CA2052978A1

CA2052978A1 - Screwable acetabular (hip joint) cup and method of its production

Info

Publication number: CA2052978A1
Application number: CA002052978A
Authority: CA
Inventors: Gerd Hoermansdorfer
Original assignee: Individual
Current assignee: Johnson and Johnson Medical GmbH
Priority date: 1990-10-09
Filing date: 1991-10-08
Publication date: 1992-04-10
Also published as: EP0480551B1; JP3209764B2; DE4031926A1; DE59107242D1; ATE132732T1; DE9112735U1; EP0480551A1; JPH04226650A

Abstract

ABSTRACT

The present invention relates to a hip-joint socket of spherical geometry that can be screwed into place, this having a special thread for cementless implantation, and a process for the production of this. In its shape, the special thread follows the convex surface formed by the nominal diameter of the socket and its bottom is almost completely matched to this spherical surface. According to the present invention, the production of the special thread by metal-cutting methods is effected by machining with several tools having different cutting edge angles relative to the thread bottom, which are moved on different paths. Because of the thread shape according to the present invention, a significantly higher primary fixation during implantation is achieved as a result of the thread tooth height that is available, and at the same time, the danger of loosening has been considerably reduced by the fact that because of the complete and large area bone contact of the socket surface, only low specific forces act on the bone bed.

Description

2 ;2~5Z:97~3 The present invention relates to an artificial hip-joint socket that can be screwed into place, this being intended for human use, and a process for producing this.

Hip-joint sockets are components of the ar~ificial joint set that is intended for human use. The hip-joint end prosthesis, like other areas of medical technology, has been characterized from its very beginnings by constant development that has been brouyht about by the attempt to copy natural function~ as perfectly and as durably as possible in order to serve the patient in the best possible way. These efforts have in no way been discouraged by setbacks encountered during efforts to arrive at the optimal geometry, the most suitable materials, or the safest surgical methods. Because of the fact that human life expactancy has been increased greatly in recent times, the replacement of hip joints has not only increased quantitatively; there has also been a demand for higher quality, for one can assume that the demand will exist for significantly longer times. Against this background, the systems that correspond to the prior art have not yet been fully optimized, even today. The present inventiQn is intended to contribute to an improvement in medical technology in one sub-area.

In principle, hip-joint sockets of this kind can be classified into two main groups, i.e., those that can be cemented into place and those that are not secured by cement. Of ~hese, the non 2 015~378 cemented hip-joint sockets are used more frequently among older age groups, and in conjunction with an uncemented shaft. 0~ this type, the one that is the most widely used is that which consists of a metallic screw--in body and a plastic insert. In such a case, the screw-in body is provided with an external thread. In addition, a distinction is made between self-cutting threads and those in which the thread must first be cut in the acetabulum.
The types described above can also classified according to various external shapes, i.e., conical, spherical, conico-spherical, and to a relatively small ~egree, cylindrical cross sections.

Meanwhile, it has been found that one cannot always guarantee a permanent success when a hip joint prosthesis is implanted.
Frequently, loosening of the hip-joint socket can be observed, and this leads to inflammatory processes and requires additional surgery. Today, a very stable primary fixing of the hip-joint socket is regarded as an extremely important precaution to be taken in order to prevent loosening o~ this kind.

Theoretically, very good primary fixing can be achieved with conical hip-joint sockets if the socket is screwsd with greater angular precision into the machined pelvis that has been milled out. In practice, however, mostly lower values are to be quoted, because even very small angular errors lead to linear contact between the supporting bone and the conical shell o~ the socketO

In addition, conical sockets entail the disadYantage that they require that large volumes of material be machined oEf because their external geometry does not correspond to the anatomy.

Spherical sockets match the anatomical shape completely. The natural bone tissue can be largely retained for the implantakion.
A further advantage is the sphere/sphere match, for because of this the angle at which the gocket is screwed into the prepared milling in the pelvis has no effect on the anchoring strength.
Spherical sockets suffer from the shortcoming that at their bottoms their threads do not follow the desired spherical shape exactly. A thread bottom that runs on the conical surface formed by the nominal diameter of the socket can, it is true, be realized with a normal screw thread (V-thread with a triangular profile) without any trouble; however, this thread shape is not adequate for the proposed use. Division of the thread profile into narrow thread lands and wide thread grooves, as is the case, for example, with wood screws, is to be aimed at because better preservation of the living bone structure is ensured because of the broad base of the thread lands on the bone side and, in addition, a division of this kind takes the relative stren~ths of the substance pairing between ~one and, for example, metal, into account. This results in the need to produce threads of this kind with a more or less trapezoidal ools, in which the cu~ting edges that cut the thread bottom are at a constant angle--at a rule 0--to the axis of the socket. The axial cross section of :
~ .:

2~ 97~

such a thread displays a stepped shape. If the thread grooves are cut deep enough to permit the soc~et to be screwed in completely, the bottom of the thread of the socket is free relative to the thread tip on the bone side, so tha~ under appropriate loading there will be a danger of loosenin~. In addition, this will require an inappropriately thick socket wall.
If the thread grooves are not cut so deeply, the phenomenon will occur whereby the thread teeth becom2 ever smaller in the direction of the socket pole and, at the ~ame time, an ever increasing amount of the bottom of the thread will extend out of the conical casing and will cause the elements to bind when the socket is screwed into position, before the socket back comes ko rest on the bottom of the pelvis. Even if, in such a case, the remaining cavity is filled with bone chips the basic shortcomi~gs of such a thread still cannot be eliminated.

A compromise solution is presented by conico-spherical hip~joint sockets that have a spherical back and conical sides, although in this case acceptable geometrical transitions between the ~onical and the spherical areas can only be achieved with very short thread lengths. This means that only a diminshed primary fixing is provided. Various designs have attempted to eliminate this shortcoming in that the socket incorporates drillings for additional attachment by means of bone screws. These measures increase both the cost of production and sur~ical costs.

" :' 37~

In general, it should be possible to produce spheric21 hip-joink sockets with the desired shape of the thread bottom by ~ine-quality casting. However, it is known that both the stxength values and the dynamic behaviour of fine-casting alloys in no way achieve the appropriate values found in forged materials. In addition, fine-quality casting is only economical if very large numbers o~ a particular item are produced.

For this reason, there has been a need to develop a screwed-in hip-joint socket with a sp~cial thread, the bottom o~ which corxesponds as completely as possible to the ~hape of the conical casing formed by the nominal diametex of the socket, a~ well as for a process to produce such hip-joint sockets.

The first part of this task has been solved in that in its development, the bottom of the thread has been largely adapted to the spherical outer shape of the hip-joint socket by secondary surfaces of that are in strips that are broken up into stages by angles and extend parallel to each other and are inclined to the development of the thread bottom.

The present invention makes available a hip-joint socket that can be screwed into place and which has greatly improved geometry of the thread and which, as a result, has important advant.ages. The hip-~oint socket according to the pxesent invention, because of its thxead bottom that matches the conical surface almost ~; , .-7~

completely permits an effectiv~ shape-locking installation into the prepared pelvis, in which connection, because of the better supporting part of the thread there is a clearly incrPased tension that is induced by the thread, and khus improved primary fixing. Contact with the bone bed, that extends over a large area and which has no load peaks over the whole of the sur~ace of the hip-joint socket according to the present invention, leads to a lower specific loading of the bone structure. Because of this, a decrease of the former loosening rates can also be anticipated.
Also of advantage is the fact that the curve ~f the thread bottom makes it possible to produce very thin-walled and thus more flexible hip-joint sockets. The angle at which the socket is screwed in is not critical and does not have any adverse ef~ect on the anchoring in the bone, so that implantation can be performed without any problems, even by surgeons with little experience.

According to the present invention, the task described above has also been solved by means of a special production process.
According to this, it is proposed that threads be machined with different tools having various cutting edge geometries relative to the thread bottom and which follow various paths, such that different sub-surfac~s are formed in tlle thread bottom, these being matched to the greatest extent possible to the surface curve of a hemisphere that corresponds to the nominal diameter.
This surface curve can be coarsely machined with two different machine tools if, for example, one of them corresponds to the smallest tangent angle of the hemispherical area swept by the thread (which thus, for example, cuts the thread bottom at an angle of OD), and the second corresponds to the largest tangent angle of the hemispherical area swept by the thread (in practice, an angle between 30 and 45).

According to the present invention, the curve of the thread bottom that comes considerably closer to the spherical shape can be achieved by means of at least three different tools, for which reason this process is preferred to that discussed above. In this case, for example, the first tool cuts the thread bottom in a sub-area with the smallest tangent angle of the hemispherical area swept by the thread (for example, 0); the second tool cuts it at the mean tangential angle of the hemispherical area (e.g.
20) swept by the thread; and the third tool does this at the greatest tangential angle of the hemispherical area (e.g. 40~) swept by the thread.

According to the present invention, an almost perfect reproduction of the hemispherical shape in the curve of the thread bottom can be achieved by means of a process in which four different tools are used for the finishing cut. Here it is preferred that, for purposes of machining, the thread bottom once again be divided into adjacent and almost equallly ~taggered sub-areas of different angles corresponding to the hemisphPrical ar~a 9 ~ 5;~97~
swept by the thread (e.g., 0, 14, 2~, 42~). When this is done, it will depend on the cutting conditions which have been found to be most favourable in practice as to the sequence in which the individual tools are used. According to the present invention, an even finer subdivision of the thread bottom angle is achievable for machining in five or more steps, although this raises the question of the cost-benefit ratio.

According to a further invention it is proposed that for the angular breakdown of the machine tools that apply to the thread bottom it is proposed that each angular step to the greater angle be made successively smaller (thread bottom angles, for example, 0, 14, 23~, 33 D ~ 42~), in order to counteract a growth of the height of the roof edge formed in the thread bottom which otherwise increases toward the pole of the socket. In addition, it is proposed that instead o~ being set at 0, the smallest thread bottom angle be set at, for example, 2~, depending on the width of the thread groove, in which case the above-named thread bottom angles, would changel for example, to 2, 13.5, 24~, 33.5~, and 42~.

During the metal-cutting production of the hip-joint socket with a thread according to the present invention, the individual tools are moved on different paths relative to the axial section of the socket, in which connection either the workpiece (socket~ is rotated and the tool is moved in one plane (lathe-turning :~S~97~
production), or the socket is fixed and the spindle with the tool rotates, when the workpiece and the spindle move on a helical line relative to each other (milling production), or both the socket and the driven tool rotate, the tool being moved simultaneously in one plane (rotary milling). In order that the desired hemispherical shaped contour is produced in the curve of the thread bottom, the positioning paths of each of tool is so set that its cutting edge point that projects furthest from it in the direction of the mid-point of the he~ispherical surface formed by the nominal diameter of the hip-joint socket is moved essentially parallel to or on this hemispherical convex surface.

From this it follows, for example, for machining with three tools, that both for the tool with the greatest as well as for the tool with the smallest cutting edge angle, the uncorrected path does not change relative to an arc, apart from parallel shifts that have to be considered, provided that the minimum or maximum value of the tangent angle of the area swept by the thread corresponds to the smallest or largest cutting edge angle~
However, the tool with the mean cutting edge angle is so moved along the path that is composed of two half arcs because first one corner of the cutting ed~e and then its other corner of the cutting edge follows the arc shape of the hemispherical convex surfac~ formed by the nominal diameter of the hip-joint socket.
As it is, the thread bottom produced in this way comes Yery close to the ideal shape.

5;~78 The present invention also proposes additional corrective measures to bring the bottom of the thread closer to the desired shape on the surface of the convex surface and also to perfect the flanks of the thread; these measures are superimposed on the individual paths which are, in principle, arc-shaped or made up of segments of arcs. These corrective measures are as follows:
1. corrections to the cutting radius;
2. shifting of the flanks;

3. corrections to the chord;

4. corrections to the roof edge.

Correction of the cutting radius results from rounding the corners of the tools. At the moment, there are no CNC commands that take the corner radii of the tools into consideration either when turning the thread or when the thread is milled. According to the present invention, this problem has been eliminated in that in order to calculate the path to be followed by the individual tools, what is used as the radius is not the half nominal diameter of the screw socket but rather the sum of this radius and the cutting radius of the tool; simultaneously, the mid-point of the cutting radius of one corner of the tool is used as a reference point in the tool description. In order to build up the program, one tool corner must be established uniformly for the whole group of cutting tools and one tool corner ~ust be established as a reference, and this must be done in the same way :

~5~g78 for each group of tools. In princlple, it is of no consequence whether this is the right or the left-hand corner. However, it must be ensured that different paths result for each two corners of a tool~

This displacement of the flanks according to the present invention is intended to ensure that the two thread flanks are cut cleanly and without any groove-like markings. To this end, it is proposed that the tool with the smallest cutting edge angle (e.g., o) is moved by means of the tool correction slightly, e.g., by an amount of 20 ~m, in the direction of the socket equator and simultaneously, tha contour description in the form of the path that is to be followed be moved by an equal amount in the exact opposite direction, to the pole of the socket, and that the tool with the largest cutting edge angle (eOg. 42~) be moved in the opposite manner in the direction of the socket pole and simultaneously the corresponding contour description be moved by the same amount to the socket equator, in each instance by an equal amount.

The chord correction that is proposed according to the present invention is advantageous if, in the absence of oth~r programming possibilities for machine control, the paths to be followed by the tools are composed of a number of small conical thread sections. Without consideration of the chord correction, the start and end points of these conical thread sections lie on the 13 21015~
convex surface formed from the nominal diameter of the screw socket, the chords so formed undercutting this convex ~urface more or less, depending on the length of the conical section.
This undercut is reduced by means of the proposed chord correction in that the designated chords are ~hifted radially outwards from the mid-point of the socket, when the angular path that is formed by the conical thread sections lies more or less centered on the arc formed by the nominal diameter of the screw socket in the section. Considered as absolute, the chord correction is relatively small. In addition, it decreases as the number of conical thread sectiQnS that are developed increases, which is to say it is almost insignificant in the case of relatively finely resolved programming.

In addition, the present invention proposes a roof edge correction. This is made necessary by the fact that, because of the proposed machining method, roof edqes are formed in the bottom of the thread, whenever two tools are involved in the finishing cut for the thread bottom. These roof edges disappear completely if the tangent angle of the thread bottom corresponds exactly with the cutting angle of the t~ol. On the other hand, they reach their maximum height if this tangent angle lies exactly between two cutting angles of the tools. If the corner radii of the tool~ ar~ now moved exactly along the convex surface form~d by the nominal diameter of the svcket, these roo~ edges will protrude outwards. With the roof-edge correction, the cutting tools are corrected radially towards the mid-pvint o~ the socket such that there i5 a fluid transition between the uncorrected area of the path described by the particular tool and the point of maximum correction in the area of the maximum roof edge. It will be advantageous to so develop the ~um of these roof edge corrections that the roof edge does not ~,xtend more than 0.1 mm relative to -the convex surface formed by the nominal diameter of the screw socket, in the area of the particular maximum roof edge height. The absolute sum of the roof edge correction decreases as the number of tools and the improved resolution of the angular steps that is associated with this increaseO

It is preferred that the hip-joint socket according to the present invention be produced by lathe work. It is recommended that the thread grooves be first roughed out with a parting tool.
During the final machining, e.g., with three parting tools, according to the present invention, their face angles are preferably staggered (e.g., 0, 21, 40~) almost evenly according to the angular area that is to be machined. of these three tools, at least one must cut the right-hand and a second must cut the left-hand thread flank. Usually, these flank angles lie between 6~ and 15. For normal set-up (the sphere mid-point at the chuck) it is recommended that the parting tool with the largest face angle be used simultaneously for machining the right-hand flank of the thread groove, whereas it is recommended 5,~78 that, vice versa, the parting tool with the smallest face angle (e.g., 0~) be used to cut the left-hand flank of the thread groove. For machining on a so-called two-axis CNC lathe, the tools are called up one after the other and the thread is started. When this is done, its own sh~pe descriptlon (shape of the thread path) must be input for each tool and co-ordinated with the tool description. The shape description must contain the desired path corrections according to the present invention.
It must also be borne in mind that when programming the thread-cutting cycle, each particular start and end point must be identical for the all the tools, in order that each tool is moved into an exactly synchronized position in the thread groove.

The present invention will be described in greater detail below on the basis of the drawings appended hereto. These drawings show the following:

igure 1: shows an axial cross section of one embodiment of a hip joint according to the present invention, and sections thereof;
Figure 2: shows ~he formation of an uncorrected path for an individual cutting plate during movement along the spherical outline, this being done on the basis of a geometrical layout.
Figure 3: shows a development of the thread bottom, greatly simplified.

16 2~ 37~

The embodiment shown in figure 1 shows the metal casing of a hip-joint socket according to the present invention, in which the edges of the thread teeth that extend to the rear as well as the required insert of plastic have been omitted for reasQns of simplification and clarity.

The hip joint socket 1 has a thread of which the four thread teeth 2, 3, 4, S form a thread groove between each other, each of which has a thread bottom 6, 7, 8 of different shape. Both the thread bottom 6 and the thread bottom 8 are shown at greater scale in the top portion of the drawing. The thread that is shown displays the result~ of final machining of the thread bo~tom with three different tools having different cutting edga angles of 0, 17~, and 34. In the enlarged view of the thread bottom, it can be clearly seen that the left-hand area 9 has been machinad by the tool with the smallest cutting edge angle (0~) and the right-hand area 10 has been machined by the tool with the mean cutting edge angle (17). In contrast to this, the enlarged view of the thread bottom 8 makes it clear that the left-hand area 11 has been machined with the tool with the mean cutting edge angle (17~, and the right-hand area 12 has been machined with the tool with the greatest cutting edge angle (34). Between the secondary surfaces formed because of the different thread bottom angles, there is, in each instance~ a roof edge; however, the position of this roof edge shifts - ~.

5~

continuously in a circular manner, an effect that cannot be seen in a two-di.mensional drawing. In the drawing, the mean khread groove with the thread bottom 7 is so located that essentially it is machined with the tool with the average cutting edge angle (17~). The small amount o~ material khat is xemoved by the two other tools in the left or right-hand lower corners of the thread bottom, respectively, is so small that it cannot be seen in the drawing. However, the figure clearly shows how well the formation of the shape of the thread bottom, which is arc-shaped in axial cross section, can be realized by means of the process according to the present invention.

The geometrical diagram shown in figure 2 is intended to illustrate the development of a single path of the tools, initially uncorrected durin~ movement along the contour, ~y carrying out the process according to the present invention, this being shown on the basis of an example. A completely solid arc is shown as a quarter circle with radius Rn~ which corresponds to the convex surface of the half side of a hip~joint socket 14 which is formed from the nominal diameter. The ~id-line axis of the hip-joint socket 14 is shown, in the usual manner, by a dot-dashed line whereas a continuous line 15 indicates the socket equator. In additionl a machine tool 16 is shown in thr~e different positions (A, B, C) during the last cut of the machining process. The cutter 17 that is ~achining the bottom of the thread has a face angle o~ 20. The two corners 18, 19 oE

2~ 9~3 the cutter are of a radius Rs~ Because of the proposed path 22 of the tool 16, whereby the corner of the cutter that projects furthest in the direction of the spherical mid-poin-t 20 o~ the socket joint 14 is moved along the convex shape and thus initially between the positions A and B the right-hand corner of the tool is moved on this convex shape and between the positions B and C the left-hand corner of the tool is moved on this same convex shape, this results in a path of movement that is made up of two semi-arcs, each with the radius Rn + Rs for the path indicated by the dashed line 22. Whereas the point of rotation for the first semi-arc coincides with the spherical mid-point 20 of the socket, a new point of rotation 21 is formed for the second semi-arc. The relative position of these two points of rotation 20, 21 to each other is identical to the relative position of the two corner radii middle points of the tool 16.
This results in the fact that for every other machine tool either the single or each second point of rotation is to be established correspondingly matched. The corrections that are further proposed according to the present invention in order to enhance the match of the thread bottom produced by this machining to the convex surface formed by the nominal diameter of the hip-joint socket are not consider2d in figure 2.

The thread bottom 23 that is shown as a development in Figure 3 is shown greatly simplified and simply serves to clarify the way it is structured. It can be seen clearly that it is made up of ;2i~S;~97~3 strip-like sub~surfaces 24, 25, 26, and 27 that run obliquely to the development and join at the roof edges. The latter is shown in the enlarged cross-sectional sections, which correspsnd in principle to those shown in Figure 1. The angle between the sub-surfaces 2~, 25, 26, 27 result, as is shown, from the cutting edge angles of the tools 16 that machine them. The bottom of the thread is adapted to the greatest extent possible to the spherical outside shape fo the hip-joint socket 1.

`:
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Claims

1. A hip-joint socket of spherical outer shape that can be screwed into place, which is provided with a self-cutting thread for cement free anchoring in the acetabulum, characterized in that its development the thread bottom to the greatest extent possible adapted to the spherical outer shape of the hip-joint socket (1) by strip-shaped secondary surfaces that are broken down by angles into steps which are parallel to each other and oblique to the development of the thread bottom.

2. A process for the machine production of the hip-joint socket as defined in claim 1, characterized in that the thread is so machined with different tools (16) with different cutting edge geometries relative to the thread bottom (6, 7, 8), on different paths, that the secondary surfaces that form the thread bottom are matched to the surface shape of a hemisphere that corresponds to the nominal diameter of the hip-joint socket to the greatest extent possible.

3. A process as defined in claim 2, characterized in that the thread is machined with at least three tools (16) having cutting edge geometries that differ relative to the thread bottom (5, 7, 8), in which connection the cutting edge angle of the cutting edges (17) of the tools (16) that machine the thread bottom are graduated within the framework of the tangent angle of the hemispherical area that is swept by the thread.

4. A process as defined in claim 3, characterized in that the cutting edge (17) of a tool (16) that machines the thread bottom corresponds essentially to the smallest tangent angle, the cutting edge of a further tool (16) that machines the thread bottom corresponds essentially to the half-tangent angle, and the cutting edge of a third tool (16) corresponds essentially to the greatest tangent angle of the hemispherical area that is swept by the thread.

5. A process as defined in one of the preceding claims, characterized in that the width of the individual tools measured between the corners (18, 19) in the axial direction of the thread is essentially equal.

6. A process as defined in one of the preceding claims, characterized in that at least when cutting the thread bottom, the tools (16) are so moved on their tracks that the point of the particular cutting edge that projects furthest in the direction of the mid-point of the hemisphere formed by the nominal diameter of the hip-joint socket is moved essentially parallel to or on this hemispherical surface.

7. A process as defined in one of the preceding claims, characterized in that the sum of the nominal radius (R) of the hip-joint socket (1) and the cutting radius (R) of the particular tool (16) is used as the radius for establishing the tracks (22) to be followed by the individual tools (16), and a corner (18, 19) of the tool (16) is used simultaneously as a reference point in the tool description of the mid-point of the cutting radius (R).

8. A process as defined in one of the preceding claims, characterized in that the tool (16) with the smallest cutting edge angle is moved by means of a tool correction slightly towards the socket equator and simultaneously the shape description in the form of the path to be followed (22) is shifted by the same amount towards the socket pole, and the tool (16) with the greatest cutting edge angle is moved in the opposite way in the direction of the socket pole and simultaneously the corresponding shape description is shifted to the pole equator by the same amount.

9. A process as defined in one of the preceding claims, characterized in that the tracks (22) that are to be followed by the individual tools (16) and which lie on sections of arcs of a circle are formed with the help of chord sections and the chords are in each instance shifted radially slightly outwards away from the mid-point (20) of the socket.

10. A process as defined in one of the preceding claims, characterized in that the tracks described by the tools (16) in the axial section of the hip-joint socket (1) are changed with the help of corrector values away from exact circular arcs in such a way that the roof edges formed in the thread bottom in sub-areas project no more than 0.1 mm outwards relative to the hemispherical surface formed by the nominal diameter of the hip-joint socket after machining.