Effects Eff ects of rigid and nonrigid nonrigid extracoronal attachments on supporting tissues in extension ex tension base partial removable dental prostheses: A nonlinear nonlinear finite finit e element study Hui-yuan Wang, MSD,a Yu-mei Zhang, DDS, PhD,b Dong Yao, MSD,c Ji-hua Chen, DDS, PhDd The Fourth Militar y Medical University, Xi’an, Xi’an, China; China; The 41st 41st Institute of the Fourth Academy of CASIC, Xi’an, China Statement of problem. Resilient 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. The purpose of this study was to compare the mechanical effects of 2 t ypes of extracoronal ex tracoronal attachments (rigid and nonrigid) in distal extension ext ension removable partial prostheses on the alveolar ridge r idge and abutment tooth periodontal ligament. Material and methods. A finite element model of a human left mandible edentulous arch distal to the second premolar was fabricated. The second premolar pr emolar 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 lev els of loading (100, 150, 200, 250, 300, and 350 N) were applied from 3 directions (axial, buccolingual, 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 recorded for 6 regions (cer vical bone, cer vical and apical periodontal ligaments, mesial and distal ridges, and mucosa). The data were divided into 2 groups according to the attachment attachment type. Paired t tests tests were used to compare the values of the 2 groups. Factorial ANOVA was used to test the difference between the loading directions (α=.05). Multiple linear regression was used to analyze the interactions among the factor s of region, direction, and level lev el (α=.05). Results. Stress distributions in the rigid and nonrigid attachment models were similar but the magnitudes were different. For all 3 loading directions, significantly different stresses in the alveolar ridge and periodontal tissue of the terminal abutment abutment were found between be tween the rigid r igid and nonrigid groups (P <.05). <.05). There were significant differences among the 3 loading directions (P <.05). <.05). In the nonrigid group, the stress ratio of the mesial to the distal area was higher than that of the rigid group from axial and mesiodistal loading ( P <.05). <.05). Linear interactions were found between the direction and level and region and level combinations ( P <.05). <.05). Movement between the patrix and matrix components increased as loading increased. The most obvious movement move ment of attachment occurred when loading was in the buccolingual direction. Conclusions. Stress Conclusions. Stress on the ter minal abutment abutment can be reduced by b y the use of an extracoronal e xtracoronal resilient attachment that allocates more loads onto the distal edentulous ridge. The lev el of loading influenced the extent ex tent of reduction. reduc tion. A resilient attachment with a universal hinge had the most move ment when loading was in the buccolingual direction. Interactions were found between direction and level, as well as region and level combinations ( P <.05). <.05). (J Prosthet Dent 2011;105:338-346) 2011;105:338-346)
The study was supported by National “11th “11th Five-Year” Scientific and Technologic Technological al Support Project (Grant No.2007BA118 No.2007BA118B05) B05) a
PhD student, Department Depar tment of Prosthodontics, School of Stomatology, The Fourth Militar y Medical University. Professor and Vice Director, Department of Prosthodontics, School of Stomatology, The Fourth Militar y Medical University. c Researcher, Researcher, National Key Laboratory of Combustion, Flow and Thermo-Structure, The 41st 41st Institute of the Fourth Academy of China Aerospace Science and Industry Corporation. d Vice Dean, Dean, Professor and Director, Department Department of Prosthodontics, School School of Stomatology, Stomatology, The Fourth Military Medical University. b
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Clinical Implications In distal extension base removable partial prostheses, the use of extracoronal resilient attachments for retention protects terminal abutment health. However, due to the motion of the attachment during function, an indirect retainer is recommended to stabilize the denture. Although implant-supported restorations are increasingly used to restore defective dentitions, the traditional partial removable dental prosthesis (PRDP) is still indicated because of, among other reasons, lower cost and inadequate remaining alveolar bone. Among different PRDPs, those with attachments, especially the extracoronal type, are considered more efficient in restoring function and providing retention.1,2 In most extracoronal attachment designs, 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 components. Some attachments, such as the resilient design, can help reduce stress on the periodontal ligament (PL) of abutment teeth, and are recommended for distal extension dentures. Previous studies have investigated the effects of attachments on extension base PRDPs. Some authors found that the resilient attachment could protect the abutment.3 Extracoronal 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-Sterngold) 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 attachments could result in torquing forces. These can be transmitted to the terminal abutment because the connect-
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ing parts are not located on the long MATERIAL AND ME THODS 8 axis of the abutment. Similar findings have also been reported for dental 3D model fabrication implant biomechanics. Nishimura et al,9 reported that rigid connectors, Mandibular bone and teeth comin particular, caused slightly higher puterized tomography (CT) data were stresses in the supporting structure obtained from a male volunteer with than nonrigid connectors. However, an intact natural dentition without other authors have indicated that obvious periodontal disease. The CT rigid contact between the attachment scan was done after obtaining ethipatrix and matrix had more advan- cal approval from the Institute Retages and could reduce the move- search Committee. A CT scanner (PQ ment of abutment teeth.10,11 Saito et 6000; Picker International, Highland al,12 found that the displacement of Heights, Ohio) was used to obtain the denture base tended to be less digital imaging and communications when the denture was designed with in medicine (DICOM) data. Using a rigid connection to the retainer and image-processing software (Mimics with cross-arch stabilization. However, 10.0; Materialise, Leuven, Belgium), most of these studies were conducted the scanned profiles in DICOM format with photoelastic analysis. Many of were translated into 3-D models and them lacked detailed data for precise were saved as stereolithography files. evaluation. Furthermore, the authors The files were imported into redid not report on stress analysis for verse engineering software (Geomagic extension base PRDPs retained by ex- Studio 8.0; Geomagic, Inc., Research tracoronal attachments. Triangle Park, NC) as polygon data. Compared with other mathemati- In the Polygon Phase, small surface cal methods, finite element analysis is holes were filled and the “Relax” and considered more accurate in analyzing “Sandpaper” commands were used the stress distribution in tissues with to smooth and flatten the model surcomplicated structures such as hu- face. In the Shape Phase, the model man alveolar bone. Load and bound- contours were detected and optiary conditions, stress, strain, and mized. Patches and grids were conthe displacement of each part of the structed on the surface. A non-unimodel for 3-dimensional (3D) finite form rational basis spline (NURBS) element analysis have been reported surface in each patch was generated in several studies.4,5,9 The purpose of by the “Fit Surface” command. Finalthis study was to determine the bio- ly, the NURBS models were converted mechanical effects of distal extension into CAD (computer-aided design) base PRDPs retained by extracoronal models. The solid model was generattachments using a nonlinear finite ated and saved as Initial Graphics Exelement method (FEM). The nonrigid change Specification (IGES) data. The attachment design was hypothesized contour of the PL was obtained in the to allocate more stress to the alveolar Polygon Phase by offsetting 0.2 mm ridge than rigid attachment, the ex- from the outer shape of the tooth. tent of which is affected by the load The IGES files of each part were ing mode. imported into computer-aided engi-
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Volume 105 Issue 5 neering software (Abaqus/CAE 6.9; Abaqus, Inc., Pawtucket, RI). An at- Table I. Material properties tachment was used (ERA; Sterngold Material Elastic Modulus, E (MPa) Dental, LLC, Attleboro, Mass) as the denture retainer. In the “Part” module Dentin 14, 15 18600 of the Abaqus software, the attachment model was generated according Periodontal ligament 16 0.0689 to the manufacturer’s instructions for Cortical bone17, 18 13700 the product. In the “Assembly” module, Boolean operations were perCancellous bone 17, 18 1370 formed to obtain different parts, inNi-Cr alloys19, 20 200000 cluding abutment, PL, alveolar bone, attachment, crown, mucosa, and Nylon 21 2400 denture base. The models for abut4500 Denture base18 ment, PL, bone, crown, and attachment matrix were merged together Mucosa18, 22 1 with the retaining boundaries, and the models for attachment patrix and denture base were merged in the same 3.0 manner.
Two groups of contact management 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 analysis. In the “Interaction” module, the surface between the patrix and matrix components of the attachment was set as a contact unit of “finite sliding”, 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 allowed. 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 sliding is minute, and contacting surfaces can undergo only minimal sliding. In the rigid group, only contact between denture base and mucosa were considered and the surfaces of patrix and matrix were tied together without any movement. In the “Property” module, material properties were assigned14–22 (Table I). In the “Mesh” module, attachment components and mucosa
0.31 0.45 0.30 0.30 0.33 0.39 0.35 0.37
ALLAE Whole Model
2.5
Contact surface, meshing, load, and boundary management
Poisson’s ratio
ALLWK Whole Model
] 2.0 3 E . 1 x 1.5 [ y g r e n 1.0 E
0.5
0 0
0.5
1.0
1.5
2.0
Time
1 ALLAE-ALLWK data changes during loading process. X axis shows 2 loading steps. Y axis shows ALL AE and ALLWK value. ALLAE/ALLWK value was within 5% at end of second loading step (0.66%). were meshed by linear hex elements principle,26 the sections farthest from (C3D8R). Crown, denture base, den- the analyzed field were constrained to tin, PL, and alveolar bone were meshed the zero boundary condition, includby modified quadratic Tet elements ing the inferior, mesial, and distal sur(C3D10M). Similar to previous stud- faces of the alveolar bone. ies,23–25 the meshing of PL and bone onto the tooth root area was refined. Convergence test and analysis In the “Load” module, the load was applied to the central fossa of The study tested for convergence the first and second molar. The load- and accuracy before analysis. Crucial ing force was respectively applied areas of the meshing model were adfrom the axial (A), mesiodistal (MD), justed, including tissues around the and buccolingual (BL) directions. Six PL and the contact area. In repeated levels of load magnitude were used solution operations, the result of the (100, 150, 200, 250, 300, and 350 meshed model with different accuN). According to the Saint-Venant’s racies was found to be similar and
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May 2011 stable. Furthermore, the results were loading directions were analyzed by evaluated using an energy time-his- factorial analysis of variance (ANOtory curve (Fig. 1). ALLAE (artificial VA, α=.05). Multiple linear regression energy) is the total energy dissipated analysis was used to determine the inas artificial strain energy and ALLWK teractions among the factors (α=.05). (external work energy) shows the toRESULTS tal structural energy in the model during the loading process. The X-Y curve showed the ALLAE/ALLWK value and When the PRDPs were retained by verified the precision of the model. the rigid and nonrigid attachments, the Under ideal conditions, ALLAE is stress was concentrated on both the alwithin 5% of ALLWK. The maximum veolar ridge and the periodontal tissue ALLAE/ALLWK was 0.66% in the last of the terminal abutment teeth. Stress loading step, which demonstrated in the cortical bone was greater than that the model were dependable. that in the cancellous bone (Fig. 2). Figure 3 shows the stress in cervical Model calculation and data analysis PL and alveolar bone for the 2 types of attachment designs. The stress distriAfter analysis, the maximum butions were similar, but more stress equivalent stress (SEQV) on the sup- appeared in the rigid design. Table II porting tissues and denture move- shows the maximum SEQV for the difments were observed and recorded. ferent attachment contacts. Table III The regions included cervical bone, indicates the stresses for the 2 attachapical and cervical PL , mesial and dis- ment groups with different loading dital area on the alveolar ridge, and the rections of the alveolar ridge (mesial mucosa. The stress value was analyzed and distal areas) and the periodontal using statistical software (SPSS 12.0; tissues (PL and alveolar bone) around SPSS Inc, Chicago, Ill). The difference the second premolar. The paired t test between the 2 groups was analyzed indicated that alveolar ridge stress in by a paired t test. Differences among the nonrigid group was lower than
that of the rigid group with axial loading (P <.001), but the result was the opposite with those of mesiodistal (P=.006) and buccolingual (P=.001) loading. Stress in the rigid group in the PL and bone was higher than that in the nonrigid group for all 3 loading directions (axial: P=.002, mesiodistal: P=.015, buccolingual: P=.001). Stress on the alveolar ridge was higher in 2 regions, the mesial and distal ends of the denture base area (Figure 4). Table IV shows the maximum SEQV ratio (mesial/distal) of the 2 ends under different loading and attachment contacts. The paired t -test showed that the ratios of the nonrigid group were higher than that of the rigid group for axial and mesiodistal loading (axial: P=.004, mesiodistal: P <.001, buccolingual: P=.098). Based on the ANOVA, the stresses at the 6 locations showed significant differences among the 3 loading directions (P <.001). Stress was not concentrated on the apical area of the alveolar bone. In the cervical bone area, the effect of loading direction was axial less than mesiodistal which was less than buccolingual (Fig.5A). In PL
A
B
C
D
E
F
2 Equivalent stress distributions in tissues and denture in nonrigid attachment (A xial loading, 350N). A, Mesialdistal section of entire model; B, Buccal-lingual section of alveolar bone; C, Mesial-distal section of denture base; D, Mesial-distal section of ERA attachment; E, Mesial-distal section of first premolar and second premolar PL; F, Buccallingual section of second premolar root and PL . Note in A, stress was not uniformly distributed around abutment tooth and wide stress distribution under distal extension base, compared with mesial area (red color indicates highest values of SEQV).
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A
B
C
D
E
F
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.
Table II. Maximum equivalent stress (MPa) of supporting tissue with 200N loading
Design
Loading Direction
Rigid
Nonrigid
Alveolar Bone
PL
Edentulous Ridge
Cervical
Apical
Cervical
Mesial
Distal
Mucosa
AL
13.10
3.66
0.86
20.29
6.04
1.29
BL
130.60
23.57
4.12
23.57
13.18
4.80
ML
24.59
1.57
0.71
3.72
3.98
0.99
AL
8.04
2.51
0.71
17.20
4.48
1.34
BL
120.57
21.03
3.45
25.01
14.01
4.67
ML
22.29
1.53
0.70
5.70
4.22
1.01
PL: periodontal ligament; AL: axial loading; BL: buccolingual loading; ML: mesiodistal loading.
Table III. Paired t test between rigid and nonrigid attachment design in different loading directions Paired Groups (Rigid and Nonrigid Group)
Mean (MPa) PT
AR
PT
AR
PT
AR
PT
AR
PT
AR
Axial loading
2.31
2.67
2.68
1.47
3.66
6.29
17
11
.002
<.001
Mesiodistal loading
0.79
–1.78
1.24
1.22
2.71
–3.35
17
11
.015
.006
Buccolingual loading
4.65
–1.61
4.83
1.33
4.09
–4.21
17
11
.001
.001
SD (MPa)
t
df
P
PT: Periodontal tissues; AR: Alveolar ridge
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Table IV. Mesial/distal ratio of maximum SEQV on alveolar ridge Axial Loading
4 Equivalent stress distributions in edentulous area ridge (Occlusal view, nonrigid attachment with 200N mesiodistal loading). Stress distributions are primarily on mesial and distal region of the ridge (red color indicates highest values of SEQV).
Mesiodistal
Buccolingual
Nonrigid
Rigid
Nonrigid
Rigid
Nonrigid
Rigid
100N
3.69
3.43
1.36
0.91
1.78
1.77
150N
3.64
3.38
1.32
0.92
1.81
1.80
200N
3.84
3.36
1.35
0.93
1.79
1.79
250N
3.82
3.35
1.42
0.95
1.88
1.79
300N
4.25
3.34
1.35
0.95
1.85
1.78
350N
4.04
3.34
1.39
0.94
1.80
1.79
250 Axial loading Mesiodistal loading Buccolingual loading
200
) a 150 P M ( V Q E 100 S
50
0
100
150
200
250
300
A
Loading (N) 38 36 34 32 30 28 26 ) 24 a P 22 M ( 20 V 18 Q 16 E S 14 12 10 8 6 4 2 0
350
7 Axial loading Mesiodistal loading Buccolingual loading
Axial loading Mesiodistal loading Buccolingual loading
6 5 ) a P 4 M ( V Q3 E S
2 1
100
150
200
250
Loading (N)
300
0
350
B
100
150
200
250
Loading (N)
300
350
C
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).
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45
220
Rigid Nonrigid
200
35
180
30
) a 160 P M (
) a P 25 M (
140
V Q E 120 S
V 20 Q E S
15
100
10
80
5
60 40
Rigid, apical area Nonrigid, apical area Rigid, cervical area Nonrigid, cervical area
40
100
150
200
250
300
0
350
Loading (N)
100
150
200
250
300
350
Loading (N)
A
B
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.
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. Nonrigid Variable
Rigid
Coefficient
t
P
Coefficient
t
P
Direction x Level
–25.57
–2.57
.012
–25.11
–2.33
.022
Direction x Region
12.93
1.39
.166
17.84
1.79
.076
Region x Level
29.49
3.27
.001
33.12
3.46
.001
Region x Direction x Level
0.30
0.03
.978
4.16
0.36
.717
tissue, stresses under the axial and mesiodistal loading directions were close and less than that under buccolingual loading (Figs. 5B, 5C). Figure 6 shows stress under the buccolingual loading. Multiple linear regressions
showed that interactions existed between the direction and level, as well as the region and level (Table V). Movement between the patrix and matrix components increased with increasing loading force. Buccolingual
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rotation was the most obvious motion when loading was from the buccolingual direction (Fig. 7). However, under axial and mesiodistal load, the displacement was reduced.
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DISCUSSION Based on the results of this study, the hypothesis that the nonrigid attachment 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 permit different movements between the component parts, which affects dental biomechanics. The nonrigid attachment used in this study had a hinging motion. Several types of attachments have hinge resilience, which allows movement around a given point, providing stress-breaking action to the denture. For example, the Dalbo attachment 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 attachment, axial force can be transferred to the long axis of the abutment. In addition, the combination of attachment and clasp can reduce the effect of horizontal force, which benefits abutment health. However, the abutment tooth can also be injured if a hinge attachment denture is inappropriately designed. In this study, 2 contact modes of attachment produced different results with the same loading force. To identify the difference, interactions between contacting parts, minute distortions of the nylon patrix, and denture movement were considered during the modeling process. However, all of these confounding factors could produce unreliable results. Therefore, a convergence test was done to ensure the dependability of the study. The results indicate that the stress distributions under the 2 attachment designs (rigid and nonrigid) were similar. Given that the patrix and matrix were bound together in the rigid at-
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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 connector in particular situations caused slightly higher stresses in the supporting 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 occlusal forces, the resilience of the attachment could be adjusted to distribute 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%, mesiodistal: 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 constant. However, the absolute value increased as loading increased. SEQV distributed mesially and distally 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 loading 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 affected by the vertical movement of the attachment, but not by rotation. This also shows that the resilience of the ERA attachment could help transfer some force to both mesial and distal 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 buccolingual loading and the stress was on the buccal and distal parts under mesiodistal loading. The biggest movement of the denture base appeared when loading was from the buccolingual direction. This indicates that the shape of the alveolar ridge had the greatest influence on rotation. In addition, certain parts (including PL, periodontal 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 recommended for most clinical situations. As an initial investigation of an attachment-retained distal extension prosthesis using the nonlinear finite element method, the study has limitations. The study was performed with a single abutment design. Results may be different with additional factors such as multiple abutments or a cross-arch major connector. The contact between the attachment matrix and patrix should be considered further because the exact friction coefficient has not been reported and this parameter was found to have a large effect in this pilot study. Aside from the loading conditions, the mechanical function of the attachment could also be affected by the supporting structures, 27 such as periodontal support, mucosal thickness, and resilience. These should be investigated further.
CONCLUSIONS Within the limitations of the study, the following conclusions were drawn: 1. Compared with rigid attachment, nonrigid attachments can reduce stress in the supporting tissues around the terminal abutment of extension base PRDPs. 2. Lateral force has the greatest effect on the terminal abutment in distal extension base PRDPs with universal hinge attachments. Interactions
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