nasal septal deviation is a common nasal deformity . it can be a congenital disorder or a consequence of nasal trauma .
deviation of the bony or cartilaginous component of the nasal septum from the midline leads to its deviation .
this results in external nasal deformity , internal nasal obstruction due to nasal airway constriction , or a combination [ 13 ] .
presently , septal deviation classification has largely been descriptive , based on nasal septal geometry and relationships between the bony and cartilaginous septa [ 47 ] .
jang et al .   presented a simplified classification of nasal deviation and the associated treatment outcome into five types based on the orientation of the bony pyramid and the cartilaginous vault .
jin et al .   presented a four - category classification of septal deviation based on the morphology , site , severity , and its influence on the external nose .
buyukertan et al .   reported a morphometric study of nasal septal deviation by separating the nasal septum into 10 segments .
they concluded that the system would constitute a new , objective , simple , and practical classification system .
i. baumann and h. baumann   argued that the existing nomenclatures of septal deviation only dealt with nasal septum deformation exclusively and were rarely used in routine clinical work .
they instead presented a method for the classification of septal deviations based upon the anatomical structures of the nasal septum and common clinical concepts .
however , the most observable nasal septal deviation classification system was proposed by rohrich et al . .
therefore , for simplicity , nasal septal deviations will be classified according to that proposed by rohrich et al . .
in order to improve the clinical outcome of septoplasty , a greater understanding of the etiopathogenesis of nasal septal deviation is necessary .
we aim to apply incremental force to a computer - generated septal model using structural modal analysis , which has also been utilized by laura et al . , who previously described a simple method for determining the fundamental mode of a vibrating ulna to approximate its dynamic response .
the objective of this study is to identify areas of high - stress and septal deformation patterns .
clinically , this may assist surgeons in the delineation of key areas for septal realignment and reconstruction .
cranial ct scans were obtained from a patient who possessed normal features  a straight nasal septum , normal occlusion and a perceivably symmetric face ( figure 1(a ) ) .
this study was performed in accordance with the guidelines of the institutional review board ( irb ) and conforms to the helsinki 's declaration .
the patient had not previously undergone septoplasty or rhinoplasty , nor subject to nasal injury .
superposition of the ct images to create a three - dimensional ( 3d ) model was conducted with mimics software ( materialise technologies , leuven , belgium ) .
an idealized model ( figure 1(b ) ) and a patient - specific finite element model were generated for the study .   from the ct scans
we chose to base the idealized model on the middle slice ( figure 1(a ) ) and measured the significant features of the nasal septum .
we then utilized these measurements to create an idealized model ( figure 1(b ) ) in the finite element analysis software , abaqus ( dassault systmes technologies , providence , ri , united states of america ( usa ) ) , where the idealized model was , subsequently , meshed .
however , to simplify analyses and gain an estimate of nasal deformation , models were prescribed with a uniform thickness of 2  mm , which is an approximate average septum thickness , as reported previously [ 1113 ] . to ensure mesh accuracy ,
convergence studies were carried out on the model . to create a more realistic representation of the septum , which incorporated thickness variation ,
a 3d patient - specific model was created from the same ct scan utilizing mimics software ( materialise technologies , leuven , belgium ) and meshed with hypermesh ( altair hyperworks , troy , mi ) .
nonhomogenous , anisotropic , nonlinear , viscoelastic behaviour  . for deformations below 20% , however , no significant changes occur within the cartilage , and it is therefore sufficiently accurate to model cartilage as a homogenous , linearly elastic material in our analyses .
utilized a similar homogenous , linear elastic material property to simulate septal l - strut deformation . to define the linear elastic model of the cartilage ,
the young 's modulus , e , and poisson 's ratio , v , are required .
however , the tensile and compressive young 's moduli are vastly different due to the structure of cartilage . according to lee et al . , the tensile modulus ranges from 2.62  mpa to 10.6  mpa , the compressive modulus ranges from 0.40  mpa to 0.83  mpa , and the poisson 's ratio ranges from 0.26 to 0.38 .
cartilage is approximately 75% water , while the other 25% consists mainly of type - two collagen fibrils and proteoglycan molecules .
the density of water is 1000  kg / m , while the other components are highly dense structures .
therefore , the density of cartilage was estimated to be 2000  kg / m .   as the relative displacement within
the septum is the main area of concern in this analysis , and since material properties affect the absolute and not the relative displacement of the septum , the average values of the elastic modulus and poisson 's ratio and an estimated value of the density were used .
the elastic modulus was assigned a value of 5  mpa , poisson 's ratio was 0.32 , and the density was 2000  kg / m . as the bony interfaces with the nasal septum 
ethmoidal , vomer , hard palate , and nasal bone interfaces ( figure 1(a))are much stiffer than the septal cartilage , most of any applied force will be absorbed by the cartilaginous septum , leaving the bony septum uninjured .
the nasal bone length overlapping the cartilaginous septum may affect the degree of nasal deformation and normally ranges from 3 to 15  mm .
however , to simplify analyses , a candidate that displayed a length that fell within this range  in this case , 14  mm  was considered , so that a typical deformation pattern could be observed .   in vivo ,
the nasal tip lies anterior to the anterior septal angle ( asa ) where the lower lateral cartilages ( llcs ) meet , although this may vary .
however , due to the small distance between the asa and the nasal tip , and for simplification in this analysis , the asa was assumed to be the nasal tip .   according to lee
, the nasal tip cartilages may be thought of as a spring and a cantilever , as they exhibit deformation recoil and elasticity .
a cantilever is a result of the unequal stability in the tripod formed by the medial crura and paired llcs , and a spring results from the llcs , which produce an upward force that is in the form of stored elastic potential energy .
the spring - stiffness constant , k , may be defined by ( 1 ) , and a spring - stiffness constant of 20  kn / m is applied in the three orthogonal axes .
a free nasal tip was prescribed as a preliminary step . a spring - supported nasal tip boundary condition , where the spring was connected between the two orange points on the nasal tip ( figure 2 ) ,
the dorsal and caudal septa were prescribed a free boundary condition .   as  frontal force to the septum
causes damage ranging from simple fracture of the nasal bones to severe flattening of the nasal bones and the septum  , two forms of frontal loading were applied 
the force and pressure applied are estimates and are inconsequential to the relative displacements of the septum . as the present intention is to determine the eigen modes , or the most likely deformed patterns of the septum , only the possible in - plane loading which will affect the resulting eigen modes
will be considered . in the case of anteroposterior loading ( figure 2 ) , a couple of forces of 1n each in both the vertical and horizontal axes were applied to the nasal tip to simulate a direct frontal punch at an angle such that the forces on the nasal tip are the most significant . in the case of dorsal and caudal septa in - plane loading ( figure 2 ) , a uniform pressure of 2000  pa was applied to both the dorsal and caudal septa .
this was to simulate a frontal punch at an angle such that both the dorsal and caudal septa components are equally significant .
every object , including the septum , has a set of eigen modes , depending on its structure and composition . in each mode , all parts of the system vibrate with the same distinct frequency , which is referred to as the system 's eigen value at that mode .
since lower modes have lower frequencies and energies , they are more likely to occur . hence , only the first 10 modes of the nasal septum were analyzed .
abaqus ( dassault systmes technologies , providence , ri ) was used to obtain the eigen mode shapes of the septum under the various loading conditions .
a general , static step is created , in which one of the two loading conditions is applied .
thereafter , a linear perturbation , frequency step is created , in which the natural frequency and the corresponding mode shape will be extracted .
the patterns of nasal septal deviation were similar to those described by rohrich et al . .
in our study , the deviation patterns were therefore classified into three groups , each with its specific sites of high stress and dislocation and possible surgical corrective procedures ( table 1 ) . through observation of all deformation patterns ,
we were also able to identify the intrinsic points of fatigue within the cartilaginous septum  the bc junction , anterior nasal spine ( ans ) , vomer - ethmoidal cartilage junction ( vej ) , and a single or couple of cracks in the quadrangular cartilage that lead to c - shaped and s - shaped nasal deformations , respectively .
these points could lead to the septum levering off the vomerine groove and , in the latter two cases , a shortening of the septum ( figure 3 ) . for an idealized model , the slanted ( figure 4(a ) ) ,
c - shaped ( figure 4(b ) ) , and s - shaped ( figure 5(a ) ) deviation patterns were all observed ( table 2 ) . in some modes , the system vibrates in - plane and therefore lacks a resultant deformation shape .
in such cases , a dash is indicated . however , due to the lack of restriction on the nasal tip , it moves relatively freely , which may not represent in the vivo conditions .   in the following idealized model , the nasal tip
is now constrained by a spring . while displaying similar patterns of deviation with a free nasal tip model , the spring - supported nasal tip model exhibits decreased displacement due to its prescribed restriction .
a patient - specific model was then analysed . by observation of the previous idealized models
the patient - specific model exhibited similar deformation patterns as the idealized nasal septal models ( table 2 and figure 5 ) .
the nasal septum is of utmost importance in the support of the  distal nose and for the maintenance of the bilateral nasal airway  .
a straight septum exists where there is force equilibrium , which may be disturbed in fracture , resulting in warping of the septal cartilage [ 18 , 26 ] . depending on the sustained trauma , the septum may deform in a myriad of patterns .
presently , however , studies have reported that septal deformation patterns may be categorized in a number of broad categories , regardless of the trauma and/or injuries sustained .
guyuron et al . , rohrich et al . , and rhee et al .  
categorized nasal deformities broadly into a septal tilt , anteroposterior or cephalocaudal c - shaped and an s- or reverse - s - shaped deformities .
unfortunately , these studies have not correlated these deviation patterns with degrees of force . through the correlation of septum deformation patterns with increasing degrees of force , as well as with areas of dislocation and fracture , preoperative planning and septoplasty may be improved . the prompt identification and management of septal fractures
, we were able to identify clinically observable nasal septal deviations and the aforementioned high - stress areas that would require stress - relief and the possible dislocation sites ( figure 3 ) .
we observed that regardless of force direction , with increasing force , the septum first tilts ( type i ) and then crumples into a c - shape ( type ii ) and finally into an s - shape ( type iii ) .
this was observed through the prevalence of the tilted septum in lower modes , while the  c - shaped  followed by the 
therefore , the lower the mode in which the deviation pattern is observed , the smaller the force required to cause this deformation , and consequently , the greater the probability of observing this pattern .
, in a sample size of 93 patients who had undergone primary septoplasty , 40% had a septal tilt , 32% had a c - shape anteroposterior septum , 4% had a c - shape cephalocaudal septum , 9% had an s - shape anteroposterior septum , and 1% had an s - shape cephalocaudal septum . in type i ,
when a tilted septum is observed , the highest stress concentration occurs at the bc junction and ans .
this suggests that with a low to moderate force , the septum dislocates en - mass from the midline vomerine groove ( figure 3 ) and levers off the bc junction to a tilted position .
this may be observed on ct and mri scans , and naso - endoscopy , where posterior buckling is frequently observed at the vej . through submucous resection
, the septum may be repositioned onto the groove , with prior resection of the cartilage tongue in the nasal floor .
the septum may then be reset to the ans ( table 1 ) . with a higher moderate force , a 
deformation is likely due to a central line of stress in the septum , bending it into two pieces .
the line of high stress may run through the anteroposterior ( figure 4(a ) ) or cephalocaudal ( figure 4(b ) ) directions .
we propose that with significant loading , intrinsic septal fractures occur by breaking the cartilaginous septum into two , leading to the clinical morphology of a c - shaped nose and shortening of the septum . in addition
, the septum will be displaced off its vomerine groove and/or the ans and will likely buckle at the vej ( figure 3 ) .
this is clinically significant as it can not be observed on ct or mri scans due to the invisibility of the septum in such modalities .
hence , a clinical observation of a c - shaped septum may be the only indication .
in addition to the corrective procedures mentioned previously , spreader grafts may be required on both sides of the septum , to assist in its straightening by providing the necessary nasal support that was relinquished when the septum fractured , thereby allowing the septum time to heal ( table 1 ) .   with a force of higher magnitude , an 
deformation may result due to multiple lines of stress leading to a septal concertina and shortening of the septum into a minimum of three overlapping pieces .
this is due to two lines of stress running in the anteroposterior direction ( figure 5 ) .
in addition to being shortened , the septum might be displaced from the vomerine groove and ans ( figure 3 ) . as with a c - shaped deformed septum ,
in addition to the aforementioned corrective procedures , longer spreader grafts will be required to brace both deformed sites to support the septum   and allow it to heal ( table 1 ) .
therefore , regardless of the deviation pattern , by relieving stress in these specific strips of concavity , in combination with the aforementioned surgical procedures , we propose that a more stable , straight septum may be achieved . despite different loading conditions ,
the nasal septum deviates in a relatively constant pattern of a septal tilt , c- and s - shaped deviations with insignificant differences between the resultant modal shapes .  
the free nasal tip and spring - supported nasal tip models responded differently to the loading conditions , specifically in mode three .
a septal tilt is observed in the free nasal tip model , while a c - shaped deformation is observed in the latter model .
as the c - shape deviation is noted to occur with higher energy and septal tilt deviation with lower energy , this finding suggests that the spring of the llcs acts to insulate and constrain the nasal tip and septum against deformation .
the protective interrelationship of the llcs to the nasal septum should be preserved during surgery .
the prevalence of modal shapes in patient - specific and idealized septal models , subject to frontal point - loading , is almost identical ( table 1 ) .
slight deviations , such as those in mode three , are expected , due to the difference in shape between the models . despite the patient - specific model exhibiting greater relative movement than the idealized model
, this difference is insignificant as the basic modal shape remains ( figure 5 ) .
the similarities observed between these two models are a testament to the accuracy of the idealized model .
it is imperative to note that nasal septal deviations are secondary to the bony vault and cartilaginous changes . for the purpose of this study ,
future research aims to combine the study of the deformations of the bony and cartilaginous septa . due to the inherent collagen fibrils and the consequent anisotropy within the cartilaginous septum
, we recognize that the prescription of a linearly elastic material model to the nasal septum material properties may not be fully representative of in vivo cartilage . in spite of this ,
an understanding of the relative displacement that occurs within the different models in different eigen modes remains beneficial in aiding surgeons to correct a deviated nasal septum .
no physical model was mechanically tested to validate the computational model in this preliminary study , which means that absolute stresses and relative stress patterns should be considered cautiously .
such an experimental validation study would typically make use of strain gages , but also infrared thermography , and global stiffness measurements [ 29 , 30 ] .
the purpose of this study was to gain a greater understanding of the septal deformation biomechanics .
we found that despite different loading directions , the septum deformed consistently into only three shapes  a tilted position , a c - shaped septum , and an s - shaped septum .
the tilted septum is seen with the least force , c shape with moderate force , and s shape with high force .
this suggests an intrinsic fracture of the septum into increased number of overlapping fragments with escalating force .
clinically , this is important information that provides insight into predictable patterns of internal septal fractures that need to be realigned and reconstructed to create a straight septum .
 
 background . with the current lack of clinically relevant classification methods of septal deviation , 
 computer - generated models are important , as septal cartilage is indistinguishable on current imaging methods , making preoperative planning difficult 
 . methods . 
 three - dimensional models of the septum were created from a ct scan , and incremental forces were applied . 
 results . 
 regardless of the force direction , with increasing force , the septum first tilts ( type i ) and then crumples into a c shape ( type ii ) and finally into an s shape ( type iii ) . in type 
 i , it is important to address the dislocation in the vomer - ethmoid cartilage junction and vomerine groove , where stress is concentrated . in types ii and iii 
 , there is intrinsic fracture and shortening of the nasal septum , which may be dislocated off the anterior nasal spine . 
 surgery aims to relieve the posterior buckling and dislocation , with realignment of the septum to the ans and possible spreader grafts to buttress the fracture sites . conclusion . by identifying clinically observable septal deviations and the areas of stress concentration and dislocation , a straighter 
 , more stable septum may be achieved . 
1. Introduction
2. Materials and Methods
3. Results
4. Discussion
5. Conclusion