The present invention relates to a method of improving a taper interface between a femoral head implant and a femoral stem implant of a femoral prosthesis, specifically but not necessarily exclusively for use in hip arthroplasty surgical procedures. The invention further relates to a femoral prosthesis joint-enhancement system for improving a taper interface between a femoral head implant which has a first taper interface and a femoral neck which has a second taper interface which is receivably engagable with the first taper interface. The invention further relates to a method of improving a taper interface between a first prosthetic implant and a second prosthetic implant, and also to a generalised prosthesis joint-enhancement system for improving a taper interface between a prosthetic implant which has a first taper interface, and a second taper interface which is receivably engagable with the first taper interface.

Prosthetic implants can be manufactured as unitary components. However, for many applications, this may not be appropriate, for example, where different parts of the prosthetic implant should be formed from different materials or manufacturing processes, or where the shape of the prosthetic implant would otherwise create manufacturing difficulties.

A good example of such a prosthetic implant is a femoral prosthetic implant, which will typically comprise a femoral stem implant, which is fitted into a resected femur of a patient in need of hip arthroplasty surgery, and a femoral head implant, which fits with an acetabulum of the patient to form the ball-and-socket joint of the leg. It is common to provide a taper fit between the femoral stem implant and the femoral head implant to allow modularity and permit getting the original hip centre of the restored correctly. Similarly, the femoral head implant may often be cobalt chrome alloy or at least in part ceramic, whereas the femoral stem implant is typically formed from a metal such as titanium, cobalt chrome alloys or stainless steel.

The taper interface between the femoral head implant and femoral stem implant can be manufactured to a high degree of precision, so that the tapers wedgingly interconnect almost perfectly. However, any slight manufacturing tolerance will result in a slight mismatch of the taper interface, which, once implanted into the patient, can lead to highly undesirable relative movement, referred to as micromotions, between the femoral head implant and femoral stem implant. When this occurs between cobalt chrome alloy heads and metal stems then electro-corrosion and wear can occur leading to metal particle and/or ion release and early pain and failure; this is known as trunnionosis and is an area that has been well researched over the past few years.

It is an object of the invention to provide an improved means of engaging taper-fit prosthetic components with one another, to obviate the above-described problems.

According to a first aspect of the invention, there is provided a method of improving a taper interface between a femoral head implant and a femoral stem implant of a femoral prosthesis, the method comprising the steps of: a) selecting a femoral head implant of a desired dimension, having a first taper interface; b) selecting a femoral stem implant of a desired dimension, having a second taper interface at a femoral neck of the femoral stem implant, the second taper interface being receivably engagable with the first taper interface; c) applying an abrasive medium to the first and/or second taper interface; d) interengaging the femoral head implant and femoral stem implant at the first and second taper interfaces; and e) generating a relative motion between the femoral head implant and femoral stem implant about an axis of the femoral neck to abrasively improve a taper fit between the femoral head implant and femoral stem implant at the first and second taper interfaces.

Femoral prosthetic implants typically interengage via a taper fit between the femoral head implant and the femoral neck of a femoral stem implant. The present method reduces discrepancies between the pre-manufactured tapers of the two implants, so that there is a significantly reduced movement between the two components. This reduces in-vivo wear and electro-corrosion at the interface, reducing the associated risks to the patient.

Optionally, there may further comprise a step d1) prior to step e), step d1) comprising engaging the femoral head implant with a femoral head implant engagement device, the relative motion being applied via the femoral head implant engagement device.

It will be appreciated that the femoral head implant is far more rotationally symmetric than the femoral stem implant, and therefore is a much more suitable candidate for engagement with the drive means for imparting the motion. In an alternative embodiment, the method may further comprise a step d1) prior to step e), step d1) comprising engaging the femoral stem implant with a femoral stem implant engagement device, the relative motion being applied via the femoral stem implant engagement device.

It will of course be possible to rotate the femoral stem implant instead of the femoral head implant, should the situation dictate that this is more appropriate.

Preferably, during step e), a load may be applied towards the first and second taper interfaces when generating the relative motion.

In some instances, steps a) to e) may be performed prior to transport of the femoral prosthesis to a surgical operating theatre.

In other instances, steps a) to e) may be performed ex vivo in a surgical operating theatre.

In yet still further instances, steps c) to e) may be performed in a surgical operating theatre, wherein at least one of the femoral head implant and femoral stem implant is in vivo.

Where the abrasive action occurs is pre-determined by the manufacturing process. It is preferred that the improvement to the taper fit be performed as close to installation into the patient as possible so as to avoid damage during transit. However, physical constraints in the operating theatre may limit options for performing the abrasive action in situ.

Steps c) to e) may be performed inside a cell or containment structure, the abrasive medium being an abrasive fluid contained within the cell or containment structure.

The provision of a contained cell or similar containment structure in which the abrasive action can occur allows for a sanitised and sterile prosthesis to be ground and implanted in situ in the operating theatre with minimal risk of contamination.

According to a second aspect of the invention, there is provided a femoral prosthesis joint-enhancement system for improving a taper interface between a femoral head implant which has a first taper interface and a femoral neck which has a second taper interface which is receivably engagable with the first taper interface, preferably in accordance with a method according to the first aspect of the invention, the system comprising: a femoral head implant engagement device having a femoral-head-implant connector and driving means for driving the femoral-head-implant connector when engaged with the said femoral head implant; and an abrasive medium which is applicable to the first taper interface of the said femoral head implant and/or to the second taper interface of the femoral neck; wherein the abrasive medium abrasively improves a taper fit between the said femoral head implant and the said femoral neck at the first and second taper interfaces.

The equipment used to implement the above method, that is, a rotational driving mechanism engagable with the femoral head, and the abrasive medium applicable to the taper interface has the advantage of being able to improve the taper engagement interface of existing femoral prosthetic implants having machined interfaces which will have a natural tolerance, again reducing the risk to the patient.

Optionally, the abrasive medium may be a biocompatible grinding or lapping medium material, for example, a diamond-based lapping paste. Any suitable hard and abrasive grind medium that is sufficiently biocompatible would be feasible.

Given that the abrasive paste material may linger in the taper interface, a biocompatible medium is much preferred so as to not cause damage to the patient following a surgical procedure.

Preferably, the femoral-head-implant connector may be a connector adapted to engage with an articulating surface of the femoral head implant.

The articulating surface of the femoral head implant is usually that which is opposite to the first taper interface, and therefore engagement with this surface will generally produce a more convenient spatial arrangement for motion.

Optionally, the femoral-head-implant connector may comprise a clamping mechanism.

Abrasive action will only be achieved correctly if the femoral head implant rotates in a phase locked manner with the femoral-head-implant connector, and therefore a clamping mechanism is highly desirable.

The driving means may comprise a manually operable rotor and/or a mechanised rotor. A mechanised rotor or machine may improve the efficiency of the abrasive process; however, a manually operable mechanism may be more suitable for use in an operating theatre or during a surgical procedure.

The femoral head implant engagement device may further comprise a load and/or torque applicator for applying a load and/or controllable torque onto the femoral head implant during motion.

The abrasion process will be far more effective if a load and/or specific torque is applied during the motion, and therefore a specific load and/or torque applicator is desirable. This could, of course, be co-located or part of the driving means. The load applicator could be part of the torque applicator, or these could be provided as discrete devices.

Preferably, the femoral neck may be a femoral neck of a femur or a femoral neck of a femoral stem implant.

Optionally, the abrasive medium may be or comprise a sterile or sterilising medium.

Advantageously, the abrasive medium may sterilise the taper interface region, at least, of the femoral prosthesis, which may reduce the risk of patient infection.

According to a third aspect of the invention, there is provided a method of improving a taper interface between a first prosthetic implant and a second prosthetic implant, the method comprising the steps of: a) selecting a first prosthetic implant of a desired dimension, having a first taper interface; b) selecting a second prosthetic implant of a desired dimension, having a second taper interface, the second taper interface being receivably engagable with the first taper interface; c) applying an abrasive paste material to the first and/or second taper interface; d) interengaging the first prosthetic implant and second prosthetic implant at the first and second taper interfaces; and e) generating a relative motion between the first prosthetic implant and second prosthetic implant to abrasively improve a taper fit between the first prosthetic implant and second prosthetic implant at the first and second taper interfaces.

It will be appreciated that the techniques outlined above are not specific to femoral prosthetic implants, and could be applied to any prosthetic implant in which there is a taper joint. In one optional arrangement, steps a) to e) may be performed prior to transport of the femoral prosthesis to a surgical operating theatre.

Alternatively, steps a) to e) may be performed ex vivo in a surgical operating theatre.

Steps c) to e) may, in a further embodiment, be performed in a surgical operating theatre, and wherein at least one of the first and second prosthetic implants is in vivo.

Steps c) to e) may be performed inside a cell or containment structure, the abrasive medium being an abrasive fluid contained within the cell or containment structure.

According to a fourth aspect of the invention, there is provided a prosthesis joint- enhancement system for improving a taper interface between a prosthetic implant which has a first taper interface, and a second taper interface which is receivably engagable with the first taper interface, preferably in accordance with a method according to the third aspect of the invention, the system comprising: a prosthetic implant engagement device having a prosthetic-implant connector and driving means for driving the prosthetic-implant connector when engaged with the prosthetic implant; and an abrasive paste material which is applicable to the first taper interface of the said prosthetic implant and/or to the second taper interface; wherein the abrasive paste abrasively improves a taper fit at the first and second taper interfaces to improve a joint connection between the prosthetic implant and a counterpart object having the second taper interface to which the prosthetic implant is being connected.

Optionally, the counterpart object may be a prosthetic implant. Alternatively, the counterpart object may be a bone of a patient.

The apparatus of the system need not be used solely in the context of improving taper fits between two prosthetic implants. Bone-to-implant grinding or lapping may also find uses and therefore the present invention is equally applicable to such contexts.

Preferably, the abrasive medium may be or comprise a sterile or sterilising medium.

According to a fifth aspect of the invention, there is provided an abrasive medium, suitable for use in a method for improving a taper interface between a prosthetic implant which has a first taper interface, and a second taper interface which is receivably engagable with the first taper interface, the abrasive medium being or comprising a sterile or sterilising medium. The abrasive medium may preferably be provided as a suspension of abrasive particles in a saline solution.

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

Figure 1 shows an exploded perspective representation of a first embodiment of a femoral prosthesis joint-enhancement system in accordance with the second aspect of the invention;

Figure 2 shows a perspective representation of the femoral prosthesis joint- enhancement system of Figure 1, being using according to a method in accordance with the first aspect of the invention;

Figure 3 shows a vertical central cross-section through the femoral prosthesis joint-enhancement system of Figure 2; and

Figure 4 shows a perspective representation of a second embodiment of a femoral prosthesis joint-enhancement system in accordance with the second aspect of the invention.

Referring to Figure 1, there is shown a femoral prosthesis joint-enhancement system referenced globally at 10 which comprises a femoral head implant engagement device 12 and an abrasive medium, illustrated as an abrasive paste material 14, and which is designed for improving a taper interface between a femoral head implant 16 and a femoral neck 18. An abrasive medium here could refer to a lapping medium, which is typically a loose or suspended abrasive, or another type of abrasive material. Grinding media are also applicable, though this would traditionally refer to bonded media only, such as a grinding wheel or sandpaper.

The femoral head implant 16 has a first taper interface 20, which will typically be formed as a tapered, but otherwise substantially cylindrical, recess. The first taper interface 20 is therefore positioned at a rim 22 of the femoral head implant 16, distal to the articulating surface 24 of the femoral head implant 16.

The femoral neck 18 is here formed as part of a femoral stem implant 26, with an end of the femoral neck 18 comprising the corresponding second taper interface 28 which is receivably engagable with the first taper interface 20 of the femoral head implant 16. In this instance, the femoral head implant 16 and femoral stem implant 26 together form the full prosthesis for use in hip arthroplasty surgical procedures.

The femoral head implant engagement device 12 is designed to permit rotational driving of the femoral head implant 16 relative to the femoral neck 18. The term rotationally driven is used, which not only covers the scenario in which the drive is applied in a single direction, such as clockwise, but is also intended to cover oscillatory, bi-directional, and/or periodic rotary motions, which tends to be used more in lapping abrasive contexts. It will be appreciated that rotary motion could be used in conjunction with axial movement as well, for example, by using a cam in a drive coupling, or similar machine capable of axial movement as well. The axial movement could be fairly small, for example only of the order of a few millimetres, but would assist with the clearance of debris from the interface site.

The femoral head implant engagement device 12 is therefore at least in part rotatable, and has a femoral-head-implant connector 30, which can be best seen in Figure 3, discussed in more detail below, which allows the femoral head implant engagement device 12 to be engaged with the femoral head implant 16. This is shown as being a receiver shaped to correspond with the shape of the articulating surface 24 of the femoral head implant 16. This may provide a wedging or interference engagement, but more likely, the femoral head implant engagement device 12 will also comprise a clamp or similar force applicator which rotationally locks the femoral head implant 16 to the femoral head implant engagement device 12. This rotational locking ensures that the femoral head implant 16 is driven in tandem with the femoral head implant engagement device 12.

The method of improving the taper interface is shown in more detail in Figure 2.

The femoral head implant engagement device 12 is connected to the femoral head implant 16 such that the femoral head implant engagement device 12 and femoral head implant 16 are rotationally locked to one another. The abrasive paste material 14 is then applied to the region between the first and second taper interfaces 20, 28. This is most likely by application to the femoral neck 18 since this will prove simpler being the external taper surface. The user will apply the abrasive paste material 14 as they best see fit, however. Examples of suitable abrasive medium 14 include diamond paste materials, though any suitable biocompatible paste medium or other grinding or lapping medium could readily be considered, such as alumina, zirconia, carborundum, and silicon nitride ceramics.

Once the abrasive paste material 14 is applied, the femoral neck 18 is engaged with the femoral head implant 16, to form the full femoral prosthesis. It will, of course, be appreciated that the application of abrasive paste material 14 and interengagement of the femoral neck 18 and femoral head implant 16 may be performed prior to engagement of the femoral head implant 16 and femoral head implant engagement device 12.

If there is any discrepancy between the first and second tapers 20, 28, however, there is potential for movement between the femoral neck 18 and the femoral head implant 16. As soon as the femoral head implant 16 and femoral neck 18 are attached to one another statically, that is, in the absence of a rotation, then no movement is apparent. However, as soon as the joint is exposed to torque or moments off-axis relative to the femoral neck 18, then micromotions can occur.

This arises as the typical engagement length between the two tapers 20, 28 is a fraction of the total length of the external taper surface. For a taper length of approximately 12mm, which is common, the engagement length is only 1mm to 2mm, which arises due to the small manufacturing variances of the female and male tapers. Here, engagement length is the portion where the taper angle conforms without major divergence between the two tapers 20, 28.

To overcome this issue and therefore consequentially increase this engagement length, the femoral head implant engagement device 12 is rotationally driven with respect to the femoral stem implant 26.

This generates an abrasive action at the first and second taper interfaces 20, 28, creating a more uniform interface region and improving the fit between the femoral head implant 16 and the femoral stem implant 26. A cross-section of the engagement is shown in Figure 3. A debris catcher may be necessary to catch any debris, in particular, but not necessarily exclusively to prevent ingress of debris into the patient’s body.

The rotation may be imparted to the femoral head implant engagement device 12 by a manual mechanism, such as a hand crank. A load can also thus be applied by the user applying the rotation, by urging the femoral head implant engagement device 12 towards the interface region. This improves the efficiency of the abrasive action, leading to a better taper interface at the first and second taper interfaces 20, 28.

This approach may be more suited towards the improvement of the taper interface between the femoral head implant 16 and femoral neck 18 in situ, that is, in the operating theatre of the surgical procedure. It may be that this is performed on the sterile nurse’s tabletop in the operating theatre. This ensures that the surgeon can ensure that the fit between the femoral head implant 16 and femoral neck 18 is as good as can possibly be achieved at the point of implant.

It is noted that this process can be performed not only external to the patient, in advance of a surgical procedure being performed, but could also in theory be performed in vivo, that is, once the femoral stem implant 26 has already been installed into a resected femur and also during a revision of the acetabular cup, or any component thereof, where the femoral head may be removed to check for potential taper wear or corrosion issues. This latter case is much more preferable than removing a well-fixed and otherwise well-functioning hip stem from the patient and minimises surgical trauma, blood, and bone loss.

In either case, the user will be careful to clear away any debris or excess abrasive medium 14 from the taper interface region prior to installation of the prosthetic implant.

Where the taper fit is optimised prior to transport to the operating theatre, it will be appreciated that a more mechanical grinding apparatus could be used. A femoral head implant engagement device 12 could thus be engaged with a rotary machine that applies load, rotation and torque and a non-manual load applicator could be used during the grinding procedure. One example of such a load applicator could be a linear actuator. Such a mechanical system could improve the taper connection to a greater degree than may be achievable by a surgeon in an operating theatre, but spatial constraints may limit the opportunity to use such equipment within an operating theatre. Advances in robotics may, of course change this.

The process of abrading the taper interface created by the first and second taper interfaces 20, 28 is thus described in terms of a femoral prosthetic implant, in which the femoral head implant engagement device 12 is used to engage with and rotate the femoral head implant 16 relative to the static femoral stem implant 26, along the axis of the femoral neck 18.

This will be appreciated as being only one exemplary embodiment of the present invention.

Firstly, whilst acknowledged as being more complex to achieve, it will be clear to the skilled person that the rotation could be imparted to the femoral stem implant 26 instead of the femoral head implant 16, in which case, a femoral stem implant engagement device is required to replace the femoral head implant engagement device 12.

Furthermore, the intended use of the present femoral prosthesis joint-enhancement system 10 is for hip arthroplasty surgery, to replace the existing ball-and-socket joint of a patient.

The present method is equally applicable to any ball-and-socket joint arthroplasty. In particular, this is applicable to a humeral arthroplasty without any significant deviation from the disclosed method for femoral arthroplasty.

The method and system could also further be applied to any prosthetic implant involving a first prosthetic implant having a first taper interface which interengages with a second prosthetic implant having a second corresponding taper interface. This might allow for segmentation of prosthetic implants into multiple components, for example, to provide different material constructions or to circumvent difficult manufacturing aspects for the prosthetic implant.

One example of such as system would be where a native hip taper has worn out, and thus the orthopaedic surgeon fits a hollow frusto-conical sleeve onto the taper of the femoral neck. A new acetabular head can then be connected to the sleeve, allowing the engagement of tapers of different sizes via the intermediate sleeve. The present invention could therefore be used to abrasively engage the tapers on both inner and outer surfaces of such a sleeve. Other prosthetic components having tapers will be apparent to the skilled person as being suitable for use in this manner.

The invention has been described thus far in relation to an abrasive paste material which is applied manually to the taper interfaces. However, there is a significant risk of debris contamination in this arrangement, as the paste is ejected centrifugally and axially during the process of the operative procedure.

An alternative embodiment of the system is shown in Figure 4. Identical or similar reference numerals are used to refer to identical or similar components of the first described embodiment, and further detailed description is omitted for brevity.

A cell 132 or similar containment structure may be provided within which the femoral head implant 116 and femoral stem implant 126 are receivable. This cell 132 could be fluidly sealed, and filled with an abrasive medium, such as the abrasive grinding fluid 114 shown. This could be a suspension of, for example, diamond particles in an otherwise sterile solution, such as a saline solution.

A femoral head implant engagement device 112, or other prosthetic implant engagement device is positioned inside the cell 132 so as to be drivably rotatable. No drive transmission mechanism is illustrated, for clarity, but it will be apparent that this will be present in the system 110.

In the operating theatre, the abrasion process can be performed in the sterile environment of the cell 132, and the cell 132 itself could be flushed with a sterile or sanitising fluid prior to surgical implantation to minimise the risk of patient contamination. The surgeon can then extract the improved prosthesis directly from the cell 132 for implantation.

The term cell here refers to a containment structure which is adapted to entirely contain the prosthetic implant and the machine which provides the rotation.

Of course, a cell or containment structure could be envisioned in which the prosthetic implant is not completely enclosed. Instead, a portion of the prosthetic implant could be enclosed to ensure that the region around the first and second taper interfaces is enclosed, without necessarily enclosing the rotary machine or the femoral stem, for instance. This containment structure could take the form of a skirt, which could be rigid or flexible, and which only encloses the joint region at the first and second taper interfaces. Such a containment structure may significantly reduce the amount of abrasive medium required, since the total volume of the containment structure will be much smaller. As previously described, the system and method can be performed in vivo or ex vivo for first and second prosthetic implants. However, taper engagement in surgical procedures is not limited to implant-to-implant engagement; implant-to-bone engagement is equally important for partial replacement procedures. This is particularly applicable for femoral head resurfacing applications where there is no reason to remove any part of a healthy femoral neck bone. This is, again, an exemplary embodiment only, and a taper interference fit can be applied to any prosthetic implant engaging with a counterpart object, be it bone, a further prosthetic implant, or another object, and the invention is not limited to femoral implants only.

For a prosthetic implant-to-bone engagement, it will be apparent that the abrasion process must be performed at least in part in vivo. The taper must be ground onto the bone, such as the existing femoral neck of the patient’s femur, and there are many bone grinding devices known in the art to achieve this.

The abrasive medium can then be applied to the taper interface of the bone or to the taper interface of the prosthetic implant, such as an internal taper interface of a femoral head implant. The prosthetic implant can then be installed onto the bone, and the prosthetic implant rotated, preferably under application of a load, to improve the taper fit.

It is therefore possible to provide a prosthesis joint-enhancement system for improving a taper interface between a prosthetic implant which has a first taper interface, and a second taper interface which is receivably engagable with the first taper interface, as well as a corresponding method of improving said taper interface. This is particularly applicable to femoral prostheses, as well as to humeral prostheses, but can be applied to other prosthetic implants having taper connections. The advantage of this system and method is that the movement between the prosthetic implant and its counterpart object, be it another implant or a bone, is minimised, and thus the risk to the patient during use is significantly reduced.

The words ‘comprises/comprising’ and the words ‘having/including’ when used herein with reference to the present invention are used to specify the presence of stated features, integers, steps, or components, but do not preclude the presence or addition of one or more other features, integers, steps, components, or groups thereof. It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination.

The embodiments described above are provided by way of examples only, and various other modifications will be apparent to persons skilled in the field without departing from the scope of the invention as defined herein.