THE INTERNATIONAL JOURNAL OF MEDICAL ROBOTICS AND COMPUTER ASSISTED SURGERY ORIGINAL Int J Med Robotics Comput Assist Surg 2013; 9: 497–502. Published online 6 November 2013 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/rcs.1557

ARTICLE

Computer-assisted zygoma reconstruction with vascularized iliac crest bone graft

Ali Modabber1* Marcus Gerressen1 Nassim Ayoub1 Dirk Elvers1 Jan-Philipp Stromps2 Dieter Riediger1 Frank Hölzle1 Alireza Ghassemi1

Abstract Background The reconstruction of zygoma is a challenge with regard to aesthetic and reconstructive demands. Methods Pre-operative CT data were imported into specific surgical planning software. The mirror-imaging technique was used. A surgical guide transferred the virtual surgery plan to the operation site, whereby it fitted uniquely to the iliac donor site. A postoperative CT scan was obtained for comparing the actual postoperative graft position and shape with the pre-operative virtual simulation.

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Department of Oral, Maxillofacial and Plastic Facial Surgery, RWTH Aachen University Hospital, Germany

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Department of Plastic, Hand and Burn Surgery, RWTH Aachen University Hospital, Germany *Correspondence to: A. Modabber, Department of Oral, Maxillofacial and Plastic Facial Surgery, RWTH Aachen University Hospital, Pauwelsstrasse 30, 52074 Aachen, Germany. E-mail: [email protected]

Results A mean difference of 0.71 mm (SD ± 1.42) for the shape analysis and 3.53 mm (SD ± 3.14) for the graft position was determined. The calculation of the closest point distance showed a surface deviation of < 2 mm for the shape analysis with 83.6% of values and for the graft position with 35.7% of values. Conclusion Virtual surgical planning is a suitable method for zygoma reconstruction with vascularized iliac crest bone graft, with good accuracy for restoring the three-dimensional anatomy. Copyright © 2013 John Wiley & Sons, Ltd. Keywords computer-assisted surgery; virtual planning; vascularized iliac crest bone graft; surgical guide; zygomatic reconstruction

Introduction

Accepted: 10 October 2013

Copyright © 2013 John Wiley & Sons, Ltd.

Malar and zygomatic arch defects often occur after cancer surgery, resection of benign maxillary tumours and trauma. The localization, extension and dimensions of the defect provide important information for the surgical treatment. Brown and Shaw (1) described a new classification for maxilla and mid-face defects. However, this classification does not include the zygomatic arch, and this region represents a complex anatomical structure. There are many therapeutic options to restore complex craniofacial defects. Microvascularized bone flaps present an excellent therapeutic alternative for zygoma reconstruction with autologous transplants. Microsurgically revascularized iliac crest bone grafts have the benefit of a rich cancellous blood supply, a large amount of bone and a compact cortex (2), providing an ideal site for the reconstruction of malar and zygomatic arch defects. The aesthetic outcome and satisfying facial appearance of the reconstruction depends on the position and shape of the graft. Three-dimensional (3D) modelling, assisted by computed tomography (CT), can be an ideal method for obtaining precise information for reconstructive surgery. The transformation of digital CT data to 3D software for the simulation of the operative field and the donor region provides a detailed and precise analysis. It serves as a diagnostic tool to plan the size, shape and exact

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Figure 1. Comparison of the actual postoperative graft position (violet) with the virtually planned situation (green): (A) bottom view; (B) frontal view

placement of the bone graft (3), which may be of important clinical benefit. Virtual pre-operative planning provides accuracy in detail without loss of information, and a number of alternative surgical approaches can be visualized (4). Using surgical planning software to simulate various surgical scenarios can be convenient and cost-effective (5). For an accurate translation of the virtual pre-operative planning to real-time surgery, operation templates fabricated by a selective laser-sintering technique provide a precise modelling of the detailed anatomical structures of the defect. Various approaches demonstrate the benefit of prefabricated 3D image templates, linking the virtual operation plan to the actual surgical procedure (6–9). Computer-assisted surgery can help facilitate the shaping procedure of the bone graft and to increase precision in order to achieve an optimal aesthetic outcome (10), as well as a shortening of transplant ischaemia time (11). The aim of this study was to determine the accuracy of computer-assisted zygoma reconstruction with vascularized iliac crest bone graft and, consequently, whether it should be routinely used for all patients undergoing bony reconstruction of malar and zygomatic arch defects. Copyright © 2013 John Wiley & Sons, Ltd.

Materials and methods After institutional approval and written, informed consent, the translation from a virtual plan to the operating site with a surgical guide was performed in a patient with a gunshot defect of the left malar and zygomatic arch. Pre-operative CT scans of the facial skeleton and iliac crest were performed with a 128 row multislice CT scanner (Somatom Definition Flash, Siemens, Erlangen, Germany). Reconstructions were carried out in a bone and soft tissue window, kernel 30/60 for head and neck and 70 for the pelvis. Acquisition of scans for head and neck is done in 0.5 mm slice thickness, for the pelvis in 1 mm slice thickness. The CT data of the facial skeleton in digital imaging and communications in medicine (DICOM) file format were imported into ProPlan CMF Planning Software (Materialise N.V., Leuven, Belgium). The process of segmentation followed, in which artifacts were removed and all bony structures of interest were isolated. A high-quality 3D visualization of the mid-face was calculated. The mirror image of the healthy side served as reference for the virtual reconstruction of the affected malar and zygomatic arch. Int J Med Robotics Comput Assist Surg 2013; 9: 497–502. DOI: 10.1002/rcs

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Computer-assisted zygoma reconstruction

Additionally, angiographic CT scans of the iliac donor site, which also allowed investigation of the arteries, were read in with the software. Depending on the vascularization, the position of the graft selected afterwards was determined at the right iliac crest. After virtual reconstruction of the defect area, as mentioned above, the best-fitting part of the iliac crest was selected. The positions of the vessels nourishing the iliac graft were also taken into account. The donor site was virtually osteotomized and replaced into the zygoma defect. After fine adjustment, the data were imported into 3-matic software (Materialise N.V.) as an STL file. Thus, based on the final plan, a surgical guide fitting uniquely to the iliac crest, and indicating the desired osteotomy lines, graft size and angulation, was designed. With the aid of the rapid prototyping selective laser-sintering method, the guide was produced out of polyamide powder and solidified using a carboxide laser. The surgical guide linked the computer-assisted surgical plan to real-time surgery. A postoperative CT scan was obtained after 6 months to compare 3D computer models of the final reconstruction with the pre-operative virtual plan. Using 3-matic software, the pre- and postoperative objects were first aligned using point registration. Thereafter, automatic global surface registration was performed; this registration is based on an

iterative closest point (ICP) algorithm. The remaining skull, after resection, was used to register the postoperative to the pre-operative situation, as it is important to use those objects that remain unchanged through the surgery. The actual postoperative graft position was compared with the virtual simulation (Figure 1). This was done based on a part comparison algorithm that measures the distance of every triangle corner of the postoperative surface against the planned surface. This resulted in a set of measurements that were analysed in a histogram. The colour map overlay histograms represented a close proximity of objects coloured green and red to show the increase of distance differences from the pre-operative plan. To compare the actual postoperative graft shape with the virtual simulation, again the global surface registration was performed, using only the actual postoperative and simulated graft (Figure 2A, B). The same process was used in 3-matic as for the graft position comparison.

Results The presented surgical method of zygoma reconstruction allowed the implementation of predetermination of the iliac

Figure 2. Comparison of the actual postoperative graft shape (violet) with the pre-operative planned shape of the graft (green): (A) lateral view; (B) medial view Copyright © 2013 John Wiley & Sons, Ltd.

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Figure 3. The surgical guide is temporarily fixed on the external side of the right iliac, using osteosynthesis screws. Arrow points to the deep circumflex iliac artery at the medial side of the iliac crest

Figure 5. Positioning and fixation of the iliac crest bone graft for reconstruction of the left malar and zygomatic arch. Arrow points to the anastomosis of the deep circumflex iliac vessel to the left temporal vessels

graft with regard to its shape, size and the site of osteotomy during surgery. Its temporary fixation on the donor site simplified the surgical procedure (Figure 3). Guided surgical sawing of the iliac crest reduced the amount of removed bone to the determined level (Figure 4) and it fitted into the zygoma defect without major adjustments (Figure 5). No complications were encountered during the surgery or healing phase. The microsurgically revascularized iliac crest bone graft showed excellent perfusion. Comparison of the pre- and postoperative 3D computer models using a part comparison algorithm showed a mean difference of 0.71 mm (SD ± 1.42) for the shape analysis and 3.53 mm (SD ± 3.14) for the graft position. The part comparison-based closest point distance of the absolute values indicated that 83.6% of surface deviation was < 2 mm for the shape analysis. The surface deviation showed a 35.7% smaller difference than 2 mm for the position analysis. The absolute count of measured points in each calculation using the part comparison algorithm was 7113 points (ED, element distribution). The colour map overlay histograms (Figure 6A, B) showed

Figure 6. The surface deviation analysis for shape (A) and position (B) of the iliac crest bone graft. The calculation showed a surface deviation of < 2 mm (red line) for the shape analysis with 83.6% of values and for the graft position with 35.7% of values. ED, element distribution

the results of the surface deviation analysis for shape and position of the iliac crest bone graft.

Discussion

Figure 4. The iliac crest bone graft, exactly sawed and osteotomized with the help of the surgical guide Copyright © 2013 John Wiley & Sons, Ltd.

There are many therapeutic options for restoring the facial skeleton. The most commonly used alloplastic implants for zygomatic reconstruction can cause postoperative complications, such as foreign body reaction, swelling, infection and replacement (12). Using autologous bone flaps such Int J Med Robotics Comput Assist Surg 2013; 9: 497–502. DOI: 10.1002/rcs

Computer-assisted zygoma reconstruction

as fibula, iliac crest or scapula is an excellent way of reconstructing complex craniofacial defects. Brown and Shaw (1) described the use of microvascular iliac crest bone grafts for the reconstruction of defects after maxillectomy, with and without involving the orbit (class I–IV). However, this classification has a weakness with respect to zygomatic defects which involve the zygomatic arch. Microvascular iliac crest bone graft also provides a perfect anatomy and bony structure to restore the curvature of the zygomatic arch (Figure 7A, B). The vascularized iliac crest graft has a short pedicle, which means that careful planning is required (13). The goal of zygoma reconstruction is to achieve the maximum aesthetic outcome. Therefore, pre-operative virtual planning can help to evaluate the defect size and the relation to neighbouring structures in order to choose the best possible reconstruction plan. The analysis of the entire facial skeleton in 3D provides more accuracy, makes an overall evaluation possible and supplies valuable information which greatly facilitates further treatment. The virtual surgery plan requires a precise 3D model, conforming to the standards of the defect, as the basis for the design of the transplant in shape, position and angulation. Different surgical tools are required from the software to perfect the virtual plan. The mirroring tool allows the use of the healthy side as a template for computational superimposition on the affected side, for the restoration of facial symmetry (14). However, the use of the mirroring tool can influence the accuracy of the pre-operative virtual plan because of the natural asymmetry of humans skulls (15). The present study, which uses the computer-assisted method ProPlan CMF (Materialise N.V.), previously described for mandibular and maxillary reconstruction with microvascular bone flaps (10,11,16), is the first for malar and zygomatic arch reconstruction. It delivers a surgical guide for intra-operative use, based on an accurate virtual operation plan, which is made with the aid of simulation before surgery. The goal was to evaluate the accuracy of this method for malar and zygomatic arch reconstruction with a microvascular iliac crest bone graft. The registration of the pre- and postoperative data for a comparison of the virtual and actual postoperative situations

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can give an idea of whether the surgery has been performed in accordance with the pre-operative virtual plan. In order to compare the 3D computer models from the final reconstruction with the pre-operative virtual plan, a postoperative CT scan was obtained after 6 months. The remodelling and resorption of the graft take place mainly in the first 6 months after transplantation (17) However, the measurements represent the result after the remodelling phase has been completed, which is very important for the long-term aesthetic outcome. A mean difference of 0.71 mm (SD ± 1.42) with 83.6% surface deviation < 2 mm for the shape analysis was calculated. It seems that the actual shape comes very close to the virtual plan. The surgical guide ensures accurate sawing of the iliac graft during the surgical procedure, with a determined transplant shape, size and number and site of osteotomies. The immediate insertion of the graft into the zygoma defect followed the explantation, as no time for shaping was necessary. The measurements for the graft position showed a mean difference of 3.53 mm (SD ± 3.14) with a 35.7% smaller difference than 2 mm in the surface deviation calculation. It appears that the exact placement of the bone graft into the zygoma defect is very difficult without predefined bony margins. Roser et al. showed a mean maximum distance of 2.00 mm (SD ± 1.12) of postoperative mandibular resection margins to that of the virtual plan (18). This reflects the fact that the anatomy and 3D structure of the zygoma seems to be more complex. However, the achieved aesthetic outcome was very satisfactory (Figure 8A, B). It appears that the natural asymmetry of faces in humans obscures some inaccuracy in surgical reconstructions to a certain degree (15). The use of computer-assisted techniques in combination with free flaps provides good functional and aesthetic results with predictable outcome (19). A surgical guide transfers the computer-based surgical plan to real-time surgery, which is the required procedure to achieve the best possible result. The clinical benefits of computer-assisted surgery are likely to outweigh the expenditure for technology (20). Surgical guides shorten transplantation time, increase precision and control and minimize the shaping process of the transplant (11). The increased accuracy ensures outcomes of constant high quality as regards both shape and aesthetics. Growing

Figure 7. Cone beam computed tomography (CBCT) of the pre-operative zygomatic bone defect (A) and postoperative result (B) of the anatomical reconstruction, with contouring accuracy of the zygomatic arch Copyright © 2013 John Wiley & Sons, Ltd.

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Figure 8. (A) Pre-operative left facial gunshot defect with affected malar and zygoma arch. (B) Postoperative facial symmetry after computer-assisted zygoma reconstruction with vascularized iliac crest bone graft

complexity in extensive bony defects may raise this method’s indication, as the surgery might be well simplified for such defect location, which is difficult to achieve through manual placement of the graft, even for experienced surgeons (18). The presented method shows that using custom-made surgery guides using vascularized iliac crest bone graft can help to restore complex areas, such as the malar and zygomatic arch, with good accuracy. We are aware that a randomized prospective trial with larger sample sizes will be required to evaluate further benefits of computer-assisted zygoma reconstructions with vascularized iliac crest bone grafts.

Acknowledgements The authors thank Annelies Genbrugge and Joris Bellinckx (Materialise N.V., Leuven, Belgium) for their valuable support.

Conflict of Interest The authors have stated explicitly that there are no conflicts of interest in connection with this article.

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Computer-assisted zygoma reconstruction with vascularized iliac crest bone graft.

The reconstruction of zygoma is a challenge with regard to aesthetic and reconstructive demands...
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