Preload and friction in an implant–abutment–screw complex including a carbon-coated titanium alloy abutment screw: an in vitro study

The load frame device

A custom load assembly (Figs. 1, 2) was used to measure the tension within the abutment screw when it was tightened. One essential requirement for this measuring method is that the two parts, which are screwed together, must not come in contact with each other. For that, a gap of 0.10 mm between the abutment and the implant was assured at the end of the tightening procedure.

Fig. 1figure 1

Photography of the measuring station

Fig. 2figure 2

Diagram of the measurement device

The upper part was constructed to hold and position the implant components and measure the preload as well as the thread-friction component of the tightening torque. With the lower part, the tightening torque was reproducibly generated.

The upper part was composed of an immovable frame with two horizontal plates rigidly held together by three side columns (Fig. 2, red frame). In this frame, a free-rotating second frame with two horizontal plates and three side columns was centrically pivoted (Fig. 2, black frame). The implant was fixed at the lower tip of the second frame. With a third frame, made of two platforms and two side columns (Fig. 2, blue frame), the second frame was mounted on a planar beam load cell (PB-75 kg-C3; Flintec, Meckesheim, Germany), which was located at the top of the device. The load cell had a load range of 750 N, and the measuring path at nominal load was 0.35 mm.

When torque was applied to the implant, the third frame loaded the load cell on the top of the device. The freely rotating second frame with the fixed implant was countered against another load cell (Single Point Load Cell 1002-K-Z, Soemer, Lennestadt, Germany) located at the bottom of the device. This load cell had a load range of 150 N, and the measuring path at nominal load was 0.4 mm. The moment of torque generated by the second frame was proportional to the thread-friction component of the implant–abutment–screw complex and to the force needed to overcome the threads’ friction. The abutment was mounted underneath the immovable frame. In the lower part of the device, the tightening torque was delivered steadily by a weight, which pulled at a disc and hereby made the disc rotate. To secure that the disc exerted accurate and reproducible torque values, the weight was dipped into a water-filled tube, and via a cord, over a pulley, the disc was rotated. During the assembly and the manufacturing, it was ensured that all parts were in line with the central loading axis.

The device’s base was a 20-mm-thick aluminum plate. All other components of the load frame were made of steel (E295, according to EN 10,027-1).

The load cells were connected with two load cell digitizing units (LDU 68.1, Hauch & Bach, Lynge, Denmark). These measuring amplifiers communicated via a RS 422/485 full duplex interface with the computer and the corresponding analyzing software (DOP 2.06, Hauch & Bach, Lynge, Denmark). With this application, the load cell digital amplifier devices were calibrated. The application could monitor values in real-time saving the measured data for a previously set interval and duration (here, 0.022-s intervals, 60-s duration).

Test protocol

25 unused titanium implant–abutment–screw units were tested, each with an implant (Replace Select Tapered implants, Nobel Biocare AG, Zürich, Switzerland), an abutment (Temporary Abutment Non-engaging, Nobel Biocare AG, Zürich, Switzerland) and a carbon-coated abutment screw (Abutment Screw Nobel Replace, Nobel Biocare AG, Zürich, Switzerland). The tests were conducted under dry conditions without lubrication. The abutment screws were torqued ten times to 25 Ncm, maintaining the torque for 60 s before loosening. The produced preload values and the force proportional to the thread-friction component were recorded.

Calculation of the coefficient of friction and the thread-friction component

According to the guideline VDI 2230 Part 1 (Verein Deutscher Ingenieure, Systematic calculation of highly stressed bolted joints—joints with one cylindrical bolt), the mathematic formula (Fig. 3) describing the relationship between the applied torque MA and the preload force FVM incorporates geometric data of the screw connection and the coefficient of friction [5].

Fig. 3figure 3

Formula describing the relationship between torque and preload

Since the introduced force was known and the preload was measured, the coefficient of friction could be calculated by solving the before-mentioned formula for it (Fig. 4).

Fig. 4figure 4

Formula for the coefficient of friction

The geometric data of the investigated screw connection were measured in SEM photomicrographs (Zeiss DSM 962 SE, Zeiss, Oberkochen, Germany) with a 50× magnification.

The load cell at the bottom of the device was used to register a force, which was proportional to the thread-friction component of the implant–abutment–screw complex. The measured force could then be converted to the moment of torque, employing the known length of the corresponding lever arm (11 cm).

Statistical analysis

The statistical software SPSS 23.0 (IBM, Chicago, USA) was used for statistical data analysis. The preload and thread-friction values were analyzed using a linear mixed model with repetition as a fixed effect and the objects as a random effect, thereby taking repeated measurements on each object into account. The significance level was chosen as α = 0.05 for each of these two parameters.

When a significant result by the global F-test occurred, the Tukey–Kramer test was used for the pairwise evaluation of two repetitions.

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