In previous studies, the implant stability and healing time had significant correlation.14 In the present study the comparison of time with bone type also indicates a significant correlation (p<0.05). This study reveals that from baseline to 90 days of healing, the stability patterns in D2, D3 and D4 bone were noticeably different, especially at the 30th day of healing (mean ISQ difference = 4.21; p <0.05). No difference was found in mean ISQ values of D1 bone at any time point (p = 0.22). These results were similar to results of Calandriello and associates,15 since the implants were submerged, the plateau effect in stability after 60 days agrees with the concept of enhanced bone formation around the implant surface and the possibility of reduced clinical healing times prior to restoration. A close examination of the 30th day of healing revealed that in all 4 bone categories a decrease in mean ISQ values was observed. An examination of the stability change between baseline and 30th day revealed no statistically significant decrease in stability for D1 bone (p>0.05). However, implants in D2, D3 and D4 bone experienced a statistically significant stability change at 30th day (p<0.05). D4 bone showed the highest difference from baseline to 30th day (7.6%). Therefore, this signifies that the quantity and location of compact and lamellar bone around the implant seem to be an important factor in providing resistance to lateral mobility, particularly at 1 month. When the implant stability in bone tissue is objectively measured, the stiffness of the tissue adjacent to and surrounding the implant will influence the stability measurement.16 With D1 bone, for example, the overall stiffness of the bone will be more because of the thick cortical layer and less trabecular core. Therefore, the majority of the implant surface will be occupied by bone with a high stiffness; hence the higher stability values. It is not surprising, then, that D4 bone showed the highest fluctuation at this stage, as the lesser bone density would contribute to lower levels of stiffness in the transducer/implant/tissue system during the healing phase. The greatest change in stability occurred between 30th and 90th day in all 4 bone groups. This change in stability might coincide with the physiologic changes reported by Roberts, 17 who inferred from the rabbit model that following implant placement humans during the early modeling phase (0 to 6 weeks) begin to develop a bridging callus of bone from the endosteum and periosteum to the surface of the implant. After 6 weeks the next stage of lamellar compaction within the loose stroma of the woven bone would begin and progress to 18 weeks. Roberts extrapolated that the lamellar compaction around implant would provide sufficient strength for loading. Of interest is the substantial 17% increase in stability (p= .000) for D4 bone from 30th day to the 90th day. After 90 days of healing, the mean ISQ values between D1 and D2 bone and that between D3 and D4 bone were similar. Thus, bone density/quality is dynamic and not static, as it continue to change with time in relation to an implant surface.18 Stability is essential throughout the healing period and later during function to permit remodelling of bone around the implant, rather than fibrous repair. Initially, primary stability prevails at the time of implant placement. This may be largely the result of primary bone contact i.e. the mechanical interlocking of the implant against the cut native bone surface.19 Secondary stability, however, results from bony modeling and remodeling around the osteoconductive titanium surface. During this healing process, primary bone contact decreases as woven bone becomes lamellar bone thereby increasing the secondary bone contact.19 The present study examined the transition in the levels of implant stability from the time of primary bone contact to the development of early secondary bone contact during the 3 months of healing. In each of the 4 bone groups, the change in the mean ISQ values from baseline to the 60th day did not achieve statistical significance. During the early transition period between primary stability and secondary stability, D1 bone had no statistically significant difference in the mean ISQ values at any time point throughout the 90 days. D2 bone did reveal statistically significant difference in the implant stability between 0 and 30th day and 30th and 60th day. However, when the implant stability at all time points was compared between D1 and D2 bone no statistically significant change was seen. Schenek has extrapolated that if stable fixation exists between the bone and implant such that the fixation avoids even minute interfragmentary movement then the implant could withstand dynamic load bearing.20 Thus, those implants that show high primary stability with limited change over time, could be immediately loaded. Also early loading of D1 and D2 bone has been advocated in the literature, especially with roughened-surface implants.21 However, according to the literature available, implants with ISQ values less than 60 need close monitoring, in the range of 60-65 can be conventionally loaded, 65-70 can be loaded early and those above 70 can be immediately loaded.22 With regard to D3 and D4 bone, the present study demonstrated statistically significant changes in implant stability between 0, 30th, 60th and 90th day. Owing to these changes, it is difficult to advocate possible immediate loading protocols for these bone types. Thus, the null hypothesis is rejected as there is a statistically significant difference in the stability of dental implants placed in different densities of bone as measured by resonance frequency analysis during the healing period.