some 20 years clinical history with a cementless , straight - stemmed hip has demonstrated that the typical preferred surface properties are : s t ten - point height of the surface is in the order of 50 to 100 microns . s q root mean square deviation of the surface is in the order of 5 to 10 microns . an example of the preferred surface features is shown in fig1 and was measured by non - contacting surface profilometry that is a technique that is able to represent a 3 - dimensional pictorial display of the surface . in the preferred surface profile , significant quantities of asperities protrude to give good initial interlock between the stem and the bone and thus imparting early post - operative stability . there is a balance of the valleys to cater for eventual bone in - growth . the surface roughness is generally achieved by blasting the surface to be roughened with a blasting medium such as alumina particles . in the preferred method described , the invention is applied to a prosthesis which has an insertion portion or stem that is to be inserted into a human bone and which extends from an operative portion . fig2 shows a femoral prosthesis to which the present invention can be applied . the prosthesis comprises an insertion portion in the form of stem 1 which extends from an operative portion provided by neck 2 on which is a tapered spigot or trunnion 3 to which a prosthetic head in the form of a bearing ball can be fitted in known manner . the roughened decontaminated surface indicated by reference numeral 4 is formed on stem 1 . the block diagram shown in fig3 illustrates the process according to the invention . prior to treating the titanium alloy prosthesis operative portion 2 and 3 of the stem is either dipped or otherwise wrapped to provide a protective covering of polyurethane or other suitable material , which extends down to the proximal end of the part of the stem to be treated and which is indicated by reference numeral 5 . thus , all the upper part of the prosthesis above line 5 is covered by the protective coat , which is indicated by broken lines 6 in fig2 . in a preferred embodiment , prior to the protective coating , however , and as shown in the flow chart of fig3 the prosthesis is first degreased , for example , by using trichlorethane as indicated in box 1 . the protective coating is now applied as indicated in box 2 , and the prosthesis is now passed to blasting as indicated in box 3 . in the example being described a guyson multiblast six - station machine is used and the prostheses are held in position by clamps , which are attached to the protected portion provided by , for example , polyurethane covering 6 . as the prosthesis is made from machined titanium alloy its outer surface has hardened so , in the method being described stem 1 is first blasted with g12 chilled iron grit for 3 minutes with a 9 mm nozzle at 6 . 5 bar air pressure . the surface is then blow cleaned with air as indicated by box 4 and is then blasted with g07 chilled iron grit , again for 3 minutes with a 9 mm nozzle at 6 . 5 bar pressure as shown in box 5 . the surface is again blown clear by compressed air as shown in box 6 and in the preferred method , the prosthesis , as shown in box 7 is placed in a bath containing 20 % nitric acid for more than 20 minutes and less than 40 minutes with ultrasonic agitation . the prosthesis is then rinsed in high pressure water as indicated in box 8 and subsequently air blown dry ( box 9 ). a second embodiment is shown in fig4 and , as indicated box 1 , the product is first degreased using for example an alkaline solution . a protective cover is applied over the spigot , as described with reference to fig3 as indicated in box 2 , and the prosthesis is now passed to the blasting station as indicated in box 3 . in the example being described a guyson multiblast six - station unit is again used as previously described . in this case , a single blast using the g12 chilled iron grit for 4 minutes with applied air pressure of 5 bar and 9 . 5 mm nozzle is employed . the surface is then blow cleaned with air at 5 bar as indicated in box 4 after which it is placed in a bath containing 20 % nitric acid for no less than 20 minutes and no more than 40 minutes with continuous ultrasonic agitation as indicated in box 5 . as indicated in box 6 , the component is then rinsed in a bath with warmed tap water for a period of 20 minutes , again with ultrasonic agitation . the process to this point has removed all of the readily solubilized iron salts . a further step to remove any insoluble salts utilizes pickling in a bath of 20 % hydrochloric acid for more than 20 minutes and less than 40 minutes with ultrasonic agitation as described in box 7 . the prosthesis is then rinsed in a warmed tap water bath with ultrasonic agitation as indicated in box 8 . a final washing in warm demineralized water again employing ultrasonic agitation completes the cleaning process as indicated in box 9 . the product is then air dried at ambient temperature for 20 minutes or until dry . both embodiments described can be operated in a manual or fully automated mode , or some appropriate combination . c si co ni cr mo s p pb fe 3 . 01 1 . 69 0 . 35 0 . 33 0 . 19 0 . 06 0 . 078 0 . 069 0 . 0016 ba - lance grit selection is determined by the surface hardness of the material for the desired surface roughness . for un - machined ( as forged ) titanium alloy it is possible to use g07 grit only to obtain the required level of surface roughness and the stages shown in boxes 3 and 4 will not be necessary . for the machined titanium alloy ( the machining operation work hardens the titanium alloy surface ) as describe above , it is necessary to use 2 grits . a first blast with g12 gave a rough , peaky surface . a second blast with g07 cuts the peaks down . if the hardness is even greater , it may be necessary to use even rougher grit to give the initial cut into the surface . the nitric acid leach does alter the roughness of the surface after blasting by a small amount . this is not , however , by attacking the surface of the alloy , but by dissolving the iron grit embedded in the titanium alloy surface . it has been shown that the roughness of the clean surface is unaltered by a further acid wash so indicating that the acid does not chemically attach the metal surface . the blasting time will depend upon the size of the implant and the metal , but is preferably between 3 to 4 minutes with a stand off distance of 10 - 50 mm . it has been found ( using edax probing for elemental analysis ) that the surface is substantially uncontaminated apart from small quantities of zirconium and molybdenum , the nitric acid acting to dissolve the residual iron grit , and this also provides a surface , which , depending upon the resting time can be s q 5 - 10 microns . ( as determined using non - contacting surface profilometry .) [ 0047 ] fig6 is an edax profile similar to that shown in fig5 of a ti - alloy but showing additional contamination by oxygen , aluminum and silicon when a known method of producing a roughened surface is used which does not include the process set forth above . comparison of fig5 and 6 clearly indicates the reduction in contamination achieved by the present invention over the previously known process . although the invention herein has been described with reference to particular embodiments , it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention . it is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims .
a method of forming a roughened , decontaminated surface on a metal article which includes the single or multiple blasting of the surface with a chilled iron grit of appropriate roughness structure followed by acid pickling to produce a contamination free surface of substantially s q 5 to 10 microns .