Mini-implant insertion based on tooth crown references: a guide-free technique

Abstract

Mini-implant insertion requires accurate surgical technique. This study shows an insertion technique using only tooth crown references; its scientific basis is evaluated radiographically. The sample consisted of 213 inter-radicular septa, evaluated in 53 bitewing radiographs. The proximal contour of adjacent tooth crowns was used to define septum width. The midpoint of the septum width was linked to the interdental contact point to determine septum midline. The distances from septum midline to mesial and distal teeth were measured to evaluate the septum midline centralization degree in two different septum heights. The difference between mesial and distal distances represented the septum midline deviation degree. The mesial and distal distances were compared by t -tests, and the septum midline deviation was correlated with septum height using Pearson’s correlation test. The mesial and distal distances were not statistically different in the midpoint of the septum height, but they were different at the apical septum height. There was a moderate correlation ( r = 0.45) between septum midline deviation and septum height. The tooth crown references evaluated on interproximal radiographs determine a high centralization degree of the septum midline on which the insertion site could be defined. The greater centralization degree was observed at the coronal septum area.

Mini-implants have been an excellent adjunct to orthodontic mechanics, but mini-implant placement between teeth roots is a critical surgical procedure when the interradicular septum is very narrow , because injurious mini-implant insertion can be associated with complications ranging from slight root contact favouring screw failure, to severe root perforation causing pulp damage or tooth loss . Recently, diagnostic resources based on advanced technologies, such as computed tomography (CT), have been used to evaluate anatomical and topographical characteristics of the interradicular septum . Several surgical guides have been developed to obtain safer mini-implant insertion between teeth roots, avoiding accidents and complications .

Surgical guides are manufactured devices used to create an artificial radiographic and clinical reference to determine adequate insertion site. CT and stereolithography were used to obtain accurate surgical guides, but these procedures require elaborated technology, greater complexity, cost and radiation exposure to the patient . Other surgical guides can be hand-made, but their accuracy depends on presurgical radiographic standardization to avoid oblique projections and distorted radiographic images, which can lead to mistaken interpretations about safe insertion sites . The simple use of anatomical structures to create clinical references for mini-implant insertion has not been explored and systematically evaluated. This study presents a surgical technique to determine mini-implant insertion sites, taking into account tooth crown references only.

Material and methods

The surgical technique for mini-implant insertion based on tooth crown references only is performed according to the following clinical procedures. An accurate diagnostic radiograph is taken to evaluate the bone tissue availability of the selected insertion site. The principles of the paralleling or bitewing technique can minimize the overlapping images associated with obliquely projected and distorted images, which do not show the actual anatomical relationships between the septum and the adjacent teeth . There is no consensus about the minimal interradicular septum width required to place mini-implants, but considering that the drill-free screw diameter should be around 1.5 mm to avoid fractures, and that excessive proximity between screw and tooth root can increase screw failure rate and root damage risks, the interradicular space should not be smaller than 2.5 mm .

The patient undergoes a presurgical prophylaxis procedure including a 0.12% chlorhexidine mouth rinse to reduce contamination by oral microorganisms during surgery. The mesial and distal interradicular septum limits are marked. To do this, a segment of dental floss is inserted into the mesial gingival sulcus and apically pulled, following the mesial surface contour of the tooth distally positioned to the interradicular septum where the mini-implant will be inserted ( Fig. 1 A ). It should always be kept under tension without contacting or surrounding the buccal or palatal tooth surfaces before being pulled apically. This clinical procedure is performed in order to stamp the gingival tissue, creating a clinical reference representative of the distal septum limit (DSL; Fig. 1 B). The same clinical procedure must be performed in the distal gingival sulcus of the adjacent tooth to create a clinical reference representative of the mesial septum limit (MSL; Fig. 1 C and D). The dental floss reliably reproduces the convergence of the proximal surfaces towards the cervical region, which can change between different teeth and/or individuals, and cannot be predicted visually because the proximal surface contours of the adjacent teeth are mainly covered by gingival tissues. The MSL and DSL marks allow the cervical width of the interradicular septum to be delimited clinically. The septum limit marks left in the gingival tissue are highlighted using a marker pencil, which is also used to indicate the midpoint between these marks, creating a clinical reference representative of the septum central point (SCP) at the crest level ( Fig. 1 E).

Fig. 1
Clinical procedures to insert mini-implants based on tooth crown references. (A–D) Dental floss is used to explore the subgingival proximal contour of adjacent teeth in order to determine mesiodistal septum limits clinically. (E and F) The central point of mesiodistal septum width (SCP) is marked and linked to CP by a cotton thread, determining the septum midline (SML). (G–I) The SML centralization degree is checked on diagnostic radiograph. The SML is used as clinical reference to insert the mini-implant. (J and K) The mini-implant is inserted, and the final radiograph shows an excellent screw positioning in the interradicular septum.

The next step is to create a septum midline (SML) through the SCP and the dental contact point (CP). Thus, the second point that defines the SML is the CP between the teeth that delimits the interradicular septum where the mini-implant will be inserted. In order to stamp the SML in the gingival tissue, a segment of dental floss is supported on the CP and stretched on the soft tissues, passing through the SCP ( Fig. 1 F). If the CP is not tight, a cotton thread thicker than dental floss can be used to ensure it is supported by the CP during SML stamping. The same tooth references can also be used on a diagnostic radiograph to evaluate SML centralization regarding MSL and DSL to avoid clinical errors due to anatomical variations as discussed later ( Fig. 1 G). Sequentially, the SML is highlighted with a marker pencil and taken as a clinical reference on which the mini-implant can be inserted in different septum heights in accordance with perceived clinical need ( Fig. 1 H).

Local anaesthesia is performed and the insertion site is marked on the SML according to the intended height. Judicious surgical exploration is performed on the cortical bone surface using a delicate manual spear-point drill to avoid insertion inaccuracy due to screw tip slip, especially if a vertically angulated screw insertion path regarding bone surface is intended ( Fig. 1 H). The mini-implant is inserted using a manual screwdriver ( Fig. 1 I and J). A postsurgical radiograph is taken to evaluate screw positioning in the interradicular septum, and the patient is instructed to maintain oral hygiene ( Fig. 1 K).

Basis of surgical technique

The mini-implant should be centrally inserted in the septum; the validity of the tooth crown references to determine a centralized SML was evaluated radiographically using interproximal radiographs. 53 posterior bitewing radiographs were randomly selected from the files of the Orthodontic Department at Bauru Dental School, University of São Paulo. To be useful, the radiographs must present minimal distortion and suitable image quality. Radiographs were selected to include only those: obtained with the aid of a bitewing X-ray positioner to reduce image distortion; showing teeth proximal surfaces with no restoration; with no overlap in the contact area or, at most, less than a quarter of the enamel thickness overlapped; and with acceptable contrast and sharpness. This resulted in 213 interradicular septa to be evaluated.

All bitewing radiographs were digitized with a SprintScan 35 Plus scanner (Polaroid, MA, USA) at 100% scale and high resolution (675 dpi), allowing an image magnification of 300% without image quality loss. Digital images were stored in Tagged Image File Format (TIFF) and the Adobe Photoshop software (Version 7.0; Adobe Systems Inc, San Jose, CA, USA) was used for radiographic image measurements.

The MSL and DSL of each interradicular septum were determined radiographically as the mesial and distal points of the proximal tooth surface at the most occlusal bone crest level, which are situated close to their analogous clinical points, where the dental floss is positioned during clinical procedures as shown in Fig. 2 . If a ligature wire with a similar dental floss diameter is deeply inserted into the gingival sulcus and pulled apically following the proximal surface contour, it can be visualized radiographically close to the mesial or distal bone crest limit ( Fig. 2 ). Considering the same anatomical references used to define the SML clinically, the SCP was marked on the bitewing radiographs as the midpoint between MSL and DSL ( Fig. 3 ). Sequentially, the SML was traced from the CP image passing through SCP, extending up to the most apical septum limit visualized in the bitewing radiograph ( Fig. 3 ). The smallest distance from SML to the mesial and distal periodontal ligament of adjacent roots (mesial distance (MD); distal distance (DD)) was measured in two different septum heights: septum mid-height (h1) and septum apical height (h2), which are visible in bitewing radiographs ( Fig. 3 ).The difference between MD and DD (MD–DD) showed the SML deviation degree towards the adjacent tooth root in h1 and h2. The smallest distance between MSL and DSL points determined interradicular septum width at the crest level, whilst the sum of MD and DD showed the interradicular septum width at h1 and h2.

Fig. 2
A 0.008 inch ligature wire was attached to the apical edge of radiographic film. Sequentially, it was inserted in the gingival sulcus and apically pulled following the mesial tooth surface in order to simulate dental floss positioning during its use in clinical procedures for determination of mesiodistal septum limits. The radiograph shows the ligature wire very close to the bone crest and distal septum limit, highlighting the reliability of this clinical procedure to evaluate septum location and width.

Fig. 3
Landmarks, line and measurements of bitewing radiograph. Landmarks: (1) MSL, mesial point of the proximal tooth surface at the most occlusal bone crest level; (2) DSL, distal point of the proximal tooth surface at the most occlusal bone crest level; (3) SCP, the mid-point between MSL and DSL; (4) CP, contact point of adjacent teeth. Line: SML, line that passes through points CP and SCP. Measurements: MD, the smallest distance from SML to mesial periodontal ligament, measured at h1 (septum mid-height) and h2 (septum apical height); DD, the smallest distance from SML to distal periodontal ligament, measure at h1 and h2.

Mini-implant success rate

The success rate obtained with the surgical technique was calculated for a standardized sample of 53 mini-implants inserted in 22 patients (13 males, 9 females) with a mean age of 20.4 years. The mean observation period was 8.52 months. A clinically perceptible mobility was the observation parameter used to define an unsuccessful mini-implant, regardless of its severity. The number of totally stable mini-implants was divided by lost mini-implants in order to obtain the success rate.

Error study

15 bitewing radiographs, totaling 67 interradicular septa, were remeasured 2 months later by the same examiner (SE). The first and second radiographic measurements were used to calculate casual and systematic errors. The casual error was calculated according to Dahlberg’s formula, S 2 = ∑ d 2 /2 n , where: S 2 is the error variance, d is the difference between repeated measurements, and n is the number of duplicate determinations. The systematic errors were evaluated with dependent t -test at P < .05.

Statistical analyses

The mesial and distal distances (MD and DD) and the SML deviation degree (MD–DD), evaluated in h1 and h2 septum heights, were compared with t -tests. The mean value and standard deviation of h1 and h2 regarding bone crest level (septum height measurements) were also calculated.The deviation degree of SML towards the adjacent tooth root (MD–DD) was correlated with the septum height measurements regarding bone crest, using Pearson’s correlation test.

The interradicular septum width, measured at the crest level, and at h1 and h2 septum heights, were compared with ANOVA followed by Tukey tests.

Results

The radiographic variables had no significant systematic error, whilst the casual errors ranged from 0.067 mm (DD measured in h1) to 0.104 mm (SML deviation calculated at h2).

The distances between SML and the periodontal ligament of adjacent tooth roots were not statistically different when the septum measurement height was 3.2 ± 0.8 mm. These distances presented significant differences when the measurement height was 6.4 ± 1.6 mm ( Table 1 ). The SML deviation degree was significantly smaller in h1 than in h2.The SML deviation degree towards the adjacent tooth root was positively and significantly correlated with septum height measurements regarding bone crest ( Table 2 ).

Table 1
Comparison of mesial and distal distances (MD and DD), and SML deviation degree (MD–DD), evaluated at different septum heights (h1 and h2) ( t -tests).
Variables MD, n = 213 DD, n = 213 P
Mean SD Mean SD
h1 (mean = 3.2 mm, SD = 0.8) 1.26 mm 0.40 1.30 mm 0.46 0.354
h2 (mean = 6.4 mm, SD = 1.6) 1.42 mm 0.59 1.62 mm 0.75 <0.01
h1 h2
Mean SD Mean SD
MD–DD 0.23 mm 0.27 0.55 mm 0.58 <0.01
Statistically significant at P < .05.

Jan 26, 2018 | Posted by in Oral and Maxillofacial Surgery | Comments Off on Mini-implant insertion based on tooth crown references: a guide-free technique
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