Eye (2009) 23, 1565–1571 & 2009 Macmillan Publishers Limited All rights reserved 0950-222X/09 $32.00 www.nature.com/eye 1 1,2 The application of MJ Millar and AJ Maloof CLINICAL STUDY stereotactic navigation surgery to orbital decompression for thyroid-associated orbitopathy Abstract Keywords: stereotactic surgical navigation; orbital decompression; small incision surgery Objectives To describe the application of stereotactic guidance, its preoperative workup, and limitations, if any, during orbital Introduction decompression surgery of the lateral orbital wall for thyroid-associated orbitopathy Surgical decompression of the orbit is an (TAO). established treatment of thyroid-associated Methods Case-controlled series of seven orbitopathy (TAO) for progressive proptosis, patients who underwent stereotactic-guided optic neuropathy, raised IOP, or aaesthetic surgical navigation during external approach rehabilitation not responding to medical balanced orbital decompression with maximal management.1–6 The mainstay of treatment debulking of the lateral wall. has been two wall decompression originally Results A progressive increase in debulking described by Walsh and Ogura4 in 1957, of the greater wing of sphenoid and exposure modified into an endoscopic formulation by of dura was noted in the series. The average Kennedy in 1990.7 Although inferomedial 1Department of proptosis reduction was 9.36 mm. No orbital decompression has afforded good results Ophthalmology, Westmead complications were encountered during any of in terms of proptosis reduction, it accrues a Hospital, Sydney, NSW, Australia the cases, nor was there any new onset large risk of postoperative diplopia, with rates postoperative diplopia. In all cases, exposure of postoperative motility imbalance as high as 2Department of 8 of dura was planned and did not present as a 80%. In fact, even with the creation of a strut at Ophthalmology, Sydney surprise. Stereotactic setup added 10 min to the ethmoid–maxillary junction, two wall Hospital, Sydney, NSW, preparation time. decompression still carries such a high risk of Australia Conclusions Stereotactic guidance improves postoperative diplopia9,10 that some surgeons anatomic localization and precision during do not even consider it to be a complication.11 Correspondence: AJ Maloof, orbital decompression, increasing confidence, Alternative techniques have been sought in Department of and reducing surgical stress. The ability to an attempt to decrease decompression-induced Ophthalmology, accurately determine the maximal limits of diplopia;1 Graham et al12 suggested a balanced Eyes and Faces, Level 9, decompression real time, while confirming decompression that requires lateralization of the 229 Macquarie Street, depth of bone removal, offers the possibility of outfractured lateral wall, whereas the lateral Sydney, NSW 2000, Australia reduced risk of iatrogenic injury. Stereotactic orbital wall has recently been suggested as the Tel: þ 1300 303 669; navigation allows for improved intraoperative region of first choice for orbital decompression Fax: þ 612 9223 4233. localization that may improve the ability to as it provides a low risk of consecutive diplopia E-mail: drmaloof@ maximally decompress the lateral wall, or severe complications, such as cerebrospinal eyesandfaces.com.au including the zygoma, orbital roof, and trigone, fluid leak.13 Despite the wide variety of and extending towards the optic nerve with innovative approaches designed by Received: 24 March 2008 Accepted in revised form: exposure of dura through smaller incisions. multidisciplinary teams, orbital decompression 9 October 2008 Eye (2009) 23, 1565–1571; doi:10.1038/eye.2009.24; remains a technically challenging procedure Published online: 5 June published online 5 June 2009 where the goal of proptosis reduction competes 2009 Stereotactic navigation surgery for TAO MJ Millar and AJ Maloof 1566 with postoperative diplopia and cosmetic requests space of the patient. The correlation of these points is of an increasingly demanding patient population. termed as registration. The accuracy of image guidance Stereotactic localization is typically employed during depends on the registration, and the computer gives an neurosurgery to assist with localization of soft tissue estimate of accuracy called the predicted accuracy. This is structures.14 Stereotactic-guided orbital bony surgery carried out using a reference plane that is rigidly fixed to differs slightly from that of neurosurgery; although soft the skull before and during surgery. Landmarks are tissues may move during surgery, the bony structures of chosen in three-dimensional image space, and matched the orbit do not move, and maintain their alignment with with landmarks in surgical space. These landmarks may the reference plane, allowing precise localization of be attached directly to the skull through titanium screws, incisions into bone during surgery. The use of image or more conventionally, by adhesive markers termed as guidance has just recently been reported for endoscopic ‘fiducial markers’ that adhere to the skin. Ideally, these orbital decompression however the technique was poorly markers should be applied to skin that is unlikely to described.15 This pilot study describes our experience of move after application or during the registration process, the application of stereotaxis to conventional small as the less the movement, the greater the accuracy. Once incision orbital decompression surgery to assist with the location of these reference points is known, that of localization of the limits of the bony lateral orbital wall. other structures in image space may be calculated with Specifically, we: (i) outline the principles of stereotactic reference to these points. The localization system guidance applied to orbital decompression surgery; (ii) comprises a workstation and a dual infra-red camera develop the protocol for application of fiducial markers mounted to a pole. An optical reference arc is attached to for stereotactic navigation of the orbit to achieve accurate the head using pinpoint fixation once the head has been orbital navigation; (iii) report the reduction in proptosis immobilized. The location of this arc is registered with and surgical outcomes using stereotactic navigation, and the workstation, and then the location of the reference compare with results from conventional decompression points with respect to the arc is registered. from the literature; and (iv) report our experience and As there are no manufacturer or published guidelines impressions with the application of stereotactic on placement of orbital fiducial markers, we undertook navigation to orbital surgery. several cases with varying placement of fudicial markers to determine the most reliable placement. We now use a modified ENT protocol for the application of these Materials and methods markers (see Figure 1) that achieves a 1-mm zone of Principles of stereotactic guidance accuracy around the orbit. Surgery was undertaken through a transconjunctival external approach to the In principle, stereotaxis involves the three-dimensional medial wall, and a lateral skin crease incision to the computerized localization of anatomical structures using lateral wall. Bone removal was carried out using an external guidance system. Preoperative imaging is high-speed surgical drills. reconstructed into a volume that can be reformatted into any plane in image space. The image space refers to this reformatted volumetric representation of the patient that Case series is stored in the computer, in comparison with the patient space, which is the real surgical space. To use the images This is a case controlled series of seven patients who of the image space, both intraoperatively and in real underwent a balanced medial and lateral wall time, the image space is correlated with the surgical decompression using stereotactic navigation. All patients Figure 1 (a and b) Intraoperative fudicial placement according to modified ENT protocol. Eye Stereotactic navigation surgery for TAO MJ Millar and AJ Maloof 1567 Table 1 Results of balanced orbital decompression using stereotactic guidance with maximal debulking down to the lesser wing of sphenoid and exposure of dura Patient Decompression technique Pre-op Hertel measurement (mm) Proptosis reduction (mm) New onset diplopia Complications 1 Balanced 21 7 No Nil 2 Balanced 21 8 No Nil 3 Balanced 27 8 No Nil 4 Balanced 26 9 No Nil 5 Balanced 28 10.5 No Nil 6 Balanced 27 12 No Nil 7 Balanced 26 12 No Nil had stable TAO, whose indication for surgery included Follow-up interval ranged from 9 months to 3 years; proptosis, intraocular pressure control, or cosmesis. measurement parameters are reported at 6 months after Information obtained included preoperative and surgery. Proptosis was reduced on average by 9.36 mm postoperative Hertel measurements, age, sex, the side of (range: 7–12 mm). Reduction in proptosis was noted to surgery, diplopia history, visual acuity, surgical improve with each sequential decompression. No complications, and follow-up period. intraoperative or postoperative complications were In this study, the lateral wall decompression is observed (see Table 1). No patient suffered new onset considered in two parts, the lateral and deep lateral postoperative diplopia, and there was no change in walls. The lateral wall includes the zygomatic bone and extraocular muscle motility postoperatively. The best- lower aspect of the frontal bone, and the deep lateral wall corrected visual acuity remained unchanged.