-value less than or equal to .05.

Results

A high degree of conformance (mean RMS < 1 mm) was observed when comparing all classes of simulated reconstruction. The closest conformance was observed for class III simulated reconstructions (mean RMS: 0.4 ± 0.2 mm). Inclusion of the condyle resulted in a reduced mean RMS conformance (class II: 0.5 ± 0.3 mm vs class IIc: 0.7 ± 0.5 mm; p = 0.01). There was no significant difference between RMS conformance distances when comparing side of simulated reconstruction. Evaluation of 3D mapping demonstrated reduced conformance with simulated reconstruction of the condyle and coronoid process.

Conclusion

The segmental mirroring technique can be used reliably to generate highly accurate three-dimensional models that may assist with mandibular reconstruction in circumstances where bony deformity limits intraoperative adaptation of a reconstruction plate. This technique is less accurate where pathology involves the mandibular condyle and, to a lesser degree, the coronoid process.

Introduction

A primary goal of oromandibular reconstruction is to preserve function through re-establishing mandibular continuity and maintaining symmetric contour (Cohen et al., 2009). Deforming pathologic processes of the mandible pose unique challenges to reconstructive surgeons as achieving optimal contour is necessary for restoring normal occlusion, which is influenced by the degree of deformation. Intraoperative adaptation of a reconstruction plate over a distorted segment of bone will result in an inadequate fit between the plate and bone graft potentially leading to malocclusion, temporomandibular joint dysfunction and an increased risk for plate extrusions (Abou-elfetouh et al., 2011, Li et al., 2011, Lee et al., 2007). However, this is also dependent on the extent and location of defect following resection (Brown et al., 2016).

A recently developed technique involves digitally removing a tumor of the mandible using a CT generated three-dimensional (3D) computer model, filling the defect with a “mirrored”, or inverted, image of the unaffected contralateral segment of the mandible and printing a 3D stereolithographic model to bend a reconstruction plate. While this technique has been reported in various case studies to be successful in reconstruction of the mandible secondary to deforming tumors (e.g. ameloblastoma, etc), trauma and osteoradionecrosis, no study has examined the accuracy of this method using quantitative parameters based on the classification of defect (Cohen et al., 2009, Lee et al., 2007, Hannen, 2006, Khalifa et al., 2016, Singare et al., 2004).

Numerous systems have been proposed for classifying mandibular defects resulting from segmental mandibulectomy. More recently, Brown et al. (2016) developed a classification system based on the most commonly encountered defects following oncologic resection (Brown et al., 2016). This system categorizes defects into four classes with subclasses for inclusion of the mandibular condyle (Table 1). The location and extent of resection often dictates the type of osteocutaneous/osseous free flap reconstruction. Other factors in determining type of flap include age, comorbidities, occupational/recreational activities and decisions regarding dental rehabilitation. The fibular, scapular and iliac crest free flaps are among the most commonly selected (Brown et al., 2016).

The objective of the study was to determine the degree of conformance between simulated mandibular defects, based on the Brown et al. (2016) classification system, and the corresponding inverted segments of the contralateral mandible using high resolution CT generated 3D computer models.

Section snippets

Study design and sample selection

The study population was comprised of patients randomly selected from an institutional melanoma database between January 1, 2005 and September 30, 2015 (Princess Margaret Cancer Centre, Department of Otolaryngology, University Health Network). To be included in the study, patients must have completed high resolution CT imaging of the head/neck with minimal metal artifacts from dentures and fillings. Patients were excluded from the study if there was evidence of bony disease involving the

Results

This retrospective cross-sectional study included a total of 10 patients (6 males, 4 females; mean age 55 ± 18; age range 31–85) that met criteria for inclusion and exclusion.

Discussion

Given that segmental mirroring has been reported as a successful technique within case reports in the literature, our hypothesis was that a high degree of conformance would be observed, but that variability would exist dependent on defect class of the mandible to be simulated for reconstruction (Cohen et al., 2009, Lee et al., 2007, Hannen, 2006, Khalifa et al., 2016, Singare et al., 2004). This was borne out by the data which demonstrated that mirrored segments from each of the four classes

Conclusion

While an overall high degree of conformance can be achieved when modeling the mandible using a segmental mirroring technique, variations exist that are dependent on the classification of defect. These variations should be taken into consideration when pre-planning for surgery. Inclusion of the condyle within a model will result in reduced conformance when using the mirroring technique. While modeling of class III reconstructions using the mirroring technique demonstrated the highest degree of

Acknowledgments

This work is funded by the Guided Therapeutics (GTx) Program, University Health Network, Strobele Family GTx Research Fund, Kevin and Sandra Sullivan Chair in Surgical Oncology, Hatch Engineering Fellowship Fund, and Princess Margaret Hospital Foundation.

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The first two authors (Joel C Davies and Harley HL Chan) equally contributed to the production of this manuscript.

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