Patent Publication Number: US-8529572-B2

Title: Head centering jig for femoral resurfacing

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is a divisional of U.S. patent application Ser. No. 10/844,152, filed on May 12, 2004, now U.S. Pat. No. 7,695,476 which claims priority to United Kingdom Patent Application No. 0310869.3, which was filed on May 12, 2003. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a head centering jig for use in the preparation of the femur in hip resurfacing procedures and to a kit comprising such a jig. 
     BACKGROUND OF THE INVENTION 
     Conventional Total Hip Replacement (THR) is a very successful procedure for the treatment of arthritis of the hip, a condition which causes considerable pain and loss of movement. As is well known, the hip is a ball and socket joint which allows the upper leg to move from side to side, back to front, and to rotate. The joint is made up of the head of the femur (the ball) which fits into the acetabulum (the socket). In a healthy hip, both the head of the femur and the acetabulum are covered with cartilage which provides a smooth surface allowing the joint to move freely. 
     The earliest Total Hip Replacement (THR) procedure, in 1938, involved an implant in which both surfaces were made from metal and was known as a metal-on-metal bearing. During the 1950&#39;s and early 1960&#39;s, a number of surgeons developed hip implants using this type of bearing, although many of these designs had a tendency to work loose early on as the techniques used to anchor them to the bone were not very successful. However, the implants that did not loosen early on have generally lasted well. 
     In the early 1960&#39;s, a British Surgeon, Sir John Charnley, developed a new type of Total Hip Replacement which is still in use today. This procedure, which is illustrated in  FIG. 1 , involves cutting the worn head off the femur and replacing it with a metal ball  10  and stem  12  in the shaft of the femur  14  and a plastic cup  16  in the pelvis  18 . Both elements are typically anchored to the bone by “bone cement”. This has become a very common surgical procedure with some 45,000 hip replacements being carried out in the UK every year. 
     The plastic used to form the cup  16  is inert and so is well tolerated by the body. Nevertheless, as the metal ball  10  rubs against the plastic cup  16 , tiny particles of the plastic are worn away. This plastic debris causes an irritation. Furthermore, as the particles get between the bone and the artificial joint, this irritation causes surrounding bone to be absorbed by the body, leading to a loosening of the artificial joint. In older people, with a lower activity level, this may not happen for twenty or more years, but in younger, more active patients, this may happen much sooner. 
     To overcome these problems in younger, more active patients requiring hip replacement, a different type of implant was needed. In 1991 a procedure for metal-on-metal (MoM) resurfacing of the hip was proposed. This has two major differences from a conventional THR. The first is that both components are made from metal, typically Cobalt Chrome. By eliminating the plastic cup of a conventional THR, and making both parts of the bearing surface of metal, the resurfaced hip is expected to last much longer and therefore to be more suitable for higher demanding patients. The second difference is that the procedure is very bone conserving, since the head of the femur is simply reshaped and “resurfaced”, rather than removed. Accordingly, should the device need replacing at some time in the future, this may provide better options for the surgeon at that time as a conventional THR can then be used. 
     A typical Resurfacing Hip is shown in  FIG. 2  to comprise a femoral head component  20  and an acetabular cup  22 . During the pre-operative planning stage, an X-ray of the hip is templated to assess the probable sizes for both the femoral head component  20  and acetabular cup  22 . Alignment of the femoral head component  20  is also determined pre-operatively and is an important part of the templating procedure. The femoral head component  20  should be positioned in neutral or slight valgus alignment. Varus positioning should be avoided. Once the template has been satisfactorily positioned with respect to the X-ray, the distance is measured from the tip of the greater trochanter  24  to the point where the axis  26  of the femoral neck  32  crosses the lateral cortex of the femoral shaft  14 . This pre-operative measurement is recorded as it indicates the position for subsequent insertion of a lateral positioning pin  28  of a head centering jig  30  (see  FIG. 3  and related description below). 
     Initial preparation of the femur may be carried out to one size larger than that templated in order to ensure that there is sufficient clearance around the femoral neck  32  for the definitive implant. Further preparation to the templated size may then be carried out if it is evident that there is sufficient clearance around the neck  32  and that the integrity of the neck will not be compromised. Sufficient clearance may occasionally still remain to allow a smaller size to be used than was suggested by templating. In this case, further preparation to the smaller size may be carried out, ensuring the use of the most appropriate size of femoral components and minimizing the amount of bone removed from the acetabulum  34 . 
     During hip resurfacing procedures it is common to prepare the femur first as this will debulk the femoral head  36  and facilitate access to the acetabulum  34 . As part of this process the mid-lateral cortex of the femur is exposed and the position of the lateral positioning pin  28  is determined based on the pre-operative measurement taken from the X-ray. The lateral positioning pin  28  is then drilled into the mid-lateral cortex of the femur, initially using a lateral approach, but angling the pin towards the femoral head  36  once the outer cortex has been penetrated, as shown in  FIG. 3 . 
     Before proceeding with head centering, the size of the femoral head component  20  can be confirmed by placing an appropriate head template around the femoral neck  32 . What is important to assess is that the appropriate clearance is available and that the femoral neck  32  will not be notched during preparation of the femur since this will result in a potential post-operative weakening of the hip. 
     In the past, once the size of the femoral head component  20  had been confirmed, a jig  30 , such as that shown in  FIG. 4 , would be used to locate the centre of the femoral head  36 . Since the femoral head  36  does not define a uniform sphere, what is important is to identify that point on the femoral head which coincides with the central axis  26  of the femoral neck  32 . 
     As shown, the head centering jig  30  of the prior art comprises a hollow guide tube  38  having a proximal end  40  and a distal end  42 . The guide tube  38  is supported by an arm  44  which locates at one end over the lateral positioning pin  28  and is provided at the other with a pawl  46  for selective engagement with a rack  48  provided on an outer surface of the guide tube. Thus, by the selective engagement and disengagement of the ratchet mechanism defined by the pawl  46  and rack  48  the guide tube  38  may be progressively advanced with respect to the arm  44 . A locking screw  50  is provided to retain the guide tube  38  in fixed relation to the arm  44  once the desired relative position has been established. 
     A sleeve  52  is rotatably mounted to the guide tube  38  at a position intermediate the distal end  42  and the rack  48 . As well as rotating, the sleeve  52  is also able to slide along the guide tube  38 , thereby enabling the sleeve to be positioned at a range of distances from the distal end  42 . A further locking screw  54  is provided to enable the sleeve  52  to be clamped longitudinally with respect to the guide tube  38  while still permitting a portion of the sleeve to rotate. To this rotating portion there is attached a projection  56  which extends in a direction perpendicular to a longitudinal axis of the guide tube  38 . A stylus  58  is slidably mounted on the projection  56  to which it may be clamped in a selected one of a plurality of predetermined positions by means of a third locking screw  60 . These predetermined positions correspond to different sizes of femoral head component  20  and are marked on the projection  56  as a series of graduations. 
     In order to identify the centre of the femoral head  36  the stylus  58  is set to the confirmed size of the femoral head component  20  and the third locking screw  60  is tightened to clamp the stylus with respect to the projection  56 . The aim  44  of the head centering jig  30  is then located over the lateral positioning pin  28  and the guide tube  38  advanced towards the femoral head  36  in a controlled manner by means of the ratchet mechanism defined by the pawl  46  and rack  48 . Centralization of the guide tube  38  is achieved by rotating the stylus  58  around the femoral neck  32 . It is critical that a tip of the stylus  58  rotates freely around the femoral neck  32  at the head/neck junction without impingement. In this way it is possible to avoid subsequent notching of the femoral neck  32 . In order to assess more easily whether the guide tube  38  is accurately centered with respect to the femoral head  36 , the sleeve  52  may be slid along the guide tube to a point where the tip of the stylus  58  coincides with the junction of the femoral neck  32  and femoral head  36 . The further locking screw  54  may then be tightened to retain the sleeve  52  in this longitudinal position with respect to the guide tube  38 . 
     Once the guide tube  38  has been properly centered it can be locked into position by gently impacting into the femoral head  36  a plurality of circumferential teeth (not shown) provided on the distal end  42 . A long pin  62  is then drilled through the guide tube  38  into the femoral head  36  before the head centering jig  30  is then disassembled and removed. If required, once the guide tube  38  has been disassembled from the arm  44 , the guide tube and stylus  58  can be positioned over the long pin  62  to carry out a final check that the long pin is correctly positioned in relation to the femoral neck  32 . 
     Thereafter, as shown in  FIG. 5 , a cannulated drill  64  is advanced over the long pin  62  as far as an appropriate line for the size of the femoral head component  20  being used. Both the cannulated drill  64  and the long pin  62  are then removed and a guide rod  66  inserted in their place. 
     As shown in  FIG. 6 , an appropriate size of head cutter  68  is advanced over the guide rod  66  as far as the junction between the femoral head  36  and femoral neck  32 , ensuring that the femoral neck is not notched. During this process, the head template can be positioned around the femoral neck  32  to protect the neck and trochanter  24 , while swabs can be used to prevent bone debris entering the soft tissue. If necessary, a larger size of head cutter  68  may be used for initial preparation of the femoral head  36  prior to final preparation with the definitive size in order to debulk the head. 
     Having removed the head cutter  68 , the guide rod  66  is also then removed and a top head guide  70  placed over the prepared head surface and advanced to the head/neck junction. As shown in  FIG. 7 , a locking screw  72  is provided to hold the top head guide  70  in place while the top of the femoral head is resected. If necessary, a short pin (not shown) can be impacted through a hole in the top head guide  70  if additional security is required during the head resection. Following resection the guide rod  66  is reinserted and a top head cutter  74  advanced along the guide rod to create a flat surface perpendicular to the neck axis  26 . As a result, the femoral head  36  acquires the shape shown in  FIG. 8 . An appropriate size of head chamfer cutter  76  is then used to create a bevel as shown in  FIG. 9 . Having thus shaped the femoral head  36 , a corresponding size of head template  78  is used to check the prepared shape as shown in  FIG. 10  and to make a mark on the head/neck junction to indicate how far the femoral head component  20  should be advanced if fully seated. Cement keyholes can be drilled into the femoral head  36  at this stage if required. 
     Having prepared the acetabulum  34  and fitted the acetabular cup  22 , the femoral head component  20  is similarly implanted to the prepared femoral head  36 . If a cemented head is used, low viscosity bone cement is poured into the femoral head component  20  up to a line at the top of a chamfer on the inside of the implant. The definitive component  20  is then placed onto the femoral head  36  and impacted into position using a head impactor  80  as shown in  FIG. 11 . Any excess bone cement may then be removed. Low viscosity cement is used in preference to high viscosity cement since high viscosity cement may prevent full seating of the femoral head component  20 . However, cementless head components are also available and these are simply impacted into position using the head impactor  80 . 
     With both components now fitted, the lateral positioning pin  28  is removed and the hip reduced while at the same time avoiding scratching the femoral head component  20  against the rim of the acetabular cup  22 . A full check is then made to ensure that there is no impingement and that the range of movement and stability are satisfactory. 
     Thus, it can be seen that accurately determining the centre of the femoral head  36  is a critical step in a hip resurfacing procedure since it determines not only the position of the long pin  62  and the guide rod  66  but also the relative positions of all the cutters and guides that are subsequently used to shape the femur. There are, however, a number of problems with the prior art head centering jig shown in  FIG. 3 . 
     Firstly, the prior art jig  30  requires the use of a lateral positioning pin  28  which must be correctly positioned with respect to the femoral neck  32 . This not only requires pre-operative planning but also the intra-operative exposure of the mid-lateral cortex of the femur. Furthermore, having been inserted, it becomes necessary to remember to remove the lateral positioning pin  28  from the femoral cortex before the wound is closed. Although this might seem obvious, experience has taught that it is a sensible precaution to attach a chain  82  or other reminder to the lateral positioning pin  28  to ensure that its removal is not overlooked. 
     Another problem with the prior art head centering jig  30  is that it is bulky since the arm  44  must extend from the lateral positioning pin  28  at the mid-lateral femoral cortex to the femoral head  36  while still allowing the stylus  58  to rotate about the guide tube  38 . 
     The prior art head centering jig  30  also requires the use of two hands, one to hold the guide tube  38  and the other to rotate the stylus  58 . 
     It would therefore be advantageous to provide an improved head centering jig which was less bulky and could be operated with only one hand. It would also be advantageous if the use of the head centering jig no longer necessitated exposure of the mid-lateral cortex of the femur. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention there is provided a jig for identifying a point on the femoral head in alignment with a central axis of the femoral neck, the jig comprising a first member defining a plane and having means for at least partially receiving the femoral neck; a guide member mounted in spaced relation with respect to the first member and defining an axis at right-angles to the plane defined by the first member, the axis intersecting the plane at a point a predetermined distance from said means for at least partially receiving the femoral neck; and elongate alignment means mounted with respect to the first member, the alignment means being spaced from and extending parallel to the axis defined by the guide member. 
     Advantageously, the guide member may comprise a hollow tube and the axis defined by the guide member may comprise a central axis of the tube. This provides for accurate positioning of the drill required to drill the long pin into the femoral head. Alternatively, the guide member may comprise a channel member. This provides the advantage of not having to disassemble the jig prior to its removal once the long pin has been drilled into the femoral head. 
     Advantageously the guide member may be mounted with respect to the first member so as to selectively detachable therefrom. This facilitates the disassembly of the jig. Preferably the guide member may be separable from the first member in a direction parallel to the axis defined by the guide member. This facilitates the removal of the guide member over the long pin once the long pin has been drilled into the femoral head. 
     Advantageously the first member may comprise a handle. This facilitates the ease with which the jig may be manipulated by a surgeon. Preferably the means for at least partially receiving the femoral neck may be adapted to so receive the femoral neck when the means is offered up to the femoral neck in a first direction, the handle extending in a direction parallel to the first direction. This increases the ease with which the jig may be used. Preferably the elongate alignment means may be spaced from the axis defined by the guide member in a second direction, the second direction being orthogonal to the direction in which the handle extends. Likewise, when the means for at least partially receiving the femoral neck is adapted to so receive the femoral neck when the means is offered up to the femoral neck in a first direction, the elongate alignment means may be spaced from the axis defined by the guide member in a second direction, the second direction being orthogonal to the first direction. This facilitates the alignment of the guide member with the central axis of the femoral neck. 
     Advantageously the elongate alignment means may be adapted to be mounted with respect to the first member in a selected one of two positions, the two positions being spaced on opposite sides of the axis defined by the guide member. This enables the jig to be adapted depending on whether the surgeon is either left handed or right handed or whether the femoral head that is being reshaped is the patient&#39;s left or right. Preferably the two positions may be mirror images of each other. 
     Advantageously the elongate alignment means may extend through the plane defined by the first member. This further facilitates the alignment of the guide member with the central axis of the femoral neck. 
     Advantageously the elongate alignment means may be adapted to be mounted with respect to the first member in a selected one of a plurality of positions spaced at different distances from the axis defined by the guide member. This enables the jig to accommodate femoral heads of different dimensions and enables the elongate alignment means to be spaced at different distances from the femoral neck in accordance with the preference of the surgeon. 
     Advantageously the elongate alignment means may comprise at least one rod. Where the elongate alignment means comprises first and second rods mounted with respect to the first member and extending parallel to and spaced from the axis defined by the guide member, the first rod may be spaced from the axis in a direction orthogonal to that in which the second rod is spaced from the axis. This facilitates the alignment of the guide member with the central axis of the femoral neck by providing a means for obtaining simultaneous alignment in two orthogonal planes. Where the elongate alignment means comprises first and second rods and where the first member comprises a handle, one of the first and second rods may extend along an axis that intersects the handle. 
     Advantageously, the first member may comprise means adapted to substantially encircle the femoral neck. This enables the first member to be more accurately positioned with respect to the femoral neck. Preferably the means for substantially encircling the femoral neck may comprise a first part hingedly mounted at a first end with respect to a second part and moveable between an open position in which an opposite end remote from the hinge is separated from the second part by a distance sufficient to receive the femoral neck and a closed position in which the parts define a wall adapted to substantially surround the femoral neck. Preferably the wall defined by the first and second parts in the closed position subtends an angle of 270° or more. Preferably, in the closed position, the arc subtended by that part of the wall defined by the first part is substantially the same as the arc subtended by that part of the wall defined by the second part. 
     Preferably, in the closed position, the first and second parts may define a circular aperture for the receipt of the femoral neck. Under such circumstances the axis defined by the guide member may intersect the plane defined by the first member at the centre of the circular aperture defined by the first and second parts. This ensures that when the elongate alignment means is aligned with the femoral neck, the axis defined by the guide member coincides with the central axis of the femoral neck. 
     Advantageously means may be provided to releasably retain the first and second parts in the closed position. This provides the advantage of preventing the first member from becoming accidentally dislodged from the femoral neck. Preferably the first and second parts are each provided with a respective aperture, the aperture of the first part being adapted to be aligned with the aperture of the second part when the first and second parts are in the closed position, the mutually aligned apertures being adapted to receive the means for releasably retaining the first and second parts in the closed position. Preferably the guide member comprises the means adapted to be received within the mutually aligned apertures to releasably retain the first and second parts in the closed position. 
     According to a second aspect of the present invention there is provided a kit for use in the resurfacing of the femoral head, the kit comprising at least one jig as previously described. 
     Advantageously, the kit may comprise a plurality of jigs, each jig being as previously described and differing from each other in terms of the dimensions of the means for at least partially receiving the femoral neck and/or the predetermined distance from said means at which the axis defined by the guide member intersects the plane defined by the first member. This provides the advantage of enabling the kit to accommodate the range of femur sizes present in the population. 
     According to a third aspect of the present invention there is provided a method of identifying a point on the femoral head in alignment with a central axis of the femoral neck, the method comprising the steps of: providing a jig comprising a first member defining a plane and having means for at least partially receiving the femoral neck, a guide member mounted in spaced relation with respect to the first member and defining an axis at right-angles to the plane defined by the first member, the axis intersecting the plane at a point a predetermined distance from said means for at least partially receiving the femoral neck, and elongate alignment means mounted with respect to the first member, the alignment means being spaced from and extending parallel to the axis defined by the guide member, exposing the femoral head and neck; offering up the first member to the femoral neck so that the femoral neck is at least partially received by said means for at least partially receiving the femoral neck; aligning the elongate alignment means with a central axis of the femoral neck; and identifying a point at which the axis defined by the guide member intersects the femoral head as the point on the femoral head in alignment with the central axis of the femoral neck. 
     Preferably the mid-lateral cortex of the femur is not exposed. 
     Advantageously the first member is offered up to the femoral neck and the elongate alignment means is aligned using only one hand. Preferably the first member is offered up to the femoral neck in a lateral approach. 
     Preferably the step of offering up the first member to the femoral neck comprises substantially encircling the femoral neck. 
     Advantageously the step of aligning the elongate alignment means comprises aligning the elongate alignment means with a central axis of the femoral neck when viewed from two orthogonal directions. 
     Preferably the method further comprises the step of drilling along the axis defined by the guide member and into the femoral head. More preferably the method further comprises the step of disassembling the jig prior to removal from the femoral neck. 
     Advantageously the jig is provided as part of a kit comprising a plurality of jigs, each jig differing dimensionally from each other jig, the method further comprising the step of selecting the appropriate jig for the size of femoral head to be surveyed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a fragmentary frontal view of a hip replacement according to a procedure of the prior art. 
         FIG. 2  is a fragmentary frontal view of a hip replacement according to a procedure of the prior art. 
         FIGS. 3-11  are fragmentary frontal views of hip replacement procedures and tools of the prior art. 
         FIG. 12  is a perspective view of a head centering jig in accordance with a first embodiment of the present invention; 
         FIG. 13  is a plan view of a locating tool used in the first embodiment of the present invention; 
         FIG. 14  is a cross-sectional view of a retaining member used in a first embodiment of the present invention; 
         FIG. 15  is a plan view of a collar used in a first embodiment of the present invention; 
         FIG. 16  is a side view of an alignment rod used in a first embodiment of the present invention; 
         FIG. 17  is a perspective view of a head centering jig in accordance with a second embodiment of the present invention; 
         FIG. 18  is a plan view, partially in cross-section, of a locating tool used in a second embodiment of the present invention; 
         FIG. 19  is a plan view of a locating tool comprising two arms used in a third embodiment of the present invention in a closed position; 
         FIG. 20  is an exploded perspective view of the locating tool of  FIG. 19 ; and 
         FIG. 21  is a perspective view of the locating tool of  FIGS. 19 and 20  in an open position. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Referring to  FIGS. 12 to 16  there is shown a head centering jig  100  comprising a substantially planar locating tool  102 . In the embodiment, shown the locating tool  102  has a spanner-like form and is symmetrical about a longitudinal axis  103 . The locating tool  102  comprises an elongate handle  104  having a rounded proximal end  106 . At a distal end, the elongate handle  104  merges with a planar head portion  108 . The head portion  108  is recessed at an edge opposite where the head portion merges with the handle  104  so as to define a mouth  112  having a pair of mutually spaced jaws  110  which extend away from the handle on either side of the longitudinal axis  103 . In order to minimize the trauma caused in using the head centering jig  100 , the head portion  108  is provided with an external profile devoid of sharp angles. Indeed, in some preferred embodiments, the external profile of the head portion  108  may be smoothly curved. By contrast, the mouth  112  is defined by a pair of confronting surfaces  105  and  107  which extend parallel to and are equally spaced from the longitudinal axis  103  and an orthogonal surface  109  which extends perpendicularly to the longitudinal axis and which, in some embodiments, may serve to join the confronting surfaces  105  and  107 . In other embodiments, such as that illustrated, the orthogonal surface  109  is joined to the confronting surfaces  105  and  107  via respective angled surfaces  111  and  113  so that all of the surfaces defining the mouth  112  subtend an oblique angle with the surface or surfaces adjacent to them. In this way the mouth  112  has an internal profile which, in plan view, approximates to a U-shape. In some embodiments this internal profile may be made smoother still such that the confronting surfaces  105  and  107  are joined by an arcuate or semicircular surface (not shown). 
     A retaining member  114  is releasably mounted to the locating tool  102  and is carried on an upper surface of the handle  104  adjacent the distal end where the handle merges with the head portion  108 . To this end, the handle  104  is provided with three apertures  115 ,  117  and  119  which are spaced in the longitudinal direction along the longitudinal axis  103 . In use the central of these three apertures  117  communicates with a threaded bore  121  provided in an undersurface of the retaining member  114 . As a result, a complimentary threaded locking screw  116  may pass through the central aperture  117  to engage the threads of the blind bore  121  and in so doing releasably secure the retaining member  114  with respect to the locating tool  102 . In order to facilitate the alignment of the retaining member  114  with respect to the locking tool  102 , the same undersurface having the blind bore  121  is also provided with two projecting studs  118  and  120 . When the retaining member  114  is correctly aligned with respect to the locking tool  102 , these projecting studs  118  and  120  are received within the outer pair of the apertures  115  and  119  provided in the handle  104 . 
     As can be seen, the retaining member  114  comprises an elongate foot  122  which is releasably attached to the locating tool  102  by means of the locking screw  116  and extends in a direction coincident with the longitudinal axis  103 , an upstanding member  124  which merges with the foot  122  and projects upwardly from the handle  104  perpendicular to the plane defined by the locating tool  102 , and an arm  126  which merges with the upstanding member  124  at an end remote from the foot  122  and extends in a plane parallel to and spaced from that defined by the locating tool  102 . The arm  126  extends in a direction parallel to the longitudinal axis  103  and, at an end remote from the upstanding member  124 , terminates in an arcuate surface. This enables the arm  126  to abut, and in some embodiments be welded, brazed or otherwise joined to the cylindrical outer surface of a hollow guide tube  128 . 
     In those embodiments in which the guide tube  128  is not joined to the arm  126 , the guide tube  128  may nonetheless be secured to the retaining member  114  by means of a collar  130  which surrounds the guide tube  128  and retains the guide tube in a friction fit and is itself attached to the arm  126  by means of a locking screw  132 . Thus, the collar  130 , as well as having a central aperture  131  for receipt of the guide tube  128 , is provided with a first radial projection  134  having a through bore  133  located at an end remote from the collar. When the guide tube  128  abuts the arcuate end surface of the arm  126  this through bore  133  communicates with a threaded bore  135  provided in the arm such that the complimentary threaded locking screw  132  may pass through the first radial projection and into threaded engagement with the bore provided in the arm  126 . 
     With the guide tube  128  joined or secured in this position, a central axis  136  of the guide tube intersects the plane defined by the locking tool  102  at a location along the longitudinal axis  103  and within the mouth  112  a predetermined distance from the orthogonal surface  109 . In those embodiments in which the internal profile of the mouth  112  approximates to a U-shape, this predetermined distance may be approximately equal to half the distance separating the confronting surfaces  105  and  107 . Likewise, in those embodiments in which the confronting surfaces  105  and  107  are joined by a semicircular surface, the central axis  136  of the guide tube  128  may intersect the plane defined by the locating tool  102  at the centre of the circle defined by that semi-circular surface. In those embodiments in which the confronting surfaces  105  and  107  are joined by an arcuate surface, the point of intersection may coincide with the centre of curvature of the arcuate surface. 
     In addition to the first radial projection  134 , the collar  130  is also provided with a second radial projection  138 . This second radial projection  138  is longer than the first and extends at right-angles to it. By definition therefore, the second radial projection  138  also extends at right-angles to the arm  126 . A plurality of apertures  140  are provided at intervals along the centre line of the second radial projection and are sized so as to slidably receive one end  142  of a first alignment rod  144 . As can be seen, the alignment rod  144  is of circular cross-section and has a diameter that is stepped at a location intermediate the ends of the rod such that, at one end  142 , the alignment rod has a diameter capable of being received through a selected one of the apertures  140  and, at the other end  146 , has a diameter which is not so capable. As a result, an annular shoulder  145  is formed at the location where the diameter of the alignment rod increases and it is this shoulder which abuts an upper surface of the second radial projection  138  and prevents the alignment rod  144  from passing completely through the selected aperture  140 . 
     Although not strictly necessary, a second alignment rod  148 , of similar construction to the first, may be arranged to pass through a selected one of a plurality of mutually aligned apertures  150  provided in the foot  122  and arm  126  of the retaining member  114  and in the handle  104  of the locating tool  102 . As will be readily understood, both alignment rods  144  and  148  extend parallel to the central axis  136  of the guide tube  128  and perpendicular to the plane defined by the locating tool  102 . Furthermore, in the same way that the first alignment rod  144  may be spaced from the axis  136  at different distances depending on the aperture  140  through which the alignment rod is received, so too may the second alignment rod  148  by appropriate selection of the mutually aligned apertures  150 . 
     One of the advantages of the head centering jig  100  shown in  FIGS. 12 to 16  is that, unlike the jig of the prior art, it does not require the provision of a lateral positioning pin. As a result the mid-lateral cortex of the femur need not be exposed during the Hip Resurfacing procedure and there is no risk of a positioning pin being left behind once the wound is closed. Instead, the only parts of the femur that need be exposed are the femoral head and femoral neck. Accordingly, once this has been achieved, in order to identify the point on the femoral head in alignment with the central axis of the femoral neck the surgeon simply selects a head centering jig appropriate to the patient. In order to accommodate the range of femur sizes present in the population it will be necessary to provide a range of jigs which differ in terms of the dimensions of the mouth  112 . The length of the arm  126  would also vary from jig to jig in order to maintain the relationship between the central axis  136  of the guide tube  128  and its point of intersection with the plane defined by the locating tool  102 . Nevertheless, this range of jigs may be made so that each jig  100  corresponds to a respective one of the standard sizes of femoral head component. 
     The head centering jig  100  is offered up to the femur using a lateral approach. In so doing, by making use of the elongate handle  104  the surgeon is able to grip the jig using only one hand. This represents another significant advantage over the head centering jigs of the prior art. The locating tool  102  is positioned so that the femoral neck is received between the mutually spaced jaws  110  and abutting the orthogonal surface  109 . With the femoral neck thus received within the mouth  112 , the plane defined by the locating tool  102  is tilted by raising and lowering the handle  104  until such time as the first alignment rod  144  is aligned parallel to the central axis of the femoral neck when viewed anteriorly. Assuming that the appropriately sized jig has been selected and that the femoral neck is a close fit between the jaws  110  of the locating tool  102 , the central axis  136  of the guide tube  128  will lie within a plane parallel to the coronal plane. By then aligning the first alignment rod  144 , the central axis  136  is brought into coincidence with the central axis of the femoral neck. Thus, using his free hand, the surgeon may then drill the long pin through the guide tube  128  and into the femoral head. 
     If additional alignment is required and if the head centering jig  100  is provided with a second alignment rod  148  then, once the first alignment rod  144  has been aligned with the central axis of the femoral neck when viewed anteriorly, the second alignment rod  148  may be aligned with a central axis of the femoral neck when viewed laterally. This alignment of the second alignment rod  148  ensures that the central axis  136  of the guide tube  128  lies within a plane parallel to the coronal plane notwithstanding any small amount of play between the femoral neck and the jaws  110  of the locating tool  102  and, if performed simultaneously with the similar alignment of the first alignment rod  144 , ensures that the central axis  136  coincides with the central axis of the femoral neck. However, as stated previously, assuming that the appropriate jig  100  has been selected and the jaws  110  are appropriately spaced, this secondary alignment should not be necessary. Even if it does prove necessary, the surgeon may choose to align the upstanding member  124  with the central axis of the femoral neck to the same effect rather than employ a second alignment rod  148 . 
     Once the long pin has been drilled into the femoral head the head centering jig  100  is disassembled and removed. In order to achieve this, the locking screw  116  is unscrewed from the blind bore  121  thereby enabling the retaining member  114  to be separated from the locating tool  102 . The retaining member  114 , the guide tube  128  and the first alignment rod  144  are then removed over the long pin in a direction parallel to the central axis  136 . If a second alignment rod  148  is provided then this too is removed from the locking tool  102  in a direction parallel to the central axis  136 . By contrast, the locking tool  102  is removed laterally in a direction perpendicular to the long pin. With the head centering jig  100  removed from the wound, the remainder of the Hip Resurfacing procedure may be performed in the manner previously described with a cannulated drill being advanced over the long pin as the next step in the reshaping of the femoral head. 
     Although the head centering jig  100  has been described as comprising a hollow guide tube  128  defining a central axis  136 , it will be apparent that in certain embodiments the hollow guide tube may be replaced by a channel member of U-shaped or V-shaped cross-section sized to receive a drill for drilling the long pin into the femoral head and capable of uniquely defining a drill axis perpendicular to the plane of the locating tool  102 . The advantage of such an embodiment is that the channel member does not surround the drill axis so that once the long pin has been drilled into the femoral head, the channel member, retaining member and locating tool may be removed from the femur in a single operation and without requiring the disassembly of the head centering jig. 
     Another advantage of the present head centering jig  100  is that the first alignment rod  144  may be located with respect to the guide tube  128  on either side of the retaining member  114 . This may be useful depending on the handedness of the surgeon or whether the femoral head that is being reshaped is the patient&#39;s left or right. Thus, for example, by unscrewing locking screw  132  from threaded bore  135 , the collar  130  may be removed from the arm  126  and inverted before being reassembled to the retaining member  114 . In this way, the second radial projection  138 , instead of projecting to the right when viewed along the aim  126  in the direction of the guide tube  128 , as shown in  FIG. 12 , projects to the left. 
     A head centering jig  200  in accordance with a second embodiment of the present invention is shown in  FIGS. 17 and 18 . Many of the features of the second embodiment, including the retaining member  114 , the guide tube  128  and the two alignment rods  144  and  148  are similar to those described in relation to the first embodiment and so will not be further described in relation to the second embodiment in which they are denoted by the same reference numerals. Nevertheless, the locating tool  202  of the second embodiment does differ from that of the first embodiment although, like the first embodiment, it comprises an elongate handle  204  having a rounded proximal end  206 . 
     Although the retaining member  114  may be secured to the handle  204  by means of a locating screw  116  in the same manner as has been described with reference to the first embodiment and, indeed, as illustrated in  FIG. 17 , the retaining member  114  may equally be attached to the handle  204  by means of the fixation mechanism illustrated in  FIG. 18 . In this arrangement the handle  204  is again provided with a pair of apertures  115  and  119  for the receipt of the projecting studs  118  and  120  provided on an undersurface of the retaining member  114 . However, whereas in the first embodiment the retaining member  114  is releasably secured to the locating tool  102  by means of a threaded locking screw  116  passing through the handle  104  in a direction perpendicular to the plane defined by the locating tool  102 , in the embodiment shown in  FIG. 18  a locking screw  201  passes through a threaded aperture located within the plane of the handle  204  and extending along an axis  203  passing through the centers of the apertures  115  and  119 . Indeed, the threaded aperture communicates with aperture  119  and, as a result, the locking screw  201  may be tightened so that an end of the locking screw  205  bears against the projecting stud  120  thereby releasably securing the retaining member  114  with respect to the locating tool  202 . 
     At a distal end, the handle  204  merges with one end of a first arcuate member  208  which defines an arcuate surface  210  having a centre of curvature  212  which lies on the axis  203 . A second arcuate member  216  is hingedly connected to the first arcuate member  208  at an end remote from the handle  204  and defines an arcuate surface  218  having the same centre and radius of curvature as the arcuate surface  210 . Indeed, between them, the two arcuate surfaces  210  and  218  subtend a combined arc of 360° and it is at this centre of curvature  212  that the central axis  136  of the guide tube  128  intersects the plane defined by the locating tool  202 . 
     The second arcuate member  216  is hinged with respect to the first arcuate member  208  about an axis  220  that extends perpendicular to the plane defined by the locating tool  202  such that the second arcuate member is moveable between an open position, shown in  FIG. 17 , and a closed position shown in  FIG. 18 . In the open position an end  222  of the second arcuate member  216  remote from the hinge axis  220  is spaced sufficiently far from the handle  204  to permit the receipt of the femoral neck. By contrast, in the closed position, the end  222  of the second arcuate member  216  abuts the handle  204  and the two arcuate surfaces  210  and  218  define a closed circle. As with the head centering jig of the first embodiment, in order to accommodate the range of femur sizes present in the population, it will be necessary, in accordance with the second embodiment, to provide a range of locating tools  202  which differ in terms of the diameter of the circle defined by the arcuate surfaces  210  and  218  in this closed position. The length of the arm  126  would also need to vary from jig to jig in order to maintain the relationship between the central axis  136  of the guide tube  128  and its point of intersection with the plane defined by the locating tool  202 . 
     In order to retain the second arcuate member  216  in the closed position the member is provided adjacent the end  222  with a bore  224  sized to receive a head  226  of a locking pin  228 . The body of the locking pin  228  is substantially cylindrical and terminates at an end remote from the head  226  in an axial projection  230 . Approximately mid-way between the head  226  and the axial projection  230  the locking pin  228  is provided with a threaded bore  232  that extends perpendicularly to a longitudinal axis of the locking pin. 
     The locking pin  228  is received within a stepped bore  234  provided in the handle  204  which communicates with the bore  224  provided in the second arcuate member  216  when the second arcuate member is in the closed position. Furthermore, the stepped bore  234  extends in a direction parallel to axis  203  and comprises a first region  236  having a diameter sufficient to receive the body of the locking pin  228  and a second region  238  of reduced diameter remote from the bore  224  sufficient only to receive the axial projection  230 . 
     In order to assemble the locking pin  228 , a helical spring or other resilient member  240  is positioned over the axial projection  230  and the spring and locking pin  228  are inserted into the stepped bore  234  so that an end of the axial projection protruding from the spring is received within the second region  238 . In this way the spring is held captive between an end of the body of the locking pin  242  and an annular shoulder  244  defined at the intersection of the first and second regions  236  and  238  of the stepped bore  234 . A slot  246  is provided in a side wail of the handle  204  so as to communicate with the threaded bore  232 . A threaded stud  248  may then pass through the slot  246  and into threaded engagement with the bore  232 . Thus, whilst the locking pin  228  is biased by the spring  240  in a first direction so as to cause the head  226  to project out of the stepped bore  234  and be received within the communicating bore  224 , movement of the stud  248  in the opposite direction compresses the spring  240  and allows the head  226  to be withdrawn from engagement with the second arcuate member  216 . As will be apparent, the movement of the locking pin  228  is limited by the engagement of the stud  248  with the opposite ends of the slot  246 . 
     If it is desired to be able to snap the second arcuate member  216  shut in the closed position without recourse to the stud  248  then this may be achieved by appropriate shaping of the head of the locking pin  226  and/or by providing the end  222  of the second arcuate member with a beveled surface  250 . In this way as the beveled surface  250  comes into contact with the head of the locking pin  226  it pushes the locking pin  228  into the handle  204  against the action of the spring  240  until such time as the bore  224  is brought into alignment with the stepped bore  234 . 
     In use, the head centering jig of the second embodiment is employed in the same manner as that of the first embodiment to identify a point on the femoral head in alignment with the central axis of the femoral neck. To this end the locating tool  202  is offered up to the femur with the second arcuate member  216  in the open position. Once the arcuate surface  210  of the first arcuate member  208  has been placed up against the femoral neck, the second arcuate member  216  is pivoted about hinge axis  220  to the closed position to encircle the femoral neck. The second arcuate member  216  is retained in the closed position with respect to the first arcuate member  208  by the receipt of the head  226  of the locking pin  228  within the bore  224 . 
     Thereafter, as with the head centering jig of the first embodiment, the plane defined by the locating tool  202  is tilted by raising and lowering the handle  204  until such time as the first alignment rod  114  is aligned parallel to the central axis of the femoral neck when viewed anteriorly. Assuming that the appropriately sized jig has been selected and that the femoral neck is a close fit within the closed circle defined by arcuate surfaces  210  and  218 , the central axis  136  of the guide tube  128  will lie within a plane parallel to the coronal plane. By then aligning the first alignment rod  144 , the central axis  136  is brought into coincidence with the central axis of the femoral neck. If additional alignment is required a second alignment rod  148  may be aligned with the central axis of the femoral neck when viewed laterally. Once the surgeon is satisfied that the central axis  136  of the guide tube  128  is aligned with the central axis of the femoral neck he may use his free hand to drill the long pin through the guide tube  128  and into the femoral head. The head centering jig  200  is then disassembled and removed as previously described before performing the remaining steps of the Hip Resurfacing procedure. 
     A locating tool  302  according to a third embodiment of the present invention is shown in  FIGS. 19 to 21 . The locating tool  302  is configured to be used with a retaining member  114 , guide tube  128  and two alignment rods  144  and  148  as described in relation to the first embodiment and these features will not be described in detail in relation to the third embodiment. 
     As shown in  FIG. 19 , like the first and second embodiments, the locating tool  302  comprises an elongate handle  304 . However, the locating tool  302  of the third embodiment differs from that of the first and second embodiments. In particular the locating tool  302  comprises a first arm  340  and a second arm  370 , as shown in  FIGS. 20 and 21 . The first and second arms  340  and  370  comprise respective first and second elongate handles  342  and  372  which are designed to interlink to form the elongate handle  304 . Each of the first and second elongate handles  342  and  372  are provided with apertures  115  and  119  for the receipt of the projecting studs  118  and  120  provided on the under surface of the retaining member  114 . The first and second elongate handles  342  and  372  may also comprise the aperture  117  for use in securing the retaining member  114  to the handle  304  by means of a locating screw  116  as previously described. Alternatively, a locking screw  201  similar to that described in relation to  FIG. 18  may be used for the securement of the retaining member  114  to the handle  304 . 
     In any event the apertures  115 ,  117  (if present) and  119  are designed such that when the first and second elongate handles  342  and  372  are interlinked the apertures are mutually aligned. 
     The first and second arms  340  and  370  are hingedly attached by means of a hinge  335  at a proximal end of the elongate handle  304  as shown in  FIG. 21 . However, it will be appreciated that such an attachment is not an essential feature of the invention. The hinge  335  preferably comprises a pin  337  and a nut  338  as shown in  FIG. 20 . 
     As shown in  FIGS. 20 and 21 , the first elongate handle  342  comprises a first surface  345  which merges at inner and outer edges  344  and  346  with inner and outer downwardly depending side walls  348  and  350 , respectively. The outer side wall  350  in turn merges with a second surface  352  at a distance d 1  from the first surface  344 , which is shown as being parallel to said first surface  344 , although it will be understood that such a parallel arrangement is not an essential feature of the invention. By contrast, the inner side wall  348  merges with an inwardly extending third surface  354  at a distance d 2 , d 2  being less than the distance d 1 , to form a step  356 . Again, although the third surface  354  is depicted as being substantially parallel to the first and second surfaces  344  and  352  the invention is not so limited. The third surface  354  terminates at an inner edge  358  at a lower inner downwardly depending side wall  360  which in turn terminates with the second surface  352 . 
     In contrast to the first elongate handle  342 , the second elongate handle  370  comprises a first surface  372  which merges at inner and outer edges  374  and  376  with inner and outer downwardly depending side walls  378  and  380 , respectively. The outer side wall  380  in turn merges with a second surface  382  at distance d 1 , from the first surface  372 . By contrast, the inner side wall  378  merges with an outwardly extending third surface  384  at distance d 2  which in turn merges with a lower inner downwardly depending wall  386  which merges with the second surface  382 . The lower inner wall  386  and third surface  384  form a step  388 . As previously discussed in relation to the first elongate handle  342 , the first, second and third surfaces  372 ,  382  and  384  need not be in a parallel arrangement. 
     At a distal end, the first and second elongate handles  342  and  372  merge with first and second arcuate members  308   a  and  308   b  which define first and second arcuate surfaces  310   a  and  310   b  having a radius of curvaturer. When the locating tool  302  is in the closed position, as shown in  FIG. 19 , the first and second arcuate surfaces  310   a  and  310   b  have a centre of curvature  312  coincident with the point at which the central axis  136  of the guide tube  128  intersects the plane defined by the locating tool  302  and which lies on the longitudinal axis  103 . 
     At their proximal ends, the first and second handle  342  and  372  are designed with tapers to allow for the first and second arms  340  and  370  to rotate freely about the hinge  335 . 
     The locating tool  302  is moveable between an open position, as shown in  FIG. 21 , and a closed position, as shown in  FIG. 19 . In the open position the first and second arcuate members  308   a  and  308   b  are sufficiently spaced apart to permit the receipt of the femoral neck. Once the locating tool  302  is in position it is closed and held in the closed position by the retaining member  114  which is mounted on the elongate handle  304 . The locating tool  302  can be additionally held in a closed position by the provision of a suitable closure means (not shown) at an end of the first and second arcuate members  308   a  and  308   b  remote from the first and second elongate handles  342  and  372 . Alternatively the locating tool  302  may be provided with arcuate members similar to the arcuate members  208  and  216  described in relation to the second embodiment. 
     As with the head centering jig of the first and second embodiments, in order to accommodate the range of femur sizes present in the population, it will be necessary, in accordance with the third embodiment, to provide a range of locating tools  302  which differ in terms of the radius of curvature r, and therefore the arcs subtended by the first and second arcuate member  308   a  and  308   b  in the closed position. The length of the arm  126  would also need to vary from jig to jig in order to maintain the relationship between the central axis  136  of the guide tube  128  and its point of intersection with the plane defined by the locating tool  302 . 
     Once the locating tool  302  is retained in the closed position by the fixation of the retaining member  114  to the elongate handle  304 , as with the head centering jig of the first or second embodiments, the plane defined by the locating tool  302  is tilted by raising and lowering the handle  304  until such time as the first alignment rod  144  is aligned parallel to the central axis of the femoral neck when viewed anteriorly. Assuming that the appropriately sized jig has been selected and that the femoral neck is a close fit within the substantially closed circle defined by arcuate surfaces  310   a  and  310   b , the central axis  136  of the guide tube  128  will lie within a plane parallel to the coronal plane. By then aligning the first alignment rod  144 , the central axis  136  is brought into coincidence with the central axis of the femoral neck. If additional alignment is required a second alignment rod  148  may be aligned with the central axis of the femoral neck when viewed laterally. Once the surgeon is satisfied that the central axis  136  of the guide tube  128  is aligned with the central axis of the femoral neck he may use his free hand to drill the long pin through the guide tube  128  and into the femoral head. The head centering jig  300  is then disassembled and removed as previously described before performing the remaining steps of the Hip Resurfacing procedure. 
     Thus it will be apparent that the present invention as herein described provides a head centering jig that is less bulky than those of the prior art and which can be manipulated using only one hand. It will also be apparent that the use of the head centering jig no longer necessitates the exposure of the mid-lateral cortex of the femur.