evaluation of 3 different fixation …...miniplates, rigid internal fixation, sagittal split ramus...

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CLINICAL DENTISTRY AND RESEARCH 2018; 42(3): 141-148 Original Research ArticleCLINICAL DENTISTRY AND RESEARCH 2018; 42(3): 141-148 Orijinal Araştırma

CorrespondenceEmre Tosun, DDS, PhD

Department of Oral and Maxillofacial Surgery,

Faculty of Dentistry, Hacettepe University,

Sıhhıye, 06100, Ankara, Turkey

Phone: 0 532 6175599

Fax: 0 312 3104440

E-mail: [email protected]

Ezgi Ergezen Özaşir, DDSResearch Assistant, Department of Oral and Maxillofacial Surgery,


Ankara, Turkey

Emre Tosun, DDS, PhDAssistant Professor, Department of Oral and Maxillofacial Surgery,


Ankara, Turkey

EVALUATION OF 3 DIFFERENT FIXATION TECHNIQUES FOR SAGITTAL SPLIT RAMUS OSTEOTOMY USING 3D FINITE ELEMENT

ANALYSIS: A PILOT STUDY

ABSTRACT

Background and Aim: The aim of this study was to compare 3 different internal rigid fixation techniques for sagittal split ramus osteotomy using 3D finite element analysis.

Materials and Methods: A computed tomography scan of a healthy patient was used to generate the geometry of the mandible. A miniplate, a monocortical screw and a bicortical screw were designed and combined with the mandible for the creation of 3 different models. A load of 200 N was applied from the incisor teeth to evaluate the stress on the fixation appliances and bone.

Results: The highest stress was detected on miniplate system and the least stress was on the inverted L system. The stress values were similar in the hybrid and inverted L systems while the hybrid system displayed the least displacement, and the miniplate system yielded the maximum displacement.

Conclusion: The results of our study suggest that the hybrid system is a safe and reliable fixation technique with sufficient stability of the fragments, and it causes relatively low stress and less displacement.

Keywords: Bicortical Screws, Finite Element Analysis,

Miniplates, Rigid Internal Fixation, Sagittal Split Ramus

Osteotomy

Submitted for Publication: 11.02.2018

Accepted for Publication : 11.26.2018

Clin Dent Res 2018; 42(3): 141-148

CLINICAL DENTISTRY AND RESEARCH 2018; 42(3): 141-148 Orijinal Araştırma

Sorumlu YazarEmre Tosun

Hacettepe Üniversitesi, Diş Hekimliği Fakültesi,

Ağız Diş ve Çene Cerrahisi Anabilim Dalı,

Sıhhıye, 06100, Ankara, Türkiye

Telefon: 0 532 6175599

Faks: 0 312 3104440

E-mail: [email protected]

Ezgi Ergezen ÖzaşirArş. Gör., Hacettepe Üniversitesi, Diş Hekimliği Fakültesi,


Ankara, Türkiye

Emre Tosun Dr. Öğr. Üyesi, Hacettepe Üniversitesi, Diş Hekimliği Fakültesi,


Ankara, Türkiye

SAGİTTAL SPLİT RAMUS OSTEOTOMİSİNDE 3 FARKLI FİKSASYON TEKNİĞİNİN SONLU ELEMAN ANALİZİ İLE KARŞILAŞTIRILMASI:

PİLOT ÇALIŞMA

ÖZ

Amaç: Bu çalışmanın amacı sagittal split ramus osteotomisinde kullanılan 3 farklı internal fiksasyon tekniğinin 3 boyutlu sonlu eleman analizi ile karşılaştırılmasıdır.

Gereç ve Yöntem: Sağlıklı bireyin bilgisayarlı tomografi görüntülerinden elde edilen mandibula modellerinde; miniplak, monokortikal vida ve bikortikal vida tasarımları 3 farklı fiksasyon tekniği oluşturmak için kullanıldı. Fiksasyon sistemleri ve kemik üzerinde oluşan stresi değerlendirmek için alt kesici dişlerden 200N kuvvet uygulandı.

Bulgular: En yüksek stres miniplak sisteminde en düşük stres ise ters L sisteminde ölçüldü. Ters L ve hibrit sistemlerinde stres değerleri benzer olmakla birlikte, hibrit sistemde daha az yer değiştirme olduğu görüldü. En fazla yer değiştirme miniplak sisteminde oluştu.

Sonuç: Bu çalışma sonucunda hibrit sistemin güvenilir bir teknik olduğu, fragmanlar arasında yeterli stabilite sağladığı, daha az stres ve yer değiştirmeye neden olduğu görülmüştür.

Anahtar Kelimeler: Bikortikal Vida, Sonlu Eleman Analizi,

Miniplak, Rijit Internal Fiksasyon, Sagittal Split Ramus

Osteotomisi

Yayın Başvuru Tarihi : 02.11.2018

Yayına Kabul Tarihi : 26.11.2018

Clin Dent Res 2018; 42(3): 141-148

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FEA OF SSRO FIXATION TECHNIQUES

CLINICAL DENTISTRY AND RESEARCH 2018; 42(3): 141-148 Orijinal AraştırmaINTRODUCTION

Bilateral sagittal split ramus osteotomy (BSSRO) is one of the most common procedures in oral and maxillofacial surgery for the treatment of mandibular deformities.1 BSSRO was first described by Obwegeser, later Dalpont and Hunsuck made the major modifications.2 Advantages of BSSRO include three dimensional (3D) movement of the distal segment of mandible, large bony contact between fragments, relatively less complication rate and relatively low risk of relapse.3-5

Skeletal relapse is one of the major problems that may occur after BSSRO.6 Up to 50% relapse has been reported after surgical repositioning, and movement at the osteotomy site has been stated as one of the possible reasons of relapse.1 The ideal fixation method for BSSRO is still controversial. Non-rigid, semi-rigid or rigid fixation methods have been used for bone fixation after BSSRO. Non-rigid fixation does not restrain the movement of bone fragments but limits the movement preventing the separation of fragments. It includes wire osteosynthesis and maxillomandibular fixation. Semi-rigid fixation is applied by one mini plate, which allows micro movements of the fragments. Although ultimate immobilization is not obtained with this method, clinical results show that satisfactory bone healing can be achieved with semi-rigid fixation. Rigid fixation is regarded as the most stabile fixation method preventing functional movements of the bone fragments. It can be obtained with various combinations of mini plates and screws. Advantages of rigid fixation include adequate stability, rapid bone consolidation, early recovery of mandibular function and easy airway maintenance.7-9 The most frequently used fixation techniques for BSSRO are miniplates and bicortical screws, miniplates in combination with bicortical screws.8,10-11

Finite element analysis (FEA) is an analytic system, which helps solve complex mechanical problems. It was first used in 1960s in air and space industry. The use of FEA in dentistry was started in 1970s for biomechanical applications such as functional evaluation of dental implants, fixation materials, and biomechanical evaluation of mandible.12-14 With FEA dislocation and movement of complex geometries, strain and compression stresses can be evaluated. The aim of this pilot study was to evaluate mechanical characteristics of 3 different fixation techniques for BSSRO by using FEA. The preliminary results of this study are considered to be the foundation of a more comprehensive

study planned ahead regarding SSRO and fixation systems.

MATERIALS AND METHODS

This study was carried out by using 3D finite element model analysis of human mandible. Computed tomography (CT) of a healthy human mandible was used to obtain 3D finite element model of the mandible. CT data were transferred to 3D-doctor software (Able Software Corporation, Lexington, MA, U.S.A.) to separate bone tissue by using Hounsfield unit values. 3D model of the mandible was generated by 3D complex rendering. Only one side of the mandible was modeled due to symmetry. Miniplates and screws were built up and matched by using Rhinoceros 4.0 software (3670 Woodland Park Ave N, Seattle, WA 98103 U.S.A.). Miniplates were designed with 2 mm thickness with 4 holes while the screws were designed with 2 mm thickness without grooves. Screw lengths of monocortical and bicortical screws were 9 mm and 16 mm, respectively.Adjusting the cortical bone to 2 mm, cortical and spongious bone were separated.15 A 7 mm advancement of BSSRO was simulated in a 3D mesh model of mandible according to the Dalpont Hunsuck modifications.16

Miniplate fixation model (mini plate and 4 monocortical screws), hybrid fixation model (mini plate, 4 monocortical screws and 1 bicortical screw) and bicortical screw fixation model (3 bicortical screws) were chosen for fixation of the segments after BSSRO. The models are shown in Figure 1.The models were fixed from the condyle and lateral ramus with a 0-degree movement. 200 N force was applied from the central incisor region to simulate bite forces. Von Mises stress values were evaluated for the stress in fixation systems, while the maximum and minimum principal stress values were evaluated for the stress in the bone.

RESULTS

The comparison regarding the von Mises stresses on the fixation systems and the bone revealed that the highest stress was on miniplate system and the least was on the inverted L system. The hybrid system displayed the least displacement while the miniplate system yielded the maximum displacement.

Conventional miniplate

The highest von Mises stress measured in the plate (3838,48 MPa) was concentrated around the osteotomy gap close to the proximal segment dissipating posteriorly (Figure 2a). The highest stress was measured in the 4th

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CLINICAL DENTISTRY AND RESEARCH

screw (1763,13 MPa) (Figure 2b), and the least stress was seen in the 1st screw (237,37 MPa) (Figure 2c). The displacement values are shown in Figure 2d.The highest maximum principal stress was around the 2nd screw in the cortical bone (180,3 MPa) (Figure 3a), and around the 4th screw in the spongious bone (52,3 MPa) (Figure 3b); the highest minimum principal stress value was measured around the 3rd screw in the cortical bone (-210,5 MPa) (Figure 3c), and around the 4th screw in the spongious bone (-49,4 MPa) (Figure 3d). The least maximum and minimum principal stress values were measured around the 1st screw both in the cortical (28,2 MPa and -33,09 MPa, respectively) and in the spongious bone (1,9 MPa and -0,9 MPa, respectively).The tension in the proximal segment around the screw close to the osteotomy was dissipating to the superior border, and the compression stress was dissipating to the inferior border of the mandible.

Hybrid system

The highest von Mises stress measured in the plate was concentrated around the osteotomy gap close to the proximal segment dissipating posteriorly. The stress was found to be shared between the plate (1345,4 MPa) (Figure 4a) and the 5th screw (1060,4 MPa) (Figure 4b), while the least stress was seen in the 1st screw (139,1 MPa) (Figure 4c). The displacement values are shown in Figure 4d.The highest maximum principal stress was around the 5th screw in the cortical bone (60,4 MPa) (Figure 5a) and around the 5th screw in spongious bone (45,7 MPa) (Figure 5b); the highest minimum principal stress value was measured around the 5th screw in the cortical bone (-44.7 MPa) (Figure 5c), and around the 5th screw in the spongious bone (-51,8 MPa) (Figure 5d). The least maximum principal stress values were measured around the 4th screw (19,4 MPa) and the least minimum principal stress was measured around the 3rd screw in the cortical bone (-22,1 MPa), while the

Figure 1. Miniplate, Hybrid and Inverted L models

Figure 2. a. Von Mises stress on the miniplate, miniplate system b. The highest Von Mises stress value on the screws, miniplate system c. The least Von Mises stress value on the screws, miniplate system d. The displacement value of the miniplate system

Figure 3. a. Maximum Principal Stress values on the cortical bone b. Maximum Principal Stress values on the spongious bone c. Minimum Principal Stress values on the cortical bone d. Minimum Principal Stress values on the spongious bone

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least maximum and minimum principal stress values were measured around the 1st screw in the spongious bone (1,2 MPa and - 0,8 MPa, respectively)The tension in the proximal segment around the screw close to the osteotomy was dissipating to the superior border and the compression stress was dissipating to the inferior border of the mandible.

Inverted L arrangement

The highest von Mises stress was measured around the 1st screw (1256,04 MPa) (Figure 6a), and the least stress was around the 3rd screw (784,8 MPa) (Figure 6b). The displacement values are shown in Figure 6c.The highest maximum principal stress was around the 1st screw in the cortical bone (71,3 MPa) (Figure 7a) and around the 1st screw in the spongious bone (53,7 MPa) (Figure 7b); the highest minimum principal stress value was measured around the 1st screw in the cortical bone (-51,6 MPa) (Figure 7c) and around the 1st screw in the spongious bone (-56,7

Figure 4. a. Von Mises stress on the miniplate, hybrid system b. The highest Von Mises stress value on the screws, hybrid system c. The least Von Mises stress value on the screws, hybrid system d. The displacement value of the hybrid system


Figure 6. a. The highest Von Mises stress value on the screws, inverted L system b. The least Von Mises stress value on the screws, inverted L system c. The displacement value of the inverted L system


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MPa) (Figure 7d). The least maximum principal stress values were measured around the 2nd screw (20,4 MPa), and the least minimum principal stress was measured around the 2nd screw in the cortical bone (-12,3 MPa), while the least maximum principal stress values were measured around the 2nd screw (31,7MPa) and minimum principal stress values were measured around the 3rd screw in spongious bone -29,8 MPa).The tension around the superior screw was dissipating to the osteotomy line, and the compression was dissipating to the inferior border of the mandible mostly localized around the inferior screw.

DISCUSSION

With the advancement in fixation methods, BSSRO has become one of the most common orthognathic procedures for mandibular advancement surgery. Several fixation methods have been described for BSSRO including bicortical screws, mini plates and plate screw combinations.17 The aim of this study was to compare 3 different fixation methods for the stress distribution around the fixation system and bone by using 3D FEA.As an analytic method to solve complex biomechanical problems, FEA helps us understand the mechanical behavior of mandible under certain conditions. 3D FEA demonstrates the stress behavior of mandible realistically by simulating the actual clinical condition.6

According to the previous reports, an ideal fixation should have maximum rigidity between the segments while causing minimum stress on the bone and fixation device itself. Extreme stress around the fixation device may cause resorption of the bone and loosening of screws, both of which lead to the failure of fixation.1, 6, 9 Miniplate systems are mostly preferred over bicortical screws in orthognathic surgery procedures since they can easily be placed intraorally, reduce the nerve injury risk and cause lower stress on the condyles.18 However, biomechanical studies have revealed that inverted L configuration provides better stability under vertical forces.19-20 Likewise, the inverted L system has been reported to be 3 times more rigid than the miniplate system.21 To combine the advantages of both systems, the hybrid system has been recommended.22

In a study comparing 9 different fixation methods, Ribeiro-Junior et al.23 reported that the inverted L system presented better stability, though not significantly better than the hybrid system. Our results also suggest that the inverted L configuration and the hybrid system both show less stress

on the fixation units and the bone adjacent to the screws. Also the stress levels of these two methods were similarly less than that of miniplate system. Sato et al.24 compared 5 different fixation systems with 3D finite element analysis; the inverted L system was found to present less stress on the bone, and the stress was distributed homogenously between the screws. Miniplate system displayed the highest stress on the fixation units and bone compared to other systems, especially around the screw in the proximal segment close to the osteotomy line. According to our results, the highest stress measured on the bone was around the screw in the proximal segment close to the osteotomy line, and the overall highest stress was measured on the screw located posteriorly in the proximal segment. Sato et al.25 used mechanic, photoelastic and FEA methods to compare the inverted L miniplate and hybrid systems. Inverted L configuration was found to be the most rigid system and the miniplate was the least rigid one. Contrary to our results regarding displacement, the highest displacement value was measured in the hybrid system in this study.25 This may be the result of the bicortical screw placement, which was located anteriorly in our study. Similarly to the findings of our study, bicortical screw used in the hybrid system was found to reduce the stress on the miniplate.25 We can also conclude that the resistance against displacement increased when bicortical screw was used additionally to a miniplate.26

According to the previous reports; the increase in displacement is related to the long term stability loss and relaps. The proper fixation of the segments may reduce the displacement and facilitate proper healing.27 In our study, the least displacement values were found in the hybrid system, which is followed by the inverted L system, while the greatest displacement values were observed in mini plate fixation models. The reported stress values measured on the miniplate system was found to concentrate over the osteotomy gap close to the proximal segment, and the screw located close to the osteotomy in the proximal segment revealed higher stress values.25 Also, the stress distribution was more homogenous in the inverted L configuration and mostly concentrated on the screws close to the osteotomy line, in accordance with our results.25

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CONCLUSION

In this study, the miniplate system presented the highest and the inverted L system presented the least stress levels, consistent with the previous reports. Although the inverted L configuration with bicortical screws presented favorable results, this system is associated with higher stress on the condyles.28 Based on our results, the hybrid system is recommended, considering the low level of stress on the fixation hardware and the bone, as well as the low displacement values. FEA may provide insights about the stress levels, which should be taken into consideration when planning the surgery. However, the fixation units and mandible are affected by the physiologic factors or inflammation clinically, suggesting that the results of this study should be supported with clinical studies.

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18. Takahashi H, Moriyama S, Furuta H, Matsunaga H, Sakamoto Y, Kikuta T. Three lateral osteotomy designs for bilateral sagittal split osteotomy: biomechanical evaluation with three-dimensional finite element analysis. Head Face Med 2010; 6: 4.

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19. Ulu M, Soylu E, Kelebek S, Dikici S, Oflaz H. Comparative study of biomechanical stability of resorbable and titanium fixation systems after sagittal split ramus osteotomy with a novel designed in-vitro testing unit. J Craniomaxillofac Surg 2018; 46: 299-304.

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25. Sato FR, Asprino L, Consani S, Noritomi PY, de Moraes M. A comparative evaluation of the hybrid technique for fixation of the sagittal split ramus osteotomy in mandibular advancement by mechanical, photoelastic, and finite element analysis. Oral Surg Oral Med Oral Pathol Oral Radiol 2012; 114: 60-68.

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evaluation of 3 different fixation …...miniplates, rigid internal fixation, sagittal split ramus...

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