Key points
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The first step in advanced diagnostics and virtual surgical planning is the generation of a virtual patient model.
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Information can be added to the virtual patient model through image manipulation for advanced diagnostic purposes.
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The virtual surgical planning is used preoperatively, but can also be used intraoperatively and postoperatively.
Introduction
The principles and limitations of orbital reconstruction have triggered technological developments in the past 2 decades. Because of the complex anatomy of the orbit and limited exposure during surgery, computer-assisted surgery (CAS) is of great added value. Several studies have shown that CAS assists the surgeon in achieving a better and more predictable treatment outcome. , , CAS consists of several preoperative, intraoperative, and postoperative components. Advanced diagnostics and three-dimensional (3D) virtual surgical planning (VSP) ensure a better inspection of the problem and the possible solutions during the preoperative phase. , This article explains the preprocessing steps required to start VSP, the benefits of advances in diagnostics, and the tools used in VSP.
Imaging and preparation
Several modalities may be encountered in imaging of the orbit and orbital contents: MRI, ultrasonography, and two-dimensional (2D) or 3D radiologic imaging (eg, radiographs or computed tomography [CT]). MRI is seldom used as the primary imaging modality after trauma: soft tissue structures can be excellently distinguished, but sensitivity for hard-tissue trauma is low. MRI is contraindicated if metallic foreign bodies may be present. Ultrasonography may provide fast evaluation of the globe, but should not be used if a rupture of the globe is suspected because of the pressure exerted on the globe during image acquisition. This pressure may lead to further acute decompensation of the eye and/or intraocular content extravasation.
CT is the modality of choice in orbital traumatology. CT has higher sensitivity for fracture detection than plain radiography and offers the additional possibility of internal hemorrhage detection. The problem of superimposition on 2D radiographic imaging and missing information of one of the dimensions is overcome by the 3D nature of the data that are produced with CT: the imaging volume is reconstructed and built up in voxels (3D pixels), each with a gray-scale value (Hounsfield unit [HU]) corresponding with the x-ray absorption within the voxel. From the image data, several planes can be reconstructed: a typical multiplanar view is made up by axial, coronal, and sagittal slices. In order to be able to distinguish existing bony ledges of the thin orbital floor and walls in the advanced diagnostics and virtual planning phase, it is recommended to use a maximum slice thickness of 1 to 1.5 mm.
An important preparation step in CAS in orbital reconstruction is to create a virtual 3D model of the bony structures and soft tissue of the patient from the basic CT slices. Typically, volume rendering is used to create a fast 3D overview. The original anatomy and fractured orbit can be easily visualized. For planning purposes, a volume render is not sufficient because it cannot be manipulated; a surface model needs to be created for this. Surface rendering is a technique that generates this virtual surface model: voxels belonging to the same anatomical structure can be selected in the image volume (segmentation) and a 3D virtual object is generated based on the selection made. Surface models can be modified and manipulated and are therefore required for VSP. Typical surface models used in the preparation step include at least a surface model of the bony structures ( Fig. 1 ) and the soft tissue exterior.
The generated surface models provide the same fast 3D overview as the volume renders and accurately represent the patient’s orbital anatomy and disorder. If required, CT images from different time points or 3D images from different imaging modalities (eg, MRI, intraoral scans, cone-beam CT, 3D stereophotogrammetry) can be combined with the CT data using (multimodality) image registration techniques. , In this way, a CT base image can be augmented with additional 3D data to create a complete and detailed virtual representation of the patient: the virtual patient model ( Fig. 2 ). The integration of accurate dental information from an intraoral scan might, for example, be useful to create a dental splint for navigation guidance during surgery. After the creation of the virtual patient model, advanced diagnostics and VSP can be performed.
Advanced diagnostics
In a dedicated software environment, further manipulation and analysis of the patient model can be performed. Advanced diagnostics is the expansion of the information that is readily available in the image data. This additional information can be obtained through image manipulation. Segmentation, mentioned earlier in relation to the generation of a surface model, is such a technique: voxels belonging to the same tissue type or anatomical structure are annotated within the image volume. This process may be done manually (coloring of the image set) or through thresholding, in which voxels greater than a certain gray-scale value (HU) are selected. This thresholding is, for instance, used in differentiating the bony structures from the soft tissues before a surface model of these bony structures is generated.
The paper-thin orbital floor and medial wall hamper the accuracy of threshold segmentation: more elaborate segmentation algorithms may be used to acquire an accurate segmentation of the orbit and its contents ( Fig. 3 ). One example of this is atlas-based segmentation, in which an atlas consisting of a patient model with presegmented anatomical structures is registered to the current patient scan ( Fig. 4 ). , , Atlas-based segmentation can provide reliable segmentation, even when CT image quality is suboptimal. Small manual adjustments might be needed to optimize the segmentation result ( Fig. 5 ), especially in the case of deviating anatomy or disorder. , Because accurate segmentation of the orbit is a prerequisite for many advanced diagnostic and virtual planning processes, much research has been performed on improvement of accuracy or user-friendliness of segmentation techniques for orbital anatomy. ,
A virtual 3D model of the segmented structures can be reconstructed similar to the process which is used to generate a hard-tissue patient model from the bony tissue segmentation. Of particular interest are the bilateral orbits, orbital contents, and, for an orbitozygomatic complex fracture, the zygomatic complex. The 3D shape of an object may be analyzed, and the volume of the object can be measured within the software. This method enables comparison of the orbital content between affected and unaffected orbits and thus quantification of the enlargement of the affected orbit. The segmented structures can subsequently be manipulated in the virtual environment in the ongoing process of adding diagnostic information to the virtual patient model. In unilateral orbital fractures, mirroring provides exact insight into the displacement of the affected orbital walls ( Fig. 6 ). , In orbitozygomatic fractures ( Fig. 7 ), segmented anatomical structures, obtained through different segmentation workflows, may be combined to obtain a complete template of the unaffected side before mirroring ( Fig. 8 ). This way, the displacement of the orbital floor, zygomatic arch, and prominence (relative to the unaffected contralateral side) can be seen from 1 mirrored object ( Fig. 9 ). , , ,
Virtual surgical planning
The indication for surgery is established on clinical findings, possibly supported by findings in the advanced diagnostics process. The goal of VSP is to reconstruct the pretraumatized anatomy as closely as possible. The VSP continues on information acquired from the advanced diagnostics process. This information in itself may comprise the surgical planning: the mirroring of the zygomatic complex provides a reconstruction of the premorbid anatomy of the affected structure. The mirrored orbit provides an adequate reconstruction for the affected orbit as well, but reduction of the dislocated bony parts is infeasible: alloplastic materials are frequently necessary to reconstruct the orbital floor and/or medial wall. The surgical planning is therefore more elaborate than mirroring alone: the optimal position of the reconstruction material (eg, a preformed titanium orbital implant) is also planned.
If a preformed titanium orbital implant is used, a virtual stereolithographic model (STL) of the implant is imported in the planning environment. The position of the implant can be manipulated in the virtual patient to find an optimal position for the implant in the current patient ( Figs. 10 and 11 ). , , Several parameters are taken into account in the positioning process: covering of the defect, support on the dorsal ledge, fixation possibility on the orbital rim, prevention of interference with existing bony structures, reconstructing the contour as closely as possible to the mirrored orbit, and bony support at the medial tip of the implant ( Fig. 12 ). The virtual patient offers the possibility to perform virtual surgery and evaluate the outcome within the patient model. Multiple possible implant positions for the imported implant can be evaluated before coming to a decision on the desired position. If necessary, additional implants, with different sizes or from other manufacturers, may be imported to compare their fit and find the implant with the optimal size and shape for the individual reconstruction ( Fig. 13 ). , ,
