The Effect Of Mandibular Advancement/setback On The Head Posture And Pharyngeal Airway Space

Existing studies have indicated that facial pattern is correlated to headposture. A mandibular retrognathia usually comes along with an extended headposture while a mandibular prognathia usually with a crouched one; on the otherhand, head posture can be used to some extent to predict the development offacial pattern: subjects with an extended head posture tend to develop aretrognathic mandible and those with a crouched one tend to allow for aprognathic mandible and vice versa.Related studies also indicate that head posture can be subject to thedimension of the upper airway. An obstructed pharyngeal airway can lead to anextended head posture and a relief from the obstruction can also bring the headposture back to normal position.Orthognathic surgery refers to a series of surgical procedures used tocorrect cranio-maxillofacial malformations by osteotomy and relocating jaws inall three planes. Based on the standpoints mentioned above, it is thus hypothesized that head posture might be changed after orthognathic surgerysince the facial pattern is changed after surgery, and such a change may alsohave potential effects on the pharyngeal airway dimension.The present study was designed as a retrospective analysis of head postureand pharyngeal airway morphological changes in patients undergoingorthognathic surgeries in the Department of Oral and Maxillofacial Surgery atthe University of Michigan Hospital. In terms of methodology, we collected allthe subjects’ pre-operative and six-week post-operative CBCT data and used theup-to-date three-dimensional cephalometry and volumetric airway measurementunder two commercially available CBCT viewers so to make all themeasurements in their truest fashion. The study was approved thru theInstitutional Review Board of the University of Michigan Health System(MedIRB, HUM00050381). The whole study was composed of five parts asfollows:Part1: The effect of mandibular advancement on the head postureAim：This part was to test the null hypothesis that head posture would notchange following a mandibular advancement.Subjects and methods: Subjects who had undergone mandibular orbimaxillary advancement were recruited and classified as the BSSO group, theBSSO with genioplasy group and the bimaxillary group. Pre-operative and6-week post-operative CBCT data were collected and imported into theInvivoDental5.1(Anatomage Inc., San Jose, CA) and the three-dimensionalcephalometric analysis was done under its3DAnalysis module. Pre-andpost-operative facial pattern and head posture were measured using a few relatedparameters：SNL (mandibular position in angle), L-N⊥FH (mandibular positionin distance), SNC2(head posture in angle), CIP-N⊥FH and CSP-N⊥FH (head posture in distance). A paired t test was used to compare the post-operativechange in all the parameters to their pre-operative counterparts; arepeated-measure ANOVA was designed to analyze the variance in theinteraction, the group and the time factors, and the association between facialpattern and head posture was analyzed using a Pearson correlation test.Results: Forty-one subjects were collected.8were BSSO advancement,12were BSSO advancement with genioplasty and21were bimaxillaryadvancement. Compared to the pre-operative values, SNL and SNC2significantly increased, L-N⊥FH and CIP-N⊥FH significantly decreased andno significant change was seen in CSP-N⊥FH. All the parameters hadsignificance in the subjecting factor, SNL、SNC2、L-N⊥FH and CIP-N⊥FHhad significance in the time factor and no parameters had significance in thegroup or interaction factor. The association of SNL and SNC2and that ofL-N⊥FH and CIP-N⊥FH had significant correlation.Conclusion: Head posture is correlated to facial pattern and will change inthe manner of decreasing the cranio-cervical inclination following a mandibularadvancement regardless other orthognathic procedures are done.Part2: Effect of mandibular advancement on pharyngeal airway spaceAim: This part was to investigate the morphological change of thepharyngeal airway space after a mandibular advancement and its relationshipwith the head posture.Subjects and method: Subjects who had undergone mandibular orbimaxillary advancement were recruited and classified as the mandibualaradvancement group and the bimaxillary advancement group. The pre-andpost-operative CBCT were imported into Dolphin Imaging11.0(DolphinImaging and Management Solutions, Chatsworth, CA). The primary parameters such as Volume, Length and minimal Cross Sectional Area (CSAmin) ofpharyngeal airway space were measured under the Sinus/Airway module, andthe secondary parameters such as the average CSA (CSAave, Volume/Length),Ratio (post-op CSAmin/pre-op CSAmin) Uniformity (CSAmin/CSAave) and airflowResistance (length/CSAave2) were calculated with related formulas. For meanscomparison, a one-sample t test was used to compare the Ratio to the hypotheticvalue1, and a paired t test was used to compare the post-operative change in allthe rest parameters to their pre-operative values; a repeated-measure ANOVAwas designed to analyze the variance in the interaction factor, group factor, timefactor and subjecting factor, and the association between PAS parameters andhead posture parameters was analyzed using a Pearson correlation test.Results: Thirty-one patients were collected;15were mandibularadvancement and15were bimaxillary advancement. Ratio is significantly largerthe hypothetic value1, the Volume and CSAaveincreased while Length andResistance decreased significantly, and no significant change was seen in theUniformity. All the other parameters had significance in the subjecting factorexcept the Ratio, the Length and the CSAaveother than the Volume hadsignificance in the time factor and no parameters had significance in the groupor interaction factor. The association of CSAaveand the second vertebrate iscorrelated, yet a stronger correlation was seen between the CSAaveand with theinter-position of the second cervical vertebrate and the mandible.Conclusion: PAS is correlated to the head posture, and after a mandibularadvancement the shape of pharyngeal airway is significantly changed andairflow resistance is significantly decreased.Part3: The effect of mandibular setback on head postureAim: This part was aimed to test the null hypothesis that head posture would not change following a mandibular setback.Subjects and method: Subjects who had undergone mandibular setback ormandibular setback with maxillary advancement were recruited. Pre-operativeand6-week post-operative CBCT data were collected and measurement wasdone in the same manner as used in Part1. A paired t test was used to compareall the parameters before and after surgery, and the relationship between facialpattern and head posture was tested using a Pearson correlation test.Results: Thirteen patients were collected;2were mandibular setback and11were mandibular setback with maxillary advancement. Compared to thepre-operative values, SNL significantly decreased and L-N⊥FH significantlyincreased; the SNC2decreased and the CIP-N⊥FH and the CSP-N⊥FHincreased in means yet the differences were not significant. No significantcorrelation was seen in facial pattern parameters and head posture parameters.Conclusion: Within such a small sample, no significant head posturechange yet can be seen following a mandibular setback and the study should berepeated in the when more cases are available.Part4: Effect of mandibular setback on pharyngeal airway space and itsrelationship with head postureAim: This part was to investigate the morphological change of pharyngealairway after mandibular setback and its relationship with head posture.Subjects and method: The sample in the Part3was used. Pre-operativeand6-week post-operative CBCT data were collected and measurement wasdone in the same manner as used in Part2. A single-sample t test was used tocompare the Ratio with the hypothetic value1. A paired t test was used tocompare all the other parameters before and after surgery, and the relationshipbetween facial pattern, head posture and pharyngeal airway was tested using a Pearson correlation test.Results: Though the differences were not significant, the means of thevolume and CSAaev increased, the means of uniformity and resistancedecreased, and the means of ratio was higher than the theoretical value1. TheCSAaveshowed a significant correlation with the inter-position of the secondcervical vertebrate and the mandible.Conclusion: Pharyngeal airway is correlated with inter-relation of facialpattern and head posture in the patients receiving mandibular setback, nosignificant change was seen in the PAS shape but airflow resistance issignificantly decreased after a mandibular setback is done.Part5: A multiple comparison of head posture and pharyngeal airway inpatients undergoing mandibular advancement/setbackAim: This part is to compare the facial pattern, head posture andpharyngeal airway in patients undergoing mandibular/setback before and aftersurgery respectively.Subjects and method: Data in the part2and3were used, that is30subject with mandibular advancement and13with setback. Parameters of facialpattern, head posture and pharyngeal airway in the advancement group werecompared with their counterparts in the setback group using an unpaired t testbefore and after surgery respectively. The PAS morphological change in themandibular advancement group was compared to that in the setback group usinga repeated-measures two-way ANOVA.Results: Before surgery, the advancement group had significantly smallerSNL and SNC2, longer L-N⊥FH and CIP-N⊥FH than their counterparts in thesetback group, no significant difference was seen in the PAS parameters; aftersurgery, the advancement still had significantly smaller SNL and longer L-N⊥FH than their counterparts in the setback group, yet the differencesdecreased, and no significant difference was seen in the head posture parametersor the PAS parameters. All the parameters had significance in the subjectingfactor except Ratio; Volume, Ratio and CSAave had significance in the timefactor but no significance in the group factor or the interaction factor, Lengthhas significance in the interaction factor but no significance in time or groupfactor, yet Uniformity had no significance in time, group or the interactionfactor.Conclusion: Facial pattern, head posture and PAS are inter-associated.Mandibular hypoplasia is usually with an extend head posture while hyperplasiausually with a crouched head posture yet both with a compromised PASdimension with an increased airflow resistance; after a mandibularadvancement/setback, extend/crouched head posture tend to regress to normalalong with the correcting the mandibular hypo/hyperplasia into a normal facialpattern, and PAS dimension was dilated leading to a decrease airflow resistance.In summary, along with the reconstruction of a normal facial pattern,the procedure of mandibular advancement/setback may have significancein correcting the abnormal head posture resulting in a facilitation ofbreathing.