Biomimetic apatites and bone cements

The regeneration of hard connective tissues (e.g. bone, teeth) is activated by the presence of specific chemical and morphological signals inducing new bone formation and organization. In this respect hydroxyapatite phases presenting bone-like composition (i.e. multiple ionic substitutions such as Mg2+, CO32-, Na+, K+, SiO44-, Sr2+), nanosized microstructure and low crystal order can be synthesized by wet methods at room temperature. These new phases closely mimic the inorganic part of bone and exhibit good ability of osteogenesis and bio-resorption in vivo. These biomaterials are very suitable to be used as regenerative bone fillers in the form of granulate or as injectable pastes.

Ionic substitutions can be applied also on reactive and metastable calcium phosphates such as tricalcium phosphate that in contact with water spontaneously transform into biomimetic, nanosized hydroxyapatite particles with elongated shape. This process is exploited to develop new injectable, bio-resorbable bone cements that can be designed for use in vertebroplasty procedures in the case of vertebral bodies weakened or diseased by traumas or osteoporosis. In these cements self-hardening occur by physical entanglement of the elongated hydroxyapatite particles. These new cements possess high osteogenic ability, open porosity and ability to be completely resorbed, also due to the presence of bio-polymers promoting injectability, hardening and new bone penetration. In particular, the partial ionic substitution of calcium with strontium can provide a specific effect in vivo that promote the bone formation and limit the bone resorption, thus restoring the physiological bone turnover. The new apatitic cements intend to replace the current use of acrylic devices that, in spite of their ability to provide early mechanical stabilization of the vertebra, are characterized by several drawbacks that severely limit their use in young and physically-active patients.