Histology

New bone growing over
and penetrating into H-A.C.

Mechanical failure at the prosthesis/bone interface, that is to say loosening, is always caused by insufficiency of the substance adjacent to the implant. An interface without a separating fibrous membrane is the only guarantee for long term clinical success.

Connective tissue which comes into contact with a metal prosthesis will never change to bony tissue. Since an intervening fibrous layer between prosthesis and bone causes an impairment of load transmission efficiency and a self perpetuating growth of this connective tissue is induced. Loosening is inevitable, only the time factor varies.

Trabeculae on both sides of the end of prosthesis indicating forces on lateral side.

Direct contact between bone and metal implant can only transfer compressive forces and none other. Transmission of all forces including tension and shear forces requires true bonding. Contact, by itself, is insufficient. The only bio-material capable of creating a substantial continuity between itself and bone is Hydroxy-apatite ceramic ensuring mineralised continuity.

The mode of ingrowth is called 'bonding osteogenesis'. Hydroxy-apatite ceramic is found to be extremely bio-compatible, degradation resistant and particularly osteotropic. Due to osteotropism the formation of new bone on the surface of the Hydroxy-apatite begins between the 5th and 7th day. Timewise this process is analogous to the osteogenesis on natural bone surfaces. The newly formed bone creates a bond with the Hydroxy-apatite ceramic. Bonding osteogenesis and lamellar bone formation are also achieved under a physiological load stress. (Meenen et al 1987).

Animal experiments with uncoated titanium alloy and Hydroxy-apatite (Supravit®) coated prostheses which were implanted into rabbit femora revealed the following results:

1. The unique biological characteristics of the Hydroxy-Apatite ceramic manifest in block and granular form are preserved if Hydroxy-apatite ceramic is used as a coating for titanium alloy.

2. Osseous tissue reacts to Hydroxy-apatite ceramic with significantly increased bone formation and shorter maturation periods.

3. With titanium new bone formation begins at a distance and only secondarily reaches the implant surface. With Hydroxy-apatite ceramic the new bone grows primarily on and directly into the ceramic surface.

4. Bonding osteogenesis on the Hydroxy-apatite ceramic surface and reparative osteogenesis on the endosteal bone surface occur simultaneously. The bilateral ossification significantly shortens the time required for osseous bridging between prosthesis and host bone; and is analogous to fracture healing.

The bridging is of special significance in relation to the stem of the prosthesis in the medullary canal.

The elasticity of the bridging absorbs the inevitable micro-movement present between prosthesis and bone. Elastic absorption of energy is not the same as micro-movement.
There is continuity of tissue that is to say bonding.

The cone between the body of the prosthesis and the stem is of decisive importance in that they, by its early mechanical fixation, permit "cradling", the all important histological process to develop in the diaphysis.

The body of the prosthesis immediately after implantation is surrounded by and in contact with the spongiosa of the metaphysis.
In normal anatomy the cancellous bone of the metaphysis transmits the body-weight load to the cortical bone of the femoral diaphysis.

Following implantation this loading is transferred to and transmitted by the interface between prosthesis and spongiosa.

Ideally the stem takes up the position of the clapper in a mute bell, i.e. it is not in contact with the endosteum. The space around the stem of one millimetre contains many osteoblastic cells and these cells, under the influence of osteotropism form bridging trabeculae vectorially arranged to convey forces between loaded stem and recipient endosteum.

Cross section of 'cradle' bridging 1mm gap between H-A.C. coating and existing trabeculae. Close up of trabeculae forming cradle.

These cradling trabeculae are only present in locations where forces are to be found and they are arranged according to the magnitude and the direction of the forces. These trabeculae are elastic and absorb the inevitable micro-movement that occurs between stem and femur. The fact that the upper end of the femur is curved means that a longitudinal compressive force, i.e. weight bearing engenders micro-movement in bending .

This relative micro-motion between stem and containing femur has never been accommodated previously and is one cause of aseptic loosening of contemporary prostheses. Similarly the discrepancy in elasticity between the host bone and the metal substrate is likewise accommodated.