Abstract:
The invention relates to a prosthetic implant comprising a main body portion having a first surface for presentation to a receptive bone surface or into a bone cavity and a second surface for receipt of an articulating joint, and a lug on, and/or a bore in, the main body portion of the implant which is adapted for attachment of a filamentary member, such as a cable. The invention also relates to a surgical tool for gripping the implant, comprising an elongate body having a first end surface for bearing on the prosthetic implant and adapted for attachment of the tool to the implant, directly or indirectly by means of the cable.

Description:
This application claims priority from Provisional application Ser. No. 60/139,664, filed Jun. 17, 1999. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a prosthetic implant and to a surgical tool for gripping the prosthetic implant during a surgical procedure. 
     BACKGROUND OF THE INVENTION 
     The replacement of all or a part of the bone surface of an articulation with a prosthetic implant has become a common surgical procedure. The procedure requires the surgeon to exercise both precision and delicacy in the placement of the prosthetic implant. However, it is frequently necessary for the surgeon also to exercise a degree of force, sometimes a vigorous force, in order to situate the prosthetic implant in a desired location on the bone surface. 
     For example, in an operation to insert a prosthetic acetabulum in a patient&#39;s pelvis the surgeon first uses a reamer to grind a cavity of appropriate size in the patient&#39;s pelvis. The surgeon then inserts an acetabular cup into the cavity. By “appropriate size” is meant a size which is selected by the surgeon as being the most appropriate for that particular patient. Normally, it is desirable to retain as much of the original healthy bone surface as possible. 
     Commercially available acetabular cups are sold in a range of sizes to suit the needs of individual patients. Generally, acetabular cups are available in sizes of from 42 mm to 62 mm diameter, with 2 mm increments between neighbouring sizes. 
     Prosthetic acetabular cups generally require the use of an insertion tool to achieve correct positioning of the prosthesis in the patient&#39;s pelvic cavity. A typical insertion tool is described in U.S. Pat. No. 4,305,394 (Bertuck). 
     There are a number of different types of prosthetic acetabular cups. Firstly, cups made from polyethylene are available. Polyethylene cups are cemented into the acetabulum and require only light pressure to seat them in the cement. Secondly, there are cups which comprise a metal shell for insertion into the pelvic cavity and a polyethylene liner for articulation with the femur. Some polyethylene cups with metal shells are implanted without cement and rely on a jam fit between the metal shell and the patient&#39;s acetabulum. Often these metal shells have surfaces or coatings which encourage bone to grow into them over time. A typical coating material is hydroxyapatite. With this type of prosthesis, the polyethylene liner is snapped or screwed into the metal shell after the metal shell has been seated in the acetabulum to form the socket part of the patient&#39;s joint. 
     Cups which rely on a jam fit require a greater force to be applied via the insertion tool than is the case with cemented polyethylene cups. This force is usually applied as a direct impact into the acetabulum, for example by use of a mallet on the free end of the insertion tool. However, it may also be necessary to adjust the angular position of the cup in the bone or to take it out if positioned incorrectly. Therefore the insertion tool must positively grip the cup by some mechanical feature. This feature cannot impinge upon the outside of the metal shell because the insertion tool may then become trapped between the shell and the bone. Also there is little room around the rim of the shell because the wall thickness of the shell is generally kept to a minimum. Therefore the insertion tool usually grips on a mechanical feature on the inner hemisphere of the metal shell. An insertion tool of this type is described in International Patent Publication No. WO 94/21199 (Howmedica Limited). The mechanical feature is usually designed so as to cause minimum compromise to the function of the prosthetic hip joint. As a result it is often not strong enough for the impaction forces applied. This often results in damage to the insertion tool, or metal shell or both. 
     A third category of prosthetic hip joint exists which is manufactured entirely from metal so that the prosthetic articulation comprises a metal on metal joint. These are nearly always implanted without cement, relying on a jam fit in the acetabulum. With this type of cup the inner hemisphere is not a convenient place to locate a mechanical feature for the insertion tool to grip on. First, it would reduce the surface area of the prosthetic articulation. Secondly, it could cause damage to the highly polished concave metal against which the ball of the patient&#39;s femur or the ball head of a femoral prosthesis is to bear. 
     Therefore, the provision of means for attaching an insertion tool to a prosthetic implant so that the attachment is sufficiently robust to withstand the impaction and other forces to which it may be subject during insertion of the prosthesis and yet which does not compromise the structural strength or the articulating properties of the prosthesis itself remains a problem in the art. 
     Another disadvantage of conventional tools is that they must be manufactured in a range of different sizes to receive acetabular cups or sheaths of the different sizes referred to above. 
     The difficulty of gripping and positioning prosthetic implants applies not only to acetabular implants but more generally to implants such as the tibial and femoral components of a prosthetic knee. 
     U.S. Pat. No. 5,376,126 (Lin) discloses use of cords to assist in fastening a femoral implant to the upper end of a resected femur. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a prosthetic implant which does not suffer from one or more of the disadvantages of the prior art devices. It is further an object of the present invention to provide a surgical tool for use with such an implant. 
     Accordingly, the present invention provides a prosthetic implant comprising a main body portion, having a first surface for presentation to a receptive bone surface or to a surgically prepared bone cavity and a second surface for receipt of an articulating joint, and filamentary member attachment means adapted for attachment of a filamentary member to the implant. 
     The filamentary member can be a monofilamentary member, such as a Kevlar™ monofilament, or a multifilament member such as a Kevlar™ polyfilamentary thread or a metallic cable made, for example, from stainless steel. Such a cable may be, for example, from about 0.5 mm thick to about 2 mm thick. If desired, the cable can be coated with an inert material, such as polypropylene. Preferably the filamentary member is formed into a closed loop. 
     In one form of prosthetic implant according to the invention the filamentary member attachment means comprises a lug on the main body portion around which a filamentary member can be looped. Alternatively the filamentary member attachment means can comprise a bore in the main body portion through which a filamentary member can be threaded. In such a case it will normally be preferred to provide plurality of such bores, for example, three such bores. 
     The invention further provides such a prosthetic implant further comprising a cable attached to the main body portion of the implant by the filamentary member attachment means. Thus in a preferred embodiment of the invention the filamentary member attachment means comprises a plurality of bores in the main body portion and a filamentary member is threaded through each of the plurality of bores in turn, the free ends of the filamentary member being secured one to another so as to form a closed loop of filamentary member. The free ends of the filamentary member can be secured to one another by means of a sleeve having first and second ends, the sleeve snugly receiving a first looped portion of filamentary member located near a first end portion of the filamentary member and inserted from the first end of the sleeve so as to project as a first loop from the second end of the sleeve with the first end portion being passed through the first loop, and the sleeve further snugly receiving a second looped portion of filamentary member located near a second end portion of the filamentary member and inserted from the second end of the sleeve so as to project as a second loop from the first end of the sleeve with the second end portion being passed through the second loop, whereby upon tension being applied to the closed loop of filamentary member the first and second end portions of the filamentary member are firmly gripped by the first and second loops respectively. 
     The invention also provides an acetabular hip prosthesis for implantation in a surgically prepared socket in a hip of a patient comprising: 
     a generally cup-shaped body having: 
     an outer generally convex surface for contact with bone of a surgically prepared socket in a hip of a patient; 
     an inner generally concave surface; 
     a peripheral bridge portion whose surface extends between the outer generally convex surface and the inner generally concave surface; and 
     at least one bore which extends from a first opening in the surface of the bridge portion through an interior portion of the substantially cup-shaped body to a second opening in the surface of the bridge portion, the bore being adapted for receipt of a filamentary member therethrough for holding the prosthesis on the end of an insertion tool to facilitate insertion of the prosthesis into the prepared socket. 
     In such an acetabular hip prosthesis there is preferably provided a plurality of bores each of which extends from a first opening in the surface of the bridge portion through an interior portion of the substantially cup-shaped body to a second opening in the surface of the bridge portion, the bore being adapted for receipt of a filamentary member therethrough for holding the prosthesis on the end of an insertion tool to facilitate insertion of the prosthesis into the prepared socket. 
     Preferably the outer generally convex surface of the cup-shaped body is provided with a coating of hydroxyapatite and also is texturised to facilitate ingrowth of bone after implantation into a surgically prepared hip socket of a patient. 
     In another aspect of the invention there is provided a protected prosthetic implant comprising a main body portion, having a first surface for presentation to a receptive bone surface and a second surface for receipt of an articulating joint, filamentary member attachment means adapted for attachment of a filamentary member to the implant, and a protective cap for covering at least part of the second surface of the main body portion of the implant. The protective cap can be attached to the main body portion of the implant by means of a cable. In this case the cable can be tensioned so that the protective cap is tightly attached to the main body portion of the implant. 
     The protective cap is preferably made of a rigid material. It can be provided with a number of holes, through which the filamentary member is threaded for attachment of the protective cap to the prosthetic implant. 
     It will often be preferred for the protective cap to be a disposable protective cap. Alternatively it can be reusable after appropriate sterilisation. 
     The invention further provides a protected acetabular hip prosthesis for implantation in a surgically prepared socket in a hip of a patient comprising: 
     (i) a generally cup-shaped metal body having: 
     an outer generally convex surface for contact with bone of a surgically prepared socket in a hip of a patient; 
     an inner substantially part spherical cavity in the metal body, the cavity having a generally concave polished surface for receipt of a ball head on a femoral prosthesis and a periphery surrounding the cavity and defining a mouth; 
     a peripheral bridge portion whose surface extends between the periphery of the cavity and the outer generally convex surface; and 
     at least one bore which extends from a first opening in the surface of the peripheral bridge portion through an interior portion of the substantially part spherical body to a second opening in the surface of the bridge portion, the at least one bore being adapted for receipt of a filamentary member therethrough for holding the prosthesis on the end of an insertion tool to facilitate insertion of the prosthesis into the surgically prepared socket; 
     (ii) a cover member adapted to bridge the cavity, the cover having apertures therethrough which correspond to the first and second openings in the metal body and are adapted for passage of the filamentary member; and 
     (iii) a filamentary member threaded through the at least one hole in the metal body and through the apertures in the cover member, the free ends of the filamentary member being secured one to another to form a closed loop of filamentary member which secures the cover to the metal body. 
     Yet again the invention provides a protected acetabular hip prosthesis for implantation in a surgically prepared socket in a hip of a patient comprising: 
     (i) a generally cup-shaped metal body having: 
     an outer generally convex surface for contact with bone of a surgically prepared socket in a hip of a patient; 
     an inner substantially part spherical cavity in the metal body, the cavity having a generally concave polished surface for receipt of a ball head on a femoral prosthesis and a periphery surrounding the cavity and defining a mouth; 
     a peripheral bridge portion whose surface extends between the periphery of the cavity and the outer generally convex surface; and 
     a plurality of bores each of which extends from a first opening in the surface of the peripheral bridge portion through an interior portion of the substantially part spherical body to a second opening in the surface of the bridge portion, the bores being adapted for receipt of a filamentary member therethrough for holding the prosthesis on the end of an insertion tool to facilitate insertion of the prosthesis into the surgically prepared socket; 
     (ii) a cover member adapted to bridge the cavity, the cover having apertures therethrough which correspond to the first and second openings in the metal body and are adapted for passage of the filamentary member ; and 
     (iii) a filamentary member threaded through each of the plurality of holes in the metal body and through the apertures in the cover member, the free ends of the filamentary member being secured one to another to form a closed loop of filamentary member which secures the cover to the metal body. 
     In addition, the invention provides a protected acetabular hip prosthesis for implantation in a surgically prepared socket in a hip of a patient comprising: 
     (i) a generally cup-shaped metal body having: 
     an outer generally convex surface for contact with bone of a surgically prepared socket in a hip of a patient; 
     a cavity in the metal body for receipt of a liner of plastics material, the cavity having a periphery surrounding the cavity and defining a mouth; 
     a peripheral bridge portion whose surface extends between the periphery of the cavity and the outer generally convex surface; and 
     a plurality of bores each of which extends from a first opening in the surface of the peripheral bridge portion through an interior portion of the substantially part spherical body to a second opening in the surface of the bridge portion, the bores being adapted for receipt of a filamentary member therethrough for holding the prosthesis on the end of an insertion tool to facilitate insertion of the prosthesis into the surgically prepared socket; 
     (ii) a liner of plastics material secured in the cavity of the cup shaped metal body, the liner having an outer surface adapted to conform to the inner surface of the cavity in the cup-shaped metal body and a substantially part spherical inner surface for receipt of a ball head on a femoral hip prosthesis; 
     (iii) a cover member having a first face adapted to bridge the cavity, the cover member having apertures therethrough which correspond to the first and second openings in the metal body and are adapted for passage of the filamentary member ; and 
     (iv) a filamentary member threaded through each of the plurality of holes in the metal body and through the apertures in the cover member, the free ends of the filamentary member being secured one to another to form a closed loop of filamentary member which secures the cover to the metal body. 
     Preferably the cavity in the cup-shaped metal body is provided with an inwardly projecting lip surrounding the mouth of the cavity and the liner is provided with a peripheral ledge which is adapted to snap fit under the inwardly projecting lip of the cup-shaped metal body. It will also often be preferred that the cavity in the cup-shaped metal body is provided with a circumferential internal ridge below and adjacent the inwardly projecting lip and wherein the liner is provided with a corresponding circumferential groove on its outer surface which is adapted to receive the circumferential ridge. It will further often be desirable for the liner to be is made from high density polyethylene. In one particularly preferred form of the invention the cover member is provided on a second face opposite its first face with an axial blind bore for location of a corresponding axial locating peg on an insertion tool. In addition the cover member can be provided on its second face with an auxiliary bore whose axis is parallel to and offset from the axis of the axial blind bore, the auxiliary bore being adapted to receive a corresponding offset peg on an insertion tool to prevent relative rotation of the acetabular hip prosthesis and cover member about the axis of the axial locating peg of the insertion tool. 
     In another aspect of the invention there is provided a surgical tool for gripping a protected acetabular hip prosthesis for implantation in a surgically prepared socket in a hip of a patient, the protected acetabular hip prosthesis comprising a generally cup-shaped metal body which has an outer generally convex surface and a cavity defining a mouth, a protective cap over the mouth of the cavity, and a closed loop of filamentary member secured to the acetabular hip prothesis, the tool comprising an elongated body having a first end for engagement with the protective cap and adjustable filamentary member engagement means for engagement with and tightening of the closed loop of filamentary member so as to locate the protective cap against the first end of the tool and to draw the acetubular hip prosthesis towards the first end surface thereby to hold the prosthetic implant securely on the first end of the tool during surgical implantation of the acetabular hip prothesis, the tool and protective cap being removable after such surgical implantation by loosening the tension in the closed loop of filamentary member and then severing the closed loop of filamentary member to allow it to be disengaged from the implanted acetabular hip prosthesis. 
     Also provided in accordance with the invention is a surgical tool for gripping the prosthetic implant of the invention, comprising an elongate body having a first end surface for bearing on the implant and means on the tool adapted for attachment thereto of a cable from the implant. 
     The attachment of the tool to the prosthetic implant is achieved by means of a filamentary member, such as a cable. The attachment may be a direct attachment, in which case a cable is passed between the implant and the tool and secured to each by the respective attachment means. The attachment may be an indirect attachment, by which is meant that the cable may be used to attach a protective cover to the prosthetic implant and the tool may then be attached to the protective cover. Embodiments of the invention which make use of this indirect attachment will be described in more detail below. 
     The filamentary member attachment means on the prosthetic implant may comprise a lug on the main body portion of the implant around which a cable or other filamentary member can be looped. Alternatively, or also, the filamentary member attachment means may comprise a bore in the main body portion of the implant through which a cable can be threaded. 
     The adjustable filamentary member attachment means on the tool may comprise a mooring peg on the tool. The mooring peg may be mounted on a carriage which is movable axially within the body of the tool to tension the cable. Alternatively the adjustable filamentary member attachment means on the tool may comprise a spring loaded arm on the tool. The arm may be mounted on a carriage which is movable axially within the body of the tool to tension the cable or other filamentary member. 
     The cable or other filamentary member may be an endless cable which may be looped around or threaded through the filamentary member attachment means. Alternatively, a length of cable may be used and secured to each filamentary member attachment means by any convenient means, such as a tie, clip, or knot. Any convenient number of cables or other filamentary members may be used and any convenient number of attachment means on the prosthetic implant and on the surgical tool may be employed. For example, in one preferred embodiment of the invention, the prosthetic implant is an acetabular cup comprising three means. Each filamentary member attachment means is preferably spaced evenly from an adjacent filamentary member attachment means around the rim of the cup. The surgical tool may also comprise three adjustable filamentary member attachment means. Each of these adjustable filamentary member attachment means is preferably spaced evenly around the circumference of the tool from an adjacent filamentary member attachment means. The filamentary member is preferably an endless stainless steel cable which passes from a first filamentary member attachment means on the cup to a first corresponding adjustable filamentary member attachment means on the tool and then on to successive respective second and third attachment means on the implant and on the tool, returning in an endless loop to the first filamentary member attachment means on the cup. 
     The surgical tool and prosthetic implant of the invention have a number of significant advantages over the prior art. The use of a cable or other filamentary member to provide, directly or indirectly, means for securing the implant to the insertion tool means that any compromise in the structural and functional integrity of the implant as a result of the attachment is minimised. The diameter of the cable may be quite small, for example from about 0.5 mm to about 2 mm, and the means for attachment of the cable to the implant may be correspondingly small. For example, a bore of from about 0.5 mm to about 2 mm in the rim of a prosthetic acetabulum of cobalt chrome will not compromise the strength of the implant to any serious degree. Similarly, the provision of lugs having sufficient protrusion to allow a cable of from about 0.5 mm to about 2 mm diameter to be looped therearound on the outer surface of the acetabulum, adjacent the rim thereof, will not interfere to any serious degree with the positioning of the implant in the bone cavity. 
     The use of a cable or other filamentary member to secure the implant to the tool also has the advantage that the visibility of the attachment area by the surgeon is maintained at a significantly high degree. It is often important for the surgeon to be able to see clearly the position of the implant with respect to the bone surface or cavity. 
     The surgical tool of the invention may further be provided with means for tensioning the cable or other filamentary member after attachment of the cable to the tool to bring the first end surface of the tool to bear upon the implant. For example, the first end surface of the tool may comprise one or more flanges for bearing on the rim of an acetabular cup. The adjustable filamentary member attachment means on the tool may comprise one or more mooring pegs on the tool, the or each mooring peg being mounted on a carriage which is movable axially within the body of the tool to tension the cable. Alternatively the adjustable filamentary member attachment means on the tool may comprise a spring loaded arm on the tool, the or each arm being mounted on said carriage. Preferably, the carriage is carried on a screw-threaded member mounted axially within the body. 
     When used, the protective cover may be a disposable protective cover, or it may be reusable after appropriate sterilisation. The protective cover is preferably of a rigid material, for example polyethylene. Preferably, the protective cover is provided with a number of holes, through which the cable is threaded for attachment to the main body portion of the prosthetic implant. The main body portion of the prosthetic implant may comprise one or more lugs, around which the cable may be looped for attachment of the main body portion to the protective cover by means of the cable. Alternatively, or also, the main body portion of the prosthetic implant may be provided with one or more bores through which the cable may be threaded for attachment of the main body portion to the protective cover by means of the cable. 
     The means for attachment of the tool to the protective cover may comprise a shaped plug on one end of the tool, the plug being receivable within a corresponding socket provided on the protective cover. Preferably, a biased locating pin is provided on the tool, the pin being locatable within a corresponding opening provided on the protective cover, the biasing urging the pin into the opening to prevent rotation between the insertion tool and the implant so that rotational forces can be applied to the implant by means of the tool when the implant is positioned or impacted. 
     Alternatively, or also, the means for attachment of the tool to the protective cover may comprise means on the tool for attachment of the cable thereto. In this case, there may further be provided means for tensioning the cable after attachment of the cable to the tool to bring the first end surface of the tool to bear upon the protective cover. The means on the tool for attachment of the cable may comprise one or more mooring pegs on the tool, the or each mooring peg being mounted on a carriage which is movable axially within the body of the tool to tension the cable. Alternatively the means for attachment of the cable may comprise one or more spring loaded arms being mounted on said carriage. Preferably, the carriage is carried on a screw-threaded member mounted axially within the body. 
     When a protective cover is used in the implant of the invention, there is no necessity to manufacture surgical tools of different sizes to correspond to different sizes of acetabular cup or sheath. It is sufficient that the protective cover comprise a receiving area for the surgical tool. When a tensioned cable is used to grip the prosthetic implant the attachment provided is robust in lateral and axial directions, whilst allowing the surgeon to retain good visibility of the implant during the surgical procedure. 
     When a tensioned cable is used to effect direct attachment between the implant and the tool, the surgeon is allowed not only a good degree of rotational and axial control over the implant whilst inserting the implant, but also the surgeon is able to extract the implant if he is not satisfied with its orientation after initial insertion. 
     When any of the embodiments of the invention are applied to the implantation of a prosthetic acetabular cup comprising a polyethylene unit and a metal shell, it is not necessary for the polyethylene liner unit to be snapped or screwed into the metal shell after it has been seated in the acetabulum. A result of this is that the polyethylene liner unit may be factory fitted reducing the risk of damage to the unit during fitting. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an isometric view of a first form of surgical tool and, in disassembled state, a first prosthetic implant in accordance with the invention and a protective cover therefor. 
     FIG. 2 shows a cross-section along the axis of the surgical tool of FIG.  1 . 
     FIG. 3 shows an isometric view of the prosthetic implant of FIG.  1  and its protective cover when assembled. 
     FIG. 4 shows a side view of the main body portion of the prosthetic implant of FIGS. 1 and 3. 
     FIG. 5 shows a partial cross-section on line A—A of FIG.  4 . 
     FIG. 6 shows an isometric, partly cut away view of a second form of surgical tool and a second assembled and protected prosthetic implant according to the invention. 
     FIG. 7 shows a plan view from above of the main body portion of the prosthetic implant of FIG. 6 when disassembled from its disposable protective cover. 
     FIG. 8 is an isometric view of a third form of surgical tool in accordance with the invention in its closed condition. 
     FIG. 9 is an isometric view of the surgical tool of FIG. 8 in its open condition. 
     FIG. 10 is a perspective view on a slightly enlarged scale of a further form of prosthetic implant according to the invention for use with the surgical tool of FIGS. 8 and 9. 
     FIG. 11 is a further perspective view, taken, fro, a slightly different angle of the prosthetic implant of FIG.  10 . 
     FIG. 12 is a further perspective view of the prosthetic implant of FIGS. 11 and 12 with the protective cover raised. 
     FIG. 13 is a plan view of a further form of prosthetic implant according to the invention. 
     FIG. 14 is a cross section on line B—B of FIG.  13 . 
     FIG. 15 is a similar cross section to that of FIG. 14 with a protective cover in place to protect the articulating surface of the prosthetic implant of FIGS.  13  and  14 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, there is shown a surgical tool  1  having an elongate body  2  and a first end surface  3  for bearing on a prosthetic implant  4 . 
     Prosthetic implant  4  comprises a main body portion  5  which is an acetabular cup manufactured from cobalt chrome and intended for use as part of a metal on metal articulating joint. The inner articulating surface  5   a  of acetabular cup  5  is highly polished and is covered (when the implant is assembled in readiness for implantation in a prepared acetabular socket of a patient) by a disposable protective cover  6  which is manufactured from a plastics material such as polyethylene. 
     As will be apparent from the foregoing description, the provision of a disposable protective cover is a preferred feature of the invention. With the tool and prosthetic implant depicted in FIG. 1, the disposable protective cover  6  may, if desired, be omitted. In this case, end surface  3  of tool  1  would be provided with one or more flanges (not shown), or would itself be of sufficient diameter, to bear directly on the rim of acetabular cup  5  when assembled. 
     When prosthetic implant  4  is assembled in readiness for implantation, a cable  7  secures disposable protective cover  6  to acetabular cup  5  by means of lugs  8  on acetabular cup  5  and holes  9  through disposable protective cover  6 . 
     Cable  7  is made of stainless steel. However, any suitable material, such as a suitable synthetic fibre, may be used for manufacture of the cable  7 . The cable may be a continuous cable, as is shown in FIG.  1 . Compression clip  10  secures the two free ends of cable  7  together in the prosthetic implant of FIG.  1 . Alternatively, cable  7  may be formed from a series of individual loops or lengths of cable. 
     In the embodiment of the invention shown in FIG. 1, three cable loops are depicted. It will be readily understood by those skilled in the art that any other practical number of cable loops (e.g. 4, 5 or 6) could alternatively be used. 
     Tool  1  comprises mooring pegs  11  for attachment of cable  7 . Mooring pegs  11  are movably mounted in slots  12  on elongate body  2 . 
     Referring to FIG. 2, it can be seen that mooring pegs  11  are mounted on a carriage  13  which is carried on screw-threaded member  14 . By rotating knob  15  at the top of tool  1 , carriage  13  can be made to move up or down from its intermediate position, i.e. the position shown in FIG.  1 . 
     FIG. 3 shows prosthetic implant  4  when assembled with its protective cover  6  held in place by means of cable  7  so as protect polished surface  5 A. It is intended that the implant  4  will be supplied to customers in assembled form. However, implant  4  may alternatively be assembled by a surgeon about to use it. Disposable protective cover  6  is placed over the rim of acetabular cup  5  and each respective loop  17  of cable  7  is hooked over a respective lug  8  of acetabular cup  5 . The disposable protective cover  6  is secured to acetabular cup  5  by drawing cable  7  through holes  9  so as to tighten each loop  17  around its respective lug  8 . Disposable protective cover  6  has on its uppermost surface a raised portion or peg  19  which is receivable within a corresponding cavity  20  in the body  2  of tool  1 . Disposable protective cover  6  may further have on its underside, in the orientation depicted, a peripheral portion of reduced thickness surrounding a downwardly projecting portion for receipt within the rim  16  of acetabular cup  5 , but this is not shown in the drawings. 
     FIG. 4 shows a side view of the acetabular cup  5  and one of the lugs  8 . FIG. 5 is a cross-section on line A—A of FIG.  4  and shows lug  8  on acetabular cup  5 . 
     Referring back now to FIGS. 1,  2  and  3 , in use of the tool  1 , the surgeon will attach the assembled prosthetic implant  4  to the tool  1  simply by bringing the disposable protective cover  6  into contact with the end surface  3  of tool  1 , so that raised portion or peg  19  is received within cavity  20 , and looping each respective loop of cable  7  around each respective receiving mooring peg  11 . Rotation of knob  15  in a clockwise direction causes carriage  13  to slide upwardly and tension each loop of cable  7 . 
     Once the prosthetic implant  4  is securely attached to the tool  1 , the surgeon may then insert the prosthetic implant  4  into a surgically prepared acetabular cavity in a hip of the patient upon whom the surgical operation is being performed. When the prosthetic implant  4  is located to the surgeon&#39;s satisfaction, he simply rotates knob  15  in an anti-clockwise direction to release the tension on cable  7 . He then unloops cable  7  from mooring pegs  11  and withdraws tool  1  from the operating area. 
     To detach disposal protective cover  6  and cable  7  from acetabular cup  5 , the surgeon may either unhook the cable  7  from each lug  8  of acetabular cup  5  or he may simply cut cable  7  and pull the cut cable through the holes  9  of disposable protective cover  6 . 
     Referring now to FIG. 6, there is shown a second form of surgical tool  101  having an elongate body  102  and a first end portion  103  for engagement with a prosthetic implant  104 . 
     Prosthetic implant  104  comprises a main body portion  105  which is an acetabular cup manufactured from cobalt chrome and intended for use as part of a metal on metal articulating joint. The articulating surface of acetabular cup  105  is covered by a disposable protective cover  106  which is manufactured from a suitable plastics material, such as polyethylene. 
     A cable  107  secures disposable protective cover  106  to acetabular cup  105  by means of tunnels  108  in the rim  116  of acetabular cup  105  and holes  109  through disposable protective cover  106 . Tunnels  108  cannot be seen in FIG. 6 but their ends are visible in FIG. 7, which shows a plan view of the acetabular cup component  105  of prosthetic implant  104 . Each tunnel  108  follows a roughly C-shaped path extends the short distance between a corresponding pair of adjacent pair of adjacent tunnel ends as illustrated in FIG.  7 . Cable  107  is tensioned by means of a clip  107   a  to ensure tight registry between protective cover  106  and cup  105 . 
     Referring back to FIG. 6, the first end portion  103  is shaped to fit through a corresponding aperture  118  in disposable protective cover  106  and so that it can then be rotated through a quarter turn so as to secure tool  101  to protective cover  106  in bayonet and socket fashion. First end portion  103  is located on the end of a barrel  119  which is rotatable inside elongate body  102  by means of knob  120 . Locating pin  111  is provided to secure tool  101  with respect to the disposable protective cover  106  and is receivable within either of corresponding openings  120   a  in disposable protective cover  106 . Pin  111  is retractable from opening  120  by manual pressure applied to lift pin  111  towards its retracted position. Pin  111  is biased towards its unretracted position by means of a coil spring (not shown) housed within chamber  121 . 
     In use of the tool  101 , the surgeon will attach the assembled prosthetic implant  104  to the tool  1  simply by passing the end portion  103  through socket  118  of disposable protective cover  106  and rotating the barrel  119  of tool  101  through a quarter turn using knob  120  so that the tool  101  is secured to the prosthetic implant  104  by means of the bayonet and socket action. Then biased locating pin  111  can be caused to snap into a corresponding one of the openings  120   a.    
     Once the prosthetic implant  104  is securely attached to the tool  101 , the surgeon may then insert the prosthetic implant  104  into a surgically prepared acetabular cavity of the patient. When the prosthetic implant  104  is located to the surgeon&#39;s satisfaction, he simply pulls locating pin  111  upwardly to release it from opening  120  and rotates elongate body  102  by a quarter turn before withdrawing tool  101  from the operating area. 
     To detach disposal protective cover  106  and cable  107  from acetabular cup  105 , the surgeon must cut cable  107  and pull the cut cable through the holes  109  of disposable protective cover  106 . 
     Referring to FIG. 8, there is shown a third form of surgical tool  201  having an elongate hollow body  202  and a first end  203  for engagement with a prosthetic implant. Towards its other end, body  202  has a handle portion  204  which is surmounted by a rotatable knob  205  attached to the upper end of an internally threaded stub shaft (not shown). Below handle portion  204  is a barrel portion  206  within which is slidably located a carriage  207  having an upwardly extending externally threaded projection (not visible in FIG. 8) which is screw threadedly engaged with the internally threaded stub shaft of rotatable knob  205 . By rotating knob  205 , carriage  207  can be caused to slide axially up or down within barrel portion  206  as desired. At its lower end carriage  207  is provided with an axial bore which receives an upwardly projecting peg mounted in a bottom part  210  of tool  201 . 
     Carriage  207  is provided with three radial slots whose upper ends open into the axial bore in the bottom of carriage  207 . In addition each radial slot has associated with it a chordal slot which extends downwardly and inwardly into the carriage  207  so as to provide a pair of bearing surfaces one on either side of each radial slot. Arms  208  having a lower hooked first end  209  are pivotally mounted near their upper ends upon carriage  207  by means of stub members which bear on the bearing surfaces provided by the chordal slots. At a short distance the stub members, each arm  208  is pivotally biased outwardly away from carriage  207  by means of a coil spring trapped between arm  208  and carriage  207 . When carriage  207  moves down into barrel portion  206 , however, the top of the axial peg in the bottom part  210  impinges upon inward projections on the upper ends of arms  208  so as to cause the arms  208  to swing inwardly until the lower hooked ends  209  are received within cut-out portions  211 . On the other hand, as carriage  207  is caused to slide axially up within barrel portion  206 , the inward projections on the upper ends of arms  208  no longer impinge on the axial peg within bottom part  210  so that the springs can cause arms  208  to pivot outwardly upon their stub members. 
     Slots  212  in barrel portion  206  allow this pivoting movement to occur and rounded enlarged portions  213  permit the stub members to pass through during assembly of the tool  201 . 
     At the first end  203  of tool  201  there is an axial peg  214  and an axially offset auxiliary peg  215 . 
     FIGS. 10 to  12  illustrate an acetabular implant suitable for use with the surgical tool  201 . This comprises an acetabular cup  216  and a protective cover  217  made from high density polyethylene. Acetabular cup  216  is made of cobalt chrome with a highly polished inner bearing surface  218  and an outer texturised surface  219  which is intended to promote bone ingrowth following implantation in a patient&#39;s hip and which is coated with hydroxyapatite. In addition, the outer surface  219  is provided with two pairs of ribs  220  which are intended to bite into the bone surrounding the patient&#39;s acetabular cavity during the implantation operation so as to prevent unwanted rotation of the implant, particularly during the early stages of the post-operative recuperation period. 
     Protective cover  217  is retained in place on acetabular cup  216  by means of cable  221 . This passes through holes  222  in protective cover  217  and through tunnels (not shown) which follow a substantially C-shaped path through the wall of the acetabular cup  216 . Protective cover  217  has on its upper side an axial bore  223  to receive peg  214  of surgical tool  201  and also an axially offset bore  224  to receive peg  215  of surgical tool  201 . 
     The ends of cable  221  are secured to one another by means of sleeve  225 . As can perhaps best be seen from FIG. 12 a first end portion is threaded upwards through sleeve  225 , is then doubled back on itself to form a first loop projecting upwardly from the upper end of sleeve  225  and passed back down through sleeve  225 , and is led around sleeve  225 , before the first free end is passed through the first loop. In a similar way the other end portion of the cable  221  is passed downwardly through sleeve  225 , is doubled back on itself to form a second loop projecting downwardly from the lower end of sleeve  225  and passed back up through sleeve  225 , and is led around sleeve  225 , before the second free end is passed through the second loop. The four portions of cable passing through sleeve  225  are a snug fit therein. Up applying tension to the cable  221 , the first and second loops are tightened and so tighten their grip upon the respective free ends of cable  221  to secure the cable  221  into a continuous loop. 
     In use, a surgeon takes surgical tool  201  with the carriage  207  in its end position nearest end  203  so that arms  208  are fully retracted and fits peg  214  into bore  223  and peg  215  into bore  224 . Next he loops the three upstanding loops of cable  221  each over a respective hooked end  209  of an arm  208 . He then rotates knob  205  clockwise to cause carriage  207  to slide axially up barrel portion  206 . In so doing the upper ends of arms  208  come out of engagement with the axial peg of bottom part  210  and the springs bias arms  208  outwardly towards the position of FIG.  9 . In so doing the hooked ends  209  pull upwardly on cable  221  and tighten it. When the knob can no longer be readily rotated by hand the cable  221  is taut and holds acetabular cup  216  and its protective cover  217  firmly on the end  203  of tool  201 . 
     After the acetabular cup has been seated in the surgically prepared cavity in the patient&#39;s hip to the satisfaction of the surgeon, he then rotates knob anticlockwise to move carriage  207  back to its start position, thereby slackening the tension in cable  221  and enabling him to disengage tool  201  from the operative area. When the tool  201  has been removed, he cuts cable  221  and withdraws it from the tunnels in acetabular cup  216  through which it has been threaded and removes protective cover  217  in readiness for insertion of the ball on the patient&#39;s femur (or the ball head of a femoral prosthesis) into the acetabular cup  216  and closure of the surgical wound. 
     Instead of using a stainless steel cable  221  it is alternatively possible to use a synthetic fibrous material of high strength such as Kevlar™ monofilament. 
     FIGS. 14 to  16  illustrate another form of acetabular implant  302  according to the invention. This comprises an outer metal cup member  302 , which is made, for example, of cobalt chrome, and an inner liner  303  made of high density polyethylene which is intended to provide the bearing surface of the ball head of a femoral prosthesis. Surrounding the upper rim of the inner surface of cup-member  302  is an inwardly projecting lip  304  below which is an adjacent circumferential ridge  305 . Inner liner  303  has a peripheral ledge on its upper face which substantially conforms in dimensions to lip  304 . It also has a circumferential groove corresponding to ridge  305 . Liner  303  is press fitted in the factory into acetabular cup member  302  with the circumferential ledge on liner  303  engaged under the lip  304  and with ridge  305  engaged in the corresponding circumferential groove in liner  303 . 
     Three short tunnels  306  each of which follows a generally C-shaped path are formed between the three adjacent pairs of tunnel openings which are visible in FIG.  14 . These tunnels are provided for the passage of a closed loop of cable, similar to cable  221 , for retaining a protective cover  307  which is intended to protect the inner surface of liner  303  during implantation by a surgeon. This protective cover  307  has an axial bore  308 , similar to bore  223  of protective cover  217 , and also an axially offset bore (not shown) similar to bore  224  of protective cover  217 , to enable the implant  301  to be used with the surgical tool  201 . 
     Instead of using one continuous loop of cable  221 , it is alternatively possible to use three separate continuous loops of cable or Kevlar™ monofilament, one for each tunnel  306 .