Background The anatomical position of the mandibular third molars (M3s) is located in the distal-most portions of the molar area. In some previous literature, researchers evaluated the relationship between retromolar space (RS) and different classifications of M3 in three‑dimensional (3D) cone—beam computed tomography (CBCT). Methods Two hundred six M3s from 103 patients were included. M3s were grouped according to four classification criteria: PG-A/B/C, PG-I/II/III, mesiodistal angle and buccolingual angle. 3D hard tissue models were reconstructed by CBCT digital imaging. RS was measured respectively by utilizing the fitting WALA ridge plane (WP) which was fitted by the least square method and the occlusal plane (OP) as reference planes. SPSS (version 26) was used to analyze the data. Results In all criteria evaluated, RS decreased steadily from the crown to the root (P < 0.05), the minimum was at the root tip. From PG-A classification, PG-B classification to PG-C classification and from PG-I classification, PG-II classification to PG-III classification, RS both appeared a diminishing tendency (P < 0.05). As the degree of mesial tilt decreased, RS appeared an increasing trend (P < 0.05). RS in classification criteria of buccolingual angle had no statistical difference (P > 0.05). Conclusions RS was associated with positional classifications of the M3. In the clinic, RS can be evaluated by watching the Pell&Gregory classification and mesial angle of M3.

Background The anatomical position of the mandibular third molars(M3)is located in the posterior space of the molars, and many two-dimensional studies on them in the previous literature. This study evaluated the relationship between retromolar space(RS) and different impacted classifications of M3 in three‑dimensional(3D) cone‑beam computed tomography(CBCT). Methods 206 mandibular third molars from 103 patients were included and separately separated into three classifications concurring to four classification criteria which included PG-A/B/C, PG-I/II/III, mesiodistal angle and buccolingual angle. 3D hard tissue models were reconstructed by CBCT digital imaging. RS was measured respectively by utilizing the fitting WALA ridge plane (WP)which was fitted by the least square method and the occlusal plane(OP) as reference planes. The SPSS (version 26) was used to analyze the data. Results In all classifications of classification criteria, RS diminished steadily from the crown to the root (p < 0.05), the minimum was at the root tip. From PG-A classification, PG-B classification to PG-C classification and from PG-I classification, PG-II classification to PG-III classification, RS both appeared a diminishing tendency (p < 0.05). As the degree of mesioangula impaction diminished, RS appeared an expanding trend (p < 0.05). RS in classification criteria of buccolingual angle had no statistical difference(p > 0.05). Conclusions RS were related to the impacted classifications of M3. With the increment of the impacted degree of M3 ,the retromolar space gradually diminished. In clinic, RS can be evaluated by watching the PG classification and mesial angle of M3.

The control of Occlusal Vertical Dimension (OVD) 1 is a demanding task for dentists. Establishment of proper occlusal vertical dimension in prosthetic treatment is a important task for clinical procedure.2 No methods are considered to be scientifically accurate in determining the reduced OVD in patients with low occlusal vertical dimension.3 Various factors need to be considered simultaneously during occlusal rehabilitation, such as vertical dimension of occlusion, occlusal contact pattern, centric relation, esthetics and phonetics. Different philosophies have been documented for occlusal rehabilitation and the choice of treatment plan depends on the skills and experience of the dentist. 4 The actual basal bone height of the reconstructed maxillary and mandible is relevant to achieve normal OVD for the prosthesis fabricated.5 This paper described a case with vertical distance problem in which we combined orthodontics with implantation to rehabilitate vertical dimension of occlusion. KEYWORDS:Orthodontic; Implant; Rehabilitate. ABBREVIATIONS:CBCT: Cone-beam computed tomography; OVD: Occlusal Vertical Dimension. TREATMENT STRATEGYA female patient who lost occlusal vertical dimension was treated with this procedure. As shown in Figures 1 and 2, teeth #25, 35, 36, 37, 38 and 46 were diagnosed as residual roots; teeth #16, 26 and 27 were diagnosed as elongation; and teeth #12, 14 and 23 were missing before treatment. The vertical distances between teeth #16, 26 and the mandibular alveolar ridge were only 2 mm respectively when the patient bit together. The distal buccal cusp of tooth #27 almost contacted the mandibular alveolar ridge, as shown in Figures 3 and 4. There was a space of 3 mm between teeth 42 and 43. CBCT images showed low-density shadow around the roots of teeth #11 and 21.

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