In Vitro Evaluation of the Removal Force of Abutments in Frictional Dental Implants

The development of modern Oral Implantology is the result of concepts added to original ideas proposed by Brånemark, but the main definition of osseointegration has been preserved.1 A great challenge of Implantology is the connection between the implant and the prosthesis (abutment). Screw-retained abutments are widely used in commercial implant systems; however, they are known to present technical complications, such as smallness of screws and the need for repeated torques due to screw loosening. In addition, the formation of a gap between implant and abutment may allow the infiltration of fluids and bacteria through the implant-abutment interface.2 

Alternatives to screw-retained abutments are currently available. In a screwless implant system, the abutment is retained by means of friction force. Some examples use taper angle values from 1 to 10 degrees.3 Intimate contact between the components provides great frictional force and retention. Frictional systems present some advantages, starting with ease of handling (no tiny parts and screws), ease of abutment preparation in intraoral adjustments, and hermetic sealing.3–6 

In screw-retained systems, connection of the components is achieved by means of known rotational force (torque) during installation.7 Although long-time clinical effectiveness has been demonstrated,4,7 doubts have been raised regarding quantification of forces applied during installation of frictional systems. The importance of this matter becomes more evident with consideration that in some cases, it may be necessary to reverse the applied connection. Thus, the objective of this in vitro study was to determine some force parameters for removal of the union of an abutment and an implant in a locking taper system with a 1.23 degree angle.

Ten Ti implants of 11 mm in length and different diameters and 10 straight abutments of 13 mm in length and different diameters (Kopp, Curitiba, Brazil) were selected (Figure 1). The implants and the abutments were made of titanium alloys (ASTM F67 and ASTM F136, respectively), with hardness values of 20 HRC and 29 HRC. According to the manufacturer, all surfaces of the implants and abutments were machined in a Swiss lathe using hard metal inserts. The internal calibration was verified by standards for Morse cone confirmation. One by one, abutments and implants, available in separate packages, were passively mounted together by a single operator prior to the application of force, with rounding up to a total of 10 implant-abutment mounts (IA), as described in Table 1. After attachment, the IA mounts were placed on an adapted support base for compression. The instrument used for delivery of compression force (BCP, Kopp, Curitiba, Brazil), shown in Figure 2, was placed on another support base (7011SN, Mitutoyo, Sao Paulo, Brazil) following the long axis of the IA mounts. Fixation of IA was performed by the impact of a body weight (m  =  0.05 kg), left from a known height (h  =  0.065 m), limited by the instrument. The potential energy (Ep) provided by each tapping was calculated by equation 1, where g is the gravitational force.

The weights of the abutments were determined by a semianalytical scale (Mark 500, Bel, Denver, Colo). The resultant potential energy was 3.18 × 10⁻² J (N.m). The removal (separation of abutment from implant) force was determined after 1, 2, 3, 4, and 5 tappings performed for each IA mount. Force values were determined with the aid of a universal testing machine (DL 30000, EMIC, Sao Jose dos Pinhais, Brazil) at a cross-head speed of 3 mm/min.

Figure 3 shows removal force values as a function of the number of tappings. It can be seen that the higher the number of tappings, the greater the removal force, although a few exceptions were found, such as mounts #7 and #10. Figure 4 shows a positive correlation between removal forces and the increasing masses of the abutments.

Table 2 presents minimum, mean, and maximum removal forces as a function of the number of tappings. For all groups, the lowest removal force was found for mount #2 after a single tapping only, while the highest value occurred in mount #8, after the fifth tapping. Removal forces tended to increase as the number of tappings increased.

Removal of the abutment in frictional implants may be of greater concern in esthetic areas, where the clinician tends to verify the position of the IA mount visually. In this implant system, the abutment itself is used as transfer for the laboratory, which means that it has to be at the right level during the impression. It has been demonstrated that tapping can intrude abutment as far as 0.1 mm.3 Nevertheless, until occlusal balance is achieved, it is possible to have the IA disconnected.

The present study confirmed a positive correlation between the number of tappings and the removal forces. Figure 3 and Table 2 show that for almost all groups, the removal force increased as tappings increased. Removal mean values were increased after 1 (111.4 N) and up to 5 tappings (294.6 N). After the second tapping, the mean removal force value increased by 53.2%; the same can be seen in subsequent tappings, but after the third tapping, removal force values increased by 40%. The fourth and fifth tappings increased the removal force value, but with less intensity, respectively, at 8.4% and 14.3%. This can be explained by increasing applied forces on the abutment, as follows: the higher the applied forces (higher number of tappings), the greater will be the intrusion of the abutment in the implant. This intimate contact created by the intrusion can make the 2 separate parts work physically as a unit, as a single body. Thus, this created unity can be interesting in many clinical situations, particularly during the stress distribution of masticatory forces (Figures 5 and 6).

This study showed some coincident data points, whereby removal force values after subsequent tappings demonstrated little difference (8 in total of 50 tests) for the same implant-abutment mount, and for a specimen (IA mount #10) in which the third tapping generated a larger force than the fourth and fifth—an unexpected result. This fact can be related to some possible alterations at the moment of application of the compression force, caused by (1) little variation in IA mount positioning during tapping, which might have generated small distortions of parallelism, and/or (2) adaptation of contacts among the internal surfaces of the implant (well) and the external surfaces of the abutment. In addition, the possibility of small irregularities in such surfaces cannot be discarded. Continuing studies with increased repetitions are necessary to clarify this issue.

Although a small number of specimens were tested, thus impairing statistical analysis, a strong tendency of correlation between mass of the abutment and removal force can be seen in Figure 4, because during activation, the mass of the abutment was incorporated to the inward movement, thereby increasing potential energy provided by the system.

Regarding actual forces applied during mastication, it is known that variations occur among individuals, but the dental literature reports high values for men (between 536 and 644 N) and lower values for women (between 358 and 449 N).8 It is important to note that these values disregard parafunctional habits, such as bruxism. Regular masticatory movements are characterized by higher compressive forces, responsible for trituration of foods, when compared with tensile forces, produced by the retention of food on the occlusal surface of the teeth and by the presence of lateral excursive movements. Within the limitations of this study, it can be concluded that the higher the number of tappings, the greater is the force needed to separate the implant-abutment mount. In the present work design, 3 activations (tappings) of the abutment were considered necessary to attain optimal stability, demonstrated by the large increase in removal force.