Journal of Dr. NTR University of Health Sciences

ORIGINAL ARTICLE
Year
: 2019  |  Volume : 8  |  Issue : 2  |  Page : 95--100

Stress distribution in implant supported mandibular ovedentures and surrounding bone using three different types of attachments – a 3D finite element analysis


Vikram R Dulala1, Lahoti Anurag1, Puralasetty Srinivasa Rao1, Kummari Subash Chander1, Aditi C Sabnis1, Asha O Reddy2,  
1 Department of Prosthodontics, Sri Balaji Dental College, Moinabad, Hyderabad, Telangana, India
2 Department of Conservative and Endodontics, Sri Balaji Dental College, Moinabad, Hyderabad, Telangana, India

Correspondence Address:
Dr. Kummari Subash Chander
Sri Balaji Dental College, Yenkapally, Moinabad, Hyderabad - 500 075, Telangana
India

Abstract

Background: Since mandibular complete denture faces problems with retention, stability, support mandibular implant supported overdenture is a more successful and durable treatment option. Different types of attachments used with mandibular overdentures improve the retention and dissipate stress as well. It is of paramount importance for the success of the prosthesis. Finite element analysis (FEA) is a very precise method for stress assessment in the bone. Materials and Methods: Mandibular 2 implant supported overdenture was considered. Noble Biocare replace implant of diameter 4.3 mm, length 11.5 mm was used in the study. 3 models were considered: Model A – Implant overdenture with Ball/O-ring attachment, Model B – Implant overdenture with Locator Attachment, Model C – Implant overdenture with Ceka attachment. A detailed, precise 3D solid model of human mandible was obtained upon which FEA could be optimized using Autodesk NETFABB ver. 8.1, a software and MAYA 3D software tool. These models were subjected to vertical, oblique and horizontal forces of 35N, 70N and 10N, respectively. Stress was assessed at different sites namely silicon cap/ring, abutment, implant–abutment interface, implant body, surrounding bone. The amount of force required to uplift the dentures was assessed. Results and Conclusion: Results showed that locator attachment showed least amount of stress generation under all three types of loads. Within the limitations of the study, it was concluded that locator attachment was better in terms of stress generation in comparison to Ball and Ceka attachment. It was the most retentive attachment in comparison to the other two attachments.



How to cite this article:
Dulala VR, Anurag L, Rao PS, Chander KS, Sabnis AC, Reddy AO. Stress distribution in implant supported mandibular ovedentures and surrounding bone using three different types of attachments – a 3D finite element analysis.J NTR Univ Health Sci 2019;8:95-100


How to cite this URL:
Dulala VR, Anurag L, Rao PS, Chander KS, Sabnis AC, Reddy AO. Stress distribution in implant supported mandibular ovedentures and surrounding bone using three different types of attachments – a 3D finite element analysis. J NTR Univ Health Sci [serial online] 2019 [cited 2019 Aug 18 ];8:95-100
Available from: http://www.jdrntruhs.org/text.asp?2019/8/2/95/263638


Full Text



 Background



Mandibular complete denture is known to face problems associated with retention, stability, support, patient's comfort.[1] Two types of implant supported prostheses are widely used to overcome these problems – implant supported fixed prosthesis and implant supported overdentures.[2] Implant supported fixed prosthesis may not be possible in all patients due to financial, medical, systemic, esthetic reasons.[3] Therefore, implant supported overdenture becomes a less invasive and successful treatment option in terms of cost-effectiveness and durability.[4] Restoration of completely edentulous mandibular arch with two implants supported overdenture is a primary treatment option.[5] Attachments are interstitial parts that connect dental implants with the overlying denture and help in resisting the dislodging forces.[6] Different types of attachments namely bar and clip, ball and O ring, locator, magnets, CEKA, etc., are available. Selection of the attachment design depends upon various factors including anatomy and morphology of jaw, desired retention, patient's maintenance and compliance.[7] Attachments not only help in improving the retention, stability of the overdenture but also affect the stress distribution to the surrounding bone.[8] Transfer of stresses to underlying implant-bone interface and surrounding alveolar bone determines the success and longevity of implant prosthesis.[9] Photoelastic analysis, strain gauge, finite element analysis (FEA) are the different methods to evaluate the stress–strain in the bone surrounding implants and related structures, FEA being very precise among all. Various studies were carried out in the past to evaluate and compare the stress distribution in implant supported overdenture and surrounding bone when using different attachment systems.[10–16] None of them compared three attachment systems namely ball-O ring, locator, Ceka attachment in terms of stress distribution. Hence, present study was carried out to evaluate and compare the stress distribution in mandibular implant supported overdenture using three different attachments – the Ball O ring, Locator, Ceka and which attachment is better in stress distribution to surrounding structures.

 Materials and Methods



A computed tomography scan of human mandible was processed to visualization module using a graphics program called Autodesk NETFABB ver. 8.1, a software developed to materialize medical images. The highly accurate 3D model in. STL format was subjected to a procedure called discretization by segment anatomy tool followed by refinement using Autodesk MAYA 3D software tool. A detailed, precise 3D solid model was obtained upon which finite element method could be optimized [Figure 1] and [Figure 2].{Figure 1}{Figure 2}

The implant used in the study was Noble Biocare replace implant of diameter 4.3 mm and length 11.5 mm. The implants were placed equidistant from midline at a distance of 10 mm. The attachments were measured and the inputs of those measurements were added in the Autodesk NETFABB ver 8.1 software to generate finite element models. In the present study, three different attachments were modeled leading to three different models: Model A Implant overdenture with Ball and O-ring attachment, Model B Implant overdenture with Locator Attachment, Model C Implant overdenture with Ceka attachment [Figure 3].{Figure 3}

Uniform 2 mm thickness of mucosa was modeled over the cortical bone on the simplified 3D model. It was not incorporated in the final anatomical 3D model as its modulus of elasticity was 1 Mpa which was several orders less than that of the surrounding structures. A mandibular complete denture was scanned and designed to fit the model of the implant and its superstructures using MAYA autodesk 3D software.

The mandibular models generated were exported to a finite element mesh and the models were refitted into the Solidworks version 2016 software to create two meshes – surface meshes using triangles and inner volumetric meshes using linear tetrahedral. The completed anatomical model consisted of a total number of 209,050 nodes and 136,493 elements. The details of individual nodes and elements are shown in [Table 1].{Table 1}

After the meshed geometric model was ready the material properties to the mandible were assigned. It was considered that bone, mucosa, implant with superstructure, acrylic resin are linearly elastic, homogenous and isotropic. Their elastic properties namely Young's modulus, Poisson's Ratio for bone, implant and attachments were determined as given in [Table 2] and incorporated in the model. Once these values were assigned, the model was a virtual replica of the real human mandible with implant overdenture. Symmetrical boundary conditions were imposed on whole of the mandible. The mandible was fixed at the base as indicated by all blue triangles.{Table 2}

The load applied to the abutment were as follows – 35N vertical load at 90° in occluso-gingival direction, 10N horizontal load at 0° in a medio-lateral direction and 70N oblique load at 120° to the occlusal plane in a labio-lingual direction, simulating the load from the muscles of mastication. The stress analysis was carried out using Solidworks version 2016 Software. After the loading was done the data obtained was analyzed using the Von Misses stress analysis chart.

 Results



The tracing of Von Mises stress field were represented in the form of color-coded bands with the help of Solidworks software. Red color represented the highest stress. Orange, yellow, light green, green, light blue, blue and dark blue colors represented the stresses in the descending order following red color. The stresses generated in silico n cap/ring of the attachment, abutment, implant–abutment interface, implant body, surrounding bone upon vertical, oblique and horizontal load of 35N, 70N, 10N, respectively were tabulated and graphically presented for interpretations [Table 3] and [Bar Diagram 1], [Bar Diagram 2], [Bar Diagram 3]. Stress distribution has been shown in FEA diagrams [Figure 4], [Figure 5], [Figure 6]. The amount of force required to uplift the dentures were presented in [Bar Diagram 4].{Table 3}[INLINE:1][INLINE:2][INLINE:3]{Figure 4}{Figure 5}{Figure 6}[INLINE:4]

The results were interpreted as follows:

Upon vertical load, locator attachment showed least amount of stress generation on silicone cap, abutment, interface of abutment and implant, implant body and the surrounding bone.Upon oblique load, locator attachment showed least amount of stress generation on silicone cap and abutment whereas Ceka attachment showed least stress generation on implant body and the surrounding bone.Upon horizontal load locator attachment showed least amount of stress generation on silicone cap, abutment and implant–abutment interface.Ball attachment showed maximum amount of stress generation over the abutment under oblique and horizontal loads.Ceka attachment showed maximum amount stress generation onto the surrounding bone under vertical and horizontal loads.

 Discussion



Various studies were carried out in the past comparing two or more attachment systems for implant supported ovrdentures.[10–16] Few of the studies compared ball O-ring attachment with bar-clip and/or locator attachment stating that ball O-ring attachment was better in terms of stress distribution.[10],[11],[15] Celik et al.[12] stated in his study that locator system showed greater stresses in relation to ball, bar-clipand bar-ball attachments. Study carried out by El-Anwar in 2015 showed that higher stress was found on ball attachment neck in comparison to locator. On the other hand, in the same study it was demonstrated that mucosa and surrounding bone showed less amount of stresses under ball attachment.[13] Abdelhamid in his study in 2014 showed that locator attachment reduces stresses on implant body and supporting structures in implant retained overdenture compared to ball attachment.[16] There was no uniformity in the results obtained by these studies in terms of which attachment is better for stress transfer. None of the above mentioned studies compared ball O-ring, locator and Ceka attachment systems with each other. Hence, the present study was carried out.

Selection of attachment will depend on various factors such as desired retention, jaw anatomy and morphology, available restorative space, patient's maintenance.[7] Ball and O-ring attachment is widely used due to qualities like ease of handling, reduced chair side time, low cost, possible application with tooth and implant supported overdentures.[13] It demonstrates less stress and bending movements than bar and clip attachment.[11] Common problems encountered with ball attachment are wearing of the O ring, difficult to use in cases with reduced inter-ridge distance, fracture of the neck region of the ball, difficult to use in case of non-parallel implants. Locator attachment has certain advantages such as self-alignment leading to ease of insertion and removal, dual retention, low vertical profile, durability and unique ability to pivot, thus increasing its resilience and tolerance for implant divergence.[17] Advantages of Ceka attachment are esthetics, reduced stress to the abutments, eliminates necessity for parallelism between abutment assemblies.[18]

Upon vertical load, minimum stresses were seen in locator attachment followed by ball attachment followed by Ceka attachment. This could be because of the resilient action of the locator attachment which helps in dissipation of stresses. Ball and locator attachment demonstrated less stresses compared to Ceka attachment in surrounding bone. Upon oblique loading, maximum stresses were found in the neck region of the ball abutment. Least stresses were observed in the locator abutment. This could be because of smaller height of the locator attachment. Minimum vertical restorative space required for locator, ball and O-ring, Ceka attachment is 8.5 mm, 10–12 mm, 12 mm, respectively.[19],[20] Crown height space (CHS) is the distance from the alveolar bone crest to the occlusal plane. More the CHS more is the lever arm effect. Due to non-axial loading lateral movements take place which proportionally increase with the CHS. This leads to stress concentration in the cervical area.[21] Since locator attachment has better pivoting action than ball and Ceka attachment, upon horizontal loading less stresses are seen in model B. FEA is a numerical and quantitative method for analyzing stress in complex structures. It evaluates the amount and pattern of stress distribution in dental structures very precisely.[22]

Limitations of the study were as follows: (1) Masticatory forces are dynamic in nature, static forces were applied in the study. (2) Bone is heterogeneous, viscoelastic, anisotropic structure. Due to difficulty in deciding the anisotropic movement, all the structures were considered isotropic, linear elastic, homogenous. (3) 100% osseo integration was assumed under delayed loading. This may vary in clinical situation. 4. A computer designed model does not mimic the clinical situation.

 Conclusion



Within the limitations of the study, it was concluded that locator attachment was better in terms of stress generation in comparison to Ball and Ceka attachment. It was the most retentive attachment in comparison to the other two attachments. Further studies and clinical co-relation needs to carried out in order to judge clinical performance of the three attachments.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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