3D vector analysis of mandibular condyle stability in mandibular setback surgery with bicortical bioabsorbable screw fixation.

Journal of Cranio-Maxillo-Facial Surgery xxx (2013) 1e6

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3D vector analysis of mandibular condyle stability in mandibular setback surgery with bicortical bioabsorbable screw fixation Jee Ho Lee a, Soung Min Kim b, Bu Kyu Lee a, Ju Hong Jeon a, Myung Jin Kim b, * a b

Department of Oral and Maxillofacial Surgery (Head: Prof. Ju Hong Jeon), Seoul Asan Medical Center, Seoul, Republic of Korea Department of Oral and Maxillofacial Surgery (Head: Prof. Jin Young Choi), Seoul National University Dental Hospital, Seoul, Republic of Korea

a r t i c l e i n f o

a b s t r a c t

Article history: Paper received 19 September 2012 Accepted 17 July 2013

Introduction: Bioabsorbable screws became widely used for stable fixation in orthognathic surgery as biomechanical technology advanced. Recently, 3D image analyses begin to be used to evaluate surgical changes. The purpose of this study was to evaluate, using 3D vector analysis, the stability of bicortical bioabsorbable screw fixation in mandibular setback using a sagittal split ramus osteotomy. Spatial change of the mandibular condyle was determined by 3D coordinates containing directional information. Materials and methods: Bicortical screw fixation was performed using either a bioabsorbable screw (25 patients) or a titanium screw (5 patients) in orthognathic surgery. Pre- and post-operative CT images (6 months after surgery) were superimposed digitally. A 3D coordinate (X, Y, Z) and vectors were employed to quantify spatial changes of the condyle and analysed statistically. Results: Measuring on 3D image showed stable error about 0.16 mm. There were no significant differences in the total spatial changes of the condyle between titanium and bioabsorbable screws with the exception of the lateralemedial direction of the condylar centre (P ¼ 0.042). The directional vector components were stable, regardless of mandibular setback. Conclusion: In 3D vector analysis, bioabsorbable screw fixation in SSRO with distal segment osteotomy shows clinically acceptable postoperative condylar position stability. Ó 2013 European Association for Cranio-Maxillo-Facial Surgery. Published by Elsevier Ltd. All rights reserved.

Keywords: Three dimensional image Bioabsorbable screw Sagittal split ramus osteotomy

1. Introduction The ability to compare surgical changes is an important consideration when evaluating surgical outcomes and for prospective treatment planning. Traditionally most analyses have employed 2D-based cephalometric images for the comparison of surgical results in orthognathic surgery. These procedures are well established and have become the standard methods for postoperative result analysis. However, 2D-based methods have several limitations, including the fact that serial cephalometric images cannot be accurately superimposed and that the ability to reproduce a patients position is not enough to exclude errors (Arat et al., 2003; Ghafari et al., 1987; Gu and McNamara, 2008). Recently, 3D image analyses of surgical changes have begun to be used in conjunction with the development of 3D computer simulations that evaluate surgical changes with regard to threedimensional movement (Cevidanes et al., 2006). Superimpositions

* Corresponding author. E-mail address: [email protected] (M.J. Kim).

of computed tomography (CT) images on stable structures such as the cranial base and skull allow 3D evaluation of dental, skeletal and soft tissue changes in both growing and non-growing patients (Cevidanes et al., 2011). Thus, digitalized superimposition has the potential to achieve accurate reproducibility for surgical comparison. Indeed, Cevidanes et al. (2005) have verified its accuracy in 3D CT models of orthognathic surgery patients. The surgical stability of orthognathic surgery mainly depends on the method of internal fixation and the postoperative changes of the mandibular condyle. Joss and Vassalli (2008) verified in a systematic review of previous studies that proper seating of the condyle was a contributing factor to surgical stability while Franco et al. (1989) suggested that alteration of the proximal segment is a relapse factor, which is related to the spatial arrangement of the muscles and their attachments. The principal internal fixation methods for orthognathic surgery are bicortical screws and monocortical miniplates (Chung et al., 2008). Previous biomechanical studies have shown that bicortical screw fixation tends to be more rigid, thereby meeting the requirement for functional loading (Chung et al., 2008; Brasileiro et al., 2012; Peterson et al., 2005). However, differences in

1010-5182/$ e see front matter Ó 2013 European Association for Cranio-Maxillo-Facial Surgery. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jcms.2013.07.005

Please cite this article in press as: Lee JH, et al., 3D vector analysis of mandibular condyle stability in mandibular setback surgery with bicortical bioabsorbable screw fixation, Journal of Cranio-Maxillo-Facial Surgery (2013), http://dx.doi.org/10.1016/j.jcms.2013.07.005

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J.H. Lee et al. / Journal of Cranio-Maxillo-Facial Surgery xxx (2013) 1e6

stability between bicortical screws and monocortical miniplates have not been previously demonstrated in 2D-based cephalometric clinical studies (Chung et al., 2008; Choi et al., 2000). For internal fixation, titanium systems have been successfully used to achieve surgical stability (Buijs et al., 2007) but these systems have several disadvantages, including a potential requirement for a second operation, interference with imaging or radiotherapy, possible growth disturbance in young patients, and thermal sensitivity. Although strength and stiffness are weaker in bioabsorbable fixations than in titanium systems, the mechanical properties of bioabsorbable materials have improved due to advances in biomaterial technology, and there are decades of consecutive studies on appropriate alternatives to titanium fixation available in the literature. For example, in maxillofacial trauma, bioabsorbable fixation has shown stable mechanical properties (Tominaga et al., 2006) and a low rate of clinical morbidity (Lee et al., 2010). However, the clinical use of bioabsorbable fixation for orthognathic surgery should be continuously evaluated, especially with regard to surgical stability. In this study, the surgical stability of bicortical absorbable screw fixation was evaluated using 3D vector analysis in orthognathic surgery performed using a sagittal split ramus osteotomy (SSRO) with distal segment osteotomy.

2. Materials and methods 2.1. Patients Thirty patients (17 males and 13 females) who had orthognathic surgery for mandible prognathism were included in the present study. All patients underwent Le Fort I osteotomy and SSRO performed by one surgeon from 2004 to 2010 in Seoul National

Table 1 Demographic data of patients.

Age Sex (M:F) Surgery a

Setback (mm) a

Titanium screw (n ¼ 5)

Biodegradable screw (n ¼ 25)

19.8 1.1 2:3 Le Fort I osteotomy SSRO setback 6.895 2.233 mm

21.9 3.5 15:10 Le Fort I osteotomy SSRO setback 7.692 2.541 mm

No statistically significant difference was shown, P-value > 0.05.

University Dental Hospital, Seoul, Korea. The age of the patients ranged from 18 to 36 years with a mean age of 21.6 years. Mandibular setback (SB) was 7.692 2.541 mm in the bioabsorbable screw group and 6.895 2.233 mm in the titanium screw group with no significant difference found between the two groups (P ¼ 0.516). The bicortical screw method was used for mandible fixation using either a bioabsorbable screw or a titanium screw (Table 1). Multislice 3D CT (SOMATOM Sensation 10Ò, Siemens, Munich, Germany) was performed before surgery with postoperative CT performed 6 months from the day of surgery. Paired CT data of each patient were collected retrospectively and used for the evaluation of surgical stability by 3D superimposition analysis. The CT scan protocol for patients was reviewed by the Institutional Review Board (IRB) of Seoul National University Dental Hospital (IRB No. CRI12013) and followed the Declaration of Helsinki guidelines. 2.2. Surgical procedures SSRO with distal segment osteotomy was performed for mandibular setback surgery using the Obwegeser-Dal Pont technique. Posterior border osteotomy of the distal segment was performed to remove the interference between the proximal and distal segment at the mandibular setback. Cutting of the distal segment was done, proportional to the amount of mandibular setback. The distal segment was separated completely and removed (Kim et al., 2002). When bioabsorbable screws (2.5 mm diameter and 12e 18 mm length; INION CPSÒ System; Inion, Tampere, Finland) were used for bicortical fixation, three screws were fixed at the retromolar area and two screws at the mandibular angle area. For titanium screw fixation (2.0 mm diameter and 12e18 mm length; Le ForteÒ System; Jeil Medical, Seoul, Korea), three screws were fixed at the retromolar area with one screw at the mandibular angle area (Fig. 1). Intermaxillary fixation was not performed postoperatively. Two elastics were engaged for 4 weeks to guide stable occlusion and prevent from excessive mouth opening. 2.3. 3D vector analysis of surgical change Pre- and post-operative CT images were superimposed digitally using the 3D simulation program (OnDemand3DÒ, Cybermed, Seoul, Korea). A 3D coordinate system (X, Y, Z) was employed to quantify spatial changes in the condyle position. Spatial changes

Fig. 1. Bicortical fixation for mandibular setback surgery. A. Bioabsorbable screws were fixed at the retromolar (three screws) and mandibular angle (two screws) area. Note that radiolucent bioabsorbable screws cannot be seen on 3D CT reconstruction images and their positions were determined using the screw holes. B. In titanium screw fixation, three screws were fixed at the retromolar area with one screw at the mandibular angle area.



were presented as vectors with coordinates to evaluate amount (distance, in mm) and direction (X: lateral and medial; Y: anterior and posterior; Z: upper and lower) (Fig. 2). Anatomical points and lines were placed on the 3D images of patients to define the vectors of the surgical changes as follows (Fig. 3): Cl: Lateral pole of condyle head; most prominent point of the lateral surface of the mandibular condyle Cm: Medial pole of condyle head; most prominent point of the medial aspect of the mandibular condyle Cx: Condyle axis; a line connecting Cl and Cm Cr: Tip of coronoid process of the mandible; most prominent point of the coronoid process C: Centre of the mandibular condyle; midpoint between Cl and Cm Px: Proximal segment axis; a line connecting C and Cr Vectors were defined by anatomical points and presented as coordinates (X, Y, Z) to evaluate the direction and amount of surgical changes as follows (Fig. 4):

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components were presented as 3D coordinate [i.e. X (lateral and medial), Y (anterior and posterior), Z (upper and lower)]. The differences between each coordinates were compared by a paired ttest (P < 0.05). Total error was calculated by measuring linear distance between each point. 2.5. Statistical analysis SPSS for Windows (version 12.0, SPSS, Chicago, IL) was used for statistical analyses. Mean dimensional changes of the bioabsorbable screw group (n ¼ 50, 25 patients) and the titanium screw group (n ¼ 10, 5 patients) were calculated and compared using a linear mixed model to account for the patient effect as each patient had two mandibular condyles; the data are presented as mean 95% confidence interval (CI). Dimensional changes to the condyle and proximal segment according to mandibular setback were analysed by the Pearson correlation method and presented as

A. CR: Vector of condylar rotational movement; quantitative and directional changes of the condyle axis (Cx) B. PR: Vector of proximal segment deviation; quantitative and directional changes of the proximal segment (Px) C. CC: Vector of condyle centre movement; quantitative and directional changes of the centre of mandibular condyle (C) After superimposition of pre- and post-operative CT was completed, the CR, PR, CC vectors were calculated. The direction and amount of these vectors were analysed to evaluate the stability of mandibular setback surgery. 2.4. 3D measurement errors The errors of pointing on 3D images were verified by repeat measuring of 6 points (Rt. Cl, Rt Cm, Rt Cr and Lt. Cl, Lt. Cm, Lt Cr). Three patients were randomly selected and a total of 18 points were measured twice with 2 weeks by a single investigator. Directional

Fig. 3. Anatomical points and lines on 3D images of the mandibular proximal segment.

Fig. 2. Superimposition of pre- and post-operative CT images for comparison in the 3D coordinate system. The 3D coordinate system, presented as vectors composed of direction (X, Y, Z) and amount (mm), was employed to measure the spatial change after surgery. (In a patient’s position, X has lateral (þ) and medial () directions, Y has anterior () and posterior (þ) directions and Z has upper (þ) and lower () directions.)


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Fig. 4. Vectors of surgical changes on superimposed pre and postoperative 3D images. A. Vector of condylar rotation movement (CR). B. Vector of proximal segment deviation (PR). C. Vector of condyle center movement (CC). Cx0 , Px0 and C0 mean the value on postoperative 3D image.

Table 2 Vectors of surgical change between pre- and post-operative CT images at the condylar head and proximal segment of the mandible.

CR

PR

CC

Total X Y Z Total X Y Z Total X Y Z

Biodegradable screw (mm 95% CI)

Titanium screw (mm 95% CI)

P-valuea

1.635 0.126 0.342 0.088 2.555 0.360 0.042 0.978 0.985 0.194 0.082 0.062

1.247 0.184 0.807 0.015 1.857 0.727 0.235 0.673 0.938 0.601 0.165 0.176

0.257 0.791 0.189 0.893 0.188 0.497 0.538 0.741 0.805 0.042 0.756 0.403

± 0.286 0.184 0.289 0.449 ± 0.432 0.446 0.260 0.762 ± 0.160 0.159 0.222 0.239

± 0.625 0.403 0.647 1.003 ± 0.968 0.998 0.581 1.704 ± 0.349 0.357 0.495 0.523

The total is the net vector change after surgery. X is a component of vector in the lateral (þ) and medial () directions, Y in anterior () and posterior (þ) directions and Z in upper (þ) and lower () directions. a Performed using the linear mixed model to compare the two groups and explain the effect of two mandibular condyles in a single patient.

the mean standard deviation (SD). Differences were considered statistically significant at P < 0.05. 3. Results 3.1. Three dimensional stability of the mandibular condyle Condylar rotations (CR) were 1.635 mm in the bioabsorbable group and 1.247 mm in the titanium group. Proximal segment changes (PR) were 2.555 mm in the bioabsorbable group and 1.857 mm in the titanium group. Spatial changes of the condylar centre (CC) were 0.985 mm in the bioabsorbable group and 0.938 mm in the titanium group. For the change of the condylar centre in the lateral and medial direction, the surgical change between two groups was significantly different (P ¼ 0.042). Bioabsorbable fixation was more stable (0.194 mm in the medial direction) than titanium fixation (0.601 mm in the medial direction) in the lateralemedial direction of the condylar centre after surgery but there were no significant differences in the total spatial changes of the condylar head and proximal segment (Table 2). Other dimensional components, with the exception of the laterale medial direction of the condylar centre, showed no differences and had the same directional changes (Fig. 5). 3.2. Correlation between setback amount and directional components of surgical change The spatial changes of the mandibular condyle and proximal segment were not significantly correlated with mandibular setback

Fig. 5. Spatial changes and patterns of directional components in condylar rotations (CR), proximal segments (PR) and condylar centres (CC). In a patient’s position, X has lateral (þ) and medial () directions, Y has anterior () and posterior (þ) directions and Z has upper (þ) and lower () directions.

in all patients. The directional vector components were stable and not affected, regardless of mandibular setback movement (Table 3). 3.3. Reproducibility of 3D measurement All points (Rt. Cl, Rt Cm, Rt Cr and Lt. Cl, Lt. Cm, Lt Cr) showed no significant differences in pointing 3D images. Mean errors of directional components were 0.063 mm in lateral and medial direction, 0.034 mm in anterior and posterior direction and 0.114 mm in upper and lower direction, respectively. Total difference was 0.165 mm (Table 4). 4. Discussion The superimposition of serial digital 3D images has the potential to enable the evaluation of surgical changes in dental and skeletal structures with unparallelled accuracy. The analyses of CT images, 3D photography and dental casts have been used for the analysis of treatment outcomes (Cevidanes et al., 2011). For orthognathic surgery, CT images have been employed to evaluate surgical changes in hard and soft tissue. In addition, multi-planar approaches, including panoramic, lateral and posteroanterior cephalometric image simulations, have become available through the superimposition of 3D CT images (Cevidanes et al., 2006). Treatment outcomes can be visualized by 3D colour mapping, which visually presents the degree of surgical changes with accessible 3D colour images of hard and soft tissue (Cevidanes et al., 2005). However, the use of spatial analysis of 3D superimposition from quantitative and directional viewpoints is not yet established for the evaluation of treatment outcomes in orthognathic surgery. When quantitative and directional analysis is required, previously established 2D based methods based on cephalometric radiographs



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Table 3 The correlation of mandibular setback with mandibular condyle and proximal segment spatial changes. CR

SB

Correlation coefficient P-value

PR

CC

Total

X

Y

Z

Total

X

Y

Z

Total

X

Y

Z

0.057 0.667

0.000 0.999

0.011 0.933

0.001 0.993

0.006 0.962

0.012 0.926

0.140 0.286

0.103 0.433

0.251 0.053

0.065 0.622

0.026 0.842

0.044 0.738

The correlation coefficients represent the statistical correlation between the mandibular setback and the spatial changes of the mandibular condyle and proximal segment after surgery. The total is the net vector change after surgery. X is a component of vector in the lateral (þ) and medial () directions, Y in anterior () and posterior (þ) directions and Z in upper (þ) and lower () directions.

Table 4 The measurement errors of pointing on 3D images. Directional component

Measuring error

Lateralemedial (X) Anterioreposterior (Y) Upperelower (Z) Total

0.063 0.034 0.114 0.165

0.074 0.040 0.134 0.124

P-value 0.094 0.485 0.583

Data presented as mean (mm) SD.

are still being used. In the present study, a coordinate system and a 3D vector were employed to calculate quantitative surgical changes and the directional movement of anatomical points. Although bioabsorbable internal fixation material has several advantages over titanium, principally due to its self-degrading and radiolucent properties, it has not been universally used. Lower mechanical property values and cumbersome procedures have limited the widespread use of bioabsorbable material. To overcome the increased mechanical instability of bioabsorbable internal fixation material, Lovald et al. (2009) studied structural modifications of strut plates and Erkmen et al. (2005) suggested the use of double plates and a triangular design for bioabsorbable fixation use in mandibular setback surgery. After orthognathic surgery, shear stress and deflection come to be concentrated on the fixation material and osteotomy gap as the patient’s mandible begins to function again (Kohn et al., 1995). It has been shown that the inferior mechanical properties of bioabsorbable screws can be improved by changing the number and geometry of bioabsorbable screws used for internal fixation. Both Choi et al. (2010) and Hwang et al. (2012) showed using mechanical simulation and finite element analysis in internal fixation of SSRO that positioning the bioabsorbable screws at the retromolar area (two or three screws) and the mandibular angle (one or two screws) could provide better biomechanical stability than a single titanium plate. However, previous mechanical simulation studies had some limitations, including omitting biological factors from actual patients, requiring that the clinical stability of bioabsorbable screws be proven for widespread clinical use in orthognathic surgery. Long-term studies on material-related complications and functional problems of bioabsorbable plates and screws reported no significant differences compared to conventional titanium fixation and concluded that bioabsorbable fixation systems were clinically acceptable for mandibular surgery (Cheung et al., 2004; Stockmann et al., 2010). Cephalometric analyses have been performed on the clinical stability of orthognathic surgery in actual patients. For example, in using a bioabsorbable plate for Le Fort I osteotomy, Kim et al. (2011) reported that anatomical points were maintained stably for more than 6 months. Choi et al. (2011) found that some degree of instability was shown in the vertical dimension of the maxilla, but the horizontal dimension was maintained stably. When SSRO was fixed with bioabsorbable plates in mandibular setback surgery, although vertical relapse was reported that was related to vertical postoperative changes, a stable fixation was in general provided (Ueki et al., 2011; Kim et al., 2009).

The stability of mandibular setback can be affected by surgical methods related to bony interference, contact surface and type of fixation (screw or plate). Yang et al. (2010) simulated the effect of three different surgical methods [i.e. conventional SSRO, intraoral vertical ramus osteotomy (IVRO) and short lingual osteotomy (SLO)] on surgical stability in a computer program; they concluded that SLO was the most favourable osteotomy for reducing displacement of the proximal segment. Ueki et al. (2008) used bent plate fixation to reduce interference between the proximal and distal segments to increase surgical stability. Although conventional SSRO might produce more bony interference of the proximal segment, thereby jeopardizing stability in comparison with other methods, it was proved that bony interference due to SSRO could be overcome by performing intentional posterior osteotomy of the distal segment to maintain long-term stability (Kim et al., 2002). In the present study, three bioabsorbable screws were fixed at the retromolar area and two at the mandibular angle area in actual patients; surgical stability was evaluated by 3D image vector analysis and was not dependant on 2D-based images. Surgical change could be observed with directional component on 3D coordinate as well as amount (mm). Measurement error on the 3D image was about 0.165 mm and showed stable reproducibility in all three dimensional directions (Table 4). With regard to bioabsorbable fixation, total change of the condyle centre was less than 1.0 mm and condyle rotation did not exceed 2.0 mm. Deviation of the proximal segment was more than 2.0 mm but did not show statistically significant difference compared to titanium fixation. In the directions of changes between the two groups, the condylar axis, proximal segment and condylar centre showed almost the same directional patterns in the 3D coordinate system; these differences were not statistically significant (Fig. 5). Setback movement in the present study did not affect the direction and amount of condyle change (Table 3). But, further studies should be considered in the case of excessive mandible setback and mandible advancement. This study has some limitations such as a retrospective design and limited number of patients, and therefore needs further researches with improved prospective design. However, it could give the 3D information on surgical change of condyle in mandibular setback surgery using bioabsorbable fixation material, beyond limited 2D based access.

5. Conclusion Patients in the current study had SSRO with intentional distal segment osteotomy, but condylar displacement and proximal segment deviation in the bioabsorbable screw group using polygonal positioning at the mandibular ramus and inferior border were relatively stable compared to titanium fixation. These surgical changes were not affected by the degree of mandibular setback. Using 3D analysis of mandibular setback surgery, bioabsorbable screw fixation in SSRO with distal segment osteotomy shows clinically acceptable postoperative condylar position stability.


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Funding Source of funding for research: none declared. Financial interest disclosure: none declared. Conflict of interest The authors disclose no financial interests, funds or any commercial associations with other people or organizations that might cause a conflict of interest with the information presented in this manuscript. References Arat ZM, Rubenduz M, Akgul AA: The displacement of craniofacial reference landmarks during puberty: a comparison of three superimposition methods. Angle Orthod 73(4): 374e380, 2003 Brasileiro BF, Grotta-Grempel R, Ambrosano GM, Passeri LA: An in vitro evaluation of rigid internal fixation techniques for sagittal split ramus osteotomies: setback surgery. J Oral Maxillofac Surg 70(4): 941e951, 2012 Buijs GJ, van der Houwen EB, Stegenga B, Bos RR, Verkerke GJ: Mechanical strength and stiffness of biodegradable and titanium osteofixation systems. J Oral Maxillofac Surg 65(11): 2148e2158, 2007 Cevidanes LH, Bailey LJ, Tucker Jr GR, Styner MA, Mol A, Phillips CL, et al: Superimposition of 3D cone-beam CT models of orthognathic surgery patients. Dentomaxillofac Radiol 34(6): 369e375, 2005 Cevidanes LH, Oliveira AE, Grauer D, Styner M, Proffit WR: Clinical application of 3D imaging for assessment of treatment outcomes. Semin Orthod 17(1): 72e80, 2011 Cevidanes LH, Styner MA, Proffit WR: Image analysis and superimposition of 3-dimensional cone-beam computed tomography models. Am J Orthod Dentofacial Orthop 129(5): 611e618, 2006 Cheung LK, Chow LK, Chiu WK: A randomized controlled trial of resorbable versus titanium fixation for orthognathic surgery. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 98(4): 386e397, 2004 Choi BH, Min YS, Yi CK, Lee WY: A comparison of the stability of miniplate with bicortical screw fixation after sagittal split setback. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 90(4): 416e419, 2000 Choi JP, Baek SH, Choi JY: Evaluation of stress distribution in resorbable screw fixation system: three-dimensional finite element analysis of mandibular setback surgery with bilateral sagittal split ramus osteotomy. J Craniofac Surg 21(4): 1104e1109, 2010 Choi JY, Kim JW, Yoo CK, Yun PY, Baek SH, Kim YK: Evaluation of post-surgical relapse in maxillary surgery using resorbable plate. J Craniomaxillofac Surg 39(8): 578e582, 2011 Chung IH, Yoo CK, Lee EK, Ihm JA, Park CJ, Lim JS, et al: Postoperative stability after sagittal split ramus osteotomies for a mandibular setback with monocortical plate fixation or bicortical screw fixation. J Oral Maxillofac Surg 66(3): 446e 452, 2008 Erkmen E, Simsek B, Yucel E, Kurt A: Three-dimensional finite element analysis used to compare methods of fixation after sagittal split ramus osteotomy: setback surgery-posterior loading. Br J Oral Maxillofac Surg 43(2): 97e104, 2005

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