Porous ceramics structured for bone-cartilage implants.

There are cases of damage to articular joints in which not only a reconstruction of the cartilage is necessary but also replacement of the underlying subchondral bone portion, the thickness of which can be as much as 1-2 mm. Are-construction of the articular surface must take into consideration the particular structure of the bone in the epiphysis position, characterised by an outward fine porosity. The structure of this external porosity does not allow cell migration from the inside to the outside of the bone surface although, at the same time, it does allow good permeation of physiological fluid, flowing out from the inside the bone. This physiological fluid transports oxygen, sugars, proteins and other molecules in order to supply nourishment to the cells (chondrocytes) which populate the layer of cartilage that overhangs the surface of the bone plate. Outgoing from the cartilage layer, the flux of physiological fluid sweeps away carbon dioxide and any other metabolic or catabolic products of the cells. The surface microporosity further serves to offer rootage to cartilaginous proteins for their natural mechanical retention in the site. Since a patent aimed at producing thin ceramic layers with a gradient of porosity (decreasing from one principal face to the opposite) has been applied for, these layers are going to be tested to evaluate the degree to which they meet the requirements. Preliminary tests carried out on samples at different but homogeneous porosity have emphasised that cell permeation of osteoblast cultures is practically stopped after some few tenths microns. The osteoblasts are shown to settle in the interior of the larger pores. They remodel the surrounding walls of the pores in which they lodge and start the production of bone nodules which are useful, for in vivo applications, in producing the desired tissue continuity between the interior of the ceramic and the underlying bone tissue, thus promoting a definitive firm setting. A cytological test, performed one week after cells had seeded on one side of the ceramic samples, has shown a provisional response towards the lodging material. In fact the cells assumed a globular shape (tending to flattening), but displaying cytoplasmatic processes of cellular interconnection. Measurements of the vitality, population and penetration into the cell pores inside the ceramic were carried out. On the basis of the results, the proposed ceramic devices with a gradient of porosity indicate their suitability for the intended function. Successive experimental steps will concern in vivo applications, simulating a repair intervention for an articular arthrosis.