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Journal of Peking University (Health Sciences) ›› 2022, Vol. 54 ›› Issue (1): 105-112. doi: 10.19723/j.issn.1671-167X.2022.01.017

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Finite element analyses of retention of removable partial denture circumferential clasps manufactured by selective laser melting

MA Ke-nan1,2,CHEN Hu2,SHEN Yan-ru2,ZHOU Yong-sheng3,WANG Yong2,SUN Yu-chun1,2,△()   

  1. 1. Institute of Medical Technology, Peking University Health Science Center, Beijing 100191
    2. Center of Digital Dentistry, Faculty of Prosthodontics, Peking University School and Hospital of Stomatology, Beijing 100081, China
    3. Department of Prosthodontics, Peking University School and Hospital of Stomatology & National Center of Stomatology & National Clinical Research Center for Oral Diseases & National Engineering Laboratory for Digital and Material Technology of Stomatology & Beijing Key Laboratory of Digital Stomatology & NHC Research Center of Engineering and Technology for Computerized Dentistry & NMPA Key Laboratory for Dental Materials, Beijing 100081, China
  • Received:2021-09-20 Online:2022-02-18 Published:2022-02-21
  • Contact: Yu-chun SUN E-mail:kqsyc@bjmu.edu.cn
  • Supported by:
    National Key Research and Development Program of China(2019YFB1706904);National Natural Science Foundation of China(51705006);Capital’s Training Project for Science and Technology Leading Talents(Z191100006119022);Program for New Clinical Techniques and Therapies of Peking University School and Hospital of Stomatology(PKUSSNCT-19A08);Open Fund of Shanxi Province Key Laboratory of Oral Diseases Prevention and New Materials(KF2020-04);PKU-Baidu Fund(2019BD021)

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Abstract:

Objective: To compare the retentions of different designs of cobalt-chromium (Co-Cr), pure titanium (CP Ti), and titanium alloy (Ti-6Al-4V) removable partial denture (RPD) circumferential clasps manufactured by selective laser melting (SLM) and to analyze the stress distribution of these clasps during the removal from abutment teeth. Methods: Clasps with clasp arm size A (1.9 mm width/1.1 mm thickness at the body and 0.8-taper) or B (1.2 times A) and 0.25 mm or 0.50 mm undercut engagement were modeled on a prepared first premolar die, named as designs A1, A2, A3, and A4, respectively. The density and elastic modulus of SLM-built Co-Cr, CP Ti, and Ti-6Al-4V were measured and given to different groups of clasps. The density, elastic modulus, and Poisson’s ratio of enamel were given to the die. The control group was the cast Co-Cr clasp with design A1, to which the density and elastic modulus of cast Co-Cr alloy were given. The Poisson’s ratio of all metals was 0.33. The initial 5 N dislodging force was applied, and the maximum displacement of the clasp along the insertion path was computed. The load was reapplied with an increment of 5 N than in the last simulation until the clasp was completely dislodged. The retentive force range of different groups of clasps was obtained. The retentive forces of the SLM-built Co-Cr, CP Ti, and Ti-6Al-4V clasps with equivalent computed retentive force range to the control group were validated through the insertion/removal experiment. The von Mises stress distributions of these three groups of SLM-built clasps under 15 N loads were analyzed. Results: SLM-built Co-Cr, CP Ti, and Ti-6Al-4V clasps with designs B1 or B2, and Co-Cr clasps with design A2 had higher retentive forces than those of the control group. SLM-built CP Ti and Ti-6Al-4V clasps with design A1 had lower retentive forces than those of the control group. SLM-built Co-Cr clasp with design A1 and SLM-built CP Ti and Ti-6Al-4V clasps with design A2 had equivalent retentive forces to those of the control group. The insertion/removal experiment showed that the measured retentive forces of these three groups of SLM-built clasps were (21.57±5.41) N, (19.75±4.47) N, and (19.32±2.04) N, respectively. No statistically significant measured retentive force difference was found among these three groups of SLM-built clasps (P>0.05). The maximum von Mises stress of these three groups of SLM-built clasps exceeded their responding yield strength except for the Ti-6Al-4V one. Conclusion: SLM-built Co-Cr circumferential clasps had higher retention than CP Ti and Ti-6Al-4V ones with the same clasp arm size and undercut engagement. The retention of SLM-built circumferential clasps could be adjusted by changing the undercut engagement and clasp arm size. If SLM-built circumferential clasps are used in clinical practice, the Ti-6Al-4V clasp with clasp arm size A and 0.50 mm undercut engagement is recommended considering the long-term use of RPD in the patient’s mouth.

Key words: Design parameters, Selective laser melting, Circumferential clasp, Retention, dental prosthesis, Finite element analyses

CLC Number: 

  • R783

Table 1

Main chemical components of SLM metallic powder"

Metal Chemical element
Co-Cr Co, 62%-66%; Cr, 23%-27%; Mo, 4%-6%; W, 4%-6%
CP Ti Ti≥99.71%
Ti-6Al-4V Ti≥87.76%; Al, 5.50%-6.75%; V, 3.50%-4.50%

Table 2

Main process parameters for different metals"

Metal Laser power output/W Scan speed/
(mm/s)		 Hatch space/
mm		 Laser beam diameter/
mm		 Layer thickness/mm Annealing temperature/℃
Co-Cr 120 800 0.085 0.065 0.03 1 190
CP Ti 120 1 000 0.080 0.065 0.03 650
Ti-6Al-4V 135 800 0.085 0.065 0.03 850

Figure 1

SLM-built metal tensile specimens A, design data; B, clasp specimens built vertically and horizontay."

Table 3

Materials properties for finite element analyses"

Materials Density/(g/cm) Elastic modulus/GPa Poisson’s ratio
Cast Co-Cr 7.6 218 0.33
SLM Co-Cr 8.7 235 0.33
SLM CP Ti 4.4 122 0.33
SLM Ti-6Al-4V 4.5 113 0.33
Enamel 3.0 84 0.33

Figure 2

Frictional contact between clasp and abutment tooth A, contact body; B, target body."

Table 4

Mesh nodes and elements of each group of clasp and abutment tooth"

Design Nodes Elements
A1 663 384 474 065
A2 666 536 476 126
B1 682 203 487 173
B2 683 390 488 282

Figure 3

Mesh of the clasp and abutment tooth"

Figure 4

Boundary conditions for finite element analyses A, force; B, displacement; C, fixed support."

Figure 5

Insertion of an SLM-built circumferential clasp on the die"

Table 5

Elastic modulus and yield strength of SLM metals"

SLM metals Build direction Elastic modulus/GPa, $\bar{x} \pm s_{\bar{x}}$ P value Rp0.2/MPa, $\bar{x} \pm s_{\bar{x}}$ P value
Co-Cr Horizontally 223±45 0.187 727±23 <0.001
Vertically 246±37 691±22
CP Ti Horizontally 129±14 0.877 608±20 0.024
Vertically 122±12 590±18
Ti-6Al-4V Horizontally 111±6 0.052 997±45 <0.001
Vertically 115±3 1 087±9

Table 6

Tensile strength and elongation after fracture of SLM metals"

SLM metals Build direction Tensile Strength/MPa, $\bar{x} \pm s_{\bar{x}}$ P value Elongation after fracture/%, $\bar{x} \pm s_{\bar{x}}$ P value
Co-Cr Horizontally 1 046±73 0.229 2.8±1.2 <0.001
Vertically 1 076±38 6.4±1.4
CP Ti Horizontally 685±21 0.010 24.3±4.9 0.061
Vertically 664±16 27.3±1.9
Ti-6Al-4V Horizontally 1 007±44 <0.001 2.2±0.7 <0.001
Vertically 1 107±11 6.8±1.2

Table 7

Retentive force range of metallic circumferential clasps with different designs"

Metal Design Retentive force range/N
Cast Co-Cr A1 15-20
SLM Co-Cr A1 15-20
A2 40-45
B1 55-60
B2 75-80
SLM CP Ti A1 5-10
A2 15-20
B1 30-35
B2 30-35
SLM Ti-6Al-4V A1 5-10
A2 15-20
B1 30-35
B2 30-35

Table 8

Measured retentive force and maximum von Mises stress of SLM-built clasps with equivalent computed retentive force range to the control group"

SLM metal Design Retention/N Max von Mises stress/MPa
Experiment, $\bar{x} \pm s_{\bar{x}}$ Finite element analysis
Co-Cr A1 21.57±5.41 15-20 803.78
CP Ti A2 19.75±4.47 15-20 872.37
Ti-6Al-4V A2 19.32±2.04 15-20 890.72

Figure 6

Stress distribution of clasps’ polishing surfaces under 15 N dislodging force A, the control group; B, SLM-built Co-Cr clasp with design A1; C, SLM-built CP Ti clasp with design A2; D, SLM-built Ti-6Al-4V clasp with design A2. Unit: ×108 Pa."

Figure 7

Stress distribution of clasps’ intaglio surfaces under 15 N dislodging force A, the control group; B, SLM-built Co-Cr clasp with design A1; C, SLM-built CP Ti clasp with design A2; D, SLM-built Ti-6Al-4V clasp with design A2. Unit: ×108 Pa."

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