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In vitro comparison of biomechanical characteristics of sagittal split osteotomy fixation techniques
Int. J. Oral Maxillofac. Surg. 2006; 35: 837–841 doi:10.1016/j.ijom.2006.03.001, available online at http://www.sciencedirect.com
Research Paper Orthognathic Surgery
In vitro comparison of biomechanical characteristics of sagittal split osteotomy fixation techniques
¨ zden1, A. Alkan1, S. Arici2, B. O E. Erdem3 1 Department of Oral & Maxillofacial Surgery, Faculty of Dentistry, The University of Ondokuz Mayıs, Samsun, Turkey; 2 Department of Orthodontics; Faculty of Dentistry, The University of Ondokuz Mayıs, Samsun, Turkey; 3Department of Oral & Maxillofacial Surgery, Faculty of Dentistry, The University of Ankara, Ankara, Turkey
¨ zden, A. Alkan, S. Arici, E. Erdem: In vitro comparison of biomechanical B. O characteristics of sagittal split osteotomy fixation techniques. Int. J. Oral Maxillofac. Surg. 2006; 35: 837–841. # 2006 International Association of Oral and Maxillofacial Surgeons. Published by Elsevier Ltd. All rights reserved. Abstract. The aim of the present study was to compare the biomechanical stability of 10 different fixation methods used in sagittal split osteotomy. Twenty-five fresh sheep mandibles were stripped of all soft tissues and sectioned at the midline. A sagittal split osteotomy with 5 mm advancement was performed on each hemimandible. The hemimandibles were randomly divided into 10 groups of 5, and then fixed with 5 different bicortical screws, 4 different miniplates with or without bicortical screws, and 1 resorbable screw configuration. All specimens were mounted on a specially designed 3-point biomechanical test model and compression loads were applied using the Lloyd LRX testing machine until 3 mm displacement was reached. Load/displacement data were gathered and compared using the Mann– Whitney U-test with Bonferroni correction (P < 0.01). The 3 bicortical screws in an inverted backward-L pattern provided the most biomechanical stability of the screw patterns tested. The miniplate fixed obliquely with 2 bicortical screws in the proximal segment provided the most biomechanical stability of the miniplate groups.
Skeletal relapse is the most common complication after sagittal split osteotomy (SSO)5,7,15. Authors agree that stability at the osteotomy site is greater with rigid fixation than wire fixation7,18,28. The advantages and disadvantages of using bicortical screws and miniplates for fixation after an SSO procedure have been well documented29,20,22. There are numerous studies1,3,4,6,9,11,22 comparing 0901-5027/090837 + 05 $30.00/0
the in vitro biomechanical performance of SSO fixation techniques, but it is still not certain which technique is the most effective. With biomechanical test models, the main problem is how to imitate the human masticatory muscles when examining the stability of rigid fixation techniques. For this purpose, a 2-point biomechanical test model (a cantilevered beam model) has
Key words: sagittal split osteotomy; rigid fixation; biomechanical stability. Accepted for publication 2 March 2006 Available online 9 May 2006
been used1,19. More recently, a 3-point biomechanical test model was developed as the best to imitate the masticatory muscles, but there is only 1 study published so far using such a model3. The aim of the present study was to compare the biomechanical stability of 10 different fixation methods following SSO using a custom-made 3-point biomechanical test model.
# 2006 International Association of Oral and Maxillofacial Surgeons. Published by Elsevier Ltd. All rights reserved.
838
¨ zden et al. O
Materials and methods
Twenty-five fresh sheep mandibles (from animals with a mean weight of 40 kg, fed with the same diet, collected from the same abattoir and slaughtered in the same way) were used in this study. The mandibles were stripped of their soft tissues and divided along the anterior midline between the central incisors. The specimens were kept moist and refrigerated at 158C until all testing was completed. The coronoid processes were removed from all hemimandibles because they caused problems in placement on the biomechanical test model and changed the biomechanical test results by absorbing the forces applied in the preliminary tests. A sagittal split osteotomy with 5 mm advancement was performed on each hemimandible. A medial osteotomy extending from the mandibular foramen to the mandibular inferior border was performed, and this osteotomy was joined with a buccal vertical osteteomy through the mandibular inferior border, differing from human sagittal split procedures. Impacted molar teeth in the osteotomy site were extracted and irregular bone processes at the bony interface were removed. The hemimandibles were randomly divided into 10 groups of 5 and fixed using 10 different techniques. These fixation groups consisted of 1 bicortical screw (group A) (Fig. 1A), 2 bicortical screws in a vertical pattern (group B) (Fig. 1B), 2 bicortical screws in a linear pattern (group C) (Fig. 1C), 3 bicortical screws in an inverted backward-L pattern (group D) (Fig. 1D), 3 bicortical screws in an inverted-L pattern (group E) (Fig. 1E), miniplate placed horizontally with 4 monocortical screws (group F) (Fig. 1F), miniplate placed obliquely with 2 bicortical screws in the proximal segment (group G) (Fig. 1G), the same as group F but with miniplate placed obliquely (group H) (Fig. 1H), the same as group G but with 1 additional bicortical screw at the inferior border (group I) (Fig. 1I), and 3 resorbable bicortical screws in an inverted backward-L pattern (group J) (Fig. 1J). All screw grooves were drilled with a machine to prevent vibration. Diameters and lengths of screws were 2.0 mm and 17 mm for titanium bicortical screws, 2.0 mm and 5 mm for titanium monocortical screws (Elektron Medikal Tic. A.s¸., Ankara, Turkey), and 2.8 mm and 16 mm for resorbable bicortical screws (Inion ltd, La¨a¨ka¨rinkatu, Tampare, Finland), respectively. Each of the hemimandibles was placed on the 3-point biomechanical test model
Fig. 1. (A) 1 bicortical screw. (B) Two bicortical screws in a vertical pattern. (C) Two bicortical screws in a linear pattern. (D) Three bicortical screws in an inverted backward-L pattern. (E) Three bicortical screws in an inverted-L pattern. (F) Miniplate placed horizontally fixed with 4 monocortical screws. (G) Miniplate placed obliquely fixed with 2 bicortical screws in proximal segment, 2 monocortical screws in distal segment. (H) Miniplate placed obliquely fixed with 4 monocortical screws. (I) Miniplate placed obliquely fixed with 2 bicortical screws in proximal segment and 1 bicortical screw at inferior border. (J) Three resorbable bicortical screws in an inverted backward-L pattern.
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compared using the Mann–Whitney U-test with Bonferroni correction (P < 0.01). Results
Fig. 2. The in vitro 3-point biomechanical test device, representing the biomechanics of the human mandible.
Fig. 3. (a) Diagram of the action of the human mandible (ARMSTRONG et al.3). The load placed on the articulating surface of the mandibular condyle (A) is caused by the action of the major elevator muscles of the mandible (B) acting on the angle of the mandible and coronoid process. (C) The force of a bolus of food on the lower incisors upon mouth closure. These loads transfer to the mandibular body, where the sagittal split osteotomy (D) has been fixed using a miniplate (E). (b) Loads in (a) are transfered to the hemimandible via arms of the 3-point in vitro biomechanical test model.
designed by the authors (Fig. 2), and exposed to compression loads that simulated masticatory loads (Fig. 3a). These loads were applied by the Lloyd LRX
testing machine until reaching 3 mm displacement (Fig. 3b). Load/displacement data were gathered and the means (medians) of 2 groups for all combinations were
Table 1. Mean, median, minimum and maximum loads for each group Group description Mean SD (X¯ S) A B C D E F G H I J
One screw Two screws, vertical Two screws, linear Inverted backward L Inverted L Horizontal miniplate with monocortical screws Oblique miniplate with 2 bicortical, 2 monocortical screws Oblique miniplate with monocortical screws Group G + 1 bicortical screw at inferior border Inverted backward L (resorbable screws)
Group A (1 screw) had the least biomechanical stability statistically (P < 0.01). Group D (3 bicortical screws in an inverted backward-L pattern) had the greatest biomechanical stability of the screw groups (P < 0.01). There was, thus, a significant difference between titanium (group D) and resorbable (group J) screws in an inverted backward-L pattern. The biomechanical stability of group G (obliquely placed miniplate with 2 bicortical screws on proximal segment and 2 monocortical screws on distal segment) was higher than that of the other miniplate groups (P < 0.01), except for group I (miniplate placed obliquely plus 1 bicortical screw at inferior border). Between group D and group I, which were identified as the most stable groups, no significant difference was noted statistically (P < 0.01). There was a significant difference between group D and group G (P < 0.01). Statistical comparison of biomechanical stability between all the groups is shown in Table 1. The median values are shown graphically in Fig. 4. Discussion
SSO has been routinely performed for the treatment of mandibular deformities. Although the osteteomy is standardized, there is no agreement on the procedure for segment fixation, the selection of which depends on the surgeon’s preference. No demonstrated technique has been proved to eliminate relapse completely. As shown by in vitro biomechanical and recent clinical studies, postoperative stability is greater
Median (min., max.)
Difference*
9.56 1.03 497.67 126.21 268.36 56.45 778.48 45.51 495.53 21.29 222.75 44.11
9.62 441.34 269.40 782.00 503.80 238.61
452.31 42.05
442.20 (411.50, 504.51)
A, C, D, F, H, I
283.52 7.34
283.10 (273.40, 293.60
A ,B, D, E, F, G, I, J
788.98 81.52
761.37 (687.76, 890.20)
A, B, C, E, F, G, H, J
510.92 92.08
540.40 (376.80, 589.70
A, C, D, F, H, I
Values are in newtons. * Compared to other groups, Mann–Whitney U-test, P < 0.01.
(8.13, 11.02) (340.95, 654.38) (182.30, 339.80) (721.98, 834.99) (469.09, 521.97) (174.60, 269.00)
B, C, D, E, F, G, H, I, J A, C, D, F, H, I A, B, D, E, G, I, J A, B, C, D, E, F, G, H, J A, C, D, F, H, I A, B, D, E, G, H, I, J
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Fig. 4. Graph of median values for 10 different fixation groups (bcs: bicortical screws, mcs: monocortical screws, mp: miniplate).
with rigid internal fixation techniques7,18,28. Cadavers14,21,22,26, polyurethane8,12, fresh sheep mandibles24,25,27 and bovine and porcine rib3,19 have been used as in vitro models for SSO previously. For experimental studies of osteotomy and rigid internal fixation in the ramus area, the sheep mandible is preferred mainly due to its similarities in format, size and structure to the human mandible, including a similar Haversian system13. In the present study, all distal segments of the hemimandibles were advanced 5 mm, in conformity with previously reported studies on this subject. An in vitro biomechanical testing study by HAUG et al.13 showed that bicortical screws in a triangular pattern provided greater resistance to vertical loads than linear or diagonal patterns when considering clinically significant parameters such as yield load, yield displacement and stiffness. None of these parameters are mentioned here; our results are similar to those of studies that evaluated these parameters under vertical loads creating 3 mm displacement2,9–11,13,17. In the preliminary study with 5 sheep hemimandibles, yield load/ displacement was not seen even with the maximum forces that the Instron machine can apply. Absorption difference between bone and polyurethane material may be the explanation for this result. For this reason, these biomechanical values were excluded from the main study. The mechanical behaviour of the mandible during mastication is still far from being fully understood, in spite of a number of studies that investigated this subject recently. ARMSTRONG et al.3 developed a 3-point biomechanical test model to inves-
tigate the fixation systems that had been used after SSO, and tested it on fresh bovine ribs. The present custom-made 3-point loading model inspired these authors, and was used to imitate the 3 main muscle forces that effect the functioning mandible. These forces are: (1) the load placed on the articulating surface of the mandibular condyle caused by the action of the major elevators of the mandible acting on the angle of the mandible and coronoid process; (2) mouth closure resulting in a load placed on the incisors by a bolus of food and (3) transfer of these loads to the mandibular body. There is still a need for further development of the test model to imitate the action of the masticatory muscles in 3 dimensions, for the complete understanding of the biomechanical effect of fixation methods used after SSO. Rigid internal fixation is preferred for stabilization in SSO because it affords a rapid return to presurgical function, better nutritional support, easier airway maintenance and reduced relapse19,22. This method has, however, been associated with risk of damage to the neurovascular bundle and imprecise condylar positioning. Monocortically fixed miniplates may be preferred because of these disadvantages. Plates are easily placed and removed transorally, and correction of condylar positioning involves the repositioning of only 2 monocortical screws20. In this study, group D (inverted backward-L pattern) and group I (miniplate placed obliquely plus 1 bicortical screw at the mandibular inferior border) were identified as the most effective, and no significant difference was found between
them. This suggests that using a miniplate as well as 3 bicortical screws in an inverted backward-L pattern does not provide any additional stability. In 1999, HAUG et al.13 compared the rigidity of 4 different L-pattern techniques using red oak as a model. The groups consisted of 3 titanium bicortical screws in an inverted-L, backward-L, inverted backward-L and L pattern. Their results showed no significant differences in rigidity between groups. However, the rigidity obtained in the inverted backward-L pattern group was statistically higher than that of the inverted-L pattern group in the present study. It is suggested that stability was decreased by applying 1 bicortical screw above the inferior border far from the osteotomy line. The 1-screw group had the least biomechanical stability as has been estimated before. This group was included in the study to evaluate the need for using miniplates or maxillomandibular fixation technique where it is not possible to place more than one screw. One screw should not be considered adequate for SSO fixation. Two bicortical screws placed in a linear or vertical pattern have been used rarely in the literature16,23. In 1992, FOLEY & BECK9 MAN reported that a single 4-hole miniplate group secured with monocortical screws had more rigidity than two 2.7mm Wurzburg screws placed bicortically, 1 cm apart, parallel to the occlusal plane. SHUFFORD et al.23 and MARCHETTI et al.16 obtained sufficient rigidity using 2.0-mm bicortical screws in a linear pattern clinically. In the present study, there were no significant differences between oblique or horizontal miniplates and the group with two 2.0-mm bicortical screws in a linear pattern, and the rigidity obtained in the group with two 2.0-mm bicortical screws in a vertical pattern was greater than that in the group with two 2.0-mm bicortical screws in a linear pattern. These results reveal that greater rigidity was obtained by applying 1 bicortical screw on the inferior instead of replacing both of the screws on the upper border of the mandible. This investigation showed that there was a statistically significant difference in rigidity between miniplates placed obliquely and horizontally with monocortical screws. It is tentatively claimed that greater biomechanical stability is obtained with a miniplate placed obliquely than horizontally. Investigation of this subject is lacking in the literature. The biomechanical stability of miniplates placed obliquely with 2 bicortical screws in proximal segments was higher than those placed horizontally or obliquely with 4 monocortical screws. In
Sagittal split osteotomy fixation techniques the light of these results, sufficient rigidity to eliminate relapse can be obtained with a miniplate placed obliquely with 2 bicortical screws in proximal segments. In an in vitro study, GOMES et al.11 compared 3 titanium bicortical screws and 3 resorbable bicortical screws in an inverted backward-L pattern using fresh sheep hemimandibles as a test model. They stated that there were no significant differences between these groups, in contrast with the present results. In conclusion, this experiment established that 3 bicortical screws in an inverted backward-L pattern provide the greatest mechanical advantage. This pattern also had greater resistance to compression loads than resorbable screws in the same pattern. A miniplate placed obliquely and fixed with 2 bicortical screws in the proximal segment may be an alternative method when 1 bicortical screw cannot be placed on the inferior border of the mandible or the inferior alveolar nerve is thought to be damaged. Acknowledgement. We thank Prof. Yu¨ksel Bek, University of Ondokuz Mayis, Faculty of Medicine Department of Biostatistics for his help with the statistics.
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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. Ardary WC, Tracy DJ, Brownridge II GW, Urata MM. Comparative evaluation of screw configuration on the stability of the sagittal split osteotomy. Oral Surg Oral Med Oral Pathol 1989: 68: 125–129. 3. Armstrong JEA, Lapointe HJ, Hogg NJV, Kwok AD. Preliminary investigation of the biomechanics of internal fixation of sagittal split osteotomies with miniplates using a newly designed in vitro testing model. J Oral Maxillofac Surg 2001: 59: 191–195. 4. Blomqvist JE, Ahlborg G, Isaksson S, Svartz K. A comparison of skeletal stability after mandibular advancement and use of two rigid internal fixation techniques. J Oral Maxillofac Surg 1997: 55: 568–574 (discussion 574–575). 5. Borstlap WA, Stoelinga PJW, Hoppenreijs MA, Van’t Hof MA. Stabilization of sagittal split advancement osteotomies with miniplates: a prospective, multicentre study with two-year follow-up. Part III Condylar remodelling
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and resorption. Int J Oral Maxillofac Surg 2004: 33: 649–655. Dolanmaz D, Uckan S, Isik K, Saglam H. Comparison of stability of absorbable and titanium plate and screw fixation for sagittal split ramus osteotomy. Br J Oral Maxillofac Surg 2004: 42: 127–132. Dolce C, Van Sickels JE, Bays RE, Rugh JD. Skeletal stability after mandibular advancement with rigid versus wire fixation. J Oral Maxillofac Surg 2000: 58: 1219–1227. Eppley BL, Sarver D, Pietrzak B. Biomechanical testing of resorbable screws used for mandibular sagittal split osteotomies. J Oral Maxillofac Surg 1999: 57: 1431–1435. Foley WL, Beckman TW. In vitro comparison of screw versus plate fixation in the sagittal split osteotomy. Int J Adult Orthod Orthognath Surg 1992: 7: 147– 151. Foley WL, Frost DE, Paulin WB. Internal screw fixation: comparison of placement pattern and rigidity. J Oral Maxillofac Surg 1989: 47: 720–723. Gomes PP, Filho RG, Mazzonetto R. Evaluation of the bending strength of rigid internal fixation with absorbable and metallic screws in mandibular ramus sagittal split osteotomy—in vitro study. Pesqui Odontol Bras 2003: 17: 267–272. Hammer B, Etlin 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. 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 1999: 57: 300–308. Kim HC, Essaki S, Kameyama T. Comparison of screw placement patterns on the rigidity of the sagittal split ramus osteotomy: technical note. J Craniomaxillofac Surg 1995: 23: 54–56. Kobayashi T, Honma K, Hamamoto S, Shingaki S, Hanada K, Nakajima T. Effects of wire and miniplate fixation on mandibular stability and TMJ symptoms following orthognathic surgery. Clin Orthod Res 2000: 3: 155–161. Marchetti C, Gentile L, Bianchi A, Bassi M. Semirigid fixation of the mandible in bimaxillary orthognathic surgery: stability after 18 months. Int J Adult Orthod Orthognath Surg 1999: 14: 37–45. 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. Moenning JE, Bussard DA, Lapp TH, Garrison BT. A comparison of relapse in bilateral sagittal split osteotomies for mandibular advancement: rigid internal fixation (screws) versus inferior border
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wiring with anterior skeletal fixation. Int J Adult Orthod Orthognath Surg 1990: 3: 175–182. 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. Rubens B, Stoelinga P, Blijdorp P, Schoenaers J, Politis C. Skeletal stability following sagittal split osteotomy using monocortical miniplate internal fixation. Int J Oral Maxillofac Surg 1988: 17: 371–376. Schwimmer A, Greeberg 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. Shetty V, Freymiller E, Mcbrearty D, Caputo AA. Functional stability of sagittal split ramus osteotomies. J Oral Maxillofac Surg 1996: 54: 601–609. Shufford EL, Colonel L, Kraut RA. Passive rigid fixation of sagittal split osteotomy. Oral Surg Oral Med Oral Pathol 1989: 68: 150–153. Suuronen R, Laine P, Lindqvist C. Sagittal osteotomies fixed with biodegradable screws: a preliminary report. J Oral Maxillofac Surg 1994: 52: 715–720. Suuronen R, Manninen MJ, Pohjonen T, Laitinen O, Lindqvist C. Mandibular osteotomy fixed with biodegradable plates and screws: an animal study. Br J Oral Maxillofac Surg 1997: 35: 341–348. Tharanon W. Comparison between the rigidity of bicortical screws and a miniplate for fixation of a mandibular setback after a simulated bilateral sagittal split osteotomy. J Oral Maxillofac Surg 1998: 56: 1055–1058. Uckan S, Schwimmer A, Kummer F, Greenberg AM. Effect on the angle of the screw on the stability of the mandibular sagittal split osteotomy: a study in sheep mandibles. Br J Oral Maxillofac Surg 2001: 39: 266–268. Van Sickels JE, Larsen AJ, Thrash WJ. Relaps after rigid fixation of mandibular advancement. J Oral Maxillofac Surg 1986: 44: 698. Ward-Booth P, Schendel SA, Hausamen JE. Maxillofacial Surgery, Vol. I. London: Churchill Livingstone 2001: 57– 75.
Address: ¨ zden Bora O University of Ondokuz Mayis Faculty of Dentistry Department of Oral and Maxillofacial Surgery Kurupelit/Samsun 55139 Turkey Tel: +90 362 3121919/3480 Fax: +90 362 4576032 E-mail: [email protected]
Research Paper Orthognathic Surgery
In vitro comparison of biomechanical characteristics of sagittal split osteotomy fixation techniques
¨ zden1, A. Alkan1, S. Arici2, B. O E. Erdem3 1 Department of Oral & Maxillofacial Surgery, Faculty of Dentistry, The University of Ondokuz Mayıs, Samsun, Turkey; 2 Department of Orthodontics; Faculty of Dentistry, The University of Ondokuz Mayıs, Samsun, Turkey; 3Department of Oral & Maxillofacial Surgery, Faculty of Dentistry, The University of Ankara, Ankara, Turkey
¨ zden, A. Alkan, S. Arici, E. Erdem: In vitro comparison of biomechanical B. O characteristics of sagittal split osteotomy fixation techniques. Int. J. Oral Maxillofac. Surg. 2006; 35: 837–841. # 2006 International Association of Oral and Maxillofacial Surgeons. Published by Elsevier Ltd. All rights reserved. Abstract. The aim of the present study was to compare the biomechanical stability of 10 different fixation methods used in sagittal split osteotomy. Twenty-five fresh sheep mandibles were stripped of all soft tissues and sectioned at the midline. A sagittal split osteotomy with 5 mm advancement was performed on each hemimandible. The hemimandibles were randomly divided into 10 groups of 5, and then fixed with 5 different bicortical screws, 4 different miniplates with or without bicortical screws, and 1 resorbable screw configuration. All specimens were mounted on a specially designed 3-point biomechanical test model and compression loads were applied using the Lloyd LRX testing machine until 3 mm displacement was reached. Load/displacement data were gathered and compared using the Mann– Whitney U-test with Bonferroni correction (P < 0.01). The 3 bicortical screws in an inverted backward-L pattern provided the most biomechanical stability of the screw patterns tested. The miniplate fixed obliquely with 2 bicortical screws in the proximal segment provided the most biomechanical stability of the miniplate groups.
Skeletal relapse is the most common complication after sagittal split osteotomy (SSO)5,7,15. Authors agree that stability at the osteotomy site is greater with rigid fixation than wire fixation7,18,28. The advantages and disadvantages of using bicortical screws and miniplates for fixation after an SSO procedure have been well documented29,20,22. There are numerous studies1,3,4,6,9,11,22 comparing 0901-5027/090837 + 05 $30.00/0
the in vitro biomechanical performance of SSO fixation techniques, but it is still not certain which technique is the most effective. With biomechanical test models, the main problem is how to imitate the human masticatory muscles when examining the stability of rigid fixation techniques. For this purpose, a 2-point biomechanical test model (a cantilevered beam model) has
Key words: sagittal split osteotomy; rigid fixation; biomechanical stability. Accepted for publication 2 March 2006 Available online 9 May 2006
been used1,19. More recently, a 3-point biomechanical test model was developed as the best to imitate the masticatory muscles, but there is only 1 study published so far using such a model3. The aim of the present study was to compare the biomechanical stability of 10 different fixation methods following SSO using a custom-made 3-point biomechanical test model.
# 2006 International Association of Oral and Maxillofacial Surgeons. Published by Elsevier Ltd. All rights reserved.
838
¨ zden et al. O
Materials and methods
Twenty-five fresh sheep mandibles (from animals with a mean weight of 40 kg, fed with the same diet, collected from the same abattoir and slaughtered in the same way) were used in this study. The mandibles were stripped of their soft tissues and divided along the anterior midline between the central incisors. The specimens were kept moist and refrigerated at 158C until all testing was completed. The coronoid processes were removed from all hemimandibles because they caused problems in placement on the biomechanical test model and changed the biomechanical test results by absorbing the forces applied in the preliminary tests. A sagittal split osteotomy with 5 mm advancement was performed on each hemimandible. A medial osteotomy extending from the mandibular foramen to the mandibular inferior border was performed, and this osteotomy was joined with a buccal vertical osteteomy through the mandibular inferior border, differing from human sagittal split procedures. Impacted molar teeth in the osteotomy site were extracted and irregular bone processes at the bony interface were removed. The hemimandibles were randomly divided into 10 groups of 5 and fixed using 10 different techniques. These fixation groups consisted of 1 bicortical screw (group A) (Fig. 1A), 2 bicortical screws in a vertical pattern (group B) (Fig. 1B), 2 bicortical screws in a linear pattern (group C) (Fig. 1C), 3 bicortical screws in an inverted backward-L pattern (group D) (Fig. 1D), 3 bicortical screws in an inverted-L pattern (group E) (Fig. 1E), miniplate placed horizontally with 4 monocortical screws (group F) (Fig. 1F), miniplate placed obliquely with 2 bicortical screws in the proximal segment (group G) (Fig. 1G), the same as group F but with miniplate placed obliquely (group H) (Fig. 1H), the same as group G but with 1 additional bicortical screw at the inferior border (group I) (Fig. 1I), and 3 resorbable bicortical screws in an inverted backward-L pattern (group J) (Fig. 1J). All screw grooves were drilled with a machine to prevent vibration. Diameters and lengths of screws were 2.0 mm and 17 mm for titanium bicortical screws, 2.0 mm and 5 mm for titanium monocortical screws (Elektron Medikal Tic. A.s¸., Ankara, Turkey), and 2.8 mm and 16 mm for resorbable bicortical screws (Inion ltd, La¨a¨ka¨rinkatu, Tampare, Finland), respectively. Each of the hemimandibles was placed on the 3-point biomechanical test model
Fig. 1. (A) 1 bicortical screw. (B) Two bicortical screws in a vertical pattern. (C) Two bicortical screws in a linear pattern. (D) Three bicortical screws in an inverted backward-L pattern. (E) Three bicortical screws in an inverted-L pattern. (F) Miniplate placed horizontally fixed with 4 monocortical screws. (G) Miniplate placed obliquely fixed with 2 bicortical screws in proximal segment, 2 monocortical screws in distal segment. (H) Miniplate placed obliquely fixed with 4 monocortical screws. (I) Miniplate placed obliquely fixed with 2 bicortical screws in proximal segment and 1 bicortical screw at inferior border. (J) Three resorbable bicortical screws in an inverted backward-L pattern.
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compared using the Mann–Whitney U-test with Bonferroni correction (P < 0.01). Results
Fig. 2. The in vitro 3-point biomechanical test device, representing the biomechanics of the human mandible.
Fig. 3. (a) Diagram of the action of the human mandible (ARMSTRONG et al.3). The load placed on the articulating surface of the mandibular condyle (A) is caused by the action of the major elevator muscles of the mandible (B) acting on the angle of the mandible and coronoid process. (C) The force of a bolus of food on the lower incisors upon mouth closure. These loads transfer to the mandibular body, where the sagittal split osteotomy (D) has been fixed using a miniplate (E). (b) Loads in (a) are transfered to the hemimandible via arms of the 3-point in vitro biomechanical test model.
designed by the authors (Fig. 2), and exposed to compression loads that simulated masticatory loads (Fig. 3a). These loads were applied by the Lloyd LRX
testing machine until reaching 3 mm displacement (Fig. 3b). Load/displacement data were gathered and the means (medians) of 2 groups for all combinations were
Table 1. Mean, median, minimum and maximum loads for each group Group description Mean SD (X¯ S) A B C D E F G H I J
One screw Two screws, vertical Two screws, linear Inverted backward L Inverted L Horizontal miniplate with monocortical screws Oblique miniplate with 2 bicortical, 2 monocortical screws Oblique miniplate with monocortical screws Group G + 1 bicortical screw at inferior border Inverted backward L (resorbable screws)
Group A (1 screw) had the least biomechanical stability statistically (P < 0.01). Group D (3 bicortical screws in an inverted backward-L pattern) had the greatest biomechanical stability of the screw groups (P < 0.01). There was, thus, a significant difference between titanium (group D) and resorbable (group J) screws in an inverted backward-L pattern. The biomechanical stability of group G (obliquely placed miniplate with 2 bicortical screws on proximal segment and 2 monocortical screws on distal segment) was higher than that of the other miniplate groups (P < 0.01), except for group I (miniplate placed obliquely plus 1 bicortical screw at inferior border). Between group D and group I, which were identified as the most stable groups, no significant difference was noted statistically (P < 0.01). There was a significant difference between group D and group G (P < 0.01). Statistical comparison of biomechanical stability between all the groups is shown in Table 1. The median values are shown graphically in Fig. 4. Discussion
SSO has been routinely performed for the treatment of mandibular deformities. Although the osteteomy is standardized, there is no agreement on the procedure for segment fixation, the selection of which depends on the surgeon’s preference. No demonstrated technique has been proved to eliminate relapse completely. As shown by in vitro biomechanical and recent clinical studies, postoperative stability is greater
Median (min., max.)
Difference*
9.56 1.03 497.67 126.21 268.36 56.45 778.48 45.51 495.53 21.29 222.75 44.11
9.62 441.34 269.40 782.00 503.80 238.61
452.31 42.05
442.20 (411.50, 504.51)
A, C, D, F, H, I
283.52 7.34
283.10 (273.40, 293.60
A ,B, D, E, F, G, I, J
788.98 81.52
761.37 (687.76, 890.20)
A, B, C, E, F, G, H, J
510.92 92.08
540.40 (376.80, 589.70
A, C, D, F, H, I
Values are in newtons. * Compared to other groups, Mann–Whitney U-test, P < 0.01.
(8.13, 11.02) (340.95, 654.38) (182.30, 339.80) (721.98, 834.99) (469.09, 521.97) (174.60, 269.00)
B, C, D, E, F, G, H, I, J A, C, D, F, H, I A, B, D, E, G, I, J A, B, C, D, E, F, G, H, J A, C, D, F, H, I A, B, D, E, G, H, I, J
840
¨ zden et al. O
Fig. 4. Graph of median values for 10 different fixation groups (bcs: bicortical screws, mcs: monocortical screws, mp: miniplate).
with rigid internal fixation techniques7,18,28. Cadavers14,21,22,26, polyurethane8,12, fresh sheep mandibles24,25,27 and bovine and porcine rib3,19 have been used as in vitro models for SSO previously. For experimental studies of osteotomy and rigid internal fixation in the ramus area, the sheep mandible is preferred mainly due to its similarities in format, size and structure to the human mandible, including a similar Haversian system13. In the present study, all distal segments of the hemimandibles were advanced 5 mm, in conformity with previously reported studies on this subject. An in vitro biomechanical testing study by HAUG et al.13 showed that bicortical screws in a triangular pattern provided greater resistance to vertical loads than linear or diagonal patterns when considering clinically significant parameters such as yield load, yield displacement and stiffness. None of these parameters are mentioned here; our results are similar to those of studies that evaluated these parameters under vertical loads creating 3 mm displacement2,9–11,13,17. In the preliminary study with 5 sheep hemimandibles, yield load/ displacement was not seen even with the maximum forces that the Instron machine can apply. Absorption difference between bone and polyurethane material may be the explanation for this result. For this reason, these biomechanical values were excluded from the main study. The mechanical behaviour of the mandible during mastication is still far from being fully understood, in spite of a number of studies that investigated this subject recently. ARMSTRONG et al.3 developed a 3-point biomechanical test model to inves-
tigate the fixation systems that had been used after SSO, and tested it on fresh bovine ribs. The present custom-made 3-point loading model inspired these authors, and was used to imitate the 3 main muscle forces that effect the functioning mandible. These forces are: (1) the load placed on the articulating surface of the mandibular condyle caused by the action of the major elevators of the mandible acting on the angle of the mandible and coronoid process; (2) mouth closure resulting in a load placed on the incisors by a bolus of food and (3) transfer of these loads to the mandibular body. There is still a need for further development of the test model to imitate the action of the masticatory muscles in 3 dimensions, for the complete understanding of the biomechanical effect of fixation methods used after SSO. Rigid internal fixation is preferred for stabilization in SSO because it affords a rapid return to presurgical function, better nutritional support, easier airway maintenance and reduced relapse19,22. This method has, however, been associated with risk of damage to the neurovascular bundle and imprecise condylar positioning. Monocortically fixed miniplates may be preferred because of these disadvantages. Plates are easily placed and removed transorally, and correction of condylar positioning involves the repositioning of only 2 monocortical screws20. In this study, group D (inverted backward-L pattern) and group I (miniplate placed obliquely plus 1 bicortical screw at the mandibular inferior border) were identified as the most effective, and no significant difference was found between
them. This suggests that using a miniplate as well as 3 bicortical screws in an inverted backward-L pattern does not provide any additional stability. In 1999, HAUG et al.13 compared the rigidity of 4 different L-pattern techniques using red oak as a model. The groups consisted of 3 titanium bicortical screws in an inverted-L, backward-L, inverted backward-L and L pattern. Their results showed no significant differences in rigidity between groups. However, the rigidity obtained in the inverted backward-L pattern group was statistically higher than that of the inverted-L pattern group in the present study. It is suggested that stability was decreased by applying 1 bicortical screw above the inferior border far from the osteotomy line. The 1-screw group had the least biomechanical stability as has been estimated before. This group was included in the study to evaluate the need for using miniplates or maxillomandibular fixation technique where it is not possible to place more than one screw. One screw should not be considered adequate for SSO fixation. Two bicortical screws placed in a linear or vertical pattern have been used rarely in the literature16,23. In 1992, FOLEY & BECK9 MAN reported that a single 4-hole miniplate group secured with monocortical screws had more rigidity than two 2.7mm Wurzburg screws placed bicortically, 1 cm apart, parallel to the occlusal plane. SHUFFORD et al.23 and MARCHETTI et al.16 obtained sufficient rigidity using 2.0-mm bicortical screws in a linear pattern clinically. In the present study, there were no significant differences between oblique or horizontal miniplates and the group with two 2.0-mm bicortical screws in a linear pattern, and the rigidity obtained in the group with two 2.0-mm bicortical screws in a vertical pattern was greater than that in the group with two 2.0-mm bicortical screws in a linear pattern. These results reveal that greater rigidity was obtained by applying 1 bicortical screw on the inferior instead of replacing both of the screws on the upper border of the mandible. This investigation showed that there was a statistically significant difference in rigidity between miniplates placed obliquely and horizontally with monocortical screws. It is tentatively claimed that greater biomechanical stability is obtained with a miniplate placed obliquely than horizontally. Investigation of this subject is lacking in the literature. The biomechanical stability of miniplates placed obliquely with 2 bicortical screws in proximal segments was higher than those placed horizontally or obliquely with 4 monocortical screws. In
Sagittal split osteotomy fixation techniques the light of these results, sufficient rigidity to eliminate relapse can be obtained with a miniplate placed obliquely with 2 bicortical screws in proximal segments. In an in vitro study, GOMES et al.11 compared 3 titanium bicortical screws and 3 resorbable bicortical screws in an inverted backward-L pattern using fresh sheep hemimandibles as a test model. They stated that there were no significant differences between these groups, in contrast with the present results. In conclusion, this experiment established that 3 bicortical screws in an inverted backward-L pattern provide the greatest mechanical advantage. This pattern also had greater resistance to compression loads than resorbable screws in the same pattern. A miniplate placed obliquely and fixed with 2 bicortical screws in the proximal segment may be an alternative method when 1 bicortical screw cannot be placed on the inferior border of the mandible or the inferior alveolar nerve is thought to be damaged. Acknowledgement. We thank Prof. Yu¨ksel Bek, University of Ondokuz Mayis, Faculty of Medicine Department of Biostatistics for his help with the statistics.
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Address: ¨ zden Bora O University of Ondokuz Mayis Faculty of Dentistry Department of Oral and Maxillofacial Surgery Kurupelit/Samsun 55139 Turkey Tel: +90 362 3121919/3480 Fax: +90 362 4576032 E-mail: [email protected]