according to the invention it is suitable to use a primary , secondary or tertiary orthophosphate and / or methyl ethyl phosphate as phosphate - containing bath and ammonium , potassium or sodium orthophosphate as primary orthophosphate . when carrying out the heat - treatment under reduced pressure , it is suitable to increase the temperature to 680 ° to 740 ° c ., preferably 710 ° to 720 ° c . according to a preferred embodiment of the process of the invention the first anodic oxide layer is formed by using an aqueous solution containing 3 to 7 % by weight , suitably 5 % by weight of disodium hydrogen phosphate , and by oxidizing with a current density of 15 to 25 ma / cm 2 , preferably 18 to 20 ma / cm 2 , until reaching a voltage of 105 to 125 v , preferably 115 to 120 v . the heat - treatment of the oxidized implants is preferably carried out in a temperature range of 300 ° to 450 ° c ., suitably 350 ° to 380 ° c ., for 5 to 120 minutes , preferably for 40 to 60 minutes . the pressure is maintained at atmospheric value in air or an inert gas , or under an overpressure of 100 to 200 pa during the heating - up period . in order to obtain a colourless , transparent oxide - ceramic coating , a vacuum heat - treatment is used at a temperature of 680 ° to 740 ° c ., preferably 710 ° to 720 ° c . in the second and third steps any of the electrolytes being useful in the first oxidation step or a mixture thereof can be employed to form the oxide layer . the value of the oxidation voltage is selected according to the colour desired of the implant coating : the colour of the coating is violet at 35 v , blue at 50 v and golden yellow at 115 v . the process according to the invention is preferably used on implants , devices of cheek , mandible and common surgery devices , fixing elements , screws and tooth roots , conveniently prepared from a high - purity metal , titanium or a titanium - base microalloy containing at least 98 % of titanium . the main advantages of the process according to the invention can be summarized as follows . a ) the oxide - ceramic coating ensures the formation of a durable and solid connection between the implant and the bone due to the relatively fast growing of the bone into the solid oxide - ceramic coating ( bone integration ). b ) in addition to a high mechanical strength , the implant ensures excellent biocompatibility . c ) in biological media the oxide - ceramic layer is less toxic than the titanium metal itself . d ) the corrosion - resistance of titanium is extraordinarily ( by several orders of magnitude ) increased by the oxide - ceramic protective coating simultaneously inhibiting metal ions , mainly titanium ( iv ), iron ( iii ), vanadium ( v ) and other ions releasing from the alloying elements , to enter the organism . e ) in case of a mechanical injury , the oxide - ceramic coating shows so - called &# 34 ; self - healing &# 34 ; properties due to spontaneous oxidation under effect of air and body fluids . f ) aesthetic demands may also be satisfied by the coating since a number of colours ( golden brown , violet , blue , greenish , golden yellow ) can be produced without the use of any dye by varying the thickness of the coating and the voltage , resp . g ) the titanium - core implants covered with an oxide - ceramic coating retain their advantageous properties inclusive of colour stability even after repeated sterilization . h ) x - ray examinations as well as the preparation of computer - assisted and mr tomograms are rendered possible or not significantly influenced by the oxide - ceramic layer . i ) it is proven by clinical results that the so - called osteointegration process occurs in a more rapid and safer manner by using implants with an oxide - ceramic layer in comparison to implants without this coating . j ) in opposition to the implants possessing a protective layer applied by known processes , e . g . plasma - spray or isothermic compression as well as to implants provided with a calcium hydroxyapatite coating ( of grade 5 according to the mohs &# 39 ; hardness scale ), a further advantage of the implants prepared by the process of the invention appears in a substantially higher hardness ( of grade 6 to 6 . 5 according to the mohs &# 39 ; hardness scale ) of the oxide - ceramic coating , which makes possible to prepare e . g . hc - type surgical screws with an excellent hardness and self - holing edge as well as titanium roots with screw construction . k ) the adhesive strength between the oxide - ceramic coating and the titanium core is significantly higher in comparison to that produced by processes known in the art since the oxide - ceramic coating is actually not a layer formed by simple &# 34 ; application &# 34 ; but a protective layer grown on the surface of the titanium core itself , which possesses stabilized crystalline oxide - ceramic properties . l ) the oxide - ceramic coating ensures uniform and reproducible layer thickness , size and shape for the implant . the process according to the invention is further illustrated by the following non limiting examples . tooth - root implants prepared from a titanium alloy used for implants , with a purity of about 99 % with composition and properties corresponding to the iso 5832 / ii ( grade 4 ) standard , are etched at 45 ° c . for 3 minutes in an aqueous solution containing . 6 % by weight of hydrogen fluoride ( hf ) and 23 % by weight of nitric acid ( hno 3 ) and then washed until fluoride - free . subsequently , the tooth - root implants are anodically oxidized at 85 ° c . in an aqueous solution containing 5 % by weight of disodium hydrogen phosphate ( na 2 hpo 4 ) with a constant current density of 20 ma / cm 2 until reaching 114 . 5 v . then the implants are further oxidized at a constant voltage for additional 60 minutes . after washing , drying and heat - treatment carried out under atmospheric pressure at 350 ° c . for 30 minutes , the implants are repeatedly anodically oxidized at 85 ° c . in an aqueous solution containing 5 % by weight of disodium hydrogen phosphate with a current density of 15 ma / cm 2 until reaching 112 v . thereafter the oxidation is continued at a constant voltage of 112 v for additional 60 minutes . at this time the current density measured in the oxidizing bath and related to the oxidized pieces amounts to a few μa / cm 2 . after washing and drying the tooth - root implants are repeatedly heat - treated at 300 ° c . under an overpressure of 50 to 100 pa for 30 minutes . the first step of the oxide - ceramic coating process is carried out as described in example 1 on so - called hc - type surgical screws used as safety (&# 34 ; emergency &# 34 ;) screws ( therefore prepared to blue colour for distinction ) prepared from microalloyed titanium ( containing about 98 . 8 % of titanium ) corresponding to the iso 5832 / ii standard , with a self - holing edge [ corresponding to the iso 9268 : 1988 ( e ) standard ], except that a solution containing 10 % by weight of disodium hydrogen phosphate is used for anodic oxidation . however , in the second step following the first heat - treatment , the screws are anodically oxidized in an aqueous solution containing 1 . 5 % by weight of ammonium dihydrogen phosphate ( nh 4 h 2 po 4 ) at 85 ° c . with a current density of 15 ma / cm 2 but only until reaching a voltage of 50 v . thereafter the oxidation is continued at a constant voltage of 50 v for additional 60 minutes . subsequently , a repeated heat - treatment is carried out at 300 ° c . under atmospheric pressure for 60 minutes . the operations carried out in the second step are repeated in a third step , except that a current density of 3 ma / cm 3 is used , instead of 15 ma / cm 2 . preparation of connecting surgical fixing sheets with a violet - coloured oxide - ceramic coating the process of example 1 is followed except that the heat - treatment in the first step is carried out at 150 ° c . for 110 minutes to obtain the oxide - ceramic coating or hole - containing sheet implants used for oral surgery and surgical fixing elements , prepared from a titanium alloy according to the iso 5832 / ii standard . however , in the second step following the first heat - treatment the connecting sheet implants are repeatedly oxidized at 85 ° c . in an aqueous solution containing 2 % by weight of sodium orthophosphate ( na 3 po 4 ) with a current density of 20 ma / cm 2 until reaching a final voltage of 35 v . then the oxidation is continued at a constant voltage of 35 v for additional 60 minutes . after washing and drying the sheet implants are heat - treated at 150 ° c . under atmospheric pressure for 110 minutes . the process described in any of examples 1 to 3 is followed , except that both heat - treatments are carried out in a vacuum oven under reduced pressure at 720 ° c . for 10 minutes in the first step and for 30 minutes in the second step . during this period nearly the total amount of the oxide - ceramic layer being on the surface of the implant is transformed to a rutile - type transparent crystalline tio 2 coating possessing a hardness of grade 6 . 5 according to the mohs &# 39 ; hardness scale . the process described in example 1 is followed , except that the first oxidation is carried out in an aqueous solution containing also 2 . 5 % by weight of disodium hydrogen phosphate ( na 2 hpo 4 ) in addition to 2 . 5 % by weight of potassium dihydrogen phosphate ( kh 2 po 4 ). the process described in example 3 is followed except that in the second step the oxidation is carried out in an aqueous solution containing also 1 % by weight of potassium orthophosphate ( k 3 po 4 ) in addition to 1 % by weight of sodium orthophosphate ( na 3 po 4 ). the process described in example 1 is followed , except that the first oxidation is carried out in a solution containing 20 % by weight of methyl ethyl phosphate to give a coating with excellent adhesiveness .
the invention relates to a new process for obtaining tissue - protective devices of bone surgery implants prepared from a medical - purity metal , titanium and / or a titanium - base microalloy containing at least 98 % by weight of titanium , by establishing a biocompatible coating on the metal surface by anodic oxidation after degreasing and chemical or electrochemical etching , which comprises carrying out the anodic oxidation of the etched implant surfaces in an aqueous solution of a phosphate concentration lower than 20 % by weight with a current density of 2 to 50 ma / cm 2 until reaching a voltage of at least 105 v , then , after washing to ion - free , heat - treating the thus - oxidized implants at a temperature between 120 ° c . and 750 ° c . for 5 to 120 minutes , and repeating once or twice the anodic oxidation and heat - treatment with the phosphate concentration , current density and temperature values as given above for the first step .