mandibular class I Kennedy removable partial dentures (RPD) with ball attachments, in ... a mandibular class I. Kennedy RPD with ball attachments, versus the same RPD .... improve unfavourable removable partial denture design. Compend.
The 5th IEEE International Conference on E-Health and Bioengineering - EHB 2015 Grigore T. Popa University of Medicine and Pharmacy, Iaşi, Romania, November 19-21, 2015
Compared Stress Levels of Removable Partial Dentures with Attachments with and without Distal Implants - a Finite Element Analysis O.C. Andrei1, C. Dăguci2, M.H. Ţierean3, T.A. Farcaşiu1, C. Farcaşiu 4, L.A. Tănăsescu1 1
UMF Carol Davila Bucharest, Department of Removable Prosthodontics, Bucharest, Romania
2 3
UMF Craiova, Faculty of Dentistry, Department of Prevention of Oro-dental Diseases, Craiova
Materials Engineering and Welding Department, Transilvania University of Brasov, Brasov, Romania 4
UMF Carol Davila Bucharest, Department of Pedodontics, Bucharest, Romania
Abstract—This paper presents a comparison between the maximum von Mises stress values an maximum displacements appeared under occlusal loading in different areas of a mandibular class I Kennedy removable partial dentures (RPD) with ball attachments, in two situations: teeth supported only, and implant and teeth supported. The first 3D model was created for class I Kennedy RPDs with only six anterior teeth as abutments. The second 3D model was created for the same situation, adding two implants, placed bilaterally, each in the second molar area. On the implants were also positioned ball attachments, providing support and retention for the denture. All materials were considered homogeneous, isotropic and having linear elasticity. Our results demonstrate important differences between the two possible prosthetic treatment solutions for the same class I Kennedy situation, showing that associating an implant for each free-end saddle reduces the maximum von Mises stress and the maximum displacement’s values in the RPD. Keywords— RPD, ball attachments, implants, FEA
I.
INTRODUCTION
The need for RPD treatment is increasing with life expectancy and general interest for oral health [1-4]. For partial edentulous cases and especially for class I and II Kennedy, RPD is considered a prosthetic treatment with good aesthetic and functional results [5,6]. For extensive RPDs, implants can add stability and retention and improve mastication. Class I Kennedy removable partial dentures are moving during mastication, suffering all sorts of displacements that are affecting the masticatory efficiency. Worst of all these displacements is the rotational movement of the denture’s free end saddles in the sagittal plane, favored in mandible by the higher resilience of the alveolar ridge mucosa. A limited number of available abutment teeth and their anterior placement, as in the case we studied, increase the amplitude of this rotation especially in the distal zone because the RPD is only mucosa-and bone-supported on large areas. An implant placed on the distal end of the free-end saddle,
bilaterally, can significantly improve the mastication, offering a better support during function. The present study aims to compare the maximum von Mises stress value under occlusal loading, for a mandibular class I Kennedy RPD with ball attachments, versus the same RPD additionally supported with two distal implants, using finite element analysis. II.
MATERIALS AND METHODS
We used finite element analysis to study stress distribution of a mandibular class I Kennedy RPD with ball attachments under occlusal loads. The six anterior teeth were covered with six united metal-ceramic crowns, milled with oral shoulders and 5 interlocks. The major connector was the double lingual bar with 5 pins matching the milled interlocks on the upper bar. All metallic parts of the denture were casted from Cr-Co Remanium GM 380 Dentaurum. The acrylic saddles were made of termopolymerizable acrylic material Acry-Pole Ruthinium, and the artificial acrylic teeth that replace the PM2, M1 and M2 were made from Acry-Rock Ruthinium. Because of the bilaterally reduced vertical space in this specific area, PM1 was made with light-cured composite (Solidex, Shofu) applied by the dental technician on the metallic matrix socket. We used two dental implants with 3.75mm diameter and 10mm length and ball attachments inserted bilaterally in the second molar distal edentulous area. The mandibular bone was considered a type III bone [7]. The Teflon female part of the attachment acts like a buffer that can be changed when it loses friction. The denture was scanned in Dental View radiology centre. The tomographic images were combined with the constant mechanical and thermal properties of the materials and were used to create the 3D model using Autodesk Inventor 2015 software. (Fig.1, Fig.2, Fig.3). The material characteristics of the 3D model are presented in table 1 and the properties of the materisls in table II. We considered all materials homogeneous, isotropic and having linear elasticity. The mesh was achieved with
978-1-4673-7545-0/15/$31.00 ©2015 IEEE
tetrahedron and brick-type finite elements. The mesh was controlled at the level of each line of the geometric model. TABLE I. THE 3D MODEL CHARACTERISTICS
Mass
0.0116394 kg
Area
7308.47 mm²
Volume
5025 mm³
Centre of mass
x=0.000418323mm
III.
RESULTS AND DISCUSSION
In case of the classical RPD with ball attachments, the highest von Mises stresses appeared when the forces were applied on the PM2 (2255.467 MPa) (Fig. 4), and the smallest (612.9293 MPa) when they were applied on M1 (Fig. 5).
TABLE II. PROPERTIES OF MATERIALS Material elastic modulus Poisson's ratio Titan 113.8 GPa 0.3 ul Ceramic 67.7 GPa 0.28 ul Cortical bone 13.7 GPa 0.3 ul Trabecular bone 0.69 GPa 0.3 ul Acrylic material 22.55 GPa 0.3 ul Composite material 16 GPa 0.33 ul Niadur 172 GPa 0.3 ul
We used in this study a paraxial force of mastication with a normal component (axial) of 160 N and a tangential one of 23.5N, values measured in a study realized in 2007 by De Las Casas [8]. We applied forces on PM2 and M1, these teeth being most stressed during mastication. The stress analysis in the dentures was made using also Autodesk Inventor 2015 software. We compared the von Misses stress levels appeared in the classical RPD with ball attachments versus the same RPD with ball attachments and distal implants. Fig. 4 Maximum von Mises stress in classical RPD when the forces were applied on the PM2
Fig. 1 The 3D Model of half mandibular RPD with attachments
Fig. 5 Maximum von Mises stress in classical RPD when the forces were applied on the M1 Fig. 2 The 3D Model of the entire mandibular RPD with attachments
Fig. 3 The 3D Model of the implant and ball attachment, a. with matrix and b. with housing
In case of the RPD with ball attachments and distal implants, the highest von Mises stresses appeared when the forces were applied on the PM2 (78.58 MPa) (Fig.6) and the smallest (73.53 MPa) when they were applied on M1 (Fig.7). We noticed a considerable decrease in stresses appeared on the RPD with implants, presumably because of the distal support provided and because of the avoidance of the rotational movement of the saddles. Also, we noticed that when we use classical RPD, the one with rotational
movements, the maximum stress level is considerably influenced by the specific place where we applied the force. On the contrary, by placing an implant in the distal area of the mandible, the place where the force is applied has almost no influence on the maximum stress level registered on the RPD; only an insignificant increase can be noticed when forces are applied on PM2.
Fig. 9 Maximum displacement for classical RPD when the forces were applied on the M1
Fig. 6 Maximum von Mises stress in implant supported RPD when the forces were applied on the PM2
In case of the RPD with ball attachments and distal implants, when the forces were applied on the PM2 (Fig.10), the maximum displacement takes place on the XX axis and have a much lowered value of 0.0064mm compared to the classical RPD; also, it is located on the lingual cusps of the PM2. When the forces were applied on M1, the maximum displacement’s direction is the same, the location is on the buccal cusps of the first molar and the value is 0.002637mm (Fig.11), also a much lowered value.
Fig. 7 Maximum von Mises stress in implant supported RPD when the forces were applied on the M1
In case of the classical RPD with ball attachments, when the forces were applied on the PM2 (Fig.8), the maximum displacement takes place on the XX axis and has a value of 0,2749mm located at the distal edge of the saddle; direction and location are the same for the M1, but having a value of 0.3691mm (Fig.9).
Fig. 8 Maximum displacement for classical RPD when the forces were applied on the PM2
Fig. 10 Maximum displacement for implant supported RPD when the forces were applied on the PM2
Fig. 11 Maximum displacement for implant supported RPD when the forces were applied on the M1
Studies have shown a real improvement in the quality of life after RPD treatment, especially for class I and
II Kennedy edentulous patients [9,10]. Using attachments improves their retention and stability, and also has major aesthetic benefits. Still, long term used RPDs present resorption risk for the alveolar bone and traumatic risk for the abutments because of the rotational movements of their freeend saddles, towards the edentulous ridge [11-14]. The existence of this rotational sagittal movement increases the stress values in denture during mastication and its Von Mises stress peaks. Additionally, on a ten years average life-span of the denture, accidents can appear such as deformations and even fracture of the major connector, decementation of the abutment crowns, fractures of the abutment teeth, loss of retention [15]. Also, patients tend to use the anterior areas for mastication, since they have a better efficiency on the teeth then on the rotational end of the saddle. Researches show that, for free-end saddle RPDs, an implant positioned in the distal area offers an additional support and retention which is essential for the long-term prognostic of the treatment and also for patient’s satisfaction [3, 16]. This solution avoids the rotation of the denture towards the mucosa, so common for classical free-end saddle RPDs, and transforms a class I or II Kennedy edentulous patient in a class III Kennedy one [17], changing completely the biodynamic of the denture during function. Additionally, patients can use greater masticatory forces and their distribution is better because it is made on the entire occlusal surface, including the distal area of the arch [18]; as a result, bone resorption decreases both on the edentulous spaces and on the abutment teeth [19]. In a study realized in 2005, Yang shows that associating an implant in a class I Kennedy edentulous case reduced maximum von Mises stress values in all support structures and also in the RPD [20]. Our study demonstrates that using implants for support in the distal area of the edentulous mandible considerably reduces the maximum displacement’s values of the RPD, both for forces applied on the PM2 and M1. It also emphasize important differences between the two possible prosthetic treatment solutions for the same class I Kennedy situation, and the possibility of reducing maximum von Mises stress and maximum displacement’s values by applying the masticatory forces more distally, on the M1. IV.
CONCLUSIONS
Using dental implants to increase the support, retention and stability of the RPD with attachments in a class I Kennedy edentulous case considerably reduces the stresses on the denture, decreases the risk of fracture and resorption and improves the patient’s perception of the quality of the prosthetic treatment. REFERENCES [1] [2] [3]
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