EP2571458A1

EP2571458A1 - Hip stem design

Info

Publication number: EP2571458A1
Application number: EP11724765A
Authority: EP
Inventors: Jean Ple; Elhadi Sari-Ali
Original assignee: Symbios Orthopedie Sa
Current assignee: Symbios Orthopedie Sa
Priority date: 2010-05-19
Filing date: 2011-04-21
Publication date: 2013-03-27
Also published as: WO2011145008A1

Abstract

The invention relates to a process for manufacturing hip stems of different neck- shaft angles (CC'D) and having an internal curve of the shaft; characterized by the fact that the maximal curvature of said shaft internal curve varies with the neck-shaft angle (CC'D). The invention also concerns hip stems obtained according to this process.

Description

Hip stem design

Field of invention

The invention relates to hip prostheses and more precisely to the manufacturing of hip stems. It also relates to set of hip stems which are made available for a patient.

State of the art

Figure 1 is a representation of a common hip stem. The definitions indicated are also common and will be used in the present text.

Sizes of a hip model and the design rule between sizes. Given the variations in anatomy of the femur, implant manufacturers have had to provide variations of hip models to allow a good fit of the stem into the femur. Traditionally these variations are proposed in the form of different sizes. Sizes are obtained from one another by variation of specific parameters. The design rule defines which parameter varies and how to characterize the geometric transformation from one size to the next.

Although it is not obvious at first, the design rule between sizes is an integral part of the design of the model because the surgeon must understand what happens when he changes from one size to the next and how this will affect the situation in his quest of better fitting the femur and placing the center of rotation in the right position.

Each model has its own design rule between sizes. Below are two examples of two different design rules between sizes. Figure 2 illustrates a first example of hip stem (Muller).

In this example the design rule between sizes is as follows:

1 . The internal curve is identical for all sizes

2. The neck shaft-angle is constant for all sizes.

3. The center of rotation has the same position for all sizes.

4. The larger size is obtained by growth of the external curve only.

5. The model has six sizes defined as 7.5, 10, 12.5, 15, 17.5, 20.

Figure 3 illustrates another example of hip stem (Zweymuller).

In this example the design rule between sizes is as follows:

1 . The neck-shaft angle is constant for all sizes.

2. The distance C'C is increased for each size.

3. Each size is obtained from the preceding one by growth in all directions.

4. The model has 12 sizes denominated from 1 to 12.

Family of hip models

Because of the large anatomical variations of the femur, hip stems with different sizes often did not permit surgeons to reproduce simultaneously the two objectives: fit the internal part of the femur and reproduce the center of rotation. To offer more possibilities, implant manufacturers introduced variations in the neck-shaft angle in addition to sizes. This introduces the notion of families. There are as many families as there are neck-shaft angle variations. Below are two examples of hip models that include several sub families. Figure 4 illustrates a CLS stem

There are three families defined by the variation of neck-shaft angle: 145°, 135°, 125°. The design rule between components is as follows:

1 . The shape of the shaft is identical for all families.

2. For each family, the internal curve is identical for all sizes

3. For each family, the neck-shaft angle which is constant at 145°, 135°, 125°. 4. For each family, the center of rotation has the same position for all sizes.

5. For each family, the larger size is obtained by growth of the external curve only.

6. Each family has 13 sizes. Figure 5 shows CLS models as offered in a commercial brochure. Figure 6 shows Zweymuller models.

In this example, the Zweymuller stem has two subfamilies the standard family and the lateralized family.

The design rule between components is as follows:

1 . The shape of the shaft is identical for the two families.

2. For each family, the neck-shaft angle which is constant.

3. For each family, the distance C'C is increased for each size..

4. For each family, each size is obtained from the preceding one by growth in all directions.

5. Each family has 12 sizes denominated from 1 to 12.

General description of the invention

An object of the present invention is to provide improved hip stems, in particular with a design which more precisely fits the femur anatomy and the center of rotation.

To this effect the invention relates to a process for manufacturing hip stems of different neck-shaft angles and having an internal curve of the shaft, designed to be inserted into the femur; characterized by the fact that the maximal curvature of said shaft internal curve varies with the neck-shaft angle.

In the present text the term "curvature" has to be understood according to the following mathematical definition:

Given any curve C and a point P on it (see figure 7) where the curvature is non- zero, there is a unique circle which most closely approximates the curve near P, the osculating circle. The radius of the osculating circle r is the radius of curvature of C at this point P. The curvature itself , K , is equal to the reciprocal of r, i.e. K = 1/r.

So the curvature of a straight line is zero and the curvature of a circle is constant. But for any other curve, in particular for an internal curve as defined in the present invention, the curvature varies.

In the present text, the expression "maximal curvature" for a given curve means the maximal value of /f along said curve. As mentioned above the invention relates to a process for manufacturing hip stems.

In addition it also concerns any hip stem or set of hip stems which may be obtained by the process according to the invention. Detailed description of the invention

The invention is discussed below in a more detailed way with examples illustrated by the following figures :

Figure 8 illustrates a set of hip stems according to the invention.

Figure 9 Illustrates the range of femoral off-set achieved for each size according to the families

Figure 8 shows a set of hip stems according to the invention wherein all stems have the same size but different neck-shaft angles CCD.

To each stem is associated a circle which corresponds to the osculating circle for the maximum curvature.

In this example it can be noted that the radius of the osculating circle increases as the neck-shaft angle CCD increases, i.e. the maximal curvature increases as the neck-shaft angle CCD decreases.

The choice of this design rule is based on the results of a three-dimensional analysis of the femur anatomy performed on 478 patients. This study allowed the inventors to define morphotypes of the intra-medullary part of the femur that correlate with the extra-medullary anatomy, especially regarding the femoral offset and the height of the femoral head center relative to the top of the great trochanter. The set of stems allows an accurate anatomical reconstruction of the hip. Description of Figure 9

Illustration of set of stems achieved with the presented designing rule. 3 families are shown (Valgus, standard, Varus). The table shows the variation of the femoral off-set according to the size and the family). The range of femoral offset hen including 3 femoral head length (-4mm, 0 and 4mm )is about 20 mm for each size.

Claims

1. Process for manufacturing hip stems of different neck-shaft angles (CCD) and having an internal curve of the shaft; characterized by the fact that the maximal curvature of said shaft internal curve varies with the neck-shaft angle (CCD).

2. Process according to claim 1 wherein said maximal curvature increases as the neck-shaft angle (CCD) decreases.

3. Process according to claim 1 or 2 wherein the hip stems have the same size.

4. Process according to claim 3 wherein said maximal curvature increases as the femoral off-set increases.

5. Hip stem obtained according to the process as defined in any of the previous claims.

6. Set of hip stems having the same size but with different neck-shaft angles (CCD) and with a shaft internal curve which is defined by a maximal curvature which varies with the neck-shaft angle (CCD).

7. Set of hip stems according to claim 6 wherein said maximal curvature increases as the neck-shaft angle (CCD) decreases.

8. Set of hip stems according to claim 6 or 7 wherein said maximal curvature increases as the femoral off-set increases.