Patent Description:
Hip resurfacing is an alternative to a conventional total hip replacement wherein both the head of a femur and the socket or the acetabulum are removed and replaced with prostheses. During hip resurfacing, only a few millimetres of the femur head are shaved down to accept a replacement head, whereas for the socket the procedure is similar to that of a hip replacement where a prosthetic acetabular cup is fixed in the acetabulum bone. Currently clinically available hip resurfacing prostheses include metal (metal-on-metal), ceramic (ceramic-on-ceramic) implants and their combination (ceramic-on-metal).

It is known that metal only prostheses can release metal wear particles into the human body and patients having metal-on-metal implants have to be regularly tested for the presence of metal ions in the blood stream. From this point of view, it is advantageous to use ceramics as an implant material as it has been demonstrated that ceramic prostheses are more biocompatible and/or biologically inert when implanted into the body.

The resurfacing heads are normally attached to the part of femur by means of forming a cement mantle inside the inner profile of the head. Occasionally, fixation failures may develop when too much prolonged stress is applied to the implant, which may occur during normal functioning of the joint. These stresses may cause loosening of the bone-cement interface and may lead to a malfunctioning of the joint resulting in implant failure.

Another problem associated with full or partial hip replacement is concerned with the relative sizes of the prosthetic head and cup. The desired cup and head size are usually determined by the size and height of the patient, and are normally smaller in females. Therefore a set of differently-sized prostheses is available to the surgeon, with both the outer diameter of the cup (which sits in the acetabulum) and the inner diameter of the cup (which fits snugly with a correspondingly sized head) typically increasing in <NUM> increments. In a hip resurfacing procedure, it is generally desirable to minimise the amount of natural bone that is removed in order to accommodate the implants. At the same time, it is desirable to maximise the head-to-neck ratio for the resurfaced femur. The resurfaced "head-to-neck ratio" is the ratio between the diameter of the resurfacing head and the diameter of the natural femoral neck. The head-to-neck ratio affects the range of motion for the resurfaced joint, which is desirably as great as possible. However, maximising the head-to-neck ratio for smaller femurs and achieving consistent scaling for the whole range of sizes is difficult to achieve. <CIT> relates to a a ceramic acetabular cup which is devoid of a separate liner and which thus utilises a single shell forming an exterior bone-interfacing surface and an inner articulating surface for receiving a prosthetic femoral head. <CIT> relates to a cup-shaped acetabular implant with a hemispherical inner cavity, and a femoral implant with a domed head, hemispherical inner cavity and anchoring elements, has a flat top section on the outer surface of the cup, which has a truncated conical inner edge and external anchoring projections. <CIT> relates to a substantially cup-shaped prosthetic device that includes an outer surface configured to operatively engage at least one of a first bone of the joint and a component, an inner surface including at least a portion configured to connect to a second bone of the joint, and at least one reservoir having an opening at each of the inner surface and the outer surface and extending therebetween.

It is therefore an object of embodiments of the invention to at least mitigate one or more of the problems associated with the prior art.

Aspects and embodiments of the invention provide a ceramic acetabular cup and a ceramic resurfacing head, as claimed in the appended claims.

In accordance with the present invention there is provided a set of differently-sized ceramic acetabular cup prostheses, each cup in the set comprising:.

Advantageously, such incrementation allows for an improved size variation of the cups, thus providing more cup size choices to the surgeons and allowing to take into account individual's morphology and thus eliminating a one-size-fits-all approach for the hip treatment strategy. The combination of the lateral offset and the difference in scaling of the outer and inner diameters of the cup, improves the range of motion of the resurfaced joint reduces the chance of impingement and potentially reduce wear on the prosthesis.

In an embodiment, the outer diameter is between <NUM> and <NUM>. In yet another embodiment, the inner diameter is between <NUM> and <NUM>. Advantageously, the range of sizes allow for better matching of the prosthesis and maintaining the desired range of movement for different patients.

In an embodiment, the cup rim comprises an outer edge and an inner edge. The cup rim preferably has an edge radius between said bearing surface edge and said cup rim inner edge, preferably an edge radius of at least <NUM>. Advantageously, such a configuration helps avoid irritation of the soft tissue traversing the cup rim, e.g. the psoas tendon.

In an embodiment, the lateral offset of the inner bearing surface is defined by fixed bearing arc of <NUM>° measured from the CBD to the inner bearing surface edge. The lateral offset may be between <NUM> and <NUM>. Advantageously, the presence of the lateral offset leads to a translation of the geometric centre of rotation and, in turn, allows for better positioning of the acetabular cup in the acetabulum.

In an embodiment, the cup thickness is between <NUM> and <NUM>. Preferably, the cup thickness is greatest at the cup pole and thinnest at the cup rim. Advantageously, this helps minimise removal of the natural acetabulum when preparing it to receive the cup.

In an embodiment, the outer surface comprises a plasma-sprayed coating. In another embodiment, the plasma-sprayed coating comprises one or more of titanium and hydroxyapatite. Advantageously, the presence of a rough and partially porous coating allows for an improved initial cup fixation as well as subsequent osseointegration and long-term fixation due to the presence of a biocompatible ceramic Ca-P component (e.g. hydroxyapatite).

In an embodiment, the outer surface is flattened at the cup pole, preferably wherein the flattened section subtends an angle of <NUM>° from the COD.

One or more embodiments of the invention will now be described by way of example only, with reference to the accompanying drawings, in which:.

<FIG> shows a cross-section of an acetabular cup <NUM> according to an embodiment of the invention. The cup has a generally hollow hemi-spherical shape and comprises an outer surface <NUM> and an inner bearing surface <NUM>. The inner bearing surface <NUM> is intended to receive a resurfaced femoral head prosthesis (<NUM> in <FIG>) The inner surface <NUM> is highly polished and provides a snug fit between the cup <NUM> and the head <NUM>. The outer surface <NUM> has an outer diameter that ranges between <NUM> to <NUM> and an outer diameter centre point (COD), wherein said diameter changes in <NUM> increments when scaling the size of the cup. The inner surface <NUM> has a bearing diameter in the range <NUM> and <NUM> and a bearing diameter centre point (CBD). The cup further comprises a cup pole <NUM> having a flattened profile with a longitudinal axis X-X' of the cup passing therethrough, and a cup rim <NUM> that is coplanar with the centre of the outside diameter (COD). The flattened profile of the cup pole is defined by an inclusive subtended angle of <NUM> -<NUM> degrees struck from the COD. The cup rim comprises an outer edge <NUM> and an inner edge <NUM>, the cup rim <NUM> has an edge radius between said bearing surface <NUM> and said cup rim inner edge <NUM>, preferably an edge radius is at least <NUM> to allow for a smooth movement of the femoral head <NUM> inside the cup <NUM>. Optionally, the cup rim <NUM> may also have an edge radius of <NUM> between the outer surface <NUM> and the outer edge <NUM> of the rim <NUM> to avoid irritation of soft tissue traversing the rim.

The inner surface <NUM> is offset laterally from the cup pole <NUM>. The lateral offset effectively translates the centre of hip rotation (in the resurfaced joint) laterally relative to the cup pole and this is desirable in maximising the range of motion in the resurfaced joint. The lateral offset of the inner bearing surface <NUM> is defined by a fixed bearing arc of <NUM>° measured from a bearing diameter centre point CBD to the inner bearing surface edge. The lateral offset can be seen as an angle θ between two lines drawn from the centre of the bearing diameter CBD to the points marking the extent in each direction of the inner conforming bearing surface <NUM>. As a result, the cup thickness is greatest at the cup pole <NUM> and thinnest at the cup rim <NUM>. The fixed bearing arc and edge radius size effectively determine the degree of lateral offset of the bearing, which then scales with increasing bearing/cup size as a result. The resulting lateral offset values, measured from the cup rim <NUM> to the CBD, range from <NUM> to <NUM> between the smallest to largest size cups.

The outer surface <NUM> of the cup <NUM> may have a rough and partially porous plasma-sprayed dual coating of titanium and hydroxyapatite (HA). This provides both initial fixation (by "scratch-fit") as well as allowing enhanced longer term fixation following bone on-growth (osseointegration). The cup <NUM> is normally implanted into a prepared acetabulum reamed to between <NUM> and <NUM> smaller than the effective diameter of the coated surface <NUM>.

A set of differently-sized acetabular cups is provided in order to accommodate the full range of differently sized patients. Whereas in a conventional acetabular cup set, their outer and inner diameters increase together in fixed increments, e.g. <NUM>, in the claimed cup set the ratio between the outer and inner diameters changes as one moves through the range of cups. Starting from the smallest cup in the set, the outer diameter increases in <NUM> increments whereas the inner diameter increases in smaller increments, e.g. <NUM>.

<FIG> shows a monobloc ceramic femoral resurfacing head prosthesis <NUM>. Said femoral head prosthesis is configured to be used with the ceramic cup <NUM> shown on <FIG> above, together forming the articulating joint of the resurfaced hip. The femoral head <NUM> is affixed to the patient's femur by cementing, and, like the acetabular cups, is provided in a range of sizes to suit individual patient anatomy. The head <NUM> comprises a generally spherical outer bearing surface <NUM> having an outer bearing diameter and a head rim <NUM>. The bearing diameter is typically in the range <NUM> to <NUM>. The head <NUM> further comprises an inner socket <NUM> having an inner surface <NUM> intended to receive a prepared femur.

The inner surface <NUM> comprises two portions: a first near cylindrical portion 204a and a second tapered portion 204b adjacent to the first portion 204a. The first portion 204a tapers with respect to a longitudinal axis Y-Y' of the head by at least <NUM>°, wherein the second tapered portion 204b tapers with respect to the longitudinal axis Y-Y' of the head by <NUM> to <NUM>°. It is understood that the inner surface <NUM> and the outer surface <NUM> form a head thickness, that is defined as the furthest perpendicular distance between the first near-cylindrical portion 204a and the outer bearing surface <NUM>, and lies in the range between <NUM> and <NUM>, depending on the size of the head <NUM>. As a consequence of the head being generally spherical, the head thickness at the rim <NUM> lies in the range between <NUM> to <NUM>, preferably being <NUM>. It is important to conserve as much bone as possible on the natural femoral head and, in turn, for the head prosthesis wall thickness to be as thin as possible, particularly in the region nearest to the natural femoral neck.

As the outer bearing surface <NUM> is generally spherical, the head rim <NUM> forms a smooth radius between an outer edge <NUM> and an inner edge <NUM>. Said radius has a size of at least <NUM>. Having a radiused rim helps avoid soft-tissue irritation and minimize edge wear. Furthermore, the inner edge <NUM> of the head rim <NUM> is also radiused, with the radius being at least <NUM>. The absence of sharp inner edges helps to avoid localised stress concentrations within the prosthesis. The outer bearing surface <NUM> and the rim edges <NUM> and <NUM> are highly polished for a better fit with the cup <NUM>.

The first near-cylindrical portion 204a comprises a first group of elongate cement pockets <NUM> in the form of elongated recesses oriented generally parallel with and spaced, preferably equispaced, around the longitudinal axis Y-Y' of the head <NUM>.

The second tapered portion 204b comprises a second group of cement pockets <NUM> that are regularly spaced around the longitudinal axis Y-Y'. Said first <NUM> and second <NUM> groups of cement pockets are elongated and between <NUM> and <NUM> in depth at their centers, with a concave radiused profile at each end running-up to the surrounding socket surface. It is also understood that other shapes of the cement pockets can be used, such as flat, square or trapezoid. While the second group of cement pockets <NUM> provides rotational stability, in a conventional manner by resisting rotation around the longitudinal axis Y-Y', the first group of pockets <NUM> provides a mechanical interlock with the cement mantle to resist both torsional and axial forces acting on the cemented femoral head <NUM>. Therefore, as well as rotational stability, the cement pockets <NUM> also provide resistance to pull-off forces owing to their alignment with respect to the longitudinal axis Y-Y'. Advantageously, the pockets <NUM> provide a local high-point for cement ingress, deeper and further from the head's axis Y-Y' than the surrounding near-cylindrical inner portion <NUM>.

The femoral head <NUM> further comprises a stem <NUM> projecting laterally inwardly from a pole of the inner surface <NUM> along the longitudinal axis Y-Y'. The stem <NUM> comprises a proximal end <NUM> and a distal end <NUM>, said distal end projects laterally beyond the head rim <NUM>. The stem <NUM> is of nearly-cylindrical shape and its diameter ranges in size between <NUM> and <NUM>. By "nearly-cylindrical" it is understood that the proximal end <NUM> of stem <NUM> is generally cylindrical, and the distal end <NUM> is tapered in the lateral direction. The length of the proximal portion can be <NUM>-<NUM>% of the overall length of the stem <NUM>. The stem <NUM> projects laterally between <NUM> and <NUM>, more preferably between <NUM> and <NUM>, beyond the level of the rim <NUM>. The distal end <NUM> of the stem <NUM> is curved with a radius of between <NUM> and <NUM>.

The protruding distal end <NUM> of the stem <NUM> can be used to assist with the correct alignment of the femoral head <NUM> during implantation. It is understood that the stem <NUM> is intended to be a clearance fit within a prepared hole in the natural femur. In the other words, the natural femur is prepared to accept the inner socket <NUM> and the stem <NUM> of the resurfacing head <NUM> prior to cementing. The prepared natural femur has a profile which broadly resembles that of the inner socket <NUM>, but leaving clearance between the natural bone and the fully-seated head <NUM> in some regions, in particular in the tapered portion 204b, to provide a maximum cement mantle thickness of approximately <NUM>.

A set of differently-sized femoral heads <NUM> is provided in order to accommodate the full range of differently sized patients. The bearing diameters of the differently-sized femoral heads increase in <NUM> increments through a range of <NUM> to <NUM>, in line with the inner diameters of the correspondingly-sized acetabular cups.

<FIG> shows a prosthetic hip joint <NUM> comprising an acetabular cup <NUM> and a femoral head <NUM>, each selected from a set of available sizes. The prosthetic hip joint <NUM> is mounted on a natural femoral neck <NUM>. As described above, starting from the smallest cup <NUM> in the set, the outer diameter thereof increases in <NUM> increments whereas the inner bearing diameter increases in smaller increments, e.g. <NUM>. The outer bearing diameter of the femoral head <NUM> matches the inner bearing diameter of the respective cup <NUM>. This results in the ratio between the outer diameter of the cup and the bearing surface of the head being different for each size in the range. For example, for the smallest cup and head in the set, the ratio is <NUM>. For the largest cup and head in the set, the ratio is <NUM>.

<FIG> shows an impingement zone Z (indicated by dotted lines in <FIG>) wherein rotation of the cup <NUM> on the head <NUM> is limited at the extremes by impingement on the natural femoral neck <NUM>. The lateral offset of the inner surface of the cup <NUM> enables the cup <NUM> to rotate further with respect to the head <NUM> before impingement occurs. This effectively increases the range of motion in the resurfaced joint.

The variation in the ratio between the outer diameter of the cup <NUM> and the outer bearing surface of the head <NUM> contributes to desirably minimizing the amount of natural bone that needs to be removed to accommodate the prostheses (this having a greater adverse effect in a smaller sized patient). In addition, the variation in ratio contributes to maximising the head-neck ratio for the resurfaced femur. This, along with the lateral offset described above, maximises the range of motion of the resurfaced joint.

Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of them mean "including but not limited to", and they are not intended to (and do not) exclude other additives, components, integers or steps.

Claim 1:
A set of differently-sized ceramic acetabular cup prostheses, each cup (<NUM>) in the set comprising:
a generally hemispherical outer surface (<NUM>) having a cup pole (<NUM>) with a longitudinal axis of the cup (<NUM>) passing therethrough, an equatorial cup rim (<NUM>) and an outer diameter having an outer diameter centre point ("COD");
a generally hemispherical inner bearing surface (<NUM>) having a bearing surface edge and a bearing diameter having a bearing diameter centre point ("CBD"); and
a cup thickness defined between the inner bearing surface (<NUM>) and the outer surface (<NUM>) of the cup (<NUM>),
wherein the CBD is laterally offset from the COD along the longitudinal axis and
characterised in that for each cup (<NUM>) in the set starting from the smallest cup (<NUM>) in the set, the outer diameter increases in <NUM> increments and the inner diameter increases in less than <NUM> increments.