Histomorphometric analysis revealed that bone growth start to develop as early as 6 weeks by lining pore cavity walls. Mineralized bone matrix was observed to penetrate 1–2 mm under the surface of the scaffolds thus providing mechanical stability of the inserted prosthesis. Healing continued by filling the entire pore volume (Fig. 2a, b). The presence of nano-hydroxyapatite particles enhanced bone growth and deposition compared to uncoated surfaces (Fig. 3a, b). Hydroxyapatite enhanced osteogenesis ability of zirconia scaffolds, and the proposed hypothesis was accepted.

Kim et al. used coated zirconia scaffolds to augment calvarial defects in a rabbit model and reported closely matching values regarding porosity and density of the prepared scaffolds and regarding the amount of newly measured bone formation [20]. However, in this study, hydroxyapatite particles were not fused to the structure of the scaffold but were used to fill the pores resulting in much quicker release once in contract with body fluid which explains the superior performance of the enriched scaffolds.

Islands of hydroxyapatite particles were observed entrapped in the mineralized bone lining the internal pores of the enriched scaffold. Similar observation regarding the solubility of hydroxyapatite was reported in a cell culture study on porous zirconia surfaces [21]. In a clinical study, micro-porous-coated zirconia scaffolds demonstrated four times higher bone ingrowth and seven times higher bone-scaffold contact compared to uncoated scaffolds inserted in the maxilla of human patient [5].

Several trials used porous titanium scaffolds as a matrix for repair of large boney defects. Enriching these scaffolds with various growth factors enhanced clinical outcome [22,23,24]. However, the design of these scaffolds remains basically a network of interconnected wire structure that acts as a matrix filling the boney defect. CAD/CAM technology allows fabrication of custom-made zirconia scaffolds with full control over the distribution, size, and percentage of the created porosity.

In this study, the scaffolds were inserted in healed wounds in the alveolar ridge of dogs. The external surface of the scaffolds was protected by a resorbable-guided tissue membrane to prevent soft tissue migration which could compete with deposition of the required boney matrix. Traces of the collagen matrix of the membrane were observed on the free surface of the scaffolds. Being CAD/CAM fabricated from special milling blocks, the design of the scaffold could easily be tailored to meet the demands of different fields. In the field of implant dentistry, the future site of dental implant could be considered in the design of the scaffold to facilitate easier placement which will be considered in further studies.