In this retrospective study, the bone level changes related to different insertion torque levels of dental implants were investigated. The measurements of bone level changes were taken at day of second stage surgery, 12–15 weeks after placement. At that point, the bony healing is completed and due to the submerged healing, any other influences than the surgical trauma due to implant placement itself could be excluded. Bone level changes at 3–12 months after implant placement was defined as early bone loss (EBL) by several authors and found to be crucial for long term implant survival [11, 27, 29]. The results in the presented study indicate that the mean marginal bone loss was minimal within the first three months of healing (mean EBL 0.25 ± 0.28 mm) and no significant differences could be observed between the high and low ITV and bone quality 1–4; therefore, the null hypothesis cannot be rejected.

Complete bony healing after tooth extraction takes 8–12 weeks; up to 90% of the crestal bone changes and bone resorption occurs in the first 12 weeks post extractionem and the same principles apply to bony healing after placing implants [10, 25]. For this reason, in the presented study implants were placed in cases with more than 12 weeks post extractionem and the implants healed 12–15 weeks before second stage surgery. The relationship between EBL exposing implant threads and consequently the risk of development of periimplantitis is a well-documented problem [11, 24, 27]. A recent 10-year prospective cohort study including some 1400 implants suggests that EBL is a predictor for long-term peri implant pathology, with a significant higher risk for periimplantitis when EBL exceeds the thresholds of 0.5 mm and 1 mm [29]. In the presented study, no significant differences in EBL were found neither regarding ITV nor ISQ and the mean EBL detected was below the thresholds defined as critical by Windael et al. [29]. These results are of major importance regarding the future implant survival and highlights the necessity of comprehending the precise factors contributing to primary stability, including its mechanism of attainment.

Primary stability of a dental implant is a prerequisite for immediate loading. Any micromotion of the implant beyond a certain threshold can impede osseointegration and ultimately result in implant failure [15].

There is a tendency for using high ITV in order to achieve high primary stability in current literature [13, 18]. From a clinical point of view, it has been made clear that high primary stability is mandatory for immediate loading, especially for single units [6, 14, 16, 26]. Carr et al. [8] have found an increased risk for clinical failure when using an ITV of less than 30 Ncm and two meta-analysis studies have found no indications for higher bone loss or complication rates when using high ITVs [5, 12].

On the other hand, there is strong evidence that excessive intraosseous tissue pressure around an implant as caused by high ITVs can induce bone damage through microfractures and cellular apoptosis. This process can trigger excessive resorption and remodeling of the healing bone, ultimately culminating in avascular necrosis [9, 26]. Until today, the literature is inconsistent regarding the question whether using high ITVs will increase EBL or lead to higher implant failure rates [4, 5, 9, 12, 13, 18, 26]. According to mathematical models, there is a linear correlation between ITV and bone compression [21]. Therefore, it is important to understand the implant design and how the forces on the bone-to-implant contact are distributed. In the presented study, the use of a finite element simulation with the herein used implants indicates that the highest pressure rates are occurring at the implant apex, whereas the crestal part of the implant distributes only minor forces to the surrounding bone, regardless of the insertion torque used (Figs. 6 and 7). This could explain why the EBL shows no statistically significant difference between high and regular torque group in the presented study.

The ITV and/or ISQ are values taken for clinically assessing the primary stability of a dental implant [3, 20, 22, 26]. ITV is measured at the time of implant placement as a single measurement. ISQ measurements can be repeated over time during the healing process and therefore can be used to monitor the bone healing [20, 26]. Several publications emphasized the importance of conducting repeated measurements, as implant stability tends to decrease during the healing process owing to resorptive phenomena in the healing bone (weakest point around week 3–4). The final stability is reached after 8–12 weeks [19, 21, 23]. The use of ITV as only measurement to define primary stability can be misleading, as a high ITV does not necessarily correlate with a high ISQ value. ITV is a measurement that defines rotational stability as caused by mechanical friction of the implant in the bone, whereas RFA analysis giving the ISQ values is measuring the lateral stiffness of the bone-to-implant interface and the stiffness of surrounding bone. In the presented study, there was only limited and moderate linear correlation between ITV and ISQ which is in accordance with existing literature [19, 26], whereas the bone density classes 1–4 according to Lekholm and Zarb [17] are correlating significantly both to ITV and ISQ. This again is in accordance with literature [3, 22, 28]. The correlation coefficient could potentially be higher if bone density had been assessed using an objective bone density assessment calculated from CBCTs [28], rather than the subjective assessment according to Lekholm and Zarb [17].

The categorisation of the bone qualities found in the clinical cases into classes 1–4 as described by Lekholm and Zarb [17] is a subjective clinically judgement of the surgeon. In the presented single center study, all surgical procedures were performed by the same skilled clinician. This eliminates a possible bias as would have been created by different clinicians with different clinical experience.

In current publications, a great variety of ITVs varying from < 10 to > 100 Ncm are described. Based on the systematic review by Lemos et al. [18] and Atieh et al. [2], the dividing criteria between high and regular torque groups was defined as 50 Ncm. To avoid possible damage either to the implant´s inner connection or to the bone by application of excessive force, a limit of 80 Ncm was set for all surgeries.

Other limitations are the retrospective character of the study and the imbalance of the study groups. However, the bone quality and associated insertion torque during surgery as patient individual parameters seem to be representative from experience.