2.1 Participants
Torque wrenches are broadly classified into two types: mechanical and digital; the former is further classified into the beam and preset types. Considering the global market share, the following eight beam types were selected: Ratchet (Institut Straumann Ag, Basel, Switzerland); Manual Torque Wrench Prosthetic (Nobel Biocare, Zürich-Frughafen, Switzerland); Ex Torque Wrench (Kyocera Medical Corporation, Osaka, Japan); GC Implant Re and Surgical Instrument Torque Wrench (Gc, Tokyo, Japan); Torque Ratchet Wrench (Ktc, Kyoto, Japan); Mono torque ratchet (Thommen, Grenchen, Switzerland); Torque wrench (Nippon Piston Ring Co, Saitama, Japan); and Biofix Torque wrench (Shofu, Kyoto, Japan) (Figure 1).
3.2 Measurement device
A screwdriver (Screwdriver Machine Unigrip 20 mm, Nobel Biocare, Japan) and a torque gauge (BTG36CN, Tohnichi, Japan) were fixed (Figure 2), and the torque values exerted by each torque wrench (actual measured torque values) were measured using the Latin square design.
3.2 Measurement of the torque value
The same examiner, who was experienced in implant treatment, applied the recommended torque value of each manufacturer’s prosthetic screws (target torque value) five times clockwise (Table 1). For investigating the influence of the location of the beam placed on the scale, the measurement was performed with the scale aligned with the upper edge, center, and lower edge of the beam (Figure 3). Additionally, measurements were recorded at 90°, 60°, and 30° to examine the effect of the angle at which the examiner read the torque value (Figure 4). The average of the five measured torque values (average measured torque value) was calculated and recorded. The bias, which was the difference between the average measured and target torque values divided by the target torque value, was used as an index of accuracy. The coefficient of variation, which was the standard deviation of the measured torque value divided by the average measured torque value, was used as an index of repeatability.
Additionally, we clarified whether the influence of the part of the beam to be adjusted to the scale and the influence of the angle at which the examiner read the torque value were related to the structure of the torque wrench. Thus, the torque value per mm of the scale, the width of the scale line, the width of the beam, and the distance between the scale and the center of the beam were measured and compared with the bias and coefficient of variation.
As a statistical method, a paired t-test was performed with Bonferroni correction. In terms of accuracy, depending on the part of the beam, the calculated bias of the lower edge, center, and upper edge of the beam was used as the dependent variable. In terms of repeatability by beam site, the coefficient of variation of the lower edge, center, and upper edge of the beam was used as the dependent variable. In terms of accuracy depending on the angle at which the examiner reads the torque value, deviations of 90°, 60°, and 30° were used as dependent variables. In terms of repeatability depending on the angle at which the examiner read the torque value, deviations of 90°, 60°, and 30° were used as dependent variables. The significance level was set at 5%. Additionally, the four items of difference in torque value between the lower edge and upper edge, coefficient of variation in the center, the difference in torque value between 90° and 60°, and coefficient of variation when viewed from 60° were set as the dependent variables. Pearson’s correlation coefficient was calculated between the dependent variable and the following four items: torque value per mm of scale, beamwidth, scale line width, and distance between scale and beam center. The significance level was set at 5% (Tables 2, 3, 4, 5).
IBM SPSS Statistics 25.0 (IBM, Chicago, USA) was used for statistical processing.