Three-dimensional Analysis Of Craniofacial Symmetry With CT And MRI Based On Integrated And Modular Organization Of Head

In recent years,computer tomography(CT),particularly cone beam computed tomography(CBCT),has been used widely.This technique has emerged with certain benefits such as accurate and comprehensive three dimensional craniofacial images.In the study of craniofacial asymmetry,the establishment of a three-dimensional coordinate system and a reference plane is especially important.Although there are a variety of methods to determine the median sagittal plane,they have their respective advantages and disadvantages.The landmark method can be operated easily,but the combination of marks is random,with no scientific basis.In addition,the probability of error increases with the number of marks.Although the external reference system does not rely on intracranial structures,it requires patients to maintain a natural head position,which is difficult for patients with craniofacial malformation.The mathematical algorithms obtain a “median” midsagittal plane,but it is a nonexistent calculated plane that does not follow the developmental axis during the evolutionary process of vertebrates.In order to find more reasonable ways to analyze craniofacial asymmetry,the authors reviewed studies of evolutionary development,clinical medicine,and molecular biology.came to the following conclusions.1.Integration of the developing brain,cranium and faceNumerous studies have demonstrated that multiple tissues including nerves,bones,and muscles form an interactive development system in vertebrates.Therefore the development of the head is a complicated,integrated and modularized process.2.Modularized development and evolvabilityThe anterior cranial base with the mid-upper face,and the posterior cranium with the lower jaw,respectively,form two craniofacial skeletal modules.Whereas the upper face above the eyebrows forms a single musculoskeletal module.The anterior cranial base and mid-upper face are significantly integrated,forming a developmental module that can promote and inhibit changes in the craniofacial morphology.The lower jaw forms a joint with the posterior cranial base and is therefore more likely to be influenced by the posterior cranial base.3.The forebrain,anterior cranial base,and midupper facial midline are regulated by the same signaling factors;thus,the three components are highly consistent.The high consistency of the forebrain,anterior cranial base,and mid-upper facial midline has been fully reflected in both physiological and pathological conditions.The prechordal plate defines the facial midline by signaling factors such as sonic hedgehog(SHH)and induces the forebrain to develop into two hemispheres,thus dividing the orbital area into two.The forebrain,anterior cranial base,and mid-upper facial midline are regulated by the same signaling molecules,thus the three are highly consistent.Based on the studies reviewed,we suggested a framework of craniofacial structures according to evolutionary development and molecular biology evidence.We suggested that the brain,cranium,and face interact to be an integrated and modularized developmental system.The cerebral midline is consistent with the facial midline.Inspired by this,the authors proposed to use the anterior cerebral falx as the reference plane for studying craniofacial symmetry.We suggested to use the cerebral falx part anterior to the hypophysial foramen as the accurate reference plane.The anterior and posterior dura have different sources of tissues as the cranium,hence they may differ from each other.In addition,a number of studies have revealed a deviation of the cerebral falx from the midline adjacent to the occiput.The cranial skeletons anterior to the pituitary fossa are derived from neural crest cells,whereas the posterior is derived from the mesoderm.The cerebral falx is a dura mater boundary formed between the two cerebral hemispheres,and it can be viewed clearly on CT and MRI images.The gold standard for brain midline shift suggests to use the falx as a reference midline and to measure the deviation of the septum pellucidum from the falx cerebrum.The method of determining the median sagittal plane based on the head symmetry axis is a new idea for three-dimensional analysis of craniofacial asymmetry.Also,the authors have introduced the modularization viewpoint for the first time to explain the aggravated asymmetry from the upper to the lower face.The authors suggested that the most functional and inherent craniofacial asymmetry is derived from the lower jaw-posterior cranium skeleton module and is transferred to the upper face via an asymmetric contraction of muscle modules on both sides and occlusal compensation.Additionally,the bilateral upper-lower jaw muscle modules may experience functional asymmetric contractions that further cause lower jaw skeleton asymmetry.The anterior cranial base and mid-upper facial module are relatively stable,especially the upper third of the face.This further confirms the conclusions made in the human head anatomical network analysis that the upper face is a single module of nerves,bones,and muscles;therefore,it has the best symmetry.The aim of this study was to determine the relationship between the anterior cerebral falx plane and the facial midline.In addition,to compare the median sagittal cerebral falx plane and the median sagittal plane constructed using the three-point method,to evaluate whether the anterior cerebral falx plane can be used as the median sagittal plane to study craniofacial symmetry.Based on the modular characteristics of the head,a threedimensional CT and MRI analysis system for craniofacial soft and hard tissue symmetry is establishedThis was a retrospective study,conducted using the radiographic data of patients attending the Department of Orthodontics at the Fourth Military Medical University,Xi'an,Shanxi Province of China.This study was reviewed and approved by the Ethics Committee at the Tong Liao City hospital.Informed consent was waived by the committee due to the retrospective nature of the study.All subjects(n=100)who fulfilled the selection criteria were randomly divided into two groups(n=50 per group;control and asymmetry).The inclusion criteria were: adults(? 18 years of age)having complete dentition and centric jaw position,with a scan ranging from the calvaria to the lower edge of the mandible.The exclusion criteria included: craniofacial injuries or bone fractures,diffuse inflammation,brain abnormalities or soft tissue tumors,grossly decayed teeth,crowding more than 3 mm or spacing more than 1 mm,a history of previous orthodontic or orthognathic treatment,and cleft lip/palate patients.In the asymmetry group,two independent researchers analyzed craniofacial midline anatomical landmarks and confirmed the visible craniofacial asymmetry.For example,the deviation of the mandibular symphysis(Me)from the median sagittal plane constructed by Cr,ANS,and Op landmarks was greater than 4 mm in the asymmetrical group.Patients in the control group were selected from the emergency room of the Tong Liao City Hospital Tongliao,Inner Mongolia,China during the period of study.Two independent researchers analyzed the CT data to rule out the presence of any craniofacial asymmetry in the control group.Deviation of the Me from the median sagittal plane constructed by Cr,ANS,and Op,was considered control at less than 2 mm.Result 1: Constructed the falx plane Successfully.Result 2: According to distances between anatomical landmark and the cerebral falx plane in the control and asymmetrical groups: In the control group,the mean distances between landmarks and the cerebral falx plane were less than 1 mm.The only exception greater than 1 mm was the mean distance between Op and the cerebral falx plane.In the asymmetry group,the mean distances from the Cr,Cl,MZF,and N to the cerebral falx plane were less than 1 mm,while the mean distances from the ANS,PNS,UI,LI to the cerebral falx plane were greater than 1 mm.Furthermore,the mean distances were gradually increased from the upper to lower face.Points below N(including N,ANS,PNS,UI,LI and Me)were significantly different between the control and asymmetrical groups.In addition,although the mean distance from Op to the cerebral falx plane was greater than 1 mm in both groups,the difference between these two groups was not statistically significant.Result 3: According to distances between anatomical landmarks and the reference plane constructed by the three-point method in the control and asymmetrical groups.:Similar to the results obtained using the cerebral falx plane as the reference,the landmarks N,PNS,UI,LI and Me showed statistically significant differences between the control group and asymmetrical group when using the medial sagittal plane(determined by three-point method)as the reference.In addition,the mean distances from PNS,UI,LI and Me to the reference plane determined by the three-point method were smaller than those to the cerebral falx reference plane.Designating ANS as zero in the three-point method concealed craniofacial asymmetry below ANS.Result 4: According to correlation between anatomical landmarks in the control group using the distance to the cerebral falx plane as the reference :in the control group,the LI was directly correlated only with Me,whereas UI was strongly correlated only with LI.Since LI is on the mandible,the normal range of Me point deviation is strongly correlated with LI,and UI was strongly correlated with LI due to the direct occlusal contact between the upper and lower teeth.However,the normal range deviation of Me from cerebral falx plane is not strong enough to cause midline deviation of the landmarks superior to the mandible.Result 5:According to correlation between anatomical landmarks in the asymmetrical group by using the distance to the cerebral falx plane as the reference: in the asymmetrical group,ANS,UI,LI were correlated with Me strongly and directly.Therefore,their deviation from the cerebral falx plane was strongly and directly correlated with Me deviation.These data suggested that most of the functional and inherent craniofacial asymmetry were derived from the lower jaw and transferred to the upper face via asymmetric contraction of muscle modules bilaterally.Result 6: According to consistency of results at anatomical landmarks using the median sagittal planes determined by the cerebral falx and the three-point method as references :The median sagittal planes determined using the two methods showed consistency in terms of Cl,MZF,N,PNS,UI,LI and Me,with the best consistency observed for MZF,LI and Me.Result 7: A CT and MRI three-dimensional analysis of craniofacial soft and hard tissues symmetry based on integrated and modular structure of head is preliminarily established.Conclusion: The anterior cerebral falx plane is a physical median sagittal cerebralcraniofacial plane that has been confirmed by evolutionary development and morphology.It can be considered a sensitive reference plane for the assessment and diagnosis of craniofacial asymmetry.The anterior cerebral falx plane is also relatively stable in patients with severe craniofacial asymmetry.These common onsets of craniofacial asymmetry can be interpreted by modular organization of human head that provides new evidence for the NHH and mosaic evolution.A CT and MRI three-dimensional analysis of craniofacial soft and hard tissues symmetry based on integrated and modular structure of head is preliminarily established.