A Comparison of Biomechanical Properties of Implant-Retained Overdenture Based on Precision Attachment Type

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Article A Comparison of Biomechanical Properties of Implant-Retained Overdenture Based on Precision Attachment Type

Małgorzata Idzior-Haufa 1, Agnieszka A. Pilarska 2,* , Wiesław H˛edzelek 3, Piotr Boniecki 4, Krzysztof Pilarski 4 and Barbara Dorocka-Bobkowska 1

1 Department of Gerodontology and Oral Pathology, Poznan University of Medical Sciences, Bukowska Street 70, 60-812 Poznan, Poland; [email protected] (M.I.-H.); [email protected] (B.D.-B.) 2 Department of Plant-Derived Food Technology, PoznańUniversity of Life Sciences, ul. Wojska Polskiego 31, 60-624 Poznan, Poland 3 Department of Prosthodontics, Poznan University of Medical Sciences, Bukowska Street 70, 60-812 Poznań,Poland; [email protected] 4 Department of Biosystems Engineering, PoznańUniversity of Life Sciences, ul. Wojska Polskiego 50, 60-627 Poznan, Poland; [email protected] (P.B.); [email protected] (K.P.) * Correspondence: [email protected]; Tel.: +48-61-848-73-08

Abstract: This paper aims to compare, in vitro, the biomechanical properties of an overdenture retained by two bar-retained implants and an overdenture retained by two bar-retained implants with ball attachments. An edentulous mandible model was prepared for the study based on the Citation: Idzior-Haufa, M.; Pilarska, FRASACO mold with two implants. In the first system, the “rider” type (PRECI-HORIX, CEKA) A.A.; H˛edzelek,W.; Boniecki, P.; retention structure and the complete mandibular denture with the matrix were made. In the second Pilarski, K.; Dorocka-Bobkowska, B. system, the “rider” type retention suprastructure was also used. In the distal part, (CEKA) clips were A Comparison of Biomechanical placed symmetrically, and a complete mandibular denture, together with the matrix on the bar, and Properties of Implant-Retained the clip patrices were made. A numerical model was developed for each system where all elements Overdenture Based on Precision were positioned and related to geometric relations, as in reality. The FEA analysis (finite element Attachment Type. Materials 2021, 14, 2598. https://doi.org/10.3390/ analysis) was carried out for seven types of loads: with vertical forces of 20, 50, and 100 N and ma14102598 oblique forces of 20 and 50 N acting on individual teeth of the denture, namely central incisor, canine, and first molar. Displacements, stresses, and deformations within the systems were investigated. Academic Editors: Maximum denture displacement in the first system was 0.7 mm. Maximum bar stress amounted to Gianmario Schierano, 27.528 MPa, and implant stress to 23.16 MPa. Maximum denture displacement in the second system Bruno Chrcanovic and was 0.6 mm. Maximum bar stress amounted to 578.6 MPa, that of clips was 136.99 MPa, and that of Giuliana Muzio implants was 51.418 MPa. Clips cause smaller displacement of the overdenture when it is loaded but generate higher stress within the precision elements and implants compared to a denture retained Received: 11 March 2021 only by a bar. Regardless of the shape of the precision element, small deformations occur that mainly Accepted: 15 May 2021 affect the mucosa and the matrix. Published: 17 May 2021

Keywords: overdenture; bar; implant; mandible; finite element analysis Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. 1. Introduction Overdentures are one of the recognized methods of prosthetic treatment of the mandible. Relatively reasonable costs and uncomplicated clinical management with a significant improvement in retention and stabilization make this type of restoration an Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. attractive treatment option for both patients and dentists [1–3]. Precision elements, such as This article is an open access article bars, ball or locator clips, telescopic crowns, and magnets [4–7], limit denture mobility both distributed under the terms and vertically and horizontally. Precision attachments used with implant-retained overdenture conditions of the Creative Commons counteract free rotation, so a greater occlusal force may be exerted because of the perceived Attribution (CC BY) license (https:// better denture stability. creativecommons.org/licenses/by/ From the biomechanical point of view, overdentures are the optimal solution because 4.0/). they allow for a more uniform physiological distribution of chewing forces and their impact

Materials 2021, 14, 2598. https://doi.org/10.3390/ma14102598 https://www.mdpi.com/journal/materials Materials 2021, 14, 2598 2 of 17

on the mucosa and alveolar process. Shortened teeth allow for a greater axial load. A variety of possible structural solutions of overdentures, concerning both the number and location of dental implants, as well as the types of clips, make rational justification for individual component application difficult. Despite the abundant amount of works in the literature, supraconstruction designs are often based on clinical tradition without addressing available theoretical background [8]. Consequently, along with other manufacturing errors or lack of follow-up care, unexpected and uncontrolled rotation of the denture, rapid wear, damage to its components, or implant overloading may happen [9]. The number of implants and placement sites depend on many factors, including anatomical conditions and the economic situation of the patient. Currently, many clinicians are inclined to use two implants. Opinions are more divided about the choice of a precision system. Two basic solutions include bar retention and single ball or locator attachments [10]. The choice of the precise element is an individual matter and depends on the conditions of the prosthetic area. When deciding on a bar retainer, we need to choose its proper shape and possible additional elements. The modification of the bar is its extension, symmetrically, distally, beyond the implants. This results in better stability and reliable anchoring of the prosthesis to the bar. According to Mericske-Stern [11], the length of the cantilever bar may be up to approx- imately 1 cm and should not exceed the area of the first premolar. In turn, Elsyad [12] believes that the optimal length of the tabs is 7 mm. The smallest stresses are then generated without significant differences around the implants. Currently, instead of a cantilever bar, ball attachments are often used, placed on both sides, distally to the implants. For a rational selection of the bar, suitable for a clinical situation, knowledge of its mechanical properties is indispensable to achieve long-term success in treatment [13]. In order to analyze the range of motion occurring within a denture and its retention element, it is required to find out the mechanical and fatigue characteristics of each component. The aim of the study is to compare the in vitro biomechanical properties of an overdenture retained by two bar-retained implants with an overdenture retained by two bar- retained implants with ball clips.

2. Materials and Methods A model of a toothless mandible based on the FRASACO mold with two OSTEO- PLANT implants 14 mm long, and 4 mm in diameter was prepared for the study. The implants were located in the anterior region. Parameters of implant elements are shown in Table1.

Table 1. Implant element parameters.

Diameter Height Platform Material (mm) (mm) BASE implant ND 4.0 14 Ti4 Conical abutment M 4.5 1.0 Ti 23 Conical abutment screw ND ND ND ND Technical cap ND ND ND POM ND—not determined; POM—poly(methylene oxide).

The first system included a “rider” bar type retention (PRECI-HORIX, CEKA) and the lower complete denture with a matrix (Figure1a). In this clinical situation, a denture is primarily retained directly on the mucosa, while the bar provides adequate retention. In the second system (see Figure1b), the “rider” bar type retention suprastructure was also used, and the ball clips (CEKA) were symmetrically placed in the distal part. A lower complete denture with the matrix on the bar and ball clip patrices was made, too (Figure2). In this system, components are the same as in the first one; however, the method of fixing the denture is different. It is retained directly on the mucosa, and the bar’s function is to stabilize it, while the clips used are to retain the whole system. Materials 2021, 14, x FOR PEER REVIEW 3 of 16

The first system included a “rider” bar type retention (PRECI-HORIX, CEKA) and the lower complete denture with a matrix (Figure 1a). In this clinical situation, a denture is primarily retained directly on the mucosa, while the bar provides adequate retention. In the second system (see Figure 1b), the “rider” bar type retention suprastructure was also used, and the ball clips (CEKA) were symmetrically placed in the distal part. A lower complete denture with the matrix on the bar and ball clip patrices was made, too (Figure

Materials 20212)., 14 ,In 2598 this system, components are the same as in the first one; however, the method of 3 of 17 fixing the denture is different. It is retained directly on the mucosa, and the bar’s function is to stabilize it, while the clips used are to retain the whole system.

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FigureFigure 1. A 1. real A real model model and aand computer a computer model model (a) of the (a ﬁrst) of the studied first system studied and system (b) of theand second (b) of studiedthe second system. studied system.

The geometry of the mandible and dentures was scanned with the Roland LPX 600 3D Laser Scanner (Roland DGA Corporation, Tulsa, OK, USA) and imported into the com- puting environment. In order to increase accuracy, three independent measurements were made from three different directions (see Figure 2). Such a procedure eliminates places where the measuring head cannot register geometry during measurement from just one direction.

Figure 2. Visualization of measurement data obtained for the mandibular mucosa model in an Figure 2. Visualization of measurement data obtained for the mandibular mucosa model in an LPX-600 scanner (in the real scale). LPX-600 scanner (in the real scale). The geometry of the mandible and dentures was scanned with the Roland LPX 600 3DThe Laser data Scanner obtained (Roland by DGA using Corporation, a 3D (three Tulsa,-dimensional) OK, USA) and importedscanner into concerning, the for in- stancomputingce, surface environment. mesh, saved In order in to an increase Standard accuracy, Tessellation three independent Language measurements (STL) format, were were made from three different directions (see Figure2). Such a procedure eliminates transferredplaces where to the the measuring Geomagic head Studio cannot register12.0 software geometry (Research during measurement Triangle from Park, just NC, USA) in whichone direction. further processing was carried out. It consisted of matching and placing individual sets ofThe measurement data obtained data by using on aeach 3D (three-dimensional) other (3D scans) scanner from concerning,three different for instance, directions so that onesurface model mesh, with saved complete in an Standard mandibular Tessellation geometry Language (Figure (STL) format,3) could were be transferredobtained. During this process, all data were filtered, the discontinuities of geometry were eliminated, and all points outside the mandible were removed (e.g., accidental elements of the background, the measuring table, etc.). Any surface distortions that arise during scanning and data placement were also removed during data processing.

Figure 3. Mandibular mucosa geometry defined with a surface triangle mesh.

The obtained surface triangle mesh was subjected to further processing. Finite Ele- ments Analysis (FEA) programs used for analysis require defining geometry as volumetric objects. For that, the next step involved changing the description of the mandibular mucosa geometry from surface triangle mesh to an object defined with Non-Uniform Ra- tional Basis Spline (NURBS) type surfaces (see Figure 4). To generate NURBS surface, contours along with division lines of particular areas of the mandible were determined. These measures allowed the surface formation of the model to be controlled well.

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Figure 2. Visualization of measurement data obtained for the mandibular mucosa model in an LPX-600 scanner (in the real scale).

Materials 2021, 14, 2598The data obtained by using a 3D (three-dimensional) scanner concerning, for in-4 of 17 stance, surface mesh, saved in an Standard Tessellation Language (STL) format, were transferred to the Geomagic Studio 12.0 software (Research Triangle Park, NC, USA) in which further processingto the Geomagic was carried Studio 12.0 out. software It consisted (Research of matching Triangle Park, and NC, placing USA) in indiv whichidual further sets of measurementprocessing data wason each carried other out. (3D It consisted scans) offrom matching three and different placing individualdirections sets so ofthat mea- one model with completesurement data mandibular on each other geometry (3D scans) (Figure from three 3) could different be directionsobtained so. During that one modelthis with complete mandibular geometry (Figure3) could be obtained. During this process, all process, all data datawere were filtered, ﬁltered, the the discontinuitiesdiscontinuities of geometry of geometry were eliminated, were elim andinated, all points and outside all points outside thethe mandible mandible werewere removed removed (e.g., (e.g., accidental accidental elements elements of the background, of the background, the measuring the measuring table,table, etc.). AnyAny surface surface distortions distortions that arise that during arise scanning during andscanning data placement and data were placement were alsoalso removedremoved during during data data processing. processing.

Figure 3. MandibularFigure 3. Mandibularmucosa geometry mucosa geometry defined deﬁned with a with surface a surface triangle triangle mesh. mesh.

The obtained surfaceThe obtained triangle surface mesh triangle was meshsubjected was subjected to further to further processing processing.. Finite Finite Ele- Ele- ments Analysis (FEA) programs used for analysis require deﬁning geometry as volumetric ments Analysis (FEA)objects. programs For that, the used next stepfor involvedanalysis changing require the defining description geometry of the mandibular as volumet- mucosa Materials 2021, 14, x FOR PEER REVIEW 5 of 16 ric objects. For that,geometry the next from surfacestep involved triangle mesh changing to an object the deﬁned description with Non-Uniform of the mandibular Rational Basis mucosa geometrySpline from (NURBS) surface type triangle surfaces mesh (see Figureto an 4object). defined with Non-Uniform Ra- tional Basis Spline (NURBS) type surfaces (see Figure 4). To generate NURBS surface, contours along with division lines of particular areas of the mandible were determined. These measures allowed the surface formation of the model to be controlled well.

Figure 4.Figure Division 4. Division of mandibular of mandibular mucosa mucosa into into areas areas andand ﬁnal final model model deﬁned defined with NURBSwith NURBS surfaces. surfaces.

To generate NURBS surface, contours along with division lines of particular areas Next, a surface mesh of the object was formulated, taking into account division lines. of the mandible were determined. These measures allowed the surface formation of the Dependingmodel to on be the controlled complexity well. of local geometry, it is possible to control mesh density. The obtained surface of the mandibular mucosa defined with NURBS was exported by using a universal computer-aided design (CAD) data exchange file. The other system components (including implants, conical abutments, bar, etc.) were modeled directly in the SolidWorks software—a CAD program, version 2013 (Dassault Systems Co., Waltham, MA, USA)—because, in the computed tomography (CT) scan and scanner image, metal elements produce a light artifact in the form of a flare, which makes the boundary of the object unclear. Geometric data of individual components were obtained from the manufacturer’s documentation and from our measurements. Spacing was positioned from CT image, scanner, and control measurements. To simplify numerical calculations, the mapping of threaded connections (outline of the helical thread line) was abandoned due to their negligible significance for the study. It would considerably prolong the calculation time, too. Ankylotic connection of the implants with the bone was assumed; that is, the thread connection was considered ideal and, hence, deprived of any mobility. Compact and spongy bone were matched to the model of the mandible with respect to the mucous membrane. A denture was developed with a 3D scanner, and data processing was analogous to the one done in the mandible modeling. The denture was adapted to the geometry of the mandible. An ideal adhesion of the denture to the prosthetic bearing area was assumed. All elements were positioned and geometrically related, as in reality (Figure 5).

Figure 5. A cross-section showing the connection of system elements with bone.

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Figure 4. Division of mandibular mucosa into areas and final model defined with NURBS surfaces.

Next, a surface mesh of the object was formulated, taking into account division lines. Depending on the complexity of local geometry, it is possible to control mesh density. The obtained surface of the mandibular mucosa defined with NURBS was exported by using a universal computer-aided design (CAD) data exchange file. Materials 2021, 14, 2598 5 of 17 The other system components (including implants, conical abutments, bar, etc.) were modeled directly in the SolidWorks software—a CAD program, version 2013 (Dassault SystemsNext, a surfaceCo., Waltham, mesh of the object MA, was USA) formulated,—because taking into, in account the computed division lines. tomography (CT) scan and Depending on the complexity of local geometry, it is possible to control mesh density. The obtainedscanner surface image, of the metal mandibular elements mucosa defined produce with NURBSa light was artifact exported in by the using form a of a flare, which makes universalthe boundary computer-aided of the design object (CAD) unclear. data exchange Geometric file. data of individual components were ob- tainedThe other from system the componentsmanufactur (includinger’s documentation implants, conical abutments, and from bar, etc.) our were measurements. Spacing was modeled directly in the SolidWorks software—a CAD program, version 2013 (Dassault Systemspositioned Co., Waltham, from MA,CT USA)—because,image, scanner, in the and computed control tomography measurements. (CT) scan To simplify numerical andcalculations, scanner image, the metal mapping elements produce of threaded a light artifact connections in the form of(outline a flare, which of the helical thread line) was makes the boundary of the object unclear. Geometric data of individual components were obtainedabandoned from the due manufacturer’s to their documentationnegligible significance and from our measurements. for the study. Spacing It would considerably pro- waslong positioned the calculation from CT image, time, scanner, too. and controlAnkylotic measurements. connection To simplify of numericalthe implants with the bone was calculations, the mapping of threaded connections (outline of the helical thread line) was abandonedassumed due; that to their is negligible, the thread significance connection for the study. was It would considered considerably ideal prolong and, hence, deprived of any themobility. calculation time,Compac too. Ankylotict and connectionspongy ofbone the implants were withmatched the bone wasto the assumed; model of the mandible with thatrespect is, the threadto the connection mucous was membrane. considered ideal A and,denture hence, deprivedwas developed of any mobility. with a 3D scanner, and data Compact and spongy bone were matched to the model of the mandible with respect to theprocessing mucous membrane. was analogous A denture was developedto the one with done a 3D scanner, in the and mandible data processing modeling. The denture was wasadapted analogous to to the the onegeometry done in the of mandible the mandible. modeling. TheAn denture ideal was adhesion adapted to of the denture to the pros- the geometry of the mandible. An ideal adhesion of the denture to the prosthetic bearing areathetic was assumed.bearing All area elements was wereassumed. positioned All and elements geometrically were related, positioned as in reality and geometrically related, (Figureas in5 ).reality (Figure 5).

FigureFigure 5. A 5. cross-section A cross- showingsection the showing connection the of system connection elements with of system bone. elements with bone.

Parameters of the oral mucosa thickness and compact and spongy bone thickness were determined by following the literature data. For the mucous membrane, a thickness of 1.5 mm was assumed, Young’s modulus E = 5 MPa and a quite high Poisson’s ratio ν = 0.49, which to some extent imitated its incompressibility [14,15]. For the cortical bone, Young’s modulus E = 17 GPa was assumed; for spongy bone, E = 600 MPa with Poisson coefﬁcient equal to 0.3 in both cases. Material features of the denture are described with E = 3500 MPa and ν = 0.35 data [16,17]. Values of basic material properties of individual elements determined based on the literature are shown in the table (see Table2)[14,15,18,19]. Materials 2021, 14, 2598 6 of 17

Table 2. Material properties.

Plasticity Young’s Poisson’s Ratio Density Part Boundary Modulus (GPa) (-) (kg (m3)−1) (N (mm2)−1) Denture 3.5 0.35 1190 ca 17.5 Matrix housing 200 0.26 517.02 206.81 Matrix 1.4–7 0.39 1330–1610 45–65 Screw 110.3 0.31 4480 744.63 Bar 2 0.3 8900 827 Conical 113.8 0.342 8900 827 abutment Implant 105.2 0.37 4510 500 Mucosus 0.005 0.49 1000 9.5 membrane Compact bone 17 0.3 1900 48 Spongy bone 0.6 0.3 380 48 Clip 113.8 0.342 8900 827

The prepared model was exported to the FEA calculation module. Finite element analysis enables evaluating the distribution of internal forces in a given object when exposed to an external load. In the loaded construction elements, there are two states: stress (loading force divided by a cross-section area, e.g., of an implant) and deformation, a change in an element geometry. When these two states are fused in one mathematical formula, a mathematical model is formulated, e.g., elasticity [20–22]. FEA was performed in the SolidWorks Simulation calculation module of the Solid- Works program. FEA mesh parameters for systems I and II are given in Table3.

Table 3. FEA mesh parameters for systems I and II.

Parameter Type System I System II Studied element Bar 1 Bar 2 Mesh type Tri-dimensional mesh Tri-dimensional mesh Mesh generator used Standard mesh Curvature-based mesh Jacobian 4 points 4 points Mesh control Defined Defined Element size 1.5 mm ND Maximal element size ND 7.86174 mm Minimal element size ND 1.25788 mm Tolerance 0.02 mm ND Mesh quality High High Total number of knots 659,329 784,006 Total number of elements 431,348 518,639 Maximal shape coefficient 263.08 509.35 Shape coefficient < 3 95.9 97.6 Shape coefficient > 10 0.23 0.12 ND: not determined.

Forces of 20 and 50 N were applied at 0◦ and 45◦ angles, as well as 100 N at 0◦ angle [16,17,23,24]. These forces correspond to the average and maximum loads on the teeth in occlusion. It should be added that, in patients using removable dentures, lower forces are produced during chewing. The range of occlusive forces from 20 to 90 N allows us to obtain a satisfactory mechanical efﬁciency for the denture. The maximum acceptable values of occlusive forces can be up to 122 N [25]. The tests simulated chewing loads when the denture is loaded not only in the vertical but also in the horizontal plane. In the ﬁrst case, the force was applied to the right central incisor at an angle of 0◦ (P1); in the second case, the same tooth was loaded with the forward force at 45◦ (P2). In the third and fourth cases, the force was directed to the right canine. In the third case, the force was applied Materials 2021, 14, 2598 7 of 17

at 0◦ (P3); in the fourth one, at 45◦, perpendicular to the tooth (P4). In the fifth, sixth, and seventh cases, the force was directed to the right first molar. In the fifth, the force was applied at 0◦ (P5); in the sixth, at 45◦, laterally toward the cheek (P6); and in the seventh, at 45◦, forward (P7). The displacements were in three directions: XYZ and net forces, stress reduction according to von Mises, and deformations were investigated. Displacement is a change in a given point position, i.e., a difference between initial and final position, e.g., extension, bending, and distortion angle. Its units are meters (m) or degrees (◦)[20–22]. To speak about displacement, one has to determine an object (a system) state called an undeformed state. It is an arbitrary state where the position of all points of an object is known. It means that the following function is known (1):

→ R0 : Ω → P (1) → where R0 is the position vector, Ω is a set of all object points, and P is a set of all space points in which an object is “suspended”. In practice, such a state occurs when no force is applied to an object. Considering other object states, it is possible to determine its points position → R1. In light of the above, displacement is expressed as the following subtraction (2):

→ → → u = R1−R0 (2)

It means that displacement is a vector ﬁeld that is assigning each object point its displacement vector. To evaluate stress, a modiﬁed von Mises criterion was applied. According to it, the material will be damaged when reduced stress exceeds tensile strength [26]. Contour maps show the distribution of stresses obtained in computer simulations. Deformation, or relative deformation, is the ratio of displacement to initial dimensions, for instance, due to tension: ∆l l − l ε = = k 0 (3) l0 l0

where l0 is the initial length, and lk is the ﬁnal length. It is a dimensionless quantity (or %) [22–26]. The obtained results were analyzed and graphically presented in tables, graphs, and contour maps.

3. Results To present the results, a force load of 50 N was selected. Contour maps show places where the greatest stress is concentrated within the bar retention and clips, as well as around the implants (Figure6). The places where the denture undergoes the most signiﬁcant displacements are also depicted (Figure7). Crucial results obtained in the tests, i.e., displacement of the denture and the stresses of the bar and implants, are shown in the graphs (Figures8–12). Materials 2021, 14, 2598 8 of 17 Materials 2021, 14, x FOR PEER REVIEW 8 of 16 Materials 2021, 14, x FOR PEER REVIEW 8 of 16

FigureFigure 6. 6.Distribution Distribution of of displacements displacements in in an an overdenture overdenture when when an an incisor incisor is is loaded loaded with with a a vertical vertical force force of of 50 50 N: N: ( a(a)) system system IFigureI and and ( b (6.b)) systemDistribution system II. II. of displacements in an overdenture when an incisor is loaded with a vertical force of 50 N: (a) system I and (b) system II.

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Figure 7. Distribution of stressvertical within force an of implant 100 N, and and precision the displacement element when of an the incisor screws is loaded exceeds with the a vertdisplicalacement force of of Figure 7. Distribution of stress within an implant and precision element when an incisor is loaded with a vertical force of Figure50 N: ( 7.a)Distribution system I and of (b stress)the system denture. within II. an However, implant and the precision use of CEKA element clips when significantly an incisor is loadedlimits the with screw a vertical displacement, force of 50 N: (a) system I and (b) system II. 50 N: (a) system I and (b) systemleading II. to protection against loosening. Greater dislocations of the denture were recorded in the first system, where the max- imumGreater value dislocations was 0.7 mm of (Figure the denture 8). In werethe second recorded system, in the the first maximum system, wheredisplacement the max- of imumthe denture value waswas 0.7 0.6 mm mm (Figure (Figure 8). 9). In The the largestsecond displacementssystem, the maximum occur when displacement an incisor of is theloaded. denture For wasforces 0.6 of mm the (Figuresame value, 9). The greater largest displacements displacements occur occur for when oblique an forces.incisor Re- is loaded.gardles Fors of forcesthe location of the of same the load,value, the greater most significantdisplacements dislocations occur for occur oblique in the forces. retromo- Re- gardleslar pads ofregion. the location With forces of the actingload, the on mostan incisor, significant especially dislocations for oblique occur forces,in the retromo-displace- larments pad areregion. found With on bothforces sides acting of the on denture.an incisor, especially for oblique forces, displace- mentsFor are forces found acting on both on sides a canine of the and denture. molar, the denture undergoes greater displacement on theFor balancing forces acting side. on Depending a canine and on mothe lar,shape the of denture the precision undergoes element greater used, displacement the value of ondisplacement the balancing changes side. Depending while the ondirection the shape of rotationof the precision remains element the same. used, Loading the value of theof displacementexamined teeth changes results while in rotati theonal direction motion of on rotation the working remains side, the pressing same. Loadingagainst the of den-the examinedture bearing teeth area, results and intilting rotati fromonal themotion YZ plane on the (~in working the XY side, plane, pressing ~around against the Y the-axis) den- on turethe bearingunloaded area, side, and which tilting results from inthe detaching YZ plane the(~in denture the XY plane,on this ~around side. The the use Y -ofaxis) CEKA on theclips unloaded results inside, ext ensionwhich resultsof the retention in detaching element, the denture which results on this in side. a more The stable use of denture, CEKA clipsless significantresults in ext displacement,ension of the especially retention element,when loading which an results incisor in anda more canine. stable Adverse denture, ef- lessfects significant of oblique displacement, forces are also especially smaller, when and thereloading is noan rotation.incisor and Mucosal canine. load Adverse onto ef-the fectsbone of remains oblique smaller, forces aretoo. also The smaller,use of CEKA and thereclips makesis no rotation. the bar itself Mucosal subjected load toonto greater the bone remains smaller, too. The use of CEKA clips makes the bar itself subjected to greater displacement, especially with forces directed to an incisor, which may bring about a larger displacement,bone load around especially the implants. with forces When directed exposing to an the incisor, denture which to a singlemay bring load, about no significant a larger Figure 8. Maximum denture displacement in the first and second system. Figurebonedifferences 8. loadMaximum around were denture thefound implants. displacement in the bone,When in boththeexposing ﬁrst cortical and the second anddenture spongy, system. to a singlefor individual load, no significantbars. How- differencesever, tests shouldwere found also includein the bone, cyclic both loads. cortical and spongy, for individual bars. How- ever, testsCEKA should clips alsoare partinclude of a cyclic system loads. that is exposed to displacements, especially when loadingCEKA a canine clips are and part incisor. of a Itsystem should that be notedis exposed that obliqueto displacements, forces are alwaysespecially less whenfavor- loadingable for a acanine clip on and the incisor. working It should side. However, be noted thatwhen oblique loading forces canine are alwaysfor the lessclip favor-on the ablebalancing for a clip side, on greater the working displacements side. However, occur for when vertical loading forces. canine The obtainedfor the clip data on show the balancingthat implants side, and greater coni caldisplacements abutments make occur very for verticalstable system forces. elements, The obtained regardless data show of the thatbar implantsused. and conical abutments make very stable system elements, regardless of the bar used.

Figure 9. Average denture displacement in the first and second system.

The greatest stress concentrate within the precision element and concern the bar. In the first system, bar stress ranges from 0.0020 to 27.552 MPa (Figure 10), obtained when a canine is loaded with a vertical force of 100 N. High values of maximum stresses when loading all teeth with a vertical force of 100 N are, however, focal, while the average values are definitely smaller (3.8260 MPa for an incisor, 3.1607 MPa for a canine, and 1.1483 MPa for a molar). Regardless of the force location, higher stresses occur for oblique forces. The largest bar stresses appear in the medial area, especially for the incisor and canine. For the

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The weakest element of this part of the system is the place where the bar is fixed, i.e., the screws, especially when loading a molar with both a vertical force of 100 N and oblique forces. For these forces, greater displacements within the screws occur on the balancing side than on the working one. When a denture retained by a “rider” bar is loaded with a vertical force of 100 N, and the displacement of the screws exceeds the displacement of the denture. However, the use of CEKA clips significantly limits the screw displacement, leading to protection against loosening.

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molar, stresses are distributed over a larger area. Moreover, during loading of the molar, there are clearly lower stresses in the bar than in an incisor or canine, and the force of 50 N does not exceed 1 MPa. In the second system, bar stresses range from 0.080 to 578.6 MPa (Figure 11). The highest stresses occur when loading a molar tooth with a vertical force of 100 N. For an incisor and molar, higher stresses occur for oblique forces (for a force of 50 N, average bar stresses for an incisor are 8.1340 MPa for vertical force and 10.7370 MPa for oblique force; for a molar, they are 4.7082 MPa for vertical force and 5.4998 and 7.5317 MPa for oblique forces). Relatively small stresses arise in the bar when loading a canine with oblique forces (for the force of 50 N, the maximum value is 13.4736 MPa, and average Materials 2021, 14, 2598 1.6251 MPa). In the second system, the precision element was extended by mounting 9 of the 17 FigureCEKA 8. clips. Maximum The maximum denture displacement stresses produced in the first in and clips second are smallersystem. than the stresses of the bar. Stress values range from 0.0538 to 136.9900 MPa for the right clip and from 0.0784 MPa to 127.6700 MPa for the left. The highest stresses occur in the right clip with a vertical force of 100 N applied on a canine and in the left clip when applying this force on an incisor. Significant maximum values of clip stresses result from their shape and are located within the narrowing. CEKA clips cause significantly higher stresses in the screws, conical abutments, and implants than when a bar is used alone. Comparative analysis of the first and second system (Figure 12) shows that the additional use of CEKA clips causes bar extension, and therefore the denture is less susceptible to displacement. Still, more stresses are generated within the bar. Both the value of the maximum and average stresses is several times greater. The distribution of stresses is changed, too. When a bar is used alone, the greatest stresses are limited, located in the medial area. CEKA clips result in a more even distribution of stresses in the bar, with a greater concentration of stresses in clips. Significant stresses in this area are also associated with the clip shape. The material narrowing site causes a notch effect. Interestingly, oblique forces applied to all teeth give rise to similar stresses in both clips. An exception is an oblique force that is directed forward to a molar and thus acting in the final phase of the chewing cycle, where higher stresses are on the balancing side. For vertical forces, the greatest stresses appear on both sides when loading an incisor. Loading a canine generates

higher stresses on the working side, whereas loading a molar causes relatively smaller FigureFigurestresses, 9. 9. AverageAverage which denture denture are asymmetrical displacement displacement andi inn the the greater first ﬁrst and and on second secondthe balancing system. system. side. The greatest stress concentrate within the precision element and concern the bar. In the first system, bar stress ranges from 0.0020 to 27.552 MPa (Figure 10), obtained when a canine is loaded with a vertical force of 100 N. High values of maximum stresses when loading all teeth with a vertical force of 100 N are, however, focal, while the average values are definitely smaller (3.8260 MPa for an incisor, 3.1607 MPa for a canine, and 1.1483 MPa for a molar). Regardless of the force location, higher stresses occur for oblique forces. The largest bar stresses appear in the medial area, especially for the incisor and canine. For the

Figure 10. Maximum bar stresses in the ﬁrstfirst andand secondsecond system.system.

MaterialsMaterials 20212021, 1414, x FOR PEER REVIEW 11 of 16 Materials 2021,, 14,, 2598x FOR PEER REVIEW 1011 ofof 1716

FigureFigure 11. 11. AverageAverage bar bar stresses stresses in in the the first ﬁrst a andnd second second system. system. Figure 11. Average bar stresses in the first and second system.

Figure 12. Maximum implant stresses in the first and second system (explanation: L—left; R—right). FigureFigure 12.12. MaximumMaximum implantimplant stressesstresses inin thethe firstfirst andand secondsecond systemsystem (explanation:(explanation: L—left;L—left; R—right).R—right). For subsequent components of the precision element and implants, the stresses are GreaterFor subsequent dislocations components of the denture of the were precision recorded element in the and first implants, system, wherethe stresses the maxi- are smaller and smaller because each element has absorbed part of the energy created by the mumsmaller value and was smaller 0.7 mm because (Figure each8). Inelement the second has absorbed system, the part maximum of the energy displacement created ofby thethe load. Of course, ideally, stresses for this part of the system would equal 0. However, the dentureload. Of wascourse, 0.6 mmideally, (Figure stresses9). The for largest this part displacements of the system occur would when equal an incisor 0. However, is loaded. the material applied for testing is real. These elements work, and the denture works; hence, Formaterial forces applied of the samefor testing value, is greater real. These displacements elements work occur, and for obliquethe denture forces. works Regardless; hence, there are low values of stress. It is essential that CEKA clips cause a significant increase in ofthere the are location low values of the of load, stress. the It most is essen significanttial that CEKA dislocations clips cause occur a insignificant the retromolar increase pad in stresses within conical abutments and screws. Proportionally higher stresses in the screws region.stresses Withwithin forces conical acting abutments on an incisor, and screws. especially Proportionally for oblique higher forces, stresses displacements in the screws are when using CEKA clips result from the screws being more stable in this system, as they foundwhen using on both CEKA sides clips of the result denture. from the screws being more stable in this system, as they undergo lesser displacement. Hence, a smaller risk of loosening. However, an increase in undergoFor forceslesser actingdisplaceme on a caninent. Hence, and a molar, smaller the risk denture of loosening. undergoes However, greater an displacement increase in stress may cause mechanical damage to be more likely. Higher stresses in conical abut- onstress the may balancing cause side.mechanical Depending damage on theto be shape more of likely. the precision Higher elementstresses used,in conical the valueabut- ments and screws generate lower stresses in the implants, too. ofments displacement and screws changes generate while lower the stresses direction in the of implants, rotation remains too. the same. Loading of Moreover, the studies showed that significant stresses were observed for the matrix the examinedMoreover, teeth the resultsstudies inshowed rotational that motionsignifican ont the stresses working were side, observed pressing for against the matrix the housing in the first and the second system. However, maximum values occurred only in denturehousing bearingin the first area, and and the tilting second from system. the YZ However, plane (~in maximum the XY plane, values ~around occurred the only Y-axis) in theon thesite unloadedmarked in side, Figure which 13. results In other in detachingplaces, stress the denturevalues are on thissmaller. side. The The place use of where CEKA the site marked in Figure 13. In other places, stress values are smaller. The place where maximumclips results stress in extension occurs is ofthe the result retention of element element, geometry, which results causing in astress more buildup stable denture, in this maximum stress occurs is the result of element geometry, causing stress buildup in this less significant displacement, especially when loading an incisor and canine. Adverse

Materials 2021, 14, 2598 11 of 17

effects of oblique forces are also smaller, and there is no rotation. Mucosal load onto the bone remains smaller, too. The use of CEKA clips makes the bar itself subjected to greater displacement, especially with forces directed to an incisor, which may bring about a larger bone load around the implants. When exposing the denture to a single load, no significant differences were found in the bone, both cortical and spongy, for individual bars. However, tests should also include cyclic loads. CEKA clips are part of a system that is exposed to displacements, especially when loading a canine and incisor. It should be noted that oblique forces are always less favorable for a clip on the working side. However, when loading canine for the clip on the balancing side, greater displacements occur for vertical forces. The obtained data show that implants and conical abutments make very stable system elements, regardless of the bar used. The weakest element of this part of the system is the place where the bar is fixed, i.e., the screws, especially when loading a molar with both a vertical force of 100 N and oblique forces. For these forces, greater displacements within the screws occur on the balancing side than on the working one. When a denture retained by a “rider” bar is loaded with a vertical force of 100 N, and the displacement of the screws exceeds the displacement of the denture. However, the use of CEKA clips significantly limits the screw displacement, leading to protection against loosening. The greatest stress concentrate within the precision element and concern the bar. In the first system, bar stress ranges from 0.0020 to 27.552 MPa (Figure 10), obtained when a canine is loaded with a vertical force of 100 N. High values of maximum stresses when loading all teeth with a vertical force of 100 N are, however, focal, while the average values are definitely smaller (3.8260 MPa for an incisor, 3.1607 MPa for a canine, and 1.1483 MPa for a molar). Regardless of the force location, higher stresses occur for oblique forces. The largest bar stresses appear in the medial area, especially for the incisor and canine. For the molar, stresses are distributed over a larger area. Moreover, during loading of the molar, there are clearly lower stresses in the bar than in an incisor or canine, and the force of 50 N does not exceed 1 MPa. In the second system, bar stresses range from 0.080 to 578.6 MPa (Figure 11). The highest stresses occur when loading a molar tooth with a vertical force of 100 N. For an incisor and molar, higher stresses occur for oblique forces (for a force of 50 N, average bar stresses for an incisor are 8.1340 MPa for vertical force and 10.7370 MPa for oblique force; for a molar, they are 4.7082 MPa for vertical force and 5.4998 and 7.5317 MPa for oblique forces). Relatively small stresses arise in the bar when loading a canine with oblique forces (for the force of 50 N, the maximum value is 13.4736 MPa, and average 1.6251 MPa). In the second system, the precision element was extended by mounting the CEKA clips. The maximum stresses produced in clips are smaller than the stresses of the bar. Stress values range from 0.0538 to 136.9900 MPa for the right clip and from 0.0784 MPa to 127.6700 MPa for the left. The highest stresses occur in the right clip with a vertical force of 100 N applied on a canine and in the left clip when applying this force on an incisor. Significant maximum values of clip stresses result from their shape and are located within the narrowing. CEKA clips cause significantly higher stresses in the screws, conical abutments, and implants than when a bar is used alone. Comparative analysis of the first and second system (Figure 12) shows that the additional use of CEKA clips causes bar extension, and therefore the denture is less susceptible to displacement. Still, more stresses are generated within the bar. Both the value of the maximum and average stresses is several times greater. The distribution of stresses is changed, too. When a bar is used alone, the greatest stresses are limited, located in the medial area. CEKA clips result in a more even distribution of stresses in the bar, with a greater concentration of stresses in clips. Significant stresses in this area are also associated with the clip shape. The material narrowing site causes a notch effect. Interestingly, oblique forces applied to all teeth give rise to similar stresses in both clips. An exception is an oblique force that is directed forward to a molar and thus acting in the final phase of the chewing cycle, where higher stresses are on the balancing side. For vertical forces, the greatest stresses appear on both sides when loading an incisor. Loading a canine generates Materials 2021, 14, 2598 12 of 17

higher stresses on the working side, whereas loading a molar causes relatively smaller stresses, which are asymmetrical and greater on the balancing side. For subsequent components of the precision element and implants, the stresses are smaller and smaller because each element has absorbed part of the energy created by the load. Of course, ideally, stresses for this part of the system would equal 0. However, the material applied for testing is real. These elements work, and the denture works; hence, there are low values of stress. It is essential that CEKA clips cause a significant increase in stresses within conical abutments and screws. Proportionally higher stresses in the screws when using CEKA clips result from the screws being more stable in this system, as they undergo lesser displacement. Hence, a smaller risk of loosening. However, an increase in stress may cause mechanical damage to be more likely. Higher stresses in conical abutments and screws generate lower stresses in the implants, too. Materials 2021, 14, x FOR PEER REVIEW 12 of 16 Moreover, the studies showed that significant stresses were observed for the matrix housing in the first and the second system. However, maximum values occurred only in the site marked in Figure 13. In other places, stress values are smaller. The place where area.maximum This place stress can be occurs exposed is the to result mechanical of element damage, geometry, but due causing to embedding stress buildup in acrylic, in this it willarea. beThis difficult place to can locate, be exposed but it does to mechanical not necessarily damage, affect but the due denture to embedding function. in acrylic, it will be difficult to locate, but it does not necessarily affect the denture function.

Figure 13. Stress concentration in matrix housing. Figure 13. Stress concentration in matrix housing. High values of stress were found in the denture. Maximum values, however, occur only in the place where the force is applied. This situation is not clinical in nature, as under High values of stress were found in the denture. Maximum values, however, occur physiological conditions, the load is applied to the denture plane and is not point-wise. only in the place where the force is applied. This situation is not clinical in nature, as under The average values of stress are incomparably smaller. Generally, there are no significant physiological conditions, the load is applied to the denture plane and is not point-wise. stresses in the denture as it undergoes dislocations rather than stresses when loaded. The average values of stress are incomparably smaller. Generally, there are no significant The deformation results concerning each case show that the subsequent components stresses in the denture as it undergoes dislocations rather than stresses when loaded. undergo only minor deformations, which are not relevant for understanding the biome- chanicsThe deformation of the overdenture. results concerning During the each deformation case show that analysis, the subsequent only the mucosa components and the undergomatrix only together minor with deformations, the housing which deserve are attention. not relevant Irrespective for understanding of the shape the of biome- the preci- chanicssion element,of the overdenture. the highest mucosalDuring deformationsthe deformation occur analysis, when an only incisor the ismucosa loaded (Figureand the 14 ). matrixIn system together I, thewith maximum the housing deformation deserve attention. values areIrrespective 0.22900 forof the a vertical shape of force the preci- of 100 N sionand element, 0.15670 the for highest an oblique mucosal force deformations of 50 N. The occur use ofwhen CEKA an incisor clips additionally is loaded (Figure does not 14).significantly In system I, changethe maximum the deformation deformation values, values which are 0.22900 are maximum for a vertical at 0.21060 force for of a100 vertical N andforce 0.15670 of 100 for Nan and oblique 0.15060 force for of an 50 oblique N. The force use ofof 50CEKA N. The clips results additionally concerning does the not first significantlyand second change system the show deformation that the mounting values, which of CEKA are maximum clips does at not 0.21060 significantly for a vertical affect the forcedeformation of 100 N and values 0.15060 of the for matrix an oblique and its force housing, of 50 causing N. The only results a slight concerning increase, the especially first andwhen second loading system the show molar that tooth the m (Figureounting 15 ).of TheCEKA highest clips deformationdoes not significantly values were affect recorded the deformationfor the forces values directed of the atmatrix the incisal and its tooth, housing, and causing when only only the a slight bar is increase, used, they especially are 0.0144 when(matrix loading housing) the molar and tooth 0.0077 (Figure (matrix) 15). for The a verticalhighest deformation force of 100 Nvalues and 0.0057were recorded and 0.0055, for respectively,the forces direc forted an obliqueat the incisal force oftooth, 50 N. and However, when only when the the bar bar is withused, CEKA they are clips 0.0144 is used, (matrixthe maximum housing) and deformations 0.0077 (matrix) are 0.0109 for a andvertical 0.0081 force for of a vertical 100 N and force 0.0057 of 100 and N and 0.0055, 0.0066 respectively,and 0.0062, for respectively, an oblique force for an of oblique 50 N. However, force of 50 when N. the bar with CEKA clips is used, the maximum deformations are 0.0109 and 0.0081 for a vertical force of 100 N and 0.0066 and 0.0062, respectively, for an oblique force of 50 N. Denture deformations occur mainly at the force application site. Similar deformation values were found for both systems. In system I, the largest deformations occur for vertical forces when the molar is loaded, namely 0.09744 (for 100 N force), and for oblique forces when the incisor is loaded, namely 0.0490 (for 50 N force). In system II, the largest deformations occur for vertical forces when loading the canine, namely 0.08876 (for 100 N force), and for oblique forces when loading the incisor, namely 0.1506 (for 50 N force). Regardless of the shape of the precision element, the denture undergoes little deformation. Acrylic is a rigid material so that the denture is displaced under load but not deformed. As expected, the element that undergoes significant deformation is, due to its highest elasticity, the mucosa. The deformation values of the mucosa are related to the displacement of the denture. Another element where increased deformation was observed is the matrix, which is the second most elastic part. For both systems, however, the deformation values are already much smaller. Larger values of deformation of the matrix in relation to the other components of the model, related to its elasticity, are the cause of wear of this element under the influence of the use of the denture.

Materials 2021, 14, 2598 13 of 17 Materials 2021, 14, x FOR PEER REVIEW 13 of 16 Materials 2021, 14, x FOR PEER REVIEW 13 of 16

Figure 14. Maximum mucosa deformations for systems I and II. FigureFigure 14.14. Maximum mucosa deformations for for systems systems I Iand and II. II.

Figure 15. Maximum matrix deformations for systems I and II. FigureFigure 15.15. Maximum matrix de deformationsformations for for systems systems I Iand and II. II. 4. Discussion 4. DiscussionDenture deformations occur mainly at the force application site. Similar deformation valuesA weresignificant found problem for both when systems. planning In systeman overdenture I, the largest is the choice deformations of an appropriate occur for A significant problem when planning an overdenture is the choice of an appropriate verticalprecision forces system. when Many the molarresearchers is loaded, consider namely dentures 0.09744 retained (for 100 by N constructions force), and for splinting oblique precision system. Many researchers consider dentures retained by constructions splinting forcesto be an when advantageous the incisor solution is loaded, [27 namely–29]. Undoubtedly, 0.0490 (for 50the N advantages force). In system of bar constructions II, the largest to be an advantageous solution [27–29]. Undoubtedly, the advantages of bar constructions deformationsinclude proper occur denture for vertical stabilization forces and when the loading ability to the regulate canine, retention. namely 0.08876 (for 100 N include proper denture stabilization and the ability to regulate retention. force),Loads and for during oblique denture forces dislocations when loading in the incisor,lateral region namely are 0.1506 reduced (for 50[30]. N force).The inci- Loads during denture dislocations in the lateral region are reduced [30]. The inci- denceRegardless of complications of the shape is also of lower the precisioncompared element, to ball attachments. the denture Magnets, undergoes which little are de- dence of complications is also lower compared to ball attachments. Magnets, which are formation.less frequently Acrylic used, is can a rigid corrode material and allow so that for the horizontal denture displacement is displaced underof the loaddenture but less frequently used, can corrode and allow for horizontal displacement of the denture not[31]. deformed. They alsoAs have expected, the lowest the elementretention that strength. undergoes It should significant be remembered, deformation however, is, due [31]. They also have the lowest retention strength. It should be remembered, however, tothat its bars highest require elasticity, more space the mucosa. occlusally, The that deformation is, between the values jaws of (13 the–14 mucosa mm), as arecompared related that bars require more space occlusally, that is, between the jaws (13–14 mm), as compared toto the ball displacement attachments (10 of– the11 mm) denture. [32]. AnotherNo adequate element space where may result increased in the deformation incorrect shape was to ball attachments (10–11 mm) [32]. No adequate space may result in the incorrect shape observedof the denture is the plate matrix, or the which position is the of second artificial most teeth. elastic This, part. in turn, For bothmay bring systems, about however, faulty of the denture plate or the position of artificial teeth. This, in turn, may bring about faulty thefacial deformation aesthetics or values impaired are alreadysound articulation much smaller. [33–37]. Larger values of deformation of the facial aesthetics or impaired sound articulation [33–37]. matrixThe in relationstudied bar to the has other a retention components shape. ofCurrently, the model, connections related to of its round elasticity, or straight are the The studied bar has a retention shape. Currently, connections of round or straight causecross- ofsections wear of are this less element and less under often theused. influence These bars of theare useonly of indicated the denture. for short implants cross-sections are less and less often used. These bars are only indicated for short implants or problems with osseointegration, as they generate fewer stresses around the implants or problems with osseointegration, as they generate fewer stresses around the implants 4.[38]. Discussion To improve retention, the “rider” bar is better. The tests have confirmed that the more [38]. To improve retention, the “rider” bar is better. The tests have confirmed that the more retentiveA significant the shape problem of the when precision planning element, an overdenture the smaller isthe the denture choice dislocations.of an appropriate The retentive the shape of the precision element, the smaller the denture dislocations. The precisionlength of system. the connection Many researchers is of great consider importance. dentures The greater retained the by length constructions of the retention splinting length of the connection is of great importance. The greater the length of the retention toarm, be an the advantageous more rigid the solution denture. [27– 29 Higher]. Undoubtedly, rigidity of the the advantages prosthetic of restoration bar constructions causes arm, the more rigid the denture. Higher rigidity of the prosthetic restoration causes includesmaller properdisplacement denture of stabilization the restoration, and theso mucosal ability to support regulate of retention. the denture is smaller as smaller displacement of the restoration, so mucosal support of the denture is smaller as

Materials 2021, 14, 2598 14 of 17

Loads during denture dislocations in the lateral region are reduced [30]. The incidence of complications is also lower compared to ball attachments. Magnets, which are less frequently used, can corrode and allow for horizontal displacement of the denture [31]. They also have the lowest retention strength. It should be remembered, however, that bars require more space occlusally, that is, between the jaws (13–14 mm), as compared to ball attachments (10–11 mm) [32]. No adequate space may result in the incorrect shape of the denture plate or the position of artificial teeth. This, in turn, may bring about faulty facial aesthetics or impaired sound articulation [33–37]. The studied bar has a retention shape. Currently, connections of round or straight cross-sections are less and less often used. These bars are only indicated for short implants or problems with osseointegration, as they generate fewer stresses around the implants [38]. To improve retention, the “rider” bar is better. The tests have confirmed that the more retentive the shape of the precision element, the smaller the denture dislocations. The length of the connection is of great importance. The greater the length of the retention arm, the more rigid the denture. Higher rigidity of the prosthetic restoration causes smaller displacement of the restoration, so mucosal support of the denture is smaller as well. However, the stresses within the precision element and implants are greater. The site of implant insertion may determine the length of the retention arm. The wider the distance between implants, the longer the connection. However, it should be remembered that when the line connecting the implants exceeds the denture plate, only individual clips are possible [37]. To extend the retention arm, it is possible to use symmetrical projections or clips distally. The results of the studies show that the use of additional clips reduces denture displacement. However, the bar with clips and implants is more loaded; the more retentive the shape of the precision element, the greater the stresses [38,39]. In the case of short implants and substantial bone atrophy, it seems more advisable to make a straight bar to increase the mucosal force transfer and relieve the implants. Studies showing that a longitudinal bar extended distally (with a twice-broken axis) causes a significant exceeding of the stress tolerated by the bone tissue can be found in the literature. Still, this study has not confirmed such a phenomenon. An increase of stress in this area was observed only, which in extreme cases cannot rule out bone resorption around the implant. A relatively smaller stress increase in the implants is related to the fact that conical abutments and screws absorb a greater part of the energy released from loading forces. This is where a marked increase in stress was noted. It seems, therefore, that the risk of loosening or even damage to these elements is greater than the disintegration of the implant or peri-implantitis [40,41]. The conducted research confirmed the reports of other authors that the greatest load occurs in the distal parts of the bar retention [42,43]. Stress concentration, however, within the clips results from their shape. The implementation of additional clips is indicated for long implants in the absence of clinical problems with osseointegration. Then, the use of direct support on implants that are ankylotically connected to the bone relieves the structure of the denture bearing area, especially in case of a mucous membrane poor susceptibility. This contributes to protecting the underlying tissues, which are not physiologically adapted to sustain direct occlusal forces, which may lead to bony atrophy in a short time [44,45]. The studies presented in this paper show the interdisciplinary character of today’s prosthetics. FEA is applied by a growing number of researchers [24,38,43,46]. Advances in virtual engineering enable making computer simulations in concrete and stable conditions, which helps avoid mistakes in experimental studies. Of course, it would be prudent to perform an experimental validation to verify the result. Confirmation of the FEA ideally would be performed in human subjects, but that is very difficult to perform stress tests in vivo. For this test, a phantom can be prepared, but it should be noted that it is a very complicated biological model with heterogeneous materials. Nevertheless, in FEA studies on dental implants, validation techniques are still rare. In his study, Chang performed a literature review of finite element analysis of dental implants. In PubMed, he found 522 articles about FEA, but only 47 papers including validation [47]. Materials 2021, 14, 2598 15 of 17

In the near future, the authors of this study plan to compare the results obtained in the presented article with clinical trials.

5. Conclusions 1. The shape of a precision element affects the biomechanical properties of an overdenture. 2. Slight stresses are observed within the studied bar-retained precision attachment connected to conical abutments and implants. 3. Ball clips cause less displacement of an overdenture when loaded. However, they generate higher stresses within the precision element and implants, compared with the overdenture retained on the bar-retained implants only. 4. Regardless of the shape of the precision element, small deformations occur, which mainly affect the mucosa and the matrix. 5. For the analyzed situation, the best solution seems to be using the bar with CEKA clips.

Author Contributions: Conceptualization, M.I.-H.; methodology, M.I.-H. and W.H.; software, P.B. and K.P.; validation, M.I.-H. and W.H.; formal analysis, A.A.P. and B.D.-B.; investigation, M.I.- H.; resources, M.I.-H., A.A.P. and K.P.; data curation, A.A.P. and B.D.-B.; writing—original draft preparation, M.I.-H.; writing—review and editing, M.I.-H. and A.A.P.; visualization, W.H., P.B. and K.P.; supervision, B.D.-B.; project administration, M.I.-H. and B.D.-B.; funding acquisition, P.B. and K.P. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: We do not provide additional data. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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