Role of Virtual Surgery in Preoperative Planning. Assessing the Individual Components of Functional...

7/31/2019 Role of Virtual Surgery in Preoperative Planning. Assessing the Individual Components of Functional Nasal Airway

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tal deviation and NVC with septoplasty alone, providesincomplete benefit and may result in a poor outcome andpatient dissatisfaction with surgery. Surgically overtreat-ing patients with NAO (ie, a shotgun approach con-sisting of septoplasty, turbinate reduction,and nasal valverepair for every patient) might expose patients to unnec-essary risk while not providing additional benefit. Thereare also financial ramifications to overtreatment, with ad-ditional cost to patients, insurance providers, and an al-

ready overburdened health care system.In todaysevidence-based medicineenvironment,thereis an emphasis on demonstrating efficacyof treatments orinterventions using validated outcome measures. In ad-dition, insurancecompaniesmorecommonlyrequirepre-authorizationforsurgeryand deny compensation forpro-cedures with questionable benefit or that are not clearlyindicated.These factorsplaceadditional pressureon healthcareprofessionalsto offer safe,effective,andpatient-centeredcare in a cost-efficient manner. In an ideal situation, na-sal surgeons would have technologically advanced toolsat their disposal to assist in the preoperative evaluationofpatientswithNAOtobetterassessthenasalairway,iden-tify problematic areas, and select the optimal surgical in-

tervention to produce the best possible outcome.Currently, such tools do not exist. The current meth-odsof objectively assessingthe nasal airway, such as acous-tic rhinometry and rhinomanometry, havelimitationsthatprevent themfrom being widelyaccepted12 and do not pro-vide much insight intopreoperative planning. Newer tech-nology adopted from engineering fields has the potentialto fill this gap. Computational fluid dynamics (CFD) is atechnique that can be used to study nasal airflow and re-sistance, as well as other aspects of nasal physiology. Usinganatomically accurate 3-dimensional (3D) digital modelscreated fromcomputedtomography (CT)or magneticreso-nance imaging, CFD simulation softwarecan calculate na-sal resistance, airflow velocity, air conditioning, and wall

shear stress. Furthermore, the computational models canbedigitallymodified to simulate thechanges created by sur-gery, and new simulations can be performed to determinewhat effect these changes produce with respect to resis-tance and other parameters of interest.13

This article presents the results of a preliminary studythat is part of a larger prospective project to correlate CFDdata with patient-reported subjective measures of nasalobstruction andexamine therole of virtual surgery in pre-dicting postoperative outcomes. The aims of this par-ticular study are to demonstrate the effect of individualcomponents of functional nasal airway surgery in a pa-tient with multifactorial obstruction and discuss the po-tential benefit of CFD-aided virtual surgery.

METHODS

PATIENT RECRUITMENT AND TREATMENT

Research methods were approved by the institutional reviewboard at the Medical College of Wisconsin, and written in-formed consent was obtained. For this study, a 53-year-oldwoman with a long-standing bilateral nasal obstruction and noprior history of nasal trauma or surgery was selected. Physicalexamination revealed the presence of rightward septal devia-

tion, bilateral inferior turbinate hypertrophy, and NVC. Modi-fied contiguous CT in the axial plane of the entire nasal cavityand external nasal soft tissues was performed before surgeryand 6 months after surgery (both times with 6.0-mm incre-ments and 0.313-mm resolution). Surgical treatment deci-sions were made by the surgeon (J.S.R.) based on the clinicalpresentation and standard of medical care. For this patient, sep-toplasty, nasal valve repair, and inferior turbinate reduction wereperformed. Thestandard external rhinoplastyapproach wasusedto gain access to the septum and lower two-thirds of the ex-

ternal part of the nose. Submucosal resection of the deviatedmidportion of the septum was performed. Septal cartilage wasthen used to createa butterfly graft, bridging the nasal dorsumand positioned beneath the lower lateral cartilages. Inferior tur-binate reductions were accomplished via submucosal resec-tion of the bone and soft tissue of the anterior half of the tur-binates. Standard postoperative care was performed, and thepatients recovery and healing were uneventful.

MODELING AND SIMULATION

Thepresurgery andpostsurgery CT scans were used to createdigi-tal 3D models of the patients nasal cavity (excluding the para-nasal 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 coregis-tered using 3D reconstructions of the skull in each scan. To iso-late the effects of the separate components of the surgery, 2 hy-brid nasal cavity models using portions of the presurgery andpostsurgery CT scans were created. To allow investigation of theintranasal component of the surgery (septoplasty and turbinatereduction) alone, a model was created using the external part ofthe nose in the presurgery model fused to the nasal cavity of thepostsurgery model. Another model was created using the exter-nal part of the nose in the postsurgery model fused to the nasalcavityof the presurgery model to allow investigation of thenasalvalve repair component of the surgery alone. Any minor mis-matches at the interfaces where the portions of the models werefused were smoothed using the image analysis software to mini-mize their effect on the results of the simulation.

The CFD modeling methods used have been described indetail elsewhere.14 Briefly,planar nostril and outlet surfaces werecreated, and the digital models were meshed with approxi-mately 4 million tetrahedral cells using ICEM-CFD 11.0 soft-ware (ANSYS Inc). Steady-state inspiratory airflow simula-tions 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 wereassumed to be stationary), (2)a pressure-inlet condition at thenostrils in which gauge pressurewas set to zero, and (3) a pres-sure-outlet condition at the outlet in which gauge pressure wassetto a negativevalue in Pascals corresponding to a targetsteady-state flow rate. The target airflow rate was 14.9 L/min, twicethe patients estimated minute volume (amount of air exhaledin 1 minute), which was obtained from body weight.15 Valuesused for the density and dynamic viscosity of air were 1.204

kg/m3 and 1.825105 kg/ms, respectively. Fluent and the post-processing software Fieldview 12.0(Intelligent Light) wereusedfor simulation analysis and visualization of results.

OUTCOME MEASURES

The outcome measures calculated by CFD analysis were bilat-eral and unilateral nasal resistance and airflow allocation. Resis-tance was calculated asp/Q, wherep was the transnasal pres-sure decrease (in Pascals) between the nostrils and the posteriorendof theseptum, andQ wasthe volumetric airflow rate (inmil-

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lilitersper second).Airflowallocationwas determined as the per-centage of the total airflow that traveled through each side of thenasal cavity. In addition, the patient was administered the NasalObstruction Symptom Evaluation (NOSE) Scale to collect infor-mation on patient-reported symptoms before and after sur-gery.16 The NOSE Scale is a disease-specific quality-of-life instru-ment forNAO that hasbeen validatedfor septoplasty11 and nasalvalve repair, for which it has also been used to measure surgicalsuccess.1,17 Finally, as part of the research protocol, unilateral vi-sual analogscale(VAS) scores fornasalairflow were determinedbeforeand after surgery. Patients were asked to cover onenostrilandrate their ability to breathe through theuncovered nostril ona scale of 0 (completely obstructed) to 10 (no obstruction). Theprocess was then repeated for the opposite nostril.

RESULTS

Before surgery, bilateralnasalresistancewasestimatedfromCFD analysis to be 0.0979 Pa/(mL/s). In the postsurgerymodel, bilateral nasal resistance was 0.0742 Pa/(mL/s), a24.2% reductionfrom presurgical resistance. In the modelwith nasal valverepair alone,bilateral nasal resistance was0.0922 Pa/(mL/s), a 5.8% reduction from presurgical re-

sistance. In the model with septoplasty and inferior turbi-nate reduction alone, bilateral nasal resistancewas 0.0788Pa/(mL/s), a 19.5% reduction from presurgical resistance(Table 1). When examined unilaterally, right-sided na-sal resistance decreased from0.2604 Pa/(mL/s) before sur-gery to 0.1217 Pa/(mL/s) after surgery, a 53.2% reduc-tion. Conversely, left-sided nasal resistance was mildlyincreased from 0.1577 Pa/(mL/s) before surgery to 0.1904Pa/(mL/s) after surgery, an increase of 20.7%. In the nasalvalve repair alone model, nasal resistance was mildly de-creasedin both sides compared withbeforesurgery,whereasin the septoplasty and turbinate reduction model, right-

sided nasalresistancewas reduced by approximately50%,but left-sided nasal resistance was increased by approxi-mately 27% compared with before surgery. The unilateralnasal resistance results of theseptoplasty andturbinate re-duction alone model approximated the results of the post-surgery model (Table 2).

Allocation of airflow between the nasal cavities wasaltered 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 alloca-

Lateral wall

1 12 2

Septum

PresurgeryScan

Level 1

PostsurgeryScan

PresurgeryScan

Level 2

PostsurgeryScan

A

B

C

D

Figure 1. Models of the patients nasal cavity (excluding the paranasalsinuses) and the soft tissues surrounding the nasal cavity created frompresurgery and postsurgery computed tomographic (CT) scans.A, Reconstructions of the right lateral wall and right side of the septum frompresurgery CT scan. B, Same views with surgical sites marked.Purpleindicates 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 frompostsurgery CT scan, coregistered with presurgery scan. D, Coronal viewsfrom presurgery and postsurgery CT scans. Yellow outline highlights border

between soft tissue and nasal air space.

Table 1. Changes in Bilateral Nasal Resistance by Model

Model

Bilateral NasalResistance,

Pa/(mL/s) Change, %

Before surgery 0.0979 1.0 [Reference]

Nasal valve repair alone 0.0922 5.8

Septoplasty and turbinatereduction alone

0.0788 19.5

After surgery 0.0742 24.2

Table 2. Changes in Unilateral Nasal Resistance by Model

Model

Unilateral Nasal Resistance, Pa/(mL/s)

Right Change, % Left Change, %

Before surgery 0.2604 1.0 [Reference] 0.1577 1.0 [Reference]

Nasal valverepair alone

0.2476 4.9 0.1479 6.2

Septoplastyand turbinatereductionalone

0.1298 50.2 0.2010 27.5

After surgery 0.1217 53.2 0.1904 20.7

Table 3. Airflow Allocation by Model

Model

Airflow Allocation, %

Right Left

Before surgery 37.1 62.9

Nasal valve repair alone 36.7 63.3

Septoplasty and turbinatereduction alone

61.0 39.0

After surgery 61.6 38.4




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tion pattern similar to the presurgery pattern (63.3%through the left side), whereas the septoplasty and infe-

rior turbinate reduction model had a pattern similar tothe postsurgery pattern (39.0%through the left side). Air-stream flow through the postsurgery nasal passage wasnoted to be qualitatively improvedas exemplifiedby morelaminar flow (video 1 andvideo 2; available at http://www.archfacial.com). Symptomatically, the patient reportedimprovement after surgery, with a reduction in NOSEscore from 60 before surgeryto 20 after surgery. TheVASscores improved as well, with improvements on the rightside from 2 before surgery to 7 after surgery and on theleft side from 3 before surgery to 10 after surgery. Com-parison of the appearance of the external part of the nosebefore and after surgery shows a qualitative improve-ment in the area of the nasal valves (video 3), and com-parison of the appearance between the actual photo-graphs and digital soft-tissue models shows excellentcorrelation (Figure 2 and Figure 3). Virtual endo-scopic view demonstrates improvement in the nasal valvearea as viewed from the vestibule (Figure 4).

COMMENT

Nasal obstruction is a complex phenomenon that likelyhas many contributing factors. Even within the domain

of anatomical nasal obstruction, there can be several po-tential problematic areas, creating a dilemma for the na-sal surgeon in deciding which components of the nasalairway to target surgically to produce the best possibleoutcome for the patient. Although experience and clini-cal judgment are essential, it would be useful to have ad-ditional assistance with preoperative planning.

In this pilot study, we examined a patient with mul-tiple anatomical causes of nasal obstruction treated sur-

gically in an effort to determine how much benefit couldbe attributed to each of the components. In this particu-lar patient, most of the benefit with respect to bilateralnasal resistance was a result of the combined septo-plasty and inferior turbinate reduction. However, addi-tional benefit in overall nasal resistance was added by theperformance of nasal valve repair. The right side, whichwas most affected by the septal deviation, showed a sub-stantial decrease in nasal resistance after septoplasty andturbinate reduction when examined unilaterally, whereasthe left side showed a moderate increase, as would be ex-

A B

C D

Figure 2. Comparison of appearance between the actual photographs anddigital soft-tissue models. Presurgery frontal view (A) compared withappearance on digital model of soft tissue of the face (B). The 6-monthpostsurgery frontal view (C) compared with appearance on digital model ofsoft tissue of the face (D).

A B

C D

Figure 3. Comparison of appearance between the actual photographs anddigital soft-tissue models. Presurgery base view (A) compared withappearance on digital model of soft tissue of the face (B). The 6-monthpostsurgery base view (C) compared with appearance on digital model ofsoft tissue of the face (D).

Inferior turbinate

Presurger y vestibule Postsurger y ves tibule

Inferior turbinate

S S

A B

Figure 4. Virtual endoscopic view from right nasal vestibule. Photographscompare the presurgery (A) and postsurgery (B) appearance. Images are

color-coded for wall shear, with blue signifying areas of low shear stress andgreen-yellow signifying areas of higher shear stress. S denotes nasalseptum. Note contact between septum and lateral wall in presurgery imagewith associated higher shear stress above and below this area.




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pected after correction of the septal deviation. The ef-fect of surgery on airflow allocation was exclusively a re-sult of the septoplasty and turbinate reduction as well.Nasal valve repair alone produced mild decreases in uni-lateral nasal resistance for both sides of the nose.

To our knowledge, this is the first study that simu-lates individual virtual surgery scenarios (nasal valve sur-gery vs internal nasal surgery) with comparison to aknown outcome. A previous article published by mem-

bers of our group

13

used a virtual surgery model and CFDtechniques to examine the effects of septoplasty and tur-binate reduction on nasal resistance and airflow. One ofthe best attempts at quantifying the nasal valve compo-nent (also known as functional rhinoplasty) separate fromother surgical manipulations was performed by Con-stantian and Clardy,18 in which they investigated the rela-tive importance of septal surgery and nasal valve repairin correcting NAO. They prospectively evaluated a groupof patientswith NAO using rhinomanometry as their pri-mary outcome measure. Patients were stratified accord-ingto the site of their obstruction as determined by clini-cal examination: internal nasal valves, external nasalvalves, septum, or any combination of these 3; patients

with turbinate hypertrophy were excluded. Surgery wasthen performed with targeting of problematic areas iden-tified preoperatively. The authors concluded that the ef-fects of NVC may equal or even surpass septal deviationas the primary cause ofNAO, in contrast to what was dis-covered in the present simulation.

Thequestion of what constitutes a successful nasal sur-gery is a key consideration. As of yet a correlation be-tween CFD data and subjective, patient-reported mea-sures has not been reported. In this particular case, thepatient reported symptomatic improvement in nasal symp-toms, as represented by her improvements in NOSE andVAS scores. Accordingly, she also exhibited improvementin bilateral nasal resistance and in unilateral resistance on

thesideaffectedbyseptaldeviationasdetermined with CFDtechniques. Interestingly,however, the patient reportedanimprovement in VAS scores on the contralateral side eventhough the nasal resistance actually increased on the leftside (lesssymptomaticside) as one would expect when theseptumis placedmore in themidline. This potentially para-doxical finding highlights the seemingly inconsistent re-lationship between objective measures, such as nasal re-sistance, and patient-reported measures and the need fora more sophisticated model of understanding perceptionsof nasal airflow with objective measures of nasal patency.

In thisparticular instance, thisparadoxicalfinding maybe explained by the following possibilities: surgical pla-cebo effect, halo effect (ie, improvement of the symp-

tomatic side overrides all), and/or threshold effect (ie,nasal resistance increased but not enough for a percep-tual change). The underlying assumption of a thresholdeffect is that there is a certain objective point at whichpatients willexperience a subjective improvement in theirobstructive symptoms. Therefore, one need only selectthe procedure or combination of procedures that wouldmeet this cutoff point, and additional procedures aboveand beyond this would be unlikely to provide addi-tional benefit. This concept of threshold effect is an in-triguing one that is highlighted by breaking down the in-

dividual componentsof our example of functional airwaysurgery. Although the nasal valve surgery effect on na-sal resistance wasless than the internal nasal surgery (sep-toplasty and turbinate reduction), the question remainswhether patients would have thought their condition hadimproved without the additional nasal valve compo-nent. Furthermore, if the left turbinate (the less symp-tomatic side before surgery) was not addressed, wouldtheresultant nasal resistance on theleft side been toohigh,

producing a sensation of nasal obstruction on that side?Would the threshold have been breached? The under-lying rationale for the left turbinate reduction before sur-gery was to mitigate the potential reverse nasal obstruc-tion sensation that may result when performing aseptoplasty with a compensatory hypertrophied turbi-nate on the less symptomatic side.

Moreover, it is unclear whether the absolute vs relativevalues of nasal resistance between sides of the nasal cavityand between presurgery and postsurgery conditions aremore paramount for measuring patient outcomes. Thereisa normal range ofnasal resistances, andcertainly markedoutliers would likely be correlated with patient-perceivednasal obstruction. However, thisnormal range is large, and

it might be that the relative values between the sides of thenasal cavity and change from the presurgery state dictatea patient-perceived improvement.

Another area of uncertainty is the importance of bal-anced airflow allocation between sides of the nose. In thepresurgery model, more airflow occurred through the leftside, whereas this pattern was essentially reversed in thepostsurgery state. The airflow allocation in the rhino-plasty alone model closely mirrored that of the presurgerymodel, whereas that of the septoplasty and inferior turbi-nate reduction model closely mirrored the postsurgerymodel. This finding suggests that, in this patient, surgeryon the internal portion of the nasal airway had a greatereffect on airflow allocation than that of external nasal sur-

gery. The clinical significance of this is unclear. Whetherimbalance in airflow allocation is an important contribut-ing factor in thesensation ofnasal obstruction hasnot beenwell studied. Additional objective measures, such as wallshear, mucosaltemperature changes, or specific airflow al-location patterns within a nasal passage, may be contribu-torsto patient-perceived improvement and successful out-comes.In essence, theobjective measuresthat correlatebestwith patient-perceived improvement may be multifacto-rial, and we are just beginning to model and understandthese components.

This study has several limitations that should be ac-knowledged. The dynamic nature of NVC was not ac-counted for in the CFD modeling because the walls of

the nasal cavity were fixed. Additional assumptions oflaminarity and that steady-state airflow was a useful ap-proximation for cyclic flow are reasonable in many casesbut may not always hold true. From a practical stand-point, a great deal of time, technical expertise, and com-puting capability are needed to construct the digital mod-els and run the simulations, which precludes its routineuse at this time. Finally, differences among patients inhealing, tissue properties, nasal sensation, and psycho-logical factors may limit the ability to precisely predictsuccessful surgical outcomes.




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A potential criticism of this particular study is the factthat the changes in both unilateral and bilateral nasal re-sistance of the postsurgery model do not match the sumof the hybrid models. As mentioned in the Modeling andSimulation section, there was not an exact match in thetransition point when the model portions were fused to-gether, with some minor modifications necessary. whichmay account for some of the difference. Another possi-bility is that there is some subtle interaction between the

surgical procedures such that their effect when com-bined is different than the sum of their individual ef-fects. The study by Constantian and Clardy18 supportsthis because their results showed that the effects of in-dividual surgical components arenotsimplyadditive whenperformed in conjunction. Regardless of the cause, thecalculated differences wererelatively small compared withthe overall large percentage changes.

Much of the literature on simulation in surgery in-volves its utility in the acquisition of surgical skills, notas a tool to guide preoperative planning. However, thecomplexity of nasal obstruction lends itself well to suchan endeavor. Furthermore, the advent of CFD technol-ogy provides a powerful tool that greatly aids the study

of nasal physiology in both normal and pathologic con-ditions, and its potential can be extended to the use ofvirtual surgery in preoperative planning. At this point,CFD-aided virtual nasal surgery is still in its infancy, androutine use is not currently practical. However, one canenvision a future in which a nasal surgeon would havethe ability to use validated CFD-aided virtual surgery toolsthat had easily obtainable imaging data and were pack-aged in a user-friendly configuration that would allowrapid, sophisticated, in-office analysis to identify prob-lematic areas and simulate the effects of surgical tech-niques in a way that would allow the surgeon to tailoran individualized surgical approach. Such a tool wouldhelpprovide safe, cost-effective, and patient-centered care

in a manner that could lead to the best possible surgicaloutcomes.

Accepted for Publication: February 8, 2012.Published Online: April 16, 2012. doi:10.1001/archfacial.2012.182Correspondence: John S. Rhee, MD, MPH, Departmentof Otolaryngology and Communication Sciences, Medi-cal College of Wisconsin, 9200 W Wisconsin Ave, Mil-waukee, WI 53226.Author Contributions: Study concept and design: Rhee,Cannon, Frank, and Kimbell. Acquisition of data: Rhee,Cannon, Frank, and Kimbell. Analysis and interpreta-tion of data: Rhee, Cannon, Frank, and Kimbell. Draft-

ing of the manuscript: Rhee and Cannon. Critical revisionof the manuscript for important intellectual content: Rhee,Cannon, Frank, and Kimbell. Statistical analysis: Can-non and Frank. Obtained funding: Rhee and Kimbell.Ad-ministrative, technical, and material support:Rhee, Frank,and Kimbell. Study supervision: Rhee and Kimbell.

Financial Disclosure: None reported.Funding/Support: This research was funded by grantR01EB009557 from the National Institutes of Health/National Institute of Biomedical Imaging and Bioengi-neering to the Medical College of Wisconsin and by a sub-contract from the Medical College of Wisconsin to theUniversity of North Carolina at Chapel Hill.Previous Presentation: Accepted for presentation at theAmerican Academy of Facial Plastic and Reconstructive

Surgery meeting at the Combined Otolaryngology SpringMeeting; April 18, 2012; San Diego, California.Online-Only Material: Videosare availableonline at http://www.archfacial.com.

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