The present invention relates to surgical cutting instruments and their methods of manufacture. Surgical cutting instruments have been proposed with hardened cutting edges. Such instruments are difficult to fabricate and often the materials used for the blade and for hardening the blade are unsuitable for use in surgery because they either can contaminate human tissue or become themselves the subject of a reaction with the human body fluids .

Titanium has been found to be a highly suitable material for surgical instruments. Titanium has a very high strength to weight ratio, and is generally inert has far as surgical processes are concerned. Titanium is hard and so provides a durable cutting edge. Nevertheless, the period for which titanium remains sharp for surgical purposes (where standards for sharpness are extremely high) is still relatively short.

The object of the present invention is to provide a surgical cutting implement of titanium having an improved cutting edge. According to the present invention there is provided a surgical cutting implement of titanium or a titanium alloy having a- cutting edge which is infused with at least one metalloid and treated to render the cutting edge amorphous. According to the present invention there is further provided a method of hardening a surgical cutting instrument of titanium or a titanium alloy comprises the steps of depositing a layer of metalloid on the cutting edge, heating the layer of metalloid and the immediately adjacent titanium or

titanium alloy substrate to melting point, to effect an infusion of the metalloid into the substrate and rapidly cooling the cutting edge to render at least part of the substrate infused with the metalloid, amorphous.

According to the present invention there is further provided a method of making a surgical cutting implement comprising the steps of forming a cutting edge substrate of titanium or titanium alloy, coating the cutting edge substrate with a metalloid layer, placing the coated cutting edge substrate in a cold environment, effecting local heating of the cutting edge substrate sufficient to allow local infusion of the metalloid into the substrate, displacing the locality of the heating progressively to cover the whole of the cutting edge substrate, the cold environment having such a low .temperature that following lo'cal heating, the cutting edge substrate is cooled sufficiently quickly for an amorphous outer layer to form, and removing part of the amorphous outer layer to provide a sharp edge for the treated substrate.

Preferably the cutting edge is lapped to remove a portion of the infused titanium or titanium alloy and to leave a sharp edge.

The depth to which the infused titanium or titanium alloy is removed is selected to be that depth at which the infused titanium or titanium alloy has reached its maximum hardness. Advantageously the method of heating and cooling the cutting edge is by submerging the cutting edge in a liquid coolant and scanning the blade with a laser. - *

Advantageously the metalloids are selected from the group consisting of Phosphorous, Boron, Carbon, Silicon and Germanium.

A surgical cutting instrument and its method of manufacture will now be described, by way of example, with reference to the accompanying diagrammatic drawings in which: Figure 1 is a section through a titanium knife blade onto which a layer of metalloid has been deposited;

Figure 2 is a section through the knife blade of Figure 1 after heating and subsequent cooling; and

Figure 3 is a section through the knife blade of Figure 1 after lapping.

The surgical cutting instrument to be described is of titanium or a titanium alloy, the preferred material being commercially available, 318 Titanium Alloy. The cutting edge of the titanium or titanium alloy is hardened by a process to be described hereinafter. In the drawings, the cutting edge of the instrument before being processed will be referred to as the substrate.

As shown in Figure 1 , the substrate 4 is coated with a layer 2 of metalloid. The term metalloid as used herein refers to that class of material which is non-metallic but which can have the properties of a metal for example Arsenic, Silicon,

Boron, Tellurium, Antimony, Bismuth, Phosphorous, and Germanium.

The layer 2 of metalloid and the immediately adjacent portion of the substrate 4 are heated to melting point. As a result, the metalloid tends to become infused into the upper portion of the substrate to form a composite titanium or titanium alloy/metalloid layer 6 (see Figure 2) . ^'

The layers 3 and 6 are then rapidly cooled, for example by subjecting them to an environment

having a temperature of less than -150°C. Because of the thickness of the composite layer, that part of the layer nearest the upper surface of the cutting edge will cool faster than that part of the composite layer which lies furthest from the upper surface. As a result, the composite layer upon cooling will progress from an upper region which is amorphous to a lower region which is crystalline, the crystalline structure becoming smaller with distance from the outer surface of the cutting edge.

The heating and cooling process is carried out by submerging the cutting edge of the instrument in a bath of liquid coolant (for example, liquid nitrogen) and then progressively scanning the cutting edge with a laser beam. In this way the cutting edge is progressively processed. The point on the cutting edge where the laser beam impinges becomes molten, and, and as soon as the beam moves on, the liquid coolant acts to effect rapid cooling. The result of this process, as shown in

Figure 2, is a three part structure consisting of the remnants of a layer of metalloid 2, a composite layer 6 which changes progressively from an amorphous structure to a large crystalline structure and then to a small crystalline structure, and the original substrate layer of unchanged titanium or titanium alloy.

The cutting edge is then lapped or otherwise treated to remove the remnants of the metalloid layer 2 and part of the composite layer to a depth in the amorphous region where the amorphous region has reached its maximum hardness (see Figure 3). This depth can be readily determined experimentally by processing a test piece of similar material simultaneously with the cutting edge and

then removing the outer surface of the test piece in small steps, making a hardness measurement after each step.

The lapping action is also conducted to render the cutting edge sharp.

The processed blade has a significantly improved hardness over the original titanium or titanium alloy blade so that its life span is extended dramatically. Figure 4 shows apparatus for heating and cooling a titanium substrate workpiece 10 coated with a metalloid layer 12. As shown a thermally insulated bath 14 is filled with liquid nitrogen 16. A support structure 18 mounted on the floor of the bath is provided with an array of pins 20 to support the workpiece 10 so that its upper surface lies just below the surface of the liquid nitrogen and its underside is for the most part exposed to direct contact with ^" the liquid nitrogen 16. A laser assembly 22 mounted above the bath 14 projects a laser beam 24 on to the upper surface of the workpiece 10. The assembly includes beam deflection means (not shown) for causing the beam 24 to perform a full scan of the surface area of the workpiece 10. The deflector means is advantageously an optical scanning arrangement of the type well known in the ar .

A tank 26 of liquid nitrogen has a discharge spout 28 for discharging liquid nitrogen into the bath 14 to maintain the level of the liquid nitrogen in the bath 14 constant. This function may be performed manually by an operator or may be automated in a manner well known in the art.

In operation the bath 14 is filled with liquid nitrogen to a predetermined level and the

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-6- workpiece is lowered into the bath 14 until it rests on the pins 20. The laser assembly 22 is energised and actuated to cause the beam 24 o progressively scan the workpiece. The heating and rapid quenching of the surface of the workpiece produces a structure as described with reference to Figure 2.

The workpiece is subsequently removed from the bath 14 and lapped to remove any residual metalloid and to provide a sharp cutting edge. It will be appreciated that instead of heating and cooling with a laser and liquid nitrogen, other methods may be used to effect surface heating of the cutting edge followed by rapid quenching.