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  Table of Contents 
Year : 2022  |  Volume : 25  |  Issue : 2  |  Page : 185-191

Effect of Framework's Manufacturing Technique on Screw's Preload of Implant Supported Prosthesis

1 Department of Prosthetic Dental Sciences, College of Dentistry, King Saud University, Riyadh, Saudi Arabia
2 Department of Dental Services, King Abdulaziz Medical City, Riyadh, Saudi Arabia
3 Ministry of Health, Prince Sultan Military Medical City, Riyadh, Saudi Arabia
4 General practitioner, Prince Sultan Military Medical City, Riyadh, Saudi Arabia

Date of Submission09-Oct-2020
Date of Acceptance06-Jan-2022
Date of Web Publication16-Feb-2022

Correspondence Address:
Dr. H Alshehri
Riyadh 11555, P.O. Box 60542
Saudi Arabia
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/njcp.njcp_619_20

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Background: Implant supported prosthesis is a common treatment modality. Nowadays, new manufacturing techniques are available to fabricate them. Aims: To evaluate the effect of different manufacturing techniques of implant supported frameworks (ISF) on the preload of abutment's screws. Materials and Methods: A mandibular edentulous acrylic model with four dental implants temporarily stabilized in the interforaminal area was used. One ISF was fabricated using the conventional technique; implants were removed from the model and reassembled into the framework; this framework served as the passively fitting framework (PF). Three additional frameworks were constructed: conventional cast framework (CF), milled framework (MF) and 3D-printed framework (3D-PF). The gap between the frameworks and the neck of the implants were recorded in microns using a digital microscope. A tightening torque (TT) of 35 N·cm was applied to all the four abutments' screws and the screw's preload was recorded using two methods, by strain gauges (SGs) that were attached to the neck of each implant and fed into a stain book in microstrain (μɛ) and by removal torque (RT) using a digital torque meter. Results: The frameworks' gap means from the lowest to the highest were PF, CF, 3D-PF, and MF. The RT was significantly lower than the TT in all frameworks (P ≤ 0.05). One-way analysis of variance (ANOVA) revealed that the PF had the lowest RT, while the CF and the 3DPF both had the highest RT, and those differences were found to be statistically significantly (P ≤ 0.05). When preload of the frameworks was recorded by SGs, one-way ANOVA revealed that PF had the highest preload value, while both 3D-PF and MF had the lowest preload values, those differences were also found to be statistically significant (P ≤ 0.05). Conclusion: The fabrication of implant-supported frameworks using milling or selective laser melting computer aided design/computer-aided manufacturing technologies did not necessarily enhance the screw's preload. This lack of enhancement could be attributed to the great amount of marginal gap in the frameworks fabricated by both techniques.

Keywords: ISF, removal torque, screw's preload, tightening torque

How to cite this article:
Alshehri H, Alotaibi H, Alshareef N, Alsenani N, Aljuma'ah L, Alshhrani W, Alsahhaf A. Effect of Framework's Manufacturing Technique on Screw's Preload of Implant Supported Prosthesis. Niger J Clin Pract 2022;25:185-91

How to cite this URL:
Alshehri H, Alotaibi H, Alshareef N, Alsenani N, Aljuma'ah L, Alshhrani W, Alsahhaf A. Effect of Framework's Manufacturing Technique on Screw's Preload of Implant Supported Prosthesis. Niger J Clin Pract [serial online] 2022 [cited 2022 Dec 2];25:185-91. Available from:

   Introduction and Literature Review Top

Placement of four implants and subsequent implant supported framework (ISF) in the edentulous mandible is a highly successful treatment option.[1] Passive fit of ISF is a major mechanical factor that affects long-term success of implant prostheses.[2],[3] Misfit leads to plaque accumulation, which may cause peri-implantitis.[4] In addition, Inadequate fit results in irregular thread contact, tension, and bending stresses of the connecting screw, which cause screw loosening and fracture.[5] Moreover, screw loosening can be affected by many other factors, among which is the preload generated during screw torqueing.[6]

Preload is defined as the initial force created within the screw after torqueing. This force is responsible for keeping the screw head tightly secured to its seat, creating what is called a clamping force.[7] Preload will cause the screw's shank and threads to elongate creating tension forces, and the elastic recovery of the screw material itself will result in more clamping force, thus preventing the separation of implant parts. However, when external forces exceed the clamping force, screw loosening will occur.[7] Each implant manufacturing company has its own recommended torqueing value for every implant system to ensure adequacy of the preload and stability of the screw.

Preload is affected by the fitting of the framework; in return, the passivity of the framework is affected by the manufacturing technique of the framework.[8] Typically, the casting method was used to manufacture ISF. However, the casting method found to produce distortions, which result in discrepancies in fit between the implant and ISF.[9] With the advancements in computer aided design/computer-aided manufacturing (CAD/CAM), new techniques were developed to make ISF that assumed to be more accurate and has precise dimension.[10] Two methods of CAM are present for fabrication of ISF, subtractive (milling) and additive using additive manufacturing (AM) or rapid prototyping techniques like selective laser melting (SLM).

Subtractive manufacturing is based on milling the raw material from a larger block by a milling machine. The CAM software automatically translates the CAD model into tool path for the milling machine.[11] AM refers to the process of building material to make three-dimensional (3D) objects from 3D model data, usually layer upon layer.[12] Many recent studies have compared these new techniques to the casting technique.[13],[14]

Since the CAD/CAM technology is recently introduced to dentistry, there is a lack in the literature in regard to the effect of those new ISF manufacturing techniques on the preload of the abutment screws. This study aims to compare the effect of different ISF manufacturing techniques (casting, milling, and SLM) on the preload of abutments' screws.

   Materials and Methods Top

Master model and PF construction

An acrylic resin model (orthodontic resin clear; Dentsply international) for a completely edentulous mandibular jaw was constructed by silicone mold, as seen in [Figure 1]. Four parallel internal connection implants (4.3 × 13 + 3 mm, ReplaceTM Select TC, Nobel Biocare) were temporarily stabilized in the acrylic model, using poly vinyl siloxane (PVS) material (putty Genie rapid set, Sultan health care).
Figure 1: Acrylic master model

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An open tray impression was taken for the model using a custom tray (Preci tray; Yeti Dental). Final impression was taken using polyether impression material (Impregum; 3M/ESPE, Seefeld), after that, poured using dental stone (GC Fuji Rock EP, GC America Inc). A resin pattern was fabricated (Pattern resin, GC Corporation) invested, casted (type IV gold Lodestar, Ivoclar Vivadent), and finished using the standard method of framework construction. To make sure that the framework is passively fitting. The implants were removed from the acrylic model, then reassembled to the constructed framework as seen in [Figure 2]. After that, it was restabilized into the acrylic jaw model using extra amount of clear orthodontic resin, as seen in [Figure 3].
Figure 2: The implants assembled in the cast metal framework

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Figure 3: The implants stabilized inside the model

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Construction of CF

On the acrylic model, another polyether final impression was taken, and stone poured to fabricate a master model. Four nonengaging gold adapt were attached to the stone master model, and the resin pattern was constructed, as seen in [Figure 4], invested and casted (type IV gold Lodestar, Ivoclar vivadent), and finished using the standard method of conventional framework construction.
Figure 4: Resin pattern constructed

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Construction of MF

The cast was sent for scanning using scanning software (Ceramill map, Amanngirrbach), and an identical ISF to the CF was designed using the designing software (Ceramill mind, Amanngirrbach). An STL file of the designed framework was obtained, as seen in [Figure 5]. The framework was milled from Co–-Cr Sinter metal (Ceramill Sintron Amanngirrbach) using milling machine (Ceramill motion 2, Amanngirrbach).
Figure 5: STL file

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Construction of a 3D-printed framework (3D-PF)

The same STL file was used for the construction of the 3D-PF using SLM. A laser beam is directed toward Co–Cr powder (Remanium® star CL cobalt-chromium alloy powder, Dentaurum) selectively to print the framework in 3D way using SLM machine (Mlab cusing, ConceptLaser).

Measurement of the marginal gap

The gap was measured for all the four frameworks. Each framework was torqued in place (35 N·cm) using the same digital torque meter. The marginal gap was measured in microns using a digital microscope (Hirox digital microscope KH-7700). The gap measurement was performed for two points buccally and two points lingually in each implant/framework connection. The readings were repeated three times for each framework, and each framework was represented by one average number to simplify the comparisons between the frameworks.

Preparation of the master model prior to the experiment

a- Strain Gauges (SGs) placement on the implants

Two SGs were attached to the neck of each implant, as seen in [Figure 6]. The vertical axis of the implants was marked and a prewired linear SG (KFG-02-120-C1-11LIM2R, Omega Engineering) was glued to the neck of each implant (Rapid Cure SG adhesive SG401, Omega Engineering). Typically, 120 Ω gauge resistance factor was confirmed with an ohmmeter (Omega Engineering). Every SG was connected to a 30-channel strain meter (TDS 303 Portable Data Logger, Tokyo Sokki Kenkyujo) in a three-wire quarter bridge arrangement, as seen in [Figure 7].
Figure 6: The SGs attached to the neck of each implant

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Figure 7: The SGs attached to the strain meter for measurement of strain on each implant

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b- Calibration of implants

A special load carrier (CARL ZEISS) was used to calibrate implants. The end of the load carrier was placed on the tip of the tightened abutment (Easy abutment NobRpl RP 0.5 mm, REF 29470, Nobel Biocare) on each implant, and the channels of the strain meter were zeroed out. A standardized load of 10 kg was applied in 1 kg increments, and the readings were recorded from the strain meter for each of the SG in micro strains (μɛ).

Preload measurement

Four abutment screws (Replace select, TC, Nobel Biocare) were used during each experiment running. The experiment was repeated five times under each framework. In which, the framework was placed on the acrylic model, the channels were zeroed and the abutment screws were torqued to the value recommended by the manufacturer (35 N·cm) using a digital torque meter (BTGE, Tohnichi), as seen in [Figure 8].
Figure 8: Digital torque meter

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The implants screws were torqued in the following sequence: 2, 3, 4, and 1, every running and after the torqueing of the last screw; the preloads' readings from all the eight channels were recorded in μɛ. The abutments' screws were detorqued in the opposite sequence (1, 4, 3, and 2, respectively) using the same digital torque meter, and the torque required to untighten the screw was recorded as the removal torque (RT).

Statistical analysis

Statistical package for the social science SPSS (v16, SPSS Inc.) was used to perform the statistical analysis. The mean of marginal gaps in microns for each framework was represented as single number to simplify the comparison. Independent sample t-test was used to compare the amount of preload loss between the tightening torque (TT) and the RT. The differences in preload between the four frameworks for both the RT and preload obtained by SG were evaluated using one-way analysis of variance (ANOVA) and post hoc multiple comparison. The significance level was set at 0.05 (P ≤ 0.05).

   Results Top

Mean and standard deviation of marginal gaps for each framework are presented in [Table 1]. The greatest gap was observed in the MF, followed by the 3D-PF, CF, and the smallest gap was observed in the PF.
Table 1: Descriptive statistics of the marginal gap of each framework

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Calculation of calibration of implants

The applied load in kilograms was converted to newton using the following equation:

Force (N) = mass (kg) × acceleration (m/s2) in which the acceleration value is equal to 9.8 m/s2.

The relation between the applied loads in newton and the obtained με from each strain gage was calculated using regression analysis. A regression equation and the coefficient of determination (R2) were calculated for each implant, and the regression plot was made to demonstrate the relationship between the applied load in newton and the obtained preload in με. The obtained regression equations for each implant was used to convert the obtained preload in με to N, as seen in [Diagram 1].

Regression equation

Implant 1: N = 1.51 – Strain (με)/1.26 R2 = 99.96%

Implant 2: N = 0.889 – Strain (με)/0.920 R2 = 99.93%

Implant 3: N = 3.04 – Strain (με)/1.01 R2 = 99.94%

Implant 4: N = 2.63 – Strain (με)/1.45 R2 = 99.96%

Difference between TT and RT under each framework

An independent sample t-test was used to compare TT and RT for each framework. In all frameworks, the RT was significantly lower than applied TT [Table 2].
Table 2: Means, standard deviation, and independent sample t-test between the applied TT and RT under each framework

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Comparison of RT among different frameworks

One-way ANOVA was used to compare RT of different frameworks. The highest mean RT was observed in the MF (25.72 ± 2.33) followed by CF (25.57 ± 1.16), PF (23.56 ± 2.45, and the least mean was found in the 3D-PF (21.93 ± 2.49). Games-Howell post hoc test revealed a statistically significant difference between all ISFs except between PF and 3D-PF and between CF and MF (P ≥ 0.05) [Table 3] and [Table 4].
Table 3: One-way ANOVA of RT among different ISFs

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Table 4: Games–Howell posthoc multiple comparisons of mean RT among different ISF

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Comparison of preload among different frameworks

Each framework was presented by one value representing the mean of all preload obtained from all SGs; the mean and SD of each framework are presented in [Table 5].
Table 5: Preload's mean and SD for each framework

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The highest preload mean was observed in PF (2681.06 ± 137.77), followed by CF (1901.9 ± 110.26), followed by the 3D-PF (1423.84 ± 148.49), and the least mean was observed in the MF (1209.33 ± 217.61). The one-way ANOVA revealed a significant effect of framework type over the obtained preload [Table 6]. The Games-Howell Post hoc multiple comparisons test showed that those differences in preload value are statistically significant between all ISF except between the 3D-PF and MF (P ≥ 0.05) [Table 7].
Table 6: One-way ANOVA of preload among different frameworks

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Table 7: Games–Howell post hoc multiple comparisons of mean preload between different frameworks

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   Discussion Top

Few laboratory and clinical studies were performed to compare conventional fabrication methods of implant-supported framework to the newly introduced CAD/CAM techniques either additive or subtractive.[10],[15] In the current study, the screws' preload of frameworks fabricated by three different techniques were compared; accordingly, the null hypothesis was rejected, that is to say that the framework's manufacturing technique did significantly affect the screw's preload.

In the current study, the RV was significantly lower in all frameworks than the applied TT and this was in agreement with other studies.[8],[16] It was reported in the literature that most of the torque used for tightening the screws is utilized to overcome friction in the surface and only 10% is used to develop preload.[17] The torque is divided between the friction and the preload of the screw in internal connection implants.[18],[19],[20] The reason behind this is that part of the applied torque will be used to flatten the micro roughness of all the metal contacting surfaces between screw's threads and the internal implant surface leading to preload reduction.[21]

The loss of preload was also explained in previous studies in which part of the torque applied during tightening will be used to overcome misfit in superstructure.[8],[21] In the current study, MF exhibited the least amount of torque loss, despite the same framework having the largest mean gap measurement among all the frameworks. It seems that the framework misfit works in favor of the RT because of the misalignment of the screws path and the framework creating an additional friction during the removal. This can also be supported by preload loss measured by SGs where it was highest in the MF.

The difference in preload obtained from SGs was significant between all frameworks except between the MF and 3D-PF ones where the highest preload was in the PF, while the lowest was observed in the MF. This sequence coincides with the amount of framework's gap measurement, where the PF had the lowest mean gap measurement, while the highest gap measurement was observed in the MF. This can be justified by the fact that the presence of misfit in the prosthesis can cause loss in the preload. In the presence of a gap between prosthesis and abutment, the preload will be utilized to bring mating surfaces close to each other or into complete contact.[22] This could explain the greater loss in the preload in the MF, which has the greatest marginal gap.[10]

Although the CAD/CAM milling technology is far more established than the printing technology, the current finding indicates that the fit along with the screws preload are the worst in the framework fabricated by this technique in comparison to all the other frameworks. This was in the contrary to one study that compared two frameworks' fabrication techniques, casting and milling. and found that the milled frameworks (MFs) showed better fit than the casted once.[23]

The better fit encountered in the frameworks fabricated by CAD/CAM technology can be due to the elimination of several fabrication steps that could introduce errors.[10],[15] However, and similar to our findings, other studies reported that the MF has more incidence of misfit than conventionally fabricated frameworks.[23],[24] One explanation of this finding can be due to the inaccuracies that can happen because of new steps introduced in the fabrication of MF such as scanning, software modelling, and framework milling.[25],[26] Another study that compared the internal and marginal fit of fixed dental prostheses fabricated by conventional, milling, and SLM techniques found that the SLM method produced the worst internal and marginal fit, while the milling technique had the best fit.[25] Surface irregularities of the mating surfaces produced by each manufacturing technique can affect the presence of microgaps, where the milled surfaces have smother surfaces than casted and sintered surfaces and this can lead to better fit.[26]

The experiment was run in a controlled setting where no contamination of blood or saliva took place, the effect of both on the preload is not known.[20],[27] Moreover, tightening sequence and different screw's materials might also have an impact on the outcome of the study.[28] Additionally, marginal fit of only ISF of each manufacturing technique was measured in this study. Future studies can include multiple frameworks and the relation of their fit to implants' preload. Finally, with the advancements of postmanufacturing computer controlled finishing techniques, future studies should be done to make better conclusions of these technologies as alternatives to conventional methods.

   Conclusions Top

The fabrication of implant-supported frameworks using milling or SLM CAD/CAM technologies did not necessarily enhance the screw's preload. This lack of enhancement could be attributed to the great amount of marginal gap in the frameworks fabricated by both techniques. Care must be taken when evaluating the preload of frameworks fabricated by different techniques as it can be easily affected by the fit of the framework.


This research project was supported by a grant from the Research Center of the Female Scientific and Medical Colleges,” Deanship of Scientific Research, King Saud University (registration number FR 0323).

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

   References Top

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  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8]

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6], [Table 7]


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