Figure 1. Models of the patient's nasal cavity (excluding the paranasal sinuses) and the soft tissues surrounding the nasal cavity created from presurgery and postsurgery computed tomographic (CT) scans. A, Reconstructions of the right lateral wall and right side of the septum from presurgery CT scan. B, Same views with surgical sites marked. Purple indicates nasal valve repair; blue, right inferior turbinate reduction; and green, septoplasty. Arrows indicate coronal levels shown in panel D. C, Reconstructions of the right lateral wall and right septum from postsurgery CT scan, coregistered with presurgery scan. D, Coronal views from presurgery and postsurgery CT scans. Yellow outline highlights border between soft tissue and nasal air space.
Figure 2. Comparison of appearance between the actual photographs and digital soft-tissue models. Presurgery frontal view (A) compared with appearance on digital model of soft tissue of the face (B). The 6-month postsurgery frontal view (C) compared with appearance on digital model of soft tissue of the face (D).
Figure 3. Comparison of appearance between the actual photographs and digital soft-tissue models. Presurgery base view (A) compared with appearance on digital model of soft tissue of the face (B). The 6-month postsurgery base view (C) compared with appearance on digital model of soft tissue of the face (D).
Figure 4. Virtual endoscopic view from right nasal vestibule. Photographs compare the presurgery (A) and postsurgery (B) appearance. Images are color-coded for wall shear, with blue signifying areas of low shear stress and green-yellow signifying areas of higher shear stress. S denotes nasal septum. Note contact between septum and lateral wall in presurgery image with associated higher shear stress above and below this area.
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Rhee JS, Cannon DE, Frank DO, Kimbell JS. Role of Virtual Surgery in Preoperative Planning: Assessing the Individual Components of Functional Nasal Airway Surgery. Arch Facial Plast Surg. 2012;14(5):354–359. doi:10.1001/archfacial.2012.182
Author Affiliations: Department of Otolaryngology and Communication Sciences, Medical College of Wisconsin, Milwaukee (Drs Rhee and Cannon); and Department of Otolaryngology–Head and Neck Surgery, University of North Carolina School of Medicine, Chapel Hill (Drs Frank and Kimbell).
Objectives To demonstrate the effect of individual components of functional nasal airway surgery in a patient with multifactorial obstruction and to discuss the potential benefit of computational fluid dynamics (CFD)–aided virtual surgery.
Methods A 53-year-old woman underwent septoplasty, turbinate reduction, and nasal valve repair. Presurgery and postsurgery digital nasal models were created from computed tomographic images, and nasal resistance was calculated using CFD techniques. The digital models were then manipulated to isolate the effects of the components of the surgery, creating a nasal valve repair alone model and a septoplasty/turbinate reduction alone model.
Results Bilateral nasal resistance in the postsurgery model was approximately 25% less than presurgery values. Similarly, CFD analysis showed reductions in nasal resistance of the virtual models: 19% reduction with intranasal surgery alone and 6% reduction with nasal valve repair alone.
Conclusions Most of the reduction in nasal resistance was accomplished with performance of septoplasty and inferior turbinate reduction. The contribution of nasal valve repair was less in comparison but not insignificant. This pilot study implies that CFD-aided virtual surgery may be useful as part of preoperative planning in patients with multifactorial anatomical nasal airway obstruction.
Nasal airway obstruction (NAO) is a common condition associated with a poor quality of life1 and numerous physician visits and medical and surgical treatments. Among the leading causes of NAO are inflammatory conditions and anatomical abnormalities, such as septal deviation, turbinate hypertrophy, and nasal valve compromise (NVC). Although the management of inflammatory medical conditions primarily consists of topical medications, including corticosteroids, antihistamines, and salve, surgery is the mainstay for treatment of anatomical abnormalities. However, the results of surgical treatment of NAO are variable. For example, after performance of septoplasty with or without turbinectomy, approximately 25% of patients may be unsatisfied with surgical results, with less than half completely satisfied.2
This patient dissatisfaction has several potential explanations. One reason is the poor correlation between subjective and objective methods of measuring NAO,3-5 which creates difficulty in analyzing and reporting the results of functional nasal surgery. Another potential reason for surgical failure is inappropriate procedure selection or failure to target the appropriate problematic components of the nasal airway.6 Patients can have multifactorial anatomical obstruction due to coexisting problems, and there can be difficulty in deciding which component(s) to address for the best functional outcome. Although there is evidence of the efficacy of septoplasty,7 turbinate reduction,8 and nasal valve repair9 individually in the treatment of nasal obstruction, data on procedures performed in conjunction are less clear. Studies10,11 involving patients with both septal deviation and turbinate hypertrophy have demonstrated no additional benefit of performing both turbinate reduction and septoplasty compared with either procedure alone.
Inappropriate surgical procedures, such as septoplasty in a patient whose main problem was, in fact, NVC, expose patients to unnecessary risk while providing little or no benefit. Undertreatment, such as treating a patient with both septal deviation and NVC with septoplasty alone, provides incomplete benefit and may result in a poor outcome and patient dissatisfaction with surgery. Surgically overtreating patients with NAO (ie, a “shotgun” approach consisting of septoplasty, turbinate reduction, and nasal valve repair for every patient) might expose patients to unnecessary risk while not providing additional benefit. There are also financial ramifications to overtreatment, with additional cost to patients, insurance providers, and an already overburdened health care system.
In today's evidence-based medicine environment, there is an emphasis on demonstrating efficacy of treatments or interventions using validated outcome measures. In addition, insurance companies more commonly require preauthorization for surgery and deny compensation for procedures with questionable benefit or that are not clearly indicated. These factors place additional pressure on health care professionals to offer safe, effective, and patient-centered care in a cost-efficient manner. In an ideal situation, nasal surgeons would have technologically advanced tools at their disposal to assist in the preoperative evaluation of patients with NAO to better assess the nasal airway, identify problematic areas, and select the optimal surgical intervention to produce the best possible outcome.
Currently, such tools do not exist. The current methods of objectively assessing the nasal airway, such as acoustic rhinometry and rhinomanometry, have limitations that prevent them from being widely accepted12 and do not provide much insight into preoperative planning. Newer technology adopted from engineering fields has the potential to fill this gap. Computational fluid dynamics (CFD) is a technique that can be used to study nasal airflow and resistance, as well as other aspects of nasal physiology. Using anatomically accurate 3-dimensional (3D) digital models created from computed tomography (CT) or magnetic resonance imaging, CFD simulation software can calculate nasal resistance, airflow velocity, air conditioning, and wall shear stress. Furthermore, the computational models can be digitally modified to simulate the changes created by surgery, and new simulations can be performed to determine what effect these changes produce with respect to resistance and other parameters of interest.13
This article presents the results of a preliminary study that is part of a larger prospective project to correlate CFD data with patient-reported subjective measures of nasal obstruction and examine the role of virtual surgery in predicting postoperative outcomes. The aims of this particular study are to demonstrate the effect of individual components of functional nasal airway surgery in a patient with multifactorial obstruction and discuss the potential benefit of CFD-aided virtual surgery.
Research methods were approved by the institutional review board at the Medical College of Wisconsin, and written informed consent was obtained. For this study, a 53-year-old woman with a long-standing bilateral nasal obstruction and no prior history of nasal trauma or surgery was selected. Physical examination revealed the presence of rightward septal deviation, bilateral inferior turbinate hypertrophy, and NVC. Modified contiguous CT in the axial plane of the entire nasal cavity and external nasal soft tissues was performed before surgery and 6 months after surgery (both times with 6.0-mm increments and 0.313-mm resolution). Surgical treatment decisions were made by the surgeon (J.S.R.) based on the clinical presentation and standard of medical care. For this patient, septoplasty, nasal valve repair, and inferior turbinate reduction were performed. The standard external rhinoplasty approach was used to gain access to the septum and lower two-thirds of the external part of the nose. Submucosal resection of the deviated midportion of the septum was performed. Septal cartilage was then used to create a butterfly graft, bridging the nasal dorsum and positioned beneath the lower lateral cartilages. Inferior turbinate reductions were accomplished via submucosal resection of the bone and soft tissue of the anterior half of the turbinates. Standard postoperative care was performed, and the patient's recovery and healing were uneventful.
The presurgery and postsurgery CT scans were used to create digital 3D models of the patient's nasal cavity (excluding the paranasal sinuses) and the soft tissues surrounding the nasal cavity, using the image analysis software Mimics 14.0 (Materialise Inc) (Figure 1). Presurgery and postsurgery models were coregistered using 3D reconstructions of the skull in each scan. To isolate the effects of the separate components of the surgery, 2 hybrid nasal cavity models using portions of the presurgery and postsurgery CT scans were created. To allow investigation of the intranasal component of the surgery (septoplasty and turbinate reduction) alone, a model was created using the external part of the nose in the presurgery model fused to the nasal cavity of the postsurgery model. Another model was created using the external part of the nose in the postsurgery model fused to the nasal cavity of the presurgery model to allow investigation of the nasal valve repair component of the surgery alone. Any minor mismatches at the interfaces where the portions of the models were fused were smoothed using the image analysis software to minimize their effect on the results of the simulation.
The CFD modeling methods used have been described in detail elsewhere.14 Briefly, planar nostril and outlet surfaces were created, and the digital models were meshed with approximately 4 million tetrahedral cells using ICEM-CFD 11.0 software (ANSYS Inc). Steady-state inspiratory airflow simulations were conducted using Fluent 12.0 software (ANSYS Inc) with the following boundary conditions: (1) a wall condition (in which velocity was set to zero at airway walls, which were assumed to be stationary), (2) a pressure-inlet condition at the nostrils in which gauge pressure was set to zero, and (3) a pressure-outlet condition at the outlet in which gauge pressure was set to a negative value in Pascals corresponding to a target steady-state flow rate. The target airflow rate was 14.9 L/min, twice the patient's estimated minute volume (amount of air exhaled in 1 minute), which was obtained from body weight.15 Values used for the density and dynamic viscosity of air were 1.204 kg/m3 and 1.825 × 10−5 kg/ms, respectively. Fluent and the postprocessing software Fieldview 12.0 (Intelligent Light) were used for simulation analysis and visualization of results.
The outcome measures calculated by CFD analysis were bilateral and unilateral nasal resistance and airflow allocation. Resistance was calculated as Δp/Q, where Δp was the transnasal pressure decrease (in Pascals) between the nostrils and the posterior end of the septum, and Q was the volumetric airflow rate (in milliliters per second). Airflow allocation was determined as the percentage of the total airflow that traveled through each side of the nasal cavity. In addition, the patient was administered the Nasal Obstruction Symptom Evaluation (NOSE) Scale to collect information on patient-reported symptoms before and after surgery.16 The NOSE Scale is a disease-specific quality-of-life instrument for NAO that has been validated for septoplasty11 and nasal valve repair, for which it has also been used to measure surgical success.1,17 Finally, as part of the research protocol, unilateral visual analog scale (VAS) scores for nasal airflow were determined before and after surgery. Patients were asked to cover one nostril and rate their ability to breathe through the uncovered nostril on a scale of 0 (completely obstructed) to 10 (no obstruction). The process was then repeated for the opposite nostril.
Before surgery, bilateral nasal resistance was estimated from CFD analysis to be 0.0979 Pa/(mL/s). In the postsurgery model, bilateral nasal resistance was 0.0742 Pa/(mL/s), a 24.2% reduction from presurgical resistance. In the model with nasal valve repair alone, bilateral nasal resistance was 0.0922 Pa/(mL/s), a 5.8% reduction from presurgical resistance. In the model with septoplasty and inferior turbinate reduction alone, bilateral nasal resistance was 0.0788 Pa/(mL/s), a 19.5% reduction from presurgical resistance (Table 1). When examined unilaterally, right-sided nasal resistance decreased from 0.2604 Pa/(mL/s) before surgery to 0.1217 Pa/(mL/s) after surgery, a 53.2% reduction. Conversely, left-sided nasal resistance was mildly increased from 0.1577 Pa/(mL/s) before surgery to 0.1904 Pa/(mL/s) after surgery, an increase of 20.7%. In the nasal valve repair alone model, nasal resistance was mildly decreased in both sides compared with before surgery, whereas in the septoplasty and turbinate reduction model, right-sided nasal resistance was reduced by approximately 50%, but left-sided nasal resistance was increased by approximately 27% compared with before surgery. The unilateral nasal resistance results of the septoplasty and turbinate reduction alone model approximated the results of the postsurgery model (Table 2).
Allocation of airflow between the nasal cavities was altered by surgery as well (Table 3). Before surgery, 62.9% of airflow passed through the left side of the nose, whereas after surgery, 38.4% passed through the left side. The nasal valve repair alone model showed an allocation pattern similar to the presurgery pattern (63.3% through the left side), whereas the septoplasty and inferior turbinate reduction model had a pattern similar to the postsurgery pattern (39.0% through the left side). Airstream flow through the postsurgery nasal passage was noted to be qualitatively improved as exemplified by more laminar
Nasal obstruction is a complex phenomenon that likely has many contributing factors. Even within the domain of anatomical nasal obstruction, there can be several potential problematic areas, creating a dilemma for the nasal surgeon in deciding which components of the nasal airway to target surgically to produce the best possible outcome for the patient. Although experience and clinical judgment are essential, it would be useful to have additional assistance with preoperative planning.
In this pilot study, we examined a patient with multiple anatomical causes of nasal obstruction treated surgically in an effort to determine how much benefit could be attributed to each of the components. In this particular patient, most of the benefit with respect to bilateral nasal resistance was a result of the combined septoplasty and inferior turbinate reduction. However, additional benefit in overall nasal resistance was added by the performance of nasal valve repair. The right side, which was most affected by the septal deviation, showed a substantial decrease in nasal resistance after septoplasty and turbinate reduction when examined unilaterally, whereas the left side showed a moderate increase, as would be expected after correction of the septal deviation. The effect of surgery on airflow allocation was exclusively a result of the septoplasty and turbinate reduction as well. Nasal valve repair alone produced mild decreases in unilateral nasal resistance for both sides of the nose.
To our knowledge, this is the first study that simulates individual virtual surgery scenarios (nasal valve surgery vs internal nasal surgery) with comparison to a known outcome. A previous article published by members of our group13 used a virtual surgery model and CFD techniques to examine the effects of septoplasty and turbinate reduction on nasal resistance and airflow. One of the best attempts at quantifying the nasal valve component (also known as functional rhinoplasty) separate from other surgical manipulations was performed by Constantian and Clardy,18 in which they investigated the relative importance of septal surgery and nasal valve repair in correcting NAO. They prospectively evaluated a group of patients with NAO using rhinomanometry as their primary outcome measure. Patients were stratified according to the site of their obstruction as determined by clinical examination: internal nasal valves, external nasal valves, septum, or any combination of these 3; patients with turbinate hypertrophy were excluded. Surgery was then performed with targeting of problematic areas identified preoperatively. The authors concluded that the effects of NVC may equal or even surpass septal deviation as the primary cause of NAO, in contrast to what was discovered in the present simulation.
The question of what constitutes a successful nasal surgery is a key consideration. As of yet a correlation between CFD data and subjective, patient-reported measures has not been reported. In this particular case, the patient reported symptomatic improvement in nasal symptoms, as represented by her improvements in NOSE and VAS scores. Accordingly, she also exhibited improvement in bilateral nasal resistance and in unilateral resistance on the side affected by septal deviation as determined with CFD techniques. Interestingly, however, the patient reported an improvement in VAS scores on the contralateral side even though the nasal resistance actually increased on the left side (less symptomatic side) as one would expect when the septum is placed more in the midline. This potentially paradoxical finding highlights the seemingly inconsistent relationship between objective measures, such as nasal resistance, and patient-reported measures and the need for a more sophisticated model of understanding perceptions of nasal airflow with objective measures of nasal patency.
In this particular instance, this paradoxical finding may be explained by the following possibilities: surgical placebo effect, “halo” effect (ie, improvement of the symptomatic side overrides all), and/or “threshold” effect (ie, nasal resistance increased but not enough for a perceptual change). The underlying assumption of a threshold effect is that there is a certain objective point at which patients will experience a subjective improvement in their obstructive symptoms. Therefore, one need only select the procedure or combination of procedures that would meet this cutoff point, and additional procedures above and beyond this would be unlikely to provide additional benefit. This concept of “threshold effect” is an intriguing one that is highlighted by breaking down the individual components of our example of functional airway surgery. Although the nasal valve surgery effect on nasal resistance was less than the internal nasal surgery (septoplasty and turbinate reduction), the question remains whether patients would have thought their condition had improved without the additional nasal valve component. Furthermore, if the left turbinate (the less symptomatic side before surgery) was not addressed, would the resultant nasal resistance on the left side been too high, producing a sensation of nasal obstruction on that side? Would the threshold have been breached? The underlying rationale for the left turbinate reduction before surgery was to mitigate the potential reverse nasal obstruction sensation that may result when performing a septoplasty with a compensatory hypertrophied turbinate on the less symptomatic side.
Moreover, it is unclear whether the absolute vs relative values of nasal resistance between sides of the nasal cavity and between presurgery and postsurgery conditions are more paramount for measuring patient outcomes. There is a “normal” range of nasal resistances, and certainly marked outliers would likely be correlated with patient-perceived nasal obstruction. However, this normal range is large, and it might be that the relative values between the sides of the nasal cavity and change from the presurgery state dictate a patient-perceived improvement.
Another area of uncertainty is the importance of balanced airflow allocation between sides of the nose. In the presurgery model, more airflow occurred through the left side, whereas this pattern was essentially reversed in the postsurgery state. The airflow allocation in the rhinoplasty alone model closely mirrored that of the presurgery model, whereas that of the septoplasty and inferior turbinate reduction model closely mirrored the postsurgery model. This finding suggests that, in this patient, surgery on the internal portion of the nasal airway had a greater effect on airflow allocation than that of external nasal surgery. The clinical significance of this is unclear. Whether imbalance in airflow allocation is an important contributing factor in the sensation of nasal obstruction has not been well studied. Additional objective measures, such as wall shear, mucosal temperature changes, or specific airflow allocation patterns within a nasal passage, may be contributors to patient-perceived improvement and successful outcomes. In essence, the objective measures that correlate best with patient-perceived improvement may be multifactorial, and we are just beginning to model and understand these components.
This study has several limitations that should be acknowledged. The dynamic nature of NVC was not accounted for in the CFD modeling because the walls of the nasal cavity were fixed. Additional assumptions of laminarity and that steady-state airflow was a useful approximation for cyclic flow are reasonable in many cases but may not always hold true. From a practical standpoint, a great deal of time, technical expertise, and computing capability are needed to construct the digital models and run the simulations, which precludes its routine use at this time. Finally, differences among patients in healing, tissue properties, nasal sensation, and psychological factors may limit the ability to precisely predict successful surgical outcomes.
A potential criticism of this particular study is the fact that the changes in both unilateral and bilateral nasal resistance of the postsurgery model do not match the sum of the hybrid models. As mentioned in the “Modeling and Simulation” section, there was not an exact match in the transition point when the model portions were fused together, with some minor modifications necessary. which may account for some of the difference. Another possibility is that there is some subtle interaction between the surgical procedures such that their effect when combined is different than the sum of their individual effects. The study by Constantian and Clardy18 supports this because their results showed that the effects of individual surgical components are not simply additive when performed in conjunction. Regardless of the cause, the calculated differences were relatively small compared with the overall large percentage changes.
Much of the literature on simulation in surgery involves its utility in the acquisition of surgical skills, not as a tool to guide preoperative planning. However, the complexity of nasal obstruction lends itself well to such an endeavor. Furthermore, the advent of CFD technology provides a powerful tool that greatly aids the study of nasal physiology in both normal and pathologic conditions, and its potential can be extended to the use of virtual surgery in preoperative planning. At this point, CFD-aided virtual nasal surgery is still in its infancy, and routine use is not currently practical. However, one can envision a future in which a nasal surgeon would have the ability to use varchfacial.2012.422.xmlalidated CFD-aided virtual surgery tools that had easily obtainable imaging data and were packaged in a user-friendly configuration that would allow rapid, sophisticated, in-office analysis to identify problematic areas and simulate the effects of surgical techniques in a way that would allow the surgeon to tailor an individualized surgical approach. Such a tool would help provide safe, cost-effective, and patient-centered care in a manner that could lead to the best possible surgical outcomes.
Correspondence: John S. Rhee, MD, MPH, Department of Otolaryngology and Communication Sciences, Medical College of Wisconsin, 9200 W Wisconsin Ave, Milwaukee, WI 53226.
Accepted for Publication: February 8, 2012.
Published Online: April 16, 2012. doi:10.1001 /archfacial.2012.182
Author Contributions:Study concept and design: Rhee, Cannon, Frank, and Kimbell. Acquisition of data: Rhee, Cannon, Frank, and Kimbell. Analysis and interpretation of data: Rhee, Cannon, Frank, and Kimbell. Drafting of the manuscript: Rhee and Cannon. Critical revision of the manuscript for important intellectual content: Rhee, Cannon, Frank, and Kimbell. Statistical analysis: Cannon and Frank. Obtained funding: Rhee and Kimbell. Administrative, technical, and material support: Rhee, Frank, and Kimbell. Study supervision: Rhee and Kimbell.
Financial Disclosure: None reported.
Funding/Support: This research was funded by grant R01EB009557 from the National Institutes of Health/National Institute of Biomedical Imaging and Bioengineering to the Medical College of Wisconsin and by a subcontract from the Medical College of Wisconsin to the University of North Carolina at Chapel Hill.
Previous Presentation: Accepted for presentation at the American Academy of Facial Plastic and Reconstructive Surgery meeting at the Combined Otolaryngology Spring Meeting; April 18, 2012; San Diego, California.
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