Dental implants are considered as a treatment for completely or partially edentulous patients, with a success rate of 98%1. Successful treatment depends on patient-related parameters and the surgical procedure2. Preoperative assessment of available bone and its quality affects the selection of surgical technique and implant site and design, which will improve the success rate of implantation34. Some authors have attributed the higher success of mandibular implants to its better bone quality5. Bone density relates to its biomechanical features, which depend on many other factors including mineralization and structure. Many classifications have been suggested for bone density6, and the most common one is a 4-type scale proposed by Lekholm-Zarb classification. This assortment is based on radiographic evaluation and bone resistance assessed by the surgeon while drilling7. Implant stability is affected by bone quality and quantity as well as implant design and operative technique2. The authors suggest that implant stability is one of the important prerequisites for osteointegration89. Different methods have been proposed for evaluation of implant stability. Although percussion and radiographic assessment are commonly used, their results are not always reliable even in cases of fibrous encapsulation10. Periotest values and resonance frequency analysis (RFA) respectively measured by periotest and Osstell device are additional methods for evaluation of implant stability11.

Studies have shown that measurement of implant stability with RFA is reliable, noninvasive, and can be used at any time after implant insertion12. The aim of this study was to compare primary and secondary stability measured by RFA between different bone types and to compare implant stability at different time points during 3 months of follow-up.

This study enrolled 59 patients (29 women and 30 men) with an age range of 35 to 78 years (mean, 54±12 years). In these patients, 65 implants were inserted in different regions. According to Lekholm-Zarb classification, patients were categorized into 4 types: Types I to IV had 9, 18, 20, and 12 patients, respectively. In our study, 3 implants failed (1 in type I and 2 in type IV). At implant installation, the mean ISQ value was 77.21±4.25 in type I, 74.40±2.12 in type II, 76.61±3.45 in type III, and 73.50±4.43 in type IV.(Table 1) One month after implant insertion, the mean ISQ values were 68.60±2.13, 70.50±2.78, 72.74±2.45, and 69.13±2.65 in types I to IV, respectively.(Table 2) The mean ISQ values at the 3-month follow-up were 74.23±2.54 in type I, 76.10±3.23 in type II, 78.70±3.75 in type III, and 73.46±3.32 in type IV.(Table 3) According to statistical analysis, these values were not significantly different at any time point (P>0.05). The implant stability of each bone type at baseline, 1 month, and 3 months after insertion were compared and showed no statistically significant difference (P>0.05).(Fig. 2)

Implant stability is one of the most important factors for successful implant treatment and seems to be crucial for osteointegration, especially for immediate loading. Primary stability is a mechanical phenomenon, while secondary stability is the result of a biological event (osteointegration)13. Studies have shown that factors including bone density, implant size and design, and operative technique may influence implant primary stability14. On the other hand, secondary stability depends on ossification around the implant fixture15. According to our results, for all bone types, stability values decreased during the first month and increased in the following two months. However, these changes were not statistically significant.

Many authors have described changes of implant stability over time. Most of these studies showed a similar pattern of consecutive reduction and increase in ISQ value. There remains debate on the timing of stability changes. Studies have reported an ISQ reduction period from the 1st to 8th week after implant installation161718. According to Simunek et al.17, only implants with low primary stability showed increase of ISQ during the healing period, while implants with high primary stability showed reduction of stability values. Martinez et al.19 showed that primary stability was different among various bone densities, but secondary stability was similar. Farré-Pagés et al.1 found a significant relationship between bone density according to Lekholm-Zarb classification and ISQ value; as a result, implant stability in type I was significantly higher than other types. Turkyilmaz et al.20, Ikumi and Tsutsumi21, and Friberg et al.22 showed a significant relationship between bone density and primary and secondary stability. The results of studies have not been consistent about the relationship among bone density and primary and secondary stability. For example, in a study by Beer et al.2, there was no statistically significant relationship between bone density and primary stability. In another study, Simunek et al.17 could not show a significant relationship between bone type and primary stability or primary and secondary stability. Similarly, in the present study, primary and secondary stability were not significantly difference among various bone types. A recent review article reported that results on the relationship between bone density and implant stability were not consistent, and this controversy could be attributed to diverse methodology of studies, different descriptions and evaluation of bone quality, and different methods of stability assessment23. Various implant sizes and designs could be another source of difference, as implants with greater diameter have higher primary stability due to additional bone-to-implant contact (BIC). Also, tapered implants have higher primary stability because of greater diameter at the crestal region19. Implant surface modification is another possible source of difference and may have an effect on osteointegration and BIC24. Although Han et al.18 did not find a relationship of implant surface modification and diameter with ISQ value. Regardless of the relationship between density and primary and secondary stability, it seems that implants with primary stability lower than a critical limit cannot achieve secondary stability. Different critical limits have been proposed, although ISQ <50 is considered an acceptable limit for primary stability, and successful integration could be possible in this group. Implants with ISQ >60 had more chance of successful osteointegration2526. In our study, failure was seen in all three implants with primary stability of ISQ <50.

According to our results, stability decreased during the first month and increased in the following two months for all bone types. These changes were not statistically significant. The ISQ values of all bone types were not statistically significant different at each time point. Studies have not shown consistent results on the relationship between bone density and implant stability. Due to various methodologies for stability measurement, density classification, different follow-up times, and diverse implant surface and design, it is very challenging to reach a conclusion about the importance of stability in osteointegration. The effects of other factors, especially implant surface and design and surgical technique, on the relationship between primary stability and osteointegration should be evaluated in future studies.