GB2534140A - Joint prosthesis supporting assembly - Google Patents

Abstract

A joint prosthesis supporting assembly 10 comprisies a sub-frame 12 to be located between a joint replacement component 20 and a resected surface of a femur or tibia, and at least one extra-cortical fixation element 14 for securing the sub-frame 12 to the bone. The extra-cortical fixation elements 14 may have a tab 14a which extends into a slot 12a in the sub-frame, and may be secured to the bone with fasteners extending through apertures (see figure 4). The sub-frame 12 provides a stable, secure platform for attachment of the joint replacement component 20. The extra-cortical fixation element 14 provides for loads to be transferred and helps to stabilise the sub-frame 12 and therefore any joint replacement component 20 attached to it. The arrangement is particularly suited for use in revision procedures, allowing the use of primary joint replacement components therein.

Description

Intellectual Property Office Application No. GII1500479.9 RTM Date:5 June 2015 The following terms are registered trade marks and should be read as such wherever they occur in this document: Tantalum (page 6) Intellectual Property Office is an operating name of the Patent Office www.gov.uk /ipo

JOINT PROSTHESIS SUPPORTING ASSEMBLY

Field of the Invention

The present invention relates generally to a joint prosthesis supporting assembly. In particular, the present invention relates to an assembly which enables extra-cortical fixation of a joint prosthesis and which comprises a sub-frame component. The invention has particular application to knee joint prostheses, especially for revision knee surgery, but is not limited to that application.

Background to the Invention

Anatomy of the knee ioint The bone regions associated with the human knee joint are: the distal end of the femur, the proximal end of the tibia, and the patella; each of these has at least one smooth articulating surface arranged for articulation on an adjacent surface.

The distal end of the femur has two smooth processes, called the medial condyle and the lateral condyle, spaced apart by an inter-condylar groove posteriorly. The groove extends towards the anterior direction to provide a shallow vertical groove between the condyles at the distal end of the anterior femur. The proximal end of the tibia has two smooth meniscal condyles, medial and lateral, which are slightly cup-shaped to receive the respective femoral condyles during articulation of the tibio-femoral joint. The patella has a smooth articulating surface posteriorly with a vertical ridge in the middle. During flexion and extension of the joint, this surface glides over the vertical groove between the anterior femoral condyles. The ridge provides stability against medial-lateral dislocation of the patella from the vertical groove. Several muscles, ligaments, and tendons provide stability and functionality of the joint.

Arthritis and knee replacement Because of several etiological reasons, the knee joint may undergo irreversible degenerative changes which may not be manageable by non-operative treatment. In such cases, it is required to replace the natural knee joint partially or totally with an artificial knee prosthesis. Primary total knee arthroplasty (TKA), also known as total knee replacement (TKR), is an effective surgical procedure for end-stage treatment of knee arthritis. Unicompartmental knee arthroplasty (UKA) is sometimes used for patients with arthritis on only one side of their knee. However, there are a growing number of patients for whom the primary prosthesis (implanted during TKA or UKA) fails for some reason and revision knee surgery is required.

Components of knee implants There are two types of knee prosthesis; primary and revision. The prosthesis used in a patient's first joint replacement surgery is called the primary prosthesis whereas a revision prosthesis is used following failure of a previously implanted prosthesis, whether that be a primary or a revision prosthesis. A typical primary total knee prosthesis consists of a femoral component, a tibial component, and a patellar component. The tibial component has a base plate, called the tibial tray, and a stem and/or few pegs extending from the tray down into the bone. When implanted, the tibial tray sits over the resected surface of the proximal tibia, and the stem (and/or pegs) goes into the metaphyseal part of the tibia. A bearing insert which articulates with the femoral component is mounted on the tibial tray, and may be either fixed or mobile with respect to the tray. The femoral component has a pair of condylar portions that articulate with the respective bearing surfaces of the tibial insert. There is a shallow vertical groove in the anterior flange of the femoral component. The patellar component slides over the anterior vertical groove of the femoral component. A typical primary unicompartmental knee prosthesis similarly consists of a femoral condylar component, a tibial tray, and a bearing insert. When a unicompartmental prosthesis fails, it may be revised with the implantation of a "primary" or "revision" total knee replacement prosthesis, but currently no unicompartmental prostheses specifically for revision surgery are commercially available.

The basic structural features of primary total knee prostheses and revision knee prostheses are similar. However, revision prostheses are generally bulkier.

Additionally, some augmentation devices may be used with the femoral and tibial components of a revision knee prosthesis. In order to achieve stability and fixation of the prosthesis, stem extensions are often used with both the femoral and tibial components in revision cases. The components of both the primary and revision systems are available in various shapes and sizes to accommodate the patient population. Large numbers of prosthesis types are available with similar principles of function. Depending on the state of the ligaments of the joint, prostheses may be designed to perform the functions of the ligaments partially or fully.

Bone defects: classification and management A certain amount of bone loss often occurs during the lifetime of a primary TKA prosthesis, due to a variety of possible factors, including implant-induced stress-shielding, wear-related osteolysis, mechanical loosening and infection. These bone defects are often characterised by contained or cavitary voids and uncontained segmental or peripheral cortical rim defects. During a revision procedure, additional bone is often lost in the removal of the failed primary prosthesis. These bony defects should be reconstructed during revision surgery in order to maintain the joint line, and to provide structural support to the revision components. In the clinical literature, classification of bone defects is widely based on the Anderson Orthopaedic Research Institute (AORI) system, which defines bone loss in the femoral or tibial bone as one of Type 1, 2A, 2B or 3, based on radiographic and intra-operative findings. Type 1 represents an intact metaphyseal bone with minor defects which does not compromise the stability of a revision component. Type 2 represents damaged metaphyseal bone (loss of cancellous bone in the metaphyseal region). Defects can occur either in one femoral condyle or one tibial plateau (2A) or in both the condyles and plateaus (2B). Type 3 represents deficient metaphyseal bone which comprises a major portion of either condyle or plateau.

Various surgical techniques and products are available to orthopaedic surgeons to reconstruct these bone defects during revision surgery. However, the final choice of treatment depends on the size and location of the defects, age and level of activity of the patients, and the quality of the remaining bone stock, among other considerations. Traditional treatment options include cement augmentation (Whittaker et al. 2008; Radnay and Scuderi, 2006), bone grafting (Whittaker et al. 2008; Radnay and Scuderi, 2006; US Pat. Nos. 4678470, 4789663, 5356629; 5510396; 5788976; 5972368), modular metallic augments (US Pat. Nos. 4936847; 5019103; 5152797; 5370693; 5387241; 5458637; US Patent Application 20110015751A1), and specific customized prostheses with different level of constraints (US Pat. Nos. 6485519; 6773461; 7572292; 7658767). In addition to the traditional methods of managing bone loss, recent developments include the use of metaphyseal filling implants made of highly porous metal (US Patent Application Nos. 20040049285A1; 2010057212A1; 2010076565A1; US Patent Nos. 4846839; 7291174; 7799085).

Description of the Prior Art

Typically, there is an amount of bone loss from the distal femur and/or the proximal tibia in revision knee patients. The defective bone regions may provide inadequate structural support to the revision prosthesis components. Cement is generally preferred to augment AORI Type-1 defects, and small Type-2 defects. It is used either alone or combined with screws and mesh (Whittaker et al. 2008; Radnay and Scuderi, 2006). Bone graft is commonly used, either with cement or without cement, to prepare a support structure for revision knee components where there is a considerable amount of bone loss. The bone grafts may be autograft or allograft, and are available in either structural or morsellised forms. Synthetic bone substitute materials can also be used as grafts which are intended to stimulate growth of healthy bone. Some examples of bone graft materials can be found in US Patents 5356629, 5510396, 5537791, 5788976, and 5972368. Additionally, US Patents 5910172, 6013080, 6142998 disclose several techniques for positioning the bone graft in the bone cavity.

Augments or wedges have also been used commonly in revision knee surgery to compensate for bone loss (W02009089581A1; EP0378928A1). US Patent 5019103 disclosed a design of wedges that can be used in the defect to partially support the revision components. A large number of augmentation blocks are available to accommodate variability of shape, size, anatomy of patients, and variable location and extent of bone losses. Some early augment designs were limited to be mounted on either the medial or lateral side of the tibial component. The invention by German et al. (US Pat. 7175665) provided a universal augment system which can be mounted on either side of the tibia, thus reducing the inventory cost of the system. Attempts have also been made to provide an efficient and secured connection between augments and the basic prosthesis components. Manginelli (US Pat 4936847) disclosed a design wherein the correct size of the required augmentation device can be determined using multiple trials of augments with different thickness. The device of desired size can then be snapped into the required position on the femoral component. US Patent 5370693 disclosed a design where the augments are cemented to the prosthesis components. Hayes (US Pat. 5458637) disclosed an attachment mechanism that uses a button and a key-hole formed completely through the tibial tray component. In many revision instances, defects of condylar bone in one side may be more advanced than the other side. Non-uniform resection of the condylar bone is required, therefore, to retain as much bone as possible in each condyle. In order to compensate for this disparity, Vinciguerra suggested the use of shims with varying thickness in one condyle or the other (US Pat. 5609645A; EP 0797417B1).

Often, there is a lack of normal bony reference feature for aligning the femoral and/or tibial components properly during revision surgery. In such cases, the medullary canal of the corresponding bone is used as a reference to align the components. However, there is wide variability in the position and orientation of the medullary canal among patients. If the stem of the component is not properly aligned with respect to the canal, in the anterior-posterior or medial-lateral direction, there will be a gap and/or overhang between the prepared bone and the prosthesis component. Offset stems are used in such cases where the medullary canal deviates from the standard location (US Pat. Nos. 5271737; 5782920; 6146424; 7544211; W00006056). McCue et al. (US Pat. 6506216) disclosed a design of a tibial tray with a modular keel which is fixable to the inferior surface of the tibial tray and adjustable along the medial-lateral direction with respect to the medial-lateral centre of the tray. Sharkey et al. (US Pat. 6923832) disclosed an improved design for fixation of the stem to the tibial tray at desired locations and orientations. Stumpo et al. (US Pat. 7628794) proposed an infinite adjustment of the stem in the anterior-posterior direction to align the femoral component properly with the medullary canal to suit a wide range of patient anatomies. This design also permits a proper alignment of the valgus angle of the femoral stem to accommodate variable anatomy of patients.

In several instances during a revision knee procedure, some amount of healthy bone is unnecessarily removed along with the damaged bone, to facilitate implantation of the metallic augments in the defective sites. Blaylock et al. (US Patent Application 200410162619) disclosed a design intended to conserve the healthy peripheral bone in situations where the metaphyseal bone has a contained defect. They described a cone-shaped tibial augment system to replace bone defects using a highly porous 'Tantalum' material. The associated US patent applications (US Patent Application Nos. 2010057212A1; 2010076565A1; EP 2394608A1) proposed porous Titanium sleeves with a stepped design to treat contained metaphyseal defects in the tibia and femur. The porous sleeves are made of foam of either commercially pure Titanium or Titanium alloy. The tibial prosthesis to be used with such a porous augment has a tibial tray and an integral stem that can be connected to a stem extension distally. A modular sleeve may be provided with a tapered bore (widest at the proximal end) to accommodate the stem extension. The proximal surface of the modular sleeve has a flat elliptical shape to fit the distal surface of the tibial tray. The sleeve has a stepped or terraced outer surface which distributes the load and stabilizes the implant.

Several implant configurations have been previously described for provision of stability of the tibial component within the proximal tibia. In some commercial primary knee replacement designs, stability is provided by a number of integral keels extending between the stem and the distal surface of the tibial tray. The stem is integral to the tray and made hollow to accommodate a stem extension.

Alternatively, stepped or terraced sleeves are also used with tibial and femoral components (US Patent Application Nos. 2010057212A1; 2010076565A1; EP 1396240B1; US20100114323A1). German et al. (US Pat. 7291174) disclosed a tibial component which has both keels and a modular sleeve. The modular sleeve has an oval proximal surface with spaced reliefs. The relief and keels are shaped, sized and positioned so that the modular sleeve can be mounted on the stem of the tibial component. The stem is received in the interior bore of the sleeve and the keels are placed in the spaced reliefs of the sleeve. The system has the freedom for the tibial component to be implanted alone, with the keel providing the stability, or for an assembly with the sleeve to be implanted where bone deficiency is present.

To make component selection flexible during surgery, the individual prosthesis components should be interchangeable to all systems. Goodfried et al. (US Pat. 7799085, US Patent Application 20100222891A1) disclosed modular knee implant systems that allow the use of a universal metaphyseal component interchangeable with all styles of distal femoral components through the use of an adaptor (Fig. 4). For use in patients with ligamentous instability of the joint, several constrained/hinged prostheses, with fixed or rotational bearings, are disclosed in US Patents 6485519, 6773461, 7572292, and 7658767. Some revision cases require an artificial patella component. Some patellar component designs are disclosed in US Patents 5181924, 7878989, and US Patent Application 2010057211A1.

Prosthesis fixation Fixation of conventional knee prosthesis components relies on either polymethylmethacrylate (PM MA) bone cement or natural bone ingrowth. US Patent 6355067 disclosed a prosthetic knee joint and a method for its fixation to the prepared bone using a cement layer of a controlled thickness, US Patent Application 20080306602A1 disclosed a cement impaction system for the tibial component and a method for its characterization, and US Patent 4479271 disclosed a technique enabling natural bone ingrowth fixation of a tibial component that incorporates a fibrous metal mesh. To enhance natural bone growth, other type of porous coated surface treatments have been disclosed in US Patents 3855638, 4550448, and 7501073.

During revision knee surgery, long-stemmed femoral and tibial components are also used to augment the implant fixation (Whittaker et al. 2008; Radnay and Scuderi, 2006). Engagement of diaphyseal bone by stem extensions is intended to enhance component fixation and assist with component alignment for the patients with compromised metaphyseal bone quality. Stem extensions are also recommended to reduce stress at the implant-bone interface when using augments or wedges. While the stems may be fixed to the medullary canal either with cement or without cement, the most common method of fixation during revision surgery at present is the use of an uncemented stem with a cemented metaphyseal part of the prosthesis component. US Patents 5480444, 5683471 and US Patent Application 20090248158A1 disclose techniques for fixation of orthopaedic implants with a combination of two different methods, cemented and uncemented.

Use of cement to fill bone defects remains a well-performing option in the short-term, and is therefore suitable for elderly and infirm patients. However, long-term performance of this technique is generally poor: it provides inferior load transfer with poor fatigue properties and does not restrict deflection of the tibial tray (Whittaker et al. 2008). Bone grafting has been successful for small bone defects, but higher failure rates have been observed for larger defects (Whittaker et al. 2008). The important advantages of bone grafting are the ability to contour the graft intra-operatively and the capacity of the graft to transfer load in a physiological manner. However, the limitations of using allografts include the theoretical transmission of viral, bacterial and prior disease, and difficulties with availability. Based on the results of a variety of techniques for bone grafting, it is clear that an ideal method of accounting for lost bone stock is yet to be developed.

Metallic augments are used for Type-1 and Type-2 defects of 5 to 10 mm, or segmental defects, with bulk allograft in larger defects. The augments are used when 40% or more of the projected implant-bone interface is unsupported by host bone or if peripheral defects involve 25% or more of the cortical rim (Whittaker et al. 2008). The advantages of augmentation include ready availability and quick application. However, these augmentation devices can only be used with specific implants available from selected manufacturer. Additionally, problems may arise from corrosion and wear debris as a result of fretting between the tray and the augment. Angled wedge augments create shear stresses and fail to load the distal bone. Step wedge augments subject the underlying interface to compressive forces, but may require further removal of bone to accommodate them (Whittaker et al. 2008). These stiff devices make the revised joint more vulnerable to stress-shielding.

A recent review article on fixation of revision TKA concluded that there is no consensus on the best-performing method for fixation of revision knee prosthesis components (Beckmann et al. 2011). Clinical studies have reported mixed results concerning whether such components should be fully cemented, partially cemented (hybrid), or uncemented. The primary advantage of cemented fixation is the achievement of immediate post-operative stability. Moreover, the risk of infection is lower than for uncemented fixation, because of the presence of antibiotics in typical bone cement. However, a larger amount of host bone must be removed to accommodate the cement layer. Generally, cemented fixation of a stem extension increases the effect of stress-shielding in the metaphyseal bone due to non-physiological load transfer. Bone remodelling under these conditions leads to bone resorption in the metaphyseal bone, enhancing the risk of failure of the revised joint replacement prosthesis. Failure of the bone-cement interface is also observed frequently in clinical studies. Reduced operating time and, hence, reduced blood loss, are the major advantages of uncemented fixation. The primary disadvantages of cementless fixation are reduced stability during the early post-operative period, unpredictable bone ingrowth onto the implant surface, and end-of-stem pain. As with the cemented stem, the use of a long uncemented stem also increases the effect of stress-shielding in the metaphyseal bone.

Need for improved design Although much attention has been paid to designing new revision knee prostheses and associated implantation techniques, there remains scope for improvement. There is a need for a design to transfer the load more physiologically (load transfer close to the joint line) in the revised knee joint. Simultaneously, there is a need for a revision system that provides sufficient stability with minimal resection of the host bone. Finally, there is a need for an improved augmentation device for the bone defects, of low weight but sufficient strength, to provide enough support to the revision components and to encourage bone ingrowth over a large surface area.

As outlined above, bone loss in patients who require revision of total joint replacements, such as that in the femur and/or tibia for knee implant recipients, occurs for a variety of reasons. When a moderate or severe degree of bone loss is identified, augmentation devices are used with the corresponding prosthesis component (e.g., the femoral component in the case of femoral bone loss) to achieve greater fixation strength, and hence post-operative stability, of the prosthesis. The most common augmentation device is a stem that extends into the bone's medullary canal, typically extending at least 10-20cm away from the joint.

There are several issues with intramedullary stems used in revision joint replacement. Firstly, bone resorption around the stem is observed due to the stress-shielding effect, which may lead to an implanted component loosening and an increased risk of re-revision. This risk is greater in the case of stems of increased length and diameter. Secondly, end-of-stem pain and pen-prosthetic fractures have been reported to occur in a small but not insignificant number of patients. Thirdly, the amount of components required in the operating theatre is large, due to a large number of options of stem diameter and stem length, and a large number of sizes of reaming tools. This leads to a greater cost burden than for the initial implantation.

References: Whittaker, J.P., Dharmarajan, R., Toms, A.D. 2008. The management of bone loss in revision total knee replacement. Journal of Bone and Joint Surgery [Br], 90(8), pp. 981-987 Radnay, C.S., Scuderi, G.R. 2006. Management of bone loss: augments, cones, offset stems. Clinical Orthopaedics and Related Research, 446, pp. 83-92 Beckmann, J., Luring, C., Springorum, R., Kiick, F.X., Grifka, J., Tingart, M. 2011. Fixation of revision TKA: a review of the literature. Knee Surgery, Sports Traumatology, Arthroscopy, 19 (6), pp.872-879

Summary of the Invention

According to a first aspect of the invention, there is provided a joint prosthesis supporting assembly comprising: a sub-frame for attachment to a joint replacement component and for attachment to a bone such that the sub-frame is disposed between the joint replacement component and the bone; and at least one extra-cortical fixation element for securing the sub-5 frame to the bone.

The sub-frame and associated extra-cortical fixation element(s) provide a stable, secure platform for the attachment of the joint replacement component to the bone, without the need for an intramedullary stem or other internal augment. By having the sub-frame as a separate modular component from the joint replacement component, the sub-frame can be patient-specific whilst the joint-replacement component can be selected from a limited range of options, thus reducing the need for a large inventory whilst also accommodating different patient needs through providing patient-specific attachment to the bone.

Additionally, the platform allows for flexibility during surgery because the surgeon can opt to select different joint replacement components, each of which would attach to the sub-frame in the same manner.

This arrangement is particularly beneficial for revision procedures, because the at least one extra-cortical fixation element can carry a major part of the loads through the joint and can securely register the sub-frame and hence the joint replacement component in place, thus reducing the need for a long, and perhaps large diameter, intramedullary stem and associated relatively large removal of bone as would otherwise be the case in conventional revision procedures, particularly for cemented fixation. In particular, the sub-frame allows the use of a conventional primary joint replacement component for a revision procedure.

The at least one extra-cortical fixation element may comprise a plate which includes one or more holes for receiving screws for securing the at least one extra-cortical fixation plate to the bone. At least one of the one or more holes may be threaded so as to receive locking screws. In one embodiment, the assembly further comprises locking screws to be used in conjunction with the threaded holes. At least one of the one or more holes may not be threaded, and hence be configured to receive conventional bone screws which do not lock to the plate.

In one embodiment, the screw holes are elongated along the plate, such that the plate may slide in relation to the screw when the screw is fixed into the bone.

The at least one extra-cortical fixation element may comprise an elongate portion for extending along an outside surface of the bone. The elongate portion of the at least one extra-cortical fixation element may be contoured to follow the surface of the bone, either solely in the longitudinal direction, or in both the longitudinal and circumferential directions. The elongate portion of the at least one extra-cortical fixation element may include features to encourage bone ingrowth.

The at least one extra-cortical fixation element may be made on a patient-specific basis, such as by rapid prototyping, to match the shape of the patient's corresponding bone.

The at least one extra-cortical fixation element may be tapered along its length, towards a narrow tip at the end away from the joint being revised.

The at least one extra-cortical fixation element may be removably attachable to the sub-frame. Alternatively, the at least one extra-cortical fixation element may form an integral part of the sub-frame. A combination of removable and integral extra-cortical fixation elements may be provided. A removable fixation element has the advantage of reducing the profile of the supporting assembly to enable insertion through a relatively small incision. Conversely, an integral fixation element has the advantage of not requiring attachment to the sub-frame during the surgical procedure.

The joint prosthesis supporting assembly may be a revision prosthesis supporting assembly, suitable for supporting a revision prosthesis component. As mentioned above, the joint replacement component may be a primary joint prosthesis component, meaning that a revision procedure can be carried out using a primary component, thereby reducing the inventory needed.

The joint replacement component may be either a tibial component or a femoral 5 component.

Where at least one of the extra-cortical fixation elements is removable, the sub-frame may further comprise a socket for receiving a part of the at least one extra-cortical fixation element that is removably attachable to the sub-frame.

The at least one extra-cortical fixation element may comprise a tab for receipt in the socket of the sub-frame. Alternatively, the sub-frame may comprise a tab with the at least one extra-cortical fixation element comprising a socket for receiving the tab. In either case, the external shape (e.g. cross-section) of the tab may closely match the corresponding internal shape (e.g. cross-section) of the associated socket. Alternatively, the tab and socket arrangement may have a tapered geometry, such that they jam together on assembly.

In one embodiment, the at least one socket comprises a three-sided slot that is open on one of: a surface of the sub-frame which, in use, is mounted to bone; and a surface of the sub-frame which, in use, is attached to the joint replacement component. The three-sided slot may be dovetail-shaped in cross section, with the narrower section of the slot being closer to the surface of the sub-frame which, in use, is mounted to bone than the surface of the sub-frame which, in use, is mounted to the joint replacement component.

The assembly may include a second extra-cortical fixation element for securing the sub-frame to the bone. The second extra-cortical fixation element may be removably secured to the sub-frame, or may form an integral part of the sub-frame.

In certain embodiments, the sub-frame is comprised of more than one component.

According to a second aspect of the invention, there is provided a sub-frame for use in a joint prosthesis, the sub-frame comprising: means for securely mounting a joint replacement component to the sub-frame; and means for securely mounting at least one extra-cortical fixation element to the sub-frame.

The sub-frame may further comprise means for accommodating fixation features of the joint replacement component.

The at least one extra-cortical fixation element may be integrally mounted to the sub-frame as a single unit.

Alternatively, the at least one extra-cortical fixation element may be removably attachable to the sub-frame. The means for securely mounting the said removable at least one extra-cortical fixation element may comprise a respective socket in the sub-frame for receiving a part of said removable at least one extra-cortical fixation element. The socket may comprise a three-sided slot that is open on one of: a surface of the sub-frame which, in use, is mounted to bone; and a surface of the sub-frame which, in use, is mounted to the joint replacement component. The three-sided slot may be dovetail-shaped in cross section, with the narrower section being closer to the surface of the sub-frame which, in use, is mounted to bone than the surface of the sub-frame which, in use, is mounted to the joint replacement component.

The means for securely mounting a joint replacement component to the sub-frame may be adapted to accommodate a central box mechanism of the joint replacement component, which is a joint replacement component that requires sacrifice of the posterior cruciate ligament.

According to a third aspect of the invention, there is provided a joint prosthesis kit comprising: the sub-frame of the second aspect of the invention; a joint replacement component; and at least one extra-cortical fixation plate for secure attachment to the sub-frame.

The kit may further comprise screws for attachment of the at least one extra-cortical fixation plate to the bone

Brief Description of Drawings

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 depicts a joint prosthesis supporting assembly and a tibial joint replacement component, where a sub-frame component of the joint prosthesis supporting assembly is indicated by hatching; Figure 2 depicts the joint prosthesis supporting assembly and tibial joint replacement component of Figure 1 from a different angle; Figure 3 depicts an exploded view of the joint prosthesis supporting assembly and tibial joint replacement component of Figures 1 and 2, showing the sub-frame and extra-cortical plate components of the joint prosthesis supporting assembly; Figure 4 corresponds to Figure 3, but from a different angle; Figures 4a and 4b show respective cross sectional and side elevation views of an alternative embodiment, with a different means of connection of a sub-frame to an extra-cortical fixation plate; Figures 4c and 4d show respective front elevation and perspective views of another alternative embodiment, having buttressed extra-cortical fixation plates that are mounted vertically to the sub-frame using a dovetailed joint; Figure 4e is a cross-sectional view through an alternative embodiment of the joint prosthesis supporting assembly and tibial joint replacement component, wherein the sub-frame comprises two block-like elements; Figure 5 depicts a joint prosthesis supporting assembly for a femoral joint replacement component, where a sub-frame component of the joint prosthesis supporting assembly is indicated by hatching; Figure 6 depicts a joint prosthesis supporting assembly and a femoral joint replacement component from a different angle; Figure 7 depicts an exploded view of a joint prosthesis supporting assembly and a femoral joint replacement component; Figures 7a and 7b show an alternative sub-frame for a truciate-sacrificing' type femoral joint replacement component, in respective side elevation and perspective views; Figure 8a-8c show three alternative extra-cortical fixation plate designs with varying taper features: tapering of thickness (Fig. 8a), tapering of width (Fig. 8b) and tapering of both thickness and width (Fig. 8c); Figure 9a depicts a joint prosthesis supporting assembly with a unitary part comprising the sub-frame and two integral extra-cortical fixation plates; Figure 9b depicts a joint prosthesis supporting assembly with a unitary part comprising the sub-frame and one integral extra-cortical fixation plate, and one removable extra-cortical fixation plate; Figure 10a depicts a joint prosthesis supporting assembly and a tibial joint replacement component with an extra-cortical fixation plate comprising a tab and a sub-frame comprising an open-sided socket adjacent to the resected bone surface to accommodate the tab; Figure 10b depicts an end view of the tab and socket of Figure 10a, showing the dovetail cross-sections; Figure 10c depicts a joint prosthesis supporting assembly and a tibial joint replacement component with an extra-cortical fixation plate including a tab, and a sub-frame with an open-sided socket adjacent to the tibial tray component; Figure 11 depicts a joint prosthesis supporting assembly and a tibial joint replacement component with a sub-frame comprising a tab and an extra-cortical fixation plate comprising a socket to accommodate the tab; Figure 12 depicts a joint prosthesis supporting assembly with an extra-cortical fixation plate attached to a sub-frame with a screw passing through a hole in the extra-cortical fixation plate into a threaded hole in the sub-frame; Figure 13 depicts an alternative fixation of an extra-cortical fixation plate to a sub-frame, with a combination of tab-and-socket geometrical mating and a screw passing through the fixation plate and into a threaded hole in the sub-frame; Figure 14a depicts a resected tibia with a postero-medial defect; Figure 14b depicts a tibia implanted with a joint prosthesis and supporting assembly, and an additional containing structure to contain the bone graft or other bone substitute material used to fill the postero-medial defect of Figure 14a; Figures 15a and 15b depict two different views of the containing structure of Figure 14b fixed to cortical bone on the opposite side with a tense trans-osseous string; Figures 16a and 16b show an alternative joint prosthesis supporting assembly and associated femoral joint replacement component for use with a truciate-sacrificing' femoral component, in which the sub-frame has a stepped configuration; Figures 17a and 17b depict alternative views of the distal part of a femur which has been implanted with a femoral component of a unicondylar joint prosthesis and an associated supporting assembly incorporating a sub-frame with one integral extra-cortical fixation plate; and Figures 18a and 18b depict alternative views of the proximal part of a tibia which has been implanted with a tibial component of a unicondylar joint prosthesis, and a supporting assembly again incorporating an associated sub-frame and one integral extra-cortical fixation plate.

Detailed Description

In current clinical practice, long intramedullary stems are used to achieve adequate mechanical stability of the joint replacement components in revision knee surgery. However, as mentioned above, the use of stiff stem-extensions increases the effect of stress-shielding (and hence risk of bone resorption and component loosening) in the region close to the joint line, among other disadvantages. In order to transfer load more physiologically (i.e., greater load transmitted through cancellous bone close to the joint line) and to simultaneously achieve component stability, use of one or more extra-cortical fixation plates (ECP) is proposed. Such plates should be of low profile, with lower stiffness than an intramedullary stem intended to have the same load-bearing capacity, but sufficiently strong to carry the part of the joint load required for post-operative stability.

Figure 1 depicts a joint prosthesis supporting assembly 10 for a tibial joint replacement component 20. The joint prosthesis supporting assembly comprises a sub-frame 12 and two extra-cortical fixation plates 14 (although only one is visible in Figure 1). Alternative embodiments comprise a single extra-cortical fixation plate 14 or more than two extra-cortical fixation plates 14. The tibial joint replacement component 20 may be a conventional tibial component used in primary Total Knee Replacement (TKR) procedures. Figure 2 depicts the same joint prosthesis supporting assembly 10 from a different angle at which both extra-cortical fixation plates 14 are clearly visible.

As shown in Figures 1 and 2, the tibial joint replacement component 20 is mounted into the sub-frame 12, as shall be described in detail below. The extra-cortical fixation plates 14 are also mounted to the sub-frame 12. The sub-frame 12 supports a primary TKR component at the correct level and orientation, even in the presence of bone loss, by transferring the joint load along the fixation plates 14, then into the strong cortex of the bone distant from the zone affected by the loosening/revision process. By using multiple-screw fixation of the plates 14 to the bone (described in detail below), there is load-transfer which results in less stress-shielding than with intramedullary fixation. Bone graft may be packed into a bone defect and be protected by the plate over it, as it transfers the majority of the load to the extra-cortical fixation plates 14 and not directly onto the bone graft below the sub-frame 12. The sub-frame 12 additionally enables use of a primary TKR component, thus reducing inventory requirements and total procedure cost. The sub-frame 12 and extra-cortical fixation plates 14 are easy to remove if required for a subsequent revision, compared with the difficulty associated with long stems, particularly those which have been cemented into the medullary canal.

The sub-frame 12 could be made of a typical metal used in joint replacement components, such as titanium alloy or cobalt-chromium-molybdenum.

The profile of the extra-cortical fixation plates 14 is designed to follow the contours of the surface of the bone so as to match the bone outer surface, certainly in the longitudinal direction, and possibly also in the circumferential direction.

However, the curvature of the bone surface varies from patient to patient. As such, it is preferable that the extra-cortical fixation plates 14 be either patient-specific or malleable enough for adjustment of their shape during the surgical procedure, although they may be designed as an optimum generic shape suitable for use with the majority of patients, or as a series of different sizes each designed for use with different ranges of bone geometry.

If the extra-cortical fixation plates 14 are patient-specific, they may be manufactured by rapid prototyping, for example, by using a suitable scanner to obtain the geometry of a patient's bone, using suitable modelling software to design the extra-cortical fixation plates 14 such that they match the bone outer surface and additive manufacturing of the extra-cortical fixation plates 14.

The extra-cortical fixation plates 14 can be made of a typical metal used for fracture fixation plates, such as titanium alloy, or cobalt-chromium-molybdenum. The fixation plates 14 could be made to be partially or entirely porous, for reduced stiffness purposes. The fixation plates 14 could also be made partially or entirely of a lower stiffness material, such as a polymer composite, in order to reduce stress-shielding.

As can be seen in Figure 1, the extra-cortical fixation plates 14 include holes 16.

These holes 16 are designed to receive screws for securing the extra-cortical fixation plates 14 to the bone.

The holes 16 may be threaded so as to receive locking screws (not shown) which lock into both the bone and the extra-cortical fixation plates 14, providing a secure attachment of the extra-cortical fixation plates 14 to the bone, as known in the art. The use of locking screws may reduce the effect of stress-shielding underneath the extra-cortical fixation plates 14. The holes 16 may instead be unthreaded so as to receive conventional cortical bone screws. A combination of threaded and unthreaded holes 16 may also be used.

The holes 16 may be elongated along the extra-cortical fixation plates 14, such that an extra-cortical fixation plate 14 may slide in relation to a screw once the screw has been fixed into the bone through the extra-cortical fixation plate 14. This reduces load transfer, which may be desirable in certain circumstances.

An additional advantage of the extra-cortical fixation plates 14 is that they act to enlarge the surface area of the joint prosthesis supporting assembly 10, which aids bone ingrowth, improving long term stability. The holes 16 help to achieve initial stability following fixation of the extra-cortical fixation plates 14 to the bone with screws.

The design of the tibial component 20 may include fixation features such as a short stem 20a (as can be seen in Figures 3 and 4) having a length similar to that of a tibial component of conventional primary TKR, facilitating load transfer close to the joint line, as opposed to the longer stem present in conventional revision TKR prostheses. Other typical fixation features include pegs and keels.

The extra-cortical fixation plates 14 can have a low profile as two recesses 40 (as can be seen in Figures 3 and 4) are prepared on the resected tibia surface 42 to accommodate two bosses 41 protruding from the bone-facing surface of the sub-frame 12, the bosses 41 containing sockets or slots 12a, such that the sockets 12a are further from the joint line than the body of the sub-frame 12.

Advantageously, the thickness of the main body 12b of the sub-frame can be reduced, meaning that less of the bone surface 42 (in this case the surface of the tibia) need be removed. This also maintains a flat upper surface on the sub-frame 12, to match the conventional preparation of the bone, which matches the flat underside of the tibial component 20.

A small cantilever portion 14a of the extra-cortical fixation plates 14 (shown in Figures 3 and 4) can be fitted inside the protruding socket 12a with a horizontal sliding motion and fixed by, for example, a dove-tail mechanism (other mechanisms shall be described in detail). This connection between the fixation plates 14 and the sub-frame 12 is what allows the plates to bear some of the load transmitted through the artificial knee joint. The extra-cortical fixation plates 14 transfer part of the joint load to the proximal tibial cortex and provide stability to the tibial sub-frame 12 for patients with bone deficiency in the region beneath the sub-frame 12. This bone deficiency may be filled with bone cement, bone graft, or other materials.

The cantilever portion 14a may be in the form of a tab 14a configured to be inserted into the socket 12a. The cross-section of the tab 14a may be such that it matches that of the associated socket 12a.

The tab 14a and socket 12a arrangement may have a tapered geometry, such that they jam together on assembly. The tapered part could be circular in cross-section, to allow angular adjustment during surgery, then jammed together by impaction. This may be in conjunction with additional anti-rotation features.

Figures 4a and 4b show such a tab and socket arrangement from different angles. The arrows in Figure 4b indicate how the position of the fixation plate 14 can be angularly adjusted prior to the jamming together of the tab 14a and socket 12a.

Although Figures 3 and 4 depict the sockets 12a within bosses protruding from the underside of the sub-frame 12, the sockets may be formed within the main body 12b of the sub-frame 12 such that there is no need for protruding bosses, as shown in Figures 4e, 9b and 10 (as described below).

The extra-cortical plates 14 typically comprise an elongate portion 14b for extending along the outside of the bone. The elongate portion 14b is preferably contoured to follow the surface of the bone, in any of the manners previously discussed (i.e., patient-specific design or optimum design), or it may be malleable so as to adjust to the bone shape and fit closely against the bone.

The screw holes 16 are typically provided in the elongate portion 14b.

The extra-cortical plates 14 may be made with an integral buttress part 14c that protrudes from the bone-facing surface to provide a load-bearing ledge on which the sub-frame 12 will rest. Figures 4c and 4d show such an integral buttress part 14c from different angles.

The tab 14a shown in Figures 4c and 4d is tapered in towards the elongate portion of the extra-cortical plate 14. The slot 12a has a corresponding shape. As such, the extra-cortical plate 14 is locked in place once the sub-frame 12 is placed on top of the extra-cortical plate 14 such that the tab is disposed in the slot 12a and once the whole assembly is placed onto a bone.

Rather than a cantilever portion 14a, the end of the extra-cortical plate 14 may be adapted such that it can clip onto the edge of the sub-frame 12.

The sub-frame 12 has an orifice 18 for receiving the stem 20a or other fixation features such as posts of the tibial component 20.

The tibial component 20 may comprise additional fixation features such as stems, keels, pegs, etc. (not shown). The sub-frame 12 may further comprise addition means for accommodating fixation features of the joint replacement component 20. The sub-frame 12 may comprise one or more further orifices (not shown) for receiving any additional fixation features such as stems, keels, pegs, etc., which may be present on the tibial component 20.

The tibial component 20 may be securely mounted to the sub-frame 12. This may be achieved via a mechanical fixation means such as screws or clips, or by moulding PMMA bone cement on assembly.

The sub-frame 12 may comprise more than one component. For example, there may be two block-like elements 12d and 12e (shown in Figure 4e) which may be on the medial and lateral sides of the stem 20a or other fixation features of the tibial component 20, and which contain similar sockets 12a for connection to the extra-cortical fixation plates.

Figures 8a, 8b and 8c show alternative designs of the elongate portion 14b of the extra-cortical plates 14.

Figure 8a shows an elongate portion 14b with a tapered thickness. Figure 8b shows an elongate portion 14b with a tapered width. Figure 8c shows an elongate portion 14b with a tapered width and a tapered thickness. All three of these designs result in the region of the extra-cortical plate 14 that is further away from the joint carrying reduced load and hence reducing the 'stress-shielding' of the bone underneath this region of the plate.

The elongate portion of the extra-cortical fixation plates 14 may include features to encourage bone in-growth, such as a porous metal surface on the surface adjacent to the bone, additional slots in the plates, and surface treatments such as hydroxyapatite coating.

The elongate portions of the extra-cortical fixation plates 14 are likely to have the following dimensions for ease of implantation, similarity to existing fixation plates, and load-bearing strength: width between 10 and 20 mm, length between 50 and 120 mm, and thickness between 2 and 5 mm.

Figures 9a and 9b show alternative types of joint prosthesis supporting assembly 10. Figure 9a depicts a joint prosthesis supporting assembly with extra-cortical fixation plates 14i secured to the sub-frame 12 by being formed integrally therewith.

Figure 9b depicts a joint prosthesis supporting assembly with one integral extra-cortical fixation plate 14i and one removable extra-cortical fixation plate 14 which can be secured to the sub-frame 12 by any suitable means. The removable extra-cortical fixation plate 14 shown is of the cantilever type, as previously discussed; however, it will be understood that any other type of removable extra-cortical fixation plate 14, as discussed herein, may be used.

Figure 10a depicts a sub-frame 12 and a removable extra-cortical fixation plate 14. The sub-frame 12 comprises a slot 12a' that is three-sided and open on a surface of the sub-frame 12 which, in use, is mounted to the resected bone surface 42. A tab 14a of the extra-cortical fixation plate 14 has a matching cross-section such that it can be inserted into and be retained by the slot 12a.

Alternatively, the three-sided slot may be open on a surface of the sub-frame 12 which, in use, is attached to the tibial component 20, as shown in Figure 10c.

In either embodiment, the three-sided slot may be dovetail-shaped (Figure 10b, shown in the context of the embodiment of Figure 10a) in cross section, with the narrower section of the slot being closer to the surface of the sub-frame 12 which, in use, is mounted to bone.

Figure 11 depicts a sub-frame 12 and a removable extra-cortical fixation plate 14. The sub-frame comprises a tab 12c and the extra-cortical fixation plate 14 comprises a corresponding socket 14d. As will be understood, this arrangement is the reverse of the above described socket (or slot) and tab arrangements and the socket 14d of the extra-cortical fixation plate 14 may comprise the features of the any of the previously described sockets (or slots 12a) and the tab 12c of the sub-frame 12 may comprise the features of any of the previously described tabs 14a. The connection between the sub-frame 12 and the extra-cortical fixation plate 14 may also be of any other type as detailed in this document, such as the use of an additional screw and threaded hole.

Figure 12 depicts a sub-frame 12 and a removable extra-cortical fixation plate 14. The sub-frame 12 comprises at least one threaded hole 15 into which a corresponding screw 115 may be inserted. The extra-cortical fixation plate 14 may have a threaded or unthreaded hole through which such a screw 115 would pass. Alternative embodiments could include other attachment arrangements, such as the use of tabs and sockets, tapers, rivets, pins and plugs. There may also be a combination of multiple means of fixation, such as a screw passing through a tab/socket connection and into a threaded hole in the sub-frame 12, as shown in Figure 13.

Although a joint prosthesis supporting assembly 10 for a tibial joint replacement component 20 is shown in Figures 1 to 4, in an alternative embodiment, a joint prosthesis supporting assembly 50 for a femoral joint replacement component 30 (as shown in Figures 5 to 7) is provided. The femoral joint replacement component 30 may be the femoral component of a conventional primary TKR.

The tibial joint replacement component 20 and femoral joint replacement component 30 may instead be specially designed revision joint replacement components. For example, the revision component may not have the bone pegs 30a, but instead have threaded holes for screw fixation of the revision component to the sub-frame component.

In the embodiment where the joint prosthesis supporting assembly 50 is designed for use with a femoral joint replacement component 30, the femoral component 30 may include zero, one or more pegs 30a (as can be seen in Figure 7) having a length similar to those of a femoral component of conventional primary TKR, facilitating load transfer close to the joint line, as opposed to the longer stem present in conventional revision TKR prostheses.

The femoral component 30 may comprise additional fixation features such as stems, keels, pegs, etc. (not shown).

The design of the sub-frame 32 of the femoral joint prosthesis supporting assembly 50 is different to that of the tibial joint prosthesis supporting assembly 10 to account for differences between the femoral joint replacement component 30 and the tibial joint replacement component 20. The sub-frame 32 may have orifices 38 for receiving the corresponding pegs 30a of the femoral component 30. The sub-frame 32 may also have a stepped configuration, as can be seen in Figure 7, and further comprise two sockets 12a which perform the same function as the sockets 12a of sub-frame 12.

The sub-frame 32 may further comprise addition means for accommodating fixation features of the joint replacement component 30. The sub-frame 32 may comprise one or more further orifices (not shown) for receiving any additional fixation features such as stems, keels, pegs, etc., which may be present on the femoral component 30.

The sub-frame 32 is configured to receive similarly designed extra-cortical fixation plates 14 to those described above in connection with the tibial implementation, although modified to suit the femoral bone surface geometry. It will be understood that all of the variations and configurations of the sub-frame 12 of the tibial joint prosthesis supporting assembly 10 and corresponding extra-cortical fixation plates 14 as described above and shown in the Figures, including the various attachment mechanisms, can be applied to the sub-frame 32 of the femoral joint prosthesis supporting assembly 50.

The femoral surface 60 (as can be seen in Figure 7) may be resected to accommodate the stepped configuration of the sub-frame 32. Advantageously, the thickness of part of the main body 32b of the sub-frame 32 can be reduced, meaning that less of the bone surface (in this case the femoral surface 60) need be removed.

The flat configuration of sub-frame 32 (shown in Figures 5, 6 and 7) is adapted to match a conventional femoral component in which the posterior cruciate ligament (PCL) has been preserved; a so-called 'cruciate-retaining' implant.

However, it will usually be the case that the tibial bone attachment of the PCL has been destroyed, and so an implant of a 'cruciate-sacrificing' type must be used. In such implants, there is often a central 'box' feature 70 between the two femoral condyles (see Figures 7a and 7b), which the sub-frame 32 would be adapted to accommodate.

The adaptation of the sub-frame to enable use of a 'cruciate-sacrificing' femoral component may comprise a stepped shape that, when viewed from the anterior or posterior directions, passes from being approximately parallel to the transverse plane across one femoral condyle, then turns through approximately degrees to stand alongside the 'box' 70, then turns back to being approximately in the transverse plane where it passes over the box of the prosthesis; this pattern being duplicated symmetrically down and across the other femoral condyle. One such configuration is shown in Figures 16a and b, in which the sub-frame 32 comprises: a first flat portion 75a across one femoral condyle and a second flat portion 75b across the other femoral condyle, the flat portions being approximately parallel to the transverse plane; a third flat portion 77 also approximately parallel to the transverse plane, but offset vertically from the first and second flat portion 75a, b so as to pass over the box 70; and first and second risers 76a, b that rise up along the sides of the box 70 to respectively connect the first and second flat portions 75a, b to the third flat portion 77.

This stepped sub-frame 32 may be adapted to take the fixation features of the conventional primary prosthesis 30, such as pegs 30a that protrude through holes 38 in the sub-frame. It may also be combined with stiffening features such as the additional thickness shown anteriorly and antero-distally in Figure 7 a, b, and/or similar features which may be positioned postero-distally and posteriorly.

The sub-frame 32 may be adapted to increase its strength in bending from medial to lateral by having further facets 71, 72 which fit against the antero-distal facet 73 and anterior facet 74 within the femoral prosthesis component, thus being high enough to pass over the central box assembly 70.

This stepped sub-frame may be adapted to take the fixation features of the conventional primary prosthesis, such as pegs that protrude through holes in the sub-frame.

The top and bottom surfaces of the sub-frame 32 may not necessarily be parallel. Specifically, there may be a sloping of the top surface relative to the bottom surface, in the anterior-posterior and/or medial-lateral directions, to provide for the artificial joint line to be at an angle to the resection cut.

The joint prosthesis supporting assemblies 10, 50 described herein may be adapted such that they cover only a part of the end-face of a damaged bone; this may be approximately one-half, for example, to treat the bone defect resulting from unicondylar damage and provide extra-cortical fixation on that side of the bone. Such unicondylar damage may occur from loosening of a unicondylar prosthesis, and the supporting assembly would thereby be appropriate for use with a unicondylar knee replacement component. A pair of these could be used together, to treat sequential damage. The unicondylar device may also be adapted to support a conventional tibial or femoral component of a TKR prosthesis, when revising a unicondylar or total knee prosthesis, if the bone damage is at one side of the tibia or femur.

Figures 17 a, b show the distal part of a femur which has been implanted with a femoral component 130 of a unicondylar (also known as unicompartmental) joint prosthesis, and a supporting assembly 150 incorporating a sub-frame 132 and one integral extra-cortical fixation plate 114. Figures 18 a, b show the proximal part of a tibia which has been implanted with a tibial component 120 of a unicondylar joint prosthesis, and a supporting assembly 110 again incorporating a sub-frame 112 and one integral extra-cortical fixation plate 114.

The components of the joint prosthesis supporting assemblies 10, 50, 130, 150 described herein may be manufactured in a variety of ways, which may include additive layer manufacturing, casting, forging, machining from a cast or forged solid, or any combination of these.

The shape of the sub-frames described herein may be adapted to fit against the bone-facing surfaces of associated joint replacement components. For example, for the tibia, the sub-frame 12 may have the same outer shape as the cross-section of the tibial bone and have, for example, a central hole 18 to receive a fixation stem 20a of the tibial component 20, while the femoral sub-frame 32 may be substantially rectangular and sized so that it fits within the femoral component 30, and may, for example, have one or more load-bearing faces 32d which match the distal and/or antero-distal or anterior or postero-distal or posterior facets 33a, 33b, 33c. This sub-frame 32 will have appropriate holes 34 or other features to accommodate fixation pegs 30a or other means which are integral to the femoral joint prosthesis 30.

The above-described embodiments each provide for a bony fixation of the prosthesis component(s) through extra-cortical fixation plates and screws, rather than through an intramedullary stem, which has the advantage of avoiding all of the aforementioned drawbacks associated with the use of an intramedullary stem associated with conventional revision joint prostheses.

The described sub-frames 12 and 32 positioned between a conventional primary implant component and bone, enables attachment of the extra-cortical fixation plates 14 in a variety of possible embodiments, such as extra-cortical fixation plates 14 with sockets or threaded or unthreaded holes 16, as described above. Such a sub-frame may also be rigidly attached to one or more of the extra-cortical fixation plates 14 plates at the time of manufacture or at any time prior to or during implantation.

Method of implantation The fundamental steps for implantation of a tibial prosthesis supporting assembly 10 and tibial component 20, following removal of the primary components, may be as follows: 1) Apply bone cement, bone graft, or other structure to fill any area(s) of bone defect.

2) Prepare bone surface at appropriate distance from desired joint line. This may involve resection cuts performed with a surgical power saw, and additional cuts for the protruding sockets 12a of the sub-frame 12, performed with high-speed burr or slot drill tools.

3) Apply bone cement, such as poly methyl methacrylate (PMMA) to the bone surface or directly to the underside of the sub-frame 12.

4) Place the sub-frame 12 in position, and establish seating with several mallet strikes. Clear away extruded cement.

5) Mount the tibial component 20 on the sub-frame 12, either with cement or any other means.

6) Mount the extra-cortical fixation plates 14 to the sub-frame 12, through the associated fixation mechanism (e.g. slide the tabs into the sockets).

7) Attach the extra-cortical fixation plates 14 to the underlying bone with screws placed through the holes 16.

The fundamental steps for implantation of a femoral prosthesis supporting assembly 50 and femoral component 30 would be similar to the above steps.

Management of bone defects or bone loss Figure 14a depicts a tibia with postero-medial defect 140. Figure 14b depicts the same tibia with a supporting assembly 10 for a tibial prosthesis 20, with an additional wire mesh 144 containing bone graft that is filling the defect 140. Compacted morsellised bone graft may be used together with the joint prosthesis supporting assemblies 10, 50. A ductile or malleable wire mesh 144, or other containing structure or feature, such as a curved porous protrusion, may be used which fits different patients and different defect sizes. Such a structure 144 may be customised intra-operatively to fit around the patient's defect 140 and the applied bone graft, if the structure 144 is made of a ductile or flexible material. Fixation of the containing structure 144 to the cortex may be achieved by screws or other fasteners. The containing structure 144 or feature may also be integrated into the sub-frame component 12 or an extra-cortical fixation plate 14. Fixation of the containing structure 144 to the sub-frame 12 or other prosthesis assembly component may be achieved through a variety of means, such as screws, tapers, plugs, and bone cement.

Additionally, a trans-osseous string 150 or threaded wire may be used to keep the containing structure 144 in place after bone grafting, as shown in Figures 15a and 15b, which show the tibia shown in Figures 14a and 14b, further comprising a trans-osseous string 150. This would provide tension from across the diameter of the tibia or femur through a tunnel, as is known in the art.

Bone graft may not be deemed appropriate for filling bone defects, and one alternative could be the use of a Trabecular metal' augmentation block (a 3-D porous structure with pore sizes comparable to those of trabecular bone, which may be made from titanium). This would not require containment as bone graft does. This augmentation block may be of uniform radius and part of a cylindrical or spherical surface, and may be made integral with the subframe.

Features and benefits of the above-described joint prosthesis supporting assemblies Functional: * Reduces risk of stress-shielding around the implant, and associated bone 30 resorption Provides sufficient implant stability Eliminates risk of ped-prosthetic fracture adjacent to the tip of a fixation stem, and end-of-stem pain Easily modified (e.g., use of different extra-cortical fixation plate sizes/shapes) Easily removed if required for re-revision Intuitive implantation technique that is comparable to common operations: primary knee arthroplasty and bone fracture fixation.

Commercial/logistical: Ability to use a company's existing line of primary implant components Reduced time/cost of development and manufacture * Reduced overall cost and inventory of revision surgery equipment Ability to build custom-shaped plates to fit an individual patient's medial and lateral bone geometry (i.e., a patient-specific product) as well as a generic plate shape that will work just as well with the use of locking screws.

* Minimal resources required for training new users.

It will be appreciated that, although specific types of extra-cortical plates 14 are shown in conjunction with specific sub frames 12 and 32 and joint replacement components 20 and 30, the different types of extra-cortical plates 14 described herein and the different types of fixations between the extra-cortical plates 14, the various types of sub-frame 12 and 32 and the various types of joint replacement components 20 and 30 may be combined in any feasible combination, as would be understood by the skilled person.

None of the existing prior art in the field of revision knee implants demonstrates the features of a sub-frame in conjunction with extra-cortical fixation plates for a revision knee tibial or femoral prosthesis. The present design is considerably less-invasive and more bone-preserving than any of the prior art.

Claims (34)

1. CLAIMS1. A joint prosthesis supporting assembly comprising: a sub-frame for attachment to a joint replacement component and for attachment to a bone such that the sub-frame is disposed between the joint 5 replacement component and the bone; and at least one extra-cortical fixation element for securing the sub-frame to the bone.
2. 2. The assembly of claim 1, wherein the at least one extra-cortical fixation element comprises a plate which includes one or more holes for receiving screws for securing the at least one extra-cortical fixation plate to the bone.
3. 3. The assembly of claim 2, wherein at least one of the one or more holes is threaded so as to receive locking screws.
4. The assembly of claim 3, further comprising locking screws.
5. 5. The assembly of claim 2 wherein the screw holes are not threaded, and are configured to receive conventional bone screws which do not lock to the 20 plate.
6. 6. The assembly of claim 2 wherein the screw holes are elongated along the plate, such that the plate may slide in relation to the screw when the screw is fixed into the bone.
7. 7. The assembly of any preceding claim, wherein the at least one extra-cortical fixation element comprises an elongate portion for extending along an outside of the bone.
8. 8. The assembly of claim 7, wherein the elongate portion of the at least one extra-cortical fixation element is contoured to follow the surface of the bone, either solely in the longitudinal direction, or in both the longitudinal and circumferential directions.
9. 9. The assembly of any preceding claim, wherein the at least one extra-cortical fixation element is made on a patient-specific basis, such as by rapid prototyping, to match the shape of the patient's corresponding bone.
10. 10. The assembly of any preceding claim, wherein the at least one extra-cortical fixation element is tapered along its length, towards a narrow tip at the end away from the joint being revised.
11. 11. The assembly of claim 7 or claim 8, wherein the elongate portion of the at least one extra-cortical fixation element includes features to encourage bone in-growth.
12. 12. The assembly of any preceding claim, wherein the at least one extra-cortical fixation element is removably attachable to the sub-frame.
13. 13. The assembly of any of claims 1 to 11, wherein the at least one extra-cortical fixation element forms an integral part of the sub-frame.
14. 14. The assembly of any preceding claim, wherein the prosthesis supporting assembly is a revision prosthesis supporting assembly.
15. 15. The assembly of claim 14, wherein the joint replacement component is a primary joint prosthesis component.
16. 16. The assembly of any preceding claim, wherein the joint replacement component is either a tibial component or a femoral component.
17. 17. The assembly of any of claim 12 and any claim dependent thereon, wherein the sub-frame further comprises a socket for receiving a part of the at least one extra-cortical fixation element that is removably attachable to the sub-frame.
18. 18. The assembly of claim 17, wherein the at least one extra-cortical fixation element comprises a tab for receipt in the socket of the sub-frame.
19. 19. The assembly of any of claim 12 and any claim dependent thereon, wherein the sub-frame comprises a tab and the at least one extra-cortical fixation element comprises a socket for receiving the tab.
20. 20. The assembly of claim 18 or 19, wherein the cross-section of the tab matches that of the associated socket.
21. 21. The assembly of either of claims 18 and 19, wherein the tab and socket arrangement has a tapered geometry, such that they jam together on assembly.
22. 22. The assembly of any of claims 17 to 20, wherein the at least one socket comprises a three-sided slot that is open on one of: a surface of the sub-frame which, in use, is mounted to bone; and a surface of the sub-frame which, in use, is attached to the joint replacement component.
23. 23. The assembly of claim 22, wherein the three-sided slot is dovetail-shaped in cross section, with the narrower section of the slot being closer to the surface of the sub-frame which, in use, is mounted to bone than the surface of the sub-frame which, in use, is mounted to the joint replacement component.
24. 24. The assembly of any preceding claim, wherein the assembly comprises a second extra-cortical fixation element for securing the sub-frame to the bone.
25. 25. The assembly of claim 24, wherein second extra-cortical fixation element is removably secured to the sub-frame.
26. 26. The assembly of any preceding claim, wherein the sub-frame is comprised of more than one component.
27. 27. A sub-frame for use in a joint prosthesis, the sub-frame comprising: means for securely mounting a joint replacement component to the sub-frame; and means for securely mounting at least one extra-cortical fixation element to the sub-frame.
28. 28. The sub-frame of claim 27, further comprising means for accommodating fixation features of the joint replacement component.
29. 29. The sub-frame of claims 27 or 28, wherein the at least one extra-cortical fixation element is integrally mounted to the sub-frame as a single unit.
30. 30. The sub-frame of claims 27 or 28, wherein the at least one extra-cortical fixation element is removably attachable to the sub-frame and said means for securely mounting said removable at least one extra-cortical fixation element comprises a respective socket in the sub-frame for receiving a part of said removable at least one extra-cortical fixation element.
31. 31. The sub-frame of claim 30, wherein said socket comprises a three-sided slot that is open on one of: a surface of the sub-frame which, in use, is mounted to bone; and a surface of the sub-frame which, in use, is mounted to the joint replacement component.
32. 32. The sub-frame of claim 31, wherein the three-sided slot is dovetail-shaped in cross section, with the narrower section being closer to the surface of the sub-frame which, in use, is mounted to bone than the surface of the sub-frame which, in use, is mounted to the joint replacement component.
33. 33. The sub-frame of any of claims 27 to 32, wherein the means for securely mounting a joint replacement component to the sub-frame is adapted to accommodate a central box mechanism of the joint replacement component, which is a joint replacement component that requires sacrifice of the posterior cruciate ligament.
34. 34. A joint prosthesis kit comprising: the sub-frame of any of claims 27 to 33; a joint replacement component; and at least one extra-cortical fixation plate for secure attachment to the sub-frame.