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 Table of Contents  
RESEARCH
Year : 2022  |  Volume : 22  |  Issue : 4  |  Page : 389-397

Evaluation of physical changes due to simulated loading on prosthetic screw supporting 4- and 6-unit implant prosthesis: An in vitro study


1 Department of Prosthodontics and Crown and Bridge, Santosh Deemed to be University, Ghaziabad, Uttar Pradesh, India
2 Department of Prosthodontics and Crown and Bridge, ITS Dental College and Hospital, Greater Noida, Uttar Pradesh, India

Date of Submission29-Jan-2022
Date of Decision15-Jul-2022
Date of Acceptance03-Aug-2022
Date of Web Publication03-Oct-2022

Correspondence Address:
Mansi Singh
Department of Prosthodontics and Crown and Bridge, Santosh Deemed to be University, Delhi-NCR, Ghaziabad, Uttar Pradesh
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jips.jips_48_22

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  Abstract 


Aim: Screw loosening is a very common cause of failures in implant prosthodontics. In order to avoid screw fracture, it is imperative to understand the mechanical behavior of the screw and the dynamics it is subjected to intraorally. The present study was conducted to qualitatively evaluate and compare the morphological changes, surface defects, and cracks observed under a scanning electron microscope (SEM) in the prosthetic screw.
Settings and Design: Two Stainless steel edentulous mandible models were fabricated on the basis of all on four and all on six concepts by using CAD design. Screw retained prosthesis were fabricated for both the models and total number of 80 prosthetic screws were made up of Ti6Al4V.
Materials and Methods: Eighty prosthetic screws (N = 80) used in four- and six-unit implant-retained cast hybrid denture were subjected to cyclic loading of 1.5 million cycles and 3 million cycles, simulating a 5 and 10 years of usage, respectively. Once the simulated cycles had been completed in all subgroups, each prosthetic screw was inspected under SEM (×150–×1000) for any changes.
Statistical Analysis: The data thus obtained were statistically analyzed using SPSS 12.0 software and P < 0.005 was considered statistically significant.
Results: The study revealed statistically significant (P < 0.005) changes (like morphological changes, surface defects, crack initiation, and propagation) in the prosthetic screws after exposing them to predefined test conditions (P < 0.001).
Conclusion: It can be concluded that the prosthetic screws need to be changed after a period of clinical use of 5 years irrespective of the number of implants used for rehabilitation. Further, the tilt of the abutment and numbers of implants also contribute to the stresses on the implant-supported prostheses.

Keywords: All-on-4™, axial implant, implant-retained hybrid prosthesis implant-supported prostheses, scanning electron microscope, screw-retained hybrid denture, tilted implant


How to cite this article:
Singh M, Bhargava A, Nagpal A, Chaudhary A. Evaluation of physical changes due to simulated loading on prosthetic screw supporting 4- and 6-unit implant prosthesis: An in vitro study. J Indian Prosthodont Soc 2022;22:389-97

How to cite this URL:
Singh M, Bhargava A, Nagpal A, Chaudhary A. Evaluation of physical changes due to simulated loading on prosthetic screw supporting 4- and 6-unit implant prosthesis: An in vitro study. J Indian Prosthodont Soc [serial online] 2022 [cited 2022 Dec 7];22:389-97. Available from: https://www.j-ips.org/text.asp?2022/22/4/389/357799




  Introduction Top


Dental implantology has undergone a revolutionary rebirth and rediscovery; therefore, implants are considered as the principal choice of treatment in selected cases.

After osseointegration is achieved around the implant, long-term clinical follow-ups have reported mechanical or biological complications.[1] One of the systematic reviews showed the survival rate of implant-supported single crowns and concluded that the overall incidence of abutment screw loosening was 7.3% in both external and internal type connections from 26 clinical studies included, while the incidence of abutment screw fracture was found to be 0.6%. Screw loosening may cause implant or screw fracture. In totaling, screw loosening or deformation also leads to micromotion at the implant–abutment interface when chewing.[2],[3] Sones[4] reported that the failure of implant components principally, if abutment screws cannot be retrieved, might necessitate the disuse of the involved implant and require conversion or remake of the prosthesis.[5]

The mechanism of screw loosening has been described in two stages. Initially, external forces cause sliding between the thread, partially relieving the stretching of the screw and reducing preload. The second causes turning of the screw in an counterclockwise direction, which leads to loss of function. These nonperformances are due to metal fatigue and occur under repeated cyclic loading at levels below than the maximum strength of material.[6],[7]

Many factors related to screw design and fabrication method may affect abutment or prosthetic screw loosening in metal-to-metal screw system; these primarily are related to preload. It was reported that the primary factor in screw loosening was not consistent; the following preload showed a difference and could affect the removal torque.[8],[9]

The most common variables that influence the joint stability are the junction of implant–abutment where the contacting parts change when the screw is tightened. Being tightened together by the screw, the microroughness of all the metal contacting surfaces slightly flattens, and the microscopic distance between contacting surfaces decreases. As an outcome of this process called “settling,” the screw loses part of its preload. Detorque value instantly after tightening is always lesser than the initial tightening torque.[10]

In additional, factors that affect abutment screw loosening include hex (internal hex system), height (or depth), platform diameter, surface condition, diameter of the screw, excessive bending, vibrating micromovement, microleakage, abutment diameter, surface coating, abutment connection, cement wash out, lateral cyclic loading, collar length, abutment angulations, inadequate tightening torque, retorque, reverse torque, and settling effect.[11],[12]

On evaluation of newly placed abutment and screw assembly as observed by Hum. There is a percentage of initial torque loss, which is higher as compared to screws that have already undergone an application of initial torque. This torque loss is inherent in any bolted joints; it is a combined effect of bolts and is about 10% during the first 24 h after installation. This could be due to gasket creep, vibration in the system, thermal expansion, and elastic interaction during bolt tightening. Hence, previously tightened screws were observed to be unstable after the application of successive torque; if the abutment screw is exposed to excessive wear and is still in place, screw replacement is a good option. Hum had also introduced a technique to accurately locate the loose abutment screw and replace it with a new one.


  Materials and Methods Top


This study qualitatively evaluated and compared the physical changes in the prosthetic screw after 1.5 million and 3 million cycles that simulate 5 and 10 years of clinical usage.

Two stainless steel edentulous mandible models were fabricated on the basis of all-on-four and all-on-six concepts using computer-aided design [Figure 1] and [Figure 2]. Implant analogs were positioned and transmucosal abutments (ADIN implant company, Israel) to depict the four- and six-unit implant-supported hybrid prosthesis. Screw-retained prostheses were fabricated for both the models [Figure 3] and [Figure 4]. A total number of 80 prosthetic screws were made up of Ti6Al4V. For the present study, there were two Group A and Group B, which were further divided into A1 and A2 and B1 and B2, based on the number of loading cycles. For each subgroup, different angulations were compared. For Group A1 and A2, there were two angulations. For Group B1 and B2, there were three angulations [Table 1].
Figure 1: CAD design (ISP on four implants). CAD: Computer-aided design, ISP: Implant-supported prostheses

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Figure 2: CAD design (ISP on six implants). CAD: Computer-aided design, ISP: Implant-supported prostheses

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Figure 3: Stainless steel model of ISP on 4 implants with hybrid prosthesis. ISP: Implant-supported prostheses

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Figure 4: Stainless steel model of ISP on 6 implants with hybrid prosthesis. ISP: Implant-supported prostheses

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Table 1: Distribution of samples

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Mandibular models were subjected to 1.5 million and 3 million cycles under a customized jaw simulator-cyclic loading machine [Figure 5] and [Figure 6]. In this machine, only vertical movement takes place and 100 N force is applied by maxillary dentulous model. Following these, all prosthetic screws were removed and replaced with a new set of screws and this process was repeated again. Once the stipulated cycles had been completed, each prosthetic screw was cleaned using acetone in ultrasonic cleaner for 10 min and then each prosthetic screw was inspected under a scanning electron microscope (SEM) from ×150 to ×1000 magnification [Model-JSM-6510 LVby JEOL USA, as shown in [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11], [Figure 12], [Figure 13], [Figure 14] to evaluate the physical changes.
Figure 5: Customized cyclic loading machine

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Figure 6: Prosthesis under cyclic loading

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Figure 7: Before tightening the prosthetic screw (×150)

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Figure 8: Before tightening the prosthetic screw (×300)

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Figure 9: Physical changes in prosthetic screw – ISP on 4 implants after 5 years of usage (1.5 million Cycles) (×300). ISP: Implant-supported prostheses

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Figure 10: Physical changes in prosthetic screw – ISP on 4 implants after 10 years of usage (3 million Cycles) (×300). ISP: Implant-supported prostheses

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Figure 11: Physical changes in prosthetic screw – ISP on 6 implants after 10 years of usage (3 million cycles) (×300). ISP: Implant-supported prostheses

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Figure 12: Physical changes in prosthetic screw – ISP on 6 implants after 5 years of usage (1.5 million Cycles) (×300). ISP: Implant-supported prostheses

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Figure 13: Physical changes in prosthetic screw - ISP on 6 implants after 5 years after 5 years of usage (1.5 million cycles) (×1000). ISP: Implant-supported prostheses

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Figure 14: Physical changes in prosthetic screw – ISP on 4 implants after 5 years after 5 years of usage (1.5 million cycles) (×170). ISP: Implant-supported prostheses

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Null hypothesis

Clinical usage of 5 and 10 years, respectively, causes no physical changes in the physical screw, retaining a four- and six-unit implant-supported prostheses (ISP).


  Results Top


Models simulating ISP on 4 implants (Group A, N = 32) were further subdivided into two subgroups, i.e., A1, N = 16 and A2, N = 16 based on the number of cycles, i.e., 1.5 and 3 million, respectively [Table 1].

On subjecting the samples in Subgroup A1 (n = 16) [Table 2] to 1.5 million cycles, thirteen (81%) out of sixteen prosthetic screws showed physical changes (7 prosthetic screws were placed over 30° angulated implants and 4 prosthetic screws were placed over straight implants). One of the prosthetic screws (6.25%) showed loosening which was placed over an implant at angle of 30° as viewed under SEM (×150–×300).
Table 2: Number of prosthetic screws that underwent physical changes in implant-supported prostheses on 4 implants (Group A, n=32) after 1.5 million cycles (Subgroup A1, n=16) and 3 million cycles (Subgroup A2, n=16): At different angulation

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On subjecting the samples in Subgroup A2 (n = 16) [Table 2] to 3 million cycles, all sixteen prosthetic screws (100%) showed physical changes (08 prosthetic screws were placed over 30° angulated implants and 08 prosthetic screws were placed over straight implants) and 4 prosthetic screws (25%) showed screw loosening (placed over 30° angulated implants) and 2 screws (12.5%) fractured (placed over 30° angulated implants) as viewed under SEM (×150–×1000).

On subjecting the samples in Subgroup B1 (n = 24) [Table 3] to 1.5 million cycles, eleven out of twenty-four prosthetic screws (45.83%) showed physical changes (06 prosthetic screws were placed over 30° angulated implants and 02 prosthetic screws were placed over angulated implants at 17°) as viewed under SEM (×150–×1000).
Table 3: Number of prosthetic screws that underwent physical changes in implant-supported prostheses on 6 Implants (Group B, n=48) after 1.5 million cycles (Subgroup B1, n=24) and 3 million cycles (Subgroup B2, n=24): At different angulation

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On subjecting the samples in Subgroup B2 (n = 24) [Table 3] to 3 million cycles nineteen out of twenty-four screws (79.83%) showed physical changes (07 prosthetic screws were placed over 30° angulated implants and 03 prosthetic screws were placed over implants angulated at 17°) as viewed under SEM (×150–×1000). There was no loosening and fracture observed in ISP on six implants.

The data thus obtained were statistically analyzed using SPSS 12 software details-SPPS Inc., Chicago, IL, USA and P < 0.005 was considered statistically significant.

On comparing the statistical data of physical changes between ISP with four implants (Group A) which showed 81.25% changes to that of ISP with six implants (Group B) which showed 45.83% changes, there was a difference of 35.42% in physical changes which was statistically significant (P = 0.003). Therefore, from a mechanical point of view, an increase the number of implants from four to six when subjected to similar loading cycles (1.5 million), there were 35.42% ± 5.25% lesser physical changes in ISP with six implants as compared to ISP with four implants, also there were no loosening of screws or fractures observed.

On comparing the statistical data between physical changes in ISP with four implants at 1.5 million cycles (Subgroup A1) that shows 81.25% changes to ISP with four implants at 3 million cycles (Subgroup A2) that shows 100% changes, there was a difference of 18.75% in physical changes in the prosthetic screws which though was not statistically significant (P = 0.042) but, from a mechanical point of view, an increase in the number of cycles from 1.5 million to 3 million there were 18.75% ± 5% more physical changes observed in the prosthetic screws that might have led to screw loosening or screw fracture as observed in one of the screws.

On comparing the statistical data between physical changes of ISP with four implants (Group A) that showed 100% changed to ISP with Six implants (Group B) that showed 79% changes, there were 21% differences observed in prosthetic screws which were not statistically significant (P = 0.024). As far as the mechanical aspect is concerned, an increase in the number of implants from four to six, and subjecting them to similar loading cycles, there were 21% ± 5.25% less physical changes in ISP with six implants.

On comparing the statistical data between physical changes of ISP with six implants at 1.5 million cycles (Subgroup B1) that showed 45.83% changes to ISP with six implants at 3 million cycles (Subgroup B2) that showed 79.83%, there were 33.33% differences observed in prosthetic screws which was again not statistically significant (P = 0.0359) but from a mechanical stand point there were more significant changes, it can therefore be inferred that the increased cyclic loading from 1.5 million to 3 million leads to 33.33% ± 5% more plastic deformation in the prosthetic screws [Table 4] and [Table 5].
Table 4: No. of prosthetic screws that underwent physical changes in ISP on 4 implants (Group A, N=32) after 1.5 million cycles (Subgroup A1, N=16) and 3 million cycles (Subgroup A2, N=16): At different loading cycles

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Table 5: No. of prosthetic screws that underwent physical changes in ISP on 6 implants (Group B, N=48) after 1.5 million cycles (Subgroup B1, N=24) and 3 million cycles (Subgroup B2, N=24): At different loading cycles

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  Discussion Top


Several in vitro studies have investigated the loosening of prosthetic screws.[7],[13],[14],[15],[16],[17],[18],[19] The present study analyzes the prosthetic retaining screws submitted to dynamic cyclic loading after 5 and 10 years of clinical use, which made it possible to appreciate the physical changes of prosthetic screws that cause a significant effect on loosening of prosthetic screws.

In some of the causes of screw loosening, the most important causes are low preload due to inappropriate torque, ill-fitting screw and implant, vertical discrepancy on the abutment–implant, cyclic load on all components of the prosthesis, and excessive occlusal force.[19],[20] The exact amount of torque on the screw is important for the ideal preload of the implant joint, which is the prosthetic abutment. The loosening or fracture of prosthetic screws is related to the difference of that of the implant–prosthetic abutment and the presence of a space between the implant connection and the prosthetic abutment, which may cause unfavorable stresses on the connecting components, implant, and bone.[21]

When a prosthetic screw is tightened, the screw becomes flattened [Figure 10] and [Figure 11] and a friction force or coefficient is generated around the screw which prevents loosening. Loosening of the screw is also related to the density of the bone that accepts the implant. This may account for the fact that the stability of the implant increases as the bone density increases. Consequently, prosthetic screw loosening is more frequent in case of implants in maxillary arch which is less dense than the mandible.[20],[22],[23]

There are few studies[7],[8],[9],[18],[24],[25] to explain the causes of screw loosening, but none of them are conclusive. Almost all agree that loosening will not happen until the friction force between the threads is reduced by some external mechanism. In the present study, friction between the thread of prosthetic screw and multiunit abutment was affected by physical changes that push the coping against the abutment overtime, the screw threads of both abutment and prosthetic screw show surface deformation and reduce the normal friction force, and consequently, the cyclic load may rotate/loosen/fracture the prosthetic screw.

Theoretical analysis suggested that there is a linear relationship between the preload and torque applied. This relationship was reported by Bickford.[18]



(Fp = preload, T = torque, P = Screw pitch, μ = friction coefficient between prosthetic screw and abutment, r = minor radius of screw, β = thread half angle)

According to the above relation, preload depends on three factors: applied torque onto the prosthetic screw, screw geometry, and friction coefficient between prosthetic screw and abutment. If possible, the torque applied should impart the maximum preload that will not show any damage to the screw surface. The torque recommended by the manufacturer depends on the material of the screw, the shape of the screw, the type of thread, the material of the prosthetic component, and the surface finishing of the thread. In the present study, the recommended torque values of 20 N-cm for abutment screw and 15 N-cm for prosthesis screw were used.[26],[21]

Various authors suggest applying a torque larger than the value recommended by the manufacturer as a means to avoid loosening. This practice is not advisable because as already discussed that preload should be in maximum limit to 80% of the tensile yield strength of the material in order to avoid screw strain and fracture during loading.[24]

The function of the friction coefficient is also somewhat conflicting: on one side, a low friction coefficient generates a higher preload for a given tightening torque; on the other hand, a low friction coefficient result in lower frictional forces opposing the opening torque. When the screws evaluated in this study were visually inspected, all of them appeared to be in good condition. The results of this study showed that, after 5 years of simulated clinical usage, 70% of prosthetic screws of ISP with four and six implants exhibited physical changes. Considering the plastic deformations observed in the screws [Figure 2], [Figure 5] and [Figure 7], it was noticeable that they present in areas in which there was more contact in between the prosthetic screw threads and the internal abutment screw threads. Continuous clinical usage creates plastic deformation (exhibited on the surface of screws) that causes crack initiation and this crack is responsible for the fracture nucleated at root of the implant threads. The fracture surfaces showed dimples (microcavities), which is suggestive of ductile fracture. Near the site of crack nucleation, the dimples presented plastic deformations caused by the compressive stress due to opening and closing of the cracks.

ISP on four implants evidently showed that all the screws exhibited physical changes after 5 years of usage, which were more pronounced after 10 years of usage. It was also observed that physical changes were accompanied by screw loosening and fracture in ISP with four implants and the terminal implants were observed to be most affected by this, which could be attributed to the fact that these implants are placed at angle of 30 degree and the entire load is directed across the implant–abutment junction to the underlying fixture trough the screw which turned out to be the weakest link and hence underwent physical changes initially which even progressed to loosening as the duration of use increased and finally a few fractured. When evaluating the ISP with six implants, all the screws showed very minimal physical deformation as compared to the ISP with four implants. However, implants which were angulated at 30° showed the maximum physical changes.

During case selection, we have two choices that means either we increase the number of implants or tilt them depending on the clinical situation:

  1. On increasing the numbers of implants, durability of prosthetic screw of ISP also increases, as observed in the study
  2. On tilting, greater stresses are generated at implant abutment junction and these are deleterious when observed at the level of the abutment screw.


Hence, from a technical point of view, tilting of the abutments reduced the durability of prosthetic screw that retained ISP as shown in the present study. Therefore, to be on safer side, increasing the number of implants can help. However, according to the current literature, perception is to go from an all-on-four configurations with distal implants placed at an angle less than or equal to 30°, which is strongly not recommended according to our observations from the prosthetic screw point only.


  Conclusion Top


Within the limitations of the study, the following conclusions can be drawn:

  1. In rehabilitation of edentulous jaws with ISP (four and six implant configurations), tilting of implants to be avoided
  2. Increasing the number of implants instead of tilting is more favorable as per observation of the present study, wherein it was evident that in increasing the tilt between 17 and 30 degree, the risk of prosthetic screw was very high
  3. In any configuration of ISP with four or six implants, the prosthetic screw is to be replaced after a period of clinical use of 5 years/1.5 million cycles in order to have a more predictable outcome of the prosthesis.


Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

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Pjetursson BE, Tan K, Lang NP, Bragger U, Egger M, Zwahlen M. A systematic review of the survival and complication rates of fixed partial dentures (FPDs) after an observation period of at least 5 years. I. Implantsupported FPDs. Clin Oral Implants Res 2004;15:625-42.  Back to cited text no. 1
    
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Jung RE, Pjetursson BE, Glauser R, Zembic A, Zwahlen M, Lang NP. A systematic review of the 5-year survival and complication rates of implant-supported single crowns. Clin Oral Implants Res 2008;19:119-30.  Back to cited text no. 2
    
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Sones AD. Complications with osseo-integrated implants. J Prosthet Dent 1989;62:581-5.  Back to cited text no. 3
    
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Goodacre CJ, Kan JY, Rungcharassaeng K. Clinical complications of osseo-integrated implants. J Prosthet Dent 1999;81:537-52.  Back to cited text no. 4
    
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Bickford JH. An Introduction to the Design and Behavior of Bolted Joints. Vol. 3. New York: Marcel Dekker Inc.; 1995. p. 515-64.  Back to cited text no. 5
    
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Haack JE, Sakaguchi RL, Sun T, Coffey JP. Elongation and preload stress in dental implant abutment screws. Int J Oral Maxillofac Implants 1995;10:529-36.  Back to cited text no. 6
    
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Martin WC, Woody RD, Miller BH, Miller AW. Implant abutment screw rotations and preloads for four different screw materials and surfaces. J Prosthet Dent 2001;86:24-32.  Back to cited text no. 7
    
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Sakaguchi RL, Borgersen SE. Nonlinear contact analysis of preload in dental implant screws. Int J Oral Maxillofac Implants 1995;10:295-302.  Back to cited text no. 9
    
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Barbosa GA, Bernardes SR, das Neves FD, Fernandes Neto AJ, de Mattos Mda G, Ribeiro RF. Relation between implant/abutment vertical misfit and torque loss of abutment screws. Braz Dent J 2008;19:358-63.  Back to cited text no. 10
    
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Spazzin AO, Henrique GE, Nóbilo MA, Consani RL, Correr-Sobrinho L, Mesquita MF. Effect of retorque on loosening torque of prosthetic screws under two levels of fit of implant-supported dentures. Braz Dent J 2010;21:12-7.  Back to cited text no. 11
    
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Hum S. Managing patients with a loose implant abutment screw. J Can Dent Assoc 2014;80:e22.  Back to cited text no. 12
    
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Schwedhelm ER, Raigrodski AJ. A technique for locating implant abutment screws of posterior cement-retained metal-ceramic restorations with ceramic occlusal surfaces. J Prosthet Dent 2006;95:165-7.  Back to cited text no. 13
    
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Secatto FB, Elias CN, Segundo AS, Cosenza HB, Cosenza FR, Guerra FL. The morphology of collected dental implant prosthesis screws surface after six months to twenty years in chewing. Dent Oral Craniofac Res 2017;3:1-7.  Back to cited text no. 15
    
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Williamson RT, Robinson FG. Retrieval technique for fractured implant screws. J Prosthet Dent 2001;86:549-50.  Back to cited text no. 16
    
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Kano SC, Binon P, Bonfante G, Curtis DA. Effect of casting procedures on screw loosening in UCLA-type abutments. J Prosthodont 2006;15:77-81.  Back to cited text no. 17
    
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Bickford JH. Introduction to the Design and Behavior of Bolted Joints. Wallengford, United Kingdom: CRC Press; 2008. p. 140-2.  Back to cited text no. 18
    
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Guzaitis KL, Knoernschild KL, Viana MA. Effect of repeated screw joint closing and opening cycles on implant prosthetic screw reverse torque and implant and screw thread morphology. J Prosthet Dent 2011;06:159-69.  Back to cited text no. 19
    
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Misch CE. Principles for abutment and prosthetic screws and screw-retained components and prostheses. In Book: Dental Implant Prosthetics. Vol. 28. St.Louis, Missouri: Elsevier; 2015. p. 724-6.  Back to cited text no. 20
    
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[PUBMED]  [Full text]  
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11], [Figure 12], [Figure 13], [Figure 14]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5]



 

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