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Study on digital teeth selection and virtual teeth arrangement for complete denture
Accepted Manuscript
Study on digital teeth selection and virtual teeth arrangement for complete denture Xiaoling Yu , Xiaosheng Cheng , Ning Dai , Hu Chen , Changjiang Yu , Yuchun Sun PII: DOI: Reference:
S0169-2607(17)30349-8 10.1016/j.cmpb.2017.11.022 COMM 4553
To appear in:
Computer Methods and Programs in Biomedicine
Received date: Revised date: Accepted date:
28 March 2017 18 November 2017 24 November 2017
Please cite this article as: Xiaoling Yu , Xiaosheng Cheng , Ning Dai , Hu Chen , Changjiang Yu , Yuchun Sun , Study on digital teeth selection and virtual teeth arrangement for complete denture, Computer Methods and Programs in Biomedicine (2017), doi: 10.1016/j.cmpb.2017.11.022
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ACCEPTED MANUSCRIPT Highlights Digital artificial teeth selection system.
Virtual teeth arrangement
Optimization method of tooth arrangement
The process of teeth alignment is introduced in detail
The means of adjustment for adjacent teeth clearance is described
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Study on digital teeth selection and virtual teeth arrangement for complete denture Xiaoling Yua , Xiaosheng Chenga, Ning Dai a,*, Hu Chenb, Changjiang Yua, Yuchun Sunb,*
a College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics, Mailbox 357, 29 Yudao Street, Qinhuai District, Nanjing 210016, PR China
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b Center of Digital Dentistry, Faculty of Prosthodontics, Peking University School and Hospital of Stomatology & National Engineering Laboratory for Digital and Material Technology of Stomatology & Research Center of Engineering and Technology for Digital Dentistry of Ministry of Health, 100081 Beijing, PR China
Abstract:
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Background and objective: In dentistry, the complete denture is a conventional treatment for edentulous patients. The computer-aided design and computer-aided manufacturing (CAD/CAM) has been applied on the digital complete denture which is developed rapidly. Tooth selection and arrangement is one of the most important parts in digital complete denture. In this paper, we propose a new method of personalized teeth arrangement. Methods: This paper presents a method of arranging teeth virtually for a complete denture. First, scan and extract the feature points of the 3D triangular mesh data of artificial teeth (PLY format), then establish a tooth selection system. Second, scan and mark the anatomic characteristics of the maxillary and mandibular cast
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surfaces, such as facial midline, the curve of the arches. With the enter information, the study calculates the common arrangement lines of artificial teeth. Third, select the preferred artificial teeth and automatically
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arrange them virtually in the correct position by using our own software. After that, design the gingival part of
the dentures on the basic of the arranged teeth on the screen and then fabricated it by using Computerized Numerical Control (CNC) technology, Rapid Prototyping (RP) technology or 3D printer technology. Finally, select artificial teeth were embedded in wax rims.
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Results: This system can choose artificial teeth reasonably and the teeth placement can meet the dentist’s request to a certain extent, whereas all the operations are based on the medical principles.
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Conclusions: The study performed here involves computer sciences, medicine, and dentistry, a teeth selection system was proposed and virtual teeth arrangement was described. This study has the capacity of helping operators
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to select teeth, which improved the accuracy of tooth arrangement, and customized complete denture.
Keywords: complete denture; artificial teeth database; teeth selection system; personalized virtual teeth arrangement
* Corresponding author. E-mail address: [email protected] (Ning Dai), [email protected] (Yuchun Sun).
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1. Introduction With the aging of the world population, the number of patients with edentulous jaw has been growing rapidly, and the complete denture is the most common and effective treatment. In addition, teeth arrangement is crucial to the function and esthetics of complete denture restoration.
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The traditional process of full denture is complicated, especially in arranging teeth manually. This process was heavily depended on the experience and technique skills of the technician, which is time-consuming and has low precision of operation and repeatability.
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Since CAD/CAM was first introduced to clinical prosthodontics in the 1980s [1-2], many researchers devoted themselves to the study of robot-aided teeth alignment of complete denture and have made some achievements [3-4].Compared with the conventional method of teeth set-up,
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the teeth alignment of complete denture with the help of robot was more reliable and flexible. However, the additional device was necessary and the irregular shape of the artificial tooth
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increase the difficulty of grasping. Besides the repeated alignment of the teeth also makes the
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devices become complicated.
In order to simplify the teeth arrangement, digital tooth set-up was proposed when CNC and
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RP have been applied in the field of prosthodontics. The fabrication of complete denture mainly
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depends on the artificial teeth from then on. Maeda et al. [5-6] proposed an approach to study the complete denture. They fabricated two plastic outer shells of dentition by 3D laser lithography and filled. But the method was not practicable. Kawahata et al. [7] scanned the complete denture surface and then fabricated the prosthodontics wax type by CNC and studied on the duplication procedure. Kanzawa and Inokosh [8-9] used a three-dimensional (3D) cone-beam computed tomography (CBCT) to scan the base and artificial teeth of one complete denture separately, and
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created a new one base on the scanning data. They fabricated an acrylic resin denture base by using RP [8] and a totally wax-free manufacturing process by combining RP and rapid milling [9]. Finally, the teeth were adapted and bonded into the milled sockets of the milled base. Sun et al. [10] obtained the 3D data of artificial teeth and edentulous, then they defined the anatomic
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landmark for artificial teeth and edentulous. After that, they formulated a principle of tooth arrangement and created individual physical flasks by using the 3D printer. However, in their study, they have not formed a complete system and it is hard to select artificial teeth according to
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the sex and age of edentulous patients.. Wang Y et al. [11] studied on the logical expression of automatic tooth selection for complete denture by CAD, but there is no discussion of different models. Wu et al. [12] described a traditional method of artificial teeth selection and arrangement
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of the complete denture, which as a reference for digital. Digital teeth arrangement and selection have significant advantages compared with the conventional arrangement: stable in the accuracy
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of teeth arrangement, free from manual errors, less chair-time and short cycle [13, 21-22].
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To date, the virtual teeth arrangement technologies have been added to the CAD of the full denture, then the denture is fabricated by using CNN, RP and 3D printer. Kattadiyil [14] reported
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the comparison between two commercial fabrication systems. In his report, both teeth were set
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virtually and used two-step method. The Dentca system printing the whole denture by using the 3D printer and the Avadent system fabricates denture base with CNC. Infante [15] obtained the required data and arranged teeth virtually on the Avadent system. Then he sent the data to the machining center. Bilgin et al. [16] investigated a one-set aligned artificial tooth (1SA) by Dental Wings system and then manufactured as one piece by using CAD/CAM and RP. But it has not been extending due to the security of printed material. Wimmer [17] proposed the position of
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artificial teeth through the Ceramill Full Denture system that can eliminate occlusal interference by itself. Finally, they use CNC to fabricate the denture base and insert the artificial tooth manually. Baba [22] reported four denture systems in detail and revealed the advantages and disadvantages of CAD/CAM complete dentures.
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Although the use of CAD/CAM &RP techniques for the fabricating of complete denture has been widely reported [15-20], the description of the virtual teeth arrangement is not detailed. The purpose of this study is to describe a new method of virtual artificial teeth arrangement for a full
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denture, which bases on a multilevel database and offers a series of optimization methods.
2. Material and methods
The patient models in the article were provided by the Department of Prosthodontics, Peking
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University School and Hospital of Stomatology, patients were elderly and edentulous. The participant voluntarily joined the study and provided informed consent.
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2.1 Artificial teeth selection system
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2.1.1 Artificial tooth database
In this paper, we establish an artificial tooth database and form a tooth selection system. The
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database contains a lot of artificial teeth datasets and the general definition function of a teeth
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dataset has the form:
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S = A B i A = L, R ; B = U,D
(1)
i=1
Artificial teeth are selected automatically and/or manually by a dentist from the database (Fig.1) that consists of several datasets of different tooth sizes, shapes, and colors for each of four types of the teeth: the upper/lower anterior teeth and the left/ right posterior teeth. The teeth in the datasets are uniquely identified by ABi, where A = {L, R}; B = {U, D}, whereas letters L, R, U, 5
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and D denote the left, right, maxillary, and mandibular teeth, respectively, and i is the number from 1 to 7 that represent central and lateral incisors, canines, first premolar and second premolar, first molar and second molar, respectively. For example, LU1 means the left maxillary central incisor. For each tooth, the database stores its 3D model information and parameterized
serves as the index of the database. 2.1.2 Definition of anatomical characteristics of artificial teeth
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anatomical features, from which the total width (w) of the teeth in the dataset is calculated and
We used a dental 3D scanner (3Shape Q-750, 0.02-mm accuracy; 3Shape, Copenhagen,
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Denmark) to scan the cervical area of the artificial teeth to obtain the triangular mesh data (STL/PLY) from which we extracted the anatomical features of teeth via our own software. The anatomical features of artificial teeth are mainly used for teeth translation and rotation in the process of teeth placement. The teeth of anterior, posterior, maxillary and mandible are different in
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location mode, thus, the characteristic parameters are not identical (Table.1 & Fig.2). To facilitate the storage and management of the artificial tooth database, an XML file for each
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tooth was built to store the teeth and their characteristics. We also create an XML file for each
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database, storing the tooth's storage path and tooth width (index) information.
Table 1 Anatomical features of artificial teeth Local
Mesial/Distal
Fullness
long axis of
Central sulcus
cheek/tongue
name
coordinate
points
sign point
tooth lma
line
points
system xoz
pnea/pfar
pf
lcs
pc/pt
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Teeth
+
+
+
－
－
－
D1-D3
+
+
－
＋
－
－
U4-U7
+
+
－
－
＋
＋
D4-D7
-
+
－
＋
＋
＋
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U1-U3
Note: “+” means “yes”, “-” means “no”
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2.1.3 Definition of anatomical characteristics of edentulous patients The edentulous patients’ anatomic features, which includes midline, lip lines, and occlusion plane, and so on, are extracted from the triangular mesh model (STL/PLY), obtained by the dental 3D scanner, and serve for the teeth set-up [10], as shown in Table 2 and Fig.3. Table 2 Anatomic features of edentulous patient Symbol
Facial midline
lm
High/low lip
Occlusion plane
Pj
Fullness surface
Middle sagittal
Pms
Cuspid lines of left/ right
llc/ lrc
Margin of base
ce
Lingual cusp curve
clt/ crt
llp/ lrp
Anterior teeth arrange curves of up/down
cua/ cda
Symbol lh/ ld Sf
Retromolar pad of left/right
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Posterior teeth arrange lines of left/right
Feature name
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Feature name
2.1.4 Teeth selection
Crest of ridge
plmrp/ prmrp Scr
Teeth set-up with all these factors: the distal surface of maxillary cuspid is consistent with the
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maxillary cuspid line in the direction of mesiodistal, the distal surface of mandibular second molar
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does not exceed the retromolar pad [3]. Teeth selection usually base on the experience of the dental technician, they predict teeth location on the wax rim and then choose teeth subjectively.
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However, unreasonable tooth selection makes it difficult to meet the teeth set-up requirement. For
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this purpose, we used the teeth database as described above to establish a digital teeth selection system. According to the experience skills of dentists, we get the appropriate teeth by comparing
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the width of the teeth and the length of the patient's arch. The calculating formula of the width of artificial teeth in a dataset is following: n
w | i 0
p
fari
p
neai
|
w
w , w , w , w ua
ud
lp
i; = 0 , 1 , 2 , . . . ,(2) n
rp
Where pnear and pfar are mesial and distal points of i-th tooth, respectively, and n is the number of teeth in the dataset. We denote the width of anterior maxillary and mandibular teeth dataset by wua,
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and wda, respectively, and the width of left and right maxillary posterior teeth by wlp and wrp. Anterior teeth selection. The maxillary anterior teeth with shape, size, and color are the main factors for the facial aesthetic of people. For maxillary anterior teeth, wua equals to the arc length between the lines of angulus oris, which is the condition of choosing teeth. We find the tooth
The mandibular anterior teeth are selected in a similar way.
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dataset which wua is less than but closest to cua (the arrangement curve of maxillary anterior teeth).
Posterior teeth selection. Posterior teeth focus on function recovery, we mainly consider its
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size and occlusion, Posterior teeth selection is similar to the anterior teeth, we choose the teeth according to the length of posterior teeth wp (wlp/wrp ) and the arrange line lp. For the purpose of better occlusion, the left opposite teeth (include the upper and lower teeth) use the upper teeth size
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as their only index in the database. Since the lengths of the posterior teeth are not necessarily
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equal, the posterior teeth of both sides are asymmetrically aligned. The formula is as follows:
w - c < 0 argmin w - l < 0 a
w wua, wud ; j = 0,1,2,...,n
p
w
aj
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argmin
pj
w , w ; j = 0,1,2,...,n lp
rp
(3) (4)
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Where waj and wpj are the the width of anterior teeth and posterior teeth of j-th datasets, and n is
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the number of teeth in the dataset, Formula (3) and (4) illustrate the selection principles of different types teeth, they are using both the teeth arrange lines or curves of patients and the size information of teeth. Likewise, the height of teeth should be considered, but because the problem is complex, this paper put it aside temporarily. As shown in Fig. 1, the automatic teeth selection, we have just described, is followed, optionally, by the manual teeth selection, in which the dental technician has to select all the types
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of teeth in turn. All the types of teeth may be or may not be of the same shape, size, and color. In dentistry, the maxillary teeth and the mandibular teeth have a combination of fixed models and use the former as the benchmark, so the size of former is used as the index of anterior teeth (Fig.1). Rich databases fulfilled the requirements of patients in different sex, personality, and age (SPA) by
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offering several options. 2.2 Arrange artificial teeth virtually 2.2.1 Artificial teeth selection
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In this study, the teeth of the complete denture are set-up virtually. Before that, we choose teeth via the teeth selection system and do some preparation. For instance, the mesial and distal point of anterior teeth are projected on the x-axis of its local coordinate system, while posterior
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teeth project on the line of the central sulcus, then the teeth are moved to arrange curves/lines by
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the process of matrix transformation and rotation.
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2.2.1 Arrangement of anterior teeth
According to the rules in this article, the margin lines of the maxillary anterior teeth are
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consistent with the smile line of the lower lip. The mesial and distal point of teeth are projected
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onto its own x-axis, and the x-axis of teeth is moved to the arranging curve of patients and overlap with each other. After that, the teeth are positioned successfully. Teeth rotate around the x-axis, and its torque angle is determined by the both of fullness sign point pf and the fullness surface Sf , while the mandibular anterior teeth relied on the teeth long axis lma and the base margin ce [1]. Here is an example of locating the maxillary anterior teeth (Fig.4, Fig.5.a). In the first place, we have to import the oral information of a patient and the database of artificial teeth. Then move
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the mesial point of LU1 to the facial midline lm and put the distal point of teeth on the cua, and rotate the teeth around its own x-axis according to the torque angle. After that, we move the mesial point of LU2 to the distal point of LU1 and made LU2 rotated to a certain torque angle, and then arranged LU3. Finally, the left anterior teeth were treated by mirror image with the median sagittal
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plane Pms as the mirror plane. The right anterior teeth were obtained. 2.2.2 Arrangement of posterior teeth
The posterior teeth were arranged in 4 steps, including tooth pretreatment, arrangement of the
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posterior teeth, lower molars, and rearrangement of the posterior teeth.
Pretreat posterior tooth. For the posterior teeth, the maxillary teeth arrangement method is adopted in the paper. We first arrange the maxillary posterior teeth, and then arrange the
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mandibular posterior teeth. Before the arranging, the mesial and distal points of the maxillary posterior teeth are projected on its own x-axis, while the mandibular posterior teeth projected on
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its own central sulcus line. The posterior arrangement line lp is generated by the both points, one is
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the distal point of incisors and the other is the middle point between the retromolar pad anterior and posterior border. The arrangement of left posterior teeth is illustrated in Fig.4 and the results
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of virtual posterior teeth arrangement as shown in Fig.5 b and c.
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Arrange the maxillary posterior teeth. First of all, arrange the first maxillary molar LU4, which means that the superposition of the local coordinate system xoz plane and occlusal surface is required. The both points of projected are moved to the arrangement line llp, and then use the same way to set-up the LU5, LU6, and LU7. The mandibular posterior teeth arrangement is similar to the maxillary teeth. The only difference is that the former has no coordinate system, and the central sulcus line lcs coincides with llp, the positioning sequence is LD4, LD5, LD6, and LD7.
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Rearrange the mandibular posterior teeth. In order to adjust the occlusion interference between the jaw of upper and lower, we need to re-adjust the position of mandibular posterior teeth according to the fitting curve of maxillary posterior teeth. The lingual cusp points pt of left maxillary posterior are used to fit the Spee’s curve clt , then lcs is moved to clt, and the mandibular
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posterior rotate around the lcs until the mandibular posterior buccal cusp region and central sulcus of maxillary posterior region contact with each other. This step increases the tooth contact between
2.2.3 Adjacent teeth gap adjustment
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the opposing teeth and aims at maximizing contacted intercuspal position.
The adjustment of the adjacent tooth gap runs through the entire tooth arrangement process.
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Since artificial teeth are rigid objects in the real world, there is no interference when teeth are inserted into the denture base, and a small gap is required to keep each other apart during the
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arrangement. Once interference exists, teeth cannot insert to the dimples in the denture base and
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they need to be polished. This not only difficult to clean up teeth but also cause the retention of Candida albicans. Therefore, we should be sure to perform this step each time when we change the
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position of teeth except the central incisor. As shown in Table 3, d means the actual interference
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distance (d01) or gap (d02), d1 ,d2 means step length (d2 is recommended by the dentist for the gap size), they are empirical value. LU2 and LU4 represent the adjustment of anterior teeth and posterior teeth respectively. Finally, the value of gap ranges from [0, d2] (Fig.6).
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Table 3 Gap adjustment of adjacent teeth Code with gap adjustment of posterior teeth(LU4)
While(d>=0) // has interference -step1 { Move Mesial point On Curve(cua,d1, leave LU1); Get Distal point On Curve(LU2, Mesial , Distal ); Rotate Up Front teeth(LU2); } While(d> d2,)// has gap, no interference- step2 { Move Mesial point On Curve(cua,d2, close LU1); Get Distal point On Curve(LU2, Mesial , Distal ); Rotate Up Front teeth(LU2); }
While( d>=0) // has interference -step1 { Translate Teeth( llp , d1 , leave LU3); } While( d> d21,) has gap, no interference- step2 { Translate Teeth( llp , d2, close LU3); }
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Note: d1>d2>0, cua/ llp indicating that the teeth move along the line/curve.
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Code with gap adjustment of anterior teeth(LU2)
2.2.4 Virtual detection and elimination of occlusal interference
The purpose of virtual occlusion adjustment is to reduce the excessive interference. According
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to the recommendations of dentists, the upper and the lower posterior teeth should keep
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micro-interference, such as 0.05 mm. There are two cases of jaw occlusion: non-interference and excessive interference. The problem of non-interference had been solved by mandibular posterior
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teeth re-arrangement. However, excessive interference problem is unsettled. The size and position
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of the interference region are recorded, and then used for NC machining to eliminate. The amount
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of interference on the triangular mesh of teeth is calculated as follows:
D = length V V '
i
i
i
(5)
In which Di is the interference intensity at a vertex on a loop of interference region, Vi is a vertex of triangular mesh of tooth, Take Vi as the original point, make a ray and intersect the opposite tooth of at the vertex. The vertex is Vi’, and length is a function of the length of the vector. As shown in Fig.7, there is occlusion interference in the lower dentition. In fact, the same
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interference is in the upper dentition but we need to deal with only one of them to reduce the excessive interference. The colored areas are interference region, and the deeper color the more interference is (Fig.7.a). The result of eliminating the interference is shown in Fig.7.b. 2.3 Teeth arrangement optimization
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It is difficult to evaluate whether the selected teeth is reasonable before arranging. However, lots of information can be obtained by observing the teeth on the screen after the teeth set-up virtually. The means of the teeth arrangement optimization including re-extraction the anatomic
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characteristics of patients, posterior teeth arrangement line adjustment, and re-selection of artificial teeth (Fig.8).
Re-extraction anatomic characteristics of the patient. It is the most complex solution to
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optimize the teeth set-up and eliminate the existing accumulative error on manual interaction. However, the following cases can only be dealt with in this way, and they are often caused by the
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wrong patient characteristics that are extracted. First, the teeth cannot be arranged successfully.
information.
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Second, the anterior teeth are out of the alveolar ridge. After that, re-import the patient feature
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Adjustment the arrangement line of posterior teeth. The arrangement line lp of posterior
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teeth and its deflection angle directly affect the contact and the force between the dentition and the cheek of patients. We can observe that the results of the tooth arrangement on the screen. If the length of posterior teeth does not match with the wax rim, then adjust the length of lp or change the deflection angle of lp outward or inward, but the position of the second molar can never exceed the retromolar pad. Then enter the next step to choose the artificial teeth again.
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Re-selection of artificial teeth. As for a patient, the results of teeth automatic selection are the same for every time, because their anatomical features are uniqueness, and it’s why the manual selection is introduced into the teeth selected system. Technicians choose teeth according to their own experience, they can maintain the anterior teeth while re-selected the posterior teeth or
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re-selected all the teeth.
3. Results
In order to verify the proposed method in this paper, experiments were performed with five
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models. The five models include the standard model and the edentulous patients. The teeth arrangement is shown in Table.4 and Fig.9. The denture products of which the teeth arranged via traditional and digital are shown in the plot (Fig.10). The development platform is Windows 7
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system, Visual Studio2008, OpenGL.
Table 4 Comparison of traditional and digital on time for teeth arrangement Sex &Age
Standard model Patient model 1
Female,82 Male,60
30
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Patient model 2
Traditional (min)
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Model label
Digital (s)
Dentist comments
6.280
Good
6.286
Good
6.474
Normal
Patient model 3
Male,68
6.079
Good
Patient model 4
Female,78
7.705
Normal
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Note: “Good” means teeth are on the alveolar ridge (ALR), the second molar does not exceed the retromolar pad.
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“Normal” means some teeth are a little bit off the ALR. “Bad” means all the teeth are out of the ALR.
4.Discussion For the moment, a number of companies have studied the virtual tooth arrangement and
manufacture of complete dentures, such as Dentca (USA), AvDent (USA), Ceramill FDS (AT), Dentalwings (DK), 3Shape (DK). However, the virtual tooth arrangement is not described in detail. Dentca [14] uses 3D to print the whole complete denture, and Dentalwings [16] prints one-set
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aligned artificial tooth (additional strength) and inserts wax based denture, the method is currently not available for clinical trials. AvDent [15] and Ceramill FDS CNC [17] process a wax based denture with teeth sockets, while we use 3D print the wax based denture. As for 3Shape, the online Video data for Full Denture Design (the software of complete denture design) shows that
adjustment of occlusal interference detection and adjustment.
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the teeth occlusal interference is not taken into account in the design, while we have the
In this study, a multilevel artificial tooth database was constructed and the tooth arrangement
arrangement were analyzed as follows:
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was performed on the basis of feature information of oral and artificial teeth. The results of tooth
Teeth morphology and position analysis. By observing them on the screen after all the
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operations (Fig.9), we can found that all the teeth are on the alveolar ridge and the position of the mandibular second molar on the inside of the retromolar pad (the point behind the tooth, Fig.9),
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and therefore, the teeth are able to fulfill the requirement of the medical rules on its morphology
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and position.
Teeth occlusion analysis. After the virtual teeth arrangement, the problem of non-interference
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has been solved to a certain extent. In addition, the excessive interference can be solved by milling
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or grinding. Therefore, the data of mandibular interference is used as the reference for post processing. Efficiency analysis. The time of automatic tooth arrangement is less than 10 seconds, while
the conventional way is 30 minutes (Table.4). Virtual teeth arrangement reduces the chair-time and saves technicians working hours. At the same time, the virtual teeth arrangement is repeatable and easy to file for the next visit. Finally, virtual artificial teeth set-up is popular with its higher
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precision and clinical applicability. We have used artificial teeth for alignment so that we can make experiments directly with artificial teeth on the market [14-15], while the 3D method is limited by materials and techniques.in the short term [14, 16-17]. In this paper, we mainly focus on general edentulous patients. For the patient who’s
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mandibular arch is too short or severe atrophy of alveolar ridge, subtracting the first premolar or second molar in the database when the minimum artificial tooth cannot meet the demand. For the anterior teeth, if its jaw relationship is normal, the teeth will be arranged with light overjet and
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overbite, otherwise, they will be arranged with deep overjet and overbite. To this end, we change the distance between the x-axis of the local coordinate system of mandibular anterior and its incisal edge. We'd move the anterior teeth's x-axis close to its incisal edge if they are light overjet
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and overbite, otherwise, move them to the opposite direction.
There are some limitations on virtual teeth placement of full denture. First of all, the error
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accumulation in feature extraction of patients and teeth has a great effect on the tooth selection
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and alignment. Besides, it is difficult to achieve balance dentures by digital design at present [22], this study can only achieve the lingualized occlusion, while protrusive balanced occlusion and
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lateral occlusion are still under research.
5. Conclusion This study presents an approach for digital tooth arrangement of full denture base on a
multi-level artificial teeth selection system. The denture with artificial teeth performs well on the clinical applicability and is easy to popularize and apply. In addition, it is possible to perform individualized patient tooth arrangement in dentistry.
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Conflict of interest None declared.
Acknowledgment
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This study was financially supported by the National Natural Science Foundation of China (No.51475004), the Natural Science Foundation of Jiangsu Province, China (No.BK20161487)， Six talent peaks project in Jiangsu Province, China(No. GDZB-034), and The Key Research and
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Development Program of Jiangsu Province (No. BE2016010-4). We also thanks very much to the journal editor and reviewers for their valuable comments to our paper.
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Removable Dentures : PartI A System for Complete Denture Fabrication, Journal of the Japan Prosthodontic Society, 37 (1993) 800-805. [6] Y. Maeda, M. Minoura, S. Tsutsumi, M. Okada, T. Nokubi, A CAD/CAM system for removable denture. Part I: Fabrication of complete dentures, international Journal of Prosthodontics, 7 (1994). [7] N. Kawahata, H. Ono, Y. Nishi, T. Hamano, E. Nagaoka, Trial of duplication procedure for complete dentures by CAD/CAM, Journal of oral rehabilitation, 24 (1997) 540-548. [8] M. Kanazawa, M. Inokoshi, S. Minakuchi, N. Ohbayashi, Trial of a CAD/CAM system for fabricating complete dentures, Dental materials journal, 30 (2011) 93. [9] M. Inokoshi, M. Kanazawa, S. Minakuchi, Evaluation of a complete denture trial method applying rapid prototyping, Dental Materials Journal, 31 (2012) 40-46. [10] Y. Sun, P. Lü, Y. Wang, Study on CAD&RP for removable complete denture, Computer methods and
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programs in biomedicine, 93 (2009) 266-272. [11] Y. Wang, P. Lv, Y. Wang, Y. Sun, Y. Zhao, Methodology of computer-aided design of automatic artificial tooth selection for complete denture, Beijing da xue xue bao. Yi xue ban= Journal of Peking University. Health sciences, 42 (2010) 108-110. [12] W.U. Guofeng Xian, D.O. Prosthodontics, S.O. Stomatology, The selection and arrangement of artificial teeth for complete dentures, Journal of Practical Stomatology, 28 (2012) 262-266. [13] A.S. Bidra, T.D. Taylor, J.R. Agar, Computer-aided technology for fabricating complete dentures: systematic review of historical background, current status, and future perspectives, The Journal of prosthetic dentistry, 109 (2013) 361-366.
systems, Journal of the California Dental Association, 41 (2013) 407-416.
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[14] M. Kattadiyil, C. Goodacre, N. Baba, CAD/CAM complete dentures: a review of two commercial fabrication
[15] L. Infante, B. Yilmaz, E. McGlumphy, I. Finger, Fabricating complete dentures with CAD/CAM technology, The Journal of prosthetic dentistry, 111 (2014) 351-355.
[16] M.S. Bilgin, A. Erdem, O.S. Aglarci, E. Dilber, Fabricating complete dentures with CAD/CAM and RP technologies, Journal of Prosthodontics, 24 (2015) 576-579.
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[17] T. Wimmer, K. Gallus, M. Eichberger, B. Stawarczyk, Complete denture fabrication supported by CAD/CAM, The Journal of prosthetic dentistry, 115 (2016) 541-546.
[18] J.B. McLaughlin, V. Ramos, Complete denture fabrication with CAD/CAM record bases, The Journal of prosthetic dentistry, 114 (2015) 493-497.
[19] A.F. de Mendonça, M. Furtado de Mendonça, G.S. White, G. Sara, D. Littlefair, Total CAD/CAM Supported Method for Manufacturing Removable Complete Dentures, Case Reports in Dentistry, 2016 (2016). [20] M.S. Bilgin, E.N. Baytaroğlu, A. Erdem, E. Dilber, A Review of CAD/CAM techniques for removable
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denture fabrication, European Journal of Dentistry, 10 (2016) 286.
[21] M.T. Kattadiyil, R. Jekki, C.J. Goodacre, N.Z. Baba, Comparison of treatment outcomes in digital and conventional complete removable dental prosthesis fabrications in a predoctoral setting, The Journal of prosthetic
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dentistry, 114 (2015) 818-825.
[22] N.Z. Baba, Materials and Processes for CAD/CAM Complete Denture Fabrication, Current Oral Health
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Reports, 3 (2016) 203-208.
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Fig.1. Artificial teeth database and selection system
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Fig.2. Anatomic features of teeth
Fig.3. Anatomic features of edentulous patient
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Fig.4. Flow chart of tooth arrangement
(a) Anterior teeth placement. (b) Maxillary posterior teeth placement. (c) Mandibular posterior teeth placement.
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Fig.5. Virtual tooth arrangement
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(a) Schematic diagram of adjustment steps
(b) Initial position
(c) Auto adjustment result (step1)
(d) Auto adjustment result (step2)
Fig.6. Adjust the gap between the adjacent teeth (LU1&LU2)
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(b) Virtual occlusion adjustment result
(a) Detection of occlusion interference
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Fig.7. Detection and adjustment of occlusion interference
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Fig.8. Optimization of virtual tooth arrangement
(a) Standard model
(b) Patient model1
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Fig.9. Virtual tooth arrangement of standard model & patient model1
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Fig.10. Conventional full denture (up) with digital design (down) finished products.
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Study on digital teeth selection and virtual teeth arrangement for complete denture Xiaoling Yu , Xiaosheng Cheng , Ning Dai , Hu Chen , Changjiang Yu , Yuchun Sun PII: DOI: Reference:
S0169-2607(17)30349-8 10.1016/j.cmpb.2017.11.022 COMM 4553
To appear in:
Computer Methods and Programs in Biomedicine
Received date: Revised date: Accepted date:
28 March 2017 18 November 2017 24 November 2017
Please cite this article as: Xiaoling Yu , Xiaosheng Cheng , Ning Dai , Hu Chen , Changjiang Yu , Yuchun Sun , Study on digital teeth selection and virtual teeth arrangement for complete denture, Computer Methods and Programs in Biomedicine (2017), doi: 10.1016/j.cmpb.2017.11.022
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Highlights Digital artificial teeth selection system.
Virtual teeth arrangement
Optimization method of tooth arrangement
The process of teeth alignment is introduced in detail
The means of adjustment for adjacent teeth clearance is described
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Study on digital teeth selection and virtual teeth arrangement for complete denture Xiaoling Yua , Xiaosheng Chenga, Ning Dai a,*, Hu Chenb, Changjiang Yua, Yuchun Sunb,*
a College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics, Mailbox 357, 29 Yudao Street, Qinhuai District, Nanjing 210016, PR China
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b Center of Digital Dentistry, Faculty of Prosthodontics, Peking University School and Hospital of Stomatology & National Engineering Laboratory for Digital and Material Technology of Stomatology & Research Center of Engineering and Technology for Digital Dentistry of Ministry of Health, 100081 Beijing, PR China
Abstract:
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Background and objective: In dentistry, the complete denture is a conventional treatment for edentulous patients. The computer-aided design and computer-aided manufacturing (CAD/CAM) has been applied on the digital complete denture which is developed rapidly. Tooth selection and arrangement is one of the most important parts in digital complete denture. In this paper, we propose a new method of personalized teeth arrangement. Methods: This paper presents a method of arranging teeth virtually for a complete denture. First, scan and extract the feature points of the 3D triangular mesh data of artificial teeth (PLY format), then establish a tooth selection system. Second, scan and mark the anatomic characteristics of the maxillary and mandibular cast
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surfaces, such as facial midline, the curve of the arches. With the enter information, the study calculates the common arrangement lines of artificial teeth. Third, select the preferred artificial teeth and automatically
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arrange them virtually in the correct position by using our own software. After that, design the gingival part of
the dentures on the basic of the arranged teeth on the screen and then fabricated it by using Computerized Numerical Control (CNC) technology, Rapid Prototyping (RP) technology or 3D printer technology. Finally, select artificial teeth were embedded in wax rims.
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Results: This system can choose artificial teeth reasonably and the teeth placement can meet the dentist’s request to a certain extent, whereas all the operations are based on the medical principles.
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Conclusions: The study performed here involves computer sciences, medicine, and dentistry, a teeth selection system was proposed and virtual teeth arrangement was described. This study has the capacity of helping operators
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to select teeth, which improved the accuracy of tooth arrangement, and customized complete denture.
Keywords: complete denture; artificial teeth database; teeth selection system; personalized virtual teeth arrangement
* Corresponding author. E-mail address: [email protected] (Ning Dai), [email protected] (Yuchun Sun).
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1. Introduction With the aging of the world population, the number of patients with edentulous jaw has been growing rapidly, and the complete denture is the most common and effective treatment. In addition, teeth arrangement is crucial to the function and esthetics of complete denture restoration.
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The traditional process of full denture is complicated, especially in arranging teeth manually. This process was heavily depended on the experience and technique skills of the technician, which is time-consuming and has low precision of operation and repeatability.
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Since CAD/CAM was first introduced to clinical prosthodontics in the 1980s [1-2], many researchers devoted themselves to the study of robot-aided teeth alignment of complete denture and have made some achievements [3-4].Compared with the conventional method of teeth set-up,
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the teeth alignment of complete denture with the help of robot was more reliable and flexible. However, the additional device was necessary and the irregular shape of the artificial tooth
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increase the difficulty of grasping. Besides the repeated alignment of the teeth also makes the
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devices become complicated.
In order to simplify the teeth arrangement, digital tooth set-up was proposed when CNC and
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RP have been applied in the field of prosthodontics. The fabrication of complete denture mainly
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depends on the artificial teeth from then on. Maeda et al. [5-6] proposed an approach to study the complete denture. They fabricated two plastic outer shells of dentition by 3D laser lithography and filled. But the method was not practicable. Kawahata et al. [7] scanned the complete denture surface and then fabricated the prosthodontics wax type by CNC and studied on the duplication procedure. Kanzawa and Inokosh [8-9] used a three-dimensional (3D) cone-beam computed tomography (CBCT) to scan the base and artificial teeth of one complete denture separately, and
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created a new one base on the scanning data. They fabricated an acrylic resin denture base by using RP [8] and a totally wax-free manufacturing process by combining RP and rapid milling [9]. Finally, the teeth were adapted and bonded into the milled sockets of the milled base. Sun et al. [10] obtained the 3D data of artificial teeth and edentulous, then they defined the anatomic
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landmark for artificial teeth and edentulous. After that, they formulated a principle of tooth arrangement and created individual physical flasks by using the 3D printer. However, in their study, they have not formed a complete system and it is hard to select artificial teeth according to
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the sex and age of edentulous patients.. Wang Y et al. [11] studied on the logical expression of automatic tooth selection for complete denture by CAD, but there is no discussion of different models. Wu et al. [12] described a traditional method of artificial teeth selection and arrangement
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of the complete denture, which as a reference for digital. Digital teeth arrangement and selection have significant advantages compared with the conventional arrangement: stable in the accuracy
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of teeth arrangement, free from manual errors, less chair-time and short cycle [13, 21-22].
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To date, the virtual teeth arrangement technologies have been added to the CAD of the full denture, then the denture is fabricated by using CNN, RP and 3D printer. Kattadiyil [14] reported
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the comparison between two commercial fabrication systems. In his report, both teeth were set
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virtually and used two-step method. The Dentca system printing the whole denture by using the 3D printer and the Avadent system fabricates denture base with CNC. Infante [15] obtained the required data and arranged teeth virtually on the Avadent system. Then he sent the data to the machining center. Bilgin et al. [16] investigated a one-set aligned artificial tooth (1SA) by Dental Wings system and then manufactured as one piece by using CAD/CAM and RP. But it has not been extending due to the security of printed material. Wimmer [17] proposed the position of
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artificial teeth through the Ceramill Full Denture system that can eliminate occlusal interference by itself. Finally, they use CNC to fabricate the denture base and insert the artificial tooth manually. Baba [22] reported four denture systems in detail and revealed the advantages and disadvantages of CAD/CAM complete dentures.
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Although the use of CAD/CAM &RP techniques for the fabricating of complete denture has been widely reported [15-20], the description of the virtual teeth arrangement is not detailed. The purpose of this study is to describe a new method of virtual artificial teeth arrangement for a full
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denture, which bases on a multilevel database and offers a series of optimization methods.
2. Material and methods
The patient models in the article were provided by the Department of Prosthodontics, Peking
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University School and Hospital of Stomatology, patients were elderly and edentulous. The participant voluntarily joined the study and provided informed consent.
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2.1 Artificial teeth selection system
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2.1.1 Artificial tooth database
In this paper, we establish an artificial tooth database and form a tooth selection system. The
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database contains a lot of artificial teeth datasets and the general definition function of a teeth
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dataset has the form:
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S = A B i A = L, R ; B = U,D
(1)
i=1
Artificial teeth are selected automatically and/or manually by a dentist from the database (Fig.1) that consists of several datasets of different tooth sizes, shapes, and colors for each of four types of the teeth: the upper/lower anterior teeth and the left/ right posterior teeth. The teeth in the datasets are uniquely identified by ABi, where A = {L, R}; B = {U, D}, whereas letters L, R, U, 5
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and D denote the left, right, maxillary, and mandibular teeth, respectively, and i is the number from 1 to 7 that represent central and lateral incisors, canines, first premolar and second premolar, first molar and second molar, respectively. For example, LU1 means the left maxillary central incisor. For each tooth, the database stores its 3D model information and parameterized
serves as the index of the database. 2.1.2 Definition of anatomical characteristics of artificial teeth
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anatomical features, from which the total width (w) of the teeth in the dataset is calculated and
We used a dental 3D scanner (3Shape Q-750, 0.02-mm accuracy; 3Shape, Copenhagen,
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Denmark) to scan the cervical area of the artificial teeth to obtain the triangular mesh data (STL/PLY) from which we extracted the anatomical features of teeth via our own software. The anatomical features of artificial teeth are mainly used for teeth translation and rotation in the process of teeth placement. The teeth of anterior, posterior, maxillary and mandible are different in
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location mode, thus, the characteristic parameters are not identical (Table.1 & Fig.2). To facilitate the storage and management of the artificial tooth database, an XML file for each
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tooth was built to store the teeth and their characteristics. We also create an XML file for each
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database, storing the tooth's storage path and tooth width (index) information.
Table 1 Anatomical features of artificial teeth Local
Mesial/Distal
Fullness
long axis of
Central sulcus
cheek/tongue
name
coordinate
points
sign point
tooth lma
line
points
system xoz
pnea/pfar
pf
lcs
pc/pt
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Teeth
+
+
+
－
－
－
D1-D3
+
+
－
＋
－
－
U4-U7
+
+
－
－
＋
＋
D4-D7
-
+
－
＋
＋
＋
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U1-U3
Note: “+” means “yes”, “-” means “no”
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2.1.3 Definition of anatomical characteristics of edentulous patients The edentulous patients’ anatomic features, which includes midline, lip lines, and occlusion plane, and so on, are extracted from the triangular mesh model (STL/PLY), obtained by the dental 3D scanner, and serve for the teeth set-up [10], as shown in Table 2 and Fig.3. Table 2 Anatomic features of edentulous patient Symbol
Facial midline
lm
High/low lip
Occlusion plane
Pj
Fullness surface
Middle sagittal
Pms
Cuspid lines of left/ right
llc/ lrc
Margin of base
ce
Lingual cusp curve
clt/ crt
llp/ lrp
Anterior teeth arrange curves of up/down
cua/ cda
Symbol lh/ ld Sf
Retromolar pad of left/right
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Posterior teeth arrange lines of left/right
Feature name
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Feature name
2.1.4 Teeth selection
Crest of ridge
plmrp/ prmrp Scr
Teeth set-up with all these factors: the distal surface of maxillary cuspid is consistent with the
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maxillary cuspid line in the direction of mesiodistal, the distal surface of mandibular second molar
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does not exceed the retromolar pad [3]. Teeth selection usually base on the experience of the dental technician, they predict teeth location on the wax rim and then choose teeth subjectively.
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However, unreasonable tooth selection makes it difficult to meet the teeth set-up requirement. For
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this purpose, we used the teeth database as described above to establish a digital teeth selection system. According to the experience skills of dentists, we get the appropriate teeth by comparing
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the width of the teeth and the length of the patient's arch. The calculating formula of the width of artificial teeth in a dataset is following: n
w | i 0
p
fari
p
neai
|
w
w , w , w , w ua
ud
lp
i; = 0 , 1 , 2 , . . . ,(2) n
rp
Where pnear and pfar are mesial and distal points of i-th tooth, respectively, and n is the number of teeth in the dataset. We denote the width of anterior maxillary and mandibular teeth dataset by wua,
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and wda, respectively, and the width of left and right maxillary posterior teeth by wlp and wrp. Anterior teeth selection. The maxillary anterior teeth with shape, size, and color are the main factors for the facial aesthetic of people. For maxillary anterior teeth, wua equals to the arc length between the lines of angulus oris, which is the condition of choosing teeth. We find the tooth
The mandibular anterior teeth are selected in a similar way.
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dataset which wua is less than but closest to cua (the arrangement curve of maxillary anterior teeth).
Posterior teeth selection. Posterior teeth focus on function recovery, we mainly consider its
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size and occlusion, Posterior teeth selection is similar to the anterior teeth, we choose the teeth according to the length of posterior teeth wp (wlp/wrp ) and the arrange line lp. For the purpose of better occlusion, the left opposite teeth (include the upper and lower teeth) use the upper teeth size
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as their only index in the database. Since the lengths of the posterior teeth are not necessarily
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equal, the posterior teeth of both sides are asymmetrically aligned. The formula is as follows:
w - c < 0 argmin w - l < 0 a
w wua, wud ; j = 0,1,2,...,n
p
w
aj
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argmin
pj
w , w ; j = 0,1,2,...,n lp
rp
(3) (4)
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Where waj and wpj are the the width of anterior teeth and posterior teeth of j-th datasets, and n is
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the number of teeth in the dataset, Formula (3) and (4) illustrate the selection principles of different types teeth, they are using both the teeth arrange lines or curves of patients and the size information of teeth. Likewise, the height of teeth should be considered, but because the problem is complex, this paper put it aside temporarily. As shown in Fig. 1, the automatic teeth selection, we have just described, is followed, optionally, by the manual teeth selection, in which the dental technician has to select all the types
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of teeth in turn. All the types of teeth may be or may not be of the same shape, size, and color. In dentistry, the maxillary teeth and the mandibular teeth have a combination of fixed models and use the former as the benchmark, so the size of former is used as the index of anterior teeth (Fig.1). Rich databases fulfilled the requirements of patients in different sex, personality, and age (SPA) by
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offering several options. 2.2 Arrange artificial teeth virtually 2.2.1 Artificial teeth selection
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In this study, the teeth of the complete denture are set-up virtually. Before that, we choose teeth via the teeth selection system and do some preparation. For instance, the mesial and distal point of anterior teeth are projected on the x-axis of its local coordinate system, while posterior
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teeth project on the line of the central sulcus, then the teeth are moved to arrange curves/lines by
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the process of matrix transformation and rotation.
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2.2.1 Arrangement of anterior teeth
According to the rules in this article, the margin lines of the maxillary anterior teeth are
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consistent with the smile line of the lower lip. The mesial and distal point of teeth are projected
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onto its own x-axis, and the x-axis of teeth is moved to the arranging curve of patients and overlap with each other. After that, the teeth are positioned successfully. Teeth rotate around the x-axis, and its torque angle is determined by the both of fullness sign point pf and the fullness surface Sf , while the mandibular anterior teeth relied on the teeth long axis lma and the base margin ce [1]. Here is an example of locating the maxillary anterior teeth (Fig.4, Fig.5.a). In the first place, we have to import the oral information of a patient and the database of artificial teeth. Then move
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the mesial point of LU1 to the facial midline lm and put the distal point of teeth on the cua, and rotate the teeth around its own x-axis according to the torque angle. After that, we move the mesial point of LU2 to the distal point of LU1 and made LU2 rotated to a certain torque angle, and then arranged LU3. Finally, the left anterior teeth were treated by mirror image with the median sagittal
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plane Pms as the mirror plane. The right anterior teeth were obtained. 2.2.2 Arrangement of posterior teeth
The posterior teeth were arranged in 4 steps, including tooth pretreatment, arrangement of the
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posterior teeth, lower molars, and rearrangement of the posterior teeth.
Pretreat posterior tooth. For the posterior teeth, the maxillary teeth arrangement method is adopted in the paper. We first arrange the maxillary posterior teeth, and then arrange the
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mandibular posterior teeth. Before the arranging, the mesial and distal points of the maxillary posterior teeth are projected on its own x-axis, while the mandibular posterior teeth projected on
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its own central sulcus line. The posterior arrangement line lp is generated by the both points, one is
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the distal point of incisors and the other is the middle point between the retromolar pad anterior and posterior border. The arrangement of left posterior teeth is illustrated in Fig.4 and the results
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of virtual posterior teeth arrangement as shown in Fig.5 b and c.
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Arrange the maxillary posterior teeth. First of all, arrange the first maxillary molar LU4, which means that the superposition of the local coordinate system xoz plane and occlusal surface is required. The both points of projected are moved to the arrangement line llp, and then use the same way to set-up the LU5, LU6, and LU7. The mandibular posterior teeth arrangement is similar to the maxillary teeth. The only difference is that the former has no coordinate system, and the central sulcus line lcs coincides with llp, the positioning sequence is LD4, LD5, LD6, and LD7.
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Rearrange the mandibular posterior teeth. In order to adjust the occlusion interference between the jaw of upper and lower, we need to re-adjust the position of mandibular posterior teeth according to the fitting curve of maxillary posterior teeth. The lingual cusp points pt of left maxillary posterior are used to fit the Spee’s curve clt , then lcs is moved to clt, and the mandibular
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posterior rotate around the lcs until the mandibular posterior buccal cusp region and central sulcus of maxillary posterior region contact with each other. This step increases the tooth contact between
2.2.3 Adjacent teeth gap adjustment
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the opposing teeth and aims at maximizing contacted intercuspal position.
The adjustment of the adjacent tooth gap runs through the entire tooth arrangement process.
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Since artificial teeth are rigid objects in the real world, there is no interference when teeth are inserted into the denture base, and a small gap is required to keep each other apart during the
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arrangement. Once interference exists, teeth cannot insert to the dimples in the denture base and
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they need to be polished. This not only difficult to clean up teeth but also cause the retention of Candida albicans. Therefore, we should be sure to perform this step each time when we change the
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position of teeth except the central incisor. As shown in Table 3, d means the actual interference
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distance (d01) or gap (d02), d1 ,d2 means step length (d2 is recommended by the dentist for the gap size), they are empirical value. LU2 and LU4 represent the adjustment of anterior teeth and posterior teeth respectively. Finally, the value of gap ranges from [0, d2] (Fig.6).
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Table 3 Gap adjustment of adjacent teeth Code with gap adjustment of posterior teeth(LU4)
While(d>=0) // has interference -step1 { Move Mesial point On Curve(cua,d1, leave LU1); Get Distal point On Curve(LU2, Mesial , Distal ); Rotate Up Front teeth(LU2); } While(d> d2,)// has gap, no interference- step2 { Move Mesial point On Curve(cua,d2, close LU1); Get Distal point On Curve(LU2, Mesial , Distal ); Rotate Up Front teeth(LU2); }
While( d>=0) // has interference -step1 { Translate Teeth( llp , d1 , leave LU3); } While( d> d21,) has gap, no interference- step2 { Translate Teeth( llp , d2, close LU3); }
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Note: d1>d2>0, cua/ llp indicating that the teeth move along the line/curve.
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Code with gap adjustment of anterior teeth(LU2)
2.2.4 Virtual detection and elimination of occlusal interference
The purpose of virtual occlusion adjustment is to reduce the excessive interference. According
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to the recommendations of dentists, the upper and the lower posterior teeth should keep
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micro-interference, such as 0.05 mm. There are two cases of jaw occlusion: non-interference and excessive interference. The problem of non-interference had been solved by mandibular posterior
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teeth re-arrangement. However, excessive interference problem is unsettled. The size and position
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of the interference region are recorded, and then used for NC machining to eliminate. The amount
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of interference on the triangular mesh of teeth is calculated as follows:
D = length V V '
i
i
i
(5)
In which Di is the interference intensity at a vertex on a loop of interference region, Vi is a vertex of triangular mesh of tooth, Take Vi as the original point, make a ray and intersect the opposite tooth of at the vertex. The vertex is Vi’, and length is a function of the length of the vector. As shown in Fig.7, there is occlusion interference in the lower dentition. In fact, the same
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interference is in the upper dentition but we need to deal with only one of them to reduce the excessive interference. The colored areas are interference region, and the deeper color the more interference is (Fig.7.a). The result of eliminating the interference is shown in Fig.7.b. 2.3 Teeth arrangement optimization
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It is difficult to evaluate whether the selected teeth is reasonable before arranging. However, lots of information can be obtained by observing the teeth on the screen after the teeth set-up virtually. The means of the teeth arrangement optimization including re-extraction the anatomic
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characteristics of patients, posterior teeth arrangement line adjustment, and re-selection of artificial teeth (Fig.8).
Re-extraction anatomic characteristics of the patient. It is the most complex solution to
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optimize the teeth set-up and eliminate the existing accumulative error on manual interaction. However, the following cases can only be dealt with in this way, and they are often caused by the
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wrong patient characteristics that are extracted. First, the teeth cannot be arranged successfully.
information.
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Second, the anterior teeth are out of the alveolar ridge. After that, re-import the patient feature
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Adjustment the arrangement line of posterior teeth. The arrangement line lp of posterior
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teeth and its deflection angle directly affect the contact and the force between the dentition and the cheek of patients. We can observe that the results of the tooth arrangement on the screen. If the length of posterior teeth does not match with the wax rim, then adjust the length of lp or change the deflection angle of lp outward or inward, but the position of the second molar can never exceed the retromolar pad. Then enter the next step to choose the artificial teeth again.
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Re-selection of artificial teeth. As for a patient, the results of teeth automatic selection are the same for every time, because their anatomical features are uniqueness, and it’s why the manual selection is introduced into the teeth selected system. Technicians choose teeth according to their own experience, they can maintain the anterior teeth while re-selected the posterior teeth or
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re-selected all the teeth.
3. Results
In order to verify the proposed method in this paper, experiments were performed with five
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models. The five models include the standard model and the edentulous patients. The teeth arrangement is shown in Table.4 and Fig.9. The denture products of which the teeth arranged via traditional and digital are shown in the plot (Fig.10). The development platform is Windows 7
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system, Visual Studio2008, OpenGL.
Table 4 Comparison of traditional and digital on time for teeth arrangement Sex &Age
Standard model Patient model 1
Female,82 Male,60
30
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Patient model 2
Traditional (min)
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Model label
Digital (s)
Dentist comments
6.280
Good
6.286
Good
6.474
Normal
Patient model 3
Male,68
6.079
Good
Patient model 4
Female,78
7.705
Normal
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Note: “Good” means teeth are on the alveolar ridge (ALR), the second molar does not exceed the retromolar pad.
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“Normal” means some teeth are a little bit off the ALR. “Bad” means all the teeth are out of the ALR.
4.Discussion For the moment, a number of companies have studied the virtual tooth arrangement and
manufacture of complete dentures, such as Dentca (USA), AvDent (USA), Ceramill FDS (AT), Dentalwings (DK), 3Shape (DK). However, the virtual tooth arrangement is not described in detail. Dentca [14] uses 3D to print the whole complete denture, and Dentalwings [16] prints one-set
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aligned artificial tooth (additional strength) and inserts wax based denture, the method is currently not available for clinical trials. AvDent [15] and Ceramill FDS CNC [17] process a wax based denture with teeth sockets, while we use 3D print the wax based denture. As for 3Shape, the online Video data for Full Denture Design (the software of complete denture design) shows that
adjustment of occlusal interference detection and adjustment.
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the teeth occlusal interference is not taken into account in the design, while we have the
In this study, a multilevel artificial tooth database was constructed and the tooth arrangement
arrangement were analyzed as follows:
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was performed on the basis of feature information of oral and artificial teeth. The results of tooth
Teeth morphology and position analysis. By observing them on the screen after all the
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operations (Fig.9), we can found that all the teeth are on the alveolar ridge and the position of the mandibular second molar on the inside of the retromolar pad (the point behind the tooth, Fig.9),
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and therefore, the teeth are able to fulfill the requirement of the medical rules on its morphology
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and position.
Teeth occlusion analysis. After the virtual teeth arrangement, the problem of non-interference
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has been solved to a certain extent. In addition, the excessive interference can be solved by milling
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or grinding. Therefore, the data of mandibular interference is used as the reference for post processing. Efficiency analysis. The time of automatic tooth arrangement is less than 10 seconds, while
the conventional way is 30 minutes (Table.4). Virtual teeth arrangement reduces the chair-time and saves technicians working hours. At the same time, the virtual teeth arrangement is repeatable and easy to file for the next visit. Finally, virtual artificial teeth set-up is popular with its higher
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precision and clinical applicability. We have used artificial teeth for alignment so that we can make experiments directly with artificial teeth on the market [14-15], while the 3D method is limited by materials and techniques.in the short term [14, 16-17]. In this paper, we mainly focus on general edentulous patients. For the patient who’s
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mandibular arch is too short or severe atrophy of alveolar ridge, subtracting the first premolar or second molar in the database when the minimum artificial tooth cannot meet the demand. For the anterior teeth, if its jaw relationship is normal, the teeth will be arranged with light overjet and
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overbite, otherwise, they will be arranged with deep overjet and overbite. To this end, we change the distance between the x-axis of the local coordinate system of mandibular anterior and its incisal edge. We'd move the anterior teeth's x-axis close to its incisal edge if they are light overjet
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and overbite, otherwise, move them to the opposite direction.
There are some limitations on virtual teeth placement of full denture. First of all, the error
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accumulation in feature extraction of patients and teeth has a great effect on the tooth selection
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and alignment. Besides, it is difficult to achieve balance dentures by digital design at present [22], this study can only achieve the lingualized occlusion, while protrusive balanced occlusion and
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lateral occlusion are still under research.
5. Conclusion This study presents an approach for digital tooth arrangement of full denture base on a
multi-level artificial teeth selection system. The denture with artificial teeth performs well on the clinical applicability and is easy to popularize and apply. In addition, it is possible to perform individualized patient tooth arrangement in dentistry.
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Conflict of interest None declared.
Acknowledgment
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This study was financially supported by the National Natural Science Foundation of China (No.51475004), the Natural Science Foundation of Jiangsu Province, China (No.BK20161487)， Six talent peaks project in Jiangsu Province, China(No. GDZB-034), and The Key Research and
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Development Program of Jiangsu Province (No. BE2016010-4). We also thanks very much to the journal editor and reviewers for their valuable comments to our paper.
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Fig.1. Artificial teeth database and selection system
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Fig.2. Anatomic features of teeth
Fig.3. Anatomic features of edentulous patient
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Fig.4. Flow chart of tooth arrangement
(a) Anterior teeth placement. (b) Maxillary posterior teeth placement. (c) Mandibular posterior teeth placement.
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Fig.5. Virtual tooth arrangement
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(a) Schematic diagram of adjustment steps
(b) Initial position
(c) Auto adjustment result (step1)
(d) Auto adjustment result (step2)
Fig.6. Adjust the gap between the adjacent teeth (LU1&LU2)
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(b) Virtual occlusion adjustment result
(a) Detection of occlusion interference
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Fig.7. Detection and adjustment of occlusion interference
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Fig.8. Optimization of virtual tooth arrangement
(a) Standard model
(b) Patient model1
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Fig.9. Virtual tooth arrangement of standard model & patient model1
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Fig.10. Conventional full denture (up) with digital design (down) finished products.
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