The stability of occlusal plane rotation during orthognathic surgery

82 © Australian Society of Orthodontists Inc. 2017 Background: Considered rotation of the occlusal plane and the maxillomandibular complex during orthognathic surgery is an approach that enables optimisation of the functional and aesthetic outcomes of surgical-orthodontic treatment for the correction of a variety of dentofacial deformities. Objectives: The objectives of this study are to: (1) retrospectively evaluate the stability of clockwise and anti-clockwise rotation of the occlusal plane during orthognathic surgery, and (2) compare the findings with the stability of orthognathic surgery in which no significant occlusal plane rotation occurs. Aims and methods: Data were collected from a sample of 31 adult patients (22 females and nine males) treated by one orthodontist in private practice and at the University of Western Australia from 1992–2008. Patients received a Le Fort I osteotomy and BSSO to reposition the maxilla and mandible, respectively. The patients were separated into three groups defined by no occlusal plane rotation; clockwise occlusal plane rotation; or anti-clockwise occlusal plane rotation. Lateral cephalograms were taken at four time periods during treatment: T1 (pre-surgery), T2 (immediately post-surgery), T3 (six weeks post-surgery), and T4 (longest follow-up). The cephalograms were digitised and analysed using a customised cephalometric analysis (QuickCeph Studio, CA, USA) and landmarks were transferred via cranial base and maxillary superimpositions for each patient from T1 sequentially to T4. Results: Clockwise rotation of the occlusal plane was highly stable one year following surgery and showed comparable results with cases treated without occlusal plane rotation. Anti-clockwise rotation of the occlusal plane was less stable and showed significant relapse of the occlusal plane angle towards the pre-surgical value during the post-surgical observation period. Conclusion: Clockwise rotation of the occlusal plane during orthognathic surgery appears highly stable. Anti-clockwise occlusal plane rotation during orthognathic surgery appears less stable. Further studies are needed to evaluate the longer-term stability of clockwise and anti-clockwise rotation of the occlusal plane during surgical-orthodontic treatment. (Aust Orthod J 2017; 33: 82-96)


Introduction
Dentofacial deformities severe enough to require surgical intervention often require bimaxillary surgery to optimise the functional and aesthetic outcomes of treatment. [1][2][3][4] Depending on the patient's facial morphology, it is sometimes necessary and/or desirable to rotate the maxillomandibular complex (MMC) and occlusal plane (OP) during surgery. The benefits of performing this type of surgery are functional, such as reducing airway resistance and improving masticatory efficiency; and secondarily aesthetic, related to the production of an angular taper to the facial form and a reduction in chin prominence of brachyfacial individuals (Figures 1-3); or increasing chin projection in severely retrognathic patients (Figure 4-6). [5][6][7] Performing bimaxillary surgery without rotating the occlusal plane may produce undesirable aesthetic outcomes that are also potentially less stable over the long term. The rotation of the occlusal plane in a clockwise direction during orthognathic surgery was initially performed on patients with hypodivergent Samuel C. Bennett: samuel.bennett@uwa.edu.au Class II type of malocclusions because it enabled a reduction in chin prominence and improved stability. 7 Clockwise rotation of the occlusal plane during surgery also produces a tapering of facial form that is aesthetically beneficial, particularly in brachyfacial female patients. 8 However, it may be more advantageous to rotate the occlusal plane in an anti-clockwise direction during surgery to gain improvements in airway volume and horizontal chin projection, especially in patients with severe chin retrusion. Whilst the potential functional and aesthetic advantages gained by surgery involving clockwise rotation of the occlusal plane are well documented, there is little literature supporting the long-term stability of anti-clockwise occlusal plane rotation during orthognathic surgery. 9,10   The rotation of the occlusal plane is a consequence of rotation of the maxillomandibular complex and, depending on the functional and aesthetic goals of treatment, the MMC may be rotated downwards and backwards (clockwise) or upwards and forwards (anticlockwise). (In order to be consistent with the literature relating to rotation of the MMC, cephalograms are viewed with the patient facing to the right; thus clockwise rotation is downwards and backwards and anti-clockwise rotation is upwards and forwards). Following superior repositioning of the maxilla, the mandible autorotates about a point posterior and inferior to cephalometric-condylion, resulting in a variable amount of rotation of the occlusal plane, depending on the amount and direction of maxillary repositioning. 13,14 Changes in the occlusal plane angle   THE STABILITY OF OCCLUSAL PLANE ROTATION DURING ORTHOGNATHIC SURGERY of greater than 2° are considered to be clinically significant. 11 The key to occlusal plane rotation and MMC is the surgical rotation of the maxilla. Depending on the functional and aesthetic goals for treatment, the maxilla can be rotated clockwise or anti-clockwise to achieve the desired outcomes and so cephalometric studies aiming to assess surgical rotation must measure changes that are produced at both anterior and posterior maxillary cephalometric landmarks during surgery. Cephalometric analysis via superimposition is an accepted method by which to assess stability following surgical-orthodontic treatment, especially when maxillary surgery is performed. [15][16][17] pterygo-masseteric sling when indicated, and (4) placement of grafting material, such as hydroxyapatite, into the osteotomy site. 10,11 Further studies are needed to re-evaluate the stability of clockwise and anti-clockwise rotation of the occlusal plane and to compare the stability of orthognathic surgery without occlusal plane rotation in light of contemporary clinical advances.
Therefore, the objectives of the present study are to (1) evaluate the stability of clockwise and anti-clockwise rotation of the occlusal plane during orthognathic surgery, and (2) compare the findings with the stability of surgical treatment undertaken without occlusal plane rotation.

Materials and methods
Thirty-one non-syndromic, relatively faciallysymmetrical, non-growing patients (22 females with a mean age of 24.5 years, and a standard deviation 9.1 years; and 9 males with a mean age of 22.8 years and a standard deviation 6.9 years) underwent surgical orthodontic treatment either at a private orthodontic clinic, or at the Orthodontic Department of the University of Western Australia. All patients were treated by the same orthodontist and the surgical procedures took place between 1992 and 2008. Four patients had a skeletal Class I malocclusion, 19 had Class II, and eight patients had a Class III skeletal pattern. Patients received a Le Fort I maxillary osteotomy and a BSSO to re-position the mandible. Twenty-four patients received a genioplasty. Rigid fixation techniques were applied in all cases and an interocclusal splint was utilised during surgery and maintained throughout the initial post-operative stage (<6-weeks post-surgery).
The cases were selected retrospectively on the basis that standard lateral cephalograms were taken in centric relation at four stages during treatment: T1 (pre-surgery: mean six weeks, range 1-18 weeks); T2 (immediate post-surgery: mean 8.5 days, range 2-20 days); T3 (six weeks post-surgery: mean 6.5 weeks, range 5-9 weeks); and T4 (longest follow-up with a minimum of six months post-surgery: mean 13.1 months, range 6-36 months).
The patients were grouped according to the following criteria: Group 1: No occlusal plane rotation (NR). Nine patients (five female, four male) with a mean age of Previous studies appear to indicate that clockwise rotation of the occlusal plane is highly stable following bimaxillary surgery. 10,11 In contrast, anti-clockwise rotation of the occlusal plane and MMC seems to be less stable. This may be attributed to factors related to the larger magnitude and direction of mandibular advancement produced by anti-clockwise rotation, the increased vertical dimension produced when the maxilla is inferiorly repositioned, and variations in osteotomy design and fixation techniques. 6,9,11,18 The stability of orthognathic surgery involving anti-clockwise rotation of the occlusal plane may be improved by (1) planning maxillomandibular movements to ensure neuromuscular adaptation following surgery, (2) the utilisation of rigid fixation techniques, (3) the use of modern osteotomy design techniques (such as the Hansuck modification), including detachment and incision of the 22.7 years (range 13-31) received less than 2° occlusal plane rotation as a result of surgery. The average length of follow-up time was 10.9 months (range 6-15) after surgery.
Group 2: Clockwise rotation of the occlusal plane (CR). Twelve patients (9 female, 3 male) with a mean age of 27.6 years (range 15-44) received (>2°) clockwise rotation of the occlusal plane during surgery. The average follow-up time was 13.9 months (range 6-36) after surgery.
The pre-surgical radiographs were traced in a semi-dark room by one author onto clear overhead transparency sheets using a 0.5 mm fine permanent marker. The cranial base and maxillary regional tracings were used to transfer cephalometric landmarks from the pre-surgical radiograph to the subsequent postsurgical treatment radiographs to improve accuracy, eliminate landmark identification errors and obviate inter-examiner assessment. 15,16,[19][20][21][22] Eight landmarks (Nasion, Sella, Horizontal Reference, Porion, Basion, Articulare, Pterygomaxillary, and Orbitale) were transferred via best-fit cranial base superimposition on the anterior wall of sella turcica and planum sphenoidale of the middle cranial fossa. Five landmarks (ANS, PNS, A-point, upper incisal edge, distal upper molar cusp) were transferred via best fit maxillary superimposition. The transfer of mandibular landmarks by superimposition was precluded due to the alteration in mandibular morphology following BSSO surgery and the inclusion of patients who underwent a concurrent genioplasty procedure. Measurement of the functional occlusal plane relative to the cranial base (SN-OP) was made in order to eliminate measurement errors due to infra-and overerupted incisors. 1,23 Each radiograph (total = 124) was scanned and digitised using QuickCeph software (CA, USA) on an Apple Macintosh computer and analysed using a customised cephalometric analysis. The analysis  created an XY co-ordinate system that enabled measurement of linear distances from horizontal and vertical reference lines. The horizontal reference line passed through Nasion at right angles to the patient's True Vertical line, as assessed radiographically. The calculation of angular and linear measurements to the nearest 0.1° and 0.1 mm respectively was made using the software. Calibration of radiographs was performed using QuickCeph software to ensure uniform scaling of the measurements. The process of digitisation of each of the four radiographs taken during treatment was repeated blindly for a random selection of five patients (total = 20 radiographs), one month following the initial digitisation to assess intraexaminer reliability and ensure accuracy of the method using the intra-class correlation (ICC) test. 21,22 Results Intra-Class Correlation (ICC) was used to evaluate the reliability of the data produced by the cephalometric superimposition and digitisation methods applied during the study. Each of the variables (linear measurements, angular measurements, and LFH%) that are presented in the following tables demonstrated an ICC of greater than 0.9, which is excellent, and indicates the data were very reliable.
Tables II-IV show the vertical, horizontal and angular changes that occurred in each of the three groups.
Linear measurements in the superior direction of the vertical plane were designated as negative (-), and linear measurements in the inferior direction of the vertical plane were positive. Linear measurements in the anterior direction of the horizontal plane were designated as positive, and linear measurements in the posterior direction of the horizontal plane were negative (-). The surgical changes were measured during the interval from T1-T2; post-surgical changes were measured during intervals T2-T3, T3-T4, and T2-T4; and the overall changes were measured during the interval T1-T4.

Group 1: No Rotation group
The No Rotation group was characterised by surgical changes involving significant measurable horizontal advancement of maxillary and mandibular cephalometric landmarks but no occlusal plane rotation during surgery. There was no significant measurable change in occlusal plane angle observed during any of the time intervals of the study.
The maxillomandibular complex tended towards superior repositioning as indicated by the negative values (Table II) from T1-T2 and T1-T4. Although these movements were not statistically significant, they represent: (1) a concomitant superior maxillary repositioning that accompanies horizontal advancement of the maxilla during surgery,  (2) mandibular autorotation during surgery, and (3) post-surgical change. Pogonion was the only variable to show significant change in the vertical plane, which was the result of genioplasty procedures. Overall from T1-T4, Pogonion moved superiorly by an average of 3.1 mm (p < 0.001). The initial surgical change at Pogonion from T1-T2 (average 2.3 mm) was not statistically significant, and the overall change was the result of surgical movement plus the accumulated post-surgical movement in the superior direction.
The maxillomandibular complex was advanced surgically in the horizontal direction, without rotation of the occlusal plane. A-point was advanced horizontally by an average 2.1 mm (p < 0.05) during surgery. Throughout the post-surgical review, there was further anterior movement observed at A-point, and overall, the average of 2.9 mm (p < 0.01) advancement was measured at A-point from T1-T4. B-point was advanced horizontally by an average 1.84 mm during surgery; however, this change was not statistically significant. Following removal of the interocclusal splint, B-point moved anteriorly by an average of 2.9 mm (p < 0.05) as a result of the mandibular rotation.
Overall, B-point advanced horizontally by an average of 3.7 mm (p < 0.01) from T1-T4. Menton moved forwards during surgery and overall by an average of 5.0 mm (p < 0.01) and 7.2 mm (p < 0.001), T1-T2  T2-T3  T3-T4  T2-T4  T1-T4 Vertical changes respectively. The greater magnitude of advancement measured at Menton compared with B-point was the result of mandibular autorotation following maxillary superior repositioning during surgery.
No significant change in lower facial height proportion was measured at any interval during the study.
The results indicate excellent stability of the maxillomandibular surgery as no significant relapse was measured at any of the variables examined during the post-surgical interval of T2-T4. The largest amount of post-surgical relapse of 2.2 mm anterior movement measured at Menton was not statistically significant, and the remaining linear variables showed ≤2 mm post-surgical change. Furthermore, the changes produced during surgery at each of the variables tested remained statistically significant for the duration of the study over the interval of T1-T4.

Group 3: Anti-clockwise Rotation group
A significant anti-clockwise rotation of the occlusal plane (SN-OP) averaging 4.6° (p < 0.001) during surgery was measured from T1-T2, as presented in Table IV. However, a significant amount of relapse of the occlusal plane angle was also measured during the post-surgical observation period T2-T4 of average 2.5° (p < 0.001). Overall from T1-T4, the occlusal plane was rotated significantly anti-clockwise by an average of 2.1° (p < 0.01).
An anti-clockwise rotation of the occlusal plane during surgery was produced by significant superior repositioning of the maxilla by a greater amount anteriorly compared with posteriorly. A-point was superiorly repositioned by an average 3.5 mm (p < 0.001) and PNS moved upwards by an average of 1.  T1-T2  T2-T3  T3-T4  T2-T4  T1- A significant increase in lower facial height proportion during surgery was measured in the Anti-clockwise Rotation group (LFH% = 2.4, p < 0.001). Although this change remained significant for the overall observation period of T1-T4 (LFH% = 1.2, p < 0.05), a significant amount of negative change (relapse) of equal magnitude was also measured during the postsurgical observation period of T2-T4 for the lower facial height percentage (LFH% = -1.2, p < 0.05). No cases of severe or measurable condylar resorption or bone remodelling were identified in this group. Figure 10 depicts the relapse of occlusal plane rotation in the anti-clockwise (CCR) group. The relapse in occlusal plane angle was statistically significant in the CCR group from T2-T3 and during the post-surgical observation period from T2-T4. By comparison, there was no significant change in occlusal plane angle in either the NR group or the CR group during the post-surgical observation period from T2-T4.

Discussion
The present study utilised similar methods of cephalometric superimposition and digitisation to previous studies on surgical-orthodontic treatment to ensure accuracy, reproducibility, and reliability of the data. 10,11,[15][16][17]25 Cephalometric tracing, superimposition and digitisation was performed by the same person and the intra-examiner reliability was tested using the ICC, according to accepted statistical testing methods for medical research. 21,22 The value of over 0.9 calculated for the ICC of each of the cephalometric variables measured indicated an excellent agreement and reliability of measured data.
In the context of post-treatment stability relating to surgical-orthodontic care, it is important to establish how stability is reported in the scientific literature. Studies of post-treatment stability describe postsurgical changes using various methods, including: (1) the percentage post-surgical change of a variable that was observed for a given period of follow-up time; (2) the percentage of patients who experience post-surgical change of a certain variable; and finally, (3) in terms of the absolute amount of post-surgical change measured at a certain variable. [9][10][11]17,[25][26][27][28][29] Each of these methods has provided valuable knowledge in the field of orthognathic surgery, and has contributed to the development of a hierarchy of stability. 9,27 A detailed discussion of stability is beyond the scope of this article; however, as a guide, orthognathic surgery involving maxillary superior repositioning, mandibular advancement (<10 mm), and maxillary advancements (<8 mm) is considered stable (i.e. approximately 20% chance of 2-4 mm post-surgical change) when moderate in magnitude.
In contrast, surgery resulting in maxillary inferior repositioning is categorised as problematic, defined by an approximately 40-50% chance of 2-4 mm postsurgical change and a significant chance of >4 mm change. 27 Furthermore, several principles for postsurgical stability have been proposed: (1) stability is greatest when the soft tissues are relaxed during surgery and least when they are stretched, (2) excellent stability requires neuromuscular adaptation, and (3) neuromuscular adaptation affects muscular length, not muscular orientation. 9 No significant occlusal plane rotation was produced during orthognathic surgery or overall in the No Rotation group. The maxillomandibular complex was advanced horizontally during surgery without occlusal plane rotation. A measurable, although statistically insignificant, amount of superior maxillary repositioning and mandibular autorotation during surgery was evident. The possible explanation for superior maxillary repositioning is twofold: (1) to maintain incisal display and adjust for upper-lip softtissue shortening following maxillary advancement, and (2) the removal of bony interferences from the posterior maxilla during surgery. 30,31 Mandibular autorotation follows as a consequence of maxillary superior repositioning during surgery. 13,14 Overall, the maxilla advanced horizontally at A-point by an average of 2.9 mm (p < 0.01); and the mandible advanced by an average of 3.7 mm (p < 0.01) and 7.2 mm (p < 0.001) at B-point and Menton, respectively. A larger amount of advancement at Menton compared with B-point was the effect of mandibular autorotation during surgery. No significant changes were measured in any of the cephalometric variables during the postsurgical observation period from T2-T4, and a ≤2 mm change in vertical and horizontal linear measurements was recorded for each of the variables (except Menton = 2.2 mm, NS), indicating excellent post-surgical stability. Furthermore, the changes produced during surgery at each of the variables tested remained statistically significant for the duration of the study over the interval from T1-T4, also indicating excellent treatment stability. These results are comparable to previous studies that have demonstrated the high stability and predictability of orthognathic surgery without occlusal plane rotation. 9,11,27,29,32 The Clockwise Rotation group was characterised by clockwise rotation and horizontal advancement of the maxillomandibular complex. The occlusal plane steepened by an average of 4.3° during surgery. A further 0.5° steepening was measured during the post-surgical interval, producing an overall clockwise rotation by 4.8°. Steepening of the occlusal plane during surgery was produced by superior repositioning and horizontal advancement of the posterior maxilla (measured at PNS and Mx6). During surgery, PNS was repositioned superiorly by an average of 2.7 mm and advanced horizontally by average 5.4 mm.
Overall, PNS was superiorly repositioned by an average of 2.3 mm and horizontally advanced by an average of 4.9 mm. There was no significant postsurgical change measured at PNS. By contrast, there was a measurable amount of post-surgical movement in the inferior direction at dental landmarks Mx1 and Mx6, such that, for the overall observation period from T1-T4, the vertical change produced during surgery at Mx6 was no longer significant (Mx6 = -1.2 mm, NS) and significant inferior movement at Mx1 was evident (Mx1 = 1.9 mm, p < 0.05), which was not produced during surgery from T1-T2. These results compare favourably with the no-rotation group and confirm the findings of previous studies. [9][10][11]27 Clockwise rotation of the occlusal plane and maxillomandibular complex during orthognathic surgery is an effective and stable treatment to correct a variety of dentofacial deformities, especially those characterised by mandibular retrognathism and deep overbite. 5,7,10,11 A decrease in the occlusal plane angle was achieved during orthognathic surgery and remained significant throughout the observation period in the Anticlockwise Rotation group. The occlusal plane angle decreased by an average of 4.6° during the interval from T1-T2. However, during the post-surgical review, significant relapse (increase) in the occlusal plane angle was measured (2.5°). Overall, from T1-T4, a significant decrease in the occlusal plane angle of 2.1° was observed throughout the study, which was clinically significant. 11 Anti-clockwise rotation of the occlusal plane was produced by significant differential superior repositioning of the maxilla by a relatively larger amount anteriorly. The mandible was surgically repositioned to match the maxilla; however, the superior repositioning of the mandible was not statistically significant, and resulted in a proportional increase in lower facial height from T1-T2 by an average of 2.4%. The significant relapse in occlusal plane angle measured during the post-surgical interval from T2-T4 may be attributed to vertical changes at maxillary landmarks (dental and skeletal), and was likely the result of settling of the occlusion. 9,10 The significant proportional decrease in lower facial height measured during the post-surgical interval T2-T4 was the result of a greater amount of vertical relapse of the anterior maxilla (A-point) compared with a vertical change at the anterior mandibular landmark, Menton. Although the vertical changes at A-point and Menton were not significant from T2-T4, the absolute difference was an average of 1.5 mm, which was significant enough to alter the LFH% value.
Overall, the change in lower facial height proportion was stable throughout the study, and measured an average increase of 1.2%. The maxillomandibular complex advanced horizontally during surgery in the anti-clockwise group by an average of 4.5 mm at A-point, 9.4 mm at B-point, and 15.1 mm at Menton. These changes were statistically significant and remained significant for the overall duration of the study from T1-T4 (A-point = 4.4 mm, B-point = 8.7 mm, and Menton = 13.6 mm), indicating excellent treatment stability, comparable with the findings of previous studies in this field. [9][10][11]27 Postsurgical relapse in the horizontal direction was not significant and measured a maximum average of 1.5 mm at Menton from T2-T4. As expected, a far greater amount of mandibular advancement was produced in the Anti-clockwise Rotation group compared with the Clockwise Rotation group and the No Rotation group for a similar magnitude of horizontal maxillary movement. 5,6,11 The present results indicate that a significant degree of relapse may follow an anti-clockwise rotation of the occlusal plane during surgical-orthodontic treatment, which is the possible result of postsurgical changes related to settling of the occlusion. 9,10 However, overall, the results also demonstrate excellent stability of the maxillomandibular complex in the vertical and horizontal planes, indicating that anti-clockwise rotation of the occlusal plane during orthognathic surgery is a viable treatment alternative for the correction of certain forms of dentofacial deformity, particularly those characterised by severe chin retrusion and airway compromise. 10,33 In these circumstances, the stability of occlusal plane rotation during orthognathic surgery may be improved by (1) ensuring maximal superior repositioning of the MMC, (2) maintaining or decreasing the patient's facial height and lower facial height proportion during surgery to promote neuromuscular adaptation, and (3) optimising surgical techniques, including the use of rigid fixation, and appropriate osteotomy design. 9,10,18,28,34,35 Inferior repositioning of the maxilla during orthognathic surgery is considered one of the least stable orthognathic procedures and is categorised as problematic. 27 Therefore, practitioners should aim to reposition the anterior maxilla superiorly to achieve occlusal plane rotation during surgery and negate the potential for relapse that would otherwise have been introduced by inferior repositioning of the posterior maxilla to produce anti-clockwise rotation. Care must also be taken during surgical planning to consider the desired aesthetic and functional goals for each patient, whilst also 'setting' the occlusal plane to the desired angulation to produce anti-clockwise occlusal rotation. These measures will ensure a balanced approach to achieving a harmonious dentofacial outcome, guided by the principles of post-surgical stability. 9,36 Although the present findings compare favourably with previous studies on the stability of orthognathic surgery with and without occlusal plane rotation, further studies with a longer follow-up and larger sample size are needed. The duration of follow-up for the present study was an average of 13 months for the three groups, with a range of 6-36 months. A longer follow-up time would be beneficial for comparison with studies demonstrating long-term stability and reporting on the influence of condylar resorption following orthognathic surgery. 10,25,27,29,37 A larger sample size may also increase the statistical power and conclusiveness of the study. However, the authors of a recent systematic review stated that the greatest difficulty encountered in their study was the scarcity of literature on the topic of occlusal plane (MMC) rotation. It was suggested that the use of randomised clinical trials (RCTs) was warranted to ensure that the quality and standard of further scientific evidence on this topic is acceptable. 38

Conclusions
The findings of the present study indicate that a clockwise rotation of the occlusal plane during THE STABILITY OF OCCLUSAL PLANE ROTATION DURING ORTHOGNATHIC SURGERY orthognathic surgery is highly stable in the year following surgery, and demonstrates comparable stability with cases in which no occlusal plane rotation was performed. Anti-clockwise rotation of the occlusal plane is less stable and a significant degree of post-surgical relapse may be expected. However, the relapse in occlusal plane angle is a result of occlusal settling and likely neuromuscular adaptation following surgery, and, overall, the results of anticlockwise rotation and moderate advancement of the maxillomandibular complex demonstrate excellent stability for the medium term.