BACKGROUND OF THE INVENTION Implants are known for use as the anchoring members of dental and orthopaedic prostheses. To this end, the implant is inserted into a bore-hole drilled into the bone tissue of a bone tissue structure at a site where a prosthesis is required. A superstructure having the prosthetic part of the prosthesis is then secured to the implant.

In the case of a dental prosthesis, the superstructure will typically consist of a spacer or transmucosal component which engages to the implant to bridge the gingiva overlying the maxilla or mandible at the implant site and the prosthetic part, e. g. a crown, bridge or denture, is then secured to the spacer. There are various other forms that the superstructure can take as is known in the art. For instance, the prosthetic part may be secured directly to the implant as in US patent No. 5,180,303 (Hornberg/Regents of the University of California).

The long-term integrity of the prosthesis is highly dependent on the successful osseointegration of the implant with the bone structure, that is to say, the remodelling of the bone tissue in the bone tissue structure into direct apposition with the implant. A study on the factors which effect the osseointegration of implants was undertaken by Professor Per-Ingvar Brånemark and co-workers and the results were published in a book entitled"Osseointegrated Implants in the Treatment of the Edentulous Taw: Experience from a 10-Year Period", Almqvist &

Wiskell International, Stockholm, Sweden, 1977. It was found by Brånemark et al that successful osseointegration depends upon inter alia the use of biocompatible materials for the implant, for example commercially pure (cp) titanium and alloys thereof, and the surgical procedure adopted, for example leaving the implant unloaded for several months before adding the superstructure.

Implants are not necessarily always used as part of a prosthesis, in some instances they can be a"stand alone"structure. As an example, implants are known for use as bone fixation screws. The success of these"stand alone"implants is also highly dependent on their successful osseointegration.

To advance the osseointegration of implants in bone tissue it has previously been proposed to deposit a calcium phosphate coating thereon as a result of calcium phosphates having (i) excellent biocompatability, (ii) the ability to initiate a rapid biological response between the bone and the implant, and (iii) provide a "scaffold"for bone growing into apposition with the implant. This bioactive property of calcium phosphates can be attributed to the fact that the main mineral constituent of bones is a calcium phosphate called hydroxyapatite (Cas (PO4) 3OH).

A problem to-date, however, has been the relatively rapid dissolution and delamination of calcium phosphate coatings deposited on implants following their implantation in bone tissue. This mitigates the contribution the coating can make to the osseointegration process.

The present invention proposes to improve on this.

SUMMARY OF THE INVENTION According to the present invention there is provided an implant for implantation in bone tissue having a calcium phosphate coating deposited on a substrate

surface of the implant which has been grit blasted.

By"grit blasted"is meant blasting of the substrate surface with particles which are sufficiently hard for surface deformation to occur. As examples there may be mentioned particles of titanium, particles of an oxide, nitride or carbide of zirconium, aluminum, silicon, boron, titanium, niobium or hafnium or a mixture of such particles. In the case of particles of zirconium oxide and silicon oxide, zirconium dioxide and silicon dioxide are preferred.

Experimental data, as will hereinafter be presented, shows that a calcium phosphate coating on a grit blasted substrate surface has a greater resistance to dissolution and delamination than a corresponding coating on a non-grit blasted substrate surface.

According to the present invention there is further provided a method of producing an implant for implantation in bone tissue having a calcium phosphate coating deposited on a substrate surface of the implant which comprises the step of grit blasting the substrate surface prior to deposition of the calcium phosphate coating thereon.

Preferably, the substrate surface of the implant is formed from a titanium-based material, e. g. commercially pure titanium or a titanium alloy.

It is further preferable that the grit blasting be carried out with particles of titanium oxide, for example titanium dioxide. In this case, the particles of titanium oxide may have a grain size within the range of 1-2000, um. For large implants such as those used in orthopaedic surgery a grain size near the upper limit of this range may be preferred whereas for dental implants a grain size towards the lower limit of this range would be preferred, for example using particles having a grain size in the range 1-300Rm, more preferably in the range

10-250u. m and even more preferably in the range 10-90Fm.

Suitable methods for grit blasting are known per se and in the particular case of titanium oxide blasting details can be found in prior International patent application publication W092/05745 (Astra AB), the contents of which are incorporated herein by reference. As indicated in W092/05745, the blasting with titanium oxide particles may be carried out by air blasting, airless (mechanical) blasting or wet blasting.

It is yet further preferable that the calcium phosphate coating be thin enough so that the coating follows the surface geometry of the substrate surface. To this end, a physical vapour deposition (PVD) technique is advocated, examples of which are vacuum evaporation, ion sputtering, ion plating and ion beam dynamic mixing. The preferred PVD technique is the ion sputtering method known as magnetron sputtering, with radio frequency (RF) magnetron sputtering being particularly preferred.

In a preferred embodiment of the invention the calcium phosphate coating has a thickness which is greater than 0.1, ut. A coating with such a coating thickness has an increased resistance to dissolution and delamination and provides enhanced osseointegration as will be shown hereinafter. As an example, the coating thickness may be in the range 1-4 llm.

In another preferred embodiment of the invention at least a part of the calcium phosphate coating has a crystalline structure. This can be achieved by heat treating the calcium phosphate coating at an elevated temperature, for instance with infrared radiation. Having a crystalline content in the coating further increases the adherence of the coating to the substrate surface of the implant after implantation as will be shown hereinafter.

In an embodiment of the invention the implant is a dental implant.

The invention will now be illustrated by the following non-limiting Examples.

EXAMPLE 1 Commercially pure titanium discs having a diameter of 12 mm and thickness of 1 mm were either ground with 500-grit silicon carbide (SiC) paper or air blasted with titanium dioxide (TiO2) particles having a grain size in the range 10-90Rm at an air pressure of 5 bar.

A 2Rm calcium phosphate coating was then deposited on the discs by RF <BR> <BR> <BR> magnetron sputtering the discs at 400 W at a pressure of 5 x 10 3 bar for 5 hours on a rotating substrate holder in an Edwards High Vacuum ESM100 sputtering unit.

After sputtering, the discs were left as sputtered or subjected to a rapid heat treatment with infrared radiation for 30 seconds in air to maximum temperatures ranging from 250°C to 700°C, the temperature being measured with a Pt-Rh thermocouple.

Analysis of the as sputtered and heat treated coatings by X-ray diffraction (XRD) with a Philips 0-20 diffractometer using CuKa-radiation, Fourier transform infrared spectroscopy (FTIR) (Perkin-Elmer), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) revealed that the as sputtered coatings on the SiC-ground and TiO2-blasted discs had an amorphous structure. The analysis further revealed that depending on the heat treatment temperature used for the coatings on the SiC-ground and Ti02-blasted discs two different states resulted, namely amorphous and amorphous-crystalline, with crystallinity

increasing with increasing heat treatment temperature.

The dissolution properties of the calcium phosphate coatings on the as sputtered and heat treated SiC-ground and Ti02-blasted discs were compared by incubating specimens of the discs in 4 ml of Simulated Body Fluid (SBF) having a pH of 7.2 at 37°C. At incubation times of 1,2,3 and 4 weeks the SBF buffer solution was refreshed and the calcium and phosphate concentrations determined by flame absorption and spectrophotometry.

The dissolution experiment demonstrated that the amorphous and amorphous- crystalline calcium phosphate coatings on the as sputtered and heat treated SiC- ground discs delaminated within one week of incubation. The amorphous calcium phosphate coatings formed on the TiO2-blasted discs, on the hand, remained present for a longer duration although were still completely dissolved within 4 weeks incubation. In contrast, the amorphous-crystalline calcium phosphate coatings on the TiO2-blasted discs were still present after 4 weeks of incubation.

This Example shows that:- 1. A calcium phosphate coating formed on a titanium surface roughened by blasting thereof with TiO2 particles has a greater resistance to dissolution and delamination than a corresponding coating formed on a titanium surface similarly roughened by grinding with SiC paper (irrespective of the calcium phosphate coating being amorphous or amorphous-crystalline).

2. An amorphous-crystalline calcium phosphate coating on a titanium surface roughened by blasting thereof with TiO2 particles has a greater resistance to dissolution and delamination than a corresponding amorphous calcium

phosphate coating on a titanium surface roughened by blasting thereof with TiO2 particles.

EXAMPLE 2 One hundred and forty four commercially pure titanium discs having a diameter of 12 mm and thickness of 1 mm were air blasted on both major surfaces with TiO2 particles having a grain size in the range 10-90. m at an air pressure of 5 bar. A calcium phosphate coating was then deposited on each disc by RF magnetron sputtering with an Edwards High Vacuum ESM100 sputtering unit using as a target material a copper disc provided with a plasma-sprayed hydroxylapatite coating (CAMCERAM (E)). The deposition process was carried out at a pressure of 5 <BR> <BR> <BR> x 104 bar using a sputter power of 400 W and deposition rate of 100-150<BR> <BR> <BR> <BR> <BR> nm/min.

The RF magnetron sputtering was carried out for durations such that one third of the discs had a calcium phosphate coating of thickness of 0.1 gm, another third a calcium phosphate coating of thickness of 1, um and the final third a calcium phosphate coating of thickness of 4 p. m.

After deposition, half of the coated specimens in each group were subjected to an infrared heat treatment for 30 sec at 425-475°C to give a total of six distinct sub- groups of twenty four calcium phosphate coated discs, namely:- (1) discs provided with an as sputtered coating of 0.1, um (CaP-0.1); (2) discs provided with a heat treated coating of 0.1, um (CaP-0. lHT); (3) discs provided with an as sputtered coating of 1. Ogm (CaP-1);

(4) discs provided with a heat treated coating of 1.0, um (CaP-lHT); (5) discs provided with an as sputtered coating of 4.0, um (CaP-4); and (6) discs provided with a heat treated coating of 4.0 um (CaP-4HT).

The coatings deposited on the discs were characterised by thin film XRD with a Philips 8-26 diffractometer using CuKa-radiation, FTIR (Perkin-Elmer), SEM and EDS from which it was observed that :- * The as sputtered coatings of the CaP-0.1, CaP-1 and CaP-4 discs had an amorphous structure.

* The heat treated coatings of the CaP-0. lHT, CaP-lHT and CaP-4HT discs had an amorphous-crystalline apatite structure with tetracalciumphosphate being detected as a second phase in the coatings of the CaP-0. lHT and CaP-lHT discs.

* All of the coatings showed an excellent coverage of the substrate surface.

* The Ca/P ratio of the coatings varied between 1.8-2.0.

In two surgical sessions the discs (after ultrasonic cleaning and sterilisation in an autoclave) were placed subcutaneously into the back of twelve female New Zealand white rabbits for implantation periods of 1,4,8 and 12 weeks. Before surgery the skin was shaved, washed and disinfected with iodine. During each surgical session six longitudinal incisions of about 1.5 cm were made parallel to the spinal column. Lateral to these incisions a pair of small subcutaneous pockets was created by blunt dissection with scissors. The discs were inserted into these pockets and the wounds were closed using Vicryl 3-0 intracutaneously. Each

rabbit thus received twelve discs meaning that at the end of the animal experiment there were six specimens from each sub-group for each implantation time.

After killing the appropriate animals at the end of each implantation period by injecting an overdose of pentobarbitalsodium (NembutalOO) peritoneally, the skin was shaved and the discs with surrounding tissues excised immediately. Three discs from each sub-group per implantation period were left in the surrounding tissues from which sections were prepared for evaluation of the tissue response to the discs by light microscopy. The three remaining discs of each sub-group per implantation period were removed from the surrounding tissues for examination by XRD, FTIR, SEM and EDS.

Light microscopy showed that after an implantation period of one week the tissue reaction to each sub-group of discs was mainly characterised by an inflammatory response. The discs were surrounded by a moderately thick loose connective tissue capsule containing many inflammatory cells and blood vessels.

After four weeks of implantation light microscopy showed that the discs from each sub-group were surrounded by a thin to medium-thin fibrous tissue capsule which was almost free of inflammatory cells and contained fibroblasts, collagen and blood vessels. Those few inflammatory cells present in the capsules were generally accumulated at the interface between the surface of the discs and the tissue capsule. The fibroblasts appeared as either active cells with a round nucleus or as highly elongated cells with a flattened nucleus. The collagen fibres of the capsule ran parallel to the surface of the discs.

At eight and twelve weeks post-implantation light microscopy showed that the tissue response became very uniform with each sub-group of discs being surrounded by a very thin connective tissue capsule usually free of inflammatory cells. At the interface there was a close contact between the capsule and surface of

the discs and no inflammatory cells could be seen.

Physicochemical analysis of the CaP-0.1, CaP-1 and CaP-4 discs post-implantation by XRD, FTIR and SEM confirmed that :- (i) the coating on the CaP-0.1 discs had dissolved after 1 week of implantation, (ii) the coating on the CaP-1 discs had dissolved after 4 weeks of implantation, and (iii) the coating on the CaP-4 discs was still present at least in part after 12 weeks of implantation.

By comparison, physicochemical analysis of the CaP-0. lHT, CaP-lHT and CaP- 4HT discs post-implantation by XRD, FTIR and SEM confirmed that:- (i) the coating of the CaP-O. lHT discs, was still present after 4 weeks of implantation, (ii) the coating of the CaP-0. lHT discs disappeared completely after 8 weeks of implantation, and (iii) the coating of the CaP-lHT and CaP-4HT discs was present at all implantation periods with the tetracalciumphosphate phase in the CaP-lHT discs disappearing.

This Example shows that:- 1. A titanium implant having a TiO2 particle-blasted surface which supports a calcium phosphate coating thereon promotes an acceptable body tissue response.

2. An amorphous-crystalline calcium phosphate coating on a titanium surface roughened by blasting thereof with TiO2 particles has a greater resistance to dissolution and delamination than a corresponding amorphous calcium phosphate coating on a titanium surface roughened by blasting thereof with TiO2 particles.

3. The rate of dissolution and delamination of amorphous and amorphous- crystalline calcium phosphate coatings on a titanium surface roughened by blasting thereof with TiO2 particles has a coating thickness dependence.

EXAMPLE 3 Thirty six screw-type implants manufactured from commercially pure titanium and having a length of 12 mm, diameter of 4 mm and outer peripheral surface air blasted with TiO2 particles having a grain size in the range 10-90pLm at an air pressure of 5 bar (Astra Tech AB, Mölndal, Sweden) were provided with infrared heat treated calcium phosphate coatings by RF magnetron sputtering as in Example 2 such that:- 1. Twelve implants had a 0.1, um thick coating (CaP-0. lHT').

2. Twelve implants had a 1 gm thick coating (CaP-lHT').

3. Twelve implants had a 4, tLm thick coating (CaP-4HT').

After the coating process the implants were cleaned ultrasonically and sterilised in an autoclave. The sterilised implants were then inserted into the trabecular bone of the medial femoral condyles of the hind limbs of twelve adult female Saanen

goats.

For insertion of the implants the animals were immobilized on their back and the hind limbs shaved, washed and disinfected with povidone-iodine. A longitudinal incision was made on the medial surface of the left and right hind femurs and the medial femoral condyles exposed for two pilot holes to be drilled with the distance between the holes being at least 1 cm. The pilot holes were gradually widened with five differently sized drills to a diameter of 3.85 mm. The bone preparation was performed with a very gentle surgical technique, using a low drill speed (1500 rpm) and continuous external cooling. The implants were inserted by hand into the expanded pilot holes and following insertion the soft tissue closed in separate layers using resorbable Vicryl 3-0 sutures. In this way each goat received four implants, two in each medial condyle of the left and right hind femurs. The bone-implant interface was evaluated after 6 and 12 weeks of implantation by sacrificing six animals at each predetermined endpoint by an overdose of Thiopental and natriumchloride and then excising the femurs.

Light microscopy evaluation of sections of the excised implants showed an uneventful healing of all implants without any sign of inflammatory response. At the marginal cortical bone level the presence of a peri-implant space was observed only occasionally and when present this space was filled with fibrous tissue. For most implants the marginal bone was level with the top of the implant. All implants were inserted for their major part in trabecular bone and the drilling procedure appeared to be very accurate.

The excised implants were also subjected to histological and histomorphometrical evaluations consisting of:- (i) A subjective description of the bone-implant interface.

(ii) Measuring the percentage of implant length at which there is direct bone-to- implant contact without an intervening soft tissue layer. This was done by making measurements along the entire length of the implant, along the first four coronal screw threads (Region 1) and along the middle part of the implant (Region 2).

(iii) Measuring the amount of bone mass. This was determined in proximity to the middle part of the implant by measuring in triangular regions inside and outside the screw thread. The measured data was related to the total area inside the screw which was slightly different for the different sections. The amount of bone mass was quantified in mm2.

The histological results for the CaP-lHT'and CaP-4HT'implants were very comparable. After six weeks of implantation an intimate contact with both cortical and trabecular bone could be observed. In the area directly adjacent to the implant new bone formation had occurred on the implant surface without any intervening soft tissue layer and remodelled lacunae with osteoblasts was visible. No accumulation of inflammatory cells next to the implant was observed. Very occasionally osteoclasts were visible at the bone-implant interface. At the cortical section the screw threads were almost completely filled with dense bone. In the femoral trabecular compartment the bone present at the implant surface had a tentacle-like appearance. The bone attached to one point, mostly at the top of the screw thread, and was growing as a thin layer over the implant surface into the screw thread. New bone formation was also observed at the apical bottom of the implants.

At twelve weeks, remodelling and compaction of the lamellar bone-implant interface had proceeded. The newly formed bone could not be, or hardly be, discerned from the surrounding"old"bone. The quality of the apposed cortical bone was very similar to the original cortical bone. Furthermore, bone in-growth

into the screw threads which were in contact with trabecular bone had increased.

The screw threads were completely covered with bone which was in close contact with the implant surface without any sign of fibrous tissue formation or an inflammatory reaction. The calcium phosphate coating had thus acted as a scaffold for bone growth.

The histological evaluation for the CaP-0. lHT' implants showed that after 6 and 12 weeks of implantation there was a similar bone formation compared with the CaP- 1HT'and CaP-4HT'implants at the cortical side. Abundant dense mature bone was closely laid down to the implant surface without intervening fibrous tissue interface. A difference was observed, however, in the trabecular bone-implant interface. After 6 weeks bone-implant contact existed only at the top of the screw threads. After 12 weeks of implantation trabecular bone in-growth into the screw threads of the CaP-0. lHT'was dramatically less pronounced than for the CaP- 1HT'and CaP-4HT'implants. The bone contact was frequently still limited to the top of the screw threads. The regeneration of bone did, though, start from the top of the screw thread and advance from this point over the implant surface into the screw thread. The CaP-0. lHT' implants also showed less bone deposition at their apical bottom with this only being completely covered with bone occasionally.

Tables 1 and 2 hereinbelow show the results of the histomorphometrical evaluation of the implants for percentage of bone contact. More specifically, Table 1 gives the bone apposition data for the various implant surfaces, implantation times and evaluation areas whilst Table 2 gives a statistical comparison of the bone apposition between the different implants using a one way analysis of variance (ANOVA) and a Tukey multiple comparison procedure. The data in Tables 1 and 2 illustrates that the coatings on the CaP-lHT'and CaP-4HT'implants result in a higher percentage of bone contact with the implant.

Table 3 hereinbelow shows the bone amount measurements for the implants at

both implantation periods provided by histomorphometrical evaluation.

Statistical testing (analysis of variance and Tukey multiple comparison) was focused on the possible existence of a difference in bone mass amount between the various implants inside the screw threads and outside the screw threads. The analysis revealed that at 12 weeks the CaP-lHT'and CaP-4HT'implants had more bone inside the screw threads than the CaP-O. lHT' implants.

This Example shows that:- 1. A titanium implant having a TiO2 particle-blasted bone contacting surface which supports a calcium phosphate coating thereon is biocompatible in bone tissue.

2. A titanium implant having a TiO2 particle-blasted bone contacting surface which supports a calcium phosphate coating thereon promotes osseointegration of the implant in bone tissue.

3. The osseointegratability of a titanium implant having a TiO2 particle-blasted bone contacting surface which supports a calcium phosphate coating thereon has a dependency on the thickness of the calcium phosphate coating.

Similar results to those outlined above in the foregoing Examples would be expected if, instead of cp titanium, the discs or implants were formed from a titanium alloy or other biocompatible material suitable for implantation in bone tissue. Similar results to those outlined above in the foregoing Examples would also be expected if grit blasting was carried out with other particles sufficiently hard for surface deformation to occur and/or a deposition technique other than RF magnetron sputtering were used.

A titanium-based substrate surface in combination with blasting with titanium- based particles such as oxide particles is expected to give the best results due to a possible synergistic effect between a calcium phosphate coating and such a substrate surface.

The Examples provide support for a conclusion that calcium phosphate coatings on implants in accordance with the present invention exhibit an unexpectedly improved resistance to dissolution and delamination post-implantation than an implant whose surface has been roughened by other means, e. g. grinding with SiC paper. This improved resistance to dissolution and delamination should in theory lead to implants in accordance with the invention also exhibiting improved osseointegration in bone tissue.

Table 1-Bone contact percentages for the various implant surfaces and evaluation areas.

Implant 6 Weeks 6 Weeks 6 Weeks 12 Weeks 12 Weeks 12 Weeks Bone Bone Total Bone Bone Total Region 1 Region 2 Implant Region 1 Region 2 Implant Surface Surface CaP-69 (10) 59 (9) 63 (7) 55 (19) 47 (16) 53 (17) O. in CaP-81 (23) 75 (20) 78 (17) 77 (11) 74 (12) 76 (4) 1HT' CaP-86 (7) 68 (21) 65 (16) 91 (6) 80 (6) 83 (6) 4HT' The values between parentheses represent standard deviations, the results being based on testing of 6 implants of each'coating thickness.

Table 2-Tukey test on implant surface variables. Variable 6 Weeks 6 Weeks 6 Weeks 12 Weeks 12 Weeks 12 Weeks Bone Bone Total Bone Bone Total Region 1 Region 2 Implant Region 1 Region 2 Implant Surface Surface CaP-0.1HT' n.s. * * * * * -CaP-1HT' CaP-0.1HT' * n.s. n.s. * * * -CaP-4HT' CaP-1HT' n.s. n.s. * * n.s. n.s. -CaP-4HT' CaP-1HT' n.s. n.s. * * n.s. n.s. -CaP-4HT'

* = stabsfical significant difference (P<0.05) n. s. = no statistical significant difference

Table 3-Bone mass (%) measurements for the various implant surfaces and implantation periods Implantation Implant Bone Mass Inside Screw Bone Mass Outside Period Thread (mm²) Screw Thread (mm²) 6 weeks CaP-O. lHT' 53,7167 (22,9243) 52,6333 (14,1327) 6 weeks CaP-lHT'46,2500 (12.0663) 47, 7167 (14,3465) 6 weeks CaP-4HT'43,8333 (21,1674) 49,8833 (20,4770) 12 weeks CaP-O. lHT'42,4667 (7,2745) 52,9500 (10,4642) 12 weeks CaP-lHT'47,0000 (4,9518) 45,5000 (3,6643) 12 weeks CaP-4HT'49,9167 (18,6154) 58,2833 (18, 8845) The values between parentheses represent standard deviations, the results being based on testing of 6 implants of each coating thickness.