Quantitative assessment of ramal bone width and the proximity of the inferior canal for the predictable insertion of ramal implants: Cone-beam computed tomography study

INTRODUCTION

Today’s orthodontics is a mash-up of new and revised concepts, procedures, techniques, and evolving ideologies. Many advancements in treatment mechanics and diagnostic procedures have contributed to the expansion of orthodontic treatment options. Advances in skeletal anchorage and associated biomechanics have provided operators with novel ways to overcome major obstacles in orthodontic mechanotherapy. Temporary anchorage devices (TADs) have enabled previously unthinkable treatment outcomes a reality. Skeletal anchorage is based on the concept that undesirable reactive side effects of orthodontic tooth movements can be eliminated if skeletal structures can absorb these reactive forces, thereby achieving the desired therapeutic goals.

The mandibular and maxillary third molars have the highest relative incidence of impaction, followed by the maxillary canines and mandibular bicuspids.^[1] Impaction among mandibular second molars is uncommon. The prevalence of this was found to be 0.03% in the general population and 2.3% in orthodontic patients.^[2,3] Unilateral impaction of the mandibular second molar is more common in males, with a clear preference for the right side.^[4] The etiology of impaction could be attributed to factors such as an ectopic position that could have hampered the path of eruption and/or a failure in the mechanism of eruption.^[5-9]

The management of such impacted teeth is exceptionally difficult for the orthodontist. In the treatment planning phase, the decision to extract or upright an impacted mandibular molar tooth presents a quandary. Several authors^[10-18] have proposed various techniques for uprighting impacted mandibular molars. Kanomi^[19] and other authors^[20-23] have presented techniques for uprighting impacted mandibular molars using titanium mini-screws placed between the roots of the mandibular teeth. These implants were effective at providing direct and indirect anchorage for space closure and opening, but they had a higher failure rate.^[24] They were not designed to deal with complex issues such as horizontally impacted mandibular molars. These implants also hindered tooth movement, had increased the risk of mobility within the bone, and increased the risk of tooth root damage. Another effective strategy proposed to address this issue is the use of extra-alveolar skeletal anchorage devices to deliver appropriate forces to difficult intraoral sites.^[5,17,25,26]

Chang et al.^[5,25] developed a stainless-steel extra-alveolar bone screw with a diameter of 2 mm and a length of 14 mm that is suitable for placement in dense cortical bone sites such as the anterior border of the ramus. According to Chang et al.,^[5,17,25] the anterior border of the ramus site of implant placement provides a more posterosuperior line of traction along the plane of impaction, which can provide an occlusal and distal force component to upright and unlock the impacted mandibular molar. The presence of vital neurovascular structures, the complex clinical anatomy of the ramus, the thick soft tissue at the site of insertion, and difficulties in clinically asserting the direction of insertion all pose challenges to utilizing this site for implant placement. Improper placement complications may cause unnecessary iatrogenic damage and may result in medico-legal consequences.^[27-42]

Although ramal implants have been successfully inserted in a variety of clinical situations, there has always been concern about their safety due to variations concerning the position of the neurovascular structures, the anatomy of the ramus of the mandible and the optimum height from the occlusal plane from which the implant is inserted.^[5,43-47] Therefore, this study was conducted to identify the optimum level for the placement of ramal extra-alveolar implants by considering the transverse bone width of the Ramus and the proximity of the inferior alveolar canal (IAC) to the site of insertion using 3-dimensional (3D) perspective.

MATERIAL AND METHODS

The protocol for the study was approved by the institutional scientific review board (SRB/SDC/ORTHO-1904/21/020). For this investigation, a retrospective study design was planned.

Cone-beam computed tomographic (CBCT) scans of patients treated for orthodontic complaints from 2019 to 2020 at the Department of Orthodontics were sourced from the records. The inclusion criteria used to select the CBCT scan for evaluation are as follows-

1. No cleft lip/palate or other craniofacial anomalies

2. No prosthesis, missing teeth, or extensive carious lesions

3. No diagnosed systemic diseases

4. No gross variations in the anatomical structures of the mandible (Unilateral Condylar Hyperplasia, Bifid mandibular condyles, etc.)

5. No gross variation in the mandibular occlusal plane.

After applying the inclusion criteria, 53 CBCT scans were selected to be evaluated in the study. The sample population was from the Dravidian race and included 32 males and 21 females between the ages of 18 and 48 years with the average age being 23.5 years.

The CBCT scans were obtained using a Carestream CS9600 unit (Carestream Health Inc., NY). The Imaging parameters of the scans were as follows:

• Exposure-120 KV, 5 mA

• Scan Time-24 s

• Voxel Size-300 µm

• Focal spot-300 µm.

The scan data was saved as digital imaging and communications in medicine (DICOM) format. The DICOM files were reconstructed into 3D images for recording measurements using CS 3D Imaging software (version 3.10, Carestream Health Inc., NY, USA).

To standardize the orientation of the 3D images, the mandibular occlusal plane connecting the mesiobuccal cusp tips of the mandibular first molars and the right mandibular central incisor tip was used as the horizontal reference plane (occlusal plane). The midsagittal plane for the scan was constructed using the crista Galli, Anterior Nasal Spine (ANS), and Opisthion.^[48]

To evaluate the maximum transverse width of the ramal bone, in this study, the central groove of the mandibular first molar was chosen as the dental reference point. To identify the central groove, first, the location of the permanent right mandibular first molar was determined by adjusting the sagittal plane slider on the CBCT scan, after which the axial plane slider was used to position the axial plane at the mandibular occlusal plane level, and finally, the coronal plane was positioned on the central groove of the mandibular molar. To evaluate the maximum transverse width of the ramal bone at six different levels (3 mm, 4 mm, 5 mm, 6 mm, 7 mm, and 8 mm) above the dental reference point. Measurements were recorded in the axial plane at these levels.

Another important factor that needs evaluation was the distance from the point of insertion of the ramal implant to the inferior alveolar nerve to avoid damage to the neurovascular bundle. In this study, the midpoint of the measured maximum transverse ramal width was selected as the reference point of insertion. The distance to the IAC was measured from the selected point of insertion at all the above-mentioned six levels and this measurement was recorded parallel to the occlusal line.^[43] To avoid any variations between the right and left sides, The measurements were performed for both the right and left sides in all subjects [Figure 1].

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Figure 1:

Procedures adopted in this study for recording the maximum transverse width of the ramus and its proximity to the inferior alveolar canal (IAC): In the coronal view, the slider (green line) is positioned on the central groove of the mandibular first molar (dental reference point), (a) On the sagittal view, a measurement of 8 mm is taken from the dental reference point perpendicular to the occlusal plane, (b) The axial plane is then positioned at that level (yellow line) (b) and the maximum transverse width of the ramus is measured on the axial plane, (c) The mid-point of the measured maximum transverse ramal width is measured on the same axial plane and is selected as the point of insertion of the ramal implant and the distance to the IAC is measured from this point (d).

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Since a dental landmark could have variation due to malocclusions causing alterations in the occlusal plane, a skeletal reference point was also used as a comparative landmark to remove any disparity. In this study, the lingula being a stable anatomic landmark was taken as the skeletal reference point.^[49,50] The axial plane was adjusted to identify the most protruding point of the lingula,^[51,52] from which the maximum transverse width of the ramus was measured at six different levels (3 mm, 4 mm, 5 mm, 6 mm, 7 mm, and 8 mm). These measurements were performed on both the right and left sides [Figure 2].

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Figure 2:

Procedures adopted to localize the most protruding point of the lingula: On the axial view of the cone-beam computed tomographic, the most protruding point of the lingula (skeletal reference point) is identified (intersection of the green and purple lines), (a) On the sagittal plane, a measurement of 5 mm is taken above the skeletal reference point, (b) the axial slider is then positioned at that level and the maximum transverse width of the ramus is measured on the axial plane (c).

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RESULTS

The descriptive statistics indicated that the greatest transverse ramal width was 12.48 ± 1.76 mm, at 3 mm above the mandibular first molar [Table 1]. The descriptive statistics also highlighted the maximum clearance to the IAC from the selected site of insertion was 9.81 ± 2.64 mm, at the level of 3 mm above the mandibular first molar [Table 1]. The oneway ANOVA test identified a significant difference in the average ramal width measured at the different levels [Table 2]. The Bonferroni post hoc test highlighted those measurements taken at the level of 3 mm above the mandibular first molar, were significantly greater than the measurements taken at 6 mm (Mean Difference [MD] – 1.21 mm), 7 mm (MD – 1.57 mm), and 8 mm (MD – 2.02 mm) above the mandibular first molar. The post hoc test also showed that the measurements taken at 4 mm above the mandibular first molar were significantly greater than the measurements taken at 7 mm (MD – 1.20 mm), and 8 mm (MD – 1.65 mm) above the mandibular first molar [Table 2]. The ANOVA test did not identify a significant difference in the proximity to IAC measured at different levels [Table 2]. The Kendall Tau and Spearman correlation tests did not identify any gender disparity in the measurements (p>0.05).

DISCUSSION

Despite the relatively low incidence of impaction, the management of impacted mandibular second molars is relatively complex when compared to other more commonly impacted teeth and has been a challenge for the orthodontist.^[53] Although numerous methods for recovering deeply impacted mandibular molars using both conventional and skeletal anchorage have been described in the literature,^[20] these methods have varying degrees of success. The use of ramal implants to disimpact impacted mandibular second molars has proven to have a significant advantage over other conventional techniques because it offers the most suitable direction of traction to disimpact impacted mandibular second molars.^[26]

Although using the ramal bone as a site for TAD placement has biomechanical and efficiency benefits, it is underutilized due to anatomical and clinical challenges. These difficulties include thick soft tissue at the site of insertion, highly mobile alveolar mucosa, difficulty maintaining oral hygiene, soft-tissue hyperplasia, and complex anatomy of the ramus.^[5,54-56] The use of a long-collared implant of 2 mm diameter and 14 mm length implant helps mitigate some of these challenges.^[5,43]

According to Chang et al.,^[5] a minimum of 14 mm implant length is recommended because the implant must clear the soft tissue by at least 5 mm for oral hygiene considerations, 5 mm length is required to penetrate the thick soft tissue, leaving only 3–4 mm of the implant to penetrate the bone.

In this study, two important biological factors for successful ramal implant placement were evaluated, the maximum transverse width of the anteromedial border of the Ramus at six different heights from the occlusal plane (3 mm, 4 mm, 5 mm, 6 mm, 7 mm, and 8 mm above the mandibular occlusal plane), and the proximity of the implant tip to the IAC at these different heights from the occlusal plane.

Results of our study indicated that the greatest average transverse width of the ramus identified was 12.48 ± 1.76 mm at a height of 3 mm above the mandibular first molar. From the measurements recorded in this investigation, it is clear that a ramal implant of 2 mm in diameter would have sufficient width of bone to anchor at all the six levels measured above the central groove of the permanent mandibular first molar (3 mm, 4 mm, 5 mm, 6 mm, 7 mm, and 8 mm). It is also been clinically evident that placing the implant at a higher level from the mandibular first molar gives the clinician more clearance to apply traction to disimpact the impacted mandibular second molar.

The second factor considered was to determine the distance from the tip of the implant to the IAC taking into consideration that 4 mm of the implant was inserted into the ramal bone.^[5] From an operator’s perspective, the iatrogenic damage due to the inferior alveolar neurovascular bundle penetration is of grave concern.^[27-35] The results of our study show that the placement of ramal implants lies in the safe zone to the IAC when measured at 3 mm, 4 mm, 5 mm, 6 mm, 7 mm, and 8 mm above the permanent mandibular first molar. Our observations were made by considering the path of insertion of the ramal implant is parallel to the occlusal plane and the occlusal line. However, considering variations in the position of the IAC between subjects the authors recommend that operators use a CBCT scan to plan implant placement such that injury to the IAC can be prevented while placing ramal implants.^[57] Some authors have recommended that angulating the implant 13–25° laterally from the occlusal line offers a greater clearance up to 7 mm from IAC thus reducing the odds of injury to the IAC.^[43] However, further investigations are required to validate such observations.

The lingula was also utilized in this study as a stable anatomical reference^[49-52] to compare and evaluate the use of the occlusal plane as a clinical reference and only the cases where there was no significant difference between the measurements of the skeletal and dental landmarks were used in this study.

One limitation of this study is that it does not account for the anatomical and morphological differences of the ramus in different skeletal and vertical facial patterns, and the sample size is insufficient to ideally ascertain any gender-based differences in ramus morphology.^[36-42] Thus, the impact of these additional factors must be evaluated in future studies. Disimpaction of an impacted tooth should be attempted in all clinical situations unless certain factors dictate surgical removal. Furthermore, there is no substitute for the longevity and functional benefit of proprioception provided by natural dentition. As a result, before dismissing an impacted tooth as a lost cause, orthodontists must carefully consider and evaluate all options.

CONCLUSION

• Ramal implants can be safely placed between the level 3–8 mm above the central groove of the permanent mandibular first molar as the ideal reference point.

• There was a statistically significant difference between the width of the ramus at 3 mm, 4 mm, 6 mm, 7 mm, and 8 mm above the central groove of the permanent mandibular first molar, and the highest transverse width of the ramus was identified at the level of 3 mm above the central groove of the permanent mandibular molar.

• In relation to the proximity to the IAC, results show an average clearance of 5.62 mm for the implant tip to the IAC, indicating that ramal implants can be safely placed between 3 and 8 mm from the central groove of the permanent mandibular molar.

• Operators should carefully consider the anatomic and morphologic parameters before the placement of the ramal implants with due consideration of the soft-tissue thickness.