GB2025238A - Prosthetic devices comprising silicon nitride - Google Patents

Abstract

The present invention relates to the use of silicon nitrides in orthopaedic endoprostheses or prosthetic devices and to methods of manufacture of same. The term "silicon nitride" includes both alpha and beta forms as well as the various sialons.

Description

SPECIFICATION Surgical endoprostheses The present invention relates to the use of silicon nitrides and sialons for surgical endoprostheses.

One of the major problems facing workers in the design of prosthetic devices is the choice of materials for construction. When prosthetic devices are placed in the body, consideration must be given to the effect of the physiological environment upon the prosthetic material, and the effect of the prosthetic material and its corrosion or degradation products upon the fluids and tissues of the surrounding environment. In this case, the result might be pain, disfunction or loss of an organ or limb or even eventual death of the patient.

Further, the prosthetic material must be capable of being formed or otherwise shaped into a variety of necessary configurations, and the prostheses must be strong enough to withstand the forces imposed on it during its life expectancy and to conform as precisely as possible to the normal functions of those natural parts for which they are substitutes.

Modern metallurgy has developed new alloys from which a wide variety of orthopedic devices are fabricated ranging from plates and screws for fractured bones to artificial hip joints. There are, however, many instances where an implant must be left within the body for the duration of the life of the patient (e.g. artificial hip joints, replacement of missing segments of structural bones or the replacement of facial bones due to disease or trauma), and metal prostheses have been found to suffer from corrosion in the exceedingly hostile environment of the body.

This environment is an aqueous solution of approximately 1 M NaCI containing organic acids, proteins, enzymes, biological macromolecules, electrolytes and dissolved oxygen, nitrogen, and carbon dioxide. The pH of the fluid is about 7.35, but drops to about 5.3 upon injury to the tissue and returns to normal in about ten days. In addition to its complex chemical make-up the activities of the various dissolved species are constantly changing. In this sort of environment, it is not surprising that long term metallic implants are apt to fail due to corrosion.

Stainless steels depend upon a closely adherent oxide surface layer for their resistance to corrosion.

This layer in turn depends upon the continuous presence of oxygen in the environment. In the body tissues it is quite common to have varying oxygenation, and thus stainless steel implants are quite susceptible to corrosion.

Even polymeric materials can be "digested" in the biological environment of the body. In fact, polymers are particularly susceptible to changes which may lead to failure. Such changes can be rearrangement of the polymer itself such as breaking of cross-links or outright chemical degradation.

No foreign material placed within a living body is ever completely compatible. The only substances which enjoy complete compatibility are those manufactured by the body itself (autogenous); any other substance is recognized as foreign, initiating some type of biological reaction. Therefore, the aim in the selection of materials for implant devices is to choose those which will perform the desired function with a minimum of adverse biological reaction.

In this regard, it has previously been suggested to use bioceramic endoprostheses. By their very nature, ceramics do not suffer from corrosion as do metals, and have great chemical stability in the extreme environment of the living body. In the case of long-term implantation, ceramics seen to hold the greatest promise.

The major constituent of bone, hydroxyapatite, is recognised as being ceramic in nature. This has been borne out by the remarkable biocompatibility of the ceramic structural materials which have been tested as prostheses in the living body. To date, the material most widely used for permanent prosthetic devices has been aluminium oxide (alumina), which offers the advantages of inertness, adequate strength and an accumulation of experience in its fabrication. However, its sensitivity to brittle fracture is greater than is desirable for many orthopaedic applications.

Most of the important properties of silicon nitride are well known, and the use of reaction-bonded Si3N4 ceramics in engineering applications are well documented. They are used where metals do not possess the required degree of strength, oxidation resistance and stability at high working temperatures, or where high electrical resistivity is essential.

The present inventors have recognised that silicon nitride and sialon ceramics offer mechanical advantages over aluminium oxide ceramics and may replace the use of Al2O3 in the design and application of orthopaedic endoprostheses.

The advantages of ceramics in this application compared to metals are: (a) the absence of corrosion; (b) the ability to be fitted without the use of methylmethacrylate cement (the latter being the cause of any problems; and (c) a lower density.

The advantages of silicon nitride compared to alumina, the most commonly used ceramic at the present time, are: (a) a lower density; (b) a high strength even when prepared in a porous state to permit tissue ingrowth; (c.) a lower elastic modulus, closer to that of bone, leading to the production of lower stresses for a given deformation; (d) excellent frictional properties when bearing on itself.

This is very important in joint replacement; (e) the ability to be formed into complex shapes by partly nitriding, machining and then completing the nitriding operation. This is facilitated by a negligible dimensional change during final nitridation; (f) a high degree of biocompatability; and (g) encouragment of bone growth. This is consistent with an earlier observation that low concentrations of silica aid the growth of young bones.

Since nitride and the sialons have excellent wear resistance and, although their method of preparation is quite different from that of traditional ceramics, they can be fabricated in complicated shapes. Being compounds of silicon, aluminium, oxygen and nit rogen, the sialons allow variations of composition and, thereby control of properties. The degree of porosity can be increased or decreased depending whether the principal objective is to promote tissue ingrowth orto provide the maximum wear resistance of bearing surfaces. Changes in grain size and the presence of second phases can influence proper- ties such as fracture toughness and impact resistance.

Silicon Nitride occurs in two crystalline forms, designated as a and ss, so that any given sample may contain either or both of these forms depending on the exact conditions of formation. Normally, both occur in a sample, in various proportions. The term Silicon Nitride, as used in this specification, including the claims, should be taken to include either or both of the a and p forms. Reference to Silicon Nitride is also to be taken as a reference to the various sialons.

Silicon Nitride may be produced by any of several methods, including: (i) Reaction bonding; (ii) Chemical vapour deposition; (iii) Cold forming and sintering; and (iv) Hot pressing (See also "The Technology and Engineering Applications of Reaction-bonded Si3N4,, by N. L. Parr and E.

R. W. May, and "Silicon Nitride a new ceramic for high temperature engineering and other applications", by N. L. Parr, Research, Vol. 13(1960) PP 261-269).

In the case of methods (i) and (ii) above, the Silicon Nitride is formed by chemical reaction during the forming process, while in methods (iii) and (iv) sintering and hot pressing are applied to a prereacted powder.

Taking the formation of Silicon Nitride prostheses bythe method of reaction bonding for example, the Silicon powder if formed by, for example, pressing in a die, extrusion or slip casting, to the shape of the desired component or a shape approximating that of the component. The formed shape is heated in an atmosphere of nitrogen, where reaction takes place to produce Silicon Nitride. If the original formed shape of the Silicon is that required in the final article, nitridation is carried outto completion. If modifications to the initial shape are required, the reaction may be interrupted at an intermediate time and temperature and operations such a sawing, drilling, grinding, or caring may be carried out. The nitridation is then completed.This ability to be machined at an intermediate production stage makes Silicon Nitride a very versatile material in its ability to reproduce complex shapes. There is negligible distortion or change in dimension during the final stage of nitridation.

The Silicon Nitride prosthesis can be provided in a porous orfully dense form. The porous postheses can be produced by a foaming technique. This method merely entails mixing the fine silicon pow derwith an organic binder, adding a commercial foaming agent and mechanically whipping the mixture. The resulting foam chemically hardens and is subsequently heated in an atomsphere of nitrogen to form a porous Silicon Nitride structure. The porous structure has the advantage of permitting bone and soft tissue ingrowth, thus resulting in direct attachment of the prostheses to the masculo-skeletal system and a strong mechanical bond, without the need for the use of methylmethacrylate cement The more porous material also has the advantage of a lower elastic modulus closerto that of normal bone.

On the other hand, fully dense silicon nitride possesses a higher strength than the porous variety and is therefore preferable where stresses are high.

Anchorage to the skeleton can still be obtained by adhesion and mechanical interlocking of the bone and ceramic. Conventional pinning, nailing, wiring or use of pressure plates can be employed. In the case of hip ball joints, the prosthetic device could be fitted with a metal shank or shaft to be driven into the end of the femur; combinations of metal and ceramics in prosthetic devices offer the advantages of both materials. The shaft may also contain a porous ceramic portion to encourage bone ingrowth, to ensure a longer-lasting fit. Alternatively, the prosthesis can be formed with a sleeve at the end to be applied to the femur, the interior of which is conical in shape and contains a number of annular grooves.

The end of the femur is then conically shaped, by means of a special tool, in order to closely fit into the conical end of the prosthesis. The annular grooves in the prosthesis encourage bone growth therein, resulting in a secure fit of the prosthesis.

Some methods of using silicon nitride in orthopaedic applications are set out below: 1. For the replacement of bone which is diseased of has been so badly damaged that removal of a portion is necessary. Sometimes called a bone gap bridge.

2. As the material of construction for all or part of a prosthesis for the replacement or repair of a joint such as the hip, knee, shoulder, elbow, wrist or ankle.

3. For lengthening a limb by insertion of a piece of silicon nitride to make up all or part of the deficiency in length.

4. By insertion ofa wedge shaped piece of the ceramic in an incision in a bone to correct abnormal curvature of a bone. (i.e. osteotomy for correction of a skeletal deformity).

5. For filling a mastoid cavity following the removal of infected spongy bone from this area.

6. To cap the stump of an amputee where the length of skin available is insufficient without removal of further bone. By the use of a cap, some of the remaining bone length can be preserved.

7. Replacement of complete: bones e.g. carpel and tarsal bones.

In any of these cases, the ceramic may be used in a porous or fully dense form. The former has the advantage of permitting bone and tissue ingrowth into the pores thus assisting in attachment of the ceramic to the skeletal framework. The more porous material would also have the advantage of a lower elastic modulus closerto that of normal bone. On the other hand, fully dense silicone nitride would possess a higher strength than the porous variety and would therefore be preferable where stresses are high. Anchorage to the skeleton can still be obtained by adhesion and mechanical interlocking of the bone and the ceramic. In cases where the prosthesis could be subject to stress or is load bearing, it is possible to include a metal shaft or metal portion bonded to the ceramic for extra strength.

The methods of using Silicon Nitride in orthopaedic applications are described in more detail below.

Joint Replacement The total replacement, or partial replacement of joints and joint surfaces in the management of degenerative joint disease is now an advanced science. It is today possible to replace almost all diseased limb joints. Without doubt the most successful results have to date been in regard to total or partial replacement of the hip joint.

Current endoprostheses utilise combinations of metals, metals and high density polyethylene, silastic materials, and more recently combinations of metals and Al2O3 ceramics and Al2O3 ceramics alone.

However the various disadvantages of conventional endoprostheses are well known and have stimulated the search for new materials and methods of fixation of the endoprosthesis to bone.

Most endoprostheses require bonding to bone using polymethylmethacrylate bone cement. The use of polymethylmethacrylate bone cement is particularly open to question. If one considers the knee joint in particular it is universally agreed that 10% of knee joint endoprostheses loosen within 2 years.

The problems of fixation of this prosthesis to bone have not been solved. One of the basic problems presently encountered in the use of all permanent orthopaedic implants is that of forming a strong and lasting attachment between living bone and the implant. Certainly some method of secure implantation without bone cement is required. Al2O3 has demonstrated excellent biocompatability and has been shown to permit direct bonding to living bone without the need for polymethylmethacrylate bone cement.

Silicon nitride ceramics offer mechanical advantages over aluminium oxide ceramics and can replace the use of Awl203 in the design and application to orthopaedic endoprostheses.

In this regard Silicon Nitride can be used in combination with conventional metallic prostheses, either to coat the surface of the metallic prosthesis, or to coat that part of the prosthesis which is inserted into the bone; in this regard there is the advantage of bone ingrowth into the Silicon Nitride layer to secure the implanted prosthesis.

Bone Replacement Where there is the need for surgery, primary or secondary malignant tumours or extensive areas of osteomyelitis present common therapeutic prob lems. Radical extirpation of the pathology may preclude presentation of a weight bearing extremity.

Extirpation of such pathology and its segmental replacement by bone graft is limited by the available bone of the donor. The use of Silicon Nitride ceramic segmental spacer endoprosthesis will permit the more extensive segmental extirpation of diseased bone and its replacement to preserve a weight bearing extremity.

Recent studies in the use of high dose Cytotoxic chemotherapy to control the development of metastase from malignant tumours has been encouraging.

It may be possible in the future to preserve a limb (previously condemned to amputation) by the combined therapy of segmental tumour extirpation, silicon nitride segmental replacement and high dose Cytotoxic therapy.

Where amputation is inevitable, Silicon nitride amputation stump endoprosthesis permits the preservation of increased stump length to facilitate prosthetic fitting. Presently amputees must attach their artificial limbs to a remaining stump by a series of belts, straps and buckles, a method which has not changed in concept for many centuries. According to the present invention, a Silicon Nitride prosthesis can be implanted directly into the remaining bone stump and exit through the muscle and skin to the exterior where the artificial limb can be attached.

Limb Length Inequality Limb length inequality from whatever cause is not an uncommon problem. It is agreed that it is not wise to lengthen a bone by more than 15% or its original length. To increase the length of a bone requires a procedure of gradual lengthening to permit continuity of bone to develop. The lengthening of bone by interposition of a Silicon Nitride segmental spacer can preclude the lengthy and often indeterminate process of conventional limb lengthening procedures.

Bone Deformity The correction of bone deformity presently requires osteotomy in association with removal of bone or insertion of bone graft to correct the deformity. Where a large segment of bone is to be replaced or when osteotomy will result in shortening of the limb the insertion of a wedge of Silicon nitride ceramic can be beneficial to preserve alignment and equality of limb length.

Cavities The filling of large cavities with bone whether from excision of infected or tumour bone creates surgical problems. Available bone graft may not be sufficient to fill the cavity. Silicon nitride ceramic permits the "filling" of large cavities.

Small Bone Replacement Currently small bones are replaced by prostheses manufactured of Silastic material. The properties of Silicon nitride are such that it can be used in the design and fabrication of small bone endopros theses (e.g. Carpal or Tarsal bones).

Amputation Stump Caps The design of prostheses is dependent upon acceptable length of remaining limb bone. When difficulty of prosthetic design occurs it may require amputation at a more proximal level. The use of Silicon nitride amputation stump caps permits the elongation of stump length, as has previously been demonstrated with the use of Awl203. This allows a prosthesis to be screwed or locked onto the ceramic cap without the need for complex and bulky harnesses.

In summary, it has been found that Silicon Nitride bioceramics simulate, to a certain extent, natural bone in its surface physical properties, and unlike prosthetic metals and polymers, allow for the attachment and anchoring of adjacent muscle tissue.

The similarity between Silicon Nitride prostheses and bone extend to their mechanical properties, which include comparable specific gravities, comparable co-efficients of friction, and comparable strength properties. The material from which the prostheses are made is highly inert and insoluble, and because the material is in its highest oxidation state, it will not corrode like metal prostheses. Thus there is no evidence of the prostheses causing allergenic or toxic reactions when implanted in the muscle-skeleton system. Further, they do not appear to promote fibroplastic responses in neighboring tissue orto have hemolitic, cellular, and proteolitic destructive characteristics, which are some of the criticisms applicable to prior art prostheses.

Although the invention has been described above with reference to preferred embodiments, it will be appreciated that numerous variations, modifications or alternatives may be substituted for specifically described features, without departing from the spirit or scope of the invention as broadly described.

Claims (8)

1. The use of silicon nitrides in orthopaedic endoprostheses or prosthetic devices.

2. An orthopaedic endoprosthesis or prosthetic device comprising silicon nitride.

3. An orthopaedic endoprosthesis or prosthetic device according to claim 2 wherein said silicon nitride is in a porous and/or a fully dense form.

4. An orthopaedic endoprosthesis or prosthetic device according to claim 2, comprising a metal member for attachment to skeletal bone capped with silicon nitride on working surfaces.

5. An orthopaedic endoprosthesis or prosthetic device according to claim 4, wherein said silicon nitride is in a porous and/or fully dense form.

6. An orthopaedic endoprosthesis or prosthetic device according to claim 2, comprising a silicon nitride member for attachment to skeletal bone capped with metal on working surfaces.

7. An orthopaedic endoprosthesis or prosthetic device according to claim 6, wherein said silicon nitride is in a porous and/or fully dense form.

8. The manufacture of a silicon nitride orthopaedic endoprosthesis or prosthetic device by any of the following methods: (i) reaction bonding; (ii) chemical vapour deposition; (iii) cold forming and sintering; and (iv) hot pressing.