Assembly of a ceramic ball and a metallic stem for a hip prosthesis.

TECHNICAL AREA

The invention relates to the assembly of a ceramic head (usually a ball) with a metal rod for a hip prosthesis. This assembly, associated with a second part, called the acetabular cavity, constitutes an internal prosthesis femoral joint.

STATE OF THE ART

It is known to use as a femoral prosthesis assembly of a ceramic femoral head and a metal rod, said rod being the one hand implanted in the femur and the other fitted into a blind recess in the head ceramic. This assembly is generally associated with a natural or artificial acetabular cup supporting the rotation of the head to form an inner ceramic prosthetic hip joint. The blind cavity in the ceramic head (female part) is usually truncated circular cross section and low taper; the male part of the femoral metal rod that comes there follow almost the same vertex angle, so that the plug-in connection is free of play between the two parts, relative rotation of the head on the shaft being avoided by various devices such as force fitting, using adhesive, keying, brazing ... Since the apex angle of the cone is low, it often happens that when the practitioner puts in place the two pieces, their alignment is not strictly proper; and instead of having a contact surface between the walls of said female and male parts the male part is in fact rests by only a few points on the wall of the recess of the female part. For the same reasons, it also happens that the male part does not fits the entire planned scope of the recess of the female part; For example, the top of the male part can lead to false at a point on the wall of the recess is not located at the bottom of said recess, and at the same time the contact surface of the male part can lead to false at a point the wall of the recess near the periphery of the ceramic head.

It also happens that both the metal and ceramic male cone recess have geometrical defects that make the contact between these two parts is made only by a few points and that their implementation is not correct.

All these problems related to the implementation by interlocking male and female parts, are that ultimately the mechanical stresses due to external forces and sustained by the ceramic head is transmitted to the male part and femur only through a few bearing points between the male part and the recess of the female part, instead of by a contact surface. Thus, the stresses to the ceramic head increased considerably at these points and cause a dramatic increase in the risk of breaking the ceramic head. Such failures usually occur in the front area, as opposed to the deep end of the recess of the ceramic head.

OBJECT OF THE INVENTION

The object of the invention is to obtain a plug-in connection between the male cone of the femoral metal shaft and the conical recess of the ceramic head, as it avoids the risk of breakage of said ceramic head due to incorrect positioning and / or to defects in geometry of said two parts in contact.

Another object is to ensure a rigorous implementation of these two parts, when assembled by the practitioner.

Another object is to have a prosthetic femoral assembly consisting of a ceramic head and a metal rod, which has better mechanical resistance to breaking through optimal distribution of stresses in the ceramic head.

DESCRIPTION OF THE INVENTION

The invention is an assembly of a ceramic head comprising a blind recess and a male tang interlocking machined there on one end of a metal rod, the assembly obtained associated with an acetabular cup forming an inner joint prosthesis hip, said assembly for an optimal distribution of mechanical stresses in the ceramic head is characterized in that the fitting of the male tenon is free game in a first frusto-conical region of the recess, called deep contact zone situated towards the center of the ceramic head and having a small apex angle and that in a second region of the recess, said non-contacting, the first extending in the direction of the periphery of the ceramic head and opening to the outside, the male tenon and the wall of said second region of the recess are not in contact.

According to the invention, the pin or machined end of the metal rod and the recess of the ceramic head in which it fits, are in contact only through at most a part of their total lateral surfaces and the contact is located in cavity bottom. Contact parts are: . with regard to the recess of the ceramic head, the part of its lateral wall located in the deepest area of ​​the recess, to the exclusion of the bottom itself. This part is tapered at an angle to the low summit . as regards the tenon fitting into the recess, the part of its lateral wall located nearest to its free end.

It thus appears that the truncated cone of the deepest area of ​​the recess and the truncated cone of the free end of the lug, cooperating with one another in the assembly, are machined with the same angle, to within the tolerances, then than in the non contact area, the surfaces of the recess and the lug, which are opposite but not in contact, are totally different.

Odds of contacting walls are such that there is no clearance between them and the male tang does not abuts on the bottom of the recess. Thus, the blind cavity of the transverse top and bottom surface of the truncated cone of the male tang should not be in contact with each other.

Usually, the deep zone of the recess as well as the wall of the male tang engaging it, have a symmetry of revolution about an axis coinciding with a radius of the ceramic head. The contacting walls are preferably frustoconical, but necessarily with apex angles virtually identical.

The total angle at the top of this common truncated cone is low and is generally between 3 and 10 ° and is preferably close to 6 °.

The height of the bearing surface on which there is contact between the side wall of the recess and the corresponding wall of the pin is generally between 15% and 80%, preferably between 25 and 70%, of the total height of the recess. This height is generally measured from a point located on the axis of revolution, at such a distance that it preserves a space of clearance between the bottom of the recess and the top of the pin to avoid that 'they abut. It is important that this space has a thickness not exceeding 2 mm. While this area of ​​care is too high, the strain on the ceramic head is moved to section expanded areas which present risks of rupture and undermines the effectiveness of the device according to the invention. Deep contact area of ​​the recess is extended toward the periphery of the head by a second area where contact with the metal rod is carefully avoided. This zone of clearance is obtained: . or preferably by machining the recess of the ceramic head in its non-contact area to increase the mean diameter, for example by machining a corner truncated cone at the top greater than that of the truncated cone of the contact area ; the post surface opposite may then be tapered with an angle smaller peak (in this case the stud may advantageously be machined to a single constant profile of a truncated cone on the whole height of the two zones) or cylindrical .Either by machining the pin, in its non-contact area, to reduce the average diameter, for example in a corner truncated cone at the top smaller than that of the truncated cone of the contact zone or in a cylinder; the recess can then maintain a constant tapered profile throughout its height.

Thus, according to the invention, it is noted that at least the recess or the peg has a profile that is not constant over their entire height; which means that the height profile of at least one of the two pieces, there is a break in the slope line of the outer surface which faces the other with the workpiece surface. In the contact area, the facing side walls of the recess and stud are in contact with their entire surface. Contact is made using a common truncated cone, non-deformable, and total since the establishment of the two parts, by simple fitting, since the contact surfaces exist when machining; the height of the contact zone is also constant from said establishment.

The thickness of the free space existing clearance, in the non-contact area between the wall of the recess and that of the tenon, in general does not exceed 2 mm and usually 1 mm and even less, it is constant or variable over the height of said non-contact area following the profiles of the second region of the recess and the pin of the area facing it are similar, for example both cylindrical but not of the same diameter, or different example of which is cylindrical and one frusto-conical.

This non-contact area has a height, measured on the axis of revolution of the recess, which is derived from the contact height seen previously; it is thus the miniumum equal to 20% of the total height of the recess of reduced headroom guard. It is therefore completely different from a non-contact area, as found in the prior art, resulting from a slight chamfer practiced on the edge at the mouth of the recess; Indeed, the height of the chamfer not exceeding 1 mm in general and is generally 0.5mm.

Once the assembly is achieved by fitting avoids the rotation of the ceramic head on the end of the metal rod with any known means, such as colage, force fitting, calvetage, brazing ...

Figures 1-3 will better understand the invention.

1 shows a head assembly ceramic - metal post according to the prior art. Figures 2 and 3 show two ways of carrying out the invention.

In Figure 1 we see the metallic rod 1 with its machined conical pin 2 designed to be fit entirely in the corresponding recess of the ceramic head 3. It is seen that the wall 4 of the recess and the wall 5 of the end of the metal rod are in contact over a height 10 which is the entire height of the recess, reduced by the small space guard 6 separating the top 7 of the metal rod 1 of the bottom 8 of the recess of the ceramic head 3, and minus the height of the chamfer 9 provided on the edge of the mouth of the recess of the head 3. the machined cone 2 of the metal rod and the recess of the ceramic head 3 being truncated s 'engaging without play into each other, giving an assembly for femoral prosthesis.

In Figure 2, showing a particular embodiment of the assembly according to the invention, it is seen that the recess of the ceramic head is conical over its entire height. By against the machined tenon 2 coming into contact with the wall 4 of the deep region of the recess is tapered to a height 11 significantly lower than the total height of the recess; this cone 2 is continued by a cylindrical part 12 which faces without touching the second zone 13 of non con tact the recess. The thickness of the space between the wall of the recess and that of the metal rod is not constant in this case.

Figure 3 represents another particular embodiment of réaisation of the invention, it is seen that the recess is frustoconical always, that the deep zone of contact 14 is lower than in the previous case, and that the second non-contact area the recess has been obtained by practicing a frustoconical taper 15 in the ceramic head, as a result of the deep zone, the angle slightly larger peak. The cone 2 of the metal rod is tapered over the entire height corresponding to the recess. In this case the thickness of the space between the wall of the recess obtained after the clearance operation and the wall of the metal rod is also not constant.

The assembly according to the invention is applicable to all ceramic discs sintered at for example based on oxides such as aluminum oxides, zirconium ..., carbides, nitrides such as silicon nitrides, borides. .. or mixtures or composite materials (eg carbon-carbon ...). The head is generally massive, that is to say does not contain any other cavity that blind obviously necessary to assembly. Similarly, the metal rod is of any nature, for example titanium, stainless steel or other alloys, it may also consist of all materials of sufficient strength for this application, for example composite materials. The invention is applied particularly to the achievement of full dentures of hip joint, the acetabular cup which takes place in the patella can be natural or artificial (metal, ceramic, composite, plastic, etc ...) it is also applicable to all connections by fitting of a metal rod in a blind cavity of a ceramic head substantially spheroidal.

<u>EXAMPLES</u>

Breaking static comparative tests were carried out on the corresponding heads in each case illustrated in Figures 1 to 3. They consist of applying a force to the head 3 ceramic, assembled with the metal rod 1-2, along the axis of the assembly until the ruin of said ceramic head; they were carried out as described in the draft standard French PR.S 90443 of 02/10/88 "Parts femoral hip prosthesis with head reported: specifications of the head and the male portion of the nesting."

Examples 1,2,3 correspond to tests performed on aluminum oxide sintered ball diameter 28 mm with a recess depth of 16.5mm; the deep zone of contact has a tapered profile with a bottom 8 of 14mm diameter, and a total apex angle of 6 °. The femoral stem is metallic titanium-based alloy and has a machined stud, its contact area has a same vertex angle as the recess, but the diameter of the top 7 is slightly higher than the bottom 8 so as to create the small space guard 6, between said top 7 and the bottom 8, whose thickness is 1mm.

Examples 4 and 5 correspond to the tests performed on spherical sintered zirconium oxide of 28mm diameter with a recess depth of 20mm and an area of ​​guard 6 0.5 mm thickness, in the same conditions as the tests of Examples 1 and 2.

EXAMPLE 1 (fig.1 already described)

It illustrates an assembly according to the prior art. The height of the contact surface between the cone 2 of the machined metal rod in the form of a truncated cone and the corresponding recess of the ceramic head 3 is 15 mm in view of the clearance gap 6 and the height of the chamfer 9 which is of 0.5 mm. So the range is 100% of the usable height of the recess. Several rupture tests were performed; the average of the tensile strength obtained was 2960 kg.

EXAMPLE 2 (FIG. 2 previously described)

The height 11 of the range is 10mm, or 65% of the total height of the recess less the thickness of the space 6 guard. As said non-contact area was obtained by machining a cylindrical 12, recessed from the end of the metal rod. The same tests gave an average breaking load of 5470 kg.

EXAMPLE 3 (FIG. 3 already described)

The height of the contact surface 14 is 5 mm, or 33% of total height of the opening minus the height of the space 6 guard. As said non-contact area was obtained by machining the second frustoconical region of the recess at an angle slightly greater than that of the truncated cone of the contact zone. The same tests gave an average breaking load of 4,600 kg.

Example 4 (Figure 1, already described)

It illustrates an assembly according to the prior art applied this time to ball joints sintered zirconium oxide. The height of the contact surface between the cone 2 of the machined metal tihge and the corresponding recess of the ceramic head is 19mm 3, after deducting the space 6 guard 1mm and height of the chamfer 9 which is 0.5mm. The range is 100% of the usable height of the recess. The same tests have led to an average value of the breaking load of 8750 kg.

EXAMPLE 5 (FIG. 2 previously described)

The height 11 of the reach is 12mm or 62% of the total height of the recess space decreased guard 6. The non-contact area was obtained by machining a cylindrical recessed 12, the lug located at the end of the metal rod. The same tests yielded an average tensile strength of 12900 kg.