Comparative analysis of internal and external-hex crown connection systems-a finite element study

has reduced stress concentrations in the ABSTRACT crown. However, because the torque is transferred through the abutment screw to the abutObjectives: The abutment connection with ment contact, changing the torque has greater the crown is fundamental to the structural effect on this hex system than the masticatory stabil ity of the implant system and to the preforce. Overall the masticatory force is more vention of mechanical exertion that can cominfluential on the stress within the crown for promise the success of the implant treatment. the external-hex system and the torque is more The aim of this study is to clarify the difference influential on the internal-hex system. in the stress distribution patterns between implants with internal and external-hex connections with the crown using the Finite Element Method (FEM). Material and Methods: The internal and external-hex connections of the Neoss and 3i implant systems respectively, are


Figure 1b
Using theoretical techniques, such as the FEM, all mechanical aspects that could affect the implant success can be evaluated.FEM has been used extensively to evaluate the performance of dental implant prosthesis [4][5][6][7][8][9][10][11][12][13][14][15].Studies by Maeda et al. (2006), Merz et al. (2000) and Khraisat et al. (2002) have all considered the behavior of the stress within the abutment screw however disregarding the stress within the crown.To date no published research appears to have investigated the stress characteristics in the crown due to an internal or external-hex system.Ultimately, the outcome of this study will facilitate dental practitioners to identify locations within the implant system that are susceptible to stress concentrations.

METHODOLOGY
The modeling and simulation herein are performed using the Strand7 Finite Element Analysis (FEA) System (2004).The first step of the modeling is to define the geometry of the implant system.This is then followed by specifying the material behavior in terms of the Young's modulus, Poisson's ratio and density for the implant and componentry.After applying the appropriate loading and restraint conditions, the c) Locations for measuring stress profile and contour a) Loading and restraint conditions (with detailed variables) internal and external-hex systems can be evaluated for their contributions to the stress characteristics within the crown.

Modelling
Data acquisition for the internal and external-hex systems are obtained from the manufacturer's data.Shown in ) are details of the Neoss (2006) and 3i (2006) systems.
Shown in ) are the detailed variables considered in this study.The implant is conical with 2 degrees of taperage, a helical thread, diameter of 4 .5 m m , a n d l e n g t h o f 1 1 m m .D i f f e r e n t f i x e d restraints are applied to the symmetrical edge of the implant system as compared to the outer edge of implant thread.The symmetrical edge is restrained from rotating around the z-axis and translating through the x-and y-axis.The outer edge of the implant thread is restrained from deforming in any direction.Note that these loading and restraint conditions are the same for both internal and external-hex systems.
For the Neoss and 3i finite element models, the total numbers of elements are respectively 13464 and 30420 for the implant, 3564 and 9108 for the abutment, 17424 and 25956 for the abutment screw, 38484 and 47052 for the crown.The total number of nodal points for the entire Neoss and 3i models are 82547 and 122688 respectively.are believed by clinicians to be critical for examining the stress levels in the crown.Note that both lines NN and II are chosen on Section AA because the highest stress magnitudes (compressive is prominent over ten-

Figure 1b
Figure 1a Figure 1c sile) occur on this plane due to the masticatory loading characteristics.For the present study a negative temperature (-10 Kelvin, K) is applied to all the nodal points within the abutment screw, causing each element to shrink.A trial and error process is applied to determine the temperature coefficient, C, for both the Neoss and 3i systems (i.e.C and C ) that can yield an equivalent Neoss 3i  and C =-0.98×10 , -1.80×10 and -2.68×10 /K, respec-3i tively.

Material Properties
The material properties used are specified in terms of Young's modulus, Poisson's ratio and the density for the implant and all associated components ( ).All material properties are assumed to be linear, homogeneous and elastic in behavior.

RESULTS DISCUSSION
Zirconia typically used as a dielectric material has proven adequate for application in dentistry.With its typical white appearance and high Young's moduli it is ideal to be used in the manufacturing of sub frames   stresses along the lines NN and II for all values of F P for the construction of dental restorations such as are shown in .crowns and bridges, which are then veneered with As found for F , when F increases the stresses conventional feldspathic porcelain.Zirconia has a M P fracture strength that exceeds that of Titanium therecalculated along the line NN increase, showing two fore it may be considered as a high strength material.peaks along the line NN (refer to ) and

3-4
However with cyclic preload and masticatory loads )).Also, as found for F , elevated stress considered the influence of the implant dimensions 3.96mm) for the Neoss system.For the 3i system the and the bone-implant bond on the stress in the survon Mises stresses are measured between locations rounding bone.However, to date no research has II (0-2.38mm),II (2.38-2.78mm),II (2.78- been conducted to evaluate the stress produced by dif-3.67mm),II (3.67-4.06mm),II (4.06-4.65mm)4-5 5-6 ferent implant to crown connections (i.e.internal and and II (4.65-5.27mm),as shown in ).

6-7
external-hex).The analysis completed in this paper uses the FEM to replicate internal and external-hex systems when subjected to both F and F loading F or F ) set to its average.

M P
In general, when the applied masticatory force, F ,

M
The mastication force F is applied on the M is increased, the von Mises stresses also increase proocclusal surface of the crown, evenly distributed portionally, because the system being analysed is linalong 378 nodal locations ( ), and orien- 272.82MPa) and 698.09MPa (1047.14-349.05MPa))) is caused by a sharp corner and sudden change in respectively.The geometrical design of the externalsection at this point.
hex system tends to induce stress concentrations, Elevated stress concentrations are identified at the located 2.89mm from the apex in this study.For this beginning of the line II ( ) and )).

3-4
system, a stress concentration at this point is also This stress peak, as can be identified in ), is induced by F , increasing the compressive stresses P caused by a sharp corner at this point.For the 3i syson the right hand side of the crown.Increasing F tem, the volume of the crown exceeds that of the P Neoss system, thereby suggesting that the 3i crown from its minimum to maximum values, for the extermay endeavor greater resistance to the applied nal-hex system, increases the stress by 485.46MPa masticatory forces.However, even though the Neoss (951.67-466.21MPa).crown has a thinner wall thickness along the line The internal-hex system has reduced stress concen-NN , reduced stresses are still evident due to the 3-4 abutments high Young's modulus.Overall, the design differences between the Neoss and 3i systems ultimately results in the 3i system having higher stresses when F is increased.is kept as a constant and its medium value, i.e. 500N is considered herein.The distributions of von Mises

Figure 1 .
Figure 1.Finite element model of internal and external-hex systems.
As indicated in) the von Mises stresses along the lines NN (NN , NN and NN ) and II 1respectively, are measured for all possible combinations of loading.Note that, for example, along the line II the beginning location of the line 1-2 is identified as II and the end as II .These locations 1 2

M
the compressive strength of 2.1GPa (Curtis et al. peaks are identified at the beginning of the line II 3-4 2005) can easily be exceeded especially for implant ( ) and )).Overall, all values of systems with external-hex connections, as confirmed F cause greater stresses along lines NN and II, than M during this study.do varying values of F .The distribution of von Mises stresses in the crown P is discussed for both the internal and external-hex systems for all combinations of masticatory and preload 4. DISCUSSION forces.Shown in ), are the von Mises FEA has been used extensively to predict the stresses measured between locations NN (

M P 3 . 1 .
Masticatory Force, F M conditions.As shown in , two stress peaks The distributions of von Mises stresses along the were revealed along the lines NN and II at locations lines NN and II for all values of F are shown in M 3.76 and 2.89mm from the top.The stress values Note that the preload, F , is set at its medium p shown were calculated with the other variables (i.e.value, i.e. 587.44N.

o 4 M
ear elastic.When F increases the stress along the M tated at 45 in the x-y plane.This induces compresline NN increases showing two peaks along the line sive stresses in the right hand side of the crown and NN (refer to )).The larger of these two tensile in the left.Varying F from 200 to 1000N for 3peaks occurs at a distance of ±3.8mm in length from the internal and external-hex systems results in a NN .This stress peak (as can be identified in change in von Mises stress of 545.64 (818.47-1

M 3 . 2 .
Preload Force, F PTo investigate the effect of different preload F , F P M
Figure 1a Masticatory force, F , is applied to the occlusal surhalf of the total magnitude because only half of the implant system is modelled.Therefore the total F M modelled is 200, 500, 1000N and F is 201.93,587.44, P between abutment and abutment screw are halved when compared with that used by van Staden et al. (2008) due to the modelling assumption aforementioned.Calculations for the abutment screw surface pressure, q, confer identical results than that found by van Staden et al. (2008).

Table 2 .
Von Mises stress (MPa) in crown (location of stress recording in brackets).