Journal of

Oral Rehabilitation

Journal of Oral Rehabilitation 2014 41; 683--691

Evaluation of different screw fixation techniques and screw diameters in sagittal split ramus osteotomy: finite element analysis method A. SINDEL*, S. DEMIRALP† & G. COLOK†

*Department of Oral and Maxillofacial Surgery, Akdeniz

University Faculty of Dentistry, Antalya, and †Department of Oral and Maxillofacial Surgery, Ankara University Faculty of Dentistry, Ankara, Turkey

SUMMARY Sagittal split ramus osteotomy (SSRO) is used for correction of numerous congenital or acquired deformities in facial region. Several techniques have been developed and used to maintain fixation and stabilisation following SSRO application. In this study, the effects of the insertion formations of the bicortical different sized screws to the stresses generated by forces were studied. Three-dimensional finite elements analysis (FEA) and static linear analysis methods were used to investigate difference which would occur in terms of forces effecting onto the screws and transmitted to bone between different application areas. No significant difference was

Introduction Sagittal split ramus osteotomy is used in the correction of many congenital or acquired deformities in the facial region (1–5). Bicortical screws are used in several surgical techniques to attached proximal and distal segments of mandible. Compression is considered minimum when screw is tightened; theoretically nerve damage risk is low. Screws have been used extensively to maintain rigid internal fixation (RIF) in orthognathic surgery since 1980s. In mandibular fracture and orthognathic surgical fixation, materials used in fixation may be deformed by the influence of the nearby tissues and mastication forces. Therefore, reliability of the fragment’s stabilities is affected negatively. Although certain limits do not exist, more than 2 mm dislocations from operation area or © 2014 John Wiley & Sons Ltd

found between 1
5- and 2-mm screws used in SSRO fixation. Besides, it was found that ‘inverted L’ application was more successful compared to the others and that was followed by ‘L’ and ‘linear’ formations which showed close rates to each other. Few studies have investigated the effect of thickness and application areas of bicortical screws. This study was performed on both advanced and regressed jaws positions. KEYWORDS: sagittal split ramus osteotomy, finite elements stress analysis, mandible Accepted for publication 30 April 2014

more than 2 mm angular changes are considered relapses. Finite element analysis (FEA) is commonly used technique for testing fixation reliability and changes in forces and material properties (6–9). In the present study we aimed to use finite element analysis for comparison three commonly used screw fixation techniques and screw diameters for stability in SSRO.

Materials and methods This research was performed with static linear analysis and three-dimensional FEA methods. Bone tissue model was obtained by using a patient tomography (Fig. 1a). Bone tissue was segmented from tomography scans according to Hounsfield values with interactive segmentation method (Fig. 1b). After segmentation doi: 10.1111/joor.12188

684

A . S I N D E L et al.

(a)

(b)

(c)

(d)

Fig. 1. Stages of the bone modelling. (a) Topographic cross-section, (b) Separation of bone tissue by Interactive Segmentation Method, (c, d) 3-dimensional model of bone and disc obtained by complex 3D rendering method.

procedure, three-dimensional model was obtained with 3D complex render method. On cortical bone model, disc of the temporomandibular joint was generated in region between condyles and temporal bone. Force transmission between upper and lower jaws was maintained by Boolean method (Fig. 1). Required bone fractures were made on obtained bone models. Backward and forward displacement procedures were carried out over generated incision lines (Fig. 2). Screws were placed over displaced bone models in three different formations. Screw fragments were scanned by NextEngine 3D scanner (Santa Monica, CA, USA). Adjustment between bone tissues and screws was performed with Boolean method, and force transmission was then maintained. Maxilla and mandibula were adapted into model in a way that fixation screws would reflect real morphology (Fig. 1). All models were considered as having linear, homogeneous and isotropic materials. Model was fixed from upper and posterior regions of maxilla so that it had 0 movement per degree of freedom. Totally 12 FEA were performed on a total of 12 regressed (7 mm regression) and advanced (7 mm advancement) jaw models (Fig. 2). Different sized (1
5 and 2 mm) bicortical screws were applied into three different forms (IL, L

and linear). For loading conditions, 200 Newton mastication forces were applied from first and second pre-molar and molar regions. Then, muscle force components (Fig. 3) were applied from points of mandibular attachments of the muscles (Table 1).

Results Results in the forward jaw models by 2-mm bicortical screws It was found that von mises stress (VMS) values obtained from L and linear formation were more than those of IL formation. For principal stresses produced in buccal cortical bone, IL had lower rates in comparison with L and linear formations. For principal stress rates (PSR) produced in lingual cortical bone, three formations had close rates to each other. L formation had higher stress rates compared with the other 2 formations. Principal stress rates in the outer layer of the spongious bone were found to have lower values in IL and linear formations compared with the L formation. Principal stress rates in inner layer of the spongious bone were found to have lower in IL formations (Table 2, Fig. 3). © 2014 John Wiley & Sons Ltd

FINITE ELEMENT ANALYSIS OF SAGITTAL SPLIT OSTEOTOMY

(a1)

(b1)

(a)

(b)

(a2)

(b2)

(a3)

(b3)

Fig. 2. Screw formations. (a) Backward position of the mandible (a1: IL formation, a2: L formation, a3: Linear formation), (b) Forward position of the mandible (b1: IL formation, b2: L formation, b3: Linear formation).

(a)

(b)

(c)

(d)

(e)

(f)

Fig. 3. Applied forces and their effects on mandible and screws. (a) The resulting pattern while maintaining three-dimensional coordinates, (b) Lingual appearance of the total forces applied to mandible, (c) Buccal appearance of the total forces applied to mandible, (d) Representative photomicrograph of stresses on screws and (e, f) Representative photomicrographs of stresses on mandible. © 2014 John Wiley & Sons Ltd

685

686

A . S I N D E L et al. Table 1. Properties of the applied muscle forces

Muscles

Total force (in Newton)

Superficial masseter muscle Deep masseter muscle Medial pterygoid muscle Anterior temporalis muscles Medial temporalis muscle Posterior temporalis muscle Superior lateral pterygoid muscle Anterior digastric muscles

190
4 81
6 174
8 158
0 95
6 75
6 28
7 40
0

Force directions in threedimensions (in Cos degrees)

Applied forces in threedimensions (in Newton)

X

Y

Z

Fx

Fy

Fz

0
20 0
54 0
48 0
14 0
22 0
20 0
76 0
24

0
88 0
75 0
79 0
98 0
83 0
47 0
07 0
23

0
41 0
35 0
37 0
04 0
50 0
85 0
64 0
94

79
7 29
2 65
2 6
9 47
8 64
6 18
5 37
6

39
4 44
5 84
9 23
5 21
2 15
7 21
8 9
7

168
3 61
8 138
2 156
1 80
0 35
8 2
1 9
4

Results in the regressed jaw models by 2-mm bicortical screws VMS values obtained in 2-mm bicortical screws in three formations were found to be closer. IL formation had the lowest PSR values on buccal cortical bone. For PSR produced in lingual cortical bone, L formation showed higher stress rates compared with those of other formations. Principal stress rates in outer layer of the spongious bone were found to have lower in L and linear formations. Principal stress rates in inner layer of the spongious bone were found to have lower in linear formations (Table 2, Fig. 3). Results in the forward jaw models by 1
5-mm bicortical screws VMS rates in screws showed an increase for IL, L and linear formations, respectively. In terms of PSR in the buccal cortical bone, IL had lower values compared with those of other formations. In inner part of the lingual cortical bone, PSR were found to be very close for all three formations. In the outer part of the spongious bone, PSR were quite lower in IL formation compared with those of other formations. In the inner part of the spongious bone, the lowest PSR were found in the IL formation (Table 2, Fig. 3). Results in the regressed jaw models by 1
5-mm bicortical screws For IL and L formations, VMS values obtained in 1
5-mm bicortical screws were found to be closer to each other. Moreover, these rates were lower than linear formation. IL formation had the lowest PSR

values on buccal cortical bone. For PSR values measured in lingual cortical bone, L formation showed higher stress rates compared with those of other formations. Outer layer of the spongious bone had lower PSR values in L and linear formations. Moreover, inner layer of the spongious bone had lower PSR values in linear formations (Table 2, Fig. 3). Comparison of the stress distributions Aforementioned VMS and PSR values in L, IL and linear formations were important to show maximum and minimum stress values in bone and screws. From surgical point of view, distribution of the stresses on bone is also important for stability. Therefore, both parameters should be kept in mind in evaluation of the results. In evaluation of the stress distribution, proportions of the values are more important than difference of the numeric values of the measurement. Close stress values measured from the area near the screws show better resistance for relapse of surgery. Analysis of the data revealed that inverted L formation was more successful in comparison with other formations (Table 3).

Discussion Two point biomechanical test models and FEA have been used extensively to analyse biomechanical features of fixation methods. Yamashita et al. (10) modelled application styles of bicortical titanium screw system by FEA and studied stresses produced in screws and bone. They compared masticatory function and neurosensory recovery patterns of bicortical screw © 2014 John Wiley & Sons Ltd

© 2014 John Wiley & Sons Ltd

D Sponge Max I_ Spong Min I_ Spong Max Backward Von Misses D Kort Min D Kort Max I_ Kort Min I_ Kort Max D Sponge Min D Sponge Max I_ Spong Min I_ Spong Max

Forward Von Misses D Kort Min D Kort Max I_ Kort Min I_ Kort Max D Sponge Min

0
2

0
2 0
3

29
8 0
0 164
1

1
2 0
5 0
4

0
2

0
7

3
8

0
2 0
1

23
0 0
1 178
8

1
0 0
5 0
6

0
1

0
8

0
2

6
8 0
0 102
4 0
5 0
3 0
5

L formation

0
2

7
3 0
2 180
1 1
1 0
3 0
3

Inverted L formation

0
0

0
4

0
8

0
6 0
4 0
4

37
4 0
3 221
8

0
6 0
1

0
4

29
6 0
3 232
6 29
0 0
3 0
3

Linear formation

Rates of with each other and compared with the stress values difference in the thickness

16
3

11
7

9
0

356
6 83
7 6
8

315
0 48
5 165
1

29
2 24
6

9
2

210
6 44
2 165
1 40
5 78
8 7
4

Inverted L formation

13
7

7
3

8
6

298
6 98
5 38
9

269
0 174
6 165
1

31
2 55
2

12
8

313
0 94
2 165
1 51
3 39
1 46
9

L formation

15
8

17
5

16
1

486
3 135
5 16
2

439
6 78
0 165
1

31
6 48
0

33
7

520
9 102
1 165
1 45
3 50
5 13
8

Linear formation

15
7

37
3

3
8

16
2 17
6 465
5

14
5 259
3 0
0

6
7 123
9

37
9

48
6 112
8 0
0 26
6 50
3 531
3

% of increase between inverted L and L formations

Rates of with each other and compared with the stress values for 1
5 mm.

14
6

138
4

86
6

62
8 37
5 58
1

63
4 55
3 0
0

1
3 12
9

163
0

66
4 8
4 0
0 11
7 29
0 70
4

% of increase between L and linear formations

3
3

49
3

79
4

36
3 61
7 136
4

39
5 60
5 0
0

8
1 95
0

262
7

147
3 130
7 0
0 11
8 35
9 86
8

% of increase between inverted L and linear formations

16
4

11
6

9
0

353
0 84
2 6
8

256
0 48
5 59
2

29
2 24
7

9
2

196
2 44
1 58
9 40
0 79
1 7
4

Inverted L formation

14
3

7
2

8
6

294
9 99
1 38
7

207
2 174
6 62
5

31
3 55
4

12
8

292
8 94
2 81
5 51
6 39
2 46
7

L formation

15
8

17
4

16
3

483
1 136
1 16
3

319
9 78
2 51
2

31
4 48
0

33
8

401
9 102
4 49
6 35
1 50
6 13
9

Linear formation

12
5

37
2

10
3

138
9

87
7

63
8 37
2 57
8

16
4 17
7 466
7 3
7

54
3 55
1 17
9

0
3 13
3

163
4

37
2 8
6 39
1 32
0 28
9 70
1

% of increase between L and linear formations

19
0 259
9 5
5

7
3 124
3

38
6

49
2 113
6 38
4 28
8 50
3 530
5

% of increase between inverted L and L formations

Rates of with each other and compared with the stress values for 2 mm.

3
5

49
8

80
6

36
8 61
6 139
0

24
9 61
3 13
3

7
7 94
3

265
3

104
8 132
2 15
7 12
3 36
0 88
2

% of increase between inverted L and linear formations

Table 2. All stress values of models fixed in backward and forward jaw positions by 1
5- and 2-mm bicortical screws. During comparisons, the minimum principal force rates become more important and instructive for us. Analysis of the data revealed that inverted L formation was more successful in decrease the stress values after bone fixation

FINITE ELEMENT ANALYSIS OF SAGITTAL SPLIT OSTEOTOMY 687

688

A . S I N D E L et al. Table 3. Stress distributions of models fixed in backward and forward jaw positions by 1
5- and 2-mm bicortical screws. Note that proportions of the values are more important than difference of the numeric values of the measurement, and close stress values measured from the area near the screws shows better resistance for relapse of surgery. Therefore, inverted L formation was found as more successful in comparison with other formations Stress values measured from the area near the screws 1
5 mm

Backward position Inverted L formation L formasyon Linear formation Forward position Inverted L formation L formasyon Linear formation

2 mm

Screw 1

Screw 2

Screw 3

Screw 1

Screw 2

Screw 3

5
36 3
61 2
33

4
11 7
21 4
09

9
48 10
83 6
42

5
18 3
51 2
25

3
97 7
15 4
04

9
15 10
67 6
30

4
89 4
30 1
55

3
69 7
39 3
09

8
59 11
69 4
64

4
86 4
15 1
53

3
66 7
08 2
99

8
53 11
24 4
52

and monocortical miniplate fixation techniques after mandibular correction. They suggested that both fixation techniques seem to provide equal comfort and reliability. Due to compression, lateral condyles may become displaced if fixation is performed with lag screw (11). Compression of the proximal and distal segments may cause inferior alveolar nerve damage. Therefore, all sharp spurs of the spongious bone in the segment surfaces should be straightened (12). No compression occurs when positional screws are tightened, and no displacement occurs in the segments or condyles, as well. Theoretically, low compression means low risk of the inferior alveolar nerve damage. One of the disadvantages of positional screw technique is that bone segments may diverge from each other if segments cannot be aligned correctly during screwing. For lag or positional screw selections, another important issue which surgeon needs to decide is that whether screws should be placed transoral or percutaneous. Transoral approach prevents skin incision, requires less procedure, less muscle elevation. In small proximal segments or in presence of third molar tooth, transoral approach becomes difficult due to bone insufficiency on lingual surface. One of the advantages of the percutaneous placement is that existing bone can be fixated vertically, as screws can be led to desired direction (11). Main goal in SSRO surgeries is to maintain long-term stabilisation regardless of which technique is used. For this purpose, many stabilisation techniques for bimaxillary fixation have been developed. For instance, Uckan et al. (13) compared

biomechanical properties of the SSRO in sheep mandibles with bicortical screws and miniplates. They found that bicortical screws are more stable. In another study, it was found that bicortical positional screws are more resistant to both molar and incisional forces by comparison of the bicortical positional screws, monocortical plate and screw systems following SSRO in synthetic mandibula models (14). Anucul et al. (1) performed complete incisions on cow bone models and investigated the stability of the monocortical plates and bicortical positional screws. Force rates were compared in terms of elastic deformation, stiffness ratio, permanent deformation and rupture load. They found that bicortical positional screws are more rigid and have fewer tendencies to deformation (1). Studies comparing RIF techniques following SSRO showed that bicortical positional titanium screw fixation stability was sufficient (4, 15, 16). It has also been reported that bicortical positional screw sizes do not cause any statistically significant difference over stability (17, 18). Studies comparing stabilities between screw formation show that IL formation is much more powerful (15). It is suggested that to place screws at an angle of 90° is ideal for bone fixation (19). Uckan et al. (13) suggested that there is no significant effect in screw angling to movement resistance compared with extra oral 90° and 60° techniques which were two different screw fixation methods applied in SSRO. Many different fixation formations have been applied for bicortical screw fixations. IL formation or three screws fixation to upper border is the most © 2014 John Wiley & Sons Ltd

FINITE ELEMENT ANALYSIS OF SAGITTAL SPLIT OSTEOTOMY commonly used methods. It was also reported that IL formation was more powerful in compressive load resistance with a mechanical system (15). Foley used compressive vertical load-loss model to show that IL was more powerful compared with triple linear screw application at upper border (15). It was also reported that three 2-mm screws in IL formation or fixation with one monocortical plate was more rigid than fixation with two 2
7-mm bicortical screws (13). It was shown that there was no difference between screw diameters for stability. Smaller sized screws are acceptable and adequate for patient comfort (17, 18). de Molon et al. (20) found no difference between 1
5and 2
0-mm screws. Theoretically, the reason of the relapse following rigid fixation is movement in osteotomy region or reposition in the condylar region (21). In the cases in which mandibula was advanced more than 6–7 mm, it was observed that there was more tendency to relapse following fixation with metallic screws (7). Leonard (11) calculated surface area between bone and screw and found 35% increase in screw size. He suggested that increased surface area might improve ability to resist the pressure. Schwimmer et al. (18) compared 2
7- and 2
0-mm sized screws for SSRO in cadaver’s mandibles. They found that 2
0- and 2
7mm screws maintained similar stability. Obeid and Lindquist (22) compared mandibular ramus positions of the 2
0- and 2
7-mm screws using dry mandibles and detected no satisfactory stabilisation difference about screw diameters. Shetty et al. (23) reported that 2
4-mm screws show better stabilisation than 2
0-mm screws. Small increases in inner diameter demonstrate significant increases in tension and torsional forces (24). Maurer et al. (25) assessed various bicortical screw configurations and diameters of FEA. They found that IL configuration is most stable. They showed that 2
0-mm screws maintain more adequate stabilisation than 1
5 mm screws in SSRO. Thicker screws make bigger gaps at application sites and increase the risk of condyle disturbance and bone microfracture. Maurer et al. (25) approved clinical usage of 1
5 mm screws in sagittal mandibula osteotomies to decrease the inferior alveolar nerve damage. Critical remarks on the methodological approach Maximum principal stress rates may give inaccurate results in FEA as these rates show the tension forces © 2014 John Wiley & Sons Ltd

on the surfaces. Tension forces cannot be reflected completely in the simulations performed in computer environment. For example, in FEA performed in mandibula, prosthesis cannot get away from mouth mucosa regardless of how much increases the forces uplifting the prosthesis inside the mouth. Therefore, it causes a profound unrealistic deformation in bone and soft tissue, and such deformations change all force rates around bone and screws. As in this example, the effects on tension of the forces applied on screws (maximum principal stress rates) are insufficient to reflect the reality and may lead us to inaccurate results. We made some stop points on bone to eradicate this handicap (unpublished data). Therefore, minimum principal forces were also taken into account during analysis to avoid any inaccurate simulations. Conventional statistical analyses are not applicable as FEA results are evaluated by mathematical calculations without variances. Mathematical model will yield the same result with each test paradigm unless the parameters of the simulation are altered. Therefore, care must be taken to analyse the measured stress rates and stress distributions (8).

Conclusion Vertical relapse is more often in patients with low mandibular plane angle. However, horizontal relapse is higher in patients with high mandibular plane angle in more than 6–7 mm advancements. Bicortical screw fixation is a way for successful stabilisation (4, 13, 15–17, 19). Another important point that minimises bone resorption and increase stabilisation is expansion of the coming forces onto a large surface. In another word, while evaluating success of screw formations, it is required to pay attention how these rates spread on to screws and bone. Our data revealed that IL application of bicortical screws fixation had minimum stress rates and better stresses distribution in comparison with other formations. The success of the IL formation was followed by L and linear formations which had close rates. We suggest that surgeon should decide how to approach the surgical area. If the surgeon selects percutaneous approach, IL formation is an ideal way for stabilisation. In addition, linear formation should be used in transoral approach due to difficulties in L or IL applications.

689

690

A . S I N D E L et al. Another aim of the present study is to investigate whether there is a need to increase the diameter of the bicortical screw applied in SSRO fixation for stabilisation. We found no significant difference between 1
5- and 2
0-mm screws in SSRO fixation. We suggest that 1
5-mm screws should be chosen at SSRO fixation to avoid complications such as nerve injury, large defect area, microfractures on bone and more disturbances on condyle. We think that further studies should be performed by FEA to evaluate advantages and/or disadvantages of RIF techniques in osteotomies.

Ethical considerations No ethical approval is needed.

Funding This research was carried out without funding.

Conflicts of interest No conflict of interest is declared.

References 1. Anucul B, Waite PD, Lemons JE. In vitro strength analysis of sagittal split osteotomy fixation: noncompression monocortical plates versus bicortical position screws. J Oral Maxillofac Surg. 1992;50:1295–1299. 2. Dal Pont G. Retromolar osteotomy for the correction of prognathism. J Oral Surg Anesth Hosp Dent Serv. 1961;19:42–47. 3. Hammer B, Ettlin D, Rahn B, Prein J. Stabilization of the short sagittal split osteotomy: in vitro testing of different plate and screw configurations. J Craniomaxillofac Surg. 1995;23:321–324. 4. Ochs MW. Bicortical screw stabilization of sagittal split osteotomies. J Oral Maxillofac Surg. 2003;61:1477–1484. 5. Rubens BC, Stoelinga PJW, Blijdorp PA, Schoenaers JHA, Politis C. Skeletal stability following sagittal split osteotomy using monocortical miniplate internal-fixation. Int J Oral Maxillofac Surg. 1988;17:371–376. 6. Joss CU, Vassalli IM. Stability after bilateral sagittal split osteotomy setback surgery with rigid internal fixation: a systematic review. J Oral Maxillofac Surg. 2008;66:1634– 1643. 7. Van Sickels JE, Larsen AJ, Thrash WJ. Relapse after rigid fixation of mandibular advancement. J Oral Maxillofac Surg. 1986;44:698–702.

8. Adiguzel O. Sonlu elemanlar analizi. Disßhekimliginde Kullanım Alanları, Temel Kavramlar ve Eleman Tanımlar. Dent J Dicle 2010;11:18–21. 9. May B, Saha S, Saltzman M. A three-dimensional mathematical model of temporomandibular joint loading. Clin Biomech (Bristol, Avon). 2001;16:489–495. 10. 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. 2007;36:118– 122. 11. Leonard MS. The use of lag screws in mandibular fractures. Otolaryngol Clin North Am. 1987;20:479–493. 12. Kim YI, Jung YH, Cho BH, Kim JR, Kim SS, Son WS et al. The assessment of the short- and long-term changes in the condylar position following sagittal split ramus osteotomy (SSRO) with rigid fixation. J Oral Rehabil. 2010;37:262– 270. 13. Uckan S, Schwimmer A, Kummer F, Greenberg AM. Effect of the angle of the screw on the stability of the mandibular sagittal split ramus osteotomy: a study in sheep mandibles. Br J Oral Maxillofac Surg. 2001;39:266–268. 14. Peterson GP, Haug RH, Van Sickels J. A biomechanical evaluation of bilateral sagittal ramus osteotomy fixation techniques. J Oral Maxillofac Surg. 2005;63:1317– 1324. 15. Foley WL, Frost DE, Paulin WB Jr, Tucker MR. Internal screw fixation: comparison of placement pattern and rigidity. J Oral Maxillofac Surg. 1989;47:720–723. 16. Murphy MT, Haug RH, Barber JE. An in vitro comparison of the mechanical characteristics of three sagittal ramus osteotomy fixation techniques. J Oral Maxillofac Surg. 1997;55:489–494. 17. Kohn DH, Richmond EM, Dootz ER, Feinberg SE, Pietrzak WS. In vitro comparison of parameters affecting the fixation strength of sagittal split osteotomies. J Oral Maxillofac Surg. 1995;53:1374–1383. 18. Schwimmer A, Greenberg AM, Kummer F, Kaynar A. The effect of screw size and insertion technique on the stability of the mandibular sagittal split osteotomy. J Oral Maxillofac Surg. 1994;52:45–48. 19. Schlicke LH, Panjabi MM, White AA 3rd. Optimal orientation of transfixation screws across oblique fractures lines. Clin Orthop Relat Res. 1979;143:271–277. 20. de Molon RS, de Avila ED, Scartezini GR, Campos JADB, Vaz LG, Gabrielli MFR et al. In vitro comparison of 1.5 mm vs. 2.0 mm screws for fixation in the sagittal split osteotomy. J Craniomaxillofac Surg. 2011;39:574–577. 21. Stroster TG, Pangrazio-Kulbersh V. Assessment of condylar position following bilateral sagittal split ramus osteotomy with wire fixation or rigid fixation. Int J Adult Orthodon Orthognath Surg. 1994;9:55–63. 22. Obeid G, Lindquist CC. Optimal placement of bicortical screws in sagittal split-ramus osteotomy of mandible. Oral Surg Oral Med Oral Pathol. 1991;71:665–669.

© 2014 John Wiley & Sons Ltd

FINITE ELEMENT ANALYSIS OF SAGITTAL SPLIT OSTEOTOMY 23. Shetty V, Freymiller E, McBrearty D, Caputo AA. Functional stability of sagittal split ramus osteotomies: effects of positional screw size and placement configuration. J Oral Maxillofac Surg. 1996;54:601–609. 24. Hughes AN, Jordan BA. The mechanical properties of surgical bone screws and some aspects of insertion practice. Injury. 1972;4:25–38.

© 2014 John Wiley & Sons Ltd

25. Maurer P, Holweg S, Schubert J. Finite-element-analysis of different screw-diameters in the sagittal split osteotomy of the mandible. J Craniomaxillofac Surg. 1999;27:365–372. Correspondence: Alper Sindel, Department of Oral and Maxillofacial Surgery, Akdeniz University Faculty of Dentistry, 07070 Antalya, Turkey. E-mail: [email protected], [email protected]

691