Bone response was evaluated using removal torque measurement and resonance frequency analysis.
demonstrated that ISQ values of implants with higher initial ISQ values can reach a plateau in shorter healing periods than with lower initial ISQ values. They concluded that such a method can serve as a useful research technique and may prove to be valuable in studying the behavior of implants in surrounding bone.12
In addition, clinical observations indicated that the final healing time was affected by individual differences and operation conditions. During the process of osseointegration, the increasing rate of the ISQ values of about 300 Hz per week. Also, when the surgery was not successful, the ISQ value showed a 12% reduction during the first 2 weeks of healing.8
Experimental implants had a significantly higher ISQ values at 2 weeks after insertion but there were no statistical significantly differences among each group. There were no statistical significant differences of the ISQ values at 4 weeks of healing among experimental groups. However, in Duncan analysis considering P = 0.059, Ca-P group showed higher ISQ values, and was interpreted as more stable compare to SLA group and anodized group respectively.
All the groups found no significant differences in ISQ values between 2 and 4 weeks after implant insertion (P > .05). Such surface-modified implants showed increased or maintained ISQ values during the initial healing period, which was interpreted as more rapid and favorable bone reaction for early loading.
In this study, ISQ values were overpassed to our expectations that we had much difficulty in analyzing the results. In general, the increasing values after 2 and 4 weeks of healing period were noticed. In addition, tested groups had increased ISQ values compare to machined group. If the discrepancy were bigger than individual different effect to results, the statistics showed the differences among the groups.
Removal torque forces have been used as a biomechanical measure of anchorage or osseointegration in which the greater forces required to remove implants may be interpreted as an increase in the strength of osseointegration.
Consequently, removal torque testing might not be the best test for the evaluation of implant fixation or the amount of bone around the implant. The underlying biomechanical phenomena in torque testing are very complex. Most of all, the shear stress condition at the interface is important. Removal torque measurements are invasive biomechanical tests that provide information on the rigidity of the implant in the bone marrow. As removal torque testing measures the shear forces of the interface between bone and implant, the results do not always show a direct relation with bone response or surface roughness. Thus, it is also necessary to measure bone to implant contact. Measurements of removal torque and percentage of bone implant contact require destruction of the study specimens; therefore, it is impossible to make direct inference of a possible threshold value to clinical success and survival rates.
At 2 weeks after insertion, the removal torque values of experimental groups showed significant difference compare to control group (P < .05). Anodized group showed the highest removal torque value, but was not significantly different compare to others. At 4 weeks, there was no significant difference (P = 0.060) in One-way ANOVA analysis, but there were significant differences between experimental and control groups in Kruscal-Wallis test. No significant difference was found among experimental groups. Maybe the difference was so little that it was a definitive factor for each introduced method.
The removal torque values of the experimental groups increased to 50% at 2 weeks, compared with machined group at 4 weeks. We inferred that surface treatment affected osteoblast reaction and the osseointegration speed.
The higher removal torque value in the experimental implants with rough surface character is in accordance with the results of the previous studies.
Like in this study, the chemically etched implant surface conferred 4× greater resistance to reverse torque rotation(in this study, 2×) as compared to the machined implant surface 2 months post-surgery in the rabbit femur.13
Ivanoff et al. reported that the removal torque was closely related with the bone-implant contact and amount of bone inside the threads.14
Greater torque rotation forces required to remove implants may be interpreted as an increase in the bone-implant contact leading to higher strength of osseointegration.
Sennerby et al.15
found no difference in removal torque values for screw-shaped implants inserted in rabbit tibiae for 6 weeks, 3 months, and 6 months; in cancellous bone (femoral intra-articular implants), there was no difference between removal torque values at 3 and 6 months.
The results in this study support the idea that modified surface has a positive effect on osseointegration and early loading of the implant. Other study about removal torque using the rabbit tibia showed the similar results.8
In this study, a conventional device as hand-controlled torque may introduce an operator error. And the implants had cutting edges for self-tapping and sometimes cortical bone grew to this cutting edge. Removal torque value can be very high in this situation. Much attention was paid to achieve monocortical fixation into the rabbit tibiae.
Removal torque testing might not be the best test for the evaluation of implant fixation or the amount of bone around the implant.
The bone response was measured with the removal torque test to determine the ability of the surface properties of surface treated implants to influence bone response.
In this study, between ISQ value and removal torque value, there was not constant ratio, but relative correlationships. Spearman's analysis was used for statistical analysis on correlationship (). ISQ values were relative related to removal torque value in SLA, anodized groups. However, there were not significant differences among them (P > .05).
Nonparametic Correlations on ISQ values and removal torque values
According to previous reports, the anodized surface showed higher removal torque values than blasted surface, in contrast to this study.17
Anodized surface may have various properties according to manufacturing methods. They had the thick oxide layers formed by anodization process. That process could change the oxide thickness of titanium implants and make the biological effect. The surface changes from an amorphous metal surface with a noncrystalline oxide to a polycrystalline metal surface with a crystalline oxide layer. The surface is heterogenous with mainly smooth area of thick oxide but separated with porous regions on a nanometer level.19
Heterogeneity of the data can be caused by variation in the in vivo animal study as well as implant location. For example, local bone conditions vary significantly between various animals. This will have a very serious effect on the results of implant bone response studies.
We concluded that surface modifications of SLA, Ca-P and anodized surface showed faster osseointegration and bone healing than machined surface. Under the limitation of this study, however, we suggest that neither anodic oxidation nor Ca-P immersion techniques have any advantage over the conventional SLA technique with respect to implant stability.