Screw loosening or fracture is a frequently occurring complication in the application of dental implants.1 Screw fracture usually occurs when a screw is fatigued by screw loosening, which is influenced by the length of the screw and/or the abutment.2,3 Poor screw retention is usually attributed to complex functional loading that induces stresses in the screw that exceed the elastic limit of the material.4 Occlusal forces applied to the abutment are indirectly conveyed to the retaining screw, and local abutment stresses can be induced by asymmetric loads, especially in abutments that do not fit precisely. When localized stress is applied continuously under a subcritical cyclic loading condition, the elastic clamping energy of the preloaded screw can decrease or even disappear.5–7
Screws should be tightened to the torque value as recommended by the manufacturer to reduce the likelihood of screw loosening.8,9 The recommended value, which usually results in an ideal preload lower than the elastic limit of the material, is normally approximately 75% of the ultimate failure torque.9,10 Preload is achieved when the screw is stretched, and an additional preload cannot be obtained when the implant material is stretched beyond its elastic limit. The elongation of the screw places the shank and threads in tension. The elastic force of the screw creates the clamping force that pulls the prosthesis and implant together, and it is usually proportional to the clamping force.7 The expected maximum biting force on a posterior single implant is known to be lower than the clamping forces.5,6 The screw preload varies with the applied torque and the geometry and material of the screw.3,6 However, the clamping force induced by the preload does not completely eliminate the possibility of screw loosening, and it can be reduced by masticatory loading, fatigue, and material yield.2
Including a washer between the abutment and the screw can reduce the likelihood of screw loosening. A washer is an axially deformable elastic ring with a relatively low stiffness, and its use increases the stored strain energy2 and the amount of rotational displacement needed to completely loosen an implant gold screw. It also lengthens the time for which the preload is maintained. Including a washer will allow the screw connection to withstand greater deformations without losing the preload and clamping force.11,12 Friction between the screw head and the washer is important to prevent the reduction of the preload, and this varies with the surface condition of the mating parts.12 Excessive torque (ie, beyond the yield strength) of the screw causes permanent deformation and can result in screw fracture over time.13,14 Washers of various shapes and materials are available, and their stiffness are largely determined by the shapes and elastic moduli of their materials. The stored strain energy of the implant assembly varies with the material stiffness.2
The purpose of this study was to elucidate the effects of using a washer in conjunction with a titanium retaining screw.
Materials and Methods
Preparation of Specimens
An external connection type of implant (US II, OSSTEM, Busan, Korea) was used in this study. A method wherein the fixture, abutment, and washer are conjoined at a butt joint was adopted (Figs. 1 and 2). Fifty samples of the external connection type of implant fixture with a diameter of 4.0 mm and a length of 11.5 mm were prepared. Fifty cement-retaining-type implant abutments with a diameter of 5.0 mm were also prepared, along with 50 titanium alloy abutment screws. One control group and four experimental groups were prepared according to the type of washer used (Table 1): control group (n = 10), no washer; group I (n = 10), 1.0-mm-thick grade 2 CP titanium washer; group II (n = 10), 2.0-mm-thick grade 2 CP titanium washer; group III (n = 10), 1.0-mm-thick Ti6Al4V titanium alloy washer; and group IV (n = 10), 2.0-mm-thick Ti6Al4V titanium alloy washer.
The abutment was connected to the fixture at a torque of 30 Ncm, as recommended by the manufacturer. A cast-metal jig with a diameter of 8.0 mm (Fig. 3) was used to secure the implant fixtures in the same position. The removal torque was measured by a digital torque gauge (Amplifier GM 70, Lorenz, Alfdorf, Germany) at a resolution of 0.1 Ncm. The removal torque was measured 10 times in each group.
One-way analysis of variance was used to determine whether mean values of the removal torque differed significantly (α = 0.05) between the groups. Data were also analyzed using Scheffé's post hoc test.
The removal torque was higher in groups with a washer (groups I–IV) than in the control group without a washer (α = 0.05) (Table 1), higher with a CP titanium washer (groups I and II) than with a titanium alloy washer of the same thickness (groups III and IV) (Table 2), and higher with a 2.0-mm-thick washer than with a 1.0-mm-thick washer, irrespective of the washer material (α = 0.05) (Table 3). It was found that the removal torque decreased gradually with repeated tightening and releasing.
Alternative materials to gold alloys have been introduced into dentistry because of their cost-effectiveness. However, their corrosion resistance is of concern.15 In addition, galvanic corrosion can occur when dissimilar alloys are in direct contact in the oral cavity, notably when a titanium implant and a low corrosion resistance alloy are in direct contact.16 This galvanic corrosion is harmful due to dissolution of alloy components and the induction of current flows that can cause marginal bone loss.17 The titanium alloy washer was more effective than the CP titanium washer in preventing screw loosening in this study. However, it should be remembered that the likelihood of galvanic corrosion is higher for a titanium alloy washer than for a CP titanium washer. Future studies should test the corrosion performances of titanium alloy and CP titanium washers and assess the harmfulness of galvanic corrosion in the oral cavity.
Including a washer between the titanium screw and abutment increased the removal torque in this study, which corresponds to an increase in the rotational displacement required to completely loosen the retaining screw. The inclusion of a washer maintained the preload responsible for the clamping force for longer period in this study and meant that the screw could withstand deformation without loosening. A washer is an elastic ring with a relatively low stiffness, and its use increases the elastic energy without an increase in the preload value. The use of a washer can slow the rate at which the clamping force decreases.
A thicker washer has a larger elastic energy, but its use also decreases the length of the engaged thread of the retaining screw, which reduces the clamping force. Therefore, the thickness of the washer used in the clinical situation should be chosen such that it does not markedly decrease the length of the engaged thread of the retaining screw. Considering that a 1.0-mm-thick washer is more difficult to fabricate and less effective, a 2.0-mm-thick washer can be recommended for clinical applications. Thicker washers could also be used if the length of the engaged thread remains the same, such as by using a retaining screw with a longer thread.
Washers made from titanium alloy and CP titanium were used in this study. Titanium alloy has a higher modulus of elasticity (ie, greater stiffness) than CP titanium, and so the removal torque was increased more when using a titanium alloy washer than when using a CP titanium alloy washer due to the difference in the stored strain energy.
The surface texture of a washer is also an important factor for preventing screw loosening, with a rougher surface exhibiting superior performance. Moreover, when torque is applied to screws and bolts with rough-textured thread surfaces, some of the applied energy is spent making the mating surfaces smoother. Therefore, friction in the thread will decrease during repeated tightening and loosening. A future study should evaluate how various surface textures of the washer and the screw affect the clamping force.
This study has shown that the removal torque of an abutment screw in the external connection type of dental implant is higher (1) when a washer is included between the titanium screw and abutment, (2) for a CP titanium washer than for a titanium alloy washer, and (3) for a 2.0-mm-thick washer than for a 1.0-mm-thick washer. Moreover, repeatedly tightening and releasing the titanium screw adversely affects the removal torque.
Otgonbold Jamiyandorj and Sungtae Kim contributed equally to this study.
This study was supported by a faculty research grant of Yonsei University College of Dentistry for 2010 (grant number: 6-2010-0096).
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The authors claim to have no financial interest in any company or any of the products mentioned in this article.