The primary stability of an implant is closely related to treatment prognosis [1,2]. Currently, this primary stability is evaluated using implant stability coefficients, that consider factors such as implant stability quotient (ISQ) and insertion torque (IT) during implant placement [3,4]. The ISQ was measured by attaching a smart peg to the implant, vibrating it with a magnetic pulse, and measuring the resonance frequency of the smart peg. The obtained frequency was converted into the ISQ using a special formula and was expressed as numbers from 1 to 100. It is easy to operate and is used clinically to evaluate implant stability. IT value is measured by the implant micromotor and the evaluation is usually performed by the peak value. In practice, IT is generated over time, and the sum of these torque values is loaded onto the bone surrounding the implant. For this reason, some studies have evaluated the primary stability as the integral value of torque-depth curve or the insertion energy during implant insertion [5-8]. The insertion energy, as evaluated in these reports, is mainly expressed as the integral value of the torque-depth curve and is evaluated as a measure of primary stability. It has been shown to correlate with the immediate bone-to-implant contact. Some surgical motor systems display this torque-depth curve and are occasionally used in clinical practice to evaluate primary stability. However, the implant length, a key parameter in insertion energy, is often evaluated by converting it into the speed of motor rotations. This has the disadvantage that the calculated implant length and actual implant length may differ due to operator variability and differences in bone condition. In this study, torque values were measured at short time intervals, to the nearest 0.1 Ncm, to better account for variables such as implant shape, length, and bone condition. The sum of these IT values, measured at 0.05 second intervals, was used to evaluate the accumulated torque value (ATV).

The American Society for Testing and Materials (ASTM) has approved artificial polyurethane bone blocks as standard material for testing orthopaedic devices and instruments and has declared it an ideal material for comparative testing of bone screws (ASTM F-1839-08) [9]. Corresponding to the different bone densities specified above, the densities of polyurethane bone blocks are also classified into four types [10-12]. According to the Misch bone density classification as D1 = 0.48 g/cc, D2 = 0.32 g/cc, D3 = 0.16 g/cc, and D4 = 0.08 g/cc [10-12]. Several studies have demonstrated that IT and ISQ are correlated with the bone strength or density of the polyurethane bone blocks [5,13-15].

The purpose of this preliminary in vitro study was to determine the usefulness of the accumulated torque value as a novel method for evaluating primary stability by the correlation between the accumulated torque value and the implant stability quotient value.

Materials

Solid rigid polyurethane-simulated bone blocks of different densities (0.48, 0.32, 0.16, and 0.08 g/cc) and size of 13 cm × 18 cm × 4 cm (SAW 1522-23/01/03/04; Sawbones^® - Pacific Research Laboratories, Inc.; Vashon Island, Washington, USA) were used.

Additionally, two different sizes of titanium alloy implant bodies were prepared: 42-10ST (diameter 4.2 mm × length 10 mm, straight-shape, mechanically polished surface - Kyocera Corp.; Kyoto, Japan) and 37-6ST (diameter 3.7 mm × length 6 mm, straight-shape, mechanically polished surface - Kyocera Corp.) (Figure 1). Surgical Pro2 (NSK-Nakanishi International; Kanuma, Tochigi, Japan) was used as the implant motor system.

Methods

The study was conducted at the Department of Advanced Prosthodontics, Hiroshima University Graduate School of Biomedical and Health Sciences, Hiroshima, Japan, between November 13, 2024 to November 22, 2024. An implant osteotomy was prepared in each simulated bone block at 2 cm intervals, and the implants were inserted (n = 10). The implant osteotomies were prepared as follows: round drill, pilot drill, and final drill with a diameter 3.4 mm for the diameter 3.7 implant and diameter 3.9 mm drill (Kyocera Corp.) for the diameter 4.2 implant (1200 rpm, according to the manufacturer’s specified drilling protocol; www.kyocera.co.jp). Implants were inserted at 35 rpm. During insertion, the implant IT value was measured at 0.05 seconds intervals. The data were output from the implant motor system (Surgic Pro2 - NSK-Nakanishi International) to an offline personal computer as an Microsoft Excel file (Microsoft Office Standard 2013 - Microsoft Corp.; Redmond, Washington, USA). The peak value was recorded as the peak torque value (PTV), and the sum of the torque values measured from the start to the end of implant insertion was recorded as the ATV (Figure 2). After implant placement, the ISQ was measured three times along each of the long and short axes, and the average value was recorded 100500 Osstell^® ISQ (Osstell AB; Gothenburg, Sweden).

Statistical analysis

Pearson’s correlation coefficient (r) was used to evaluate the correlation between ATV-PTV and PTV-ISQ. The ATV, PTV, and ISQ values for each density were compared using t-tests. Data were expressed as mean and standard deviation (M [SD]). Statistical significance level was defined at P < 0.05. The calculations were performed using GraphPad Prism version 7.0 software (GraphPad Software Inc.; La Jolla, CA, USA).

Colour maps of implantation torque values

The implantation torque values were recorded as shown in Figure 3. The higher the density of the bone block, the larger the colour map displayed. Also the larger the implant size, the larger the colour map was displayed. The vertical axis of the graph is the virtual implant length calculated from the rotation speed and time. Although all implants were inserted at 35 rotations, the time required for insertion was not constant, and it was shorter at D4.

In the initial stage of implant placement, the resistance is weak, and the torque value is low, and the resistance gradually increases as the contact area with the bone block increases. The PTV is affected by the block density. The block had a uniform structure, the PTV was recorded when the implant was placed at the deepest depth and the contact area between the block and the implant was at its maximum.

Implants were inserted at 35 rpm, however the time required for insertion was not constant, depending on the bone block density. The implant length derived from the rotational speed and time differed from the actual length of the implanted fixture. In D4 blocks, the insertion process proceeded easily, resulting in a recorded length shorter than the actual depth. Conversely, in D1 blocks with high density and hardness, it had high resistance and, leading to potential rotation without insertion. Thus, implant length derived from rotational speed and insertion time was not precise and may introduce errors in the integration value of torque-depth curves when adjusting insertion depth.

Comparison of measurements between implants

The ISQ of 42-10ST was D1 = 73.9 (1.3), D2 = 65.3 (1.3), D3 = 45.6 (2), D4 = 30.6 (1.5), that of 37-6ST was D1 = 67.5 (1.6), D2 = 54.6 (1.4), D3 = 30.2 (0.7), D4 = 10.6 (2.1), and it was higher in 42-10ST than 37-6ST in each block. The PTV of 42-10ST was D1 = 30.6 (2.6) Ncm, D2 = 9.4 (0.6) Ncm, D3 = 1.5 (0.2) Ncm, D4 = 0.9 (0.1) Ncm, that of 37-6ST was D1 = 26.6 (1.7) Ncm, D2 = 6.9 (0.5) Ncm, D3 = 1.1 (0.1) Ncm, D4 = 0.3 (0.1) Ncm, and it was higher in 42-10ST than 37-6ST in each block. The ATV of 42-10ST was D1 = 4479 (369) Ncm, D2 = 1163 (107) Ncm, D3 = 147.5 (46.7) Ncm, D4 = 76.5 (27.8) Ncm, that of 37-6ST was D1 = 1850 (168) Ncm, D2 = 441.3 (44.5) Ncm, D3 = 54.1 (12.8) Ncm, D4 = 11.7 (11.6) Ncm, and it was higher in 42-10ST than 37-6ST in each block (Table 1).

Correlation

The ATV increased with the density of the simulated bone block. A strong correlation was observed between the PTV and ATV values in both implants (42-10ST [P < 0.05, r = 0.99], 37-6ST [P < 0.05, r = 0.99]) (Figure 4). A correlation was observed between ATV and ISQ (42-10ST [P < 0.05, r = 0.81], 37-6ST [P < 0.05, r = 0.83]) (Figure 5).

This study focused on the IT value as an evaluation of the primary stability of implants and investigated a new evaluation method based on the IT value over time using simulated bone blocks. Insufficient initial fixation may result in a longer healing time or an increased risk of implant disintegration. However, excessively high torque can lead to stress concentration, which may not always produce good outcomes [16-21]. Excessive stress during implant insertion may cause bone resorption, and high IT does not always guarantee positive clinical results [17].

Duyck et al. [22] observed that a high IT of > 45 N increased the risk of bone microfractures, resulting in a greater bone resorption response at the molecular and cellular levels, leading to a significant loss of bone stability within the first stage of healing. Therefore, in implant treatment, it is important to select an appropriate drilling protocol based on bone condition [23].

However, these reports were based on PTV values. The PTV is an evaluation of the value cut-off at a certain moment of the torque usually recorded at the final moment of insertion. Therefore, it did not evaluate all the stresses applied to the bone. The stress applied to the bone during implantation can be comprehensively evaluated by accumulating the IT values applied over time.

In this study, we used simulated bone blocks with a structure like that of cancellous bones. These bone blocks are classified according to Misch classification, and these blocks also showed a significant difference in torque value in our drilling evaluation [14,15]. Implants with different diameters and lengths were examined, implants with larger diameters and lengths exhibited higher ISQ, PTV, and ATV. Furthermore, among the same implants, the higher the block density, the higher the values for all parameters. The PTV and ISQ values were consistent with those in previous reports and correlated well with each other [24,25]. Also, the ATV had a strong correlation between PTV and ISQ. This was because the simulated bone block had a uniform structure.

The ISQ levels of the successfully integrated implants ranged from 57 to 82 [26]. The ISQ value is an evaluation that is influenced by the entire surrounding bone that supports the implant body. A correlation was observed between the ISQ value and the ATV; the higher the integrated torque value, the higher the ISQ value. Therefore, the ATV may also be used to evaluate the entire stress applied around the implant.

For the uniformly structured simulated bone blocks used in this study, a strong correlation was observed between the ATV and PTV. However, in situations where bone density is not uniform, such as in human bones, it might be suggested that evaluation of the ATV may be useful. ATV offers an advantage as it remains unaffected by variations in insertion time or depth changes, allowing for accurate measurement. Currently, as implant bodies with various diameters and lengths are used, the evaluation of ATV may provide a more detailed evaluation of primary implant stability.

Limitations

This study has several limitations:

The accumulated torque value changed according to the density of the simulated bone block for different-sized implants, and a correlation was observed with existing implant stability evaluations. This novel method may be useful for evaluating the stability of primary implants.