Alveolar bone remodeling following orthodontic closure of missing mandibular first molar spaces: a CBCT-based morphometric study

Abstract

Background: Loss of the mandibular first molar is a common and detrimental clinical issue that often leads to alveolar bone resorption in the edentulous area, complicating future prosthetic or orthodontic interventions. This study aimed to evaluate the efficacy of orthodontic closure of missing mandibular first molar spaces and its impact on alveolar bone remodeling.

Methods: Fifteen patients (17 sites) who underwent orthodontic closure of mandibular first molar spaces were retrospectively analyzed using cone-beam computed tomography (CBCT). The mesiodistal and buccolingual angulations of the mandibular second molars were measured in sagittal and coronal planes, respectively. The alveolar bone height was reflected by the distance from the cemento-enamel junction to the alveolar bone margin (CEJ-BM) at six sites on both the second premolars and second molars. Moreover, CBCT before and after orthodontic treatment were superimposed, and changes of alveolar bone width were obtained at three levels (3 mm, 6 mm and 9 mm below the cemento-enamel junction plane) and at three sections (mesial, middle and distal) within the edentulous region. Changes in alveolar bone volume within the extraction site were also calculated. Statistical analysis was performed by paired t-test.

Results: Orthodontic space closure significantly improved second molar angulations in both the mesiodistal (p < 0.001) and buccolingual (p < 0.01) angulations. In 57.35% of measured sites, alveolar bone height increased, with significant elevations at the distobuccal, distolingual, and mesiobuccal sites of the second molar (p < 0.05). Alveolar bone width significantly increased at 3 mm and 6 mm below the CEJ plane after orthodontic closure (p < 0.05), accompanied by a notable increase in bone volume (before treatment: 2922.0 ± 479.0 mm³, after treatment: 3003.8 ± 489.6 mm³, p < 0.05).

Conclusions: Orthodontic closure of mandibular first molar spaces promotes alveolar bone remodeling, including improvements in angulation, bone height, width, and volume. This approach offers a biologically advantageous alternative for managing molar loss, enhancing both function and long-term periodontal support.

Keywords: CBCT; alveolar bone remodeling; mandibular first molar loss; orthodontic space closure.

Fig. 1

Angulations of mandibular second molars. a Mesiodistal angulation of mandibular second molar. It was defined as the lower posterior angle (in red) between the long axis of the tooth and the posterior occlusal plane (in yellow) in thesagittal plane. b Buccolingual angulation of mandibular second molar. It was defined as the angle between the long axis of the tooth and the posterior plane (in yellow) in the coronal section. The long axis of the mandibular second molar was defined as the line passing through the central fossa of the molar crown and the root furcation for multiple roots or the root apex for a single root (in blue)

Fig. 2

Measurements of alveolar bone height. a The CEJ-BM distance was recorded at six sites on both the second premolar and second molar: mesiobuccal, central buccal, distobuccal, mesiolingual, central lingual, distolingual. b Magnification of (a). c-e The CEJ-BM distance of the second premolar was measured in the coronal plane of three cross-sectional views: through the mesial marginal ridge (mesiobuccal, mesiolingual), the mid-point of the crown (central buccal, central lingual), and the distal marginal ridge (distobuccal, distolingual). f-h The CEJ-BM distance of the second molar was measured in the coronal plane of three cross-sectional views: through the mesial root (mesiobuccal, mesiolingual), the root furcation (central buccal, central lingual), and the distal root (distobuccal, distolingual)

Fig. 3

Measurements of alveolar bone width. a Pre-treatment (in grey) and post-treatment (in green) CBCT scans superimposition. b The alveolar bone widths were measured at the mesial, middle, and distal sections in missing mandibular first molar regions across three vertical levels: 3 mm (in red), 6 mm (in blue), and 9 mm (in yellow) below the cemento-enamel junction (CEJ) plane (in orange). c-e Measurements at the level 3 mm (in red), 6 mm (in blue), and 9 mm (in yellow) below the CEJ plane before treatment. f-h Measurements at the level 3 mm (in red), 6 mm (in blue), and 9 mm (in yellow) below the CEJ plane after treatment. i-k Superimposition CBCT scans of pre-treatment (in grey) and post-treatment (in green) at the level 3 mm (in red), 6 mm (in blue), and 9 mm (in yellow) below the CEJ plane

Fig. 4

Measurement of alveolar bone volume. a-b Process of segmenting tooth and alveolar bone in Mimics: mask based on bone in red, mask based on tooth in blue. c 3D reconstruction figure of segmenting tooth and alveolar bone. d Process of overlapping the mandibular bones from the pre- and post-treatment in Geomagic, and the region of interest were taken from the same position. e-f Measurement of alveolar bone volume before (in red) and after (in blue) treatment

Fig. 5

Changes of the inclination of the second molars. a Statistical diagram of mesiodistal angulation of second molars before and after orthodontic treatment. b Statistical diagram of buccolingual angulation of second molars before and after orthodontic treatment

Fig. 6

Changes of alveolar bone height of adjacent tooth. a Schematic diagram of the change in alveolar bone height of adjacent tooth before and after orthodontic treatment. b Distribution of vertical bone loss of adjacent tooth at each site. c Statistical diagram of alveolar bone height of second premolars before and after orthodontic treatment. d Statistical diagram of alveolar bone height of second molars before and after orthodontic treatment

Fig. 7

Bone remodeling of orthodontic treatment. a Statistical diagram of alveolar bone width at the level 3 mm below the CEJ plane before and after orthodontic treatment. b Statistical diagram of alveolar bone width at the level 6 mm below the CEJ plane before and after orthodontic treatment. c Statistical diagram of alveolar bone width at the level 9 mm below the CEJ plane before and after orthodontic treatment. d Statistical diagram of bone volume before and after orthodontic treatment. e 3D reconstruction figure of the simulated alveolar bone remodeling pattern: placement of the post-treatment teeth into the pre-treatment alveolar bone. f 3D reconstruction figure of the actual alveolar bone remodeling pattern: post-treatment alveolar bone and teeth

Fig. 8

Illustration of orthodontic closure with alveolar bone remodeling. Illustration of alveolar bone remodeling patterns from the sagittal plane, the horizontal plane and the cross section of the second molar