Abstract:
An acetabular rim cutter has an annular cutting face provided on a rotatable carrier. A coupler is provided for attaching the carrier to a rotational power source. A guide element is axially aligned with the cutting face to locate and align the cutting face on the acetabulum with which it is to be used. Preferably, the guide element is axially movable in relation to the cutting face and a resilient element is provided for biasing it axially away therefrom.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates to an acetabular rim cutter which can be used for cutting the outer rim of an acetabulum to form an annular seating surface to receive a flanged acetabular cup or a rim cutter and acetabulum reamer which has the dual function of reaming an acetabulum and then cutting its rim. Such a flanged cup is sold by Stryker Corporation as the Exeter™ Contemporary Flanged Cup. 
   Rotary cutters are known but use of such rotary cutters for treating the rim of an acetabulum is difficult because the surgeon has difficulty in locating the cutter with sufficient accuracy. 
   There are also difficulties in cutting the rim of an acetabulum so that the annular seating surface is a predetermined distance from the base of the acetabulum and that the part-spherical bearing surface of the cup is at a desired depth from the cut seating surface so that there is a regular seating surface. The present invention is intended to overcome some of the difficulties referred to above. 
   SUMMARY OF THE INVENTION 
   According to one aspect of the present invention an acetabular rim cutter comprises an annular cutting face provided on a rotatable carrier. A coupler is provided for attaching the carrier to a rotation power source. A guide element is axially aligned with the cutting face to locate and align the cutting face on the acetabulum with which it is to be used. 
   Preferably the guide element is axially movable in relation to the cutting face, and a resilient element is provided for biasing it axially away therefrom. 
   Thus, the guide element can be of a part-spherical shape which locates in the acetabulum and ensures that the axis of the annular cutting face is correctly aligned. 
   In a preferred embodiment the guide element is freely rotatable in relation to the rotatable carrier and it can be made from, for example, a synthetic plastics material. 
   The annular cutting face is preferably provided with radial cutting teeth. 
   An operating handle can be provided which is axially aligned in relation to the rotatable carrier and which is free to rotate in relation thereto thus providing a method for the surgeon to operate the equipment. 
   The rotatable carrier can be connected to an extension the end of which is adapted to receive the rotation source and on which the operating handle is located. 
   A depth cutting control system is also preferably provided which act to determine the depth and cut in relation to the base of the acetabulum with which it is to be used. 
   In order to provide a modular construction a number of guide elements can be provided which are used respectively to provide different depths of cut and which can be of different diameters to appropriately suit the acetabulum which is to be cut. 
   The apparatus can also include an acetabulum surface reamer which is adapted for attachment to the rotation means in place of the rotatable carrier and the guide element. 
   These aspects of the invention are provided by an acetabular cutter for preparing an acetabulum which has a hollow handle, a drive shaft rotatably mounted within the hollow handle with the shaft having a rotating head at a first end and a second end coupled to a power source. The power source may be an electrical or pneumatic powered hand piece. An acetabular rim cutter is connected to the drive shaft rotating head at a first end of the head with the cutting having teeth thereon concentric about a rotational axis of the head a preferably lying in a plane such as to be able to cut a circular groove around the rim of the acetabulum. A guide element is mounted on the handle for engaging the acetabulum and extending beyond the first drive shaft end. The guide element is axially movable with respect to the shaft of the drive axis and is spring-biased against the acetabulum so that the guide element may be placed within the acetabular cavity and the rim cutting element moved axially with respect hereto. The guide element is preferably dome shaped. Both the dome element and the rim cutter may be removably connected to the rotating head of the drive shaft so that cutters and elements of different sizes can be utilized. These cutter elements and drive tools may be supplied in a kit which includes one or more rotary drive tools, a plurality of dome shaped cutting elements which are used for preparing the acetabulum and a plurality of rim cutting elements to cut a groove around the cavity prepared by the dome shaped cutting elements. A plurality of rotary drive tool handles can be used of different lengths and even of different configurations such as being S-shaped or curved. A plurality of dome shaped members may also be provided. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention can be performed in various ways but one embodiment will now be described by way of example and with reference to the accompanying drawings in which: 
       FIG. 1  is a side elevation of an acetabular rim cutter according to the present invention; 
       FIG. 2  is a cross-sectional view on the lines  2 - 2  of  FIG. 1 ; 
       FIG. 2A  is a cross-sectional view of  FIG. 1  with the rim cutter spaced from the guide element; 
       FIG. 3  is a cross-sectional end view on the line  3 - 3  of  FIG. 2 ; 
       FIG. 4  is a side view of the rim cutter with the rotatable carrier removed; 
       FIG. 5  is an isometric view of the rim cutter with the rotatable carrier removed; 
       FIG. 6  is an enlarged side view of part of a first end of the rim cutter with the rotatable carrier removed; 
       FIG. 7  is an end view of the rotatable carrier showing the annular cutting face; 
       FIG. 8  is an enlarged diagrammatic side elevation of locking apparatus for determining the depth of cut; 
       FIG. 9  is a view similar to  FIG. 7  with the parts in a second position; 
       FIG. 10  is a partial side view showing the apparatus ready for use; 
       FIG. 11  shows a prosthetic acetabular cup in place in a reamed acetabulum; 
       FIG. 12  is a side view of an acetabulum reamer which can be attached to the rotation system in place of the rotatable carrier; and, 
       FIG. 13  is a bottom plan view of the acetabulum reamer shown in  FIG. 11 . 
   

   DETAILED DESCRIPTION 
   As shown in the drawings an acetabular rim cutter according to a preferred embodiment of the present invention comprises an annular cutting face  2  which has radial cutting teeth  3  and which is provided on a rotatable carrier  4 , the construction of which is most clearly shown in  FIG. 6 . The carrier  4  is attached to a hollow drive spindle  5  which has a triangular shaped end piece  6 , most clearly shown in  FIGS. 4 ,  5  and  6 . The carrier  4  has a hexagonal shaped opening  7  with corners  8  and which is dimensioned so that the distance of the wall between a first corner  8  and the next but one corner  8  circumferentially spaced around the hexagon is slightly greater than the length of the sides of the triangular end piece  6  between its corners  10  which are chamfered to accommodate the corners  8  of the hexagon. As best seen in  FIG. 6  each corner  10  has a re-entry slot  10   a  one wall of which is provided by a corner  10  and the other of which is a projecting lug  11 . The width of the slot  10   a  is adapted to receive and locate the wall of the carrier between two of the corners  8 . Thus the carrier  4  can be passed over the triangular end piece  6  and then angularly rotated so that each of the corners  10  of the triangular end piece are aligned with one of the sides of the hexagon  7  between two of the corners  8 . Movement in axial directions is prevented by the walls of the slot  10   a.    
   Extending over the spindle  5  is a locking sleeve  12  which is biased towards the carrier by a spring  13  and the end of which is of similar shape to the triangular shaped end piece  6  but its corners  12   a  have flats  12   b  which are dimensioned to engage three of the sides of the hexagon  7  of the carrier  4 . The corners  12  also have slats  14  which can engage the abutment  11 . In  FIG. 6  the sleeve is in its withdrawn position adjacent the spring bias and ready to receive the carrier  4 . With the carrier in position the spring  13  will push the sleeve in the direction of the arrow A so that the flats  12   b  will engage the carrier so that it will rotate with the sleeve  12 . 
   Located on the hollow drive spindle  5  is an extension shaft  15  on which is a freely rotatable operating handle  16  which has a pair of arcuate prongs  17 , the inner diameters of which are less than the diameter of the extension shaft  15 , and which engage a portion  26  of the shaft  15  which is of reduced diameter. The end of the handle and the prongs  17  are held in position by the resilient effect. The prongs  17  are also held by a retainer  27   
   The outer end of the extension shaft  15  carries a boss  18  for connecting it to a rotary power drive of known type and which is indicated by reference letters PD in  FIG. 2 . 
   Also mounted on the extension shaft  15  is a sleeve  19  which extends into the locking sleeve  12  and retains the spring  13 . 
   The hollow drive spindle  5  has a bore  20  in which is mounted a location shaft  21  the outer end  22  of which is screw threaded for attachment to a guide element  23  which has a part-spherical shape. The inner end  24  of the bore  20  has a location groove  25  which can be engaged by a resiliently biased plunger  28  which is carried in the triangular end piece  6  (as shown in  FIG. 3 ). 
   The shaft  21  is biased to the right, as shown in  FIG. 2 , by means of a compression spring  30  mounted between the part-spherical guide element  23  and the hollow drive spindle  5 . In  FIGS. 2 and 2A  the compression spring  30  is shown in a compressed position with the inner end  24  of the shaft  21  pushed into the full length of the bore  20 . With the compression spring  30  in its expanded position the guide element  23  is pushed out to the position shown in chain lines on  FIG. 2  in the direction of the arrow  31 . 
     FIGS. 8 and 9  show how the spring loaded plunger  28  operates. The plunger has an engagement ball  32  which is pressed against the location shaft  21 . The shaft  21  is first pushed into the bore  20  until the groove  25  is engaged by the plunger ball  32  which acts to hold it in position. The guide element is thus held in the position shown in chain lines in  FIG. 2 . When pressure is applied via the handle  16  and with the guide element in the acetabulum the shaft  21  moves up the bore and the ball  32  merely presses against the side of the shaft. The movement of the bore is against the pressure of the spring  30 . 
   In an alternative construction (not shown) the spring loaded plunger  28  can be replaced by a resilient O-ring which can engage the groove  25  and can be mounted on the bore  20 . 
   A location guide  35  of known kind is provided on the handle  16 . 
     FIGS. 10 and 11  show how the acetabular rim cutter, according to the present invention, can be used. The intention is to trim the outer edges  39  of an acetabulum  40  by machining an annular seating surface  41  as shown in  FIG. 11 . A flanged acetabular cup having an outwardly projecting flange  43  can now be seated on the annular seating surface  41 . In the arrangement shown the cup is a close fit in the acetabulum but it will be appreciated that if, for example, the cup was to be held by cement then there could be an appropriate gap and spacers between the cup and the bone, as shown by chain lines  45 . 
     FIG. 10  shows the operative end of the acetabular rim cutter according to the present invention and the same reference numerals are used to indicate similar parts to those shown in the other FIGS. The guide element  23  carried on the shaft  21  is in its extended position as caused by the spring  30  being expanded and it will be seen that the guide element  23  is axially aligned with the annular cutting face  2  by the rotatable head or carrier  4 , although the guide element  23  is free to rotate in relation to the carrier  4 . With power applied through the shaft  15  the carrier is rotated and is guided by the guide element  23  to the appropriate position to cut the annular bearing surface  41 . The precise angles of cut can be determined in known manner using the guide  35 . 
   The depth of cut of the annular bearing surface  41  is determined by the length of the shaft  21  which can be arranged so that the depth of the cut in relation to the base of the acetabulum  44  can be previously determined according to the length of the shaft  21 . Thus, the surgeon can be provided with a series of cups of different shapes and shaft lengths to provide a modular construction which can be used in different circumstances depending upon the requirements of the type of acetabular cup used and the surgical implications. 
   The length of the shaft  21  therefore acts as depth cutting control means. 
   The guide element  23  can be made of any suitable material, for example metallic material. 
     FIGS. 12 and 13  show an acetabulum reamer which can be used with the acetabulum rim cutter. The reamer  50  has a dome shaped portion  51 , the outer surface of which is substantially part-hemispherical, and which is provided with a number of openings  52  which can be of any suitable shape but are shown as circular. These openings are provided with sharp corners  53  so that they will have a reaming effect when the reamer is placed in an acetabulum and rotated. This type of reamer is sometimes referred to as being of a “cheese-grater type”. 
   The reamer has a base  54 , the plan shape of which is best shown in  FIG. 13 . It will be seen that the shape is similar to the shape of the carrier  4  although it does not have the teeth  3 . The same reference numerals are used to indicate similar parts to the carrier  4 , thus, it has a hexagonal shaped opening  7  with corners  8 , the dimensions are the same as those of the carrier. 
   The reamer can therefore be used with the rim cutter by removing the rotatable carrier  4  and replacing it with the reamer  50  thus enabling the apparatus to have a dual purpose. 
   The reamer can be attached first and used to ream the acetabulum with which it is to be used, the reamer can then be removed and replaced by the rim cutter. 
   The acetabular rim cutter or rim cutter and acetabulum reamer combined may particularly, although not exclusively, adapted for use with apparatus as set for in U.S. Pat. Nos. 5,879,402 and 6,264,698 for implanting an acetabular cup utilizing a loading tube. 
   The surgical technique on an exposed acetabulum cup will now be described. 
   The true medial wall of the acetabulum is identified by using reamers or a long-handled gouge to remove any curtain ostephytes and expose the true medial wall of the acetabulum. 
   Powered reamers are then used, increasing in size in 2 mm increments to remove cartilage and subchondral bone. Care is taken not to thin the walls excessively. A note is made of the largest reamer used to clear the rim of the acetabulum. Often it is useful to revert to a smaller reamer to further expose trabecular bone; particular attention is paid to the rim of the acetabulum since it is important to achieve interdigitation of bone in this area. 
   A step drill is used to make multiple holes. The smaller part of the drill is used all around the rim of the socket, the larger part of the step makes multiple holes in the dome of the ilium, pubis and ischium. Care is taken not to perforate the walls. If this does occur then morcellised bone graft is packaged into the hole to prevent egress of cement during cementation. 
   A cup with an outside diameter 2 mm smaller than the largest reamer used is usually the appropriate implant. The rim cutter marked with the same size as the cup is attached to a power tool. This will cut a groove in the periphery of the acetabulum of the appropriate diameter for the flange. The hemispherical guide of the rim cutter centralizes the cutter in the reamed socket and sets the depth of the rim and thus the position of the cup. The orientation of the rim cutter is shown by the alignment rod on the device. Alternately, the orientation may be accomplished by a computer-aided optical navigation system (see U.S. Pub. No. 2004/0153080. Accurate inclination and flexion can be ensured by using a navigation device on the shaft of the rim cutter. The rim cutter is advanced to the fullest extent allowed by exerting pressure against the spring between the dome and the cutting ring. Any debris created including the innermost fibers of the transverse ligament are excised. The flange is trimmed to the diameter defined by the chart to fit within the rim. A trial reduction may be carried out if desired. 
   An aspirator device is turned on at this point and, to save time, the nursing staff asked to start mixing the cement during final preparation of the bone. Thorough lavage of the socket is carried out to clean the interstices of the trabecular bone of marrow and fat. Fluid is aspirated out of the wing of the ilium by the aspirator. When the bone is clean, hydrogen peroxide soaked gauze swabs are packed into the socket to further clean the bone and promote haemostasis. 
   The cement may be handled approximately 3.5 minutes after mixing (Simplex™ P cement at 20 degrees centigrade). Immediately before the cement is placed in the socket, bone graft reamings may be impacted against the smooth cortical medial wall since the cement cannot adequately gain fixation against a surface of this sort. After introduction of the cement bolus excess material is removed so the surface of the cement lies with a slightly concave surface within the mouth of the socket. This step prevents the escape of surplus cement into the soft tissues when the acetabular pressurizer is applied. 
   The cement pressurizer consists of a saline filled balloon which can be inflated from a reservoir in the handle of the device. The pressurizing technique entails inflating the balloon so that it conforms perfectly with the mouth of the socket. Full body weight is applied to the device to drive the cement into the bone and, by maintaining pressure, protect the bone cement interface from back-bleeding from the host bone. The aspirator also serves to suck cement into the wing of the ilium and remove blood from the interface. If there is excessive blood loss through the aspirator the level of vacuum should be reduced. 
   The pressurizer is applied as soon as the cement has been placed in the socket and full pressure is maintained until the cement viscosity has risen to a level of viscosity suitable for the insertion in the cup—usually about 6 minutes after the commencement of mixing. In the elderly, or where it has been possible to expose a larger surface area of trabecular bone, it is possible that so much cement is pressurized into the acetabulum that a further bolus is required on top of the initial bolus of cement. This will become apparent when the pressurizer is removed. If more cement is to be used, then the existing cement should be dried before it is applied. 
   A flanged prosthetic cup is then inserted, as trialed, using an introducer and an axial pusher to drive the cup to the seated position. This insertion will likely require significant force and there should be a constant flow of cement around the edge of the cup. Excess cement can be removed with a small curette. The timing is perfect when the cup flange is delivered to the final seated position just as it is impossible to further advance the cup into the viscous cement. The post-operative radiograph should show good cement penetration and no radiolucent lines in any zone. The principle is to cut a rim around the periphery of the acetabulum to a set depth. The flange of the socket seats into this rim thus guaranteeing the correct orientation and depth of insertion of the socket if the rim cutter has been used correctly. Accurate positioning can be ensured since the rim cuter can be navigated to the desired position. Another benefit of this technique is that it allows the possibility of carrying out a trial reduction to test stability and leg length before cementation; the trimmed socket may be introduced into the acetabulum supported only by the flange resting in the rim and the hip reduced with the trial femoral component in place. 
   Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.