[Bone reaction to contact with a granulated titanium surface. Apropos of 101 total hip prostheses with six years follow-up].

PURPOSE OF THE STUDY

The failures of cemented hip prosthesis have prompted research into a new process of fixation and a new design of the system. Stability is achieved by adherence the bone onto a blasted titanium surface. The shell is implanted without screws. The stem has an anatomical shape to improve metaphyseal fixation of the femur.

MATERIAL AND METHODS

The rugosity of the blasted titanium surface which has an average depth of 50 millimicrons allows new bone-growth without any fibrous interposition. Bone ingrowth starts from the third week onward and within two years a homogenous bone sheath is produced. Several experiments on animals have shown the quality of the bone-growth onto this blasted surface. The bone reaction has the same qualities as a hydroxy-apatite coating. It is of better quality than other titanium surfaces. The press-fit obtained by the shape allows this bone-growth. The reliability of this biological fixation is enabled for the shell by the perfect distribution of loads due to the hemispherical shape and the greatest possible thickness of the polyethylene. For the stem, the purely metaphyseal fixation appears to be a very important factor to enable painless long term stability. Another advantage of this method of fixation is that the prosthesis can be removed without any damage to the bone and can be reimplanted without any artificial fixtures with excellent results.

RESULTS

Histological results. Within a few weeks a new strip of bone will settle onto the porous metal without any fibrous interposition. This strip is connected to the cortical bone by small brackets of bone which gradually thicken over the months and guarantee anchorage. Radiological results. In 1 per cent of all cases a border or a gap has formed around the shell; there is no migration. Around the proximal part of the stem thin ribs in tension appeared at the beginning and over the months the width of these increased. Within two years they had filled the space between the metal and the cortical bone. Around the distal part of the stem, which is smooth, there is no bone-growth. Clinical results. In nearly 94 per cent of cases a perfect result was obtained which could only be achieved if the prosthesis was perfectly stable. There was mild pain in only 4 per cent of cases. Revision and observation. Two revisions for ceramic head fracture confirmed that it was possible to remove the prosthesis without damage to the bone.

DISCUSSION

A correct pressure distribution on bone would appear desirable for the two components. For the acetabulum a thick layer of polyethylene and for the stem a metaphyseal anchorage are desirable.

CONCLUSION

It is perhaps too early at this stage to speculate on the long term results. Nevertheless as of now we observe that the principle of a metaphyseal fixation significantly decreases the incidence of mild pain. On the other hand, the follow-up of nearly seven years of bone fixation on a blasted titanium surface would seem sufficient to show us that this method has proven to be a significant improvement.