WO2004028413A1

WO2004028413A1 - Femoral component for a hip prosthesis

Info

Publication number: WO2004028413A1
Application number: PCT/CH2003/000650
Authority: WO
Inventors: Walter Ramseier
Original assignee: Walter Ramseier
Priority date: 2002-09-30
Filing date: 2003-09-29
Publication date: 2004-04-08
Also published as: AU2003264227A1

Abstract

The invention concerns a femoral component (2) for a hip prosthesis comprising an elongated anchoring structure which is adapted to be inserted in a femur canal (6) and anchored therein. This anchoring structure comprises at least two elongated anchoring elements which are disposed peripherally around a central axis and engaged with locking means arranged to displace them radially outwards and bring them in firm contact with the inner wall of the femur canal. In a preferred embodiment, the locking means are locking cones (8, 9, 10) disposed along the central axis, which are engageable with corresponding ramps (11, 12, 13) on the anchoring elements. Preferably, a proximal part of the anchoring structure is inserted into a bore (5) of a connecting element (3) with a neck and a joint head (4), so that the anchoring structure is simultaneously fixed to the bone (1) and to this connecting element (3) by actuating the locking means.

Description

Femoral component for a hip prosthesis

The invention relates to a femoral component for a hip prosthesis according to the preamble of claim 1.

In total hip arthroplasty both the joint head and the corresponding acetabulum are replaced by artificial components. The femoral component of the prosthesis comprises an elongated stem adapted to be inserted into the femur canal and connecting means with a neck portion for connecting a joint head to this stem. It is important that the stem is securely anchored in the femur canal and one known approach to accomplish this anchoring is the use of bone cement. However, the loosening of the implant due to thermal and toxic degradation of the cement and local bone necrosis continues to be a problem and for the change of cemented hip implants, a literature overview shows a clinical failure rate of 12 to 29 % in a follow-up period of 2.1 to 8.9 years. It has therefore been suggested to dispense with the cement and .to rely on a press fit and biological fixation of. the stem in the femur canal. Stems for such "dry" implantation have a structured surface which incites bone accretion thereto. Yet the expectation that these prostheses would last longer than the cemented ones has not been fulfilled. Uncemented prostheses break out clearly more often than the other ones.

It is an object of the invention to provide a femoral component having an anchoring structure with improved long term stability. This object is attained with a femoral component with the characteristics of claim 1. Preferred embodiments are exposed in the dependent claims.

The stem of the femoral component is hereinafter more generally referred to as an elongated anchoring structure. This anchoring structure comprises at least two elongated anchoring elements extending from its proximal end to its distal end, which means that they are essentially parallel to one another. These elements are disposed around a central axis of the anchoring structure. With respect to this axis, the anchoring elements have radially inner and radially outer surfaces. The latter, hereinafter also referred to as anchoring surfaces, define the diameter of the. anchoring structure. In an unlocked state of the anchoring structure, the anchoring elements are in a radially inner position, allowing the insertion of the anchoring structure into a femur canal. Locking means are then actuated in order to bring the anchoring structure in its locked state, i.e. move the anchoring elements in a radially outer position, in which their anchoring surfaces are in firm contact with the inner wall of the femur canal. For best centering and fixation, an anchoring structure with at least three and preferably with exactly three anchoring elements is used.

In a preferred embodiment the anchoring elements are individual parts. This allows their unobstructed radial displacement over the entire length of the anchoring structure, and their firm engagement with the femur by the substantially entire portion which is inserted into the femur canal. ^'Another advantage of individual anchoring elements is, that any stress of the material of portions connecting the anchoring elements is avoided. Finally the production of individual anchoring elements is cheaper, they may even be made of a hard synthetic material, which allows a dramatic reduction of production costs.

A preferred concept of the locking means comprises one or more locking cones arranged along the central axis. These cones radially support the anchoring elements on axial ramps provided on their radially inner surfaces in positions corresponding to the positions of the cones. The locking cones are axially displaceable relative to the anchoring elements and actuation means are provided to cause such a displacement of the cones in their respective pointing directions. The pointing direction of a cone is the axial direction which is indicated by its thinner end, i.e. the tapering of the cones. As a result of this displacement, the anchoring elements slide on the locking cones to their radially outer positions.

Preferably, the angle of inclination of the ramps corresponds to the angle of the cones they are engaged with and has preferably also a certain length, so that there is a relatively large surface on which the ramps are in contact with the locking cones. However, this is not indispensable. If the locking cone has a long sliding surface, the corresponding ramp may be a very short section of a cam which may take various shapes. A cam which touches the locking cone in one point has at least in that point an angle of inclination which corresponds to the cone angle. In the same way, the locking cone may be very short if the corresponding ramps on the locking elements have long sliding surfaces.

Actuation means are provided in order to cause said axial displacement of the locking cones and bring the anchoring structure from its unlocked state to its locked state. These actuation means preferably comprise an actuation shaft which is axially engaged with one or more locking cones pointing in the same direction. A nut, screwed to a threaded portion of the actuation shaft, is arranged to axially engage with the anchoring elements when screwed against the pointing direction of these locking cones. Therefore this screwing action causes an axial displacement of the actuation shaft with its locking cones in their pointing direction relative to the anchoring elements, so that the latter slide on the locking cones towards their radially outer positions. Depending on the configuration, the actuation shaft may push or drag the locking cones and hence be under positive or negative tension. In the locked state of the anchoring structure, this tension determines the force exerted by the anchoring elements on the femur wall, which may be adjusted using a dynamometric key.

In a preferred embodiment the anchoring elements are forced radially inwards, against the locking cones, by at least one resilient element. This is particularly useful for an anchoring structure with individual anchoring elements . The resilient element stabilizes the entire structure and thereby facilitates its controlled positioning in the femur canal. Another advantage is that this resilient element retracts the anchoring elements from the wall of the femur canal when the locking cones are retracted, so that the femoral component can be removed from the femur canal easily even years after its implantation. Of course the resilient element needs to be of sufficient strength to fulfil this function. An annular spring surrounding all the anchoring elements has proved useful for this purpose, preferably one such spring is applied at each end of the anchoring structure.

In a preferred solution for the connection of a joint head to the anchoring structure, only a distal part of this structure is adapted to be inserted into a femur canal, while a proximal part is arranged to be received and securely anchored in an opening in a connecting element. In the locked state the distal part of the anchoring elements is in contact with the inner wall of the femur canal while their proximal part is in contact with the inner wall of said opening in the connecting element, thereby fixing the connecting element to the femur. This means that the connections of the anchoring structure with both the femur bone and the connecting element are fastened in a single operation by actuation of the actuation means. Preferably at least two locking cones are used, one in the proximal part and one in the distal part, in order to ensure the quality of both connections. These locking cones are preferably placed at the proximal and distal ends of the anchoring structure, respectively.

For a reliable connection between the anchoring elements and the connecting element it is preferable to provide these parts with correspondingly shaped contact surfaces, for example a cylindrical bore in the connecting element and a corresponding circular arc profile of the radially outer surface of the anchoring elements.

The outer diameter of the proximal end of the anchoring structure is defined by the size of the opening in the connecting element. In order to ensure at the same time the reliable anchoring of the distal part of the anchoring structure in the femur canal, this canal is preferably predrilled to an appropriate diameter.

Further objects and aspects of the invention will be apparent from the following description of preferred exemplary embodiments with reference to the accompanying drawings, in which:

Fig. 1 shows a sectional view of a femoral component of a hip prosthesis implanted in a bone,

Fig. 2 shows just the three anchoring elements of figure 1, surrounded at both ends by annular springs, represented in the upper part of the drawing in ___. _,,_.

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an axial view (from distal) and in the lower part of the drawing in a side view,

Fig. 3 shows an axial view like in the upper part of Fig. 2, of three anchoring elements as in a modified version with laterally protruding lips,

Fig. 4 shows the top view of an annular spring,

Fig. 5 shows a sectional view of the femoral component of Fig. 1 in its unlocked state, inserted into a femoral canal and ready to be locked, with an exploded representation of the^' elements used for this locking operation,

Fig. 6 shows a sectional view according to Fig. 1 and a tool used to lift off a cap from the connection element,

Fig. 7 shows a sectional view according to Fig. 1 after removal of the connection element, and an unlocking tool used to push the locking shaft back in its unlocked position,

Fig. 8 shows a detail of the unlocking tool of Fig. 7.

Figure 1 shows a bone 1 with an implanted femoral component 2. A distal part of an anchoring structure is locked in the predrilled femur canal 6 while its proximal part is locked in a cylindrical bore 5 of a connecting element 3, which connects a joint head 4, fixed on its medial neck portion, to said anchoring structure.

The anchoring structure comprises three individual, elongated anchoring elements 7, only two of which are visible in this drawing, one left and one right of the axis. These anchoring elements are radially supported by central locking cones 8, 9, 10 on axial ramps 11, 12, 13, which ramps are provided on the radially inner surfaces of these anchoring elements in positions and with inclination angles corresponding to those of the locking cones 8, 9, 10 respectively.

Both the locking cone 8 at the distal end and the locking cone 9 in the middle point in proximal direction and are integral parts of an actuation shaft 14. The third locking cone 10, which is of substantially the same shape as the locking cone 8, is used as a nut: it has an internal thread 17 and is screwed to a corresponding thread at the end of the actuation shaft 14 and tightened. Alternatively the locking cone 10 might have a slightly bigger hole, the threaded end of the actuation shaft passing through this hole and being fitted at its proximal end with an ordinary nut. The radial force exerted by the anchoring elements 7 on the surrounding wall of the bone 1 and of the connecting element 3 depends on the angles of the cones 8, 9, 10 and the tension of the locking shaft 14. The latter can be adjusted by rotation of the threaded locking cone 10.

The locking shaft 14 has in the middle a circumferential groove 15 and corresponding cams 16 are provided on the radially inner surfaces of the anchoring elements 7, which are received in said groove 15 in the unlocked state of the anchoring structure (see Fig. 5) . A segment of the actuation shaft 14 distal of said groove 15, which includes the conical flank of the groove 15, constitutes the locking cone 9, and the surfaces of said cams 16 include the ramps on which the anchoring elements 7 slide over this medial . locking cone 9 when the anchoring structure is brought from its unlocked state to its locked state. In fact, the locking ____^_,,_^

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cones 8 and 9 have different inclination angles and therefore also different lengths, the cone 9 in the middle being shorter and followed by a cylindrical shaft portion on which the corresponding ramps 12 are supported in the locked state. This means that the locking cones are not necessarily conical over their full length, but may comprise a conical portion followed by a cylindrical portion. Generally speaking, the term "locking cone" is used in a broad sense to designate a core element with an outer surface having a conical section. In the same way the ramp 12 comprises an inclined surface portion followed by a surface portion which is parallel to the central axis.

The three anchoring elements 7 are forced radially inwards, against the locking cones, by means of annular springs 18a, 18b. In the unlocked state these springs maintain the anchoring elements 7 in radially inner positions, giving the anchoring structure a diameter which is slightly smaller than the diameter of the predrilled bone canal, so that the anchoring structure can easily be inserted into this canal. A second advantage of these annular springs 18a, 18b is that in the event of a future unlocking operation, in which the locking cones are axially retracted, the springs will radially retract the anchoring elements 7 from the wall of the femur canal 6 and thus bring the anchoring structure back to its unlocked state for unproblematic retraction of the femoral component.

The bore 5 passes through the connecting element 3 in order to provide access to the actuation means (here in particular the threaded locking cone 10) from the proximal side. The length on which the anchoring structure is received in this bore 5 is limited by a circlip ring 19 locked in a circμlar recess near the proximal end of the bore 5. This end is closed by a press-fit cap 20 and under this cap 20 a u- shaped leaf spring 21 is arranged with one of its legs rotationally interlocking with that cap 20 and the other with the locking cone 10, thereby securing the latter against unintended loosening. The distal end of the anchoring structure is provided with a cover piece 22 which is press-fit into a hole in the distal locking cone 8 and which forms a homogenous surface with the anchoring elements 7.

Figure 2 shows the three anchoring elements 7 of the anchoring structure of Fig. 1, in positions corresponding to the locked state. As is apparent from the upper axial view, these anchoring elements are individual parts: Besides the locking cones and the surrounding bore walls (none of which is shown in this figure) they are only connected by the annular springs 18a and 18b, which surround them at the proximal and distal end. Their anchoring surfaces are in a distal part provided with spikes 23, which are radially pressed into the bone as the anchoring structure is tensioned, i.e. brought in its locked state. The prosthesis thereby passes on to the state of an implant and strengthens the concerned bone area. In the proximal part, the same locking surfaces are provided with a longitudinal toothing 24 for a firm connection with the connection element 3 (see figure 1) .

In this locked state, the outer surface of the anchoring structure is of cylindrical shape: the anchoring elements 7 are parallel to the central axis and their anchoring surfaces have a circular arc profile with a radius 25 which matches the radius of the predrilled bone canal.

Near their proximal end, the anchoring elements have radial holes 26, the function of which will be explained with reference to Fig. 7. Figure 3 shows an alternative version of the anchoring elements 7, which are also represented in positions corresponding to an anchoring structure in its locked state. Each of these locking elements 7 has on one of its long sides a laterally protruding lip 27, which covers the gap between that side and the adjacent long side of the neighbouring anchoring element. This is important to prevent the bone from growing into these gaps in order to make sure that the anchoring structure can be unlocked conveniently even after many years .

Figure 4 shows a top view of one of the annular springs 18, which are arranged hear the proximal and distal ends of the anchoring structure. Preferably they are made as strong as necessary for retracting the anchoring elements from the inner wall of the femur canal when the anchoring structure is unlocked.

Figure 5 shows the anchoring structure in its unlocked state, inserted into the predrilled femur canal and ready to be locked therein. The cams 16 on the inner surfaces of the anchoring elements 7 are received in the circumferential groove 15 in the middle of the actuation shaft 14 and the locking cone 9, which forms the distal flank of this groove 15, is engaged with the axial ramps 12 formed by flanks of said cams 16. The proximal locking cone 10 has not been installed yet, but this does not affect the stability of the unlocked anchoring structure since the radially inner surfaces of the anchoring elements are in this state directly supported by the cylindrical actuation shaft 14 and held against this shaft by the annular springs 18. The diameter of this unlocked anchoring structure, which is_ defined by the diameter of the actuation shaft plus twice _,_. _,,_.

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the thickness of the anchoring elements, is only slightly smaller than the diameter of the predrilled femur canal.

In order to lock the anchoring structure in the bone, the internally threaded locking cone 10 is screwed to the correspondingly threaded proximal end 28 of the actuation shaft 14 and tightened with an appropriate driving tool 29. This tool has three axial pins 30 and the locking cone 10 has three corresponding axial holes (not shown) , allowing the tool to rotationally interlock with this cone. A dynamometric driving tool may be used in order to adjust the tension of. the anchoring structure to the appropriate value. With this single locking operation the anchoring structure is simultaneously fastened both to the bone 1 and to the connecting element 3.

After the locking of the anchoring structure, the cap 20 is press-fit to a cylindrical seat 31 on the proximal side of the connection element 3 which is concentric with the bore 5 in order to close this bore 5. The u-shaped leaf spring 21 between the locking cone 10 and the cap 20, already mentioned in the description of Fig. 1 above, has pins 32 corresponding to the pins of the tool 29 on one of its legs and one pin 33 near the end of its other leg, which are thereby engaged with the corresponding holes in the locking cone 10 and in the cap 20 respectively.

Figure 6 illustrates the use of a lifting tool to remove the cap 20, which is necessary to regain access to the locking cone 10 in order release the actuation shaft and thereby unlock the anchoring structure. The tool is a stick 34 with a pin at its tip having the same diameter as the safety pin 33 on the leaf spring 21. The tool pin is pressed in the hole of the cap 20 in order to disengage the safety pin 33 from this hole and then inclined as indicated by arrow 35 in order to lift off the cap 20 by this lateral lever action.

Figure 7 illustrates the use of an unlocking tool 36 in order to push the distal locking cone 8 (see Fig. 1) back to the position it had in the unlocked state. The tool 36 comprises a conical body 37 with an internal thread in its upper part and a slightly wider bore in its lower part, and a correspondingly threaded bolt 38 which is screwed through this upper part of the body 37. After the removal of the locking cone 10 the lower end of the tool 36 is brought in that same place. The bolt 38 is then screwed down, pushing three pins 39 in the wall of the body 37 radially outwards (see Fig. 8 for this detail) into the radial holes 26 in the anchoring elements (see Fig. 1) , thereby axially locking the tool body to these anchoring elements. The bolt 38 is then screwed further down, against the proximal end of the actuation shaft 14, so that the distal locking cone 8 is released.

It will be apparent to the one skilled in the art that various modifications may be conceived without departing from the spirit and scope of the invention as defined in the claims .

Claims

1. A femoral component (2) for a hip prosthesis, comprising an elongated anchoring structure adapted to be at least partly inserted in a femur canal (6) and anchored therein and connecting means (3) for connecting a joint head (4) to this anchoring structure, characterized in that the anchoring structure comprises at least two elongated anchoring elements (7) extending from its proximal end to its distal end, which are disposed around a central axis, this anchoring structure having an unlocked state in which the anchoring elements (7) are in a radially inner position, allowing the insertion of the anchoring structure into the femur canal and a locked state in which the anchoring elements (7) are in a radially outer position in order to bring their radially outer anchoring surfaces in firm contact with the inner wall of the femur canal (6), and locking means arranged to displace the anchoring elements (7) from said inner position to said outer position.

2. A component (2) according to claim 1, characterized in that the elongated anchoring elements (7) are individual elements .

3. A component (2) according to one of claims 1 or 2, characterized in that it comprises at least three and preferably exactly three anchoring elements (7) .

4. A component (2) according to one of claims 1 to 3, characterized in that the anchoring surfaces are parallel to the central axis, have a circular arc profile and are adapted to be engaged with the inner wall of a femur canal (6) .

5. A component (2) according to one of claims 1 to 4, characterized in that in the part of the anchoring structure which is arranged to be inserted into a femur canal (6), the anchoring surfaces are provided with spikes (23) .

6. A component (2) according to one of claims 1 to 5, characterized in that ^• the anchoring elements (7) have on one of their long sides a laterally protruding lip (27) which covers in the locked state the gap between that side and the adjacent long side of the neighbouring anchoring element (7) .

7. A component (2) according to one of claims 1 to 6, characterized in that the locking means comprise one or more locking cones (8, 9, 10) arranged along the central axis, which radially support the anchoring elements (7) on axial ramps (11, 12, 13) provided on the radially inner surfaces of these anchoring elements (7) in positions and with angles of inclination corresponding to those of the said locking cones (8, 9, 10), and actuation means arranged to axially displace the locking cones (8, 9, 10) in their respective pointing directions relative to the anchoring elements, so that these anchoring elements (7) slide on the locking cones (8, 9, 10) to their radially outer positions.

8. A component (2) according to claim 7, characterized in that the anchoring structure comprises both at its proximal and at its distal end at least one locking cone (8, 10) and corresponding ramps (11, 13) on the anchoring elements (7) .

9. A component (2) according to one of claims 7 or 8, characterized in that the anchoring elements (7) are forced radially inwards against the locking cones (8, 9, 10) by at least one resilient element, preferably by at least one annular spring (18a, 18b) surrounding the anchoring elements .

10. A component (2) according to claim 9, characterized in that at least one resilient element is provided at the proximal (18a) and at the distal end (18b) of the anchoring structure respectively.

11. A component (2) according to one of claims 7 to 10, characterized in that the actuation means comprise an axial actuation shaft (14) which is axially engaged with a first group of one or more locking cones (8, 9) pointing in one axial direction and a nut screwed to a threaded portion of the actuation shaft (14), which nut is arranged to axially engage with the anchoring elements (7) when it is screwed against the pointing direction of said cones and thereby to axially displace the actuation shaft (14) with its locking cones (8, 9) relative to the anchoring elements (7) , so that the anchoring elements (7) slide on the locking cones to their radially outer positions.

12. A component (2) according to claim 11, characterized in that the nut is arranged to axially engage with the anchoring elements by means of a second group of one or more locking cones (10), .which point in the opposite direction.

13. A component (2) according to claim 12, characterized in that the first group of locking cones (8, 9) points in proximal direction and is disposed distal of the second group of locking cones (10) and fixed to the actuation shaft (14) .

14. A component (2) according to claim 12 or 13, characterized in that the second group of locking cones consists of a single locking cone (10) disposed at the proximal end of the anchoring structure and that the nut is an integral part of this locking cone (10), which is screwed on the threaded proximal end of the actuation shaft (14) .

15. A component (2) according to one of the claims 11 to 14, characterized in that the first group of locking cones (8, 9) consists of a first locking cone (8) at the distal end of the actuation shaft (14) and a second locking cone (9) in the middle of this shaft.

16. A component (2) according to claim 15, characterized in that a circumferential groove (15) with conical flanks is provided in the middle of the locking shaft (14) and corresponding cams (16) and provided on the radially inner surfaces of the anchoring elements (7) in a position that the cams are received in this groove (15) in the unlocked state of the anchoring structure, wherein the second locking cone (9) is a segment of this shaft (14) distal of said groove (15) and includes a conical flank of the groove, and wherein the ramps (12) , on which the anchoring elements slide over the second locking cone (9) when the anchoring structure is brought in its locked state, are part of the surfaces of said cams (16) .

17. A component (2) according to one of the preceding claims, characterized in that the connecting means comprise a connecting element (3) with a medial neck portion with means for fixing the joint head (4) to it and with a distal opening (5) adapted to receive a proximal part of the anchoring structure, so that in the locked state the proximal and distal parts of the anchoring surfaces are in contact with the inner surfaces of said opening (5) and of the femur canal (6) respectively, thereby fixing the connecting element (3) to the femur (1) .

18. A component (2) according to claim 17, characterized in that the opening in the connecting element is a cylindrical bore (5) and that a proximal part of the anchoring surfaces has a circular arc profile with a radius corresponding to the radius of said bore and, preferably, is provided with a longitudinal toothing (24) .

19. A component (2) according to claim 17 or 18, characterized in that the opening (5) in the connecting element (3) extends through the connecting element, and that the actuation means of the anchoring structure are accessible from the proximal end of this opening.

20. A component according to one of claims 7 to 16 and one of claims 17 to 19, characterized in that the connecting means comprise a cap (20) arranged to be press-fit into the proximal end of the opening in the connecting element (3) , and an axially resilient element (21) which is arranged between this cap (20) and the nut (10) and rotationally interlocking with both of them in order to secure the nut (10) against unintended rotation.

21. A component (2) according to claim 20 characterized in that the cap (20) has an axial hole near its periphery adapted to receive a lifting tool for the removal of the cap and that the resilient element is a u-shaped leaf spring (21) having two legs, one of them being rotationally engaged with the nut (10) and the other one having a pin (33) at its free end, which is arranged to be engaged with said hole in the cap (20) .