Centraliser

Abstract: A centraliser (1) is provided with a hollow body (2) and a plurality of equally spaced arms (4). The arms (4) extend in the form of cup handles from the hollow body (2). Each arm (4) has a variable cross-section along its length with the cross-section becoming increasingly ovoid towards one end of the arm (4).

Description: This invention relates to a centraliser for a prosthetic implant, in particular a prosthetic femoral stem for implantation in a patient's femur for hip replacement surgery. Centralisers or spacers are provided for fitting to the distal end of a femoral hip replacement stem in order to keep the implant stem away from the internal surface of the cavity of the bone in which the stem is to be inserted. In the case of stems which are cemented in the bone cavity, there is a space between the stem and the internal surface of the cavity of the bone in which bone cement is placed. Controlling the position of the stem within the surrounding bone cement mantle is vital to the long-term survivability of the replacement joint. Cement can be deposited in the bone cavity and the stem with the attached centraliser is then inserted. It is important to try to obtain an even and intact cement mantle around the stem. Known centralisers are in the form of caps which fit over the distal end of the stem and centralisers which are fixed inside the drilled end of a stem. Centralisers are also known, for example as described in US patent No. 4,658,351, which are of ring form which can have a tapered inner surface corresponding to the tapered surface of the distal end of the femoral stem on which the centraliser is located. US patent No. 3,793,650 describes a centraliser or spacer which has spring members which extend from the stem for contact with the wall of the bone cavity. European patent No. EP04274448 describes a centraliser or spacer in the form of a cap for insertion on the end of a hip stem with fins or wings extending outwardly from the cap which are adapted to fold circumferentially and inwardly towards the body portion of the cap. European patent application No. 0554987A discloses centralisers which are attached to the femoral stem via a sheath. The centraliser either has abutments in the form of tangentially projecting wings (Figure 4) or it has radially and upwardly extending arms (Figures 5 and 6). European patent application No. 0483057A describes a centraliser shaped in the form of a pyramid having an equilateral triangle as base. The triangular pyramid base is attached to the femoral stem and the pyramid apex faces the bone cavity. French patent application No. 2772595A discloses a centraliser comprising a central portion for attaching to a prosthesis (such as a femoral stem) and flexible branches projecting from the central portion. Each branch has a tip which presses against the prosthesis and a central bend which is applied against the inner surface of the bone cavity. UK patent application No. 2104391A describes a plastics spacer comprising a hollow body for receiving the prosthesis and resilient fingers extending radially and upwardly from the body. The fingers may be linked to the body by thin webs forming wings. According to the present invention, there is provided a centraliser for use on a prosthetic component to be inserted in a bone cavity, the centraliser comprising a hollow body with a plurality of spacer means projecting from the body, wherein the spacer means are in the form of arms extending outwardly from the hollow body and having a variable cross-section along the length of the arms. The hollow body may have one open end and be in the form of a cap. Alternatively, the hollow body may have two open ends and be in the form of an elongated ring. The hollow body may be in the form of a truncated cylinder. Preferably, each arm has a first end attached to the hollow body. A second end of each arm may also be attached to the hollow body. Preferably, the arms have a cross-section which varies from a circle becoming increasingly ovoid along the length of the arms. Preferably, the plurality of arms are equally spaced circumferentially around the hollow body. There may be three equally spaced arms. Preferably, the arms are attached to the hollow body such that they extend parallel to the central axis running through the hollow body. An embodiment of a centraliser in accordance with the present invention is now described by means of example only, with reference to the drawings in which: Figure 1 is a perspective view of a centraliser in accordance with the present invention; Figure 2 is a cross-section of an arm of the centraliser of Figure 1 through line A-A; Figure 3 is a cross-section through an arm of the centraliser of Figure 1 through line B-B; and Figure 4 is a cross-section of an arm of the centraliser of Figure 1 through line C-C. Referring to the drawings, a centraliser 1 is provided with a hollow body 2 and a plurality of arms 4 extending outwardly from the body 2. The body 2 is of elongated annular form and may be in the form of a truncated hollow cylinder. The body 2 has an outer surface 22 and an inner surface 20. The inner and outer surfaces 20, 22 are joined by an upper edge 6 of the body 2 and a lower edge 10 of the body 2. The upper edge 6 of the body 2 is generally circular and has a greater diameter than the lower edge 10 of the body which is also generally circular. In this example, the centraliser 1 has three arms 4 equally spaced around the body 2. Each arm 4 has a first end 24 which is attached to the body 2 adjacent the upper edge 6 of the body 2. The second end 26 of each arm is attached adjacent the lower edge 10 of the body 2. Each arm 4 curves outwardly from the body 2 to form a D-shape. The arms 4 extend parallel to a central axis through the body 2. The arms 4 resemble cup handles equally spaced around the body 2. Each arm 4 has a cross-section which varies from the first end 24 to the second end 26 of each arm 4. A top portion 14 of each arm 4 has a generally circular cross-section which provides stiffness to the arm 4. A mid-portion 16 of each arm 4 has an increasingly ovoid cross-section and a lower portion 18 of each arm 4 has a thin ovoid cross-section. The change in cross-section along the arms 4 increases the flexibility of the arms 4 towards the second end 26 of the arms 4. It is an aim of the present invention to provide a centraliser which fits a range of prosthetic hip stems with no provision made for stem subsidence within the centraliser. The centraliser 1 is formed of a plasticised PMMA material which is capable of bonding with a surrounding PMMA cement mantle. In the present example, the centraliser 1 is designed to fit in bone cavities of 10mm-20mm diameter at 2mm increments. The size of the centraliser 1 is linked with the stem size and the outer form linked with the distal reamers to be used with the cemented stem. The centraliser 1 wedges on a distal end of a femoral stem with the femoral stem contacting the inner surface 20 of the centraliser 1 at about 10mm-15mm from the distal end of the stem. The centraliser 1 can fit into bone cavities ranging from the stem size plus a 2mm cement mantle to the stem size plus a 4mm cement mantle. This variation in size can arise if a surgeon decides to undersize the stem for a prepared bone cavity. The centraliser 1 adapts to a varying size of bone cavity by having flexible arms 4. The flexibility of the arms 4 is restricted to prevent undersizing of the cement mantle, in other words, the minimum cement mantle should be 2mm at the distal end. The centraliser 1 of the present invention achieves the above requirements by having a plurality of arms 4 which give constrained flexion due to the variable cross-section along the arms 4. As the arms 4 are constrained at both ends 24, 26, this ensures that the centraliser 1 provides resistance to insertion in undersized medullary cavities. This ensures that surgeon's choice of canal and stem size is controlled and that the stem size to canal size is matched. However, should the surgeon require to oversize the stem for a given medullary cavity, he can do so by overcoming the resistance to the undersized arms 1 of the centraliser 1. Improvements and modifications can be made to the foregoing without departing from the scope of the present invention.

Claims: A centraliser (1) for use on a prosthetic component to be inserted in a bone cavity, the centraliser (1) comprising a hollow body (2) with a plurality of spacer means projecting from the body (2), wherein the spacer means are in the form of arms (4) extending outwardly from the hollow body (2) and having a variable cross-section along the length of the arms (4), characterised in that each arm (4) has a first end (24) and a second end (26) attached to the hollow body (2), and a mid-portion not attached to the hollow body (2). A centraliser (1) as claimed in claim 1 wherein the hollow body (2) has one open end and is in the form of a cap. A centraliser (1) as claimed in claim 1 wherein the hollow body (2) has two open ends and is in the form of an elongated ring. A centraliser (1) as claimed in claim 3 wherein the hollow body (2) is in the form of a truncated cylinder. A centraliser (1) as claimed in any of the preceding claims wherein the arms (4) have a cross-section which varies from a circle becoming increasingly ovoid along the length of the arms (4). A centraliser (1) as claimed in any of the preceding claims wherein the plurality of arms (4) are equally spaced circumferentially around the hollow body (2). A centraliser (1) as claimed in any of the preceding claims wherein there are three equally spaced arms (4). A centraliser (1) as claimed in any of the preceding claims wherein the arms (4) are attached to the hollow body (2) such that they extend parallel to the central axis running through the hollow body (2). A centraliser as claimed in any of the preceding claims wherein there are three equally spaced arms.

Inventors: HIERNARD BRUNO, REVIE IAN CRAWFORD, HIERNARD, BRUNO, REVIE, IAN CRAWFORD
Applicants: JOHNSON & JOHNSON MEDICAL LTD, JOHNSON & JOHNSON MEDICAL LIMITED
Application year: 1999
Main classification: A61F2
IPC codes: A61F2