The use of porous ceramics as bone graft substitutes (BGS) has been under consideration for over 30 years [1]. In particular calcium phosphates such as hydroxyapatite (HA) have been promoted as a result of their osteoconductive properties, i.e. that they stimulate bone apposition within their macroporous structures.

It is well established that both pore size [1] and pore connectivity [2] are critical morphological elements for a successful BGS. Thus biologically ‘optimal’ structures, with relatively large levels of porosity (> 70%) are consequently low in mechanical strength, with typical UCS values of between 1–8 MPa depending on the precise level of porosity and the pore size distribution. The aim of this investigation was to study the biological response to a porous HA with a relatively low level of macro-porosity (64%), but which possessed a highly interconnected micro-pore structure within the HA struts.

Phase pure porous HA implants were manufactured using a novel technique [3] with a mean macro-pore size of 230 ìm and a mean pore interconnection size of 110 μm. Cylindrical specimens 4.5 mm in diameter were implanted in the distal femur of 6 month New Zealand White rabbits and retrieved for histological and histomorphometric analysis at 4 weeks. The mineral apposition rate (MAR) was determined through the administration of fluorochrome labels at 1, 2 and 3 weeks.

After 4 weeks new bone had penetrated deep within the macro-pore structure and at high magnification osteocyte-like cells were observed occupying micro-pores within the ceramic struts. Furthermore, there was a significant increase in the MAR of bone formed within and surrounding the PHA (5.21 ìm.day-1, 4.42 ìm.day-1) as compared to the normal turnover rate of control bone (2.07 ìm.day-1, 2.09 ìm.day-1) during weeks 1-2 and 2–3, respectively.

The micro-porous network within the scaffold struts clearly influenced the host response. This could be linked to an associated increase in roughness or surface area, or it may reflect the greater level of strut permeability underlining the importance of nutrient transfer and the promotion of angiogenesis in scaffolds for bone repair.