NL1020435C2 - Hip prosthesis. - Google Patents

Description

Title: Hip prosthesis

The invention relates to hip prosthesis and more particularly to a total hip replacement prosthesis.

Treatment of degenerative hip arthritis by conventional total hip replacement is, essentially, very successful, with a typical 90% survival rate after 10 years. However, there are a number of clinical complications that can occur. These include: early dislocation (typically 3-4%), mechanical loosening and osteolysis due to debris of worn polyethylene. The harmful effect of polyethylene debris was increasingly understood during the 1990s, and therefore there has been a renewed interest in metal-to-metal prostheses, so that the use of polyethylene is avoided.

Two types of total metal-to-metal hip replacement prostheses are currently available for clinical use. These are: 15 total hip replacement prostheses with a handle with a small diameter (22-32 mm) and surface replacement prostheses with a large diameter (35 mm +). Both types are made from high-carbon cobalt chromium and require a high degree of sphericity and surface finish.

A metal-to-metal prosthesis with a large diameter would appear to have a number of advantages over a metal-to-metal prosthesis with a small diameter. These advantages include: resistance to dislocation (ie more stability), more anatomical (ie providing a better bone load and, therefore, bone integrity) and, most importantly, an improved ability to achieve a natural lubrication regime by entraining pseudo synoval liquid between the two metal connected surfaces. As a result of this improved lubrication, metal on metal prostheses with a large diameter exhibit theoretically lower wear rates than metal on metal prostheses with a small diameter.

However, metal-to-metal surface replacement prostheses with a large diameter are only suitable for the younger patient, where the collum femoris is strong enough to withstand fracture. 90% of all people over 60 suffer from osteoarthritis and 200 million people worldwide suffer from osteo-porosis. Therefore, surface replacement may be contraindicated in the elderly patient who would risk a collum femoris fracture following, and exacerbated by, implantation of a surface replacement prosthesis.

According to the present invention, a total hip replacement prosthesis is provided, provided with a femoral component and an acetabulum cup, the femoral component being provided with a femoral stem and a head defining a first, partly spherical hinge surface, and wherein the cup is provided with a recess defining a second, partially spherical hinge surface, the hinge surfaces of the head and the cup being formed of metal and the head having a diameter of at least 35 mm and a radius that is between 25 and 300 µm smaller than the radius of the recess in the bowl.

In this way it is possible to provide a prosthesis that has a number of advantages over conventional total hip prosthesis prostheses. These benefits may include: a. A self-lubricating regime between the head and the bowl so that wear is minimized.

B. the absence of polyethylene and, therefore, polyethylene debris.

c. a reduced risk of postoperative dislocation due to the use of a relatively large head.

3 d. an improved anatomical bone load as a result of at least maintaining the anatomical dimensions of the femoral head and bowl.

Preferred and / or optional measures are described in claims 2 to 10.

The invention will now be described in more detail, by way of example, with reference to the accompanying drawing, in which:

Figure 1 is a cut-away perspective view of an embodiment 10 of a total hip prosthesis prosthesis according to the present invention,

Figure 2 is a side view, partially in section, of the femoral component of the hip prosthesis as shown in Figure 1, and

Figure 3 is a sectional view through the acetabulum cup of the hip prosthesis shown in Figure 1.

Reference is made to the drawings in which the total hip replacement prosthesis is provided which is provided with a femoral component 10 and an acetabulum cup 11. The femoral component 10 is provided with a femoral stem 12 which is formed to enter the femoral cavity of the femur, and a head 13 defining a first, partially spherical hinge surface 14.

The acetabulum cup 11 has a recess 16. The recess 16 defines a second, partly spherical hinge surface 15 which is preferably hemi-spherical or substantially hemi-spherical. The bowl 11 furthermore has an outer surface 17 with a suitable diameter to fit into the acetabulum of a patient.

The femoral component 10 and bowl 11 can be implanted using bone cement. Alternatively, one or both can be implanted using a porous ingrowth coating for direct fixation to the guest bone.

4

An annular edge 18 extends between the outer surface 17 and the hinge surface 15. The surface of the edge 18 is flat. The edge 18 is provided with at least one and preferably three insertion recesses 19 placed at equal angles from each other (shown in Figure 3). The recesses 19 only extend to the edge 18 and are spaced apart from the outer surface 17 and the inner surface 15. The recesses 19 are adapted to receive an insertion aid (as described in GB-A-2333961) around the bowl 11 in the to insert and place the acetabulum of a patient.

In order to improve the fixation of the acetabulum cup 11 in the acetabulum of the patient, the outer surface of the cup may be provided with a structure and / or may comprise a number of key teeth (not shown). A pin 21 located above the center can also extend from the outer surface 17 of the cup. As mentioned above, the acetabulum bowl can be implanted using a porous ingrowth coating for direct fixation to the guest bone. To ensure that the bowl 11 remains in the acetabulum before it is fixed by bone ingrowth, the bowl is shaped such that the equatorial diameter of the outer surface is larger than the polar diameter dp of the outer surface 17. The acetabulum is cleared up to the same size as the polar diameter dp and the bowl 11 can then be driven into the acetabulum due to the larger equatorial diameter of the bowl.

The bowl 11 and the head 13 of the femoral component 10 are both made of metal and more preferably of high carbon cobalt chrome. However, one or both of the bowl 11 and the head 13 can be made of a suitable biocompatible material and be provided with a metal lining that determines the hinge surface (s).

The head 13, which in practice is referred to in the art as a large diameter head, has a diameter of at least 35 mm and becomes in particular provided with a diameter that is between 40 and 56 mm. A large diameter head reduces the risk of postoperative dislocation and improves anatomical bone load. The radius of the head is between 25 and 300 µm smaller than the radius of the recess 16 in the bowl 11 and optimally between 100 and 200 µm smaller than the radius of the recess 16. This radial mismatch between the bowl 11 and the head 13 is deliberately made in the cup and head to improve the ability of the prosthesis to achieve a natural lubrication regime by entraining pseudo synovial fluid between the hinge surfaces 10 and as a result of this improved lubrication, the wear rate of the prosthesis is reduced. Furthermore, the head 13 will rest on the pole of the bowl 11.

The sphericity of the head (as defined by ISO 7206-2 1996 section 4.1.1.) Is less than 10 µm.

The femoral stem 10 is typically made of a titanium alloy, or a cobalt chrome alloy or stainless steel. The head 13 is connected to the femoral stem by a tapered tap 22 of the type which in practice is called a Morse cone. Alternatively, the head 13 may be an integral part of the shank 10.

The surface finish of the hinge surfaces of the head 13 and the recess 16 is less than 0.05 µm.

A range of head and bowl diameters can be made available to match the size of the patient. In this case, the internal diameters of the bowls and the diameters of the heads will vary according to the external dimensions of the bowls that match the size of the patient's acetabulum.

The above-described embodiments are given by way of example only and various modifications will be apparent to those skilled in the art 6 without departing from the scope of the invention as set forth in the following claims.

Claims (10)

1. A total hip replacement prosthesis provided with a femoral component and an acetabulum cup, wherein the femoral component is provided with a femoral stem and a head defining a first, partly spherical hinge surface, and wherein the cup is provided with a recess which defines a second, partially spherical hinge surface, wherein the hinge surfaces of the head and cup are made of metal and the cup has a diameter of at least 35 mm and a radius that is between 25 and 300 µm smaller than the radius of the recess in the bowl. 10 A hip prosthesis according to claim 1, wherein the radius of the head is between 100 and 200 µm smaller than the radius of the recess in the cup. 3. A hip prosthesis as claimed in claim 1 or 2, wherein the sphericity of the head is less than 10 µm. A hip prosthesis according to any one of the preceding claims, wherein the surface finish of the hinge surfaces of the head and the recess in the cup is less than 0.05 µm. 20 A hip prosthesis according to any one of the preceding claims wherein the metal is a high carbon cobalt chromium alloy. 6. A hip prosthesis according to any one of the preceding claims, wherein the equatorial diameter of the outer surface of the cup is larger than its polar diameter. 7. A hip prosthesis according to any one of the preceding claims, wherein the cup has an annular edge between its outer surface and its hinge surface and wherein the edge has at least one recess which can be engaged by an insertion device for the purpose of inserting and placing the cup in the acetabulum of a patient. A hip prosthesis according to any one of the preceding claims, wherein the head is adapted to rest on the pole of the cup. 10 A hip prosthesis according to any one of the preceding claims, wherein the head is permanently connected to the femoral stem. 10. A hip prosthesis according to any of claims 1 to 8, wherein the head is connected to the femoral stem by a tapered tap.