Effect of two corticotomy protocols on periodontal tissue and orthodontic movement

Abstract Objective To compare two corticotomy surgical protocols in rats to verify whether they alter conventional orthodontic movement. Methodology Sixty Wistar rats were divided into three groups – orthodontic movement (CG), orthodontic movement and corticotomy (G1) and orthodontic movement with corticotomy and decortication (G2) – and euthanized after 7 and 14 days. Tooth movement (mm), bone volume fraction and bone volume ratio to total volume (BV/TV), and bone mineral density (BMD) were evaluated by micro-CT. The total amount of bone was measured in square millimeters and expressed as the percentage of bone area in the histomorphometry. The number of positive TRAP cells and RANK/RANKL/OPG interaction were also investigated. Results Day 14 showed a statistically significant difference in orthodontic tooth movement in CG compared with G1 (7.52 mm; p=0.009) and G2 (7.36 mm; p=0.016). A micro-CT analysis revealed a difference between CG, G1 and G2 regarding BV/TV, with G1 and G2 presenting a lower BV/TV ratio at 14 days (0.77 and 0.73 respectively); we found no statistically significant differences regarding BMD. There was a difference in the total amount of bone in the CG group between 7 and 14 days. At 14 days, CG presented a significantly higher bone percentage than G1 and G2. Regarding TRAP, G2 had more positive cells at 7 and 14 days compared with CG and G1. Conclusion Corticotomy accelerates orthodontic movement. Decortication does not improve corticotomy efficiency.


Introduction
With an increasing number of adult patients seeking orthodontic treatment, orthodontists are constantly searching for treatments to accelerate orthodontic movement that are predictable and have few complications. 1 Such techniques include reducing the treatment period using self-ligating bracket systems; 2 wires with memory (NiTi); 3 direct electric currents or magnet; 4 micro implants; 5 surgical interventions 6 and administration of local or systemic medications. 7 Among surgical interventions is corticotomy -an intentional bone injury limited to a cortical portion of the alveolar bone with minimal penetration into the medullary bone. 5 It is indicated to speed up corrective orthodontic treatment and facilitate executing mechanically difficult orthodontic movement, as well as to correct moderate to severe skeletal occlusions and decrease treatment time. 6,8,9 Fast orthodontic treatments are essential as shorter treatments are more acceptable to patients, and long-term treatments have been associated with negative results, such as an increased risk of dental cavities, 10 periodontal disease, 11 root resorption 12 and pulp reactions. 13 Accelerated osteogenesis has sparked interest in the orthodontic community and animal studies have demonstrated the biological reactions of bone remodeling and periodontal tissues when associated with corticotomy and orthodontic movement. 1 During bone remodeling, a phenomenon known as regional acceleratory phenomenon (RAP) occurs. Frost 14 (1983) described it as faster tooth movement due to reduced resistance of the cortical bone through the surgical procedure; Yaffe, Fine, Binderman 15 (1994) described RAP as a temporary explosion of localized remodeling of soft and hard tissues, i.e. a regeneration that rebuilds the bone, thus restoring its normal state. Sebaoun, et al. 16 (2008)  Thus, this study aimed to evaluate two surgical corticotomy protocols by an experimental model involving rats. The null hypothesis was that there is a similarity in orthodontic movement between two treatments with different amounts of surgical injury.

Methodology
The study was approved by the CEUA n º 08/2015-ICT-SJC-UNESP. Sixty male Wistar rats (Rattus norvegicus, albinos) aged 90 days and weighing 300 g on average were kept in plastic cages, at room temperature (22ºC) for a 12-hour light cycle with standard diet and water ad libitum.
They were randomly divided into three groups: CG (n=20) received conventional orthodontic treatment; G1 (n=20) received a less invasive corticotomy treatment followed by orthodontic treatment; and G2 (n=20) received corticotomy with decortication and orthodontics, a more invasive protocol.

Sample size
The sample size needed to determine tooth movement (primary outcome) calculated by a previous study 20 with an alpha level of 5% and power of 80% was 17 animals/group. To ensure sufficient samples, the final n-sample comprised 20 animals/group.

Orthodontic appliance placement
The orthodontic devices were installed on the animals' lower first molar, as previously described. 21 After anesthesia, a 0.12 mm alloy wire (Morelli, Sorocaba, SP, Brazil) was inserted between the first and second right molars. A closed-coil steel spring Surgical procedure G1 and G2 animals underwent surgery just after installation of the orthodontic appliance. G1 received trichotomy of the masseter muscle region, and the area was disinfected with chlorhexidine digluconate.
A 10-mm incision from the labial commissure region towards the mandible angle was performed, exposing the buccal cortical bone of the first lower molar roots. 23 Corticotomy was performed using the tip Images were renamed to appropriately blind the examiner. Histomorphometric analysis was performed according to Dibart,et al. 18 (2014)  The samples (n=6/group/antibody) were immunohistochemically processed as previously described. 21 We performed indirect immunoperoxidase using the following primary antibodies: anti-receptor activator of nuclear factor-κB (anti-RANK, 1:100,

Results
Macroscopic measurement of orthodontic movement Groups G1 and G2 showed statistically significant differences in orthodontic movement compared with Cross-section of the first molar mandibular with geometric area and total amount of bone outlined. CG, which received the orthodontic appliance only (p<0.01) ( Table 1).

Computerized microtomography
Regarding BV/TV, all groups showed no significant intragroup differences between 7 and 14 days; however, at 7 days, CG differed significantly from G2, and G1 was similar to CG and G2. At 14 days, CG differed significantly from G1 and G2. As for BMD, there were no significant inter-or intragroup differences ( Table 2).  Uppercase letters: statistically significant difference in intragroup comparison by two-way ANOVA. Lower case letters: statistically significant difference in intergroup comparison by two-way ANOVA. M: mean; SD: standard deviation; Me: median; BV/TV: bone volume/ total volume; BMD: bone mineral density.

Table 2-Micro-CT measurements of bone mineral fraction and bone mineral density
Images represent the right mandible teeth, where corticotomy (G-blue arrow), and corticotomy and decortication (G2-red arrow) were performed. Yellow arrows indicate the space created between the first and second molars in the vestibular region of the transverse plane.

Histomorphometric analysis
The histomorphometric analysis revealed a significant difference in the total amount of bone present in CG at 14 days when compared with the 7 th day (p=0.001). Other groups exhibited no such behavior. The intergroup analysis showed no significant differences at 7 days; however, at 14 days, CG presented a significantly higher total amount of bone than G1 (p=0.003) and G2 (p=0.001    analysis showed no differences between times in any of the groups. In the intergroup analysis, CG had a significantly higher percentage of bone volume than G1 (p=0.004) and G2 (p=0.002) at 14 days (Table 3).

Immunohistochemical analyses (RANK, RANKL, OPG, and TRAP)
In the immunohistochemical markers analysis, absolute values of RANK, RANKL, and OPG showed no significant differences in intra-and intergroup comparisons ( Figure 4). In the TRAP analysis, G2 presented more positive cells, both at 7 days (p=0.04) and 14 days (p=0.04), compared with CG and G1 ( Figure 5).

Discussion
Some studies suggest that corticotomy be performed as a way of accelerating orthodontic movement to reduce total treatment period. Therefore, it was hypothesized that combining orthodontic movement with corticotomy surgery could increase the amount of tooth movement. We compared two surgical protocols: one with corticotomy and another with corticotomy and decortication, in an experimental model to evaluate the process of bone remodeling at the tomographic, histological and immunohistochemical levels.
We chose rats as an experimental model because this is a relatively well-established model due to its low cost, easy storage, good availability and easy logistics. 25   an commonly used method, 21,27,30,31,32 and micro-CT analyses evaluated bone microarchitecture and BMD.
In this study, both groups that received corticotomy experienced accelerated orthodontic movement, and although one surgical procedure was more invasive than the other, both showed a similar orthodontic movement rate. This may relate to the use of the piezo-incision, with its micrometric and selective slices; the piezo-electric blade allows safe and precise osteotomies with no osteonecrosis damage. Two systematic reviews 9,19 showed that corticotomy can increase the orthodontic tooth movement rate due to an altered physiological response through the RAP with minimal side effects. Complications include root resorption, loss of tooth vitality, periodontal sequelae, dentin hypersensitivity, pain and swelling. More studies evaluating the side effects are needed to reach a solid consensus on its use.
The micro-CT analysis revealed no significant difference in the BV/TV between 7 and 14 days within any of the groups; but when we performed an intergroup comparison, both at 7 and 14 days, CG presented a higher BV/TV than G1 and G2. A decrease in BV/TV can be attributed to transient osteopenia, which occurs after bone lesions. Regarding BMD, there was no significant difference between groups, but the changes that occurred may suggest an increase in spinal cord space, an influx of inflammatory cells, and increased vascularization in the region. 17 The similar BMD indicates that the observed bone pattern remained harmonic. Although BDM remained unaltered, the surgical procedure was sufficient to accelerate orthodontic movement in G1 and G2.
These results corroborate a study conducted by Dibart,et al. 18 (2014)

Conclusion
According to our results, corticotomy accelerates orthodontic movement. Adding decortication to corticotomy does not improve its efficiency, suggesting Effect of two corticotomy protocols on periodontal tissue and orthodontic movement J Appl Oral Sci. 2020;28:e20190766 9/10 that the less invasive technique should be selected for such purposes.