Purpose of this study was to evaluate the influence of miniscrew dental root proximity on its late stability degree. 40 miniscrews were inserted between maxillary second premolars and first molars for anterior retraction. Post-surgical radiographs were used to measure the septum width in the insertion site, insertion height, distal and mesial distance from miniscrew to dental root, and the smallest distance between miniscrew and dental root. The miniscrews were divided in two groups according to septum width: ≤3 mm (20 miniscrews) and >3 mm (20 miniscrews). The soft tissue in the insertion site, sensitivity during load, plaque around the miniscrew, and evaluation period were also considered. The results showed no significant difference in miniscrew mobility degree and success rate between groups. Miniscrew dental root proximity did not influence the stability and success rate when the distance between the miniscrew and dental root indicated no periodontal ligament invasion. The overall success rate was 90% and no variable was associated with miniscrew failure. Nevertheless, patient sensitivity was frequently associated with some degree of mini-implant mobility. The septum width did not influence the stability and success rate of this anchorage system, but the extreme root proximity did.
The use of miniscrews as anchorage to obtain intermaxillary fixation or orthodontic movement has become routine in maxillofacial surgery (trauma and orthognathic patients) and orthodontics, due to its high predictability and scientifically proven benefits. Failures of 10–20% of these anchorage devices has been a challenge for researchers and clinicians, who have sought to identify the main risk factors involved in order to improve the success rate of this anchorage system.
Several risk factors have been related to the stability of miniscrews, such as the angle and site of insertion, cortical thickness and quality of alveolar bone, the presence of attached gingiva, miniscrew characteristics, primary stability degree and load intensity, and oral hygiene and inflammation of peri-implant tissue. Recent studies have considered the proximity of the miniscrew and its contact with the dental root as the most important risk factors for failure of this anchorage system. Few studies have analysed this risk factor and some of them have presented conflicting results.
The aim of this study was to compare the stability and success rate of self-drilling miniscrews inserted into the inter-radicular septum with critical and non-critical dimensions, and to evaluate the influence of miniscrew dental root proximity on the degree of stability of these anchorage devices.
Materials and methods
The sample consisted of 21 patients (12 females, 9 males, mean age 16.99 years, standard deviation (SD) 5.08 years) who received miniscrews specifically intended to provide skeletal anchorage for anterior retraction. The patients satisfied the following criteria: maximum anchorage requirement; bone availability in the anatomical region selected to receive the miniscrews; no need for tooth extraction adjacent to the miniscrew insertion site; and absence of any local or systemic condition that could influence the stability of miniscrews such as periodontal disease, smoking and diabetes. 40 miniscrews with the same dimensions (Absoanchor, 1.5 mm outer diameter, 1.9 mm head diameter, 7 mm length, self-drilling thread, Dentos, Daegu, Korea) were inserted into the inter-radicular septum between the maxillary second premolars and first molars.
The insertions were performed by the same orthodontist (SEB) and followed the surgical protocol described by Estelita et al. which utilizes a coaxial radiographic positioner (CRP) associated with a three-dimensional radiographic-surgical guide (3D-RSG), providing adequate miniscrew positioning regarding adjacent tooth roots ( Fig. 1 ). Immediate loading (100–250 g) was applied to all miniscrews using elastic chains.
The 40 miniscrews were divided according to the inter-radicular septum width in which they were inserted: group 1 (critical areas) consisted of 20 miniscrews inserted in a septum width ≤ 3.0 mm; and group 2 (non-critical areas) had 20 miniscrews inserted in a septum width > 3.0 mm.
Measurement of post-surgical radiograph
The pre- and postsurgical radiographs were standardized with the CRP connected to the 3D-RSG and followed the bitewing technique principles ( Fig. 1 C). The radiographs were scanned with a SprintScan 35 Plus scanner 35 mm (Polaroid, Boston, MA, USA) with a resolution of 675 dpi and a 1:1 ratio. Digital images were stored in tagged image file format (TIFF), and Photoshop software (version 7.0, Adobe Systems, San Jose, CA, USA) was used for radiographic measurements by the same investigator (MPG) with an accuracy of 0.1 mm. The high resolution scan allowed an image increase of up to 300% without loss of quality. The following variables were evaluated: distal and mesial distances of miniscrew to dental root ( Fig. 2 A ); septum width at the insertion site (SWI; Fig. 2 B); and insertion height (IH; Fig. 2 B).
The shortest mesial and distal distances (MD and DD) between the miniscrew and the inner edge of the lamina dura of the adjacent teeth were measured (smallest distance from miniscrew to dental root, SDMR; Fig. 2 A) because miniscrews can be inserted in the inter-radicular septum centre or appear displaced towards an adjacent root.
The initial MD and DD were calculated using the miniscrew head diameter owing to it overlapping over the screw thread in the radiographs. This decreased the distances by 0.22 mm on each side. Due to this, a direct contact between mini-implant and tooth root only could be assumed when a negative value of MD or DD > 0.47 mm was found, since periodontal ligament width is about 0.25 mm (0.22 + 0.25 = 0.47).
Degree of stability
Miniscrew stability was assessed by monthly measurements from the time of insertion (primary stability) until its removal, through a horizontal mobility estimating method with the aid of an adjustable telescopic rod (ATR, Fig. 3 ). Measurement of the linear (mm) amount of horizontal mobility was carried out with a digital calliper (Mitutoyo 500-144B, Mitutoyo, Japan) and an orthodontic tension gauge (Correx series 040-712-00, Dentaurum Orthodontics).
The method consisted of the following steps. First, it was necessary to define an aleatory reference point, such as the distal hook of the canine bracket. The ATR length was adjusted according to the distance between the miniscrew head and the chosen point. For this, part c of the ATR ( Fig. 3 A) was connected to the miniscrew head ( Fig. 4 A ), while the tip of part b touched the chosen reference point ( Fig. 4 B). The device was locked, maintaining this distance (distance from the miniscrew to the distal hook of the canine bracket) through the screw-lock (part a). This length was defined as the first step, and measured with a digital calliper ( Fig. 4 C).