CA1258354A

CA1258354A - Implantable prosthesis having a coated porous surface

Info

Publication number: CA1258354A
Application number: CA000486965A
Authority: CA
Inventors: Jack E. Parr; Phillip J. Andersen
Original assignee: Bristol Myers Co
Current assignee: Bristol Myers Co
Priority date: 1984-08-27
Filing date: 1985-07-17
Publication date: 1989-08-15
Also published as: GB8517417D0; BE903000A; US4863475A; MX163111B; ES546488A0; FR2569566B1; IT1182817B; ES8700045A1; ES294804Y; JPS61106149A; GB2162065B; GB2162065A; FR2569566A1; IT8548470A0; NL194114B; NL8502127A; DE3527136A1; KR860001587A; KR880001652B1; DE3527136C2

Abstract

ABSTRACT
An implantable prosthesis including a base member, such as the stem of a hip joint prosthesis, having a porous region on its surface, and the region of porosity being coated with a bioabsorbable material, such as .alpha.-tricalcium phosphate, which enhances permanent bone ingrowth into the region. A method of manufacture of the prosthesis includes the steps of providing a coating material and applying the material to at least a portion of the porous surface of the base member, while providing energy sufficient to transform the material to a state in which it is bioabsorbable. In the preferred embodiment the material is plasma sprayed onto the porous surface of the base member.

Description

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~A-~T--*N3~ET~ P~R---PROl)U~T~- T~E~l~OP-,, BAC~GROUND OF THE INVENTION

Field of the Invention. The invention in general relates ~o medical prosthese6 for implantation in a body, and more particularly to prosthe6es which are fixed within the body by ~; means of bone ingrowth into the pro~the~es.
Desc~iption of the Prior Art. Medical devices such as bone plate6, nail~, pin6, ~crew~, and joint pro~the~e~ have been commonly implanted in~o the skeletal ~tructure of human~ and animal~ for many years to join the parts of a frac~ured bone or to replace missing or damaged skeletal parts. Often it i~
intended that these parts become a permanent part of the body.
In ~uch cases it i~ important that the par~ be stro~ly and permanently fixed to the 6keletal bone 6truc~ure.
It has been known for more than 10 years tha~ 6trong permanent fixation can be obtained by the u6e of a porous 6urface , on the whole or a portion of the part, provided that the mean pore ~i~e exceeds about 50 micrometer~. However, the older~
I methods o~ bone fixation, primarily fixation by friction fit or I with methyl methacrylate bone cement, ~till are the most i~ predominately u~ed methods of fixation, in spite of the fact that the loosening of friction fitted and cement bonded pros~heses parts over time remains a significant medical problem. This i~
~because in order to obtain good fixation by means of bone 25 ,'ingrowth, the patient must refrain from applying force or loading ~on the cheletal im~lant until the bone growth occUrs~ whereac in ~!
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the case of friction fit or cement bonding 6keletal loading can take place almost immediately.
In this diselo6ure, the word~ bioab~orbable and ~esorable mean ~hat ~he sub~tance to whioh the ~erm i~ applied i8 broken down, absorbed, or otherwl6e removed by the host body chemi6try in amounts sufficient to alter the physical 6~ructure of the portion of the devi~e which is bioab~orba~le within a time period less than the pe~iod it takeR for ~ones to fully mend. It is un~er~ood tha~ all material~, even steel, are ~ligh~ly absorbed by the body chemi6try, however, such slight ab~orbtions, which do not alter the phy~ical 6tructure of the bioab60rbable device over the period of bone healing, are not ~ncluded in ~he terms bioabsorbable and resorbable. It is also underEtood that although a material i8 bioabsorbable, ~ome small amount of it may ~5 remain in the body for longer period~.
one method shown to encourage earlier bone inglowth into , porous surfaces of implants i8 the ~oating of porou6 metal fiber6 . with hydroxyapa~i~e by dipping the fiber into a water 61urry of hydroxyapatite and drying. Hydroxyapaeite i8 not generally ~ considered to be re~orbable in the human body. It wa~ found tha~
I the hydroxyapatit2 encourages mor~ rapid ingrow~h of bone into the porous metal surf~ces ~or a time pQriod of up to four weeks after ~mplan~, but that the effect was shor~ term, in tha~ the llamount of bony tissue ~lthin the pores declined after the ~our 25 1l week perlod, P. Ducheyne, et al. "Effect of Hydroxyapatite ,~Impregnation on Skeletal Bonding of POrOUB Coated Implants'' Journal of_ Blomedical Materials ~e~earch, Vol. 14, 225-237 ; (1980). In addition to the fact that the in~r~ased ingrowth was, ~ not permanent, it has been found that the hydroxyapatite en~ourage6 fibrous rather than bony tis~ue growth at the fixation , '~
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si~e over long ~ime period6, which can result in loosening of the prosthe~es. Further, the hydroxyapaplte coating in the ~ucheyne et al. article i8 relatively fragile and can be ea~ily bro~en under normal handling for commercial produ~ts, and thus it does not lend it6elf to widespread com~ercial u~e.
The followlng United S~ates patents relate ~o the aspects of the pre6ent invention as indicated. United State6 Patent No. 3,605,123 is6ued to H. Hahn di6closes tbe plasma spraying of a metal porous aurface onto a pro~ehe~is. Uni~ed State~ Paten~ Nos. 3,892,648 and 3,892,649 issued to David C.
Phillip~ et al. disclose the elec~rodepoæition o~ bone and collaqen on implants or ints a plastic me~h on implants to stimulate bone attachment to the implant.
United State~ Patent No. 3,919,723 di~closes the embedding of calcium and phosphate atoms i~ the ~urface of a;
ceramio implant by heating the implant and embedding it in a mQlt of cal~ium phosphate material. It i8 speci~ically indicated that temperatures in exce6s of 1300 C should not be used since the ,~ calcîum pho~phate material decomposes at higher temperatures.
20 1 This decompo~ition at higher temperatures i8 oharacteri6tic of ;calcium phosphate ~aterials. See E. Hayek and H. Newesely, Inorqanic Synthesis 7 (1963) ~3.
! uDited States Patent No. 4,Z02,055 i~sued to Reiner et~
I'lal. discloses the ~ombining of a bioabsorbable, bioactive calcium 25 I pho~phate with a polymer on the Rurface of a prosthesis, to ,create bone ingrowth into the polymer. United State6 Patent Nos.
1,4.365.357 and 4.373,217 issued to Draenert disclose the combininq ~of absorbable tricalcium phosphate material with bone cement to llcreate bone growth into the cement. Each of the above three 30 ¦¦ patents involve the incorporatlon of the absorbable material into I
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I! the material out of which the ~urface i~ compo~ed, and ,j contemplate that the porou6 sur~ace iB created by ~bsorption of 'I ~he absorbable ma~erial thereby leaving pores in the ~urface in the po6ition of the ~a~ancy created by the abfiorption of the absorbable ma~erial. None of the6e patentg suggeRt the coating of an already porou6 surface with an ab~orbable ma~eri~l in order to enhance bone growth.
United Statas Patent No. 4,33~,92~ i~sued to Kummer et al. di~clo~e6 the addition of a bioabsorbable layer .1 to lmm ! thick on an implant surface; the intention ~ ~o create loo~ening of the implant as the material ~ absorbed. The pro~thesis on which the absorbable layer is placed i6 ~pecifically non-porou6, ~ince bone ingrowth i8 to be discouraged.

., , SUMMARY OF TH~ INVENTION
It i8 an ob3ect of the invention to provide a prosthesi6 that re~ult~ in rapid and permanent bone ingrowth into a porou~;
surface thereby providing early. strong, and permanent fixation-of the implant into the 6keletal structure.
, I~ is a further object of the invention to provide a 20 ~i pros~he6is, and a ~ethod of manufacturing the prosthesis, whl~h;
il overcome the disadvantage6 of ~he prior art prostheses that were Il intended to be permanently af f ixed to bone.
',, The invention provides a part for attachment to skeletal, bone comprising a ba~e member having a porou% region on the 25 1¦ 6urface thereof, and a bioabsorbable coating on at least a ¦ portion of the region of porosity, which material enhance6 permanent hone ingrowth into the region of poro~ity.
!l It ha6 been found that the bafie ~ember doe6 not loose~
las the porous surface coating i8 ab60rbed as suggefited in the il _ , 35i~

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peior art. Rather, the ab~orbable material, in a manner not entirely understood, encourage6 fa~er and stronger bone ingrowth into the porou~ ~urface which it covers. Further, the ingrowth . stimulated remains permanently, and is a bony material, rather than a fibrous, weak ti6sue as has been ~ound to occur ~ith hydroxyapatite coating~.
The invention provides a method of manufac~uring a p~o~thetic part for u6e as a body implant comprising the steps of: providing a base member having a porous 6urface region on which bone attachment i~ de6ired; prov~ding a material which after heating to a high te~perature cool~ ~o a ~tate in which it is bioab~orbable; heating the material to a temperature greatsr than 1350C and applying it to at lea6t a portion of the porou~
. surface region of the base member. Preferably the material 15 ~, include6 at lea~t one sub6tance ~elected from ~he group con6isting of hydroxyapatite and ~ -trical~ium pho~phate and upon heating and cooling the material tran6~0rms ~o primarily ; ~-tricalcium pho~phate. It has been found that the high temperature application of the coating re~ults in a coating that i~ both bioab~orbable and more adherent than ~he prior art coatings. It has been ~ound that the coating i6 sufficiently ;' a~herent to enable the parts to be box~d, s~eriliæed, and handled l generally for implantinq purposes ~ithout loosening from ~he base llmember.
25 ¦' Thus, the invention has 601ved a problem long recognized ¦in the prior art. Numerou~ other a~pects, feature~, object~, and advantage6 of the ~vention will now become apparent from the following detailed description when read in conjunction with ~he ,.accompanying drawings, in which:

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BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 i~ a 8ide view of a body implan~ according ~o one embodiment of the invention;
I Fig. Z shows a perspective view of another embodimen~ of , the inventlon;
Fig. 3 ~hows a draftsman'~ rendering of an actual photograph of a coated porous surface of the type gmployed in the embodiment of Fig. 1 maqnified about 20 times:
~ ig. 4 i~ a draft~man's rendering of an ac~ual scanning electron micrograph of a ~etallorgraphic ~ection of ~he material oP Fig. 3 magnified 63.5 ~ime~, the surface of the specimen being at the lower right in the Fig.: and Fig. 5 i~ a draftsman'~ render~ng of an actual photograph of a section as in Fig. 4. but magnified 94a times and showing only a portion of one of the metal wire~ forming the , porous 6urfa~e and the coating of ~he ~ire.

,~ DESCRIPTION OF THE PREFERRED EMBODIMENT
Refereing to Flg. 1, there i8 ~hown a prosthetlc part ,l intended to be used as a body implant. The pa~ticular pro6thetic 20 1l part shown i6 a hip pro~thesi~. Such pro6the~es conventionally include a Bmooth, ball-~haped head 10 which forms one surface of the hip joint. and which i8 intended to rota~e wlthin a socket in the hip, a neck 14 and a base member or ~tem 15. The stem 15 i~
llintended for anchoring within the femur of the human or animal~
25 l~patient. In orde~ to enhance the fixation of the stem 15 within the femur, a reqion 16 of the surface of ~tem 15 i8 porou~. ln llthe embodiment of Fi~. 1, the porou~ region 16 of ~he stem ¦Icomprises a ma~ composed of compresfied short metal ~i~erfi, su~h ¦las described in ~nited Stat96 Patent No. 3,906,550 issued to !
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~ illiam Ro~toker and ~orge Galante. According to the inYent~on.
; the me~al fiber me~h 17 i6 coated with a bioab60rbable material I a~ will be di~cus~ed in more detail below.
Fig. Z 6hows a 6econd exemplary embodimant of ~he invention. The pa~t 20 intended to be implanted i~ an acetabular hip prosthe~i6, commonly referred to as a hlp cup. The hip cup 20 in~ludes a smooth, spherical urface 21, whieh form~ ~he other ` one of the surfa~es of a hip joint and whieh i8 intended to receive ~he spherical ~urface 10 of the pro~the~is of Fig. 1.
The exte~nal surface 23 of the hip cup Z0 i~ intended to be implanted within the acetabulum and is covered with a porou6 metal surface layer 24 by the proce6s of pla~ma ~praying. The plasma ~praying process is describQd in United S~ate~ Patent ~o.
3,605,123 i66ued to H. Hahn. A~cording ~o the invention the porous layer 24 includes a bioabsorbable material on a~ lea~t a portion of it6 surface, which material enhances ~he permanent, bone growth into the region of poro6ity. The coating will be described in further detail belo~.
Another commonly u6ed porous coating with whlch the~
20 ~' invention may be in~orporated i~ a ~intered coating of metal~
p~rticles.
The above types of porous surfaces are intended only as' examples, and it should be understood tha~ any of ~he variou~
Itypes of porou~ surfaces used for fixation of par~ imp~anted in 25 ' the body may be incorporated in the invention.
' Turning now to ~ig. 3, a porous surface mesh. such a~
'Imat 17, i8 6hown magnified approximately 20 times. The me6h 17 ,is composed of short metal ~ibers 18 compre6~ed ~oqether. and i,coated with a bioabsorbable material 19 which in the embodiment 30 I,shown is primarily -tricaleium pho~phate.

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il A section of ~he mesh of 17 cut ~hrough and examlned by ,l æcanning elec~ron microscopy, i8 ~hown in Fig. 4. The surface of the me~h (the direc~ion poin~lng out o~ the ~canning electron micrograph in Fig. 3) is pointing to the lower right hand corner in ~ig. 4. The scanning electron micrograph of the æection i8 enlarged 63.5 times. The typical cut sections of the wires are ~hown aæ at 31 and 32. The elongationfi of the wirs section6 a~
at 32 i8 due to the fact that the plane o the cut i8 at an oblique angle to the wire diameter. Apparent 1008e particle6. as at 33, are not actually loose but merely represent a particle that iR connected out of the plane of the section. The ~urface~
34, 35, 37 o the wires are coated with ~he n-tricalcium phosphate 33, 36, and 38. In the embodiment ~hown the maximu~
~' i coating thi~kness i6 approximately 30 micron6 wi~h an average , thickneæs of 20 micron~ on the exterior æurface6, ~ueh aæ 37, of the outer wires of ~he me~h. The ~oating lntrudes approximately .020 inches (S00 microns) into the mesh with the ~ontinunity of the coating decreasing from the outæide toward the inside. Fig., 6how6 another vi~w of the embodiment of ~ig. 4. except maqnified to g48 times.
The coated fiber metal parts fihown in Figs. 1, 3, 4, and ~5 are made in the following manner. Commercially pure ~ire i6 ¦ formed ~nto pads ~nd i~ sintered into place on the prosthesiæ
l~according to the proce~s described in United State~ Pa~ent No.
25 j,3,906,550. The megh i8 then plasma ~prayed with a materlal ehat, after 6praying and coolinq, i8 bioabsorbable. Similarly, the coated parts 20 shown in Fig. 2 may be made by fir6t plasma spraying the metal surface onto the base member (under 6urPace i24), and then plaæma æpraying onto the reæulting poroufi ~ur~ace a coating of a ~ubstance which cools to a bioab~orbable ma~erial.

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Metal cylinde~s were made açcording to the abGve-described process. The wire u~ed was commercially pure ' titanium having a diameter of approximately .0] inches and ~a~
cut into about 1 inch length6. The me~h had a thicknes6 o approximately Zmm and ~a6 sin~ere~ to a central ti~anium rod with a threaded portion to permit attachment ~o a me~hanical testing device. The cylinder6 had a diameter of 9.5mm and a length of 50mm. The fiber me6h cylinders ~ere then plasma 6prayed wi~h a ~ material comprising approxima~ely 50% hyd~oxyapatite and 50 ~-tricalciu~ phosphate. The conven~ional plasma spraying proce~s as des~ribed in United States Patent No. 3,~05,123 was u6ed.
`, Subsequent analysis of the plasma sprayed materi~l by x-ray i diffraction showed tha~ the 50/50 hydroxyapati~e ~ -tricalciu~
I'l phosphate material had t~ans~ormed to primarily ~-tricalciu~
15 1 phosphate. Some ~ -tricalcium phosphate and hydroxyapatite remained. The diffraction data also showed some lines that have not yet been identified, but are Shought to be a high tem2erature calcium pho~phate. The diffraction line width indicated that the ~, crystallite size was smaller than that in con~entional ceramic.
~ The cylinders were implanted in canine femurs along with similar cylinders treated with a number of other material~ that have been described as osteogenic in the literature, 6uch a~
~-tricalci~m phosphate, demineralized bone powder, and l autogeneous bone ~nd marrow. After lengths of time extending lfrom ~wo weeks ~o BiX weeks, the animals were sacri~iced and the samples were pulled out of the ~emur~. The samples made by the technique according to the invention had the highest pull out strength. The high pull out ~treng~h is obviou61y indicati~e of ~the gtrongest fixation, and is generally con6idered in the art to ,' ~g_ '1 , . . .

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be indica~ive of imp~oved bony ingrow~h. Trhe 8a~pl2s made according to the invention were able to be handled according to normal procedure6 for packaging and implanta~ion in ~he animal~
I, without degradation.
5 ~ It i6 believed that the ~ransformation of the hydroxyapatite/~ -tricalcium pho~phate ma~erial into ricalcium pho~phate i8 due to the fact that the plasma ~praying p~oces6 provides transformation energy to the materlal.
With thi~ in ~iew, other methods of applying the material which will provide energy for the transformation may be u~ed to apply the coating, such as 6puttering, electrophore6is, electrostatic spraying, etc.
Novel parts for implantation in the human body and the method for making the part6, which yield improved fi~ation and bony ingrowth, and which have numerous other features and advanta~es, ha~e been de6cribed. While the above description of the invention has been referenced to a few particular', embodiment~, it i8 evident that, now that the advantage of;
li coating a porou~ 6urface of a prosthesis with bioab~orbable 20 I~ material ha~ been disclosed, those s~illed ln the art can now make numerou6 use6 of and modification~ of and departure6 from lthe specific embodiment~ described hereln without departing ~rom ¦'the inventive concept6. For example, the coated porou~ surface llcan be used in combinat~on with many types of implantable 25 11 fixators other than tho~e de~cribed, as for example with knee~
Ii pro~theses, bone plates, 1ntramedullary rods, etc. In ad~ition, ¦other bioabsorbable materials 6uch as the calcium pyropho~phates lor polylactic acids ma~ be used. Similarly, other t~ic~ne88e6 ,Ithan tho6e specifically described may be al80 u~ed. Similarly.
30 1l the base membet of the pro~the~is and~or the pOrOU8 layer may be ll ___ ,, ~25~3~
i made o~ me~als, such as cobalt-chrome steel alloy, stainless 8teel, etc., and other ma~erials suitable for implantation in bodles. ~t is clear that now that the principles of the 1 invention have been disclosed, tho6e skilled ln the art can ~ubstitute numerous other equivalent parts. Consequently the invention is ~o be ~onstrued as em~racing each and every novel feature and novel combination of features wi~hin the ~ppended claim~.
What we claim i8:

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Claims

CLAIMS:

1. A method of manufacturing a prosthetic part for use as a body implant comprising the steps of:
providing a base member having a porous surface region on which bone attachment is desired;
providing a coating material of hydroxyapatite and B-tricalcium phosphate; and applying said coating material directly to said porous surface region for adherence to said porous surface region, while providing energy sufficient to transform said coating material to a state in which it is bioabsorbable, said step of applying comprising heating said coating material to a temperature greater than 1350' and applying it to said porous surface region, said coating material upon receiving said provided energy during the application thereof transforms to primarily a-tricalcium phosphate on said porous surface region and said coating material includes the a-tricalcium phosphate only after the application of the coating material to said porous surface region.

2. The method as described in claim 1 wherein said coating material intrudes into said porous surface region in decreasing continuity from the outside of said porous surface toward the inside of said porous surface.