Method and apparatus for implanting an acetabular cup
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a method and instrumentation for implanting an acetabular cup which is to be held in place by polymethyl-methacrylate (PMMA) bone cement. 
     2. Description of the Prior Art 
     It is known, for example, to provide a flexible flange around the rim of an acetabular cup to pressurize the cement, but this only occurs as the cup approaches its final position in the acetabulum. The flexible flange touches the acetabular margin tending to occlude cement flow. 
     Another device for pressurizing cement is known as the Exeter Acetabular Pressurizer. In this arrangement a water filled seal mounted on the end of a handle is used after the acetabulum has been filled with cement, but before the cup is introduced, the seal covers the acetabulum and the surgeon pushes on the handle thereby forcing the doughy cement in the holes, ridges and trabeculae of the acetabulum. When the instrument is removed and the pressure is released, blood flow can squeeze between the cement and the bone. 
     It is also known to provide cup positioning instruments with long pointers (vertical, horizontal and at right angles to the patient&#39;s center line of body symmetry) to enable the surgeon to control cup orientation reasonably well provided the pelvis does not move. Linear positioning of the cup is determined by eye in a medio-lateral and anterio-posterior sense. If the natural acetabulum is well formed, an eyeball judgment can be acceptable, but many acetabulae are deformed or eroded, in which case eyeball judgment can be confused. 
     A further method is to use what are usually referred to as acetabular spacers. These can be lugs or PMMA spacers (see for example, U.S. Pat. Nos. 4,883,490 and 4,955,325) fixed to the outside of the acetabular cup or can be press studs pushed into the bone base of the natural acetabulum. These prevent the cup from being pushed too deeply into the acetabular socket and can control medio-lateral and anterio-posterior positioning. However, they force some degree of centralization of the cup. If the surgeon chooses to place the cup eccentrically to make use of a particular area of solid bone support, acetabular spacers may not permit such a placement. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method and apparatus for overcoming some of the difficulties referred to above, to allow pressurization of the cement so that good holding is obtained and to provide a method of operation which will ensure accurate location of the cup in the socket. 
     According to the present invention a method of implanting an acetabular cup includes preparing an acetabular socket to receive the cup, placing bone cement in the socket before or after locating a loading tube in line with the socket and inserting the acetabular cup through the tube and into the socket by pushing it down the tube with an inserter to a predetermined position therein defined by a stop element acting on the inserter or attachment thereto. 
     Where the cup is provided with a flanged rim, the stop element can be arranged within the bore of the tube and the flange rim and the stop element can be constructed to allow the cup to pass the stop element but engage the inserter. Another method may include using a tube and an inserter having a stop element which engage an abutment on the tube located at a position spaced away from the end of the tube which is adjacent the socket. If desired, the position of the stop element can be adjusted prior to use. 
     The method may also include locating a flexible skirt assembly around the rim of the socket with the tube extending from the flexible skirt, and then inserting the acetabular cup through the tube and into the socket. 
     With these arrangements, it is possible to either fill the acetabulum with doughy cement and then reposition the skirt and tube assembly or the tube on the bone using markers to ensure that a previously chosen position and orientation are reestablished. Alternatively, the cement can be injected, for example by a cement gun, through the tube. The surgeon can then push the cup into the cement with the inserter to create a pressure and ensure that the cement fills all the openings in the socket. If the flexible skirt assembly is used it can be secured around the rim of the socket preventing the escape of cement and allowing it to be pressurized. The flexible skirt can be held in place by means of screws, pins and/or staples, or finger pressure alone. 
     The method also includes allowing some of the pressurized cement to escape from the socket and through a controlled orifice in the skirt. With this arrangement, the tube can be integral with the flexible skirt or it can be removably connected. Preferably, the flexible skirt is made from a synthetic plastics material and if it is to be left in place, could be polyethylene, polypropylene, polyurethane, or even a resorbable material for example, polyglycolic acid, or polylactic acid. 
     In order to locate the cup, the surgeon can control the release of cement through the orifice in the skirt, and he can push the cup with the inserter thereby feeling the desired level of cement pressure. If he is not able to reach the desired predetermined position when the stop elements are activated, he can release the pressure on the cement, allowing a little more cement to escape while the cup progresses further down the inside of the tube. An expandable tube can be used with a system for adjusting its diameter. 
     An advantage of the invention is that no spacers are required which may cause discontinuities in the cement mantle. The arrangement also enables the surgeon to accurately locate the position and attitude of the cup. An apparatus can also be included for positioning and orienting the tube or the tube and skirt on the bone into which the cup is to be implanted. 
     The invention also includes apparatus for use in the method set forth above which comprises a loading tube adapted for alignment with an acetabular socket, an inserter adapted to fit into the tube and stop elements which can act on the inserter or attachment thereto to limit movement of the inserter down the tube to a predetermined position therein. The stop element can be provided on part of the bore of the tube and act on an inner end of the inserter. This construction is particularly applicable when a flanged socket is used, the socket being provided with a gap in the flange. 
     In an alternative construction, stop elements can be provided on the inserter which engage an abutment on said tube located at a position spaced away from the end of the tube which is intended to be adjacent the acetabular socket when in use. This construction can be used with sockets having a flange or a plain upper rim. 
     These and other objects and advantages of the present invention will become apparent from the following description of the accompanying drawings, which disclose several embodiments of the invention. It is to be understood that the drawings are to be used for the purposes of illustration only and not as a definition of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings, wherein similar reference characters denote similar elements throughout the several views: 
     FIG. 1 is a diagrammatic cross-sectional view of one example of the apparatus showing the method of operation; 
     FIG. 2 shows a construction of a variable diameter tube; 
     FIG. 3 is a diagrammatic side view of another form of apparatus according to the invention; 
     FIG. 4 is a diagrammatic side view of another form of the apparatus which would be supplied as a series of designs with a wedge or sloping orientation to accommodate deformed acetabular socket rims; 
     FIGS. 5, 6 and 7 show how a step can be machined around the rim of the acetabulum showing how the apparatus can be used if the bony rim is eroded; 
     FIG. 8 shows in diagram form how an orientation guide can be provided; 
     FIGS. 9, 10 and 11 show three steps in performing the method and using the apparatus when using a trial cup; 
     FIG. 12 is a diagrammatic side view of apparatus similar to that shown in FIG. 3, but without the use of a skirt; and, 
     FIGS. 13, 14, 15 and 16 are isometric views of alternate forms of inserter. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the figures, there is shown a general form of the apparatus generally denoted as 100, an acetabular socket being indicated by reference numeral 1 into which an acetabular cup 2 is to be fitted. 
     In order to fit the cup into the socket 1, in which it is to be held by bone cement in this embodiment, a flexible skirt assembly 3 is utilized. This skirt assembly 3 is made as an integral unit from a synthetic plastics material, for example polyethylene, polypropylene, polyurethane, or even a resorbable material, for example polyglycolic acid, or polylactic acid. The assembly comprises a flexible skirt 4 which is shown with corrugations 5. The skirt has a central opening 6 and is provided with a loading tube 7, the inner bore of which is substantially the same diameter as the opening 6. The lower end of the loading tube is formed with an inwardly projecting abutment 8. The acetabular cup 2 has an annular flange 9 having a gap 14 which is dimensioned to allow the abutment 8 to pass through it. 
     To implant the cup 2, the acetabular socket 1 is first prepared in the usual way for example, by reaming. The flexible skirt assembly 3 is now placed in the position shown in full lines in the drawing and the skirt 4 is bent and fixed to the bone around the acetabular opening by pins, screws or staples indicated by reference numeral 10. The deformed position of the skirt when in its secured position is indicated in broken lines. If desired, the skirt 4 can be preformed to the required shape. The relative positioning on to the acetabular bone can be controlled by positioning guides, with or without fixing. 
     Prior to fastening it in position, however, the position and orientation of the tube 7 are determined during the early preparatory stages of the operation and once this has been determined, the skirt 4 can be trimmed if necessary and fastened in position. It will be seen that on the right-hand side of FIG. 1 the width of the skirt 4 has been trimmed down to assist in location. 
     The prepared acetabulum is now filled with cement through the piston tube 7, the cement being pushed into place by a manually operated inserter in the form of a piston 11. The piston is removed when the cement is partially set and in an adequately doughy stage. The cup 2 is immediately inserted down the tube 7 using the same or an alternate inserter and thus the cement is pressurized from the moment the cup touches it. The gap 14 in the cup flange 9 is aligned with the abutment 8. In the arrangement shown, the piston 11 is used to push the cup into place the piston descending until a peripheral portion of the edge of its front face 16 engages abutment 8 thereby preventing the surgeon from pushing the cup in too far and thus locating it at a predetermined position. 
     As will be seen from the figures, no spacers are required in the cement mantle which now surrounds the cup and is indicated by reference numeral 12. 
     A cement flow release arrangement is provided by the provision of an orifice 13 in the skirt 4. The hole in the skirt is occluded, for example by the surgeon&#39;s thumb, to thus allow the pressure to build up and then allow excess cement to escape. With the cup 2 in place and the cement fully cured the tube is cut away and the remainder of the skirt left implanted. Alternatively, the tube and skirt can be removed in their entirety. 
     In the arrangement described above, the piston tube is integral with the skirt 4 but if desired it could be made as a separate removable item. The use of the skirt 4 also enables the position and orientation of the cup to be determined prior to actual insertion, and allows the surgeon to place it precisely where he wants it in the acetabular socket. 
     It will, of course, be necessary to provide skirts in a choice of internal shapes and sizes to suit the chosen cup shape and size. The loading tube can also be provided in a similar choice of diameters, but an expandable tube could be set to the required diameter and fixed with a circumferential strap, an inserter, for example, in the form of a piston of appropriate dimensions being employed. Such a construction is shown in FIG. 2. The tube 7 is split and overlaps at its edges as indicated by reference numeral 107 and is held at the required diameter by a strap 108, a larger size piston 109 is also shown. 
     FIG. 3 shows another method and construction in which the cup can be accurately located in the socket and the same reference numerals are used to indicate similar parts to those shown in FIG. 1. With this construction, however, stop means are provided in the form of an adjustable disc 17 carried on a piston rod 18 of the piston 11. The disc 17 is held by a grub screw 19 passing through the disc and bearing against the piston rod 18. The screw 19 can be released to allow adjustment of the disc 17 on the rod 18. 
     As will be seen from FIG. 3, a cup 2 with a full projecting annular flange 9 is used but the position of the cup in the socket 12 is located by adjustment of the disc 17 which acts to limit the movement of the piston 11. As with the construction shown in FIG. 1, the tube 7 and the flange 3 can be removed completely after the cement has cured or the tube can be cut away and the flange 3 left in place. If desired, an adjustable loading tube as shown in FIG. 2 can be used. 
     FIG. 4 shows a construction of combined skirt 4 and tube 7 which can be used for deformed/eroded acetabular rims. The tubes and skirts can be supplied as a series of designs with a wedge or slope orientation to accommodate different angles of deformity, the slope angle in FIG. 4 is indicated by reference numeral &#34;A&#34;. 
     As can be seen in FIG. 4, in this construction the length of the lower end of the inner bore wall 15 varies about the diameter to provide the angled effect shown. A piston 11 and stop element are used as shown in FIG. 3, but if desired the stop element of FIG. 1 could be utilized, a suitable abutment being provided on the inner bore wall. An adjustable tube as shown in FIG. 2 can be used if desired. 
     The series of designs could, of course, also be provided in different sizes as well as in wedge/ramp angles. This construction can also be provided by arranging for the skirt 4 and lower part of the tube wall 15 to be deformable so that the desired shape and wedge/ramp angle is formed by the operating surgeon. The natural bony rim of the acetabulum is not a uniplanar circular feature and if desired, the surgeon can machine a step around the rim to receive a sealing feature or the like. 
     FIGS. 5 and 6 show how a trepanning cutter 20 is used to machine the rim depth to provide a step 21 inside the natural bony rim. The natural depression in the rim of the acetabulum is indicated by reference numeral 22. FIG. 7 shows how a skirt 23 can be used which provides an inwardly directed ledge 24 when placed on the step 21. The skirt extends upwardly and then outwardly and a removable loading tube 25 is placed in position on it. As will be seen from FIG. 7, the deformable skirt 23 is large enough to extend around and cover the natural dip 22 and prevent loss of cement when the piston 11 is operated. Once again, the stop element can be as shown in FIG. 1 or FIG. 3, and an adjustable tube can be used if desired. 
     The tube 25 can be arranged so that it is a relative push fit into the walls of the step 21 where they are covered by the upstanding portion of the skirt 23. It will be appreciated that a fixed tube 25 fixed to the skirts 4 could be used if desired. 
     In present surgical techniques, positioning and orientating an acetabular cup is sometimes achieved using a cup introducer/inserter with two alignment pointers, one of which points at 90° across the center line (axis of symmetry) of the patient and the other points vertically. This controls only orientation. Position and depth of insertion are chosen by the surgeon by eye. 
     In the construction shown in FIG. 8, the same reference numerals are used to indicate parts as shown in FIGS. 1 and 2 and an orientation guide 30 is provided which has a carrier 31 which surrounds the tube 7 and a long handle 32. The handle 32 carries adjustable alignment pointers 33, 34. The flexible skirt 4 can be steered by the handheld alignment and orientation guide 30 which is provided with a handle grip 35. The skirt is flexible enough to cope with deformed bony rims around the natural acetabulum and the alignment and orientation guide keeps everything in position and maintains the skirt 4 tight against the bone. 
     FIGS. 8, 9, 10 and 11 show an alternate method of employing the apparatus. An important task in this type of operation is to define the desired position and orientation of the cup. This is usually done by putting a trial or dummy implant in place. Theoretically, it could be determined before the operation using X-rays. 
     When using the present invention, it is necessary to place the tube and skirt such that the cup will eventually be positioned and tilted as required. The tube and skirt might be placed over a trial/dummy cup fixed and then the trial/dummy removed. Alternatively, the surgeon might remove the trial/dummy first and immediately fit the tube and skirt on the basis that his eyeball judgment of position and orientation is good enough. 
     FIG. 9 shows a prepared acetabulum 1 into which a trial acetabular cup 40 has been fitted. At this stage the surgeon chooses the position of center and orientation. 
     FIG. 10 shows how the surgeon can now fit the flexible skirt 4 around the trial cup 40 and the trial cup can then be removed, as indicated by arrow 41, without disturbing the skirt 4. After the trial cup 40 has been removed, leaving the flange in place, there is space for the loading tube (with or without an internally projecting abutment 8 according to the apparatus used) to be assembled to the skirt 4, for example as shown in FIG. 11, and for the implant equivalent to the trial cup 40 but necessarily smaller in size to be put in position. 
     FIG. 11 shows one way of carrying out the procedure referred to above by using a separable loading tube 42 which is pushed into place within the opening in the skirt 4, the flexible construction of the skirt enabling the tube to be pushed into place with a tight, sliding fit. If desired, a depth stop abutment ring 43 can be provided to control the insertion of the tube into the skirt, as shown in FIG. 11. Due to the deformities of the natural acetabulum, this abutment ring 43 will not necessarily PG,10 contact the skirt around the full periphery. If desired, the abutment ring 43 could be a tight sliding fit on the tube 42 so that it could be moved under pressure on the tube to provide a further adjustable depth stop. 
     The lower end of the tube can have an abutment to provide stop means, or an arrangement as shown in FIG. 1 can be used. The tube can be cut away once the cup has been inserted or can be simply dismantled from the cup and skirt. 
     In FIG. 12 another method of implanting the cup 2 is shown and the same reference numerals as those used in FIG. 3 are again used to indicate similar parts. With this method no skirt 4 is used. The surgeon first prepares the socket 12 in the usual way and then employs a trepanning cutter in the manner shown in FIG. 9 and 10 to provide a step 21 inside the natural bony rim. The outer wall 50 of the step 21 is dimensioned so that the outer wall 51 of the tube 7 is a push fit onto the step 21. 
     In order to implant the cup the surgeon first places the tube 7 in position on the step 21 and the cement is then pushed into place by manual operation of the piston 11. The piston is removed when the cement is partially set and in an adequately doughy stage. The cup is now immediately inserted down the tube 7 and the cement is pressurized from the moment the cup touches it. The cup is pushed further downwards into the socket 12 to the desired position provided by the adjustable disc 17. 
     A tube and piston construction as shown in FIG. 1 can alternatively be used in a similar manner. An adjustable tube 7 can be used, for example, of the kind shown in FIG. 1. Once again, it will be seen from the drawing that no spacers are required in the cement mantle 12. 
     FIG. 13 shows an alternative form of inserter which can be used for pushing the cup 2 into place. This inserter comprises a handle 55 secured to a rod 56 having a splayed end 57 secured to a ring 58. The diameter of the ring 58 is equivalent to the diameter of the annular flange 9 of the acetabular cup 2 with which it is to be used and this inserter is therefore employed with the construction shown in FIG. 1. The cup is pushed down the tube until the peripheral portion of the edge of the front face of the ring 58 engages the abutment 8. 
     FIG. 14 shows another alternate construction, again for use in the construction shown in FIG. 1, which comprises a tube 60, the outer diameter of which is similar to the inner diameter of the bore of the tube 7, and thus provides an end surface 61 which can be of substantially the same diameter as the annular flange 9 on the cup 2. A handle 62 is provided and the inserter is used in a similar way to that described with regard to FIG. 13. 
     FIG. 15 shows an alternative form of inserter which can be used with the type of construction shown in FIGS. 3 and 12 and consists of an elongate rod 65, the upper end of which is provided with a handle 66, and carries a disc 67 similar to the disc 17 shown in FIG. 1. The disc is again held by a grub screw 68 and the inserter is employed in a similar manner, the lower end 69 of the inserter resting against the inner bearing surface of the cup 2. If desired, a button as shown in broken lines 7, can be provided on the end 69 to spread the load over the cup surface. 
     FIG. 16 shows yet another alternative construction of inserter, again for use in the type of construction shown in FIG. 3 which comprises a tube 71 provided with a handle 72. The tube carries a descending ring of screw threaded holes 73 into which a screw threaded abutment 74 can alternatively be engaged. Thus the rising ring of holes 73 provide different height positions on the length of the tube 71. 
     The lower end 75 of the tube is of similar diameter of the annular flange 9 on the cup 2 so that when the inserter is used, the lower end 75 pushes the cup down the tube until the adjustable abutment 74 engages the upper end of the tube, in a similar manner to the way the disc 67 engages the upper end of the tube with the embodiment shown in FIG. 15. 
     The essence of the present invention is to provide continuous cement pressurization during the fitting of the socket and accurate location of the cup in the acetabular socket. 
     It will be appreciated that variations of the various embodiments described can be made by incorporating the different details and the method may vary by incorporating the different methods described together.