UCSF

Introduction: The anatomic limits of the alveolar bone defines the boundaries of orthodontic movement and challenging these limits may cause undesirable effects on the periodontal tissues [1, 2]. The most critical orthodontic tooth movements include dental arch expansion and incisor buccal-lingual movements [1, 3]. Prior to computed tomography, visualization of labial/buccal and lingual bone plates was not possible due to superimpositioning of structures in 2D radiography and gingival covering of the bone during clinical examination. With the advent of computed tomography, dental professionals were able to visualize what conventional radiographs never showed, the thickness and level of the labial/buccal and lingual alveolar bone. Hyperdivergent patients present with a thinner symphysis thickness than do hypodivergent and normo-divergent patients [4]. With this thinner amount of bone coverage, orthodontic tooth movement in hyperdivergent patients may lead to an increase in bone loss around the roots of the lower incisors.

Objectives: 1) To test the reliability of a novel technique to measure changes in the supporting alveolar bone around lower incisors. This was quantified using cone beam computed tomography to measure vertical levels of the labial, lingual, and interproximal alveolar bone as well as taking perimeter measurements at standardized axial slices and calculating the approximate percent of the root surface area covered by any measurable amount of bone (bone thickness was not evaluated). 2) To utilize this novel technique to evaluate the changes in alveolar bone support for the roots of lower incisors before and after orthodontic treatment in subjects with high mandibular plane angles.

Materials and Methods: The sample consisted of n=20 hyperdivergent patients (SN-MP ≥ 39 degrees which is 1 standard deviation above the norm), with a full complement of permanent dentition, who underwent comprehensive orthodontic treatment in a university setting. There were 7 males and 13 females. The ages of the subjects at the beginning of treatment ranged from 13.2 to 42.6 years with a mean age of 17.6 years. Treatment time took an average of 2.2 years. 10 subjects underwent premolar extraction in conjunction with their orthodontic treatment and 10 had no extractions. Pre-treatment crowding was estimated from pre-treatment photos and ranged from mild to severe (1-3 mm = mild, 4-6 mm = moderate, >/= 7 mm = severe). 9 were mild, 8 were moderate, and 3 were severe. Using three-dimensional cone-beam computed tomography taken before and after treatment on the same imaging machine and uploaded to Dolphin Imaging, the bone surrounding each lower incisor root was measured in all 3 planes of space (sagittal, coronal, and axial). After following a standardized orientation protocol, each lower incisor root was analyzed to determine the location of specified zones: No Bone Zone (NBZ), Partial Bone Zone (PBZ), Full Bone Zone (FBZ). Specific measurements were obtained: Highest Vertical Bone Height (HVBH), Root Length (RL), Lowest Vertical Bone Height (LVBH), Root perimeter (RP), Defect perimeter (DP). Calculations were performed to determine: Bone Coverage Area (BCA), Root Partial Bone Zone (rPBZ), Axial Radicular Bone Coverage (ARBC), and Total Root Bone Coverage (TRBC). Statistical analyses were performed. As this is an unpublished method originally proposed by Dr. Jeffery Miller, details having to do with each measurement were specified by us. We also modified certain elements of the methods so as to be more accurate and complete. Further, the original method as outlined could not be used on teeth with fenestrations, an issue that we were able to resolve in this study.

Results: On average, the total root bone coverage of lower incisors significantly decreased after orthodontic treatment by approximately 10% and root length shortened an average of 0.9 mm. Intra-agreement appeared to be good-to-excellent on root length and HVBH. Intra-agreement appeared to be fair in regards to TRBC. There were a significantly greater amount of both buccal and lingual dehiscences found after orthodontic treatment, while the number of fenestrations generally decreased. Due to power issues, analyses performed were not found to be significant.

Discussion: The accuracy of our measurements was affected by the voxel size of 0.25 mm. More testing is needed to support the reliability of this method and to compare the findings for hyperdivergent individuals with a control group. There are bony limitations to tooth movement. In patients that are diagnosed to have compromised boney housing prior to orthodontic treatment, care should be taken in the treatment planning, treatment execution, and post-treatment evaluation phases to ensure that long-term outcomes for our patients are as favorable as possible.

Conclusions: During orthodontic treatment, patients with hyperdivergent mandibles can experience significant negative changes in bone coverage around lower incisor roots. Dehiscences increased during orthodontic treatment. Using a sagittal slice to examine root bone coverage can significantly under-represent the amount of bone surrounding the root 3 dimensionally (e.g., does not take into consideration inter-radicular bone). The boundaries of lower incisor tooth movement are compromised in hyperdivergent patients and significant sagittal tooth movements may cause adverse sequelae. This type of approach must be carefully monitored to avoid negative iatrogenic effects.