Metal implant with anchoring surface

Details Metal implant with anchoring surface Metal implant with anchoring surface

1999-05-20

2001-03-13

Mancene, Gene (Department: 3732)

Prosthesis (i.e., artificial body members), parts thereof, or ai

Implantable prosthesis

C623S023500

Reexamination Certificate

active

06200346

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to a metal implant, by way of example for bones, the implant having a surface with a coarse structure of elevations and depressions, and to methods for producing the surface.
U.S. Pat. Nos. 4,673,409 and 4,608,052 show a surface in which, starting from a smooth metal surface, pillars are produced by the material in that a laser is used to liquefy and vaporize material around the pillars in the form of channels of cylindrical blind holes placed adjacent to one another in a row. The upper faces of these pillars, which lie in the originally smooth metal surface, form support surfaces for a primary anchoring in the bone tissue; and the hollow spaces surrounding them can accommodate bone tissue that grows in later or bone cement. This type of metal processing is enormously complicated and expensive since the excess material must be vaporized at the low efficiency of a laser and since the focal point of the laser optics must continually be adjusted very precisely in its distance to the surface. Furthermore, during the spontaneous melting and vaporization, metal sprays arise which are in no regard desirable so that U.S. Pat. No. 5,264,530 proposes to apply a protective coating prior to the laser drilling which can lager be removed and which is also bored through and which prevents liquid metal sprays from striking the surface that it covers. Solvent soluble binders with metallic oxides which are heat resistant at high temperatures are considered as typical protective coatings. Nevertheless it is, of course, not possible to prevent metal sprays being deposited in the already cut channels.
SUMMARY OF THE INVENTION
It is an object of the invention to provide surfaces for a good anchoring of a metal implant and simultaneously to develop methods which permit a rational manufacture of such surfaces.
This object is attained by premeating the surface with a network of protruding ribs which form nodes and interstices or meshes having an interstice width of 2 mm to 0.4 mm, by the interstices each forming a recess in the form of a section of a spherical cavity, and by the coarse structure having a fine structure for the anchoring.
A surface of this kind has the advantage that a network of ribs is available for the primary anchoring whose edges can take up shear forces at the spherical cavities independently of the direction, and the edges of which, as a sum, have an enormous length for taking up shear forces due to the small dimensions and due to the dense arrangement of the round cavities. The depth of a round cavity, which can grow to a bit more than the radius of a hemispherical shell, is provided as the anchoring depth for ingrowing bone matter.
For the primary anchoring of non-cemented prostheses, a bone bed is hollowed out in a bone which corresponds to a slightly reduced envelope surface at the surface of the prosthesis. In such a case it continues to be advantageous for the primary anchoring if the ribs form a saddle between two nodes, since the round cavities partially penetrate one another at their edges. With such an arrangement it is ensured that, when the normal force between the surface and the bone matter increases, first the edges at the nodes and then the edges at the saddle points penetrate into the bone matter and form a support surface which rapidly increases with the penetration depth, which does not lie in the envelope of the surface and which prevents sliding. If the indentation V of the saddle points with respect to the nodes amounts to 0<V<50% of the depth of the round cavities, then a perforation of the bone structure by individual pillars can be avoided and a pronounced upper surface acting as a sliding surface and coinciding with an envelope can be avoided. A surface of this kind is particularly suitable as a direct anchorage for an implant in bones if it consists of body-compatible material such as titanium or titanium alloys, for example. In metals such as, for example, cobalt-chromium alloys, this surface can be used for an anchoring in bone cement. Through the network-shaped arrangement of the ribs and through the enlargement of their cross-section at increasing depth, filigree interstices with an interstice width down to 0.4 mm can be used even in the relatively soft pure titanium.
The spherical cavities can be arranged with their midpoints at spacings which correspond to a grid. This is, however, not obligatory. Since even the mixed form with narrow ribs without saddle points and with ribs with saddle points is suitable as a coarse structure, the spherical cavities may also intersect insubstantially or just fail to intersect in a “wild” arrangement. Likewise the spherical cavities, with insubstantial intersection, can also be layered In a worm-like manner or lined us adjacent to one another, e.g. in the form of closely adjacent spiral windings on the outer side of a hip joint shell, or in the arrangement of a projected fir branch, for example. A coarse structure of the surface of this kind, on which the spherical cavities are placed next to one another in a row as on a branch from which needles project laterally, has the advantage that ingrowing bone tissue can also build up blood vessels for its subsistence through the presence of worm-like sections along the axes of the worm-like structures due to the lesser rib height.
It is advantageous for the anchoring if the filigree network structure can be produced on curved or inclined surfaces in a rational manner. This is done in accordance with the characterizing portion of the independent claim
7
by the metal surface being covered over with an electrochemically active protective lacquer, with holes of diameter smaller than the interstice width or the diameter of the later spherical cavities respectively being left at a spacing of the later interstices; by the surface which is provided with holes in the protective lacquer being exposed to an electrolytic liquid for a limited time in order to produce sections of spherical cavities by electrochemical erosion or etching which are separated from one another by narrow ribs; by the remaining protective lacquer being removed; and by the coarsely structured surface being provided with a fine structure, for example by means of sand blasting. A further rational manufacturing method for an intermediary surface as a preliminary stage of the surface described in claim
1
results from the characterizing features of claim
9
in that the implant is first coated over by an adherent, electrochemically active protective lacquer which can be vaporized by a laser of a low energy density without the underlying metal being affected thereby. On the contrary, for sufficiently long wavelengths of laser light, the reflection by the metal can be exploited as well, in that the radiation is reflected back into the protective lacquer and in that less radiation is absorbed by the metal. In addition, a substantially lower powered and less expensive laser can be used than for the metal processing. Due to this energy threshold between the protective lacquer and the implant, the maintenance of a definite focal length in order to ablate the protective lacquer is much less critical. Thus holes can be produced in the protective lacquer without the focusing distance being precisely maintained, and by using a laser beam which is incident at an inclination to the surface. A field of holes is produced in the protective lacquer by the laser beam through relative motion between the surface and the laser head and through path dependent triggering of the laser beam, with the holes having the same center to center spacing as the later interstices but having a smaller diameter than the later spherical cavities.
In a further step, which is common to all manufacturing methods, this surface, which is provided with holes in the protective lacquer, is exposed to an electrically conductive liquid, an electrolyte, in order to produce cup-like depressions in the region of the holes with its help by electrochemical erosion and/or etching. Starting with the diame

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