The lower resistance to fracture in the IP group of narrow platform titanium implants with an external hexagonal connection was significant only in the 2.5:1 CIR group. Besides, the mean total values show no significant difference in fracture resistance between the control and test samples (Table 2). CIR seems to be a much more relevant variable than IP since both the IP and the control implants show significant reductions in mechanical resistance in the 2.5:1 and 3:1 CIR groups when compared to the 2:1 (Table 3). Indeed, while IP reduced the mean fracture strength by 132.87 N, a higher CIR (2.5:1 or 3:1) led to a mean difference of 525.48 N or 707.68 N, respectively. These results are relevant since clinicians must be aware that performing IP in narrow diameter implants with an unfavorable CIR might lead to fractures.
In order to increase the external validity of the study, the pressure, time and number of strokes needed to perform the IP were not standardized.
Surface roughness was not an outcome in this study and has not been addressed, although the IP test and control surfaces were recorded with SEM (Figs. 6e, 6f respectively). Using the same IP protocol, significant differences were reported between the test and control surfaces, with a mean height of 0.1 µm (σ = 0.02) for tests and 0.76 µm (σ = 0.08) for controls.
With no surprise, significant differences due to the IP procedure were observed at all the reference points in the macroscopic analysis and there was no perforation of the inner threads. The implant diameter at each of the 3 reference points showed a reduction that ranged from 0.37 mm (95% CI: 0.33 to 0.40 mm) to 0.46 mm (95% CI: 0.42 to 0.49 mm) in the IP test samples. Other authors with similar IP protocols have reported lower reductions. These discrepancies might be explained by differences in the degree of polishing, but are more likely to be the result of different implant geometry, namely thread depth and model. Thus, further studies with different implants should be carried out, since their design and material are likely to affect the implant’s resistance to fracture.
A similar amount of change was found at each reference point (P > 0.05 in all cases; one-way ANOVA), regardless of the crown length group of the implant, thus showing that the implantoplasty was similar across all these groups. These seem to show the procedure to be easy to reproduce.
Previous reports have claimed that implant diameter affects stress fatigue behavior and that when subjected to IP, dental implants will attain a critical stress point at lower loadings[16, 27, 28]. The present results corroborate this finding, as lower resistance to fracture was observed in the IP groups (Table 2). All the IP groups showed less resistance to fracture than the control groups, but this difference proved to be significant only for one of the CIR groups (2.5:1). Hence, narrow platform implants seem to be structurally weakened by IP procedures, although the most relevant risk factor for mechanical complications in the presence of 50% vertical bone loss seems to be CIR, as the mean fracture resistance values dropped to almost half from CIR 2:1 to 2.5:1 (mean difference 590.02N, 95% CI: 371.36N to 808.68N) and by 61.6% from CIR 2:1 to 3:1 (mean difference 745.75N, 95% CI: 527.09N to 964.41N) (Table 3).
In both the IP and the control groups, resistance to fracture decreased with increasing CIR, although the differences were only significant between CIR 2:1 and the other two groups (Table 3). No significant differences were observed between CIR 2.5:1 and 3:1 despite the resistance to fracture of the latter being lower in both the IP and the control implants (Control: 815.22 N vs. 606.55 N; IP: 621.68 N vs. 465.95 N).
In the present study the area mostly affected was the platform, and all the control implants broke at this point, suggesting that the platform is more fragile than the body in narrow fixtures. In the IP group with a 2:1 CIR, some fractures occurred in the body (n = 4) and prosthetic screw (n = 1), suggesting that IP reduces the mechanical resistance of the implant body. However, when higher CIRs were tested the stress seemed to be directed towards the platform and the prosthetic connection, and therefore all the fractures occurred in this area. Other studies using regular platform implants have found that implants subjected to IP usually break at the implant body, and although IP does not seem to decrease the maximum compression force of regular diameter external connection implants significantly, it clearly weakens the implant body.
The present study has some limitations related to its in vitro design. Firstly, the IP procedures were performed by hand to simulate real-life conditions, instead of using a milling machine. Although this might compromise the standardization of the implant reduction slightly, it increased the external validity of the outcomes.
In addition, static compressive loads at a 30º angle do not replicate the daily complex oral function of patients. However, the methodology employed complied with guideline UNE-EN ISO 14801:2016 (third edition), allowing comparison with previous studies, except for the implant vertical exposure. Nevertheless, future research should include dynamic fatigue tests to determine the clinical relevance of the fracture resistance.
According to Gibbs et al. the maximum human clenching force covers a wide range, from 98N to 1243N, and is affected by several factors including age, gender and tooth support. The top of this range would fracture all the samples except for the controls with a 1:2 CIR (1276.16N (σ = 169.75)).
Bite force seems to decrease from molar to premolar and to incisor. Maximum bite forces measured in male subjects are higher than those of female subjects according to Umesh et al.. The same authors found maximum bite forces of 744N in molars, 371N in premolars and 320N in incisors.
Considering these outcomes and comparing them with the present data, IP procedures with an CIR of 2:1 (mean fracture strength 1211.70 N ± 281.64) would present a low fracture risk regardless of implant position, and fracture risk would be of concern after IP in molar regions with an CIR of 2.5:1 (mean fracture strength 621.68 N ± 86.28 N) or 3:1 (mean fracture strength 465.95 N ± 68.57 N).
However, it is important to stress that, although no statistically significant differences were found between IP and control groups with a 3:1 CIR, the mean resistance value of narrow implants in this situation was the lowest (465.95 N ± 68.57 N). Thus, clinicians should consider doing a risk-benefit analysis in such cases since implant fractures are more likely to occur. Nevertheless, as the Young modulus of different titanium alloys and ceramic implants is variable, further research is needed to determine the resistance to fracture of new materials used for dental implants.