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Table of Contents
Year : 2018  |  Volume : 8  |  Issue : 1  |  Page : 13-17

Accuracy of Implant Torque Wrenches after Clinical Service

Department of Prosthodontics, V S Dental College and Hospital, Bengaluru, Karnataka, India

Date of Web Publication5-Mar-2018

Correspondence Address:
Siddhartha Pincha
Department of Prosthodontics, V S Dental College and Hospital, K. R. Road, Bengaluru - 560 004, Karnataka
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jid.jid_14_17

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Aim: The purpose of this study was to determine the accuracy of two types (friction style and spring style) of mechanical torque wrenches after clinical use. Materials and Methods: Four mechanical torque wrenches in clinical use were collected. Two were friction style and two were spring style. All the wrenches had been in use for at least 6 months without being recalibrated and were tested to check their accuracy in delivering target torque values. The data obtained were statistically analyzed using Student's t-test. Results: After clinical use, there was no statistically significant difference in the accuracy between the two groups (P < 0.05). Conclusions: Within the limitations of this study, it was concluded that after clinical use, the accuracy of both the type of wrenches was within clinically acceptable limits. All torque wrenches delivered torque values within 10% of the target torque value.

Keywords: Implants, torque, torque wrenches

How to cite this article:
Pincha S, Shetty S, Mithra AN. Accuracy of Implant Torque Wrenches after Clinical Service. J Interdiscip Dentistry 2018;8:13-7

How to cite this URL:
Pincha S, Shetty S, Mithra AN. Accuracy of Implant Torque Wrenches after Clinical Service. J Interdiscip Dentistry [serial online] 2018 [cited 2018 Mar 23];8:13-7. Available from: http://www.jidonline.com/text.asp?2018/8/1/13/226635

   Clinical Relevance to Interdisciplinary Dentistry Top

Accurate delivery of torque to implant prosthetic screws is important to prevent prosthetic complications. Both under torquing and over torquing can affect the long term success of the prosthetic screw joint.

   Introduction Top

The use of osseointegrated implants has been a successful long-term solution in partially or completely edentulous patients. However, implant restorations are not free from complications. Implant complications include framework and prosthesis resin fracture, denture tooth wear, and chipping of veneering material.[1] Moreover, a frequent complication is prosthetic screw loosening.[2],[3],[4],[5] Loose screws may add further complications such as screw fracture, nonretrievable fragments, and even the removal of implants.[5]

Screw loosening is a commonly reported complication associated with implant prosthodontics.[2],[3],[4],[5] Inadequate torque delivery,[6],[7] embedment relaxation,[7],[8] and metal fatigue[9] are among the various reasons of unintentional screw loosening. Precalibrated torque wrenches are used in implantology to accurately set the tightening force to the abutment screw. Manufacturers suggest the use of precalibrated torque delivery to reduce the incidence of unintentional screw loosening.[3],[9],[10],[11]

The understanding of screw mechanics is important to know why screw loosens and thereby provide optimum results with implant-supported restorations. The various causes attributed to screw loosening include inadequate torque delivery,[7] embedment relaxation,[7] poorly machined components, inadequate prosthesis fit, excessive loading, screw design, and restoration design.[11],[12] A screw joint can be described as two parts which are tightened together by a screw, such as the abutment and the implant or the crown and the implant.[11] The forces trying to separate the components of the screw joint are called joint separating forces, and the forces keeping the components together are called the clamping force.[11] Screw loosening happens when the joint separating forces exceed the clamping force. Hence, to reduce the incidence of screw loosening, clamping force should be increased and the joint separating forces should be decreased. Tightening the screw produces the initial clamping force, and the clamp load is proportional to the tightening torque. When the screw is torqued, a clamping force is produced between the screw head and its seat as friction between opposing threads increases and screw elongation occurs.[13] Preload is the force or tension created within the screw by the applied torque and is equal to the clamping force.[11] Insufficiently tightened screws can become loose, thereby resulting in fracture of the screw and failure of the screw and possibly failure of the prosthesis.[11],[14],[15]

Conversely, excessive tightening can cause permanent deformation of the screw shank, stripping of the screw threads, screw loosening, and even screw fracture.[11],[14],[15] The delivery of optimum torque or target value to the screw joint is an important factor to prevent screw loosening as well as to provide adequate preload. Recommended preload torque value ranges from 10 to 35 Ncm and preload torque depends on several factors such as screw material, applied torque, screw head design, abutment surface, abutment material, and also the lubricant.[11],[16] Theoretically, in a screw joint, torsional fracture of the screw occurs just before the maximal preload is generated. Hence, a safety margin is established minimizing the risk of screw fracture while maximizing the preload.[11]

The torque delivered by a torque wrench should be within ±10% of the target value.[17] The mean torque values of new friction-style and spring-style torque wrenches were found to be within 10% of their target torque values.[18] Two studies examined the accuracy of friction-style and spring-style torque wrenches as received, and both of the studies found no differences between them.[19],[20] Another previous study reported spring-type torque wrench to be more accurate than friction-style ones.[21] One more study demonstrated that torque wrenches should be activated slowly, over 4 s when using a friction-type torque wrench.[15] The age of the device, frequency of use, and debris and corrosion in the components of the torque device may result in inaccurate torque delivery to the screw.[22]

Very few investigations are available about the effect of clinical use on different types of torque wrenches, and the results are conflicting.[6],[18],[23] The purpose of this study was to investigate the effect of clinical use on the accuracy of friction-type and spring-type torque wrenches. The hypothesis was that there is no significant difference between the accuracy of friction-type and spring-type torque wrenches in delivering their target torque values after clinical use.

   Materials and Methods Top

Implant torque wrenches from two different dental implant manufacturers (Hi-tech and Nobel Biocare) were included to evaluate their accuracy in delivering target torque values after their clinical use. Four wrenches, friction style (n = 2) and spring style (n = 2) [Figure 1], in clinical use for at least 6 months without being recalibrated were included in the study. A digital torque gauge was used to measure the peak torque values, and the output was recorded in Ncm. These four wrenches were tested to check their accuracy in delivering target torque values. The target torque values for friction style (Hi-tech) were checked at 10, 20, 30, and 40 Ncm, whereas the target torque values for spring style (Nobel Biocare) were 15, 25, and 35 Ncm.
Figure 1: Torque wrenches: Spring type (top) and friction type (bottom)

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Proper driver inserts were selected and fixed in the chuck of the digital torque gauge for each type of wrench. The driver insert was connected to the wrench, and the torque indicator on the digital gauge was set at zero. The digital torque gauge was firmly fixed to a tabletop to prevent any movement [Figure 2]. The operator used one hand to stabilize the driver and the other hand to apply force to the implant torque wrench.[24]
Figure 2: Digital torque gauge fixed on table top

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The operator was standardized by executing 100 repetitions on a new torque wrench for each type of mechanism (friction and spring). These new torque wrenches used for standardization were not included in this study. The operator applied the needed force to the torque wrenches until friction-type devices were released at their preset torque value or until the spring-type devices bent till the preset limit on the arm of the torque wrenches [Figure 3] and [Figure 4].[24] For each device, the peak torque value with the digital torque gauge was recorded, and the operator was blinded from readings. The test was repeated ten times for each wrench.[24]
Figure 3: Spring-type torque wrench being tested

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Figure 4: Friction-type torque wrench being tested

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Statistical analyses were performed using Statistical Package for Social Sciences version 22.0 released 2013 (Armonk, NY: IBM Corp.) for Windows. Median absolute differences (difference between target torque and measured torque) were evaluated. In addition, the percentage of deviation (Per dev = [absolute difference/target torque] ×100) was calculated and compared with the target torque value preset by the manufacturers. Differences in median absolute torque values between spring and friction type wrenches were compared using Student's t-test (P< 0.05) [Table 1] and [Table 2].
Table 1: Summary data for accuracy measurements of torque wrenches (deviation from target torque value)

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Table 2: Comparisons of deviation from target torque values between torque wrench types

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   Results Top

Torque data from the four torque wrenches obtained were collected. Descriptive summary data of the median absolute difference and minimum and maximum differences between the measured torque and the targeted torque values for each manufacturer are shown in [Table 1]. The median absolute difference and the percentage of deviation of the measured torque value from the targeted torque value are also shown in [Table 1]. The median absolute difference value was used to make comparisons because the mean absolute difference may not accurately reflect the central tendency because of skewing.[24]

[Table 2] represents the comparisons between the torque wrench type (friction style and spring style). Spring-style torque wrench was different than friction-style torque wrench regarding their accuracy (P< 0.05). However, there were no statistically significant difference between the two types of torque wrenches (P > 0.05).

   Discussion Top

This study was conducted to determine whether a specific type of torque wrench applied a clinically suitable torque. For this study, torque values within ±10% of their target torque value were defined as the level of accuracy of tested torque-limiting devices. The null hypothesis was accepted because there was no significant difference found between the ability of friction-style torque wrenches to achieve their target torque values than that of spring-style torque wrenches values, and both types were found within ±10% of target torque values.

However, the previous studies had contradictory results. Yilmaz et al.[24] evaluated the accuracy of friction-style and spring-style mechanical torque limiting device after clinical service and found that the spring type was more accurate than the friction style. Standlee et al.[18] assessed the accuracy of Nobel Biocare, Straumann ITI, and DynaTorq ITL torque wrenches and concluded that the mean torque values of Straumann ITI and DynaTorq ITL were within ±10% of their target torque values and that Nobel Biocare showed the maximum variation for target torque values. In the present study, friction type was more accurate than the spring type with no significant difference, although all devices fell within ±10% of target torque values preset by manufacturers and so results are not clinically relevant. This difference in the results could be because of intraoperator error, as the torque application mechanism in spring style depends on the manual dexterity of the operator. Furthermore, the difference in results could be because of the difference in frequency of use of each device, and number of sterilization cycles each device underwent.

The results of this study show that the torque wrenches applied lower torque than the target values [Table 1] except the friction type at 30 Ncm. This can be explained by the findings of McCracken et al.[15] They stated that the different mechanisms of the spring-style and friction-style torque wrenches may influence the accuracy of these devices. A flexible metal arm is used to apply a precaliberated torque value in a spring-style torque wrench, and the applied torque depends on the flexibility of this arm and the distance to which it is pulled away from the body. Whereas, multiple moving parts and mechanical moving connections, including a spring and a ball, constitute a friction-style torque wrench. Corrosion in these components affects the flexibility of the device, resulting in higher torque delivery.[15] The lower torque values may be because of loosening of these components with use, resulting in more flexibility.

According to the manufacturers' recommendations, the use of an approved lubricant and the device in the broken position of the head (for the friction-style) should be followed before each sterilization.[15] The results of this study suggest that sterilization and clinical use of torque wrenches in an institutional environment may be associated with an alteration of torque value from the target torque value preset by the manufacturer. Both friction-style and spring-style torque wrenches should be regularly calibrated to prevent complications that may occur due to the application of over-or-under torque forces to implant screws.[24]

However, there were some limitations of the study, as the application of torsional force could not be standardized and was subject to intraoperator error, and multiple number of manufacturers should be evaluated in further studies.

   Conclusions Top

Within the limitations of the study:

  1. After clinical use, the accuracy of both the type of torque wrenches was clinically acceptable. All torque wrenches delivered torque values within 10% of the target torque value
  2. Torque-limiting devices should be routinely calibrated during clinical service according to the instructions of their manufacturer.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

   References Top

Papaspyridakos P, Chen CJ, Chuang SK, Weber HP, Gallucci GO. A systematic review of biologic and technical complications with fixed implant rehabilitations for edentulous patients. Int J Oral Maxillofac Implants 2012;27:102-10.  Back to cited text no. 1
Jemt T. Failures and complications in 391 consecutively inserted fixed prostheses supported by Brånemark implants in edentulous jaws: A study of treatment from the time of prosthesis placement to the first annual checkup. Int J Oral Maxillofac Implants 1991;6:270-6.  Back to cited text no. 2
Jemt T, Laney WR, Harris D, Henry PJ, Krogh PH Jr., Polizzi G, et al. Osseointegrated implants for single tooth replacement: A 1-year report from a multicenter prospective study. Int J Oral Maxillofac Implants 1991;6:29-36.  Back to cited text no. 3
Naert I, Quirynen M, van Steenberghe D, Darius P. A six-year prosthodontic study of 509 consecutively inserted implants for the treatment of partial edentulism. J Prosthet Dent 1992;67:236-45.  Back to cited text no. 4
Zarb GA, Schmitt A. The longitudinal clinical effectiveness of osseointegrated dental implants: The Toronto study. Part III: Problems and complications encountered. J Prosthet Dent 1990;64:185-94.  Back to cited text no. 5
Dellinges M, Curtis D. Effects of infection control procedures on the accuracy of a new mechanical torque wrench system for implant restorations. J Prosthet Dent 1996;75:93-8.  Back to cited text no. 6
Jörnéus L, Jemt T, Carlsson L. Loads and designs of screw joints for single crowns supported by osseointegrated implants. Int J Oral Maxillofac Implants 1992;7:353-9.  Back to cited text no. 7
Budynas RG, Nisbett JK. Shigley's Mechanical Engineering Design. 8th ed. New York: McGraw Hill; 2008. p. 425-9.  Back to cited text no. 8
Patterson EA, Johns RB. Theoretical analysis of the fatigue life of fixture screws in osseointegrated dental implants. Int J Oral Maxillofac Implants 1992;7:26-33.  Back to cited text no. 9
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. 10
McGlumphy EA, Mendel DA, Holloway JA. Implant screw mechanics. Dent Clin North Am 1998;42:71-89.  Back to cited text no. 11
Binon P, Sutter F, Beauty K, Brunski J, Gulbransen H, Weiner R. The role of screws in implant systems. Int J Oral Maxillofac Implants 1994;9:48-63.  Back to cited text no. 12
Standlee JP, Caputo AA. Accuracy of an electric torque-limiting device for implants. Int J Oral Maxillofac Implants 1999;14:278-81.  Back to cited text no. 13
Cehreli MC, Akça K, Tönük E. Accuracy of a manual torque application device for Morse-taper implants: A technical note. Int J Oral Maxillofac Implants 2004;19:743-8.  Back to cited text no. 14
McCracken MS, Mitchell L, Hegde R, Mavalli MD. Variability of mechanical torque-limiting devices in clinical service at a US dental school. J Prosthodont 2010;19:20-4.  Back to cited text no. 15
Hill EE, Phillips SM, Breeding LC. Implant abutment screw torque generated by general dentists using a hand driver in a limited access space simulating the mouth. J Oral Implantol 2007;33:277-9.  Back to cited text no. 16
Mahshid M, Saboury A, Fayaz A, Sadr SJ, Lampert F, Mir M, et al. The effect of steam sterilization on the accuracy of spring-style mechanical torque devices for dental implants. Clin Cosmet Investig Dent 2012;4:29-35.  Back to cited text no. 17
Standlee JP, Caputo AA, Chwu MY, Sun TT. Accuracy of mechanical torque-limiting devices for implants. Int J Oral Maxillofac Implants 2002;17:220-4.  Back to cited text no. 18
Britton-Vidal E, Baker P, Mettenburg D, Pannu DS, Looney SW, Londono J, et al. Accuracy and precision of as-received implant torque wrenches. J Prosthet Dent 2014;112:811-6.  Back to cited text no. 19
L'Homme-Langlois E, Yilmaz B, Chien HH, McGlumphy E. Accuracy of mechanical torque-limiting devices for dental implants. J Prosthet Dent 2015;114:524-8.  Back to cited text no. 20
Vallee MC, Conrad HJ, Basu S, Seong WJ. Accuracy of friction-style and spring-style mechanical torque limiting devices for dental implants. J Prosthet Dent 2008;100:86-92.  Back to cited text no. 21
Gutierrez J, Nicholls JI, Libman WJ, Butson TJ. Accuracy of the implant torque wrench following time in clinical service. Int J Prosthodont 1997;10:562-7.  Back to cited text no. 22
Santos GC Jr., Passos SP, Coelho Santos MJ. Accuracy of mechanical torque devices for implants used in Brazilian dental offices. Int J Prosthodont 2011;24:38-9.  Back to cited text no. 23
Yilmaz B, L'Homme-Langlois E, Beck FM, McGlumphy E. Accuracy of mechanical torque-limiting devices for dental implants after clinical service. J Prosthet Dent 2015;114:378-82.  Back to cited text no. 24


  [Figure 1], [Figure 2], [Figure 3], [Figure 4]

  [Table 1], [Table 2]


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