Development and first clinical application of automated virtual reconstruction of unilateral midface defects

Computer-assisted surgery is used for decision making, treatment, and quality control throughout the reconstruction process of unilateral midface defects. The current approaches exploit the symmetry of the face by mirroring the intact side on the defect side using various segmentation methods. All commercially available implementations, however, are somewhat time consuming and dependent on the level of expertise of the user. We present a method for automatic reconstruction of unilateral midface defects using registration.

To reconstruct a skull by registration, the defect volume has to be virtually deleted from the skull. This modified data set is then mirrored and registered onto the original, defect-free skull. The fusion of these two skulls is the virtual reconstructed skull bridging the defect. Reconstruction by registration was performed for 24 different skulls without motion or dental restoration artifacts. Subsequently, simulation was performed with four accurately defined, various-sized, defects of the orbito-zygomatic complex. The results of the automated virtual reconstructions were compared with those obtained for the same defects as determined using conventional atlas-based planning software (iPlan). To simulate various clinical situations, four groups each containing six skulls were evaluated: the complete skull, midface and neurocranium, midface and lower jaw, and midface alone. The differences were compared using the similarity coefficients of Sørensen-Dice and Jaccard. Statistical analyses were performed using the t-test and Mann-Whitney U test.

The reconstruction results were similar for all the groups. The Sørensen-Dice coefficients of similarity for all reconstructed skulls were 0.869 and 0.874 for the registration and atlas-based reconstructions, respectively. The corresponding Jaccard coefficients were 0.774 and 0.781, respectively. Atlas-based reconstruction showed significantly better results in group 3 (midface and lower jaw) alone.

Virtual automated reconstruction by registration had equivalent accuracy to conventional atlas-based reconstruction across a spectrum of defects, from simple orbital to complex orbito-zygomatic defects. However, for those involving the midface and lower jaw, atlas-based reconstruction showed significantly better results. Although the new approach is somewhat hardware demanding, it is user independent, dispensing with the need for time-consuming adjustments to the results of planning. The first clinical application of registration reconstruction revealed performance equivalent to that of the conventional approach.