Zygomatic-maxillary cortical bone thickness in hyper, normo and hypodivergent patients

ABSTRACT Objective: The aim of this study was to evaluate the thickness of the zygomatic-maxillary cortical bone using computed tomography in different skeletal patterns. Methods: A total of 54 patients of both sexes, divided into three groups according to the vertical skeletal pattern, were evaluated for cortical bone thickness of the anterior slope of the zygomatic process of the maxilla, using cone beam computed tomography. Measurements were made at 2mm, 4mm, 6mm, 8mm and 10mm above from first molar mesial root apex. Vertical skeletal pattern was determined by Frankfurt mandibular angle (FMA). Results: The hyperdivergent pattern had the lowest cortical thickness value, nevertheless, no patient in the hyperdivergent group presented cortical thickness exceeding 2mm, and no patient in the hypodivergent group presented cortical thickness less than 1mm. However, the correlation between cortical thickness and mandibular plane angle was weak and not significant. Conclusion: Although higher prevalence of thick cortical was observed in the hypodivergent patients, and thin cortical groups in the hyperdivergent group, the vertical skeletal pattern could not be used as determinant of the zygomatic-maxillary cortical thickness.


INTRODUCTION
The use of miniplates and other temporary anchorage devices (TADs), have increased the possibilities of orthodontic movement, such as intrusion and distalization of anterior and posterior teeth. 1,2 Some studies have demonstrated success in the treatment of patients considered borderline for the indication of orthognathic surgery, when treated with the aid of these devices. However, the stability of TADs depends on the quality and thickness of the cortical bone, which may be related to the skeletal pattern of the patient. 3,4 Miniplate fixation is obtained by mechanical retention in the cortical bone, therefore, justifying the dependence on adequate bone thickness 5 . Studies have suggested that patients with a vertical growth pattern tend to present lower thickness values of the buccal and lingual bone plates at the level and above the apex of permanent teeth, when compared with patients with a horizontal growth pattern. However, there are few studies specifically evaluating the area of the zygomatic pillar. [4][5][6][7] Cone beam computed tomography enables cortical bone thickness measurement in a real proportion, without presenting distortions and with a relatively lower dose of radiation, compared to traditional computed tomography. The imaging Costa JV, Ramos AL, Iwaki Filho L -Zygomatic-maxillary cortical bone thickness in hyper, normo and hypodivergent patients resource is fundamental for measuring the cortical thickness, especially in the zygomatic-maxillary region, which has been widely used for insertion of TAD devices. [8][9][10] Thus, the aim of this study was to evaluate the zygomatic-maxillary cortical bone thickness in different vertical skeletal patterns, using cone beam computed tomography images.

MATERIAL AND METHODS
The study sample consisted of volumetric computed tomogra- Complete eruption of the permanent teeth from the right second molar to the left second molar was an inclusion criteria.
Women at the stage of menopause and patients with craniofacial anomalies were excluded from the study.  (Fig 1). 10,12,13,14 Measurement of the skeletal growth pattern was made on the lateral images (from the tomography), using the cephalometric variable FMA (Frankfurt Mandibular Plane Angle). Therefore, subjects with an angle between 21º and 29º were classified as normodivergent; those with an angle smaller than 21º or larger than 29º, were classified as hypodivergent and hyperdivergent, respectively. 4,15 The sample was then divided into three groups: Group 1)

RESULTS
The results of systematic and random errors demonstrated reduced values (0,38 to 0.63), that were not significant.
The t-test for dependent samples showed no significant difference between the cortical bone thickness values between the right and left sides (p < 0.05) ( Table 1). There were also no significant differences between the sexes within each group, or between the groups ( Table 2). For these reasons, the groups were treated by using the means without distinction between sides and sex.     There was statistically significant difference between the mean values of buccal cortical bone thickness between the hyperdivergent and normodivergent patients, in the areas closer to the root apex (at 2mm, 4mm and 6mm). However, at 8mm and 10mm, there was no difference between groups (Table 3).
Furthermore, Pearson correlation test between the buccal cortical bone thicknesses and the skeletal growth pattern (FMA) presented low values, that were not significant (Table 4).

DISCUSSION
Temporary anchorage devices have allowed tooth movements in patients considered borderline cases for orthognathic surgery. However, skeletal anchorage may not be stable, and this fact could be related to the cortical bone thickness, which differs among patients. 3,4 In this context, more studies about zigomatic cortical thickness are being encouraged, and the present study aimed at clarifying the anatomic variability of these areas. 4,6,16 Recently, the importance of cortical thickness and bone density for the insertion of temporary anchorage in the infrazygomatic crest region and the mandibular ramus was reported, relating to possible failure of these devices. 13,16 The hyperdivergent pattern may present thin cortical thickness values, as previously reported. 6 In fact, the buccal and lingual cortical bone in hypodivergent patients (ranging from 1.0mm to 2.6m) was thicker in comparison with that of hyperdivergent patients (ranging from 0.08 to 0.64mm) in the present study, however with no statistical significance. 4,15 A tendency for thicker than 2.5mm cortical bone was observed in dry skulls mandibles from Japanese and Indian subjects. 6 In the present study, this trend was observed; however, closer to the maxillary sinus (at 8 mm and 10 mm), the mean difference among the facial patterns was not significant (Table 3). No patient in the hyperdivergent group presented a thickness greater than 2 mm; and in the same way, no patient in the hypodivergent group presented a cortical thickness of less than 1 mm (Fig 2). It is worth emphasizing that these differences in behavior of the cortical bone were more evident up to distance of 6 mm above the root apex of the permanent maxillary first molars (Table 3). It is expected that higher insertion of the TAD relates to lower thickness of cortical bone for all the skeletal patterns. [16][17][18] Costa JV, Ramos AL, Iwaki Filho L -Zygomatic-maxillary cortical bone thickness in hyper, normo and hypodivergent patients The results demonstrate that there was no significant correlation between zygomatic cortical bone thickness and skeletal pattern (FMA). Therefore, an individualized evaluation would be necessary, since one hyperdivergent patient may have a thick or a thin cortical bone, as well as a hypodivergent patient may have either thick or thin cortical thickness for TAD insertion.