FR2703583A1 - Femoral implant for hip prosthesis - Google Patents

Abstract

The femoral implant for a hip prosthesis comprises a stem (1) angularly extended by a neck (1a). This stem (1) has, in its proximal part (1b), on the external metaphyseal side (1c), a protrusion (2) designed in shape and cross-section to interact, after impaction of the femoral stem (1) in the medulary cavity, under the base of the greater trochanter, while being capable of compressively stressing the external cortical region of the femur, so that the action of all the muscles is taken into account.

Description

Dour femoral implant hancha prosthesis
In a perfectly known manner, a femoral implant comprises a rod extended angularly by a neck capable of receiving directly or in an attached manner, a spherical head cooperating with the acetabular cavity of the iliac bone.

 The shapes given to this type of prosthesis are optimized according to the resulting hip force. However, it is very difficult to know precisely, the forces that act on the joints.

 In an attempt to define the forces acting on the hip joints, theoretical models have been designed. This is particularly the case for the PAUWELS biomechanical hip model. According to this model, the resulting load on the femoral head is inclined on the order of 17 with respect to the vertical, considering a single muscular load of the gluteus medius on the greater trochanter and the offset weight in a unipodal station.

 More particularly, the resulting force of the hip, generated by the muscular force which acts on the hip joint and the partial weight, composed of the trunk and the leg in suspension, acts on the joint at an angle of the order of 17 in the frontal plane. Under these conditions, the resultant leaves the femur, in particular the femoral neck, in the region of the lower buttress of the femoral neck. This results in a bending load which acts on the cortex, with high median compression forces and lower lateral traction forces.

 FIG. 1 shows a femoral stem implanted in the medular canal of a femur, the shape of this stem being determined as a function of the load resulting from the plane model of PAUWELS. We see that the resultant (R) makes an angle (a) of the order of 17 with respect to the vertical (XX ') and leaves the femur in the area of the buttress of the femoral neck.

 It therefore appears that the generally accepted design of a hip prosthesis is to adapt the rod essentially to the calcar because this area is considered to be the most important for load transfer, considering compression on the median side and tension on the lateral side of the metaphysis of the femur.

A recent biomechanical hip model from FROHLING, taking into account all the muscles, shows that the result of the forces exerted on the head of the femur makes an angle of the order of 20 to 25 relative to the vertical. This angle was validated by the measuring rod of
BERGMANN. The result of this angle is a compression load transfer on the median side as well as on the lateral side. Under this assumption, the femur is fully compressed.

 In addition, in many cases, the cementless stems are depressed until they are in close contact with the middle and lateral parts of the cortex so that, given the conical shape of the metaphysis of the femur, the rod is fixed in a conical manner.

 This conical fixation generates compression on the median side as well as on the lateral side.

 It is also recognized that a proximal load transfer should be achieved, in order to avoid distal stresses likely to generate atrophy of the proximal femur.

 Such proximal fixation requires close cortical contact of the proximal part and relatively free contact of the distal part of the stem. The conical shape of the proximal femur implies a conical shape of the stem at the proximal level.

 Given the variety of femoral shapes at the proximal level, it is not possible to use standard femoral stems, but only custom stems. However, such custom rods are relatively expensive and cannot be used routinely, and are not available for traumatic patients requiring immediate surgery.

 In an attempt to remedy these drawbacks, modular systems have been proposed which can be adapted to each femur.

 These systems emerge for example from the teaching of patents EP 0085147 and FR 8505348 which define a modularity over the entire length of the rod in order to reduce the stock. The lateral module has a straight shape, so that it is not possible to obtain a metaphyseal fixation, but only a diaphyseal one.

 Mention may also be made of patent FR 8911677 which discloses a proximal / distal modularity in order to ensure load transfer at the metaphyseal level. To obtain a proximal conical adjustment, this system must provide a large range of proximal modules. Otherwise, the result obtained is not satisfactory and does not constitute a relative transfer of the price charge by adapting the shape to the individual proximal medullary canal. In addition, it is difficult to remove the distal module during revision, so that one of the essential prerequisites of the hip prosthesis is not taken into account.

 The object of the invention is to remedy these drawbacks in a simple, safe, effective and rational manner.

 The problem which the invention proposes to solve is to optimize the shape of a hip prosthesis and more particularly of the femoral stem, taking into account the action of all the muscles acting on the hip joint, in order to obtain a controlled proximal fixation, by adaptation of the proximal part of the rod, to the individual shape of the lateral cortical shoulder of the femur. This problem turns out to be particularly important, since tests have shown that there are no tension forces, but essentially compression forces on the proximal lateral part of the femur, taking into account the resulting force. hip.

 To solve such a problem, taking into account that the result of the forces exerted on the head of the femur is directed entirely in the internal part connecting the femoral head to the trochanteric mass, it was designed and developed, a femoral stem which presents, at the level of its proximal part, on the external metaphyso side1, a prominence determined in shape and section to cooperate, after impaction of the femoral stem in the medular canal, under the base of the greater trochanter, being able to stress, in compression, the external cortex of the femur.

 Taking into account the different sizes of femurs, and in order to respond to all pathological cases, the prominence consists of an added element.

 To resolve the problem posed of being able to adapt the added element to the different sizes of femurs, the added element and the outer edge of the proximal part of the rod have complementary arrangements capable of ensuring the removable fixing of said element.

 Advantageously, the arrangements are formed on an oblique segment established angularly with respect to the vertical axis of the rod.

 The problem posed of ensuring a removable assembly between the insert and the proximal part of the femoral stem is solved in that the arrangements are constituted by a complementary dovetail assembly system presented by the oblique segment and the reported item.

 To solve the problem of ensuring the locking in translation of the element on the corresponding part of the rod, the dovetail assembly system has a longitudinal profile of tapering tapered section.

 Preferably, the insert is introduced through the bottom of the prosthesis. Various means of locking in translation of the element on the prosthesis body are also provided. These means can also be configured to provide an adjustable fastening in translation, thus making it possible to have a single element which can adapt to different pathological cases. These means can be constituted by a notching system or by a worm screw system.

 In another embodiment, the added element can have a determined shape in order to be reversible, always with the aim of reducing the number of elements depending on the pathological case.

The invention is set out below in more detail using the appended drawings, in which:
Figure 1 is a sectional view of a femur equipped with a femoral prosthesis according to the prior art, and whose shapes correspond to the plane model of PAUWELS.

 Figure 2 is a perspective view before assembly of the components of the femoral stem according to the invention.

 Figure 3 is a front view of the prosthesis. Different profiles of the metaphyseal filling insert have been illustrated in dashed lines.

 Figure 4 is a view corresponding to Figure 1, showing the impact of a femoral stem according to the invention and the result of the forces at the joint.

 The femoral implant comprises, in a known manner, a rod (1) extended angularly by a lateralized neck (la). The rod (1) has, at its proximal part (lob) on the metaphyso-external side (tic), a prominence (2). This prominence (2) is determined in shape and section to cooperate, after impaction of the femoral stem (1) in the medullary canal, under the base of the greater trochanter (F1). The aim of this prominence (2) is to solicit in compression the external cortex of the femur, to take into account the action of all the muscles which is exerted at the level of the hip joint.

 Advantageously, this prominence consists of an attached element (2), shaped in shapes and dimensions, to constitute a metaphyseal filling element. This element (2) is detachably fixed on the outer edge (here) of the proximal part of the rod, at the level of the metaphysis. For this purpose, the element (2) and the external edge (here) of the proximal part, have, in combination, complementary arrangements of removable fixing.

 These arrangements can be formed by various known and appropriate means. For example, these arrangements are constituted by a dovetail system (2a) (1c2), without excluding other equivalent systems.

 Preferably, the external edge (here) of the proximal part intended to receive the metaphyseal filling element, constitutes an oblique segment established angularly, relative to the vertical axis.

 As shown in Figure 2, this oblique segment (here) directly presents, during the manufacture of the rod (1), a male dovetail assembly profile (1c2). In a complementary manner, the underside of the insert (2) has a recessed recess (2a) of cross section at the dovetail, so as to be engaged in sliding, along the profile at the tail of dovetail (1v2). Obviously, a reverse conception can be envisaged.

 To ensure the locking in translation of the element (2), on the inclined segment (it), the longitudinal edges of each of the dovetail profiles (2a) and (1c2) are not parallel, but delimit a surface tapering tapering width towards the outer edge of the stem.

 As shown in Figure 2, the insertion of the insert (2) is carried out from the outer edge (tic), so that in this case, the insert (2) is put in place before introduction of the prosthesis. However, it is not excluded to introduce the element (2) from the other side of the prosthesis, in order to put this element (2) only after the prosthesis (1) has been put in place. In this case, a notching system, screws or the like can ensure the locking in translation of the element (2) in an adjustable manner.

 As shown by the broken lines in Figure 3, the rod (1) can receive inserts (2) of different shapes and profiles, in order to obtain a complete lining on the metaphyso-external side, at the base of the large trochanter, depending on the anatomical shape of the femur considered. Likewise, this added element (2) can have a determined shape, so as to be reversible so as to be able to treat with a single element, several pathological cases.

 The femoral stem as defined and illustrated, can have any type of coating and profile in section, with a view to its impaction with or without cement. It should be noted that the shape of the distal part of the rod can also have different embodiments. Likewise, the lateralized neck (la) can be formed directly or in an attached manner at the end of the rod (1).

 The advantages clearly emerge from the description, in particular it is emphasized and recalled that after impaction of the femoral stem, the result of the forces exerted is located entirely in the internal part of the femur connecting the femoral head to the trochanteric mass, so as to apply compression, internal and external cortices.

Claims (8)

-1- Femoral implant for hip prosthesis, comprising a rod (1) angularly extended by a neck (la), characterized in that the rod (1) has, at its proximal part (lob), on the external metaphyso side (it), a prominence (2) determined in shape and section to cooperate, after impaction of the femoral stem (1) in the medular canal, under the base of the greater trochanter, being able to urge, in compression, the external cortex of the femur, so that the action of all muscles is taken into account. -2- Implant according to claim 1, characterized in that the prominence consists of an insert (2). -3- Implant according to claim 2, characterized in that the insert (2) and the outer edge (tic) of the proximal part of the rod (1) have complementary arrangements capable of ensuring the removable fixing of said element. -4- Implant according to claim 3, characterized in that the arrangements are formed on an oblique segment (here) established angularly relative to the vertical axis of the rod (1). -5- Implant according to claim 4, characterized in that the arrangements are constituted by an assembly system in complementary dovetail (1c2) (2a) presented by the oblique segment (il1) and the attached element (2) . -6- Implant according to claim 5, characterized in that the dovetail assembly system has a longitudinal profile of decreasing conical section. -7- Implant according to claim 6, characterized in that the dovetail assembly system is combined with a locking system and adjustment in translation of the insert. -8- Implant according to claim 2, characterized in that the insert has a shape and profile in section determined to be reversible in order to treat several pathological cases.