Background: Surface characteristics of dental implants, such as roughness and chemistry, have a direct influence on bone response. Previous studies have shown that changes in implant microtopography may increase in vivo bone-to-implant contact and lead to improved bone anchorage. Sinus lift procedure is a safe and predictable method to gain bone height for implant placement. Diverse bone substitutes have been used for sinus lifting, with results comparable to those achieved when using autogenous bone.
Aim: To micro-tomographically evaluate, in a challenging bone model (sinus lift procedure with anorganic bovine bone and immediate implant placement), the biologic response to a new implant surface, modified by the addition of nano-hydroxyapatite, comparing it to a double acid etched implant surface.
Material and Methods: Sinus lifting procedures were performed bilaterally in 8 rabbits. Bone windows were prepared using a trephine bur, Then, the sinus mucosa was elevated, and the implant sites were prepared 3 mm mesially to the windows. Before placing the mini-implants, anorganic bovine bone grafts were inserted into both sinuses. After that, mini implants were randomly installed in prepared sites: in one sinus, double acid etched surface (DAA group); in the other, nano-hydroxyapatite surface (NHA group) implant. Collagen membranes were positioned over the windows and flaps were sutured. The rabbits were euthanized 4 weeks after surgery and biopsies were collected for analysis. Tomographic Tridimensional Bone to Implant Contact (IS/TS), Bone Volume Fraction (BV/TV), Trabecular Thickness (Tb.Th), Trabecular Number (Tb.N), and Total Porosity (To.po) were evaluated in the bone between and outside (adjacent bone) implant threads. Intra and Inter-groups differences were assessed by Student's t-test (p<0.05). 
Results: Healing was uneventfully for all animals. After 4 weeks, for the bone analysis inside the threads, IS/TS for double acid etched and nano-hydroxyapatite implants showed comparable results, with mean values of 9,95 ± 0,17% vs. 10,01 ± 0,10% (p > 0.05), respectively; the mean values of Total Bone Porosity was 75,77 ± 3,47% for NHA and 76,74 ± 3,08% for DAA (p > 0.05); for BV/TV, mean values were 24,23 ± 3,17% for NHA group and 23,26 ± 3,08% for DAA group (p> 0.05); for Tb.Th, there were not significant differences between NHA (0,060 ± 0,003mm) versus DAA groups (0,061 ± 0,004mm); for Tb.N, NHA group mean was 3.80 ± 0.43 and DAA group mean was 3.74 ± 0.39, without statistically significant differences (p>0.05). Bone analysis for outside the threads also showed no statistically significant differences between groups for all parameters, and no intra-group (inside versus outsite thread bone evaluation) statistically significant differences were observed (p>0.05 for all parameters).
Conclusion and Clinical implications: It was concluded, in this pilot study assessing early implant evaluation in a challenging bone model, that micro-tomographical bone parameters were similar both for double acid etched and nano-hydroxyapatite surface implants. Further studies analyzing longer healing times and with histomorphometric evaluation are suggested.

Microtomographic evaluation of a new implant surface in a challenging bone model. Study in rabbit