In order to improve hydroxyapatite (Ha) quality as a bone substitute, two types of Ha were developed based on a new and original technique: Ha with graduated porosity (G-Ha) and porous “carbonated” Ha (C-Ha). Ha cylinders were implanted into the femoral diaphysis of NZW rabbits. Before implantation the materials were characterised by XRD, porosimetry, SEM and thermic and mechanical analysis. Macroscopic, radiographic and histologic analysis were performed on the specimens at standard intervals after surgery (1-3-6- and 12 months).

G-Ha proved to be morphologically more similar to bone tissue because of the graduated porosity that mimes the two natural components of bone (cortical-scarce porosity and spongious-high porosity). The C-Ha was chemically more similar to bone because of the CO₃- substitution, which is a normal substitute in natural bone.

Both materials achieved good mechanical strength, in particular the pseudo-cortical portion of G-Ha. Interconnected porosity was also observed in both materials. Newly formed bone appeared earlier in C-Ha (1–3 months). At 1 year C-Ha demonstrated quiescent bone and significant degradation. The G-Ha was scarcely reabsorbed but showed active osteogenesis in the surrounding living bone. Graduated porosity improved the mechanical interaction with bone over time, while the carbonation improved the temporal interaction and Ha resorption.

Porous Ha was found to be a promising bone substitute and also a reliable drug-delivery carrier.

Aims: Hydroxyapatite (HA) is widely known in orthopaedic surgery and is proved to be safe and effective in bone substitution. Actually synthetic HA is a merely reproduction of the chemical constitutes of the natural HA (Ca, P). New technologies demonstrate that it is possible to assembly new materials starting from the primary microscopic unit (nano molecules) with a process called “bottom to the top”, realizing macromolecules biologically active and smart. Even bone is nano structured in HA crystalline units (20–40 nm) regularly oriented upon collagen fibres (300 nm).

Methods: This study concern a new nano structured HA realized by an auto assembled process “biologically inspired”, like in human bone formation, of nano HA crystals and collagen as to realize a new material very similar to natural bone. Samples of the nano-HA were tested in living bone in vivo (rabbits) and compared with a synthetic Mg-HA (Ha added with Magnesium). Macroscopic, radiographic, light microscopic and SEM analysis were performed periodically. New osteogenesis, bone ingrowth and ongrowth, bone apposition rate were evaluated up to 12 weeks. :The preliminary results of this study showed for both the biomaterials optimal bone apposition and biocompatibility. In the first month an earlier osseointegration was observed in traditional Mg-HA samples. The histological examination revealed a primary direct bone apposition from the surrounding living bone. The Nano-HA samples showed a slower secondary bone apposition, may be because of the initial larger gap and consequent lesser direct contact between the material and the guest bone. No adverse effect or reactive phenomenon were observed.

Conclusions: This study demonstrates the reliability of this new nano structured HA that demonstrates to be biologically active and useful in bone substitution. Further studies will reveal new promising improvements in bone substitution with interesting multidisciplinary innovations.