ORIGINAL ARTICLE
The Use of Cone Beam Computed Tomography in the Postoperative Assessment of Orbital Wall Fracture Reconstruction Kim Tsao, MBBS, BDS,* Andrew Cheng, MBBS, BDS,* Alastair Goss, DDSc, FRACDS,* and David Donovan, MBBS, FRANZCR† (J Craniofac Surg 2014;25: 1150–1154) Purpose: Computed tomography (CT) is currently the standard in postoperative evaluation of orbital wall fracture reconstruction, but cone beam computed tomography (CBCT) offers potential advantages including reduced radiation dose and cost. The purpose of this study is to examine objectively the image quality of CBCT in the postoperative evaluation of orbital fracture reconstruction, its radiation dose, and cost compared with CT. Materials and Methods: Four consecutive patients with orbital wall fractures in whom surgery was indicated underwent orbital reconstruction with radio-opaque grafts (bone, titanium-reinforced polyethylene, and titanium plate) and were assessed postoperatively with orbital CBCT. CBCT was evaluated for its ability to provide objective information regarding the adequacy of orbital reconstruction, radiation dose, and cost. Results: In all patients, CBCT was feasible and provided hard tissue image quality comparable to CT with significantly reduced radiation dose and cost. However, it has poorer soft tissue resolution, which limits its ability to identify the extraocular muscles, their relationship to the reconstructive graft, and potential muscle entrapment. Conclusions: CBCT is a viable alternative to CT in the routine postoperative evaluation of orbital fracture reconstruction. However, in the patient who develops gaze restriction postoperatively, conventional CT is preferred over CBCT for its superior soft tissue resolution to exclude extraocular muscle entrapment. Key Words: Cone beam computed tomography, orbital fractures, radiation dosage
What Is This Box? A QR Code is a matrix barcode readable by QR scanners, mobile phones with cameras, and smartphones. The QR Code links to the online version of the article.
From the *Departments of Oral and Maxillofacial Surgery and †Radiology, Royal Adelaide Hospital, Adelaide, Australia. Received August 24, 2013. Accepted for publication January 7, 2014. Address correspondence and reprint requests to Kim Tsao, MBBS, BDS, Department of Oral and Maxillofacial Surgery, Royal Adelaide Hospital, North Terrace, Adelaide, South Australia 5000, Australia; E-mail: [email protected] The authors report no conflicts of interest. Copyright © 2014 by Mutaz B. Habal, MD ISSN: 1049-2275 DOI: 10.1097/SCS.0000000000000747
1150
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rbital fractures are common in the setting of trauma. Isolated orbital fractures constitute 4% to 16% of facial fractures. If one includes orbital fractures that extend beyond the confines of the orbit into other facial bones, the incidence approaches 30% to 55%.1,2 Orbital fractures may result in cosmetic sequelae such as enophthalmos and dystopia, as well as functional sequelae such as diplopia, infraorbital nerve paresthesia, and dysfunction of the nasolacrimal drainage system. Surgical intervention is often required to correct or prevent the development of these complications. Many different materials have been used for the reconstruction of orbital wall fractures. Autogenous bone grafts and alloplastic grafts are most commonly used.3,4 Postoperative imaging is frequently used to assess the adequacy of surgical reconstruction. Computed tomography (CT) is the most commonly used modality as it provides excellent resolution of the bony orbit. However, patients with orbital fractures commonly have other associated craniofacial injuries for which they have already received multiple CT scans and a significant cumulative radiation dose. An example may include intracranial hemorrhage requiring serial imaging for monitoring. The primary concern with exposure to ionizing radiation in the craniofacial region is the development of cataracts. In the past, it was believed that a high threshold of radiation (>2 Gy) was required to produce cataracts. It was also believed that the passage of time diluted the effects of radiation and the risk of cataracts.5 However, recent evidence suggests that the necessary radiation dose for cataractogenesis is much lower than previously thought. Furthermore, the risk of cataracts likely follows a linear non-threshold relationship, and the protraction of radiation exposure does not attenuate the risk of cataractogenesis.6 In other words, any amount of radiation contributes to a cumulative risk of cataracts that persists with time. Cone beam computed tomography (CBCT) offers potential advantages over CT in the setting of postoperative imaging of orbital reconstruction, including a lower radiation dose and reduced costs. CBCT provides craniofacial skeletal imaging with high geometric accuracy in all planes. However, it provides relatively poor soft tissue contrast compared with CT. CBCT has been used in the preoperative diagnosis of orbital fractures, in the intraoperative guidance of orbital reconstruction, and in the postoperative imaging of midface fractures.7–9 To our knowledge, the use of CBCT in the postoperative assessment of orbital fracture reconstruction has not yet been described in the literature. The purpose of this study is to examine objectively the image quality of CBCT in the postoperative evaluation of orbital fracture reconstruction, its radiation dose, and cost compared with CT.
PATIENTS AND METHODS Four consecutive patients with orbital fractures diagnosed on preoperative CT in whom surgery was indicated underwent orbital reconstruction.
The Journal of Craniofacial Surgery • Volume 25, Number 4, July 2014
Copyright © 2014 Mutaz B. Habal, MD. Unauthorized reproduction of this article is prohibited.
The Journal of Craniofacial Surgery • Volume 25, Number 4, July 2014
CBCT in Orbital Fracture Reconstruction
FIGURE 3. Preoperative orbital CT for patient 2. FIGURE 1. Preoperative orbital CT for patient 1.
Patient 1 A 76-year-old male retiree with postural hypotension fell onto his face. CT showed a right zygomatico-maxillary complex fracture and an associated orbital floor blowout fracture with maximal dimensions of 18 mm transversely and 20 mm anteroposteriorly with 10 mm of displacement (Figs. 1, 2). The eye examination was unremarkable. He underwent open reduction and internal fixation of the zygomatico-maxillary complex fracture and orbital floor reconstruction with a titanium-reinforced polyethylene implant (MEDPOR TITAN; Stryker Corporation, Portage, MI). Postoperative review showed restoration of facial symmetry and normal eye examination.
Patient 2 A 42-year-old male police officer was struck in the face with a fist during combat training. CT showed an isolated right orbital floor blowout fracture with maximal dimensions of 17 mm transversely and 20 mm anteroposteriorly with 10 mm of displacement (Figs. 3, 4). On examination, there was restricted right eye movement and diplopia on upward gaze. He underwent orbital floor reconstruction with an iliac crest bone graft. Postoperative review showed a normal eye examination.
Patient 3 A 22-year-old male university student was assaulted, sustaining multiple punches and kicks to the face. CT showed a right zygomatico-maxillary complex fracture and an associated orbital floor blowout fracture with maximal dimensions of 20 mm transversely and 20 mm anteroposteriorly with 15 mm of displacement (Figs. 5, 6). The eye examination was unremarkable. He underwent an open reduction and internal fixation of the zygomatico-maxillary complex fracture and orbital floor reconstruction with an iliac crest bone graft. Postoperative review showed restoration of facial symmetry and normal eye examination.
Patient 4 A 58-year-old unemployed woman fell down a flight of stairs while intoxicated with alcohol. CT showed bilateral orbital wall
FIGURE 2. Postoperative orbital CBCT for patient 1.
fractures. On the left, there was an orbital floor blow-out fracture with maximal dimensions of 25 mm transversely and 15 mm anteroposteriorly with 10 mm of displacement, and a medial wall blow-out fracture with maximal dimensions of 13 mm vertically and 15 mm anteroposteriorly with 8 mm of displacement (Figs. 7, 8). On the right, there was an orbital floor blow-out fracture with maximal dimensions of 15 mm transversely and 15 mm anteroposteriorly and 3 mm of displacement. On examination, there was enophthalmos and restricted eye movements on the left associated with diplopia in all directions. The right eye was unremarkable. After discussion of the risks and benefits of surgery with the patient, it was decided that larger left orbital fractures would be managed surgically and the smaller minimally displaced right orbital fracture would be managed conservatively initially, with the option of delayed reconstruction if visual symptoms persisted. She underwent left orbital reconstruction with preformed titanium orbital plate (MatrixORBITAL; Synthes Inc.,West Chester, PA). Postoperative review showed a normal eye examination. Postoperatively, orbital CBCT was performed for each patient (CS 9300; Carestream Health Inc., Rochester, NY).
RESULTS Postoperative CBCTwas feasible in all 4 patients. Each CBCT was evaluated for the ability to provide objective information regarding the adequacy of surgical reconstruction as listed in Table 1. In all patients, CBCT provided excellent resolution of skeletal landmarks and the different grafts used (bone, titanium-reinforced polyethylene implant, and preformed titanium plate)—this allowed accurate assessment of all objective measures as listed in Table 1. CBCT also allowed the volume of the reconstructed orbit to be measured and compared with the unaffected side. In all patients, the volume of the reconstructed orbit was comparable to the unaffected side (Table 2). The main shortcoming of CBCT is its poorer soft tissue resolution compared with CT. It is difficult to distinguish the extraocular muscles from the surrounding retrobulbar fat. It is also difficult to determine the relationship between the extraocular muscles and the reconstructive graft/implant. The adequacy of reduction of herniated orbital soft tissue back into the orbit proper can still be assessed by the absence of soft tissue density in the maxillary sinus below the graft.
FIGURE 4. Postoperative orbital CBCT for patient 2.
© 2014 Mutaz B. Habal, MD
Copyright © 2014 Mutaz B. Habal, MD. Unauthorized reproduction of this article is prohibited.
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The Journal of Craniofacial Surgery • Volume 25, Number 4, July 2014
Tsao et al
FIGURE 5. Preoperative orbital CT for patient 3.
The radiation dose for an orbital CBCT was estimated to be 70 μSv. The cost of each orbital CBCT was 113 Australian Dollars.
FIGURE 7. Preoperative orbital CT for patient 4.
This study shows postoperative CBCT can be used to objectively assess the adequacy of reconstruction of orbital wall fractures. CBCT provides comparable bony resolution at a significantly reduced radiation dose and cost compared with conventional CT. However, the soft tissue resolution was inferior to CT. The aim of surgical reconstruction of the orbital wall fracture is the prevention or treatment of enophthalmos, dystopia, and diplopia. The pathophysiology underlying these phenomena remains controversial; however, commonly cited mechanisms include loss of bony support, increased bony orbital volume, and decreased intraorbital soft tissue volume.10,11 A fracture through the orbital wall results in loss of bony contour and support for the globe. In this regard, the retrobulbar bulge at the posterior medial orbital floor or “key area” is considered to be particularly critical as it influences the anteroposterior and vertical position of the globe. Failure to reconstruct this area is thought to be a leading cause of unsatisfactory outcomes.11 An increase in the bony orbital volume may also result in loss of support for the globe. A correlation between increased bony orbital volume and enophthalmos has been demonstrated. A 1-cm3 volume increment produces between 0.47 and 0.89 mm of enophthalmos.12–14 Enophthalmos 2 mm or greater can be expected with a volume increase of 13% and 18%.13,15 A decrease in the intraorbital soft tissue volume may also result in loss of support for the globe. This may occur as a result of herniation of intraorbital soft tissue contents or fat atrophy. However, to date there is no conclusive evidence that intraorbital soft tissue volume changes are associated with enophthalmos.10,15 Considering these mechanisms, the objectives of surgical reconstruction should include the restoration of orbital wall contours and continuity, restoration of the premorbid bony orbital volume, and reduction of prolapsed soft tissue back into the orbit proper. Thus, the selected postoperative imaging modality should allow the surgeon to accurately assess whether or not these objectives have been achieved, with minimal side effects and cost. In this series of patients, postoperative CBCT provided excellent resolution of bony landmarks and a variety of grafts, which permitted accurate assessment of postoperative orbital wall contours and continuity. The postoperative bony orbital volume can also be calculated and compared with the unaffected side. The accuracy
of volume measurement is limited by the fact that there is no anterior wall of the orbit and the investigator must define an imagined anterior boundary, which may lead to under- or overestimation. In addition, there is normally a 7% to 8% volume difference between bilateral orbits in an individual.16,17 The main disadvantage of CBCT is poorer soft tissue resolution compared with CT. The reduction of herniated orbital soft tissue can still be assessed by the presence or absence of soft tissue density in the maxillary sinus below the graft. However, it is difficult to distinguish the extraocular muscles from the surrounding retrobulbar tissues and to determine their exact relationship with the reconstructive graft/implant. This may limit the utility of CBCT in the patient who develops restriction of eye movements postoperatively. In this scenario, CBCT may not be able to identify impingement of the graft/implant on the extraocular muscles and reliably exclude muscle entrapment as the cause of gaze restriction. In the patient who develops gaze restriction postoperatively, CT should be preferred over CBCT for its superior soft tissue resolution and the ability to identify muscle entrapment due to graft impingement, which would warrant graft repositioning or removal. The advantages of CBCT over CT are reduced radiation dose and cost. Patients with orbital fractures frequently have other concomitant craniofacial injuries and have received multiple CT scans and a significant radiation dose. The primary concern about radiation exposure to the craniofacial region is the risk of cataracts. Radiation-induced posterior subcapsular cataracts have long been documented in the literature.18 It appears to result from radiation injury to dividing cells in the lens epithelium and disruption of differentiation that leads to the deposition of granular debris instead of new lens fibers.19–22 Earlier studies of atomic bomb survivors suggested radiation-induced cataract was a deterministic effect with a minimum dose threshold of approximately 1.5 Gy.23–26 Based on these and other studies, the International Commission on Radiological Protection (ICRP) assumed the minimal dose required for cataracts to be 0.5 to 2.0 Gy received as a single dose or 5 Gy received as fractionated doses over time.5 However, recent studies show the lens is more radiosensitive than previously thought. Epidemiological studies on atomic bomb survivors and Chernobyl clean-up workers show a dose threshold of 0.1 to 0.8 Gy.27–29 Furthermore, these studies suggest there is actually no minimum threshold but a linear, non-threshold relationship. This result is compatible with other studies that demonstrate a statistically significant increase in lens opacities and cataracts for doses as low as 0.008 Sv.30–32 The lens of children are more even radiosensitive than adults. In a study
FIGURE 6. Postoperative orbital CBCT for patient 3.
FIGURE 8. Postoperative orbital CBCT for patient 4.
DISCUSSION
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© 2014 Mutaz B. Habal, MD
Copyright © 2014 Mutaz B. Habal, MD. Unauthorized reproduction of this article is prohibited.
The Journal of Craniofacial Surgery • Volume 25, Number 4, July 2014
CT include significantly lower radiation dose and cost. Its main disadvantage is poorer soft tissue resolution compared with CT. It is difficult to distinguish the extraocular muscles and to determine their relationship with the reconstructive graft. This limits its utility in the patient who develops gaze restriction postoperatively, as muscle entrapment could not be reliably excluded as a cause. In these patients, conventional CT should be preferred over CBCT.
TABLE 1. Adequacy of Reconstruction Orbital fracture Graft position Graft contact with fracture defect margins Graft contour Intraorbital soft tissue reduction into orbit proper Orbital bony volume Associated ZMC fracture Fracture reduction Fixation position 2D and 3D bony symmetry
REFERENCES
of children aged less than 12 months with skin hemangiomas treated with radiotherapy, posterior subcapsular opacities were detected after 0.06 to 0.12 Gy.33 Studies on radiation workers suggest the protraction of radiation dose does not attenuate risk.30,34 A linear non-threshold relationship and the lack of risk attenuation with protraction of exposure suggests any radiation dose, no matter how small, contributes to a cumulative risk for cataracts which persists with time.6 Based on the recent evidence, the ICRP has revised the threshold dose for cataractogenesis to 0.5 Gy for both acute and fractionated exposures. The recommended annual dose limit for the lens is 20 mSv.35 The reported radiation dose from a conventional CT scan of the orbits varies widely in the literature, ranging from 25 to 103 mSv.36–38 In our own institution, the radiation dose from an orbital CT is estimated to be 20 to 30 mSv. In this series of patients, the radiation dose from orbital CBCT was estimated to be 70 μSv. This is comparable to another published study.7 In Australia, the cost of a conventional CT of the orbits is 250 Australian Dollars according to the national Medicare Benefits Schedule. The cost of an orbital CBCT is significantly lower at 113 Australian Dollars. The value of routine postoperative imaging following facial fracture repair remains controversial. Some authors argue there is a low incidence (
The Use of Cone Beam Computed Tomography in the Postoperative Assessment of Orbital Wall Fracture Reconstruction Kim Tsao, MBBS, BDS,* Andrew Cheng, MBBS, BDS,* Alastair Goss, DDSc, FRACDS,* and David Donovan, MBBS, FRANZCR† (J Craniofac Surg 2014;25: 1150–1154) Purpose: Computed tomography (CT) is currently the standard in postoperative evaluation of orbital wall fracture reconstruction, but cone beam computed tomography (CBCT) offers potential advantages including reduced radiation dose and cost. The purpose of this study is to examine objectively the image quality of CBCT in the postoperative evaluation of orbital fracture reconstruction, its radiation dose, and cost compared with CT. Materials and Methods: Four consecutive patients with orbital wall fractures in whom surgery was indicated underwent orbital reconstruction with radio-opaque grafts (bone, titanium-reinforced polyethylene, and titanium plate) and were assessed postoperatively with orbital CBCT. CBCT was evaluated for its ability to provide objective information regarding the adequacy of orbital reconstruction, radiation dose, and cost. Results: In all patients, CBCT was feasible and provided hard tissue image quality comparable to CT with significantly reduced radiation dose and cost. However, it has poorer soft tissue resolution, which limits its ability to identify the extraocular muscles, their relationship to the reconstructive graft, and potential muscle entrapment. Conclusions: CBCT is a viable alternative to CT in the routine postoperative evaluation of orbital fracture reconstruction. However, in the patient who develops gaze restriction postoperatively, conventional CT is preferred over CBCT for its superior soft tissue resolution to exclude extraocular muscle entrapment. Key Words: Cone beam computed tomography, orbital fractures, radiation dosage
What Is This Box? A QR Code is a matrix barcode readable by QR scanners, mobile phones with cameras, and smartphones. The QR Code links to the online version of the article.
From the *Departments of Oral and Maxillofacial Surgery and †Radiology, Royal Adelaide Hospital, Adelaide, Australia. Received August 24, 2013. Accepted for publication January 7, 2014. Address correspondence and reprint requests to Kim Tsao, MBBS, BDS, Department of Oral and Maxillofacial Surgery, Royal Adelaide Hospital, North Terrace, Adelaide, South Australia 5000, Australia; E-mail: [email protected] The authors report no conflicts of interest. Copyright © 2014 by Mutaz B. Habal, MD ISSN: 1049-2275 DOI: 10.1097/SCS.0000000000000747
1150
O
rbital fractures are common in the setting of trauma. Isolated orbital fractures constitute 4% to 16% of facial fractures. If one includes orbital fractures that extend beyond the confines of the orbit into other facial bones, the incidence approaches 30% to 55%.1,2 Orbital fractures may result in cosmetic sequelae such as enophthalmos and dystopia, as well as functional sequelae such as diplopia, infraorbital nerve paresthesia, and dysfunction of the nasolacrimal drainage system. Surgical intervention is often required to correct or prevent the development of these complications. Many different materials have been used for the reconstruction of orbital wall fractures. Autogenous bone grafts and alloplastic grafts are most commonly used.3,4 Postoperative imaging is frequently used to assess the adequacy of surgical reconstruction. Computed tomography (CT) is the most commonly used modality as it provides excellent resolution of the bony orbit. However, patients with orbital fractures commonly have other associated craniofacial injuries for which they have already received multiple CT scans and a significant cumulative radiation dose. An example may include intracranial hemorrhage requiring serial imaging for monitoring. The primary concern with exposure to ionizing radiation in the craniofacial region is the development of cataracts. In the past, it was believed that a high threshold of radiation (>2 Gy) was required to produce cataracts. It was also believed that the passage of time diluted the effects of radiation and the risk of cataracts.5 However, recent evidence suggests that the necessary radiation dose for cataractogenesis is much lower than previously thought. Furthermore, the risk of cataracts likely follows a linear non-threshold relationship, and the protraction of radiation exposure does not attenuate the risk of cataractogenesis.6 In other words, any amount of radiation contributes to a cumulative risk of cataracts that persists with time. Cone beam computed tomography (CBCT) offers potential advantages over CT in the setting of postoperative imaging of orbital reconstruction, including a lower radiation dose and reduced costs. CBCT provides craniofacial skeletal imaging with high geometric accuracy in all planes. However, it provides relatively poor soft tissue contrast compared with CT. CBCT has been used in the preoperative diagnosis of orbital fractures, in the intraoperative guidance of orbital reconstruction, and in the postoperative imaging of midface fractures.7–9 To our knowledge, the use of CBCT in the postoperative assessment of orbital fracture reconstruction has not yet been described in the literature. The purpose of this study is to examine objectively the image quality of CBCT in the postoperative evaluation of orbital fracture reconstruction, its radiation dose, and cost compared with CT.
PATIENTS AND METHODS Four consecutive patients with orbital fractures diagnosed on preoperative CT in whom surgery was indicated underwent orbital reconstruction.
The Journal of Craniofacial Surgery • Volume 25, Number 4, July 2014
Copyright © 2014 Mutaz B. Habal, MD. Unauthorized reproduction of this article is prohibited.
The Journal of Craniofacial Surgery • Volume 25, Number 4, July 2014
CBCT in Orbital Fracture Reconstruction
FIGURE 3. Preoperative orbital CT for patient 2. FIGURE 1. Preoperative orbital CT for patient 1.
Patient 1 A 76-year-old male retiree with postural hypotension fell onto his face. CT showed a right zygomatico-maxillary complex fracture and an associated orbital floor blowout fracture with maximal dimensions of 18 mm transversely and 20 mm anteroposteriorly with 10 mm of displacement (Figs. 1, 2). The eye examination was unremarkable. He underwent open reduction and internal fixation of the zygomatico-maxillary complex fracture and orbital floor reconstruction with a titanium-reinforced polyethylene implant (MEDPOR TITAN; Stryker Corporation, Portage, MI). Postoperative review showed restoration of facial symmetry and normal eye examination.
Patient 2 A 42-year-old male police officer was struck in the face with a fist during combat training. CT showed an isolated right orbital floor blowout fracture with maximal dimensions of 17 mm transversely and 20 mm anteroposteriorly with 10 mm of displacement (Figs. 3, 4). On examination, there was restricted right eye movement and diplopia on upward gaze. He underwent orbital floor reconstruction with an iliac crest bone graft. Postoperative review showed a normal eye examination.
Patient 3 A 22-year-old male university student was assaulted, sustaining multiple punches and kicks to the face. CT showed a right zygomatico-maxillary complex fracture and an associated orbital floor blowout fracture with maximal dimensions of 20 mm transversely and 20 mm anteroposteriorly with 15 mm of displacement (Figs. 5, 6). The eye examination was unremarkable. He underwent an open reduction and internal fixation of the zygomatico-maxillary complex fracture and orbital floor reconstruction with an iliac crest bone graft. Postoperative review showed restoration of facial symmetry and normal eye examination.
Patient 4 A 58-year-old unemployed woman fell down a flight of stairs while intoxicated with alcohol. CT showed bilateral orbital wall
FIGURE 2. Postoperative orbital CBCT for patient 1.
fractures. On the left, there was an orbital floor blow-out fracture with maximal dimensions of 25 mm transversely and 15 mm anteroposteriorly with 10 mm of displacement, and a medial wall blow-out fracture with maximal dimensions of 13 mm vertically and 15 mm anteroposteriorly with 8 mm of displacement (Figs. 7, 8). On the right, there was an orbital floor blow-out fracture with maximal dimensions of 15 mm transversely and 15 mm anteroposteriorly and 3 mm of displacement. On examination, there was enophthalmos and restricted eye movements on the left associated with diplopia in all directions. The right eye was unremarkable. After discussion of the risks and benefits of surgery with the patient, it was decided that larger left orbital fractures would be managed surgically and the smaller minimally displaced right orbital fracture would be managed conservatively initially, with the option of delayed reconstruction if visual symptoms persisted. She underwent left orbital reconstruction with preformed titanium orbital plate (MatrixORBITAL; Synthes Inc.,West Chester, PA). Postoperative review showed a normal eye examination. Postoperatively, orbital CBCT was performed for each patient (CS 9300; Carestream Health Inc., Rochester, NY).
RESULTS Postoperative CBCTwas feasible in all 4 patients. Each CBCT was evaluated for the ability to provide objective information regarding the adequacy of surgical reconstruction as listed in Table 1. In all patients, CBCT provided excellent resolution of skeletal landmarks and the different grafts used (bone, titanium-reinforced polyethylene implant, and preformed titanium plate)—this allowed accurate assessment of all objective measures as listed in Table 1. CBCT also allowed the volume of the reconstructed orbit to be measured and compared with the unaffected side. In all patients, the volume of the reconstructed orbit was comparable to the unaffected side (Table 2). The main shortcoming of CBCT is its poorer soft tissue resolution compared with CT. It is difficult to distinguish the extraocular muscles from the surrounding retrobulbar fat. It is also difficult to determine the relationship between the extraocular muscles and the reconstructive graft/implant. The adequacy of reduction of herniated orbital soft tissue back into the orbit proper can still be assessed by the absence of soft tissue density in the maxillary sinus below the graft.
FIGURE 4. Postoperative orbital CBCT for patient 2.
© 2014 Mutaz B. Habal, MD
Copyright © 2014 Mutaz B. Habal, MD. Unauthorized reproduction of this article is prohibited.
1151
The Journal of Craniofacial Surgery • Volume 25, Number 4, July 2014
Tsao et al
FIGURE 5. Preoperative orbital CT for patient 3.
The radiation dose for an orbital CBCT was estimated to be 70 μSv. The cost of each orbital CBCT was 113 Australian Dollars.
FIGURE 7. Preoperative orbital CT for patient 4.
This study shows postoperative CBCT can be used to objectively assess the adequacy of reconstruction of orbital wall fractures. CBCT provides comparable bony resolution at a significantly reduced radiation dose and cost compared with conventional CT. However, the soft tissue resolution was inferior to CT. The aim of surgical reconstruction of the orbital wall fracture is the prevention or treatment of enophthalmos, dystopia, and diplopia. The pathophysiology underlying these phenomena remains controversial; however, commonly cited mechanisms include loss of bony support, increased bony orbital volume, and decreased intraorbital soft tissue volume.10,11 A fracture through the orbital wall results in loss of bony contour and support for the globe. In this regard, the retrobulbar bulge at the posterior medial orbital floor or “key area” is considered to be particularly critical as it influences the anteroposterior and vertical position of the globe. Failure to reconstruct this area is thought to be a leading cause of unsatisfactory outcomes.11 An increase in the bony orbital volume may also result in loss of support for the globe. A correlation between increased bony orbital volume and enophthalmos has been demonstrated. A 1-cm3 volume increment produces between 0.47 and 0.89 mm of enophthalmos.12–14 Enophthalmos 2 mm or greater can be expected with a volume increase of 13% and 18%.13,15 A decrease in the intraorbital soft tissue volume may also result in loss of support for the globe. This may occur as a result of herniation of intraorbital soft tissue contents or fat atrophy. However, to date there is no conclusive evidence that intraorbital soft tissue volume changes are associated with enophthalmos.10,15 Considering these mechanisms, the objectives of surgical reconstruction should include the restoration of orbital wall contours and continuity, restoration of the premorbid bony orbital volume, and reduction of prolapsed soft tissue back into the orbit proper. Thus, the selected postoperative imaging modality should allow the surgeon to accurately assess whether or not these objectives have been achieved, with minimal side effects and cost. In this series of patients, postoperative CBCT provided excellent resolution of bony landmarks and a variety of grafts, which permitted accurate assessment of postoperative orbital wall contours and continuity. The postoperative bony orbital volume can also be calculated and compared with the unaffected side. The accuracy
of volume measurement is limited by the fact that there is no anterior wall of the orbit and the investigator must define an imagined anterior boundary, which may lead to under- or overestimation. In addition, there is normally a 7% to 8% volume difference between bilateral orbits in an individual.16,17 The main disadvantage of CBCT is poorer soft tissue resolution compared with CT. The reduction of herniated orbital soft tissue can still be assessed by the presence or absence of soft tissue density in the maxillary sinus below the graft. However, it is difficult to distinguish the extraocular muscles from the surrounding retrobulbar tissues and to determine their exact relationship with the reconstructive graft/implant. This may limit the utility of CBCT in the patient who develops restriction of eye movements postoperatively. In this scenario, CBCT may not be able to identify impingement of the graft/implant on the extraocular muscles and reliably exclude muscle entrapment as the cause of gaze restriction. In the patient who develops gaze restriction postoperatively, CT should be preferred over CBCT for its superior soft tissue resolution and the ability to identify muscle entrapment due to graft impingement, which would warrant graft repositioning or removal. The advantages of CBCT over CT are reduced radiation dose and cost. Patients with orbital fractures frequently have other concomitant craniofacial injuries and have received multiple CT scans and a significant radiation dose. The primary concern about radiation exposure to the craniofacial region is the risk of cataracts. Radiation-induced posterior subcapsular cataracts have long been documented in the literature.18 It appears to result from radiation injury to dividing cells in the lens epithelium and disruption of differentiation that leads to the deposition of granular debris instead of new lens fibers.19–22 Earlier studies of atomic bomb survivors suggested radiation-induced cataract was a deterministic effect with a minimum dose threshold of approximately 1.5 Gy.23–26 Based on these and other studies, the International Commission on Radiological Protection (ICRP) assumed the minimal dose required for cataracts to be 0.5 to 2.0 Gy received as a single dose or 5 Gy received as fractionated doses over time.5 However, recent studies show the lens is more radiosensitive than previously thought. Epidemiological studies on atomic bomb survivors and Chernobyl clean-up workers show a dose threshold of 0.1 to 0.8 Gy.27–29 Furthermore, these studies suggest there is actually no minimum threshold but a linear, non-threshold relationship. This result is compatible with other studies that demonstrate a statistically significant increase in lens opacities and cataracts for doses as low as 0.008 Sv.30–32 The lens of children are more even radiosensitive than adults. In a study
FIGURE 6. Postoperative orbital CBCT for patient 3.
FIGURE 8. Postoperative orbital CBCT for patient 4.
DISCUSSION
1152
© 2014 Mutaz B. Habal, MD
Copyright © 2014 Mutaz B. Habal, MD. Unauthorized reproduction of this article is prohibited.
The Journal of Craniofacial Surgery • Volume 25, Number 4, July 2014
CT include significantly lower radiation dose and cost. Its main disadvantage is poorer soft tissue resolution compared with CT. It is difficult to distinguish the extraocular muscles and to determine their relationship with the reconstructive graft. This limits its utility in the patient who develops gaze restriction postoperatively, as muscle entrapment could not be reliably excluded as a cause. In these patients, conventional CT should be preferred over CBCT.
TABLE 1. Adequacy of Reconstruction Orbital fracture Graft position Graft contact with fracture defect margins Graft contour Intraorbital soft tissue reduction into orbit proper Orbital bony volume Associated ZMC fracture Fracture reduction Fixation position 2D and 3D bony symmetry
REFERENCES
of children aged less than 12 months with skin hemangiomas treated with radiotherapy, posterior subcapsular opacities were detected after 0.06 to 0.12 Gy.33 Studies on radiation workers suggest the protraction of radiation dose does not attenuate risk.30,34 A linear non-threshold relationship and the lack of risk attenuation with protraction of exposure suggests any radiation dose, no matter how small, contributes to a cumulative risk for cataracts which persists with time.6 Based on the recent evidence, the ICRP has revised the threshold dose for cataractogenesis to 0.5 Gy for both acute and fractionated exposures. The recommended annual dose limit for the lens is 20 mSv.35 The reported radiation dose from a conventional CT scan of the orbits varies widely in the literature, ranging from 25 to 103 mSv.36–38 In our own institution, the radiation dose from an orbital CT is estimated to be 20 to 30 mSv. In this series of patients, the radiation dose from orbital CBCT was estimated to be 70 μSv. This is comparable to another published study.7 In Australia, the cost of a conventional CT of the orbits is 250 Australian Dollars according to the national Medicare Benefits Schedule. The cost of an orbital CBCT is significantly lower at 113 Australian Dollars. The value of routine postoperative imaging following facial fracture repair remains controversial. Some authors argue there is a low incidence (
