Journal of Cranio-Maxillo-Facial Surgery 43 (2015) 186e191
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In vitro biomechanical evaluation of fixation methods of sagittal split ramus osteotomy in mandibular setback Ji-Su Oh, Su-Gwan Kim* Department of Oral and Maxillofacial Surgery, School of Dentistry, Chosun University, Gwangju, 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 23 April 2014 Accepted 22 October 2014 Available online 3 December 2014
The purpose of this study was to compare different internal fixation techniques with resorbable or titanium fixation system used in sagittal split ramus osteotomy (SSRO) for mandibular setback. Synthetic polyurethane hemimandible replicas were used. The distal segment was repositioned in a 5-mm setback position. The hemimandibles were divided into the following eight groups: resorbable miniplate (A), titanium miniplate (B), resorbable hybrid; resorbable miniplate and bicortical screw (C), titanium hybrid; titanium miniplate and bicortical screw (D), resorbable mixed hybrid; resorbable bicortical screw and titanium miniplate (E), titanium mixed hybrid; titanium bicortical screw and resorbable miniplate (F), three resorbable bicortical screws (G) and three titanium bicortical screws (H). The compression loads were applied on the incisal edge. The compression loads (N) at 5- and 10-mm displacement were analyzed (P < 0.05). Miniplate groups (A and B) were significantly weaker than the other groups (C ~ H). Group (H) had greatest biomechanical stability. There was no significant difference between group (D) and group (H) at 10-mm displacement. Group (G) showed a lower load than group (H) and group (D). There was no significant difference between the hybrid and mixed hybrid groups. It was concluded that additional placement of a bicortical screw, either titanium or resorbable with a miniplate may provide significantly better stabilization. © 2014 European Association for Cranio-Maxillo-Facial Surgery. Published by Elsevier Ltd. All rights reserved.
Keywords: Internal fixation Orthognathic surgery Prognathism
1. Introduction Sagittal split ramus osteotomy (SSRO) is the most popular orthognathic surgical procedure for treatment of congenital and developmental deformities of mandible (Wolford, 2000; Yang et al., 2007). The broad contact surface of the segments increases the contact of cancellous bone. It also promotes bone healing and €ckmann et al., 2011). Morepostoperative mandibular stability (Bo over, rigid internal fixation in SSRO can permit early mandibular function (Sato et al., 2010). Numerous modifications of the fixation methods in SSRO have been proposed (Erkmen et al., 2005a; Oba et al., 2008; Joss and Vassalli, 2009). Fixation should be stable in healing period and
* Corresponding author. Department of Oral and Maxillofacial Surgery, School of Dentistry, Chosun University, 375, SeoSukDong, DongGu, GwangJu, 501-759, Republic of Korea. Fax: þ82 62 228 7316. E-mail address: [email protected] (S.-G. Kim).
tolerant to masticatory forces (Bohluli et al., 2010). Stable fixation in SSRO decreases postoperative relapse (Joss and Vassalli, 2008). Adequate fixation can provide sufficient resistance to the forces that cause micromovements across the osteotomy site (Brasileiro et al., 2012). Rigid fixation provides skeletal stability, fast bony healing, early recovery of mandibular function, and easier airway maintenance (Murphy et al., 1997; Erkmen et al., 2005a, 2005b; Ellis and Esmail, 2009). Rigid fixation methods using bicortical screws and miniplates with monocortical screws are widely used in SSRO (Alpha et al., 2006; Kim et al., 2012; Sato et al., 2012). Spiessl (1974) introduced rigid fixation method in SSRO using three bicortical screws with two above and one below the mandibular canal through transcutaneous approach. Rigid fixation can prevent excessive stress that may cause bone resorption and screw loosening (Erkmen et al., 2005b; Ellis and Esmail, 2009). However, temporomandibular dysfunction due to displacement of the condyle and irreversible nerve damage from excessive compression can occur. Also, the transcutaneous approach can cause a skin scar (Erkmen et al., 2005a, Matsushita et al., 2011). In
http://dx.doi.org/10.1016/j.jcms.2014.10.023 1010-5182/© 2014 European Association for Cranio-Maxillo-Facial Surgery. Published by Elsevier Ltd. All rights reserved.
J.-S. Oh, S.-G. Kim / Journal of Cranio-Maxillo-Facial Surgery 43 (2015) 186e191
addition, if a secondary osteotomy is performed in distal segment, there may be no lingual cortex to place the bicortical screw (Ellis and Esmail, 2009). An advantage of monocortical fixation is that an intraoral approach is possible. The injury of inferior alveolar nerve is lessened as compared with bicortical fixation (Yamashita et al., 2007). If the location of condyle during surgery is wrong, it is easier to readjust (Rubens et al., 1988; Stoelinga and Borstlap, 2003; Joss and Vassalli, 2009). However, monocortical fixation has insufficient stability and intermaxillary fixation (IMF) which may lead to muscular atrophy, airway obstruction risk and temporomandibular joint problems (Choi et al., 2005) is needed (Ribeiro-Junior et al., 2010; Aymach et al., 2011). Especially, the tension of the medial pterygoid muscle and suprahyoid muscles by insufficient stability may cause mandibular clockwise rotation result in relapse (Matsushita et al., 2011). Many in vitro experimental studies have been performed (Erkmen et al., 2005a; Bohluli et al., 2010; Ribeiro-Junior et al., 2010; Sato et al., 2010; Aymach et al., 2011; Matsushita et al., 2011; Brasileiro et al., 2012; Sato et al., 2012), however the ideal method of fixation has not yet been established (Sato et al., 2010). Specifically, few studies on resorbable fixation have been performed. In addition, most studies have focused on mandibular advancement surgery, and few have addressed setback surgery. The purpose of this study was to compare biomechanical effectiveness of various fixation methods in SSRO for mandibular setback. 2. Materials and methods 2.1. Materials Since human and animal mandibles have different bone quality or quantity in each sample, samples cannot be standardized. Therefore, synthetic polyurethane hemimandible replicas (model No. 8332; Synbone®, Malans, Switzerland), which have been successfully used in vitro studies (Van Sickels et al., 2005; RibeiroJunior et al., 2010; Aymach et al., 2011) were used in this study. A Titanium fixation system (LeForte; Jeil Medical Corp., Seoul, Korea)
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and a resorbable fixation system (BioSorbTMFX, Linvatec Biomaterials Ltd, Tampere, Finland) used in this study. BioSorbTMFX fixation system is made of self-reinforced (70% L-lactide, 30% DLlactide) poly lactic acid. The titanium miniplate used for monocortical fixation in this study was four-hole miniplate 1 mm in thickness and 4.5 mm in width. The resorbable miniplate was a four-hole miniplate 1.2 mm in thickness and 5.5 mm in width. Screws of diameter 2.0 mm and length 6 mm were used for monocortical fixation, and screws of diameter 2.4 mm and length 14 mm were used for bicortical fixation. The support jig for loading test was customized. 2.2. Methods A 5-mm setback was performed and interference was removed. The hemimandibles were divided into 8 groups (n ¼ 10). The experimental groups were divided as follows (Fig. 1): A. Resorbable monocortical group (resorbable miniplate with four monocortical screws); B. Titanium monocortical group (titanium miniplate with four monocortical screws); C. Resorbable hybrid group (one resorbable bicortical screw and a miniplate with four monocortical screws); D. Titanium hybrid group (one titanium bicortical screw and a miniplate with four monocortical screws); E. Resorbable mixed hybrid group (one resorbable bicortical screw and a titanium miniplate with four monocortical screws); F. Titanium mixed hybrid group (one titanium bicortical screw and a resorbable miniplate with four monocortical screws); G. Resorbable bicortical group (three resorbable bicortical screws fixed in an inverted-L shape); H. Titanium bicortical group (three titanium bicortical screws fixed in an inverted-L shape). As described by Brasileiro et al. (2012), the fixation was performed using a transparent acrylic mold. The miniplate was positioned parallel to occlusal plane. In the hybrid groups, bicortical
Fig. 1. Hemimandibles fixed using different fixation methods.
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screw was placed in retromolar region 5 mm distant from second molar. Three bicortical screws were fixed in an inverted-L shape with two above and one below the mandibular canal in the bicortical groups. 2.3. Loading tests The hemimandibles fixed with different methods were stabilized in support apparatus (Fig. 2A). In this custom-fabricated support jig, the distal segment can move freely but the condyle and ramus of proximal segment are fixed to avoid movement. The universal testing machine AG-IS (Shimadzu, Kyoto, Japan) was used for loading tests. A vertical compression force was applied to the lower incisor edge of the distal segment at a rate of 5 mm per minute (Fig. 2B) because it is more critical to evaluate incisal loading than lateral loading for in vitro evaluation (Van Sickels et al., 2005; Brasileiro et al., 2012). The loading forces in Newtons (N) for incisal displacement of the distal segment from 0 to 10 mm were measured, and the means with standard deviations of the data were calculated. The loading forces (N) for 5-mm and 10-mm incisal displacement of distal segment were recorded and processed statistically.
resistance (Fig. 3). The contrast of mean loading values among groups at 5-mm displacement did not differ greatly from the contrast of values at 10-mm displacement. Group H showed a steeper increase in loadedisplacement curves from 0 to 10 mm, while the resorbable monocortical group showed a slight increase in loadedisplacement curves. 3.2. The comparison of resorbable and titanium fixation system The loads of titanium groups (D, H) were significantly higher than those of the resorbable groups (C, G) (P < 0.05). However, there were no significant differences between the monocortical groups (A, B). Comparison of the mixed hybrid groups showed that group F was more stable than group E, but the difference was not statistically significant. Group D had a significantly higher stability compared with group E (P < 0.05). Also, the loads of group F were significantly higher than those of group C (P < 0.05). These results indicate that the titanium bicortical screw has higher resistance than the resorbable bicortical screw. 3.3. The comparison by fixation methods
Analysis of variance was performed by the test of normality and test of homogeneity of variances (Levene's test, P > 0.05). The collected data was compared through ANOVA and HSD Tukey's test for multiple comparisons between groups. For statistical analysis, SPSS version 20.0 (SPSS Inc., Chicago, IL, USA) was used and probabilities of less than 0.05 were accepted as significant.
The monocortical groups (A and B) had significantly lower mechanical resistance compared with the other groups (P < 0.05). The mechanical resistance of group C was significantly lower than that of group G (P < 0.05). No significant difference was found between group G and the mixed hybrid groups (E and F). In contrast, the difference between group D and group H was not statistically significant at 10-mm displacement, and the load values of groups D and H were significantly higher than those of the other groups (P < 0.05).
3. Results
4. Discussion
Analysis of variance using the Levene's test showed that data were normally distributed (P > 0.05). The results of ANOVA showed that there was a statistically significant difference among the groups at 5-mm and 10-mm displacement (P < 0.001).
The hybrid technique was first suggested by Schwartz and Relle (1996). This fixation method combines the advantages of bicortical and monocortical fixation (Schwartz and Relle, 1996). Brasileiro et al. (2009, 2012) described a detailed method and its advantages. The proximal segment and distal segment are fixed using miniplate and monocortical screws. Then additional fixation with single bicortical screw is performed at the retromolar region. The bone quality and width in the retromolar region are excellent, and an intraoral approach can be used, so it is the most recommended fixation position of bicortical screws (Schwimmer et al., 1994). Van Sickels et al. (2005) reported that there is no great advantage to
2.4. Statistical analysis
3.1. The values of loading force to displacement The average loads, standard deviations and multiple comparisons of significant differences among groups at 5-mm and 10-mm displacement are summarized in Tables 1 and 2. Group H was found to be the most rigid, while group A had the lowest mechanical
Fig. 2. The loading test: A) Custom-fabricated support jig, B) Application of vertical compression force at lower incisor edge of the distal segment.
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Table 1 The mean loading force values for 5 mm vertical displacement and statistical analysis summary among groups using Tukey's test. Group description
Mean ± SD (N)
A B C D E F G H
15.71 17.07 22.31 29.74 23.67 25.29 26.11 34.36
Resorbable monocortical group Titanium monocortical group Resorbable hybrid group Titanium hybrid group Resorbable mixed hybrid group Titanium mixed hybrid group Resorbable bicortical group Titanium bicortical group
± ± ± ± ± ± ± ±
1.97 3.23 2.99 2.48 2.15 4.05 3.94 3.98
Comparison between groups* C, D, E, F, G, H C, D, E, F, G, H A, B, D, F, G, H A, B, C, E, F, H A, B, D, H A, B, C, D, H A, B, C, H A, B, C, D, E, F, G
*Significant difference between the groups (HSD Tukey's test, P < 0.05). A: resorbable miniplate with 4 monocortical screws, B: titanium miniplate with 4 monocortical screws, C: resorbable 1 bicortical screw and miniplate with 4 monocortical screws, D: titanium 1 bicortical screw and miniplate with 4 monocortical screws, E: resorbable 1 bicortical screw and titanium miniplate with 4 monocortical screws, F: titanium 1 bicortical screw and resorbable miniplate with 4 monocortical screws, G: inverted-L shaped 3 resorbable bicortical screws, H: inverted-L shaped 3 titanium bicortical screws.
adding more than one bicortical screw. The advantages of the hybrid technique are as follows (Brasileiro et al., 2009, 2012): 1) excellent mechanical resistance, 2) low condylar torque, 3) low incidence of injury of the inferior alveolar nerve, and 4) intraoral approach. Micromotion at the osteotomy site causes failure of miniplate fixation; this micromotion can be reduced by hybrid technique (Alpha et al., 2006). Many studies have reported that mechanical resistance was significantly increased with the hybrid technique compared to monocortical fixation (Ozden et al., 2006; Brasileiro et al., 2009; Sato et al., 2012). Also, as the stress concentrated on the miniplate is distributed (Sato et al., 2010), the entire fixation system is stabilized (Sato et al., 2012). This is because the displacement of segments is prevented by resistance of the bicortical screw to the axial and shear stress (Sato et al., 2012). A triangular fixation using three bicortical screws is excellent for resisting stress (Erkmen et al., 2005b; Ming-Yih et al., 2010) and above all, an inverted-L shape fixation is known to be most stable (Erkmen et al., 2005a; Ozden et al., 2006). Brasileiro et al. (2012) showed that adding one bicortical screw in the retromolar region with monocortical fixation produced identical stabilization to an inverted-L fixation. In this study, the titanium bicortical screws group showed higher values than the titanium hybrid group, but there was no significant difference at 10-mm displacement.
Table 2 The mean loading force values for 10 mm vertical displacement and statistical analysis summary among groups using Tukey's test. Group description
Mean ± SD (N)
A B C D E F G H
29.37 34.15 40.25 55.07 43.26 48.39 45.36 59.48
Resorbable monocortical group Titanium monocortical group Resorbable hybrid group Titanium hybrid group Resorbable mixed hybrid group Titanium mixed hybrid group Resorbable bicortical group Titanium bicortical group
± ± ± ± ± ± ± ±
4.09 3.95 6.45 8.09 5.63 5.57 4.31 6.51
Comparison between groups* C, D, E, F, G, H C, D, E, F, G, H A, B, D, F, G, H A, B, C, E, F, G A, B, D, H A, B, C, D, H A, B, C, D, H A, B, C, E, F, G
*Significant difference between the groups (HSD Tukey's test, P < 0.05). A: resorbable miniplate with 4 monocortical screws, B: titanium miniplate with 4 monocortical screws, C: resorbable 1 bicortical screw and miniplate with 4 monocortical screws, D: titanium 1 bicortical screw and miniplate with 4 monocortical screws, E: resorbable 1 bicortical screw and titanium miniplate with 4 monocortical screws, F: titanium 1 bicortical screw and resorbable miniplate with 4 monocortical screws, G: inverted-L shaped 3 resorbable bicortical screws, H: inverted-L shaped 3 titanium bicortical screws.
Fig. 3. Mean loading values and standard deviations.
The displacement of condyle can be prevented and the neurovascular compression caused by excessive contacts can be minimized. Since the bicortical screw is located above inferior alveolar nerve, this also avoids injury to the nerve (Shetty et al., 1996; Brasileiro et al., 2009). It does not need transcutaneous approach using trocar, so facial nerve injury and skin scars can be avoided (Sato et al., 2012). However, this hybrid technique has limitations. If there is interference between the segments, proximal segment needs to be trimmed or a second osteotomy should be performed in the retromolar region of distal segment (Ellis, 2007; Ellis and Esmail, 2009). This is important to prevent the displacement of condyle and relapse due to interference. However, the hybrid technique cannot be used if there is no bone to fix the bicortical screw in this case (Brasileiro et al., 2012). In addition, shift of plate fixation in facial asymmetry is needed (Ming-Yih et al., 2010). It was reported previously that bicortical fixations are more mechanically resistant than monocortical (Chuong et al., 2005; Ozden et al., 2006; Brasileiro et al., 2009; Sato et al., 2010, 2012). Anucul et al. (1992) reported that bicortical fixation was an average of 3 times more resistant than monocortical fixation. Brasileiro et al. (2012) reported bicortical fixation was at least 40% more rigid than monocortical fixation. Pereira et al. (2013) demonstrated that the bicortical screw had the greatest loading resistance values, and that the miniplate had 60% lower resistance compared to bicortical screws. In this study, the resorbable bicortical group had 55% higher resistance and the titanium bicortical group had 73% higher resistance than each monocortical group. Shearing force has the greatest impact on the stability of the segments (Brasileiro et al., 2012). The excessive shear force in monocortical plate fixation may transform the mandibular segment postoperatively and cause decreased stability leading to nonunion or incomplete union (Chung et al., 2008; Brasileiro et al., 2012). Fixation failure can occur by migration of the screws and bending of a bone plate (Ellis and Esmail, 2009). Because the symphysis is displaced inferiorly and posteriorly due to pulling action of suprahyoid muscle and occlusal loading, the distal segment is rotated in clockwise direction. Also, since the ramus is rotated upward and forward due to the influence of masticatory muscles, the proximal segment becomes rotated counterclockwise (Ellis and Esmail, 2009; Brasileiro et al., 2012). For this, the fixed area with a miniplate is a concentrated force that creates mandible twisting between segments (Sato et al., 2010). Bicortical fixation can better resist such shearing stress than monocortical fixation (Fujioka et al., 2000). In contrast, some studies reported that the proximal segment is rotated inward by bicortical fixation, leading to condylar positional change, so it represents a large condylar torque (Ueki et al., 2005; Kim et al., 2012). The low resistance of miniplate has advantages
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such as low compression between segments and low condylar torque (Sato et al., 2010). Joss and Vassalli (2009) reported that bicortical and monocortical fixation showed only slight differences in the short term, but long-term relapse rates were higher for bicortical screw fixation. In clinical studies, despite the high resistance of bicortical screws there are no great differences in stability between bicortical and monocortical fixation (Choi et al., 2005; Chung et al., 2008; Joss and Vassalli, 2009), and the failure tendency of miniplate fixation was not increased (McDonald et al., 1987). Sufficient stability can be attained only with miniplate fixation (Sato et al., 2010) because of significant reduction of masticatory force after surgery (Throckmorton and Ellis, 2001). Also, short-term stability was guaranteed by rigid fixation, but longterm stability depends on functional and neuromuscular factors (Aymach et al., 2011). Resorbable osteosynthesis materials have the advantages of radiolucency and reduced stress shielding because of a progressive decrease in strength and stiffness. However, they have lower mechanical strength than titanium, so resorbable osteosynthesis has not yet replaced its metallic counterpart (Stockmann et al., 2010). Surgeons also hesitate to use resorbable system for orthognathic patients because of longer operation time and micromovement caused by low initial stability (Paeng et al., 2012). Cilasun et al. (2006) reported that there were no significant differences in stability when comparing poly-L-lactic acid/polyglycolic acid (PLLA/PGA) resorbable bicortical screws and titanium bicortical screws in SSRO. Ferretti and Reyneke (2002) also reported that resorbable screws could be used instead of titanium screws in SSRO. There were no significant differences between titanium screw group and resorbable screw group over 8 years long-term clinical study (Stockmann et al., 2010). Paeng et al. (2012) reported that bicortical resorbable screws showed clinically stable outcome after SSRO but are weak in open bite tendency and show a vertically less stable result than titanium screws. In addition, these authors used five resorbable screws and four titanium screws for fixation. They suggested that the bony contact between segments is important in resorbable screw fixation, and if the bony contact is minimal, the stability of resorbable screws is reduced. In this study, the resorbable bicortical screws group showed significantly lower values than the titanium bicortical screws group and titanium hybrid group. The loading force values of the resorbable bicortical screws group were lower than those of the titanium mixed group (titanium bicortical screw and resorbable miniplate), although the difference was not significant. 5. Conclusion Since this study was an in vitro evaluation of various fixation methods using resorbable plates and bicortical screws, it had limits in that it could not reproduce the stress direction in actual patients and anatomic structures including muscle interaction. Also, the clinical stability of segments after SSRO is influenced by many factors. Simply increasing mechanical stability does not ensure good results. This study focused only on the stress of the fixation material itself. In addition, the properties of resorbable and titanium fixation systems vary according to the manufacturer. However, this study is meaningful as a first step towards clinical use of hybrid technique fixation with resorbable osteosynthesis, and further consideration should be given to use of a hybrid technique with a resorbable plate and bicortical screws. Acknowledgments This study was supported by research fund from Chosun University, 2014.
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Spiessl B: Osteosynthesis in sagittal osteotomy using the Obwegeser-Dal Pont method. Fortschr Kiefer Gesichtschir 18: 145e148, 1974 €hm H, Driemel O, Mühling J, Pistner H: Resorbable versus titanium Stockmann P, Bo osteosynthesis devices in bilateral sagittal split ramus osteotomy of the mandible e the results of a two centre randomised clinical study with an eightyear follow-up. J Craniomaxillofac Surg 38: 522e528, 2010 Stoelinga PJ, Borstlap WA: The fixation of sagittal split osteotomies with miniplates: the versatility of a technique. J Oral Maxillofac Surg 61: 1471e1476, 2003 Throckmorton GS, Ellis 3rd E: The relationship between surgical changes in dentofacial morphology and changes in maximum bite force. J Oral Maxillofac Surg 59: 620e627, 2001 Ueki K, Marukawa K, Shimada M, Nakagawa K, Yamamoto E: Change in condylar long axis and skeletal stability following sagittal split ramus osteotomy and intraoral vertical ramus osteotomy for mandibular prognathia. J Oral Maxillofac Surg 63: 1494e1499, 2005 Van Sickels JE, Peterson GP, Holms S, Haug RH: An in vitro comparison of an adjustable bone fixation system. J Oral Maxillofac Surg 63: 1620e1625, 2005 Wolford LM: The sagittal split ramus osteotomy as the preferred treatment for mandibular prognathism. J Oral Maxillofac Surg 58: 310e312, 2000 Yamashita Y, Mizuashi K, Shigematsu M, Goto M: Masticatory function and neurosensory disturbance after mandibular correction by bilateral sagittal split ramus osteotomy: a comparison between miniplate and bicortical screw rigid internal fixation. Int J Oral Maxillofac Surg 36: 118e122, 2007 Yang XW, Long X, Yeweng SJ, Kao CT: Evaluation of mandibular setback after bilateral sagittal split osteotomy with the hunsuck modification and miniplate fixation. J Oral Maxillofac Surg 65: 2176e2180, 2007
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In vitro biomechanical evaluation of fixation methods of sagittal split ramus osteotomy in mandibular setback Ji-Su Oh, Su-Gwan Kim* Department of Oral and Maxillofacial Surgery, School of Dentistry, Chosun University, Gwangju, 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 23 April 2014 Accepted 22 October 2014 Available online 3 December 2014
The purpose of this study was to compare different internal fixation techniques with resorbable or titanium fixation system used in sagittal split ramus osteotomy (SSRO) for mandibular setback. Synthetic polyurethane hemimandible replicas were used. The distal segment was repositioned in a 5-mm setback position. The hemimandibles were divided into the following eight groups: resorbable miniplate (A), titanium miniplate (B), resorbable hybrid; resorbable miniplate and bicortical screw (C), titanium hybrid; titanium miniplate and bicortical screw (D), resorbable mixed hybrid; resorbable bicortical screw and titanium miniplate (E), titanium mixed hybrid; titanium bicortical screw and resorbable miniplate (F), three resorbable bicortical screws (G) and three titanium bicortical screws (H). The compression loads were applied on the incisal edge. The compression loads (N) at 5- and 10-mm displacement were analyzed (P < 0.05). Miniplate groups (A and B) were significantly weaker than the other groups (C ~ H). Group (H) had greatest biomechanical stability. There was no significant difference between group (D) and group (H) at 10-mm displacement. Group (G) showed a lower load than group (H) and group (D). There was no significant difference between the hybrid and mixed hybrid groups. It was concluded that additional placement of a bicortical screw, either titanium or resorbable with a miniplate may provide significantly better stabilization. © 2014 European Association for Cranio-Maxillo-Facial Surgery. Published by Elsevier Ltd. All rights reserved.
Keywords: Internal fixation Orthognathic surgery Prognathism
1. Introduction Sagittal split ramus osteotomy (SSRO) is the most popular orthognathic surgical procedure for treatment of congenital and developmental deformities of mandible (Wolford, 2000; Yang et al., 2007). The broad contact surface of the segments increases the contact of cancellous bone. It also promotes bone healing and €ckmann et al., 2011). Morepostoperative mandibular stability (Bo over, rigid internal fixation in SSRO can permit early mandibular function (Sato et al., 2010). Numerous modifications of the fixation methods in SSRO have been proposed (Erkmen et al., 2005a; Oba et al., 2008; Joss and Vassalli, 2009). Fixation should be stable in healing period and
* Corresponding author. Department of Oral and Maxillofacial Surgery, School of Dentistry, Chosun University, 375, SeoSukDong, DongGu, GwangJu, 501-759, Republic of Korea. Fax: þ82 62 228 7316. E-mail address: [email protected] (S.-G. Kim).
tolerant to masticatory forces (Bohluli et al., 2010). Stable fixation in SSRO decreases postoperative relapse (Joss and Vassalli, 2008). Adequate fixation can provide sufficient resistance to the forces that cause micromovements across the osteotomy site (Brasileiro et al., 2012). Rigid fixation provides skeletal stability, fast bony healing, early recovery of mandibular function, and easier airway maintenance (Murphy et al., 1997; Erkmen et al., 2005a, 2005b; Ellis and Esmail, 2009). Rigid fixation methods using bicortical screws and miniplates with monocortical screws are widely used in SSRO (Alpha et al., 2006; Kim et al., 2012; Sato et al., 2012). Spiessl (1974) introduced rigid fixation method in SSRO using three bicortical screws with two above and one below the mandibular canal through transcutaneous approach. Rigid fixation can prevent excessive stress that may cause bone resorption and screw loosening (Erkmen et al., 2005b; Ellis and Esmail, 2009). However, temporomandibular dysfunction due to displacement of the condyle and irreversible nerve damage from excessive compression can occur. Also, the transcutaneous approach can cause a skin scar (Erkmen et al., 2005a, Matsushita et al., 2011). In
http://dx.doi.org/10.1016/j.jcms.2014.10.023 1010-5182/© 2014 European Association for Cranio-Maxillo-Facial Surgery. Published by Elsevier Ltd. All rights reserved.
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addition, if a secondary osteotomy is performed in distal segment, there may be no lingual cortex to place the bicortical screw (Ellis and Esmail, 2009). An advantage of monocortical fixation is that an intraoral approach is possible. The injury of inferior alveolar nerve is lessened as compared with bicortical fixation (Yamashita et al., 2007). If the location of condyle during surgery is wrong, it is easier to readjust (Rubens et al., 1988; Stoelinga and Borstlap, 2003; Joss and Vassalli, 2009). However, monocortical fixation has insufficient stability and intermaxillary fixation (IMF) which may lead to muscular atrophy, airway obstruction risk and temporomandibular joint problems (Choi et al., 2005) is needed (Ribeiro-Junior et al., 2010; Aymach et al., 2011). Especially, the tension of the medial pterygoid muscle and suprahyoid muscles by insufficient stability may cause mandibular clockwise rotation result in relapse (Matsushita et al., 2011). Many in vitro experimental studies have been performed (Erkmen et al., 2005a; Bohluli et al., 2010; Ribeiro-Junior et al., 2010; Sato et al., 2010; Aymach et al., 2011; Matsushita et al., 2011; Brasileiro et al., 2012; Sato et al., 2012), however the ideal method of fixation has not yet been established (Sato et al., 2010). Specifically, few studies on resorbable fixation have been performed. In addition, most studies have focused on mandibular advancement surgery, and few have addressed setback surgery. The purpose of this study was to compare biomechanical effectiveness of various fixation methods in SSRO for mandibular setback. 2. Materials and methods 2.1. Materials Since human and animal mandibles have different bone quality or quantity in each sample, samples cannot be standardized. Therefore, synthetic polyurethane hemimandible replicas (model No. 8332; Synbone®, Malans, Switzerland), which have been successfully used in vitro studies (Van Sickels et al., 2005; RibeiroJunior et al., 2010; Aymach et al., 2011) were used in this study. A Titanium fixation system (LeForte; Jeil Medical Corp., Seoul, Korea)
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and a resorbable fixation system (BioSorbTMFX, Linvatec Biomaterials Ltd, Tampere, Finland) used in this study. BioSorbTMFX fixation system is made of self-reinforced (70% L-lactide, 30% DLlactide) poly lactic acid. The titanium miniplate used for monocortical fixation in this study was four-hole miniplate 1 mm in thickness and 4.5 mm in width. The resorbable miniplate was a four-hole miniplate 1.2 mm in thickness and 5.5 mm in width. Screws of diameter 2.0 mm and length 6 mm were used for monocortical fixation, and screws of diameter 2.4 mm and length 14 mm were used for bicortical fixation. The support jig for loading test was customized. 2.2. Methods A 5-mm setback was performed and interference was removed. The hemimandibles were divided into 8 groups (n ¼ 10). The experimental groups were divided as follows (Fig. 1): A. Resorbable monocortical group (resorbable miniplate with four monocortical screws); B. Titanium monocortical group (titanium miniplate with four monocortical screws); C. Resorbable hybrid group (one resorbable bicortical screw and a miniplate with four monocortical screws); D. Titanium hybrid group (one titanium bicortical screw and a miniplate with four monocortical screws); E. Resorbable mixed hybrid group (one resorbable bicortical screw and a titanium miniplate with four monocortical screws); F. Titanium mixed hybrid group (one titanium bicortical screw and a resorbable miniplate with four monocortical screws); G. Resorbable bicortical group (three resorbable bicortical screws fixed in an inverted-L shape); H. Titanium bicortical group (three titanium bicortical screws fixed in an inverted-L shape). As described by Brasileiro et al. (2012), the fixation was performed using a transparent acrylic mold. The miniplate was positioned parallel to occlusal plane. In the hybrid groups, bicortical
Fig. 1. Hemimandibles fixed using different fixation methods.
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screw was placed in retromolar region 5 mm distant from second molar. Three bicortical screws were fixed in an inverted-L shape with two above and one below the mandibular canal in the bicortical groups. 2.3. Loading tests The hemimandibles fixed with different methods were stabilized in support apparatus (Fig. 2A). In this custom-fabricated support jig, the distal segment can move freely but the condyle and ramus of proximal segment are fixed to avoid movement. The universal testing machine AG-IS (Shimadzu, Kyoto, Japan) was used for loading tests. A vertical compression force was applied to the lower incisor edge of the distal segment at a rate of 5 mm per minute (Fig. 2B) because it is more critical to evaluate incisal loading than lateral loading for in vitro evaluation (Van Sickels et al., 2005; Brasileiro et al., 2012). The loading forces in Newtons (N) for incisal displacement of the distal segment from 0 to 10 mm were measured, and the means with standard deviations of the data were calculated. The loading forces (N) for 5-mm and 10-mm incisal displacement of distal segment were recorded and processed statistically.
resistance (Fig. 3). The contrast of mean loading values among groups at 5-mm displacement did not differ greatly from the contrast of values at 10-mm displacement. Group H showed a steeper increase in loadedisplacement curves from 0 to 10 mm, while the resorbable monocortical group showed a slight increase in loadedisplacement curves. 3.2. The comparison of resorbable and titanium fixation system The loads of titanium groups (D, H) were significantly higher than those of the resorbable groups (C, G) (P < 0.05). However, there were no significant differences between the monocortical groups (A, B). Comparison of the mixed hybrid groups showed that group F was more stable than group E, but the difference was not statistically significant. Group D had a significantly higher stability compared with group E (P < 0.05). Also, the loads of group F were significantly higher than those of group C (P < 0.05). These results indicate that the titanium bicortical screw has higher resistance than the resorbable bicortical screw. 3.3. The comparison by fixation methods
Analysis of variance was performed by the test of normality and test of homogeneity of variances (Levene's test, P > 0.05). The collected data was compared through ANOVA and HSD Tukey's test for multiple comparisons between groups. For statistical analysis, SPSS version 20.0 (SPSS Inc., Chicago, IL, USA) was used and probabilities of less than 0.05 were accepted as significant.
The monocortical groups (A and B) had significantly lower mechanical resistance compared with the other groups (P < 0.05). The mechanical resistance of group C was significantly lower than that of group G (P < 0.05). No significant difference was found between group G and the mixed hybrid groups (E and F). In contrast, the difference between group D and group H was not statistically significant at 10-mm displacement, and the load values of groups D and H were significantly higher than those of the other groups (P < 0.05).
3. Results
4. Discussion
Analysis of variance using the Levene's test showed that data were normally distributed (P > 0.05). The results of ANOVA showed that there was a statistically significant difference among the groups at 5-mm and 10-mm displacement (P < 0.001).
The hybrid technique was first suggested by Schwartz and Relle (1996). This fixation method combines the advantages of bicortical and monocortical fixation (Schwartz and Relle, 1996). Brasileiro et al. (2009, 2012) described a detailed method and its advantages. The proximal segment and distal segment are fixed using miniplate and monocortical screws. Then additional fixation with single bicortical screw is performed at the retromolar region. The bone quality and width in the retromolar region are excellent, and an intraoral approach can be used, so it is the most recommended fixation position of bicortical screws (Schwimmer et al., 1994). Van Sickels et al. (2005) reported that there is no great advantage to
2.4. Statistical analysis
3.1. The values of loading force to displacement The average loads, standard deviations and multiple comparisons of significant differences among groups at 5-mm and 10-mm displacement are summarized in Tables 1 and 2. Group H was found to be the most rigid, while group A had the lowest mechanical
Fig. 2. The loading test: A) Custom-fabricated support jig, B) Application of vertical compression force at lower incisor edge of the distal segment.
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Table 1 The mean loading force values for 5 mm vertical displacement and statistical analysis summary among groups using Tukey's test. Group description
Mean ± SD (N)
A B C D E F G H
15.71 17.07 22.31 29.74 23.67 25.29 26.11 34.36
Resorbable monocortical group Titanium monocortical group Resorbable hybrid group Titanium hybrid group Resorbable mixed hybrid group Titanium mixed hybrid group Resorbable bicortical group Titanium bicortical group
± ± ± ± ± ± ± ±
1.97 3.23 2.99 2.48 2.15 4.05 3.94 3.98
Comparison between groups* C, D, E, F, G, H C, D, E, F, G, H A, B, D, F, G, H A, B, C, E, F, H A, B, D, H A, B, C, D, H A, B, C, H A, B, C, D, E, F, G
*Significant difference between the groups (HSD Tukey's test, P < 0.05). A: resorbable miniplate with 4 monocortical screws, B: titanium miniplate with 4 monocortical screws, C: resorbable 1 bicortical screw and miniplate with 4 monocortical screws, D: titanium 1 bicortical screw and miniplate with 4 monocortical screws, E: resorbable 1 bicortical screw and titanium miniplate with 4 monocortical screws, F: titanium 1 bicortical screw and resorbable miniplate with 4 monocortical screws, G: inverted-L shaped 3 resorbable bicortical screws, H: inverted-L shaped 3 titanium bicortical screws.
adding more than one bicortical screw. The advantages of the hybrid technique are as follows (Brasileiro et al., 2009, 2012): 1) excellent mechanical resistance, 2) low condylar torque, 3) low incidence of injury of the inferior alveolar nerve, and 4) intraoral approach. Micromotion at the osteotomy site causes failure of miniplate fixation; this micromotion can be reduced by hybrid technique (Alpha et al., 2006). Many studies have reported that mechanical resistance was significantly increased with the hybrid technique compared to monocortical fixation (Ozden et al., 2006; Brasileiro et al., 2009; Sato et al., 2012). Also, as the stress concentrated on the miniplate is distributed (Sato et al., 2010), the entire fixation system is stabilized (Sato et al., 2012). This is because the displacement of segments is prevented by resistance of the bicortical screw to the axial and shear stress (Sato et al., 2012). A triangular fixation using three bicortical screws is excellent for resisting stress (Erkmen et al., 2005b; Ming-Yih et al., 2010) and above all, an inverted-L shape fixation is known to be most stable (Erkmen et al., 2005a; Ozden et al., 2006). Brasileiro et al. (2012) showed that adding one bicortical screw in the retromolar region with monocortical fixation produced identical stabilization to an inverted-L fixation. In this study, the titanium bicortical screws group showed higher values than the titanium hybrid group, but there was no significant difference at 10-mm displacement.
Table 2 The mean loading force values for 10 mm vertical displacement and statistical analysis summary among groups using Tukey's test. Group description
Mean ± SD (N)
A B C D E F G H
29.37 34.15 40.25 55.07 43.26 48.39 45.36 59.48
Resorbable monocortical group Titanium monocortical group Resorbable hybrid group Titanium hybrid group Resorbable mixed hybrid group Titanium mixed hybrid group Resorbable bicortical group Titanium bicortical group
± ± ± ± ± ± ± ±
4.09 3.95 6.45 8.09 5.63 5.57 4.31 6.51
Comparison between groups* C, D, E, F, G, H C, D, E, F, G, H A, B, D, F, G, H A, B, C, E, F, G A, B, D, H A, B, C, D, H A, B, C, D, H A, B, C, E, F, G
*Significant difference between the groups (HSD Tukey's test, P < 0.05). A: resorbable miniplate with 4 monocortical screws, B: titanium miniplate with 4 monocortical screws, C: resorbable 1 bicortical screw and miniplate with 4 monocortical screws, D: titanium 1 bicortical screw and miniplate with 4 monocortical screws, E: resorbable 1 bicortical screw and titanium miniplate with 4 monocortical screws, F: titanium 1 bicortical screw and resorbable miniplate with 4 monocortical screws, G: inverted-L shaped 3 resorbable bicortical screws, H: inverted-L shaped 3 titanium bicortical screws.
Fig. 3. Mean loading values and standard deviations.
The displacement of condyle can be prevented and the neurovascular compression caused by excessive contacts can be minimized. Since the bicortical screw is located above inferior alveolar nerve, this also avoids injury to the nerve (Shetty et al., 1996; Brasileiro et al., 2009). It does not need transcutaneous approach using trocar, so facial nerve injury and skin scars can be avoided (Sato et al., 2012). However, this hybrid technique has limitations. If there is interference between the segments, proximal segment needs to be trimmed or a second osteotomy should be performed in the retromolar region of distal segment (Ellis, 2007; Ellis and Esmail, 2009). This is important to prevent the displacement of condyle and relapse due to interference. However, the hybrid technique cannot be used if there is no bone to fix the bicortical screw in this case (Brasileiro et al., 2012). In addition, shift of plate fixation in facial asymmetry is needed (Ming-Yih et al., 2010). It was reported previously that bicortical fixations are more mechanically resistant than monocortical (Chuong et al., 2005; Ozden et al., 2006; Brasileiro et al., 2009; Sato et al., 2010, 2012). Anucul et al. (1992) reported that bicortical fixation was an average of 3 times more resistant than monocortical fixation. Brasileiro et al. (2012) reported bicortical fixation was at least 40% more rigid than monocortical fixation. Pereira et al. (2013) demonstrated that the bicortical screw had the greatest loading resistance values, and that the miniplate had 60% lower resistance compared to bicortical screws. In this study, the resorbable bicortical group had 55% higher resistance and the titanium bicortical group had 73% higher resistance than each monocortical group. Shearing force has the greatest impact on the stability of the segments (Brasileiro et al., 2012). The excessive shear force in monocortical plate fixation may transform the mandibular segment postoperatively and cause decreased stability leading to nonunion or incomplete union (Chung et al., 2008; Brasileiro et al., 2012). Fixation failure can occur by migration of the screws and bending of a bone plate (Ellis and Esmail, 2009). Because the symphysis is displaced inferiorly and posteriorly due to pulling action of suprahyoid muscle and occlusal loading, the distal segment is rotated in clockwise direction. Also, since the ramus is rotated upward and forward due to the influence of masticatory muscles, the proximal segment becomes rotated counterclockwise (Ellis and Esmail, 2009; Brasileiro et al., 2012). For this, the fixed area with a miniplate is a concentrated force that creates mandible twisting between segments (Sato et al., 2010). Bicortical fixation can better resist such shearing stress than monocortical fixation (Fujioka et al., 2000). In contrast, some studies reported that the proximal segment is rotated inward by bicortical fixation, leading to condylar positional change, so it represents a large condylar torque (Ueki et al., 2005; Kim et al., 2012). The low resistance of miniplate has advantages
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such as low compression between segments and low condylar torque (Sato et al., 2010). Joss and Vassalli (2009) reported that bicortical and monocortical fixation showed only slight differences in the short term, but long-term relapse rates were higher for bicortical screw fixation. In clinical studies, despite the high resistance of bicortical screws there are no great differences in stability between bicortical and monocortical fixation (Choi et al., 2005; Chung et al., 2008; Joss and Vassalli, 2009), and the failure tendency of miniplate fixation was not increased (McDonald et al., 1987). Sufficient stability can be attained only with miniplate fixation (Sato et al., 2010) because of significant reduction of masticatory force after surgery (Throckmorton and Ellis, 2001). Also, short-term stability was guaranteed by rigid fixation, but longterm stability depends on functional and neuromuscular factors (Aymach et al., 2011). Resorbable osteosynthesis materials have the advantages of radiolucency and reduced stress shielding because of a progressive decrease in strength and stiffness. However, they have lower mechanical strength than titanium, so resorbable osteosynthesis has not yet replaced its metallic counterpart (Stockmann et al., 2010). Surgeons also hesitate to use resorbable system for orthognathic patients because of longer operation time and micromovement caused by low initial stability (Paeng et al., 2012). Cilasun et al. (2006) reported that there were no significant differences in stability when comparing poly-L-lactic acid/polyglycolic acid (PLLA/PGA) resorbable bicortical screws and titanium bicortical screws in SSRO. Ferretti and Reyneke (2002) also reported that resorbable screws could be used instead of titanium screws in SSRO. There were no significant differences between titanium screw group and resorbable screw group over 8 years long-term clinical study (Stockmann et al., 2010). Paeng et al. (2012) reported that bicortical resorbable screws showed clinically stable outcome after SSRO but are weak in open bite tendency and show a vertically less stable result than titanium screws. In addition, these authors used five resorbable screws and four titanium screws for fixation. They suggested that the bony contact between segments is important in resorbable screw fixation, and if the bony contact is minimal, the stability of resorbable screws is reduced. In this study, the resorbable bicortical screws group showed significantly lower values than the titanium bicortical screws group and titanium hybrid group. The loading force values of the resorbable bicortical screws group were lower than those of the titanium mixed group (titanium bicortical screw and resorbable miniplate), although the difference was not significant. 5. Conclusion Since this study was an in vitro evaluation of various fixation methods using resorbable plates and bicortical screws, it had limits in that it could not reproduce the stress direction in actual patients and anatomic structures including muscle interaction. Also, the clinical stability of segments after SSRO is influenced by many factors. Simply increasing mechanical stability does not ensure good results. This study focused only on the stress of the fixation material itself. In addition, the properties of resorbable and titanium fixation systems vary according to the manufacturer. However, this study is meaningful as a first step towards clinical use of hybrid technique fixation with resorbable osteosynthesis, and further consideration should be given to use of a hybrid technique with a resorbable plate and bicortical screws. Acknowledgments This study was supported by research fund from Chosun University, 2014.
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