CA1206701A

CA1206701A - Osteal prosthesis and the production thereof

Info

Publication number: CA1206701A
Application number: CA000431134A
Authority: CA
Inventors: Gerald Lord
Original assignee: Individual
Current assignee: Individual
Priority date: 1982-06-29
Filing date: 1983-06-24
Publication date: 1986-07-02
Also published as: ATE24663T1; US4623349A; EP0098224A1; DE98224T1; FR2529077A1; JPS5946956A; JPH0355137B2; FR2529077B1; ES285248Y; DE3368867D1; EP0098224B1; ES285248U

Abstract

ABSTRACT OF DISCLOSURE
An osteal prosthesis having a outer surface splined in a direction generally parallel to its mean axis, said prosthesis being capable of being secured in the bone without cement, the bone tissue developing so as to at least partly occupy the interior or hollow between the splines, said prosthesis being characterized simulta-neously by the facts that:
- the cross-sectional profile of the splines 7 is such that it imparts a bone tissue retaining capability, and - the longitudinal interior surface or hollow of the splines 7, which are non-cylindrical, has a shape which tapers from the end 4 of the prosthesis (lower end) which is the most deeply embedded in the bone to the other end 3 (upper end), the prosthesis being thus capable of being extracted from the bone tissue by a tractive movement.

Description

~2~)6~
, The present invention relates to the field of osteal prostheses. The object of the invention is to provide a new prosthesis and a method for producing the latter.
The general technique of implanting metal parts for replacing or consolidating bones or osteal arti-culations is already known. Of course, the shape of the implant must be adapted to the concerned bone part. This is for example the case of prostheses adapted ~or the articulations of the hip or knee.
Metal implants were first employed fixed in the living bone by acement, in particular a cement based on acrylic resin. This fixing technique employing cement results in many drawbacks, among which may be men-tioned a defective performance of the prosthesis owingto risk o~ the cement breaking up and intolerance on the part of the organism and the bone tissue of the foreign body formed by the cement.
This is why prostheses were proposed the securing of which did not require the use of cement. For this purpose, the surface of the implant was rendered rough or porous so as to permit the development or growth of the bone tissue on the surface o~ the prosthesis itself and thereby ensure its anchoring in the bone. The draw-

2~ backs of the technique of 5ecuring with cement are pr~c-tically completely eliminat~d by this technique, but the latter may be still further improved . By way of illustra-tion, it may be estimated that the use of cement results, ~ . ...

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in the case of hip prostheses, in more than 25% of failures owing to the loosening of the implant in the bone. This percentage of failures is substantially reduced when a porous or rough surface is provided on the implant, but in at least 10% of the cases it is necessary for many various reasons to intervene surgically again in order to replace the prosthesis. But, inthis case, the pros-thesis cannot be simply extracted, since~ owing to the extremely strong anchoring effect created by the development or growth of the bone tissue, a delicate surgical interven-tion is necessary with a cutting of the bone in order to remove and replace the prosthesis.
By way of prior documents illustrating the prior art, there may be mentioned German pàtent 837,294.

French patent 72,27,966 (publication 2,194,123). V.S.
patent 3,905,047 and French patent application published under n 2,356,465. The teaching of these various patents may be briefly summarized as follows.
German patent 837,294 relates to a prosthesis whose shaft (or stem)has an outer surface which is splined generally in a direction parallel to the means axis.
No precise information is given concerning the splines which may have any shape, number and si~e. Further, their profile may have an acute angle or be rounded. In fact, 25 these splines cannot ensure a sufficient retaining~effect on the part of the bone tissue and, moreover, the rectilinear longitudinal profile o~ these spline does not allow an extraction of the prosthesis stem under satisfactory conditlons.
French patent 72,27,966 (publication 2,194,123) proposes a prosthesis adapted to be secured without cement and comprising a porous outer surfaceO Once placed in position, such a prosthesis cannot be extracted. Further, the conditions under which such a prosthesis is obtained result in a risk of corrosion of the surface. The latter is also rendered more fragile which, under the effects of the high stresses, may create weak zones and finally result in the fracture of the prosthesis.
U.S. patent 3,905,047 concerns a prosthesis having surface irregularities. Once placed in position 9 such a prosthesis cannot be extracted by a simple tractive ~oroe owing to the development of the bone tis~ue, which constitutes one o~ the features claimed by the patent.
French patent àpplication published under N 2,356,465 relates in particular to a prosthesis compri-sing on its outer surface a covering constituted by balls or spherules. The latter are distributed in a random manner so as to ensure a solid anchoring effect. Such a prosthesis cannot be extracted in the case of an accident without a special surgical intervention.
Therefore, at the present time there are clearly no osteal prostheses to the knowledge of the appli~-cant which may be secured in position without cement,which are capable of being effectively retained in the bone tissue after development of the latter and yet which be may/extracted~ if required, without the obligation to e~fect ~6'7~.

a delicate surgical intervention An object of the invention is to provide a new prosthesis which simultaneously satisfies all these requirements.
Another object of the invention is to produce such a prosthesis under industrial conditions which result in a product having a strength much higher than that of known prostheses. In particular, the production method results in no weakening of the surface of the prosthesis.
Broadly, the invention provides an osteal prosthesis which has an outer surface splined in a direc-tion generally paralel to lts mean axis and is capable Qf being secured without cement, the bone tissue developing so as to occupy at least partly the interior o~ the splines, said prosthesis being characterized in that simultaneously:
- the prcfile (cross-section) of the splines is such that it imparts a bone tissue-retaining oapability, and - the longitudlnal intension hollow surface of the apIines, which are non-cyllndrical, has a shape which tapers from the~end of the prosthesis (lower end) which is the most deeply inserted in the bone to the other end (upper end) 9 SO thst the prosthesis is capable of being extracted from the bone tissue by :a tractive move-ment. ;
The prosthesis~according:to the inventionis an improvement of the subject matter of German patent 837,294 in that lt affords, in combination9 the desired ~6~

anchoring effect for the bone tissue and the possibility of extraction.
The cross-sectional profile of the splines must impart a high bone tissue-retaining capability.
In contrast to the teaching of German patent 837,294, this pro~ile therefore must not be open but include, at least partly, a narrowed partS so that the developing bone tissue may be trapped in the bottom of the splines.
For this purpose, the cross-sectional profile may be cur-vilinear or circular or an association of these two types of profile, it bein8 possible to interconnect the arcs by rectilinear parts. In combination with this cross-sectional profile, the splines formed on the outer sur~aoe of the prosthesis a¢cording to the invention have a taper in the longitudinal direction, or, in other words, a 91ight conicity between the end embedded in the bone and that end which, as the case may be, must be taken hold of for the extraction of the prosthesis. The advantageous results of the invention are obtained with conicities of relatively low vàlue. A slope of 1% to 10% has been ~ound suitable.
The invention is applicable to all kinds of metal implants capable ofacting as prostheses or used in the treatment of bones. Particularly interesting results are obtained in the case of prostheses of the hip or knee and, in a general way, any prosthesis incIuding~a part~
in the shape of a stem. Such a stem may be reotillnear or have a certain curvature, so that lt may better match the shape of the bone in which it is intended to be - :

67~.

anchored. Further, the outer sur~ace of the stem is not necessarily in the shape of a regular cylinder. It is even in many cases advantageous, in particular in order to satisfy requirements of the strength of materials, to shape the stem so as to give it a progressively in-creasing thickness, at least in its upper part. Some of the splines extend throughout the length of the stem and some others may~ if they extend from the upper part, lead to an edge of the stem without extending to its end.
In another aspect, the invention provided a method for producing a splined prosthesis ~element by the general lost W2X investment casting technique, comprising constructing a w~ pattern of the element to be produced9 effeQting a refractory moulding on said pattern so as to constitute a mould, and casting liquid metal in said mould so as to produce the desired el~ment, said method being charac~erlzed in that, before the re-fractory moulding, there are pIaced in position in the flutes of the wax pattern, rods which have a cross-sec-tional proflle corresponding to that of the splined of the final element and which have a slight conicity from one end to the other so as to produce~ after the casting of the metal in the refractory mould, an element with tapering splines.
In the case, for example, of a prosthesis element comprising a stem, the stem is first made in wax at the dimensions of the element to be reproduoed by pro-viding on the periphery of the stem concavs grooves parallel to the axis. In addition, rods are made for example ~rom polystyrene or waX, which have the length of the splines to be produced and a slight conicity from one end to the other. The cross-sectional profile of the rods corresponds to that of the splines of the final element. These rods are assembled, in particular by adhesion, in the concave grooves of the stem. In this way a wax pattern of the element to be produced is constructed.
This wax pattern is then provided with a refractory co-vering in the known manner so as to form a refractorymould and the final element is produced by casting~liquid metal in such a mould from which all the wax was previous-ly r~Q.~oved.
In another aspect, which correspQnds to a preferred manner of carrying out the invention, the inven-tion provides a method for pro~ucing a .spllned prosthesis element, comprising constructing by the known general lost wax investment casting technique a smooth metal blank which is slightly oversized relative to the transverse dimensions of the element to be produced, trea~ing said blank in succession by hot ~orging, then by cold forging, until the`final dimensions of the desired element are obtained, and forming on said element by an electro-machining tapered longitudinal splines having the desired cross-sectional profile.
Such a method comprises first produci~ng by the lost wax investment casting~technique a pattern of an element, for example a stem, which has a smooth outer 67~

surface and includes a slight extra allowance relative to the transverse dimensions of the element to beobtained.
By way of a concrete example, in respect of a stem, of about 15 mm in diameter, an extra allowance of 1 mm per surface is suitable. The as-cast blanks are then hot-for~ed so as to produce an intermediate element which still has a slight extra allowance (in the foregoing dimen-sioned example, this may be about 0,50 mm). The final calibration giving the final dimen~ions is effected by cold forging. With this forged ste~ having a smooth outer surface, the splines are produced by removing metal with the electromachining technique. For this purpose, a tool is used which is shifted in the longitudinal direction o~ the stem so as to cut the splines therein. The machining may be e~ected by successive passes with tools having different profiles. In a modification which is preferred from the economical point of view, a single tool is used in one pass so as to produce the desired cross-sectional profile of the splines, namely a profile having an optlmum retàining effect on the bone tissue when the latter develops or grows.
The me~hod just described is the most interes-ting in practice. Indeed, in starting with an as-cast blank, the hot and cold-forging steps very substantially improve the mechanical properties of the element. The electromachinin~ technique wholly respects the mechanical properties and may be carried out with conventional equip-ment which merely need to include a tool satisfying the _ g _ ~L2067~'~

requirements of the invention.
The invention will now be illustrated, without in any way being limited, by the following description which concerns a prosthesis adapted to be inserted into the femur (prosthesis of the hip), with reference to the accompanying drawings in which:
- fig. 1 is a front elevational view of the prosthesis;
- fig. 2 is a sectional view taken along line II-II of fig. 1;
- fig. 3 is a ~iew of the interior or holl~w of a spline in the direction of arrow III of fig. 1;
~ fi~. 4 is the cross-sectional profile of a spline to an enlarged scale;
- fig. 5 is similar to fig. 4, showing a modi~ication of the profile;
- fig. 6 is a diagram illustrating a method for producing the prosthesis acoording to the invention, and - figs. 7 and 8 are diagrams illustrating another preferred method for producing the prosthesis aooording to the invention, Figs. 1 and~2 show diagrammatically a pros-thesls element~adapted to be inserted~into the femur.~
Generally, it oomprises two parts, a stem 1~and a part 2 which, for the sake of convenience,~ will be termed "neck".
In the chosen example9 the stem 1 has a length of about 200 mm and a transverse dimension of approximately 15 mm.

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Fig. 2 shows more clearly, in section, the slightly ellip-tical shape of the stem 1. The end 4 is the end which is the most deeply embedded into the bone of the femur.
The end 3 is shaped as shown in fig. 1 so as to af~ord maximum strength. The stem 1 also includes cavities two of which, 5, 6 are shown.
According to the in~ention, the stem 1 has flutes 7. Some of the latter, in particular in the median part, extend throughout the length of khe stem 1. Other splines, in particular in the vicinity of the upper part

3 or of the cavities 5, 6, do not extend to the end 4 but stop at the corresponding outer edge of the stem.
Tt is also clear from figs~ 1 and 2 that a part 8 of the neck in the extension of the stem 1 i3 also splined.
Thls part 8 i9 restricted to one face of the neck 2.
The essential feature of the invention is that the splines 7, 8, have a cross-sectional profile which is capable of effectively retaining the bone tissue when it develops or grows after the insertion of the prosthesls. Figs.~4 and 5 show examplss of suitable profiles. In fig. 4 there are seen two adjacent splines 7a, 7b defining there between a bottom 9 or inner spline surface. It can be seen in fig. 4 that the inner surfaoe 9 is defined by a ourYe which outwardly tapers as indlcated at 10 before di~erging to form the spline proper.
It is the oonformation o~ the interior surfs-ce which ensures the anchoring effect and the bone tissue retaining capability. It will be understood that ths ~2C)~7&'~

drawing in fig. 4 has been highly magnified (magnifica-tion about ten times).
Fig. 5 shows a modification with two adjacent splines 7c, 7d. The spline bottom or hollow 11 has two bosses 12, 13 approaching each other in the upper part.In this case also, the desired retaining effect is ensured.
According to another fundamental feature of the invention which iæ combined with that concerning the cross~sectional profile of the spline, the longitudinal profile of the latter is not cylindrical but, on the contra-ry tapered. As illustrated in fig. 3~ the in~er surface of a spline 7 tapers from the end 4 the most deeply embedded in the bone to the end 3 in the vicinity of the neck 2.
It will be understood that the representation of fig, 3 is diagrammatic. ln praotice, with the aforementioned stem dimensions (namely: length 200 mm, diameter 15 mm) the transverse dimension of a spline hollow in the vicinity of the end 4 is for example 1.5 mm while it is 1.3 ~m in the vicinity of the end 3. This downwardly divergent profile ~nables the prosthesis to be extracted if required.
For the purpose of the extractlon, a pair of recesses 14 (see figo 1) may be provided on the neck 2 to enable the surgeon to insert a tool for extracting the prosthesis.
Fig. 6 diagrammatically illustrates a method for producing a prosthesis stem of the type shoh~ in flgs.
1 to 5. This method employs the general lost wax invest~
ment casting technique. Fig. 6 shows the stem 15 of wax ~ 12 -~67~

which is made with the dimensions of~ the element to be produced. It has throughout its periphery concave grooves 16 parallel to the general axis of the stem 15.
Rods 17 of polystyrene or wax are constructed which have a cross-sectional profile similar to the splines of figs. 4 and 5 and a tapering longitudinal profile.
Thus their diameter at the end 18 is larger than that at the end 19. These rods are individually placed in position by adhesion in the concave grooves 16 of the wax stem. The wax pattern obtained in accordance with the illustration of fig. 6 is then covered with a refrac-tory material in the known manner. Liquid metal may be cast in the refractory mould to produce the final pros-thesis element.
It is preferred to produce the prosthesis according to the invention by a method in which there is first of all produoed by the lost wax inveskment casting technique a st~m pattern whose outer sur~ace is smooth and whose transverse dimensions are slightly increased relative to those of the final element to be obtained.
With the aforementioned flgures~ (length of the stem 200 mm and diameter 15 mm~ the extra allowance on the wax pattern may be about 1 mm on each side, namely 2 mm in 1l. A
metal stem blank is then made with such an extra allowance.
The blank is then hot-forged to produce an intermediate element haYing less extra allowance, for example about 0.50 mm per side relative to the final element. The final calibration is achived by cold-forging which considerably ~L2~

impro~es the mechanical properties of the prosthesis.
The splined final shape of the prosthesis according to the invention is obtained by removing metal by an electro-machining technique. The latter is di.agrammatically illus-trated in figs. 7 and a. Fig. 7 shows in section thestem 21 of the prosthesis to be produced. The latter is held horizontally in a fixing tool 22 which leaves exposed only the upper half 21a of the stem 21. The lon-gitudinal grooves or spline hollows 23 are obtained by the use of a pair of electrodes 24, 25. The active and of each electrode covers one-half o~ the upper part 21a of the stem so that this upper part 21ais completely co-vered by the electrodes, as shown in fig. 7. For the purposes of the electro-erosion or electro-machining, the electrodes 24, 25 approach the part 21a (arrows 26) in a converging motion, after which the two electrodes are withdrawn (arrows 27). The ele~ent (stem 21) is then turned round and the same operation is carried out on the other half 21b.
Flg. 8 shows to an enlarged scale the oons-titution of an electrode element 24, 25. There are seen two adjavent splines 2~, 29l separated by a groove or spline hollow 30.~ In the chosen example, the groove 3Q
has a profile resulting from the combination of aros of a circle 31 and 32 of opposite curvature. ~The bottom of the groove is formed by a flat surface 33.
The element 34 of the e1ectrode7 whioh is made from copper, is designed to machine the pro~ile 317 7~L

32, 33 and comprises for theis purpose arcuate parts 35, 36 and an end part 37. Inner passages are provided in the electrode element 34 to permit the passage of an elec-trolyte as shown by the arrows in fig, 8. Thus it can be seen that the element 34 has a main passage 39, two lateral passages 40, an axial passage 41, and two other lateral passages 42. Owing to the ~hape of the electrode element 34 and to the circulation of the electrolyte (which may be brine), the metal of the prosthesis stem is remored in acoordance with the profile desired for the groove 30 between the splines 28, 29 both in the transverse direc-tion and in the longitudinal direction of this groove.
It will be understood that the production methods desoribed hereinbefore merely constitute examples.
A perqon skilled in the art will understand that modiPica-tions may be envisaged in particular to adapt the produc-tion method to the type of the material of the prosthesis.
Thus the rough prosthesis element or blank may also be obtained by hot-forging a cylindri~cal bar of suitable dimension3. The hot-~orging~requires a number of passes whichdepends on the type of material. Subse-quent to the forging, an element is obtained which has high meohanical properties.
~ The Poregoing techniques are most suitable for stainless ~steels such as chrome -cobalt steels~, for example that of the grade HS21. It will be understood that other materials may be employed and in partioular other metal alloys.

Claims

WE CLAIM:

1. In an osteal prosthesis having a mean axis and on outer surface splined in a direction generally parallel to said mean axis, said prothesis being capable of being fixed in the bone without cement since bone tissue develop so as to occupy at least partly the interior of the splines; the improvement wherein simultaneously;
- the splines have such a cross-sectional profile that it imparts a bone tissue-retaining capability and - the splines are non-cylindrical and define longitudinally extending interior hallow surface which has a shape which tapers from an end of the prosthesis (the lower end) which is intended to be the most deeply inserted in the bone to an opposite end (upper end) so that the prosthesis is capable of being extracted from the bone tissue by a tractive movement.

2. A prosthesis according to claim 1, wherein the cross-sectional profile of the splines is such as to define at least partly a narrowed part in said hollow.

3. A prosthesis according to claim 2, wherein the cross-sectional profile is curvilinear.

4. A prosthesis according to claim 2, wherein the cross-sectional profile of said splines is circular.

5. A prosthesis according to claim 2, wherein the cross-sectional profile of said splines has a combina-tion of curvilinear and circular portions.

6. A prosthesis according to claim 3, wherein rectilinear portions of said cross-sectional profile inter-connect said curvilinear and circular portions.

7. A prosthesis according to claim 1, wherein the splines have a taper in a longitudinal direction of the prosthesis in other words a slight conivity between said and intended to be the most deeply inserted in the bone and said opposite and which may be taken hold of for extracting the prosthesis from the bone.

8. A prosthesis according to claim 7, wherein the conicity is 1% to 10%.

9. In a method for producing a splined pros-thesis element by a general lost wax investment-casting technique, comprising constructing a wax pattern corres-ponding to the element to be produced, effecting a refrac-tory moulding on said pattern so as to constitute a mould and casting liquid metal in said mould to produce the desired element; the improvement comprising placing in position in hollows defined by the splines in the wax pattern, before the refractory moulding rods which have a cross-sectional profile corresponding to the cross-sectional profile of the splines of the final element and which have a slight conicity from one end of the prosthesis element to an opposite end so as to produce, subsequent to the casting of the metal in the refractory mould, a prosthesis element with splines which are tapered longitu-dinal of the prosthesis element.

10. A method according to claim 9, wherein said nods are of polystyrene.

11. A method according to claim 9, wherein said nods are of wax.

12. A method for producing a splined pros-thesis element comprising forming by a known general lost wax investment casting technique a smooth metal blank having a slight extra allowance relative to transverse dimensions of the prosthesis element to be produced, treating said blank in succession with hot-forging and then with cold-forging until the obtainment of the final dimensions of the desired prosthesis element, and forming on said element by electro-machining longitudinally exten-ding splines having a longitudinal taper and the desired cross-sectional profile.

13. A method according to claim 12, wherein, in the course of the electro-machining, at least one pass with the aid of two electrodes is effected, said electrodes operating in a plunging direction in a convergent motion and having a profile corresponding to that of the splines.

14. A method according to claim 12, comprising operating in succession on halves of the prosthesis element which is held horizontally in tooling.

15. A method for producing a splined prosthe-sis element by hot-forging a cylindrical bar.