comparacion ataches rigidos y resilientes

The Journal of Prosthetic Dentistry Wang et al

Statement of problem. Resilient (nonrigid) and non-resilient (rigid) attachments are used in extension base partial removable dental prostheses for retention. However, the biomechanical effects of these 2 types of retainers on the terminal abutment and supporting tissues, which may influence clinical treatment planning, have not been compared.

Purpose. The purpose of this study was to compare the mechanical effects of 2 types of extracoronal attachments (rigid and nonrigid) in distal extension removable partial prostheses on the alveolar ridge and abutment tooth peri-odontal ligament.

Material and methods. A finite element model of a human left mandible edentulous arch distal to the second premo-lar was fabricated. The second premolar was the terminal abutment for an attachment-retained denture. Two types of attachments (rigid and nonrigid) were modeled in the study. For the nonrigid attachment, there was movement between the patrix and matrix component of the attachment, but there was no movement between the 2 component parts for the rigid attachment. Six levels of loading (100, 150, 200, 250, 300, and 350 N) were applied from 3 direc-tions (axial, buccolingual, and mesiodistal) on the central fossa of the first and second molars. Denture motion and stress distributions of denture supporting tissues were observed. Maximum equivalent stress values (SEQV) were re-corded for 6 regions (cervical bone, cervical and apical periodontal ligaments, mesial and distal ridges, and mucosa). The data were divided into 2 groups according to the attachment type. Paired t tests were used to compare the values of the 2 groups. Factorial ANOVA was used to test the difference between the loading directions (=.05). Multiple linear regression was used to analyze the interactions among the factors of region, direction, and level (=.05).

Results. Stress distributions in the rigid and nonrigid attachment models were similar but the magnitudes were differ-ent. For all 3 loading directions, significantly different stresses in the alveolar ridge and periodontal tissue of the ter-minal abutment were found between the rigid and nonrigid groups (P

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Wang et al

Clinical ImplicationsIn distal extension base removable partial prostheses, the use of ex-tracoronal resilient attachments for retention protects terminal abut-ment health. However, due to the motion of the attachment during function, an indirect retainer is recommended to stabilize the denture.

Although implant-supported res-torations are increasingly used to restore defective dentitions, the tra-ditional partial removable dental prosthesis (PRDP) is still indicated because of, among other reasons, lower cost and inadequate remain-ing alveolar bone. Among different PRDPs, those with attachments, es-pecially the extracoronal type, are considered more efficient in restoring function and providing retention.1,2 In most extracoronal attachment de-signs, the matrix component is cast with the abutment crown and the patrix is embedded into the denture base. Retention force comes from the interaction between these 2 compo-nents. Some attachments, such as the resilient design, can help reduce stress on the periodontal ligament (PL) of abutment teeth, and are recommend-ed for distal extension dentures.

Previous studies have investigat-ed the effects of attachments on ex-tension base PRDPs. Some authors found that the resilient attachment could protect the abutment.3 Extra-coronal resilient attachment (ERA)-retained dentures have been shown more beneficial to abutment teeth than bar retainers.4,5 Heckmann et al,6

showed that more stress is transferred onto the denture-bearing area with a resilient attachment than with a rigid one; and Kratochvil et al,7 found that a Dalbo attachment (APM-Stern-gold) distributed more stress to the alveolar ridge and reduced stress on the abutment tooth. Moreover, some studies on attachment-retained distal extension dentures have shown that non-resilient extracoronal attach-ments could result in torquing forces. These can be transmitted to the ter-minal abutment because the connect-

ing parts are not located on the long axis of the abutment.8 Similar findings have also been reported for dental implant biomechanics. Nishimura et al,9 reported that rigid connectors, in particular, caused slightly higher stresses in the supporting structure than nonrigid connectors. However, other authors have indicated that rigid contact between the attachment patrix and matrix had more advan-tages and could reduce the move-ment of abutment teeth.10,11 Saito et al,12 found that the displacement of the denture base tended to be less when the denture was designed with a rigid connection to the retainer and with cross-arch stabilization. However, most of these studies were conducted with photoelastic analysis. Many of them lacked detailed data for precise evaluation. Furthermore, the authors did not report on stress analysis for extension base PRDPs retained by ex-tracoronal attachments.

Compared with other mathemati-cal methods, finite element analysis is considered more accurate in analyzing the stress distribution in tissues with complicated structures such as hu-man alveolar bone. Load and bound-ary conditions, stress, strain, and the displacement of each part of the model for 3-dimensional (3D) finite element analysis have been reported in several studies.4,5,9 The purpose of this study was to determine the bio-mechanical effects of distal extension base PRDPs retained by extracoronal attachments using a nonlinear finite element method (FEM). The nonrigid attachment design was hypothesized to allocate more stress to the alveolar ridge than rigid attachment, the ex-tent of which is affected by the load-ing mode.

MATERIAL AND METHODS

3D model fabrication

Mandibular bone and teeth com-puterized tomography (CT) data were obtained from a male volunteer with an intact natural dentition without obvious periodontal disease. The CT scan was done after obtaining ethi-cal approval from the Institute Re-search Committee. A CT scanner (PQ 6000; Picker International, Highland Heights, Ohio) was used to obtain digital imaging and communications in medicine (DICOM) data. Using image-processing software (Mimics 10.0; Materialise, Leuven, Belgium), the scanned profiles in DICOM format were translated into 3-D models and were saved as stereolithography files.

The files were imported into re-verse engineering software (Geomagic Studio 8.0; Geomagic, Inc., Research Triangle Park, NC) as polygon data. In the Polygon Phase, small surface holes were filled and the Relax and Sandpaper commands were used to smooth and flatten the model sur-face. In the Shape Phase, the model contours were detected and opti-mized. Patches and grids were con-structed on the surface. A non-uni-form rational basis spline (NURBS) surface in each patch was generated by the Fit Surface command. Final-ly, the NURBS models were converted into CAD (computer-aided design) models. The solid model was gener-ated and saved as Initial Graphics Ex-change Specification (IGES) data. The contour of the PL was obtained in the Polygon Phase by offsetting 0.2 mm from the outer shape of the tooth.

The IGES files of each part were imported into computer-aided engi-

340 Volume 105 Issue 5


neering software (Abaqus/CAE 6.9; Abaqus, Inc., Pawtucket, RI). An at-tachment was used (ERA; Sterngold Dental, LLC, Attleboro, Mass) as the denture retainer. In the Part module of the Abaqus software, the attach-ment model was generated according to the manufacturers instructions for the product. In the Assembly mod-ule, Boolean operations were per-formed to obtain different parts, in-cluding abutment, PL, alveolar bone, attachment, crown, mucosa, and denture base. The models for abut-ment, PL, bone, crown, and attach-ment matrix were merged together with the retaining boundaries, and the models for attachment patrix and denture base were merged in the same manner.

Contact surface, meshing, load, and boundary management

Two groups of contact manage-ment were used in this study. In the nonrigid group, the surfaces between the patrix and the matrix, as well as between the denture base and the mucosa were defined as contacted elements in the finite element analy-sis. In the Interaction module, the surface between the patrix and matrix components of the attachment was set as a contact unit of finite slid-ing, and the friction coefficient was set to 0.3. In finite sliding, there is separation and sliding between the 2 surfaces and arbitrary rotation is al-lowed. The contact area between the denture base and alveolar ridge was considered as small sliding, with the friction coefficient set to 0.1.13 In small sliding, the rotation or slid-ing is minute, and contacting surfaces can undergo only minimal sliding. In the rigid group, only contact between denture base and mucosa were con-sidered and the surfaces of patrix and matrix were tied together without any movement.

In the Property module, ma-terial properties were assigned1422 (Table I). In the Mesh module, at-tachment components and mucosa

were meshed by linear hex elements (C3D8R). Crown, denture base, den-tin, PL, and alveolar bone were meshed by modified quadratic Tet elements (C3D10M). Similar to previous stud-ies,2325 the meshing of PL and bone onto the tooth root area was refined.

In the Load module, the load was applied to the central fossa of the first and second molar. The load-ing force was respectively applied from the axial (A), mesiodistal (MD), and buccolingual (BL) directions. Six levels of load magnitude were used (100, 150, 200, 250, 300, and 350 N). According to the Saint-Venants

principle,26 the sections farthest from the analyzed field were constrained to the zero boundary condition, includ-ing the inferior, mesial, and distal sur-faces of the alveolar bone.

Convergence test and analysis

The study tested for convergence and accuracy before analysis. Crucial areas of the meshing model were ad-justed, including tissues around the PL and the contact area. In repeated solution operations, the result of the meshed model with different accu-racies was found to be similar and

1 ALLAE-ALLWK data changes during loading process. X axis shows 2 loading steps. Y axis shows ALLAE and ALLWK value. ALLAE/ALLWK value was within 5% at end of second loading step (0.66%).

3.0

Ener

gy [

x1.E

3]

Time

1.5

2.0

2.5

0.5

0

1.0

0.50 1.0 1.5 2.0

ALLAE Whole ModelALLWK Whole Model

Dentin14, 15

Periodontal ligament16

Cortical bone17, 18

Cancellous bone17, 18

Ni-Cr alloys19, 20

Nylon21

Denture base18

Mucosa18, 22

18600

0.0689

13700

1370

200000

2400

4500

1

0.31

0.45

0.30

0.30

0.33

0.39

0.35

0.37

Elastic Modulus, E (MPa) Poissons ratioMaterial

Table I. Material properties

341May 2011

Wang et al

stable. Furthermore, the results were evaluated using an energy time-his-tory curve (Fig. 1). ALLAE (artificial energy) is the total energy dissipated as artificial strain energy and ALLWK (external work energy) shows the to-tal structural energy in the model dur-ing the loading process. The X-Y curve showed the ALLAE/ALLWK value and verified the precision of the model. Under ideal conditions, ALLAE is within 5% of ALLWK. The maximum ALLAE/ALLWK was 0.66% in the last loading step, which demonstrated that the model were dependable.

Model calculation and data analysis

After analysis, the maximum equivalent stress (SEQV) on the sup-porting tissues and denture move-ments were observed and recorded. The regions included cervical bone, apical and cervical PL, mesial and dis-tal area on the alveolar ridge, and the mucosa. The stress value was analyzed using statistical software (SPSS 12.0; SPSS Inc, Chicago, Ill). The difference between the 2 groups was analyzed by a paired t test. Differences among

loading directions were analyzed by factorial analysis of variance (ANO-VA, =.05). Multiple linear regression analysis was used to determine the in-teractions among the factors (=.05).

RESULTS

When the PRDPs were retained by the rigid and nonrigid attachments, the stress was concentrated on both the al-veolar ridge and the periodontal tissue of the terminal abutment teeth. Stress in the cortical bone was greater than that in the cancellous bone (Fig. 2).

Figure 3 shows the stress in cervical PL and alveolar bone for the 2 types of attachment designs. The stress distri-butions were similar, but more stress appeared in the rigid design. Table II shows the maximum SEQV for the dif-ferent attachment contacts. Table III indicates the stresses for the 2 attach-ment groups with different loading di-rections of the alveolar ridge (mesial and distal areas) and the periodontal tissues (PL and alveolar bone) around the second premolar. The paired t test indicated that alveolar ridge stress in the nonrigid group was lower than

that of the rigid group with axial load-ing (P



3 Equivalent stresses of PL and alveolar bone in rigid and nonrigid attachment (Loading level: 200N; with rigid design in left and nonrigid design in right; red color indicates highest values of SEQV). A, PL, axial load; B, Alveolar bone, axial load; C, PL, buccolingual load; D, Alveolar bone, buccolingual load; E, PL, mesiodistal load; F, Alveolar bone, mesiodistal load.

B

E

C

F

A

D

Table II. Maximum equivalent stress (MPa) of supporting tissue with 200N loading

Table III. Paired t test between rigid and nonrigid attachment design in different loading directions

Rigid

Nonrigid

PL: periodontal ligament; AL: axial loading; BL: buccolingual loading; ML: mesiodistal loading.

AL

BL

ML

AL

BL

ML

13.10

130.60

24.59

8.04

120.57

22.29

DirectionLoading Alveolar Bone PL Edentulous Ridge

Cervical

1.29

4.80

0.99

1.34

4.67

1.01

Mucosa

0.86

4.12

0.71

0.71

3.45

0.70

Cervical

3.66

23.57

1.57

2.51

21.03

1.53

Apical

20.29

23.57

3.72

17.20

25.01

5.70

Mesial

6.04

13.18

3.98

4.48

14.01

4.22

DistalDesign

Axial loading

Mesiodistal loading

Buccolingual loading

PT: Periodontal tissues; AR: Alveolar ridge

PT

2.31

0.79

4.65

AR

2.67

1.78

1.61

PT

2.68

1.24

4.83

AR

1.47

1.22

1.33

Mean (MPa)Paired Groups(Rigid andNonrigid Group)

SD (MPa)

PT

3.66

2.71

4.09

AR

6.29

3.35

4.21

t

PT

17

17

17

AR

11

11

11

df

PT

.002

.015

.001

AR

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Wang et al

4 Equivalent stress distributions in edentulous area ridge (Occlusal view, nonrigid attachment with 200N mesiodistal loading). Stress distribu-tions are primarily on mesial and distal region of the ridge (red color indicates highest values of SEQV).

5 Effect of loading levels on maximum equivalent stress (SEQV) of supporting tissues in nonrigid group. Extent of SEQV changes differently in each areas of the model. A, Alveolar bone (Cervical area); B, PL (Apical area); C, PL (Cervical area).

Table IV. Mesial/distal ratio of maximum SEQV on alveolar ridge

100N

150N

200N

250N

300N

350N

Nonrigid

3.69

3.64

3.84

3.82

4.25

4.04

Rigid

3.43

3.38

3.36

3.35

3.34

3.34

Axial

Loading Nonrigid

1.36

1.32

1.35

1.42

1.35

1.39

Rigid

0.91

0.92

0.93

0.95

0.95

0.94

Mesiodistal

Nonrigid

1.78

1.81

1.79

1.88

1.85

1.80

Rigid

1.77

1.80

1.79

1.79

1.78

1.79

Buccolingual

250

SEQ

V (

MP

a)

Loading (N)

150

200

100

50

0100 150 200 250

Axial loadingMesiodistal loadingBuccolingual loading

300 350

38

SEQ

V (

MP

a)

Loading (N)

222018

30282624

323436

16141210

86420

100 150 200 250


300 350

7

SEQ

V (

MP

a)

Loading (N)

5

4

6

3

2

1

0100 150 200 250


300 350

A

B C



tissue, stresses under the axial and mesiodistal loading directions were close and less than that under bucco-lingual loading (Figs. 5B, 5C). Figure 6 shows stress under the buccolingual loading. Multiple linear regressions

7 Rotation movement of ERA attachment under buccolingual loading. Rotation of attachment was most obvious with 350N load. Image shows movement changes in different loading levels.

Table V. Multiple linear regression on interaction variables.

Direction x Level

Direction x Region

Region x Level

Region x Direction x Level

Coefficient

25.57

12.93

29.49

0.30

t

2.57

1.39

3.27

0.03

Nonrigid

Variable P

.012

.166

.001

.978

Coefficient

25.11

17.84

33.12

4.16

t

2.33

1.79

3.46

0.36

Rigid

P

.022

.076

.001

.717

showed that interactions existed be-tween the direction and level, as well as the region and level (Table V).

Movement between the patrix and matrix components increased with in-creasing loading force. Buccolingual

rotation was the most obvious mo-tion when loading was from the buc-colingual direction (Fig. 7). However, under axial and mesiodistal load, the displacement was reduced.

6 Effect of buccolingual loading level on maximum equivalent stress (SEQV) of PL and alveolar bone in rigid and nonrigid attachment designs. A, Alveolar bone, cervical area; B, PL.

240SE

QV

(M

Pa)

Loading (N)

200

180

220

160

140

120

100

80

60

40100 150 200 250

RigidNonrigid

300 350

45

SEQ

V (

MP

a)

Loading (N)

35

30

40

25

20

15

10

5

0100 150 200 250

Rigid, apical areaNonrigid, apical areaRigid, cervical areaNonrigid, cervical area

300 350

BA

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Wang et al

DISCUSSION

Based on the results of this study, the hypothesis that the nonrigid at-tachment design can allocate more stress to the alveolar ridge than a rigid attachment was confirmed. The stress distribution was affected by loading. Among different loading conditions, maximum buccolingual loading had the greatest effect on the periodontal tissues.

Different attachment designs per-mit different movements between the component parts, which affects den-tal biomechanics. The nonrigid attach-ment used in this study had a hinging motion. Several types of attachments have hinge resilience, which allows movement around a given point, pro-viding stress-breaking action to the denture. For example, the Dalbo at-tachment belongs to the hinge type and permits vertical movement with limited hinge movement.18 The ERA permits universal hinged movement. Its retention partially depends on the interaction between the patrix and matrix components. In this attach-ment, axial force can be transferred to the long axis of the abutment. In addi-tion, the combination of attachment and clasp can reduce the effect of horizontal force, which benefits abut-ment health. However, the abutment tooth can also be injured if a hinge at-tachment denture is inappropriately designed.

In this study, 2 contact modes of attachment produced different re-sults with the same loading force. To identify the difference, interactions between contacting parts, minute dis-tortions of the nylon patrix, and den-ture movement were considered dur-ing the modeling process. However, all of these confounding factors could produce unreliable results. Therefore, a convergence test was done to en-sure the dependability of the study.

The results indicate that the stress distributions under the 2 attachment designs (rigid and nonrigid) were sim-ilar. Given that the patrix and matrix were bound together in the rigid at-

tachment, attachment resilience was concluded as the major factor for the difference and not the attachment structures. The difference should lie in the magnitude of the stress values.

Nishimura et al9 reported that the least stress was observed when using a nonrigid connector, and a rigid con-nector in particular situations caused slightly higher stresses in the support-ing structure. That result is similar to the findings of this study. The authors thought that limiting the stress would help protect the alveolar ridge from excessive load; that under certain oc-clusal forces, the resilience of the at-tachment could be adjusted to dis-tribute the force properly between the abutment and alveolar ridge.

The cervical region of the bone around the abutment tooth showed higher sensitivity to loading. The maximum SEQV of the rigid group was higher than that of the nonrigid group (axial: 57.3% to 64.3%, mesio-distal: 6.4% to 10.5%, buccolingual: 5.8% to 8.3%). Therefore, regardless of how the force level changed, the stress ratio of these 2 groups was con-stant. However, the absolute value in-creased as loading increased.

SEQV distributed mesially and dis-tally on the alveolar ridge. The mesial stress was affected by the forces from both the abutment and the denture base, whereas the stress distributed in the distal area might have been caused by vertical movement of the distal denture base. The maximum SEQV ratio of the mesial and distal area of the rigid group was less than that of the nonrigid group when load-ing was from the axial and mesiodistal directions. This result indicates that the loads transferred by the nonrigid attachment were higher in the mesial area. The resilience, therefore, is af-fected by the vertical movement of the attachment, but not by rotation. This also shows that the resilience of the ERA attachment could help trans-fer some force to both mesial and dis-tal ends of the alveolar ridge.

Loading along the buccolingual direction had the greatest effect on

the supporting tissues. The maximum SEQV was distributed to the buccal and lingual parts of the periodontal tissue under both axial and buccolin-gual loading and the stress was on the buccal and distal parts under mesio-distal loading. The biggest movement of the denture base appeared when loading was from the buccolingual di-rection. This indicates that the shape of the alveolar ridge had the greatest influence on rotation. In addition, certain parts (including PL, periodon-tal bone and the residual ridge) did not show compromised periodontal support in the model. Therefore, even if a resilient attachment is used, a cross-arch major connector is recom-mended for most clinical situations.

As an initial investigation of an attachment-retained distal extension prosthesis using the nonlinear finite element method, the study has limita-tions. The study was performed with a single abutment design. Results may be different with additional fac-tors such as multiple abutments or a cross-arch major connector. The con-tact between the attachment matrix and patrix should be considered fur-ther because the exact friction coeffi-cient has not been reported and this parameter was found to have a large effect in this pilot study. Aside from the loading conditions, the mechani-cal function of the attachment could also be affected by the supporting structures,27 such as periodontal sup-port, mucosal thickness, and resil-ience. These should be investigated further.

CONCLUSIONS

Within the limitations of the study, the following conclusions were drawn:

1. Compared with rigid attach-ment, nonrigid attachments can re-duce stress in the supporting tissues around the terminal abutment of ex-tension base PRDPs.

2. Lateral force has the greatest ef-fect on the terminal abutment in dis-tal extension base PRDPs with univer-sal hinge attachments. Interactions



exist between direction and level and region and level.

3. Movement of the component parts in the attachment is most af-fected by buccolingual loading.

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13.Kibi M, Ono T, Dong J, Mitta K, Gonda T, Maeda Y. Development of an RPD CAD system with finite element stress analysis. J Oral Rehabil 2009;36:442-50.

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15.Aydin AK, Tekkaya AE. Stresses induced by different loadings around weak abutments. J Prosthet Dent 1992;68:879-84.

16.Ruse ND. Propagation of erroneous data for the modulus of elasticity of periodontal ligament and gutta percha in FEM/FEA papers: A story of broken links. Dent Mater 2008;24:1717-9.

17.Yang HS, Chung HJ, Park YJ. Stress analysis of a cantilevered fixed partial denture with normal and reduced bone support. J Pros-thet Dent 1996;76:424-30.

18.Daas M, Dubois G, Bonnet AS, Lipinski P, Rignon-Bret C. A complete finite element model of a mandibular implant-retained overdenture with two implants: comparison between rigid and resilient attachment con-figurations. Med Eng Phys 2008;30:218-25.

19.Ho MH, Lee SY, Chen HH, Lee MC. Three-dimensional finite element analysis of the effects of posts on stress distribution in dentin. J Prosthet Dent 1994;72:367-72.

20.Augereau D, Renault P, Pierrisnard L, Bar-quins M. Three-dimensional finite element analysis of the retention of fixed partial dentures. Clin Oral Investig 1997;1:141-6.

21.Pellizzer EP, Verri FR, Falcn-Antenucci RM, Goiato MC, Gennari Filho H. Evaluation of different retention systems on a distal extension removable partial denture as-sociated with an osseointegrated implant. J Craniofac Surg 2010;21:727-34.

22. Wada S, Wakabayashi N, Tanaka T, Ohyama T. Influence of abutment selection in maxillary Kennedy Class II RPD on elastic stress distribution in oral mucosa: an FEM study. J Prosthodont 2006;15:89-94.

23.Clement R, Schneider J, Brambs HJ, Wun-derlich A, Geiger M, Sander FG. Quasi-au-tomatic 3D finite element model generation for individual single-rooted teeth and periodontal ligament. Comput Methods Programs Biomed 2004;73:135-44.

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Corresponding author:Dr Ji-hua ChenDepartment of Prosthodontics, School of StomatologyThe Fourth Military Medical University145 Changle Xi RoadXian, ShaanxiCHINAFax: +86-29-84776329E-mail: [email protected]

Copyright 2011 by the Editorial Council for The Journal of Prosthetic Dentistry.

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Effects of rigid and nonrigid extracoronal at tachments on supporting tissues in extension base partial removable dental prostheses: A nonlinear finite element studyMATERIAL AND METHODS3D model fabricationContact surface, meshing, load, and boundary managementConvergence test and analysisModel calculation and data analysis

RESULTSDISCUSSIONCONCLUSIONSREFERENCES

comparacion ataches rigidos y resilientes

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