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In vitro comparison of 1.5 mm vs. 2.0 mm screws for fixation in the sagittal split osteotomy
Journal of Cranio-Maxillo-Facial Surgery 39 (2011) 574e577
Contents lists available at ScienceDirect
Journal of Cranio-Maxillo-Facial Surgery journal homepage: www.jcmfs.com
In vitro comparison of 1.5 mm vs. 2.0 mm screws for fixation in the sagittal split osteotomy Rafael Scaf de Molon a, *, Érica Dorigatti de Ávila a, Guilherme Romano Scartezini a, Juliana Alvares Duarte Bonini Campos b, Luis Geraldo Vaz c, Mario Francisco Real Gabrielli a, Valfrido Antônio Pereira Filho a a b c
Department of Diagnosis and Surgery, São Paulo State University e UNESP, Rua Humaitá, 1680, 14801-903 Araraquara (SP), Brazil Department of Biostatistics, Araraquara Dental School, UNESP, Rua Humaitá, 1680, 14801-903 Araraquara (SP), Brazil Department of Dental Materials and Prosthesis, Araraquara Dental School, UNESP, Rua Humaitá, 1680, 14801-903 Araraquara (SP), Brazil
a r t i c l e i n f o
a b s t r a c t
Article history: Paper received 4 December 2009 Accepted 26 November 2010
Purpose: Numerous “in vitro” investigations have been conducted to evaluate the role of screw size and pattern in determining optimal resistance to deformation, often these have been controversial. The aim of this study was to evaluate the effect of screw size and insertion technique on the stability of sagittal split osteotomies. Materials and methods: This study used twenty polyurethane replicas of human hemimandibles with a prefabricated sagittal split ramus osteotomy (SSRO). The hemimandibles were stabilized with 1.5 mm and 2.0 mm titanium screws inserted in an inverted L configuration. All specimens were tested to determine the strength and stability of the fixation. Results: In all cases there was failure of the synthetic bone before there was any evidence of screw failure. There were no significant differences in the load necessary to make the construct fail between the 1.5 or 2.0 mm screw sizes. Conclusion: There was no statistically significant difference between the strengths achieved with screws of 1.5 and 2.0 mm diameters for fixation of SSRO performed in synthetic mandibles. There was no fracture of the 1.5 mm or 2.0 mm diameter screws in any of the tests. 1.5 mm diameter screws in an inverted L pattern have as much stability and mechanical resistance as a 2.0 mm screw, may be safely used for this procedure. Ó 2010 European Association for Cranio-Maxillo-Facial Surgery.
Keywords: Bone screws Mandibular advancements Orthognathic surgery Sagittal split osteotomy
1. Introduction The sagittal split ramus osteotomy (SSRO) of the mandible has been performed for correction of dentofacial deformities and malocclusion for the last 50 years. During this time several modifications have been proposed to reduce morbidity and improve stability. Fixation with miniplates and/or screws improves stability in most instances and allows early return to preoperative function (Anacul et al., 1992; Hammer et al., 1995; Murphy et al., 1997; Ochs, 2003). Moreover, the use of stable fixation favours the maintenance of the airway in the immediate postoperative period, improves the nutritional support and reduces discomfort for the patient (Trauner and Obwegeser, 1957; Wolford et al., 1987).
* Corresponding author. Tel.: þ55 16 3301 6359. E-mail address: [email protected] (R. Scaf de Molon).
Lag screws are recognized as one of the most efficient fixation devices in orthopedic surgery, because they provide a maximum amount of interfragmentary compression with a minimum amount of implanted material (Haug et al., 1999; Ochs, 2003; Brasileiro et al., 2009). Schwimmer et al. (1994) affirmed that interfragmentary compression can decrease the loads supported by the screw due to transmission of some of the shearing and rotational forces through the underlying bone, thereby decreasing stresses on the screw interface. Compressive stresses can be maximized by pretapping the screw hole, which increases the holding power of the screw. The use of so-called position screws (nonlagged) increases the load on the screw because maximum interfragmentary compression is not achieved Lag screw fixation for stabilization of the SSRO of the mandible was introduced by Spiessl (1976), but due to an increase in neurosensory disturbances as a result of the compressive forces caused by the lagging technique and to the potential for condylar displacement, some authors have proposed alternative fixation
1010-5182/$ e see front matter Ó 2010 European Association for Cranio-Maxillo-Facial Surgery. doi:10.1016/j.jcms.2010.11.008
R. Scaf de Molon et al. / Journal of Cranio-Maxillo-Facial Surgery 39 (2011) 574e577
methods. The use of three bicortical screws, either in lag screw or positional screw techniques is currently accepted as a very rigid and predictable way to fixate the SSRO. There are numerous screw patterns that can be used, taking advantage of location, angulation, and regions of maximum bone contact and thickness. The perpendicular insertion of positional screws in the inverted L configuration is one of the most critically evaluated of the rigid internal fixation techniques, and is considered the “gold standard” by which other forms of fixation have been compared in most biomechanical experiments (Foley et al., 1989). Kim et al. (1993) evaluated through photoelastic analysis four different screw placement patterns employed for fixation of the SSRO. This study reported that the use of a three screw in triangular (inverted L) or oblique linear pattern seemed to be the optimal method of fixation, because potential damage to the surrounding structures could develop in the presence of large stress. Foley et al. (1989) showed by means of biomechanical compressive load tests that the screws used in an inverted L pattern promote higher resistance to displacement than screws positioned in a linear pattern at the superior border. Similarly, Kim et al. (1995) compared the rigidity of three different screw placement patterns employed to fixation of SSRO in cadaver mandible and concluded that disposition in inverted L was stronger than the linear pattern and oblique linear pattern. Another factor affecting the holding strength of a screw is the external diameter of the thread. Scientific literature presents studies comparing the stabilization of SSRO of the mandible for different diameters of screws: 2.7 mm, 2.4 mm and 2.0 mm (Obeid and Lindquist, 1991; Shetty et al., 1996; Leonard, 1987; Schwimmer et al., 1994). However, the geometric disposition of the screws seems to affect the stability and resistance to functional forces more than the diameter of the screws and the use of higher screw diameters may not be necessary (Obeid and Lindqvist, 1991). Considering that there are few studies comparing the effect of screw diameters this in vitro investigation was developed to compare stability o SSRO after fixation with 1.5 and 2.0 mm bicortical screws.
575
Fig. 1. Synthetic hemimandible fixed in inverted-L position with 2.0 mm bicortical screws.
2. Materials and methods This study used 20 polyurethane replicas of human hemimandibles (Nacional, Jaú, SP, Brazil) with a prefabricated SSRO. Each hemimandible from the same lot was then prepared, rigidly attached with the distal segment repositioned in a 5 mm advancement position and the setting stabilized by means of bicortical screws in inverted L pattern. The screws (MDT, Rio Claro, SP, Brazil) used were 1.5 mm and 2.0 mm with self-tapping titanium screws. The specimens were divided into two groups. In the first group, 10 synthetic hemimandibles were fixed with three bicortical titanium screws, of 2.0 mm diameter (Fig. 1). The second group presented 10 synthetic hemimandible fixed with three bicortical titanium screws of 1.5 mm diameter, totaling 60 screws (Fig. 2). All the screws were applied in an inverted L pattern with two screws placed on the tension or alveolar side of the osteotomy and one screw placed on the inferior or compression side of the osteotomy, installed perpendicular to the cortical bone. The length of the screw was determined to cross, both cortical and overcome at least 1 mm of cortical internal (15 mm length), in both groups. The distance between the screws on the top border was approximately 13 mm, being placed in areas that had better contact between the cortical. The most posterior bicortical screw was inserted 5 mm under the retromolar alveolar crest. The inferior screw was installed along the bottom border, in place where cortical thickness was greater and which has showed greater
Fig. 2. Synthetic hemimandible fixed in inverted-L position with 1.5 mm bicortical screws.
Table 1 Rupture strength of osteotomized mandible (N). Groups
Rupture load
2.0 mm 1.5 mm
166.70 193.94
contact area. The distance between the anterior superior screw and inferior screw was approximately 15 mm, keeping 5 mm away from the osteotomy line. The synthetic hemimandibles were mounted on a resin block. The montage was performed by placing the resin in the sandy phase in a wax mould, positioning the hemimandible until final polymerization, allowing standardization of pieces and facilitating their fixation in the testing machine. The hemimandibles were rigidly stabilized in the posterior border and the mandibular condyle, allowing free movement of the distal segment while increasing loads were applied. The hemimandibles fixed by a block of resin were placed in a steel support and fixed to the base of the testing machine. In the headstock of the assay machine the load cell was attached to a sensor of force or “load”. The maximum capacity of the cell was 1 kN with sensitivity of 5 g being the force applied by threading
576
R. Scaf de Molon et al. / Journal of Cranio-Maxillo-Facial Surgery 39 (2011) 574e577
Table 2 Analysis of data. GI
Maximum load (N) Maximum tension (Mpa) Tension rupture (Mpa) Load rupture (N) Along rup. (mm)
GII
Mann-Whitney
Average d
Median
CV(%)
Average d
Median
CV(%)
p
185.02 4.32 18.88 4.32 12.57 6.34 178.85 4.35 16.37 8.43
181.10 18.48 13.28 166.70 18.30
22.86 22.88 50.41 23.86 51.53
232.95 7.19 23.78 7.26 18.20 6.13 228.54 7.86 16.08 5.77
204.53 20.80 15.45 193.94 15.18
30.25 30.53 33.69 33.68 35.87
0.1797 0.1797 0.2002 0.2002 1.0000
a bolt. To test the sample, vertical forces of compression (10 mm/ min) were directed to the incisal edge between central and lateral mandibular incisors until occur failures in the fixation or fracture in the hemimandible (Kohn et al., 1995). All testing was performed on an MTS servo hydraulic testing machine (MTS Systems Inc, Minneapolis, MN). The data were transmitted directly from the load cell to a computer that generated a spread sheet of data of gallows vs. displacement. The data on the force necessary to bring instability and failure in the constructed was measured and calculated with its shunting line standard for each group as collected in Newton (N). The statistical analysis was done through parametric testing. The variables maximal load, maximal tension, rupture load, rupture tension and displacement were collected and the comparison between groups was analysed with the Mann-Whitney test. The adopted level of significance was of 5%. 3. Results The average force for failure of the construct did not present significant differences as shown in Table 1. The detailed results for the stability of the individual systems, plotted as a function of load with regard to the amount of displacement, and statistical comparisons among groups are shown in Table 2. None of the variables showed a statistically significant effect on the strength of the osteotomies, although the time of rupture was higher for the 2.0 mm screws. In all cases there was failure of the synthetic bone before there was any evidence of screw failure. Based on these observations there were no significant differences in the load necessary to make the construct fail between 1.5 and 2.0 mm screw sizes. 4. Discussion Although there are differences in the modulus of elasticity of fresh and synthetic bone, synthetic models of human mandible were used in this study. This was done mainly because they are easy to obtain, less costly and allow for standardization (Bredbenner and Haug, 2000). The literature demonstrates variability in the geometry and mechanical properties between different regions of the mandible and between different cadaveric mandibles (Rho, 1991; Bredbenner and Haug, 2000). Additionally, the material properties of human and animal bone are influenced by age, as well as by genetic, environmental and nutritional factors (Rho, 1991). The synthetic mandibles used present smaller values for insertion torque and pullout strength than cadaveric bone. Resistance, as an isolated factor, is not a characteristic enough to evaluate the quality of a material (Bredbenner and Haug, 2000). Bone is constituted of cortical and medullary portions which vary in thickness and height depending on the region. In the same way, it possesses changeable modulus of elasticity in its extension (Rubo, 2004). In 1984, Jeter et al., had used 2.0 mm screws for internal fixation SSRO and observed no screw related injuries to the inferior alveolar nerve and no need for screw removal in a sample of 20 patients. This turned to be a classical study that led most surgeons to used
2.0 mm screws for stable internal fixation of the SSRO. After few years, Leonard (1987) calculated that an increase in thread diameter from 2.0 mm to 2.7 mm results in a 35% increase in the surface area of the boneescrew interface. The greater interface area increased the ability of the screw to bear greater compressive loads before pullout. Foley and Beckman (1992) compared the rigidity of fixation when using three 2.0 mm screws in the inverted L pattern; two 2.7 mm bicortical screws and a four-hole miniplate with 2.0 mm monocortical screws. The results showed no significant difference between the inverted L 2.0 mm screws and the miniplate. The rigidity of those two groups was higher when compared with two 2.7 mm bicortical screws. Schwimmer et al. 1994, compared 2.7 and 2.0 mm screws for fixation of sagittal osteotomies in 10 cadaver mandibles. This study indicates that 2.0 mm diameter screws provide the same degree of stability as 2.7 mm screws. A study from Obeid and Lindqvist (1991) used six dry mandibles to compare 2.0 and 2.7 mm screws positioned in the mandibular ramus. The screws were unroofed and the authors observed that countersinking resulted in 55% of the 2.7 mm screws and 27% of the 2.0 mm screws having no threads engaging the buccal cortex and thus acting as lag screws. The smaller diameter 2.0 mm screws had more threads engaged in cortical bone. On the lingual cortex, although both diameters had good cortical engagement, several 2.7 mm screws were engaging by one thread. Those observations might account for the similar stability results frequently described when comparing 2.0 mm and 2.7 mm screw diameters. Shetty et al. 1996, using reproducible mandible analogs, reported greater stability with 2.4 mm screws than with 2.0 mm screws. Screw strength and mechanical resistance were determined by the strength of the material and core diameter of the screw. The torsional sheer strength was related to the cube of the diameter and tensional strength to the square of the diameter. Small increases in core diameter markedly enhance tensile and torsional strengths (Hughes and Jordan, 1972). In concordance, Maurer et al. (1999) evaluated different bicortical screw configurations and diameters through finiteelement-analysis, and concluded that when bite forces have been applied, the inverted L configuration was the most stable. Also they showed that the 2.0 mm screw diameter can provide sufficient stability at the osteotomy site after SSRO as well the screws with 1.5 mm diameter. Additionally these authors showed the 1.5 mm screws may withstand forces of up to 89.5 N, where the forces exerted during bite do not exceed these values. 5. Conclusion The screws of higher thickness require greater availability of space for its insertion and increase the possibility of torque to the condyle and micro fractures of the bone. This study validates the clinical use of 1.5 mm position screws as fixators for sagittal osteotomies of the mandible, reducing the possibility of injuries to the inferior alveolar nerve and the tactile perception. In conclusion, there was no statistically significant difference between 1.5 and 2.0 mm screws for fixation of SSRO performed in
R. Scaf de Molon et al. / Journal of Cranio-Maxillo-Facial Surgery 39 (2011) 574e577
synthetic mandibles. There was no fracture of the 1.5 mm diameter screws in any of the tests. The 1.5 mm diameter screws in inverted L have as much stability and mechanical resistance as the 2.0 mm screw, and may be safely used for this procedure. Acknowledgments The authors acknowledge with sincere thanks, the “MDT e Implantes Ortopédicos” by provided free of charge of titanium screws for research. References Anacul 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 50: 1295, 1992 Brasileiro BF, Grempel RG, Ambrosano GMB, Passeri LA: An in vitro evaluation of rigid internal fixation techniques for sagittal split ramus osteotomies: advancement surgery. J Oral Maxillofac Surg 67: 809e817, 2009 Bredbenner TL, Haug RH: Substitutes for human cadaveric bone in maxillofacial rigid fixation research. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 90: 574e580, 2000 Foley WL, Beckman TW: In vitro comparison of screw versus plate fixation in the sagittal split osteotomy. Int J Adult Orthodon Orthognath Surg 7: 147e151, 1992 Foley WL, Frost DE, Paulin WB, Tucker MR: Uniaxial pullout evaluation of internal screw fixation. J Oral Maxillofac Surg 47: 277, 1989 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 Cranio Maxillofac Surg 23: 321, 1995 Haug RH, Barber E, Punjabi AP: An in vitro comparison of the effect of number and pattern of positional screws on load resistance. J Oral Maxillofac Surg 57: 300e308, 1999 Hughes AN, Jordan BA: The mechanical properties of surgical bone screws and some aspects of insertion practice. Br J Accid Surg 4: 15, 1972 Jeter TS, Van Sickels JE, Dolwick MF: Modified techniques for internal fixation of sagittal ramus osteotomias. J Oral Maxillofac Surg 42: 270, 1984
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Kim HC, Essaki S, Kameyama T: Comparison of screw placement patterns on the rigidity of the sagittal split ramus osteotomy: technical note. J Cranio Maxillofac Surg 23: 54e56, 1995 Kim HC, Essaki S, Kameyama T, Yamamoto K: Photoelastic analysis of internal screw fixation after sagittal split ramus osteotomy. J Cranio Maxillofac Surg 21: 266e270, 1993 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 53: 1374, 1995 Leonard MS: The use of lag screws in mandibular fractures. Otolaryngol Clin North Am 20: 479e493, 1987 Maurer P, Holwek S, Schubert J: Finite-element-analysis of different screw-diameters in the sagittal split osteotomy of the mandible. J Cranio Maxillofac Surg 27: 365e372, 1999 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 55: 489e494, 1997 Obeid G, Lindquist CC: Optimal placement of bicortical screws in sagittal split-ramus osteotomy of mandible. Oral Surg Oral Med Oral Pathol 71: 665e669, 1991 Ochs MW: Bicortical screw stabilization of sagittal split osteotomies. J Oral Maxillofac Surg 61: 1477e1484, 2003 Rho JY: Mechanical properties of human cortical and cancellous bone [dissertation]. Dallas (TX): University of Texas Southwestern Medical Center, 1991 Rubo JH: Análise das tensões em prótese sobre implantes pelo método de elemento finito: estudo paramétrico de sete variáveis clínicas. (Livre-Docência em Odontologia). Bauru: Universidade de São Paulo. Faculdade de Odontologia de Bauru, p. 101, 2004 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 52: 45e48, 1994 Shetty V, Freymiller E, McBrearty D, Caputo AA: Functional stability of sagittal ramus split osteotomies: effects of positional screw size and placement configuration. J Oral Maxillofac Surg 54: 601, 1996 Spiessl B: Rigid internal fixation after sagittal split osteotomy of the ascending ramus. In: Spiessl B (ed.), New concepts in maxillofacial bone surgery. New York: Springer e Verlag, 115e122, 1976 Trauner R, Obwegeser H: The surgical correction of mandibular prognathism and retrognathia with consideration of genioplasty II. Oral Surg Oral Med Oral Pathol 10: 787, 1957 Wolford LM, Bennett MA, Rafferty CG: Modification of the mandibular ramus sagittal split osteotomy. Oral Surg Oral Med Oral Pathol 64: 146, 1987
Contents lists available at ScienceDirect
Journal of Cranio-Maxillo-Facial Surgery journal homepage: www.jcmfs.com
In vitro comparison of 1.5 mm vs. 2.0 mm screws for fixation in the sagittal split osteotomy Rafael Scaf de Molon a, *, Érica Dorigatti de Ávila a, Guilherme Romano Scartezini a, Juliana Alvares Duarte Bonini Campos b, Luis Geraldo Vaz c, Mario Francisco Real Gabrielli a, Valfrido Antônio Pereira Filho a a b c
Department of Diagnosis and Surgery, São Paulo State University e UNESP, Rua Humaitá, 1680, 14801-903 Araraquara (SP), Brazil Department of Biostatistics, Araraquara Dental School, UNESP, Rua Humaitá, 1680, 14801-903 Araraquara (SP), Brazil Department of Dental Materials and Prosthesis, Araraquara Dental School, UNESP, Rua Humaitá, 1680, 14801-903 Araraquara (SP), Brazil
a r t i c l e i n f o
a b s t r a c t
Article history: Paper received 4 December 2009 Accepted 26 November 2010
Purpose: Numerous “in vitro” investigations have been conducted to evaluate the role of screw size and pattern in determining optimal resistance to deformation, often these have been controversial. The aim of this study was to evaluate the effect of screw size and insertion technique on the stability of sagittal split osteotomies. Materials and methods: This study used twenty polyurethane replicas of human hemimandibles with a prefabricated sagittal split ramus osteotomy (SSRO). The hemimandibles were stabilized with 1.5 mm and 2.0 mm titanium screws inserted in an inverted L configuration. All specimens were tested to determine the strength and stability of the fixation. Results: In all cases there was failure of the synthetic bone before there was any evidence of screw failure. There were no significant differences in the load necessary to make the construct fail between the 1.5 or 2.0 mm screw sizes. Conclusion: There was no statistically significant difference between the strengths achieved with screws of 1.5 and 2.0 mm diameters for fixation of SSRO performed in synthetic mandibles. There was no fracture of the 1.5 mm or 2.0 mm diameter screws in any of the tests. 1.5 mm diameter screws in an inverted L pattern have as much stability and mechanical resistance as a 2.0 mm screw, may be safely used for this procedure. Ó 2010 European Association for Cranio-Maxillo-Facial Surgery.
Keywords: Bone screws Mandibular advancements Orthognathic surgery Sagittal split osteotomy
1. Introduction The sagittal split ramus osteotomy (SSRO) of the mandible has been performed for correction of dentofacial deformities and malocclusion for the last 50 years. During this time several modifications have been proposed to reduce morbidity and improve stability. Fixation with miniplates and/or screws improves stability in most instances and allows early return to preoperative function (Anacul et al., 1992; Hammer et al., 1995; Murphy et al., 1997; Ochs, 2003). Moreover, the use of stable fixation favours the maintenance of the airway in the immediate postoperative period, improves the nutritional support and reduces discomfort for the patient (Trauner and Obwegeser, 1957; Wolford et al., 1987).
* Corresponding author. Tel.: þ55 16 3301 6359. E-mail address: [email protected] (R. Scaf de Molon).
Lag screws are recognized as one of the most efficient fixation devices in orthopedic surgery, because they provide a maximum amount of interfragmentary compression with a minimum amount of implanted material (Haug et al., 1999; Ochs, 2003; Brasileiro et al., 2009). Schwimmer et al. (1994) affirmed that interfragmentary compression can decrease the loads supported by the screw due to transmission of some of the shearing and rotational forces through the underlying bone, thereby decreasing stresses on the screw interface. Compressive stresses can be maximized by pretapping the screw hole, which increases the holding power of the screw. The use of so-called position screws (nonlagged) increases the load on the screw because maximum interfragmentary compression is not achieved Lag screw fixation for stabilization of the SSRO of the mandible was introduced by Spiessl (1976), but due to an increase in neurosensory disturbances as a result of the compressive forces caused by the lagging technique and to the potential for condylar displacement, some authors have proposed alternative fixation
1010-5182/$ e see front matter Ó 2010 European Association for Cranio-Maxillo-Facial Surgery. doi:10.1016/j.jcms.2010.11.008
R. Scaf de Molon et al. / Journal of Cranio-Maxillo-Facial Surgery 39 (2011) 574e577
methods. The use of three bicortical screws, either in lag screw or positional screw techniques is currently accepted as a very rigid and predictable way to fixate the SSRO. There are numerous screw patterns that can be used, taking advantage of location, angulation, and regions of maximum bone contact and thickness. The perpendicular insertion of positional screws in the inverted L configuration is one of the most critically evaluated of the rigid internal fixation techniques, and is considered the “gold standard” by which other forms of fixation have been compared in most biomechanical experiments (Foley et al., 1989). Kim et al. (1993) evaluated through photoelastic analysis four different screw placement patterns employed for fixation of the SSRO. This study reported that the use of a three screw in triangular (inverted L) or oblique linear pattern seemed to be the optimal method of fixation, because potential damage to the surrounding structures could develop in the presence of large stress. Foley et al. (1989) showed by means of biomechanical compressive load tests that the screws used in an inverted L pattern promote higher resistance to displacement than screws positioned in a linear pattern at the superior border. Similarly, Kim et al. (1995) compared the rigidity of three different screw placement patterns employed to fixation of SSRO in cadaver mandible and concluded that disposition in inverted L was stronger than the linear pattern and oblique linear pattern. Another factor affecting the holding strength of a screw is the external diameter of the thread. Scientific literature presents studies comparing the stabilization of SSRO of the mandible for different diameters of screws: 2.7 mm, 2.4 mm and 2.0 mm (Obeid and Lindquist, 1991; Shetty et al., 1996; Leonard, 1987; Schwimmer et al., 1994). However, the geometric disposition of the screws seems to affect the stability and resistance to functional forces more than the diameter of the screws and the use of higher screw diameters may not be necessary (Obeid and Lindqvist, 1991). Considering that there are few studies comparing the effect of screw diameters this in vitro investigation was developed to compare stability o SSRO after fixation with 1.5 and 2.0 mm bicortical screws.
575
Fig. 1. Synthetic hemimandible fixed in inverted-L position with 2.0 mm bicortical screws.
2. Materials and methods This study used 20 polyurethane replicas of human hemimandibles (Nacional, Jaú, SP, Brazil) with a prefabricated SSRO. Each hemimandible from the same lot was then prepared, rigidly attached with the distal segment repositioned in a 5 mm advancement position and the setting stabilized by means of bicortical screws in inverted L pattern. The screws (MDT, Rio Claro, SP, Brazil) used were 1.5 mm and 2.0 mm with self-tapping titanium screws. The specimens were divided into two groups. In the first group, 10 synthetic hemimandibles were fixed with three bicortical titanium screws, of 2.0 mm diameter (Fig. 1). The second group presented 10 synthetic hemimandible fixed with three bicortical titanium screws of 1.5 mm diameter, totaling 60 screws (Fig. 2). All the screws were applied in an inverted L pattern with two screws placed on the tension or alveolar side of the osteotomy and one screw placed on the inferior or compression side of the osteotomy, installed perpendicular to the cortical bone. The length of the screw was determined to cross, both cortical and overcome at least 1 mm of cortical internal (15 mm length), in both groups. The distance between the screws on the top border was approximately 13 mm, being placed in areas that had better contact between the cortical. The most posterior bicortical screw was inserted 5 mm under the retromolar alveolar crest. The inferior screw was installed along the bottom border, in place where cortical thickness was greater and which has showed greater
Fig. 2. Synthetic hemimandible fixed in inverted-L position with 1.5 mm bicortical screws.
Table 1 Rupture strength of osteotomized mandible (N). Groups
Rupture load
2.0 mm 1.5 mm
166.70 193.94
contact area. The distance between the anterior superior screw and inferior screw was approximately 15 mm, keeping 5 mm away from the osteotomy line. The synthetic hemimandibles were mounted on a resin block. The montage was performed by placing the resin in the sandy phase in a wax mould, positioning the hemimandible until final polymerization, allowing standardization of pieces and facilitating their fixation in the testing machine. The hemimandibles were rigidly stabilized in the posterior border and the mandibular condyle, allowing free movement of the distal segment while increasing loads were applied. The hemimandibles fixed by a block of resin were placed in a steel support and fixed to the base of the testing machine. In the headstock of the assay machine the load cell was attached to a sensor of force or “load”. The maximum capacity of the cell was 1 kN with sensitivity of 5 g being the force applied by threading
576
R. Scaf de Molon et al. / Journal of Cranio-Maxillo-Facial Surgery 39 (2011) 574e577
Table 2 Analysis of data. GI
Maximum load (N) Maximum tension (Mpa) Tension rupture (Mpa) Load rupture (N) Along rup. (mm)
GII
Mann-Whitney
Average d
Median
CV(%)
Average d
Median
CV(%)
p
185.02 4.32 18.88 4.32 12.57 6.34 178.85 4.35 16.37 8.43
181.10 18.48 13.28 166.70 18.30
22.86 22.88 50.41 23.86 51.53
232.95 7.19 23.78 7.26 18.20 6.13 228.54 7.86 16.08 5.77
204.53 20.80 15.45 193.94 15.18
30.25 30.53 33.69 33.68 35.87
0.1797 0.1797 0.2002 0.2002 1.0000
a bolt. To test the sample, vertical forces of compression (10 mm/ min) were directed to the incisal edge between central and lateral mandibular incisors until occur failures in the fixation or fracture in the hemimandible (Kohn et al., 1995). All testing was performed on an MTS servo hydraulic testing machine (MTS Systems Inc, Minneapolis, MN). The data were transmitted directly from the load cell to a computer that generated a spread sheet of data of gallows vs. displacement. The data on the force necessary to bring instability and failure in the constructed was measured and calculated with its shunting line standard for each group as collected in Newton (N). The statistical analysis was done through parametric testing. The variables maximal load, maximal tension, rupture load, rupture tension and displacement were collected and the comparison between groups was analysed with the Mann-Whitney test. The adopted level of significance was of 5%. 3. Results The average force for failure of the construct did not present significant differences as shown in Table 1. The detailed results for the stability of the individual systems, plotted as a function of load with regard to the amount of displacement, and statistical comparisons among groups are shown in Table 2. None of the variables showed a statistically significant effect on the strength of the osteotomies, although the time of rupture was higher for the 2.0 mm screws. In all cases there was failure of the synthetic bone before there was any evidence of screw failure. Based on these observations there were no significant differences in the load necessary to make the construct fail between 1.5 and 2.0 mm screw sizes. 4. Discussion Although there are differences in the modulus of elasticity of fresh and synthetic bone, synthetic models of human mandible were used in this study. This was done mainly because they are easy to obtain, less costly and allow for standardization (Bredbenner and Haug, 2000). The literature demonstrates variability in the geometry and mechanical properties between different regions of the mandible and between different cadaveric mandibles (Rho, 1991; Bredbenner and Haug, 2000). Additionally, the material properties of human and animal bone are influenced by age, as well as by genetic, environmental and nutritional factors (Rho, 1991). The synthetic mandibles used present smaller values for insertion torque and pullout strength than cadaveric bone. Resistance, as an isolated factor, is not a characteristic enough to evaluate the quality of a material (Bredbenner and Haug, 2000). Bone is constituted of cortical and medullary portions which vary in thickness and height depending on the region. In the same way, it possesses changeable modulus of elasticity in its extension (Rubo, 2004). In 1984, Jeter et al., had used 2.0 mm screws for internal fixation SSRO and observed no screw related injuries to the inferior alveolar nerve and no need for screw removal in a sample of 20 patients. This turned to be a classical study that led most surgeons to used
2.0 mm screws for stable internal fixation of the SSRO. After few years, Leonard (1987) calculated that an increase in thread diameter from 2.0 mm to 2.7 mm results in a 35% increase in the surface area of the boneescrew interface. The greater interface area increased the ability of the screw to bear greater compressive loads before pullout. Foley and Beckman (1992) compared the rigidity of fixation when using three 2.0 mm screws in the inverted L pattern; two 2.7 mm bicortical screws and a four-hole miniplate with 2.0 mm monocortical screws. The results showed no significant difference between the inverted L 2.0 mm screws and the miniplate. The rigidity of those two groups was higher when compared with two 2.7 mm bicortical screws. Schwimmer et al. 1994, compared 2.7 and 2.0 mm screws for fixation of sagittal osteotomies in 10 cadaver mandibles. This study indicates that 2.0 mm diameter screws provide the same degree of stability as 2.7 mm screws. A study from Obeid and Lindqvist (1991) used six dry mandibles to compare 2.0 and 2.7 mm screws positioned in the mandibular ramus. The screws were unroofed and the authors observed that countersinking resulted in 55% of the 2.7 mm screws and 27% of the 2.0 mm screws having no threads engaging the buccal cortex and thus acting as lag screws. The smaller diameter 2.0 mm screws had more threads engaged in cortical bone. On the lingual cortex, although both diameters had good cortical engagement, several 2.7 mm screws were engaging by one thread. Those observations might account for the similar stability results frequently described when comparing 2.0 mm and 2.7 mm screw diameters. Shetty et al. 1996, using reproducible mandible analogs, reported greater stability with 2.4 mm screws than with 2.0 mm screws. Screw strength and mechanical resistance were determined by the strength of the material and core diameter of the screw. The torsional sheer strength was related to the cube of the diameter and tensional strength to the square of the diameter. Small increases in core diameter markedly enhance tensile and torsional strengths (Hughes and Jordan, 1972). In concordance, Maurer et al. (1999) evaluated different bicortical screw configurations and diameters through finiteelement-analysis, and concluded that when bite forces have been applied, the inverted L configuration was the most stable. Also they showed that the 2.0 mm screw diameter can provide sufficient stability at the osteotomy site after SSRO as well the screws with 1.5 mm diameter. Additionally these authors showed the 1.5 mm screws may withstand forces of up to 89.5 N, where the forces exerted during bite do not exceed these values. 5. Conclusion The screws of higher thickness require greater availability of space for its insertion and increase the possibility of torque to the condyle and micro fractures of the bone. This study validates the clinical use of 1.5 mm position screws as fixators for sagittal osteotomies of the mandible, reducing the possibility of injuries to the inferior alveolar nerve and the tactile perception. In conclusion, there was no statistically significant difference between 1.5 and 2.0 mm screws for fixation of SSRO performed in
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synthetic mandibles. There was no fracture of the 1.5 mm diameter screws in any of the tests. The 1.5 mm diameter screws in inverted L have as much stability and mechanical resistance as the 2.0 mm screw, and may be safely used for this procedure. Acknowledgments The authors acknowledge with sincere thanks, the “MDT e Implantes Ortopédicos” by provided free of charge of titanium screws for research. References Anacul 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 50: 1295, 1992 Brasileiro BF, Grempel RG, Ambrosano GMB, Passeri LA: An in vitro evaluation of rigid internal fixation techniques for sagittal split ramus osteotomies: advancement surgery. J Oral Maxillofac Surg 67: 809e817, 2009 Bredbenner TL, Haug RH: Substitutes for human cadaveric bone in maxillofacial rigid fixation research. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 90: 574e580, 2000 Foley WL, Beckman TW: In vitro comparison of screw versus plate fixation in the sagittal split osteotomy. Int J Adult Orthodon Orthognath Surg 7: 147e151, 1992 Foley WL, Frost DE, Paulin WB, Tucker MR: Uniaxial pullout evaluation of internal screw fixation. J Oral Maxillofac Surg 47: 277, 1989 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 Cranio Maxillofac Surg 23: 321, 1995 Haug RH, Barber E, Punjabi AP: An in vitro comparison of the effect of number and pattern of positional screws on load resistance. J Oral Maxillofac Surg 57: 300e308, 1999 Hughes AN, Jordan BA: The mechanical properties of surgical bone screws and some aspects of insertion practice. Br J Accid Surg 4: 15, 1972 Jeter TS, Van Sickels JE, Dolwick MF: Modified techniques for internal fixation of sagittal ramus osteotomias. J Oral Maxillofac Surg 42: 270, 1984
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