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Design and Development of Indigenous Dental Implant System: From Research to Reality.

Introduction

Dental implant is an artificial substitute placed inside the jawbone to provide support to an artificial prosthetics for replacing missing teeth. The current status of the dental implant success is the result of the continuous and sustained effort of researchers since middle of the last century.

In India approximately 1.5 to 2 lacks implant are placed every year and market size is still growing exponentially but still there is no Indian FDA certified manufacturer of dental implant system and as all implants are imported from outside so dental implants are costly and still not affordable to the masses of India. In the present research work a new dental implant system was conceptualized, designed and developed as per the need of Indian population scenario at IIT Delhi in collaboration with clinical partner Institute Maulana Azad Institute of Dental Sciences, New Delhi and funding agency CSIR. The main objective of research in dental implantology is to design a dental implant system which can induce the stronger and functional bone implant interference at rapid rate. The initial success of endosseous dental implant depends on primary and secondary stability after surgical placement and long term success depends on the load transfer ability of the system. Design of this newer system was patented as per application no US2011/ 0117522A1. This patented design includes buttress macro thread on implant body and micro thread thread in cervical portion. This combination provides improved biomechanics resulting decrease stresses at crestal bone and implant interface. Table 1 provides the regulatory classification of dental implant system.

The present research work includes a systematic research approach as per FDA guideline document to develop a new dental implant system.

Raw material testing

Tensile testing of raw material was done as per ASTM Standard E8/ E8M-16a. Table 2 shows the material for different components of dental implant system and their tensile strength. Other standard raw material testing wer done as per ISO/TR 10451.

FEA analysis

An initial FE analysis was conducted to validate the Implant abutment connection and to observe the stress pattern during insertion torque application and to prevent the cervical blowout fracture of Ti-6Al-4V alloy in the thin region about the internal implant abutment connection. On analysis it was found that failure stresses were seen to be concentrating in the line angles of the internal hexagon; therefore a slight fillet was incorporated.

FEA comparison was made between the different thread designs under compression, tension as shown in Table 3 and it was deduced that: Buttress thread design was least detrimental in terms of the Von Mises stresses generated at the implant abutment interface. However Reverse Buttress showed the least amount of stresses at the bone implant interface at both the crestal and cortico-cancellous interface level. However maximum number of failures takes place at the implant abutment interface, therefore to reduce the stresses at this interface, Buttress design was selected. Moreover there is a better self-tapping nature of Buttress design, which further reinforces the selection. Incorporation of microthreads into the collar of the implant with buttress design led to a dramatic reduction of stress in the bone at both the crestall & cancellous level. Further the comparison of Buttress with micro thread design against all other thread design (Classical V and Reverse Buttress) showed lower Von Mises stresses at all locations with more even & wider distribution under compression, tension and moment forces. Largest dimension of implant showed slightly higher stresses at implant abutment interface than smallest dimension, however the stresses in largest implant is lower at the crestal level and corticocancellous interface. Fig. 1 shows the stress at implant abutment interface, crestal bone interface and cortico-cancellous interface in different thread designs.

Production of Dental Implant Components

Dental implants (fig 2 A) were produced by machining of titanium rod or bar on a multi axis control turn-mill using a combination of several cutting tools programmed accordingly and 100% inspection was done with Gauges (shown in fig 2 B and C) and vision microscope for critical dimensions as per ISO 129-1 and ISO 14405-2.

Steps and procedure for the optimization of surface modification for Dental Implants

Cleaning

After machining Dental Implants were first cleaned in mild detergent to remove oil and grease from the machined parts and then cleaned in deionized water and dried in vacuum oven.

Sandblasting by alumina powder

In next step, Implant surface were blasted in sandblaster cabinet at different processing parameters so that (a) To obtainn Ra value in the range of 1.5 to 2 microns, (b) To produce a micro topography (pits) in the range of 2-10 microns. (c) Profile of micro thread and macro thread should not be changed, (d) Uniform average roughness on all surfaces can be acheived.

Following processing parameters of sandblasting were studied and their Ra values are shown in table 4. Grit size a) Small Grit b) Large Grit; Blasting pressure a) 4 bar b) 5 bar; Blasting time a) 10 sec b) 20 sec.

In vision microscope was used to check the profile of micro and macro threads of dental implant at various process parameters so that best parameter can be choosen at which there not much change in profiles of threads along with optimum Ra value otherwise they would lose their function. Fig 3 shows profile image of machined as well as different parameters blasted dental implants. As it can be seen from these figures that by increasing grit size and blasting pressure there loss of micro thread geometry. Blasting at 4.0 bar pressure for a duration of 20 second with large grit particles doesn't much change the profile of microthread as shown in fig.
EDAX--Percentage composition

Element    Weight %   Weight % [sigma]   Atomic %

Caiboa      5.124          0.740          15.093
Oxygen      8.619          2.989          19.065
Aluminum    4.002          0.220          5.249
Titanium    78.075         2.651          57.685
Vanadium    4.181          0.459          2.904


Acid etching

Main objective of acid etching is to produce a sub micron to nano topography while preserving the Roughness value obtained from sand blasting. Acid etching also removes any remaining embedded alumina particles and contaminated oxide layer formed during machining procedd from surface of titanium dental implant and also makes the surface bioactive. Two steps acid etching procedure was employed for obtaining desired topography. In first step oxide layer was removed with hydrofluoric acid and in second step conc. acids (Hydrochloric and sulfuric acids) were used at different temperatures and for different time durations to produce the porous microtopography. In fig 4 topography of dental implant surface etched with different acids are shown. In fig 4A & B, dissolution of oxide layer by HF is seen along with prefrential dissolution of alpha phase of alloy exposing beta crystals. Subsequent etching by HCl as shown in fig 4C & D leads to formation of cracks at grain boundaries but still not able to produce porous topography and remove beta crystals from surafce as exposed beta crystals are unstable. Etching in Sulfuric acid produces removal of beta crystals from surface but this acid alone is not capable to produce porous surface morphology as shown in fig 4E & F. Fig 4G & H shows Highly Porous topogyaphy with dual size distribution and no visible beta phase on the surface.
                   MEAN DISTANCE    PERCENTAGE
SNO    SPECMEN/   FROM CRESTS IN       BONE
        WEEKS        RECTANGLE      FORMATION

1      12 weeks   415.62 [micro]m     80.5%
2      6 weeks    424.63 [micro]m     25.6%


The effect of temperature of acids was studied and from fig 5 it can be seen that best porous morphology can be achieved at 80pC temperature of acids.

Duration of acid exposure was also studied and it was concluded that at room temperature acid etching there is no much poros topography even after 8 min of exposure but at higher temperature best porosity can be acheives at 3-5 min duration of exposure and then after that there is coalesing of pores into larger pores as the duration of etching is incresed as shown in fig. 6 & 7.

Characterization of surface of Finally surface modified and cleaned dental implant surface

Topography evaluation by Atomic Force Microscopy (AFM)

In Fig. 8 topography of machined, blasted and blasted plus acid etched samples under AFM is shown. The formation of pits and craters of size around 10-20 g is observed in both sand blasted as well as sand blasted plus acid etched samples. Within these pits and crater there is formation of sub-micron topography by selective acid etching proces s.

Roughness (Ra) value

3D Optical profilometer was used for determination of roughness value of dental implant at various steps processing conditions i.e. machined, sand blasted and sand blasted plus acid etched. Ra value of machined surface is below 1g, sand blasted surface is 1.7-1.9[micro] and sand blasted and acid etched one is 1.5-1.7[micro] Fig. 9 shows AFM profile image of machined, blasted and blasted plus acid etched dental implants

After acid etching there is slight decrease in Ra value as peaks obtained after sand blasting are reduced due to acid etching process and simultaneously there are numerous small peaks also visible due to generation of submicron topography by acid etching.

Mechanical Testing

Fatigue testing (as per ISO 14801:2007)

The testing machine used in this fatigue (shown in fig 1.) System survived five million cycles at the fatigue load limits of 225 NCm thus qualifying the ISO 14801 standard of testing the reliability of a dental implant. Cause of failure of the implant system at higher load levels was identified as the hex region in the abutment component which displayed maximum stress under the loading conditions however the implant survived at all the load levels. Load values for mastication of routine food products are reported to be in range of 70-150 N therefore this implant system can be potentially successful for replacement of teeth.

Torsion Test as per ISO 13498

The torsional yield strength and maximum torque of the implant body and connecting part interface are determined by clamping the implant body and connecting in a testing device. The abutment was fixed on the implant body using the abutment screw at tightening torques of 30 NCm. The ends of the implant body and abutment were fixed the rigid clamping device as shown in fig. The implant external geometry was modified so that they should be easy to clamp with the holders. The implant and abutment were connected by abutment screw at a tightening torques of 30 NCm. The retaining portions of the implant and abutment body were clamped into the specimen holders of the testing device. Torque was applied continuously to the connecting part at a rate of 6 degree rotation per minute and record of torque versus rotation angle curve was obtained at the connected monitor interface. Torsional yield strength and the maximum torque was recorded on six test specimens and averaged. The torsional yield strength was determined by the 2[degrees] offset method using the torque versus rotation angle curve produced. On the torque versus rotation angle curve, a pointed was located corresponding to 2[degrees] offset of the rotation angle and a line was drawn parallel to tangent line. Then point of intersection was located with the torque versus rotation angle curve. The maximum torque was determined by the largest value of torque on the torque versus rotation angle curve.

In vivo study

60 Implant were placed in White New Zealand rabbit's femur and evaluated after 6 and 12 weeks. After nacropsy histological, micro CT and histomorphometric evaluation was done which revealed no adverse effect and an extremely favorable bone implant contact. These studies were done at Toxicology Lab and Animal Centre, Shriram Institue of Industrial Research, New Delhi

Clinical Trial

36 patients (26 cases and 10 controls) were involved in initial clinical trials and 92% success rate of case and 90% success rate of control was observed. There was no statistical difference between intial implant stability and insertion torque and ISQ value after 3 months.

References

[1.] Osteointegration: Associated Branemark Ossointegration Centers 2010. Available from: http://www.branemark.com/Osseointegration.html." [Online].

[2.] D. Morton, R. Jaffin, and H.-P. Weber, "Immediate restoration and loading of dental implants: clinical considerations and protocols.," Int. J. Oral Maxillofac. Implants, vol. 19 Suppl, pp. 103-8, Jan. 2004.

[3.] Schwartz Z, Swain LD, Marshall T, Sela J, Gross U, Amir D. Calcif Tissue Int 1992; 51: 429-37." [Online].

[4.] L. F. Cooper, "Biologic determinants of bone formation for osseointegration: Clues for future clinical improvements," J. Prosthet. Dent., vol. 80, no. 4, pp. 439M49, Oct. 1998.

[5.] D. Buser, R. K. Schenk, S. Steinemann, J. P. Fiorellini, C. H. Fox, and H. Stich, "Influence of surface characteristics on bone integration of titanium implants. A histomorphometric study in miniature pigs.," J. Biomed. Mater. Res., vol. 25, no. 7, pp. 889-902, Jul. 1991.

[6.] S. Hansson and M. Norton, "The relation between surface roughness and interfacial shear strength for bone-anchored implants. A mathematical model.," J. Biomech., vol. 32, no. 8, pp. 829-36, Aug. 1999.

[7.] J. E. Davies, "Understanding peri-implant endosseous healing.," J. Dent. Educ., vol. 67, no. 8, pp. 932-49, Aug. 2003.

[8.] L. F. Cooper, "Biologic determinants of bone formation for osseointegration: clues for future clinical improvements.," J. Prosthet. Dent., vol. 80, no. 4, pp. 439-49, Oct. 1998.

[9.] D. A. Puleo and A. Nanci, "Understanding and controlling the bone implant interface.," Biomaterials, vol. 20, no. 23-24, pp. 2311-21, Dec. 1999.

[10.] Z. Schwartz, C. H. Lohmann, J. Oefinger, L. F. Bonewald, D. D. Dean, and B. D. Boyan, "Implant surface characteristics modulate differentiation behavior of cells in the osteoblastic lineage.," Adv. Dent. Res., vol. 13, pp. 38-48, Jun. 1999.

[11.] T. Albrektsson and A. Wennerberg, "Oral implant surfaces: Part 1-review focusing on topographic and chemical properties of different surfaces and in vivo responses to them.," Int. J. Prosthodont., vol. 17, no. 5, pp. 536-43, Jan.

[12.] A. J. Garcia and C. D. Reyes, "Bio-adhesive surfaces to promote osteoblast differentiation and bone formation.," J. Dent. Res., vol. 84, no. 5, pp. 407-13, May 2005.

[13.] T. J. Webster and J. U. Ejiofor, "Increased osteoblast adhesion on nanophase metals: Ti, Ti6Al4V, and CoCrMo.," Biomaterials, vol. 25, no. 19, pp. 4731-9, Aug. 2004.

Pankaj Chauhan (1,2), Veena Koul (2), Naresh Bhatnagar (1)

Mechanical Engineering Department, Centre for Biomedical Engineeringjndian Institute of Technology Delhi, India

* Coresponding author.

E-mail address: dr.prachichauhan@gmail.com (Pankaj Chauhan)

Caption: Figure 1: Comparing Von mises stresses in different thread types

Caption: Figure 2: A) Machined Dental Implant; B) Go and No Go Gauges (Internal Thread); D) Go and No Go Gauge (External Thread)

Caption: Figure 3: Profile images of Dental Implants under Vision Microscope A) machined B) 5 bar 10 sec large grit C) 4 bar 20 sec large grit D) 5 bar 20 sec small grit

Caption: Figure 4: SEM Images at different magnifications of Sandblasted dental implant A) acid etched with HF for 1 min at room temperature at 2000x and 10000x B) acid etched with HF+HCl for 3 min at 80[degrees]C at 2000x and 10000x C) acid etched with HF+H2SO4 for 3 min at 80[degrees]C at 2000x and 10000x D) acid etched with HF+ HCl+H2SO4 for 3 min at 80[degrees]C at 2000x and 10000x

Caption: Figure 5: SEM Images of Sandblasted dental implant acid etched with HF + HCl+[H.sub.2]S[O.sub.4] for 3 min at 2000x 5000x and 10000x at A) 40[degrees]C B) 60[degrees]C at C) 80[degrees]C and D) 100[degrees]C

Caption: Figure 6: SEM Images at different magnifications of Sandblasted dental implant acid etched for A)1 min. at room temperature at 5000x, 10000x and 20000x B) 8 min. at room temperature at 5000x, 10000x and 20000x

Caption: Figure 7: SEM Images at different magnifications of Sandblasted dental implant acid etched with HF+ HCl+H2SO4 for A) 1 min at 80[degrees]C at 2000x and 5000x B) 3 min at 80[degrees]C at 2000x and 10000x C) 5 min at 80[degrees]C at 2000x and 10000x

Caption: Figure 8: Topography Images by AFM (A. Machined, B. Sandblasted, and C. Sand blasted and acid etched)

Caption: Figure 9: Ra value measurement (A. Machined, B. Sand blasted, C. Sand blasted and acid etched)

Caption: Figure 10: XRD Spectrum and Raman Spectra

Caption: Figure 11: EDAX Spectra

Caption: Figure 12: XPS Spectra

Caption: Figure 13: Bending in abutment (a) as predicted by simulation (b) as observed from experiment

Caption: Figure 14: Load Vs. No of Cycle Diagram for Dental Implant

Caption: Figure 15: Torque vs. Rotational angle curve

Caption: Figure 16: L929 cells after contact with 100% extract of NMI-A

Caption: Figure 17: Cell adhesion study on machined and acid etched surface of dental implant

Caption: Figure 18: Clinical Trials Images
Table 1: Risk class and the applicable classification rule according
to Principles medical Devices classification as per GHTF guidelines

                                                      GHTF
Device Category     US FDA     US FDA    Risk      MDD Risk    Rule
Name              Regulation    Code    Class    GHTF   MDD   Number
                    Number              US FDA

Implant,           872.3640     DZE       2       C     2B    Rule 8
  Endosseous
Root Form
Abutment           872.3630     NHA       2       C     2B    Rule 8
  implant
Accessories,       872.3980     NDP       1       A      1    Rule 5
  implant

Table 2: Material used in fabrication of different components
of dental implant system and their respective tensile strength

                                 Tensile
Component          Material      Strength
                                  (MPa)

Denial Implant   ASTM Grade 23     10S9
Cover Screw      ASTM Grade 5      1052
Abutment Screw   ASTM Grade S      1052
Abutment         ASTM Grade 5      1052

Table 3: Comparison of different thread types under compression,
Tension & moment

S.No.       Parameter &       Classical V   Reverse
             Location                       Buttress

I)          COMPRESSION
A)       Implant Abutment        41.4        36.51
             Interface
B)      Crestal bone level       18.34       11.94
C)      Cortico-cancellous       30.44       24.39
             Interface
II)           TENSION
A)       Implant Abutment        40.47       37.57
             Interface
B)      Crestal bone level       18.34       44.3 S
C)      Cortico-cancellous       30.99       63.34
             Interface
III)          MOMENT
A)       Implant Abutment       425.34       421.62
             interface
B)      Crestal bone level      227.03       120.68
C)      Cortico-cancellous       9.59         9.18
             interface

S.No.       Parameter &       Buttress     Buttress &
             Location                     Micro threads

I)          COMPRESSION
A)       Implant Abutment       30.94         38.49
             Interface
B)      Crestal bone level      27.15         18.48
C)      Cortico-cancellous      85.33         18.85
             Interface
II)           TENSION
A)       Implant Abutment       33.95         38.44
             Interface
B)      Crestal bone level      27.15         18.48
C)      Cortico-cancellous      85.33         22.29
             Interface
III)          MOMENT
A)       Implant Abutment      278.94        396.093
             interface
B)      Crestal bone level     139.97         70.27
C)      Cortico-cancellous      30.39         8.57
             interface

Table 4: Ra value of Dental Implant Surface under different
processing parameters

           Machined            Small grit

Samples       --      4 bar    5 Bar    4 bar    5 bar
                      10 sec   10 sec   20 sec   20 sec

Ra value     0.4       1.3      1.8      1.4      1.9

                    Large grit

Samples    4 bar    5 bar     4bar     5bar
           10 sec   10 sec   20 sec   20 sec

Ra value    1.6      2.2      1.8      2.3

Table 7: List of biocompatibility studies done on
experimental dental implant as per FDA regulation

S.No   Test                                Sample Size   Standard
                                                         followed

1      Cytotoxicity                        6 implants    ISO 10993
2      Sensitization--by Guinea Pig        30 implants   ISO 10993
       Maximization Method
3      Irritation--Animal intiacutaneous   56 implants   ISO 10993
       Reactivity Test
4      Acute Systemic Toxicity             40 implants   ISO 10993
3      Implantation Study                                ISO/TS 22911
6      Genotoxicity                                      ISO 10993
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Title Annotation:Original Article
Author:Chauhan, Pankaj; Koul, Veena; Bhatnagar, Naresh
Publication:Trends in Biomaterials and Artificial Organs
Date:Jan 1, 2018
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