Advances in computer modeling and simulation technologies have the potential to provide facial plastic surgeons with information and tools that can aid in patient-specific surgical planning for rhinoplasty. Finite element modeling and computational fluid dynamics are modeling technologies that have been applied to the nose to study structural biomechanics and nasal airflow. Combining these technologies with patient-specific imaging data and symptom measures has the potential to alter the future landscape of nasal surgery.
Key points
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Computer modeling and simulation technologies have the potential to provide facial plastic surgeons with information and tools that can aid in patient-specific surgical planning for rhinoplasty.
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Finite element modeling and computational fluid dynamics (CFD) are modeling technologies that have been applied to the nose to study structural biomechanics and nasal airflow.
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Patient-specific computational models can be modified to simulate surgical changes or perform virtual surgery. CFD tools can then be used to study the effects of these changes on nasal function and, in the future, aid in surgical planning and in predicting surgical outcomes.
Introduction
Among all of the procedures in facial plastic surgery, rhinoplasty demands the highest level of understanding in aesthetics, soft and hard tissue dynamics, and the delicate interplay between form and function. Adding to the complexity of this procedure are individual patient factors that can impact patient outcomes, including variable anatomy and medical comorbidities. The techniques currently used have evolved through many years of hard work, ingenuity, and experimentation of numerous rhinoplasty surgeons. Collectively, this comprises decades of knowledge that has largely been developed through individual surgeon experience and, undoubtedly, trial and error.
Advances in computer-based modeling and simulation are now providing ways to better study and understand individual anatomy, tissue dynamics, and specific surgical techniques. Modeling has been used in engineering fields for decades and has helped engineers design complex processes and products for numerous industries. Methods for finite element modeling (FEM) and computational fluid dynamics (CFD) were first introduced in the 1950s and 1960s, and limited to applications within various engineering fields. As computing technology has advanced, applications for modeling and simulation have slowly expanded to medicine and, more recently, applied to nasal anatomy and rhinoplasty techniques.
Surgical modeling is not an entirely new concept. Craniofacial surgeons have been performing model surgery for decades, using cephalometric measurements and physical resin molds to plan and design orthognathic surgery and facial skeletal dimensions preoperatively. Using model surgery, they could make better informed decisions about the specific maneuvers needed to achieve the desired outcome for a patient, specific to that patient’s anatomy and clinical requirements. In short, this type of model surgery minimizes the guesswork needed to achieve a desired result. Similarly, computer modeling and simulation techniques have the potential to provide facial plastic surgeons with information that could aid in patient-specific surgical planning.
The accessibility to affordable, yet powerful hardware and software has fueled the emergence of very sophisticated computer modeling tools. Historically, modeling of biological hard and soft tissue modifications with endless variation and intrinsic tissue properties has been challenging. Medical imaging has been a key transformative technology in this regard because computed tomography (CT), magnetic resonance imaging (MRI), and ultrasound can now achieve extreme high levels of resolution and detail. This has facilitated the development of computations and simulations that were never previously possible. Additionally, commercially available software programs now provide tools to manipulate imaging data to simulate surgical modifications of specific anatomy.
Over the past several years, there have been an increasing number of studies using computer modeling tools to study the nose. This article reviews the specific modeling technologies of FEM and CFD, and their application to nasal surgery.
Introduction
Among all of the procedures in facial plastic surgery, rhinoplasty demands the highest level of understanding in aesthetics, soft and hard tissue dynamics, and the delicate interplay between form and function. Adding to the complexity of this procedure are individual patient factors that can impact patient outcomes, including variable anatomy and medical comorbidities. The techniques currently used have evolved through many years of hard work, ingenuity, and experimentation of numerous rhinoplasty surgeons. Collectively, this comprises decades of knowledge that has largely been developed through individual surgeon experience and, undoubtedly, trial and error.
Advances in computer-based modeling and simulation are now providing ways to better study and understand individual anatomy, tissue dynamics, and specific surgical techniques. Modeling has been used in engineering fields for decades and has helped engineers design complex processes and products for numerous industries. Methods for finite element modeling (FEM) and computational fluid dynamics (CFD) were first introduced in the 1950s and 1960s, and limited to applications within various engineering fields. As computing technology has advanced, applications for modeling and simulation have slowly expanded to medicine and, more recently, applied to nasal anatomy and rhinoplasty techniques.
Surgical modeling is not an entirely new concept. Craniofacial surgeons have been performing model surgery for decades, using cephalometric measurements and physical resin molds to plan and design orthognathic surgery and facial skeletal dimensions preoperatively. Using model surgery, they could make better informed decisions about the specific maneuvers needed to achieve the desired outcome for a patient, specific to that patient’s anatomy and clinical requirements. In short, this type of model surgery minimizes the guesswork needed to achieve a desired result. Similarly, computer modeling and simulation techniques have the potential to provide facial plastic surgeons with information that could aid in patient-specific surgical planning.
The accessibility to affordable, yet powerful hardware and software has fueled the emergence of very sophisticated computer modeling tools. Historically, modeling of biological hard and soft tissue modifications with endless variation and intrinsic tissue properties has been challenging. Medical imaging has been a key transformative technology in this regard because computed tomography (CT), magnetic resonance imaging (MRI), and ultrasound can now achieve extreme high levels of resolution and detail. This has facilitated the development of computations and simulations that were never previously possible. Additionally, commercially available software programs now provide tools to manipulate imaging data to simulate surgical modifications of specific anatomy.
Over the past several years, there have been an increasing number of studies using computer modeling tools to study the nose. This article reviews the specific modeling technologies of FEM and CFD, and their application to nasal surgery.
Finite element modeling
FEM is a computational technique used to quantitatively study the biomechanics of a structure and provides a method to analyze structural stress, strain, and energy distributions on 3-dimensional (3D) structures. The first multicomponent FEM of the nose incorporating bone, cartilage, and skin-soft tissue was reported by Manuel and colleagues. Over the past several years, there have been an increasing number of studies applying FEM to various nasal constructs in addition to studying effects of specific rhinoplasty techniques.
Nasal Septum and Dorsum
Some of the initial studies applying FEM to the nose studied biomechanics of the septal L-strut. Lee and colleagues created several models by altering material properties of the septum and nasal tip support to determine the overall deformation and stress distribution in the L-strut. They found that the most consistent points of maximum stress were the bony-cartilaginous junction and the nasal spine, highlighting the importance of maintaining adequate cartilage support within the L-strut at these 2 locations. In more recent work, they further analyzed the caudal segment of the septal L-strut and highlighted the importance of maintaining at least 1 cm of septal cartilage width along the inferior portion of the L-strut, at the junction with the anterior spine. In another recent study, Tjoa and colleagues used FEM to simulate wound healing forces and surgical maneuvers that may lead to the inverted V-deformity.
Cephalic Trim
FEM modeling of the nose has also been used to study effects of lower lateral cartilage resection on the overall mechanical stability of the nose and nasal cartilages. In an initial study, Oliaei and colleagues developed an FEM of 3 different lower lateral cartilage widths, simulating differing amounts of cephalic resection. Using this model, they showed that there was no statistically significant decline in structural support of the cartilage when a minimum 6 mm width of lateral crus was maintained, suggesting that this width could potentially resist contractile forces related to postoperative scar tissue. In a more recent study, Leary and colleagues applied FEM to study the potential impact of cephalic resection on the strength and stability of the lateral crus. They identified the common clinical problem of alar retraction after cephalic trim, and used FEM techniques to better understand the complex forces and factors that contribute to this complication. As they pointed out, objective analysis of rhinoplasty maneuvers is difficult to perform on patients due to the overall long period of time during which changes in nasal shape occur. Unfortunately, a limitation of current modeling techniques is the overall lack of experimental data to simulate these complex wound healing processes.
Nasal Tip Support
Other studies have applied FEM to investigate nasal tip dynamics and support. Shamouelian and colleagues examined relative contributions of 2 major tip support mechanisms: attachment between the lower and upper lateral cartilages (scroll region) and attachment of the medial crura to the caudal septum. Computer models were modified by removing various intercartilaginous connections to simulate various rhinoplasty maneuvers (transfixion and intercartilaginous incisions). Each model was then subjected to a nasal tip force to simulate nasal tip depression. Results of this modeling showed disruption of the medial crura attachment to the caudal septum had a greater impact on nasal tip support compared with disruption of the scroll region. In another study, FEM was used to study how columellar strut graft size, shape, and attachment to the medial crura affect nasal tip support. Interestingly, suture placement to fixate the graft was found to be just as important as the strut size, with the most important point of fixation at the proximal or anterior portion of the columellar strut graft to the medial crura. In addition to studying cartilage biomechanics, FEM has also been used to evaluate the influence of columellar scar shape on the stress distribution within the tissue following an external rhinoplasty approach and supported the current practice of an inverted-V shaped columellar incision.
Computational fluid dynamics
While FEM provides an analysis of biomechanical forces within the nose, CFD enables a quantitative analysis of nasal airflow and offers several advantages over traditional objective measures. A myriad of tools have been used by clinicians to evaluate nasal airway obstruction (NAO) in the clinical setting. These include patient-reported questionnaires, physical examination maneuvers, and objective tests such as rhinomanometry and acoustic rhinometry. These tests do little to identify specific anatomic problems for correction, have low correlation with patient symptoms, and at best are capable of producing surgical failure rates as high as 37%. This high failure rate has been attributed to the lack of a gold standard to diagnose the extent and cause of NAO, and lack of tools to aid surgeons in predicting surgical success accurately.
The complexity of the nasal airway is well suited to the creation of a computational tool to aid surgeons in the diagnosis and treatment of NAO. CFD is a well-established, powerful tool that can be used to model and analyze the biophysics of nasal airflow. With the availability of powerful bioengineering computer-aided design software, anatomically accurate 3D computational models can now be generated from CT or MRI data. CFD software can then be used to analyze these models and calculate various anatomic and physiologic measures, including nasal airflow, resistance, air conditioning, and wall shear stress. Furthermore, these 3D computational models can be modified to simulate surgical changes or perform virtual surgery. CFD tools can then be used to study the effects of these changes on nasal function and in the future, potentially aid in surgical planning and in predicting surgical outcomes.
CFD computations are based on known airflow dynamics and grounded in basic physical laws of fluid flow, such as the conservation of mass (continuity equation) and the conservation of momentum (Navier-Stokes equations). The latter is derived from Newton’s second law applied to a fluid element. In general, given a tube of any shape and the physical conditions producing airflow through the tube (called boundary conditions), the Navier-Stokes equations can be solved to obtain information about the flow such as velocity, pressure distribution, allocation of the flow to different regions within the tube, forces exerted on the walls (shear stress), how much the flow swirls (vorticity), and turbulence. However, airflow in the nose is complicated by the irregular 3D shape of the nasal cavity, areas of marked constriction, abrupt changes in direction of airflow, and areas in which the dimensions of the airway are under muscular and vascular control. These factors impose some limitations on the interpretation of nasal resistance measurements because the nasal airway cavity cannot be represented as an ideal tube by the simplest physical laws of fluid flow.
Computational Fluid Dynamics Workflow
The process of developing a patient-specific CFD model begins with raw CT or MRI data, which then goes through a process of segmentation to create an initial 3D model using medical imaging software, such as Mimics (Materialise, Plymouth, MI, USA) ( Fig. 1 ). To solve the equations that govern fluid flow, each 3D nasal model must be divided into a large number of small cells in which air velocity and pressure can be defined. This is accomplished by creating a mesh with approximately 4 million tetrahedral cells using ICEM-CFD (ANSYS Inc, Canonsburg, PA, USA). Airflow simulations for flow rates corresponding to normal resting breathing are conducted using Fluent (ANSYS Inc, Canonsburg, PA, USA). The following boundary conditions are often used to determine the steady-state airflow field: (1) a wall condition (zero velocity, stationary wall assumed) at the airway walls, (2) a pressure-inlet condition at the nostrils with gauge pressure set to 0, and (3) a pressure-outlet condition at the outlet with gauge pressure set to a negative value in pascals that generates the target steady state inhalation rate of 15.0 L/min. This flow rate represents a healthy adult breathing at rest. Additional details on the differential equations, computational algorithms, and air physical properties used can be found in previous publications. Figures, printouts, diagrams, and other visualizations of CFD model results can be made using the visualization software package Fieldview (Intelligent Light, Lyndhurst, NJ, USA), as well as the with the visualization capabilities within Fluent.
