Virtual Surgical Planning for Orbital Reconstruction.

The advent of computer-assisted technology has revolutionized planning for complex craniofacial operations, including orbital reconstruction. Orbital reconstruction is ideally suited for virtual planning, as it allows the surgeon to assess the bony anatomy and critical neurovascular structures within the orbit, and plan osteotomies, fracture reductions, and orbital implant placement with efficiency and predictability. In this article, we review the use of virtual surgical planning for orbital decompression, posttraumatic midface reconstruction, reconstruction of a two-wall orbital defect, and reconstruction of a large orbital floor defect with a custom implant. The surgeon managing orbital pathology and posttraumatic orbital deformities can benefit immensely from utilizing virtual planning for various types of orbital pathology.

INTRODUCTION

Over the past 20 years, advances in radiographic imaging and computer technology have allowed for application of image-guided surgery in orbital reconstruction. Conversely, volumetric analyses of anatomical structures have been utilized for the design of standardized anatomic implants for orbital reconstruction and custom patient-specific implants for complex orbital and midfacial defects. These advances have allowed for improved efficiency, accuracy, and safety in the surgical management of orbital pathology. The purpose of this review is to discuss the applications of computer-guidance and advanced computed tomography (CT)-imaging in the management of various conditions affecting the bony orbit. Four illustrative case examples are presented: Orbital decompression for thyroid-eye disease, midface reconstruction following a zygomaticomaxillary complex (ZMC) injury, reconstruction of a posttraumatic two-wall orbital defect, and reconstruction of a large orbital floor defect with a custom alloplastic implant.

THYROID EYE DISEASE

Thyroid eye disease (TED) is an autoimmune inflammatory disorder of the orbit that results in proptosis, diplopia, eyelid retraction, exposure keratopathy and in severe cases, optic nerve compression.1 The pathogenesis of TED has not been entirely elucidated however it is most likely due to the activation of thyroid stimulating receptors by circulating thyrotropin receptor antibodies. Activation of these receptors in the orbit results in deposition of hyaluronan and resulting extraocular muscle enlargement as well as hypertrophy of orbital adipose tissue.123 As the orbital contents enlarge, the patient is at risk for sight-threatening compressive optic neuropathy. Various treatment methodologies including steroids, orbital radiation, immunomodulators, and selenium have been used. However, refractory or sight-threatening cases of TED usually require surgical decompression.3

Orbital decompression has been a mainstay of TED treatment since the 1950s. The variability in outcomes particularly in terms of proptosis and ocular alignment has resulted in the development of numerous surgical techniques for decompression with no clear consensus on which is most effective.4 The recent development of computer-assisted surgical planning and execution has the potential to result in more efficacious, consistent and predictable decompressions. Multiplanar CT scans with three-dimensional (3D) reconstruction allow the surgeon to carefully evaluate the individual anatomy and identify specific bone segments for resection. Intraoperatively, the CT images and the patient's anatomy are virtually overlapped allowing for stereotactic navigation throughout the case.5 This allows the surgeon to check his/her distance from important anatomical landmarks and to measure the extent of bony resection in order to carefully match preoperative planning.6 Postoperative imaging and assessment may be used to objectively quantify the magnitude of decompression and correlate this with clinical outcomes. The objective data provided by computer-assisted orbital decompression has the potential to result in safer, more efficacious and consistent decompressions. In the clinical example shown [Figures 1 and 2], real-time image guidance was utilized to allow for extensive, asymmetric decompression of the bilateral orbits (left: Lateral orbital wall, right: Medial and lateral walls).

Figure 1

Preoperative (top panels) and postoperative (bottom panels) frontal and submentovertex views of a male patient with thyroid orbitopathy who underwent computer-assisted orbital decompression

Figure 2

Preoperative (left panels) and postoperative (right panels) demonstrating the three-dimensional computer planning for orbital decompression and superimposition of the expected result over the postoperative images (right middle and right bottom panels)

PERIORBITAL TRAUMA

Injuries to the bony orbit are common among patients sustaining craniomaxillofacial trauma.789101112 The external orbital framework is disrupted in several different types of facial fractures (e.g. ZMC, frontal bone, Le Fort III). The integrity of the internal orbit can likewise be disrupted, either in isolation or as a component of complex midfacial or upper facial injury. The orbital skeleton provides support for the globe and ocular adnexa and houses neurovascular structures critical to normal visual sensory function. While disruption of the orbital framework can compromise these structures, such injuries are, fortunately, rare. Conversely, surgical management of orbital skeletal injuries is fraught with potential peril, as dissection, bony mobilization, or placement of intra-orbital hardware may damage the globe or critical neurovascular structures. In this regard, advances in computer imaging have enhanced the surgeon's ability to safely dissect the internal orbit, have allowed for the design and manufacture of standard implants for orbital reconstruction, planning for correction of secondary deformities, quantitative assessment of fracture reduction and volume restoration, and have improved the ability to visualize, in real-time, the orbital anatomy during dissection.13141516171819202122232425262728293031

ZYGOMATICOMAXILLARY COMPLEX FRACTURES

ZMC fractures sometimes termed zygoma fractures or orbitozygomatic fractures, are among the most common type of facial fracture.78910 High-or low-energy mechanisms can disrupt the articulations of the zygomatic bone with the frontal bone (frontozygomatic suture), temporal bone (zygomatic arch), sphenoid bone (sphenozygomatic suture), and maxilla (zygomaticomaxillary buttress). The 3D relationship between these articulations was not initially understood in the management of these injuries, which were commonly referred to as “tripod” fractures. Advances in CT have improved the understanding of these injuries and the complex 3D anatomy that needs to be restored for successful management.78910111213 Restoration of the sphenozygomatic articulation in the lateral orbit remains the most reliable predictor of a successful reduction but is difficult to assess without surgical access to the internal orbit.712272829 Furthermore, management of the orbital floor remains controversial among patients with ZMC fractures.2829 The use of real-time imaging (mini C-arm, CT) has allowed for assessment of anatomic alignment and orbital floor integrity following reduction, improving operating times and decreasing patient morbidity from unnecessary orbital exploration.13202123 In the case presented, an orbitozygomatic fracture was repaired utilizing virtual planning to establish the appropriate position of the right zygoma. To accomplish this, the left zygoma complex was digitally rendered from a 3D CT reconstruction and subsequently mirrored and placed onto the right midface [Figure 3]. The reconstituted virtual position of the zygoma served as a template and was then used to guide intra-operative positioning of the displaced fracture. Virtual planning also allowed for visualization of the sizable orbital floor defect, which required orbital exploration and reconstruction using an anatomical plate.

Figure 3

Three-dimensional and axial computed tomography, as used for operative planning for a displaced orbitozygomatic fracture. In the preoperative images (top panels), the displaced zygoma (top left) was repositioned by mirroring the left zygoma and virtually positioning the mirrored bone (top middle and top right panels). The postoperative images (bottom panels) demonstrate anatomic alignment of the zygomatic articulations and reconstruction of the orbital floor

ISOLATED ORBITAL FRACTURES

Injuries to the orbital floor are commonly encountered among surgeons affiliated with trauma centers that treat facial injuries. Orbital floor injuries may occur in isolation or in conjunction with higher level Le Fort injuries, ZMC fractures, naso-orbital-ethmoid fractures, panfacial trauma, and often occur with fractures of the medial orbital wall. The primary goal for orbital reconstruction is to restore the premorbid orbital volume. In this regard, image-guidance technology has been useful for the design of anatomic orbital implants, particularly for two-walled defects involving the floor and medial wall.14151617181924 Custom implants can also be utilized for reconstruction of irregular defects or when there is a significant bone loss. Real-time intra-operative image guidance is a useful to aid in dissection of the orbital floor and medial wall, allowing the surgeon to assess accurately position relative to the superior orbital fissure, optic canal, and the anterior and posterior ethmoidal foramina, thereby decreasing the risk of damage to critical arterial, venous, and nervous structures in these areas. In the first clinical example, virtual planning was used to assess the size and position of the orbital implant required for reconstruction of a posttraumatic defect involving the right medial orbital wall and floor [Figures 4 and 5]. The unaffected orbit was used for comparison to allow for adequate reconstitution of orbital volume. The preoperative enophthalmos was predictably corrected; the postoperative image fusion demonstrates appropriate positioning of the implant along the orbital floor and medial wall, with a stable landing on the posterior ledge. In the second clinical example, a patient with a large, posttraumatic orbital floor defect with significant volume change and marked enophthalmos underwent presurgical planning and design of a custom titanium/polyethylene implant for orbital reconstruction [Figures 6 and 7]. The postoperative result demonstrates marked improvement in globe position, with excellent positioning of the implant along the fracture margins.

Figure 4

Preoperative (top panels) frontal and submentovertex views of a male patient with an orbital floor and medial wall fracture who underwent computer-assisted planning for orbital reconstruction. In the postoperative (bottom panels) photos, the enophthalmos has been corrected and the globe position is symmetric

Figure 5

Preoperative (left panels) and postoperative (right panels) demonstrating the three-dimensional computer planning for orbital reconstruction with an anatomic implant. The implant is rendered within the software and virtually placed within the defect. This allows the surgeon to position the implant and compute the orbital volume relative to the unaffected side. Intra-operatively, placement of the implant in the appropriate position can be ensured with the use of real-time navigation

Figure 6

Preoperative (top panels) frontal and submentovertex views of a female patient with a large right orbital floor fracture with significant enophthalmos. Computer-assisted planning was utilized to design a custom implant for reconstitution of orbital volume for enophthalmos correction. Postoperatively (bottom panels), the patient has has no orbital dystopia or enophthalmos

Figure 7

Preoperative (left panels) and postoperative (right panels) images, demonstrating the three-dimensional computer planning and subsequent reconstruction of a large orbital floor defect with an anatomic titanium-alloplastic hybrid implant. The preoperative images are utilized to compute the volume of the defect relative to the unaffected side. The custom implant is then designed to restore the orbital volume and correct the globe position

CONCLUSIONS

Advances in computer technology and imaging have improved the accessibility and efficacy of orbital reconstruction for defects involving the bony orbit. Due to the complex shape of the internal orbit, proximity of critical soft tissue structures, and small margins of error, the orbital skeleton is an ideal anatomic region for virtual planning and real-time intra-operative navigation. Utilization of this technology has the potential to improve the surgical treatment of common problems, make challenging clinical cases more accessible and predictable, and has enormous utility as an adjunct for trainee instruction.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.