Splinted Fixed Partial Denture

Corresponding author: M. Z. Bendjaballah E-mail: [email protected]

Journal of Bionic Engineering 9 (2012) 336342

The Effect of Non-Contact Conditions in a Splinted Fixed Partial Denture on the Load Sharing Mechanism: A Finite Element Study

M. Z. Bendjaballah

Biomedical Technology Department, College of Applied Medical Sciences, King Saud University, Riyadh, Saudi Arabia

1 Introduction

The use of fixed bridges in partial edentulism is very common because it provides a physiologically sound treatment for the greatest number of patients[14]. Studies have presented Fixed Partial Dentures (FPDs) as a good alternative in the treatment of missing teeth in both bounded and unbounded cases[58]. The contact problem of the denture base with the alveolar ridge has been tackled few times over the last three decades[911]. As early as 1983, Jacobson et al.[12] indicated in a review article that tissues supporting full denture base should tolerate the functional stresses without promoting dis-comfort to patients. In case they are less resistant to long-term changes or, are unable to tolerate stress, they should be relieved by providing a clearance between alveolar ridge and the denture base. From hygienic standpoint, and according to Block and Kent[4], a 4 mm to 5 mm clearance known as hygienic space, needs to be incorporated in order to prevent gingival problem in Removable Partial Dentures (RPDs). In fact, most pa-tients who had received implants for dental restoration

have lost teeth because of caries and periodontal disease. In clinical practice, clearance is sometimes set uninten-tionally when the dentist fail to attain a perfect fit con-dition between denture base and alveolar ridge[13]. Conversely, when the prosthodontist sets the clearance deliberately, this is usually to allow FPDs to compete with their RPDs counterparts on hygienic matters[4].

The load transmission mechanism is addressed in some research works through both experimental and numerical approaches. Menicucci et al.[9] used strain- gauged load cells to evaluate the in vivo load transfer in mandibular-retained overdentures. The study was car-ried out on three patients and tested two different types of denture anchorage conditions; one allowing gap (non-contact condition) and the second preventing gap (contact condition). In the non-contact condition, the results showed that the occlusal load was reduced and shifted from supporting teeth to the alveolar ridge through the mucosa. In the contact condition, patients reported more comfort and felt that they could exert greater occlusal force.

Similarly, using strain gauges on four cadaveric

Abstract A computer-aided design model for a fixed partial denture was constructed and used in a finite element analysis to study the

overall load sharing mechanism between the fixed partial denture and oral structures while the denture base rested on the al-veolar ridge. To investigate the consequences of non-contact conditions, three additional models were generated incorporating a uniform clearance of 0.125 mm, 0.25 mm, and 0.5 mm, respectively. A 100 N static load located at the free end of the prosthesis was applied while the distal portion of the jaw was set fixed. The results show that whilst releasing the ridge almost entirely, the presence of the clearance drastically increased the load on the splinting teeth. A pull-out force on the canine tooth of about 44 N was computed, accompanied by a mesio-distal moment of about 500 Ncm. The combination of which was similar to the tooth extraction maneuver performed by the dentist. In contrast, the second premolar was found to bear a push-in force of almost 115 N. The first molar, though barely solicited in the contact condition, became substantially loaded in non-contact conditions, which validates the choice of sacrificing three teeth to support the denture.

Keywords: finite element analysis, splinted fixed partial denture, hygienic space, load transfer mechanism, contact problem Copyright 2012, Jilin University. Published by Elsevier Limited and Science Press. All rights reserved. doi: 10.1016/S1672-6529(11)60122-4

Bendjaballah: The Effect of Non-Contact Conditions in a Splinted Fixed Partial Denture on the Load Sharing Mechanism: A Finite Element Study 337

mandibles, Yamashita et al.[14] measured the cortical bone surface strains before and after the incorporation of 3-unit FPDs while applying up to 250 N on each tooth individually. When the load was applied on a tooth that was not involved in the FPD therapy, no differences were found before and after FPD placement. Conversely, a difference of less than 100 strains was found when-ever the FPD retainers were loaded, leading to the con-clusion that the 3-unit FPD therapy does not alter the overall deformation pattern of the mandible during loading.

Two experimental studies performed by Sadowsky and Caputo[10,11] measured the biologic behavior of 2 or 3 implants retaining different designs of cantilevered mandibular dentures with and without edentulous ridge contact. The stresses that developed in the denture under the applied load of 66.7 N were monitored photoelasti-cally and recorded photographically. While all prostheses designs demonstrated low stress transfer to the implants, the supporting prostheses exhibited the highest stress level for the simulated tissue non-contact conditions.

A three-dimensional (3D) Finite Element (FE) study performed by Lin et al.[6] of a tooth-implant sup-ported FPD explored the loading condition, number of splinting teeth, and connector type on the stresses de-veloped in the implant, bone and prosthesis. Their re-sults indicated that stresses in a non-rigid connection (non-contact condition) increased by 3.4 folds. In addi-tion, the use of an extra support tooth in a three-unit tooth-implant FPD was found to be helpful only when the prosthesis withstands lateral occlusal forces. In an-other recent FE study on FPDs mounted in distal exten-sion areas, the load transfer mechanism revealed that the alveolar ridge carries most of the acting load (about 90%) in a contact condition while the teeth share the remaining part of the load[5]. Such an overload of the alveolar ridge may lead to bone resorption and increase the risk of denture failure[912].

Swain and his research group[1516] explored re-cently the bone remodeling response of the mandible subjected to masticatory loads before and after the in-sertion of a 3-unit FPD. Using the Frosts mechanostat theory, the authors were able to predict the resorption, equilibrium and apposition bone volume fractions, re-spectively for the different 3D FE models investigated. The Minimum Effective Strain (MES) which enables the dynamics of bone turnover to reach equilibrium was set

according to Frosts theory between 800 strains to 2000 strains. The computed results show that the incorpora-tion of an FPD induced greater strains in the mandibular bone characterized by a larger fraction of bone volume displaying strains higher than 800 strains, which was suggestive of a potential bone remodeling initiation[15]. The research group was also able to develop a time-dependant algorithm for FPD-induced mandibular remodeling by relating the mechanical stimulus in terms of the equivalent strain to the Hounsfield Unit (HU) of the bone morphology. According to the authors[16], the procedure is expected to help the prosthodontist as-sessing the treatment through the prediction of the dif-ferent zones of FPD-driven bone resorption and apposi-tion, thereby optimizing FPD design to enhance lon-gevity and reliability of future FPDs.

While imposing a clearance between the denture and the alveolar ridge is expected to relieve the mandible, its effect on the splinted teeth remain yet unexplored. The aim of this study is to investigate the consequences of introducing different levels of clearance on the amount of load transmitted to the alveolar ridge via the mucosa as well as the supporting teeth.

2 Method

2.1 The CAD models An initial CAD model of a section of a mandible

with an idealized but realistic geometry was developed based on a set of Computer Tomography (CT) images. Mimics 14.0 (Materialise NV) was used to process and edit the CT images to construct 3D models of the cor-tical and cancellous bone layers, canine teeth, and premolars. The models were then exported to Solid-Works 2010 (Dassault systems) in which the remaining constituents, the Periodontal Ligaments (PDLs) and a thin layer of mucosa, were generated. With the same CAD software, a bridge made of a Ni-Cr alloy was developed and secured to the prepared teeth. The bridge also constituted a framework that carried a porcelain denture to substitute for the missing molars. While some patients complain of noises with the use of por-celain teeth, prosthodontists still prefer them to the acrylic ones because porcelain has an outstanding color stability and an excellent resistance to wear[17]. In a primary control model, the denture was set to rest ex-actly on the alveolar ridge (fitted model). Then, to ex-plore the effects of clearance on the overall load sharing

Journal of Bionic Engineering (2012) Vol.9 No.3 338

mechanism in the FPD, three additional models were generated in which, gaps of 0.125 mm, 0.25 mm and 0.5 mm were set between the denture base and alveolar ridge (see Fig. 1). The later models were referred to as C125, C250 and C500, respectively while the term Fitting was used to denote the primary model. 2.2 The FE models

In the present study, an intricate multi-body contact problem is investigated. Therefore, special focus was directed upon the contact conditions that realistically mimic the interactions of the oral structures in vivo. A default bonded contact condition was set for all inter-acting model constituents except for the splinted FPD and the mucosa interface. A more realistic free node-to-surface contact was set for the latter interaction, aimed to prevent penetration while permitting sliding between the master (lower surface of the denture) and slave (upper surface of the mucosa) surfaces. For the current stage of the analysis, due to the lack of informa-tion concerning the precise material properties for some of the model components, all the materials were as-sumed to be linear, elastic, homogeneous and iso-tropic[1823]. The required mechanical properties of the materials used in the study are shown in Table 1[24].

Fig. 1 A CAD model showing the different constituents, and a section view that reveals the clearance between the alveolar ridge and the denture. Table 1 Material properties used for various model components

Material Elastic Modulus Poissons ratio

Tooth (Dentine) 13 GPa 0.30

Periodontal ligament 2 MPa 0.45

Mucosa 3.45 MPa 0.45

Ni-Cr 200 GPa 0.33

Porcelain 140 GPa 0.28

Cortical bone 13.4 GPa 0.30

Cancellous bone 1.37 GPa 0.31

For each CAD model, a high quality mesh as shown in Fig. 2 was generated on the Ansys 12.1 FE software, which fully interfaces with the SolidWorks program. An appropriate element type (10 Node-tetrahedral solid elements) and a degree of refinement characterized by an adequately small element size were sought in different contact areas to ensure convergence of the results[25]. A fixed restraint condition was applied to the inferior cortex of the mandible while a 100 N vertical load was distrib-uted on the free end of the denture. This magnitude of load represents an average of the normal masticatory forces reported in the literature[2628]. The distal location of the load on the denture was chosen in purpose to rep-licate the most overwhelming conditions on the support-ing teeth due to their respective large lever arms. Static nonlinear stress analyses were conducted, and the most significant results were saved.

Fig. 2 The FE mesh of the splinted FPD.

3 Results and discussions

3.1 Free-body diagram and equilibrium The load sharing mechanism discussed herein fo-

cuses on the way the applied external load spreads through the oral structures until it is equilibrated by the reaction force at the inferior border of the mandible. Fig. 3 shows a schematic representation of the Free-Body Diagram (FBD) of the FPD for the fitting model in the occlusal direction. The force and moment equilibria were satisfied in all directions and for all computed cases. Moreover, it was noted that most of the applied force crosses the mucosa (89 N) while the canine tooth and second premolar exhibit a pull-out force and a push-in force of 14 N and 23 N, respectively. The first premolar faintly participates to the load sharing by transmitting only about 2 N to the underlying bone.

The bar chart in Fig. 4 concurrently presents the


results for the fitting and the clearance models. Varying the set clearance from 0.125 mm to 0.5 mm, reduced the contribution of the mucosa to the load transmission mechanism and increasingly overloaded the canine tooth and the second premolar to reach a pull-out force of about 44 N and a push-in force of nearly 115 N, respec-tively for the C500 model.

Splinting the bridge over three teeth appeared, at first glance, to be obsolete in the contact condition as it burdens only the two outermost supporting teeth. The sacrifice of the innermost tooth (first premolar) be-comes more requisite in non-contact conditions because the tooth substantially participates in the load sharing mechanism, transmitting to the underlying PDL and bone, increasingly high loads of 11 N, 17 N and 25 N in the C125, C250 and C500 non-contact cases, respec-tively.

Aside from the predominant occluso-apical force components acting on the supporting teeth, moderate mesio-distal force components, particularly affecting the canine tooth and the second premolar, are computed. Such force components become more prominent to reach at the highest clearance, 17 N and 21 N for the canine and second premolar, respectively. The farthest location of the canine tooth with respect to the load site, led to the application of about 500 Ncm mesio-distal moment. The combination of such a moment with the previously cited pull out force (~44 N) is quite similar to the tooth ex-traction maneuver exerted by the dentist. Interestingly, the computed push-in load that is even larger than the external load (~115 N), acting upon the second premolar is endorsed by a less important mesio-distal moment of about 350 Ncm due to the shorter force lever-arm rep-resented by the exposed length of the denture. While the denture length is found to affect the load transmission mechanism, the denture curvature has negligible effect on the load sharing mechanism. Only minute buccolin-gual moments and forces are computed for the contact and non-contact models.

As a result, to the combination of loads acting on the canine, this tooth is found to drift in the mesio-distal direction at the final load of 100 N as shown in Fig. 5. The exposed canine crown displaces mesially (~0.18 mm) while the secured canine root moves less distally (~0.04 mm). The canine center of rotation is found to be roughly located at one quarter of the canine length from the root tip, assuming a linear variation of the tooths mesio-distal displacement.

100.0 N

89.0 N22.9 N2.3 N

14.2 N

100.0 N Fig. 3 The FBD for the fitted model shows the load transmission through the different FPD constituents in the occluso-apical direction.

CanineFirst premolarSecond premolarMucosa

C125 C250 C50060

40

20

0

20

40

60

80

100

120

Occ

luso

-api

cal f

orce

(N)

Occ

lusa

l A

pica

l

Fitting

Fig. 4 The occluso-apical force components born by the different FPD constituents for contact and various non-contact conditions.

Fig. 5 Canine tooth drifting in the mesio-distal direction.


3.2 Contact behavior and denture compliance The compliance of the bridge and porcelain den-

ture and the contact behavior between the different FPD constituents are responsible for the above-described load sharing mechanism. The overall bending stiffness of the FPD depends on whether the denture rests on the mucosa or not. In Fig. 6, the change in the slope clearly indicates when the contact between the denture and the mucosa begins. Such contact is computed to initiate under loads of approximately 20 N, 50 N and 80 N for the C125, C250 and C500 models, respectively. While suspended, the FPD has a stiffness of approximately 160 Nmm1 (see the slope of the curves for the C125, C250 and C500 models in the early stages). Whenever the denture gets in contact with the alveolar ridge, the stiffness rises to ~800 Nmm1 (see the slope of the curves for the fitting model and for the C125 and C250 models in their final stages). For the C500 model, be-cause the contact initiates only under a load of 80 N and was not fully established at 100 N, the stiffness is found to be slightly lower than that depicted in the two pre-ceding cases. The alveolar ridge transmitted, thus, only 5 N through its most distal area. To increase the load transfer via the ridge, the denture and bridge should be compliant enough to achieve a contact with the alveolar ridge in an earlier loading stage. The computed stiffness of 160 Nmm1 should be reduced to at least 100 Nmm1, particularly when a substantial clearance is sought.

Fig. 7 illustrates the contact status for the above studied cases. While the investigation of the contact problem shows a well-established wide-ranging brown area (sticking contact status) for the contact condition, it displayed increasingly smaller sticking contact areas with larger clearance conditions, compensated by in-creasingly larger yellow areas (near contact status), characterized by close but not touching set of contact surface pairs. Favorably, a drop in the transmitted load accompanies such a predicted decline in the sticking (effective) contact area. While the maximal contact stress of approximately 1 MPa is depicted in the mucosa for the fitting model, the remaining clearance models displayed, also on the mucosa, less but closer contact stress values.

Occ

lusa

l den

ture

def

lect

ion

(mm

)

Fig. 6 The occlusal deflection of the porcelain denture versus the applied load for the different denture/ridge contact cases.

Over constrainedFarNearSlidingSticking

Fitting


C125 Over constrainedFarNearSlidingSticking

C250


C500 Fig. 7 The contact status between the denture and the mucosa.


4 Conclusions

The instauration of a clearance between the denture and ridge to relieve the mucosa and/or to promote oral hygienic is a key factor in the success of the prosthesis. The amount of clearance and the compliance of the denture itself are the most important parameters that govern the multi-body contact occurring between the model constituents. Among the parameters, dictating the load transmission mechanism, are the forces and mo-ments acting on the prepared teeth and the resulting displacements. Such information is evidently helpful in selecting the appropriate clearance to be set between the denture and the alveolar ridge and in making the deci-sion on the number of teeth to sacrifice in support of the denture (either two or three). Therefore, within the limit of this study the following conclusions can be drawn:

(1) The sacrifice of three teeth is obsolete when the denture rests perfectly on the ridge because the tooth in the middle (the first premolar) bears only ~2 N. The author recommends the sacrifice of only two teeth since using three supporting teeth would expose the middle tooth to stress shielding that eventually leads to bone atrophy.

(2) Providing clearance unintentionally or deliber-ately induces a drastic increase in the load bearing of the outermost teeth and of the middle tooth. Hence, the use of three teeth to support the denture becomes mandatory.

(3) The denture behaves as a type 1 lever, with the fulcrum located between the first premolar and the ca-nine tooth. Applying a distal load to the denture in non-contact conditions is found to overload both the canine tooth and second premolar. The large pull-out force acting on the canine tooth accompanied by a sub-stantial mesio-distal moment is similar to those per-formed by the dentist during tooth extraction. A push-in load that is even higher than the external load acts upon the second premolar.

(4) The dentures curvature is found to have negli-gible effect on the load sharing mechanism. Very minute buccolingual forces and moments were computed for the fitting and the clearance models.

(5) To enhance the load sharing contribution of the alveolar ridge in non-contact conditions, a suitable compliance of the denture and bridge needs to be care-fully engineered through the selection of an appropriate denture geometry and material. The denture and bridge

must be compliant enough to achieve a contact with the alveolar ridge in the early loading stages. Concurrently, a thorough investigation of the stresses developed in the denture and bridge should be carried out to make sure they remain below the yield values.

Acknowledgment

This work was supported by the King Abdulaziz City for Science and Technology (KACST), Grant Number AT-282.

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Splinted Fixed Partial Denture