Acessibilidade / Reportar erro

Effects of temporal muscle detachment and coronoidotomy on facial growth in young rats

Abstract

This study analyzed the effects of unilateral detachment of the temporal muscle and coronoidotomy on facial growth in young rats. Thirty one-month-old Wistar rats were distributed into three groups: detachment, coronoidotomy and sham-operated. Under general anesthesia, unilateral detachment of the temporal muscle was performed for the detachment group, unilateral coronoidotomy was performed for the coronoidotomy group, and only surgical access was performed for the sham-operated group. The animals were sacrificed at three months of age. Their soft tissues were removed, and the mandible was disarticulated. Radiographic projections-axial views of the skulls and lateral views of hemimandibles-were taken. Cephalometric evaluations were performed, and the values obtained were submitted to statistical analyses. There was a significant homolateral difference in the length of the premaxilla, height of the mandibular ramus and body, and the length of the mandible in all three groups. However, comparisons among the groups revealed no significant differences between the detachment and coronoidotomy groups for most measurements. It was concluded that both experimental detachment of the temporal muscle and coronoidotomy during the growth period in rats induced asymmetry of the mandible and affected the premaxilla.

Temporal Muscle; Mandible; Maxillofacial Development; Growth and Development


ORAL SURGERY

Effects of temporal muscle detachment and coronoidotomy on facial growth in young rats

Fernanda Engelberg Fernandes GomesI; Rogério Bonfante MoraesII; João Gualberto de Cerqueira LuzIII

ISchool of Dentistry, Universidade de São Paulo - USP, São Paulo, SP, Brazil

IIGraduate Program in Dental Sciences, School of Dentistry, Universidade de São Paulo - USP, São Paulo, SP, Brazil

IIIDepartment of Maxillofacial Surgery, Prosthodontics and Traumatology, School of Dentistry, Universidade de São Paulo - USP, São Paulo, SP, Brazil

Corresponding Author Corresponding Author: João Gualberto de Cerqueira Luz E-mail: jgcluz@usp.br

ABSTRACT

This study analyzed the effects of unilateral detachment of the temporal muscle and coronoidotomy on facial growth in young rats. Thirty one-month-old Wistar rats were distributed into three groups: detachment, coronoidotomy and sham-operated. Under general anesthesia, unilateral detachment of the temporal muscle was performed for the detachment group, unilateral coronoidotomy was performed for the coronoidotomy group, and only surgical access was performed for the sham-operated group. The animals were sacrificed at three months of age. Their soft tissues were removed, and the mandible was disarticulated. Radiographic projections—axial views of the skulls and lateral views of hemimandibles—were taken. Cephalometric evaluations were performed, and the values obtained were submitted to statistical analyses. There was a significant homolateral difference in the length of the premaxilla, height of the mandibular ramus and body, and the length of the mandible in all three groups. However, comparisons among the groups revealed no significant differences between the detachment and coronoidotomy groups for most measurements. It was concluded that both experimental detachment of the temporal muscle and coronoidotomy during the growth period in rats induced asymmetry of the mandible and affected the premaxilla.

Descriptors: Temporal Muscle; Mandible; Maxillofacial Development; Growth and Development.

Introduction

The coronoid process of the mandible is the insertion site of the temporal muscle. Any disturbance of this site can potentially affect mandibular and facial growth. Mandibular growth has been associated with a primary growth center represented by the mandibular condyle and with a secondary growth center influenced by the action of the masticatory muscles.1 An injury to these structures may have adverse effects on mandibular growth.2,3

Experimental studies have demonstrated that temporal muscle detachment during a growth period leads to a reduction in the size of the coronoid process itself.4 The anatomy of the coronoid process reflects the relative development of the temporal muscle.5 Additionally, in growing rats, bilateral resections of the masseter muscle induced an inferior mandibular rotational pattern, while bilateral resections of the temporal muscle induced a superior rotational pattern.6 However, it is known that when the temporal muscle is removed from its cranial origin, no coronoid process changes take place. This is explained by the maintenance of an intact blood supply to this structure.7

Hypertrophy of the coronoid process is associated with limitations in mandibular movement, and coronoidectomy is considered the treatment of choice.8,9 However, coronoidotomy has been recommended because it causes less surgical trauma and less postoperative fibrosis and because the sectioned part of the coronoid heals onto the mandibular ramus in a posterior position.10 An experimental study in rats demonstrated that after coronoidotomy, the coronoid process readily reattaches to the mandibular ramus by callus formation.11 However, no studies have examined the effects of coronoidotomy on facial growth in experimental animals.

Some clinical situations involve the coronoid process or the temporal muscle insertion. Fractures of the coronoid process are relatively rare and are generally caused by an indirect mechanism.12 The possibility of reflex fractures has been reported.13 In general, conservative management allows the symptoms to resolve.12,13 Furthermore, surgical procedures in the mandibular ramus region for access to neoplasms and for reconstructions frequently require detachment of the masseter and medial pterygoid muscles and often the temporal muscle.14 However, the role of the temporal muscle in facial bone growth is not well understood.

The purpose of this study was to analyze the effects of unilateral detachment of the temporal muscle and coronoidotomy on facial growth in young rats.

Methodology

The study animals were 30 one-month-old female Wistar rats. All the animals were fed an ordinary diet of rodent feed (Labina, Agribands Purina, Paulínia, Brazil) and water. They were distributed into three groups:

  • detachment (n = 10),

  • coronoidotomy (n = 10) and

  • sham-operated (n = 10).

The study was approved by the local research ethics committee.

Under general anesthesia induced by 10 mg/kg body weight of xylazine hydrochloride (Rompum, Bayer, Porto Alegre, Brazil) and 25 mg/kg body weight of ketamine hydrochloride (Dopalen, Vetbrands, Paulínia, Brazil), a single dose of 16,000 UI of benzylpenicillin (Benzetacil, Fontoura-Wieth, Itapevi, Brazil) was given, and the right side was shaved and cleansed with a povidone-iodine solution (Riodeine, Rioquímica, São José do Rio Preto, Brazil). A 1-cm preauricular incision was made and followed by blunt dissection through the masseter muscle just below the zygomatic arch, and the lateral surface of the mandibular ramus was exposed. The detachment group underwent complete detachment of the temporal muscle from the coronoid process. In the coronoidotomy group, the coronoid process was sectioned using a surgical bur. The sham-operated group was submitted to exposure of the mandibular ramus. The procedures were concluded by suturing in layers. The animals were sacrificed at three months of age, and their heads and mandibles were carefully macerated. Following formalin fixation, radiographs with axial projections of the skull and lateral projections of the hemimandibles were obtained. These were taken with a dental machine (Spectro II, Dabi-Atlante, Ribeirão Preto, Brazil) at 56 kV and 10 mA, with an exposure time of 0.4 s for the skulls and 0.3 s for the hemimandibles. Periapical films were used (Ektaspeed Plus, Eastman Kodak, Rochester, USA).

The radiographs were subjected to a computerized cephalometric evaluation and were digitized using an optical reader (Fotovix II, Tamron Co, Tokyo, Japan). Measurements were obtained with Imagelab software (Softium Informática, São Paulo, Brazil). Using skull radiographs, the following distances were measured bilaterally:

  • tympanic bulla to the mesial root of the first molar (TB-MR),

  • tympanic bulla to infraorbital foramen (TB-IF) and

  • infraorbital foramen to incisal point (IF-IP) (Figure 1).


On the radiographs of the hemimandibles, the following distances were measured bilaterally:

  • condylar process to angular process (CP-AP),

  • distal face of the third molar to antegonial notch (TM-AN),

  • lower insertion of incisor to angular process (II-AP) and

  • incisor apex to condylar process (IA-CP) (Figure 2).


To evaluate the differences between the mean values for the right and left sides in each group, the paired student's t-test was used, while analysis of variance (ANOVA) and Tukey's tests were used for the mean values of the three groups. Statistical Package for the Social Sciences (SPSS) Version 16.0 software was used to conduct the analyses (SPSS Incorporated, Chicago, USA). The level of significance was set at 5% (p < 0.050).

Results

Macroscopic examination of the specimens revealed facial asymmetry with deviation of the mandible to the right side in the detachment and coronoidotomy groups and discrete asymmetry in the sham-operated group. The main macroscopic findings for the macerated specimens are shown in Figure 3.


The mean values of the distances found on the axial radiographs of the skulls are presented in Table 1. There was a significant difference between sides in the following measurements:

  • TB-IF in the detachment and sham-operated groups and

  • IF-IP in the detachment and coronoidotomy groups.

Comparing the maxillary measurements among groups, the ANOVA demonstrated that there were differences in all measurements except the IF-IP on the left side (Table 2). According to Tukey's test, there were no significant differences between the detachment and coronoidotomy groups, but there were significant differences between these groups and the sham-operated group.

The mean values of the distances found on the lateral radiographs of the hemimandibles are presented in Table 3. There was a significant difference between sides for all measurements except II-AP.

Comparing the mandibular measurements among groups, the ANOVA demonstrated that there were differences in all measurements on the right side (Table 4). Tukey's test showed no significant differences between detachment and coronoidotomy groups, but there were significant differences between these groups and the sham-operated group for most measurements. Only the IA-CP measurement showed a significant difference between the detachment and coronoidotomy groups.

Discussion

The effects of temporal muscle detachment or coronoidotomy on the growth of the maxilla and mandible were analyzed in young rats. Thus, when the animals reached adult age, radiographic projections were obtained and used to perform cephalometry on a computer system. Through statistical analyses, it could be seen that changes occurred in both the mandible and the maxilla. The final result was shortening of the maxilla and asymmetry of the mandible associated with either homolateral detachment of the temporal muscle or coronoidotomy.

Mandibular deformities comprising a reduction in size of the coronoid process have been related to disinsertion of the temporal muscle.4 However, the present study detected actual asymmetry of the mandible, likely because of the refined measurements taken and the statistical analyses used. Gross modifications of the mandible are reported after temporal muscle resection in young rats, supporting the functional matrix theory of mandibular growth.5,15 Another significant finding was shortening of the maxilla. The possibility of maxillary growth influencing mandibular growth and vice versa by occlusal intercuspation has been described.1,16 Occlusal disturbances have been described as the main cause of asymmetry of the maxilla after experimental fractures17 and as a complication of condylar fractures.18

Cephalometric evaluations based on radiographs of the skulls and hemimandibles of dissected specimens using a computerized system lead to reliable measurements.19 The distances in this study were similar to those in other studies.17,19,20 A new distance—incisor apex to condylar process—was used to improve the evaluation of the mandibular ramus, and this measurement demonstrated the negative influence of detachment. Mandibular distances between sides were significantly different in all the groups, but the comparison among groups demonstrated no significant differences between the detachment and coronoidotomy groups, both of which were significantly different from sham-operated group.

The effects of coronoidotomy were similar to those of temporal muscle detachment. After coronoidotomy, the sectioned part of the coronoid heals onto mandibular ramus by callus formation.10,11 The effects of muscle reattachment allow new bone formation to envelop the reorganizing tendon,21 and this may have influenced the similar results for the detachment and coronoidotomy groups. To the best of our knowledge, no previous studies in the literature have evaluated this aspect.

It has been reported that the effects of masticatory muscle action on mandibular growth in the angular and condylar processes in rats is intense.22 Similar findings have been reported for pigs, with bone apposition on the posterior, inferior and lateral borders of the mandible.23 In sum, studies have demonstrated that the lateral pterygoid muscle has an effect on the growth of the condylar process,24 the masseter muscle has an effect on the angular process,16 and the temporal muscle has an effect on the growth of both the coronoid process5 and the entire mandibular ramus and body, as demonstrated.

Conclusion

Temporal muscle detachment and coronoidotomy in young rats had the same effects on facial growth. Asymmetry of the mandible was induced and led to shortening of the premaxilla.

Received for publication on Feb 06, 2012

Accepted for publication on May 08, 2012

Declaration of Interests: The authors certify that they have no commercial or associative interest that represents a conflict of interest in connection with the manuscript.

  • 1. Tsolakis AI, Spyropoulos MN, Katsavrias E, Alexandridis K. Effects of altered mandibular function on mandibular growth after condylectomy. Eur J Orthod. 1997 Feb;19(1):9-19.
  • 2. Luz JGC, Araújo VC. Rotated subcondilar process facture in the growing animal: an experimental study in rats. Int J Oral Maxillofac Surg. 2001Dec;30(6):545-9.
  • 3. Cruz DZ, Rodrigues L, Luz JGC. Effects of detachment and repositioning of the medial pterygoid muscle on the growth of the maxilla and mandible of young rats. Acta Cir Bras. 2009 Mar-Apr;24(2):93-7.
  • 4. Rahme J, el-Danaf A, Chassagne JF, Maxant P, Stricker M, Flot F. [The influence of the masticatory muscles on craniofacial growth. A microsurgical study in the rat]. Rev Stomatol Chir Maxillofac. 1987;88(2):108-15. French.
  • 5. Mavropoulos A, Bresin A, Kiliaridis S. Morphometric analysis of the mandible in growing rats with different masticatory functional demands; adaptation to an upper posterior bite block. Eur J Oral Sci. 2004 Jun; 112(3):259-66.
  • 6. Navarro M, Delgado E, Monje F. Changes in mandibular rotation after muscular resection. Experimental study in rats. Am J Orthod Dentofacial Orthop. 1995 Oct;108(4):367-79.
  • 7. Carter GM, Harkness EM. Alterations to mandibular form following motor denervation of the masseter muscle. An experimental study in the rat. J Anat. 1995 Jun;186(Pt 3):541-8.
  • 8. Hall RE, Orbach S, Landesberg R. Bilateral hyperplasia of the mandibular coronoid processes: A report of two cases. Oral Surg Oral Med Oral Pathol. 1989 Feb;67(2):141-5.
  • 9. Kai S, Hijiya T, Yamane K, Higuchi Y. Open-mouth locking caused by unilateral elongated coronoid process: report of case. J Oral Maxillofac Surg. 1997 Nov;55(11):1305-8.
  • 10. Gerbino G, Bianchi SD, Bernardi M, Berrone S. Hyperplasia of the mandibular coronoid process: long-term follow-up after coronoidotomy. J Cranio-Maxillofac Surg. 1997 Jun;25(3):169-73.
  • 11. Allan PG, Reade PC, Steidler NE. Healing following coronoidotomy in rats. Int J Oral Maxillofac Surg. 1989 Apr;18(2):109-13.
  • 12. Rapidis AD, Papavassiliou D, Papadimitriou J, Koundouris J, Zachariadis N. Fractures of the coronoid process of the mandible. An analysis of 52 cases. Int J Oral Surg. 1985 Apr;14(2):126-30.
  • 13. Philip M, Sivarajasingam V, Shepherd J. Bilateral reflex fracture of the coronoid process of the mandible. A case report. Int J Oral Maxillofac Surg. 1999 Jun;28(3):195-6.
  • 14. Obwegeser HL. Temporal approach to the TMJ, the orbit, and the retromaxillary-infracranial region. Head Neck Surg. 1985 Jan-Feb;7(3):185-99.
  • 15. Farias-Neto A, Martins APVB, Rizzatti-Barbosa CM. The effect of loss of occlusal support on mandibular morphology in growing rats. Angle Orthod. 2011 Sep 8; [Epub ahead of print]. Available from: http://www.angle.org/doi/pdf/10.2319/060711-373.1
  • 16. Rodrigues L, Traina AA, Nakamai LF, Luz JGC. Effects of the unilateral removal and dissection of the masseter muscle on the facial growth of young rats. Braz Oral Res. 2009 Jan-Mar;23(1):89-95.
  • 17. Rodrigues L, Corrêa L, Luz JGC. Effects of the condylar process fracture on facial symmetry in rats submitted to protein undernutrition. Acta Cir Bras. 2011 Apr;26(2):88-93.
  • 18. Becking AG, Zijderveld SA, Tuinzing DB. Management of posttraumatic malocclusion caused by condylar process fractures. J Oral Maxillofac Surg. 1998 Dec;56(12):1370-4.
  • 19. Del Campo AI, Elizondo MM, Magnelli LM, Valadez AS, Ontiveros DS. Craniofacial development in rats with early resection of the zygomatic arch. Plast Reconstr Surg. 1995 Mar;95(3):486-95.
  • 20. Teixeira VCB, Teixeira ACB, Luz JGC. Skeletal changes after experimentally displaced condylar process fracture in growing rats. J Craniomaxillofac Surg. 2006 Jun;34(4):220-5.
  • 21. Cioffi I, van Ruijven RJ, Michelotti A, Langenbach GEJ. Degree of mineralization at the attachment of lateral pterygoyd. Anat Rec (Hoboken). 2010 Aug;293(8):1387-92.
  • 22. Kiliaridis S. Masticatory muscle influence on craniofacial growth. Acta Odontol Scand. 1995 Jun;53(3):196-202.
  • 23. Sarnat BG, Robinson IB. Experimental changes of the mandible. A serial roentgenographic study. J Craniofac Surg. 2007 Jul;18(4):917-25.
  • 24. Whetten LL, Johnston LE Jr. The control of condylar growth: An experimental evaluation of the role of the lateral pterygoid muscle. Am J Orthod. 1985 Sep;88(3):181-9.
  • Corresponding Author:

    João Gualberto de Cerqueira Luz
    E-mail:
  • Publication Dates

    • Publication in this collection
      10 July 2012
    • Date of issue
      Aug 2012

    History

    • Received
      06 Feb 2012
    • Accepted
      08 May 2012
    Sociedade Brasileira de Pesquisa Odontológica - SBPqO Av. Prof. Lineu Prestes, 2227, 05508-000 São Paulo SP - Brazil, Tel. (55 11) 3044-2393/(55 11) 9-7557-1244 - São Paulo - SP - Brazil
    E-mail: office.bor@ingroup.srv.br