Abstract:
A rotary mill is disclosed comprising a body portion ( 4 ) having a milling surface ( 10 ) and a central bore ( 20 ) extending along an axis of rotation ( 18 ) of the body portion ( 4 ); and a guide portion ( 6 ) having a guide body ( 22 ) and a guide peg ( 24 ) extending from the guide body ( 22 ), the guide peg ( 24 ) operable to be received in the central bore ( 20 ) of the body portion ( 4 ), the guide body ( 22 ) having at least one alignment feature ( 28, 30 ) which is the same as that of a prosthesis component. A method of implanting a unicondylar femoral component is also disclosed, the method comprising, a) reaming the femoral condylar surface to accept the unicondylar femoral component; b) drilling peg holes for affixing the unicondylar femoral component; c) affixing a guide portion of a rotary mill onto the prepared condylar surface using the drilled peg holes; d) reaming a portion of bone anterior to the affixed guide portion; e) removing the guide portion from the bone; and f) affixing a unicondylar femoral component to the prepared condylar surface.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a 371 U. S. National Stage of International Application No. PCT/GB2012/050159, filed on Jan. 26, 2012 and published as WO 2012/101441 A1 on Aug. 2, 2012. This application claims priority to British Patent Application No. 1101377.8, filed on Jan. 27, 2011. The disclosures of the above applications are incorporated herein by reference in their entirety. 
     The present invention relates to rotary mills and similar rotary cutting devices and particularly but not exclusively relates to rotary mills for use in preparing a bone for total or partial joint replacement surgery. 
     BACKGROUND 
     It is known to replace all or part of a knee joint in which the joint surfaces have deteriorated, for example as a result of osteoarthritis. Such deterioration usually starts in only one of the tibeo-femoral compartments and may spread to the other at a later stage. Replacement of only one compartment of the joint can therefore be sufficient to provide prolonged relief from symptoms. Damaged bearing surfaces are replaced by a unicompartmental prosthesis which comprises a femoral implant and a tibial implant (usually metallic), which interface through a (polyethylene) bearing component disposed between the two implants. 
     A unicompartmental or partial knee replacement (PKR) helps to conserve undamaged bone and restores more natural movement to the joint. Also, owing to the small size of the prosthesis, the surgery may be less invasive than a total knee replacement (TKR). However, the design requirements for partial knee replacement prostheses are more demanding than those for total knee replacement prostheses. Unlike in a total knee replacement, where one or more ligaments can be discarded and the mechanics of the knee can be simplified, in a unicompartmental knee replacement, all the ligaments in the joint must be retained and restored to their natural tensions and the bearing component must be completely unconstrained. 
     During articulation of the knee, and particularly when the joint is at full extension, the bearing component can impinge on femoral condylar bone tissue superior to the femoral implant, as illustrated in  FIG. 1 . Such impingement of the polyethylene bearing component onto the bone can lead to post operative pain, damage to the bearing, increased wear and eventual failure. It is therefore essential to remove a sufficient amount of anterior bone on the femoral condyle during the implantation procedure to prevent such impingement from occurring. 
     Orthopaedic surgeons conventionally use a bone chisel to manually remove the anterior bone. However, such a manual procedure can easily be forgotten during surgery and, even when carefully completed, results in an undesirable non uniform bone edge and in the removal of an uncertain and varying amount of bone. 
     SUMMARY OF INVENTION 
     According to the present invention, there is provided a rotary mill comprising a body portion having a milling surface and a central bore extending along the rotary axis of the body portion; and a guide portion having a guide body and a guide peg extending from the guide body, the guide peg operable to be received in the central bore of the body portion, the guide body having at least one alignment feature which is the same as that of a prosthesis component. 
     By forming the guide body to have at least one alignment feature which is the same as that of a prosthesis component, the guide portion can be applied to the bone and fixed in place using the pre prepared fixation features that will hold the prosthesis component in place (for example peg holes or a recess for a flange). The guide body thus provides a precise reference of the final location of the prosthesis component to be implanted. The guide peg thus guides the body portion to mill an area of bone that is in a precise and predetermined location with respect to the eventually implanted prosthesis component. 
     A further advantage to the guide body being formed in this manner is that no additional bone must be removed in order for it to be fixed on the bone surface. The guide portion, having at least one alignment feature which is the same as that of a prosthesis component, can fit into the necessary recesses already formed in the bone to accommodate the final prosthesis component. This is in contrast to conventional guided mills which require a dedicated hole to be drilled to accommodate a separate guide rod. 
     The at least one alignment feature may be an attachment peg or may be a pair of attachment pegs. The attachment pegs may be operable to be received in pre prepared prosthesis peg holes. 
     The guide body may have substantially the form of a trial prosthesis component and may in fact comprise a trial prosthesis component. 
     The guide body may include at least one nodule, protruding from a surface of the guide body and operable to abut a corresponding abutment surface on the body portion. The nodules may thus act as depth stops to ensure a precise amount of bone is removed and avoid excessive bone removal. 
     The guide peg may comprise an abutment surface operable to abut a corresponding abutment surface in the central bore of the body portion. The guide peg may thus not only act to guide the angle at which the body portion mills bone surface but may also act as a depth stop to limit bone removal. 
     The abutment surface may comprise a distal surface of the peg or the abutment surface may comprise an outwardly projecting annular shoulder. 
     The corresponding abutment surface of the central bore may comprise a base of the bore or may comprise an inwardly projecting annular shoulder. 
     The guide peg may project from a predetermined region of, and at a predetermined angle to the guide body. In this manner the region of bone to be removed may be precisely determined and fixed by the construction of the guide portion, facilitating accuracy and repeatability of milling. 
     The guide peg may be adjustable on the guide body, allowing the surgeon a degree of freedom in selection of the milled area, and to accommodate for different patient geometries. 
     The guide body may be operable to be connected to additional surgical tools, thus facilitating and providing reference for additional bone removal steps. 
     The guide body may comprise a trial femoral prosthesis component which may be a trial unicondylar femoral prosthesis component. 
     The guide peg may project from an anterior portion of the guide body and may be operable to guide reaming of a region superior to the anterior edge of the guide body. 
     The guide peg may project from the guide body at an angle of between 25 and 40 degrees to the axis of the attachment peg. The angle may vary according to the size of the rotary mill. 
     The guide body may be operable to be connected to a posterior osteophyte guide. 
     The rotary mill may further comprise additional guide portions, each guide portion being of a different size so as to match differently sized prosthesis components that are employed for patents of differing sizes. 
     The rotary mill may further comprise additional guide portions, each guide portion having a different length, and or angle of extension of guide peg, thus also accommodating different patent geometries. 
     The body portion of the rotary mill may comprise a rotary body and a guide shaft at least partially received within the rotary body. 
     The guide shaft may comprise an inner portion telescopically received within an outer portion and a biasing element acting between the inner and outer portions. 
     The guide shaft may be received within an axial bore which may be formed in the rotary body. The bore may be a blind bore. 
     The biasing element may comprise a spring which may be mounted about the inner portion of the guide shaft. 
     The body portion may further comprise cooperating protrusions formed on the rotary body and the outer portion of the guide shaft, operable to engage one another as a depth stop. 
     The cooperating protrusions may comprise annular shoulders which may be formed on an inner surface of the rotary body and an outer surface of the outer portion of the guide shaft. 
     The outer portion of the guide shaft may comprise a substantially hollow shaft, a distal end of which may comprise the central bore of the body portion, operable to receive the guide peg. 
     The cutting surface of the body portion may be formed on an annular cutting tool which may be removably attached to the rotary body. 
     According to another aspect of the present invention, there is provided a method of implanting a unicondylar femoral component comprising,
         a) reaming the femoral condylar surface to accept the unicondylar femoral component;   b) drilling peg holes for affixing the unicondylar femoral component;   c) affixing a guide portion of a rotary mill onto the prepared condylar surface using the drilled peg holes;   d) reaming a portion of bone anterior to the affixed guide portion;   e) removing the guide portion from the bone; and   f) affixing a unicondylar femoral component to the prepared condylar surface.       

     The rotary mill may be a rotary mill according to the first aspect of the present invention. 
     According to another aspect of the present invention, there is provided a rotary cutting tool comprising a rotary body having a cutting surface and a guide shaft at least partially received within the rotary body, wherein the guide shaft comprises an inner portion telescopically received within an outer portion, and a biasing element acting between the inner and outer portions. 
     The biasing element of the tool thus acts to damp the telescoping motion of the guide shaft and hence, when received within the rotary body, damps progression of the rotary body along the guide shaft. This damping action can assist a surgeon with fine control of cutting or milling operations. 
     The guide shaft may be received within an axial bore formed on the rotary body. The bore may be a blind bore. 
     The rotary cutting tool may further comprise cooperating protrusions formed on the rotary body and the outer portion of the guide shaft, which protrusions may be operable to engage one another as a depth stop. 
     The protrusions may for example comprise annular shoulders which may be formed on an inside surface of the rotary body and an outer surface of the outer portion of the guide shaft. 
     The biasing element may comprise a spring. The spring may be mounted about the inner portion of the guide shaft. The spring may act between an end of the outer portion of the guide shaft and an end cap formed on the inner portion of the guide shaft. The end cap may be formed by an end of the axial bore in which the guide shaft is received. 
     The outer portion of the guide shaft may comprise a hollow shaft, a first end of which may receive the inner portion and a second end of which may be operable to receive a guide peg. 
     The cutting surface of the rotary cutting tool may be formed on an annular cutting plate which may be removably attached to the rotary body. 
     An end of the rotary body may terminate in an annular receiving plate, which may be operable to engage the cutting plate. 
     The rotary cutting tool may further comprise cooperating formations on the receiving plate and cutting plate, which may be operable to secure the cutting plate to the receiving plate. 
     The cooperating formations may comprise protrusions, for example screw heads, and appropriately shaped recesses. The formations may alternatively or further comprise magnetic elements. 
     Another end of the rotary body may terminate in a drive shaft which may be operable to engage a drive element such as a rotary drill. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the following drawings, in which: 
         FIG. 1  shows impingement of a meniscal bearing on anterior femoral bone. 
         FIG. 2  is a perspective view of a rotary mill. 
         FIG. 3  is a perspective view of a body portion of a rotary mill. 
         FIG. 4  shows a guide portion of a rotary mill in position on a femur. 
         FIG. 5  shows a rotary mill in position on a femur. 
         FIG. 6  illustrates bone removal by a rotary mill. 
         FIGS. 7 and 8  illustrate prosthesis components in position on a femur, with and without bone removal. 
         FIG. 9  illustrates an implanted unicondylar prosthesis. 
         FIG. 10  is a sectional view of a rotary cutting tool. 
         FIG. 11  is a sectional view of the tool of  FIG. 10  in a compressed condition. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIGS. 2 to 4 , a rotary mill  2  comprises a body portion  4  and a guide portion  6 . The body portion  4  comprises a rotary body  8  that terminates at one end in an annular milling surface  10 . The milling surface comprises a series of milling teeth  12  that extend from the surface  10 . In the illustrated embodiment, the milling surface  10  is formed on an annular shoulder  14  that protrudes outwardly from the rotary body  8 . An abutment surface  15  extends radially inwardly of the projecting milling surface  10 . At a second end of the rotary body  8  an integral shank  16  extends along an axis of rotation  18  of the rotary body. The shank  16  is suitable for attachment to the chuck of a surgical drill (not shown). A bore  20  extends through the rotary body  8  along the axis of rotation  18  of the rotary body  8 . 
     The guide portion  6  comprises a guide body  22  and a guide peg  24 . The guide body  22  comprises a trail unicondylar femoral prosthesis component. The guide body thus comprises a curved condylar plate  26  and two attachment pegs  28 ,  30 . The attachment pegs  28 ,  30  protrude from a bone contacting surface  32  of the condylar plate  26  at locations and angles precisely matching those of a similarly sized prosthesis component. The guide body can thus be attached to a prepared femoral condylar surface in the same manner and using the same drilled peg holes as for a prosthesis component. In this manner, the attachment pegs  28 ,  30  serve to align the guide body with the eventual location of the femoral prosthesis, referencing off the pre prepared drilled femoral peg holes. The guide peg  24  is a cylindrical peg that protrudes from an opposite, outer surface  34  of the condylar plate  26 . The guide peg  24  extends from an anterior portion of the condylar plate  26  along an axis that is substantially normal to the adjacent condylar plate surface  34 . The guide peg  24  projects at an angle of between 25 and 40 degrees to the axis of the attachment pegs. The precise angle is selected according to the size of the guide portion and associated anatomy, as discussed in further detail below. The guide peg  24  is dimensioned to be slidably and rotatably received within the central bore  20  of the rotary body  8 . With reference also to  FIG. 6 , two nodules  27 ,  29  protrude from the outer surface  34  of the condylar plate  26 . The nodules  27 ,  29  are formed on opposite sides of the anterior portion of the condylar plate  26 , in the region of the guide peg  24 . 
     The rotary mill  2  is used to remove anterior bone on the femoral condyle prior to implantation of a unicondylar femoral prosthesis. First, the condylar surface is prepared to receive the prosthesis, including resection of the entire condylar surface and drilling of femoral peg holes. The guide portion  6  of the rotary mill  2  is then fixed on the femoral condyle by inserting the attachment pegs  28 ,  30  into the pre drilled femoral peg holes. The guide portion  6  can be seen in position on the femoral condyle in  FIG. 4 . The body portion  4  is then seated on the guide portion  4 , the guide peg  24  being receiving within the bore  20  of the rotary body  8 . The shank  16  of the body portion is then attached to a surgical drill and the body portion  4  is guided to mill the bone superior to the anterior edge of the condylar plate  26  of the guide portion. The guide peg  24  guides the orientation of the body portion  4 , ensuring that the milling surface  10  removes the bone tissue from the correct location. The body portion  4  advances along the guide peg  24  as bone tissue is removed. As the body portion  4  advances, the nodules  27 ,  29  act as stop pegs, upper surfaces of the nodules  27 ,  29  abutting the stop surface  15  that extends radially inwardly from the annular milling surface  10  and preventing further movement of the body portion  4 , thus limiting the amount of bone that is removed. The body portion  4  is dimensioned so as to ream only the anterior bone superior to the guide portion. As can be seen from  FIGS. 5 and 6 , the milling surface  10  does not disturb the adjacent soft tissues and so causes minimal damage or disruption to the surrounding structures, removing only that bone which is desired to be removed. As illustrated particularly in  FIG. 5 , the guide peg  24  is angled such that, when fully seated on the guide peg  24 , the body portion  4  has only reamed the bone superior to the anterior edge of the guide portion  6 . The reamed area of bone can be seen at area  38  on  FIG. 6 . 
     Additional tools can then be attached to the guide portion if necessary. For example, a posterior osteophyte guide  40  can be attached posteriorly to the guide portion  6 . The osteophyte guide is a slotted tool that may be used to guide a chisel to remove osteophytes from the posterior area of the femoral condyle, helping to prevent femoral loosening. 
     Once all necessary bone removal has been completed, the guide portion  6  of the rotary mill  2  is removed and the appropriate prosthesis component is implanted.  FIGS. 7 and 8  illustrate the area  38  of bone that is removed by the rotary mill  2 . On  FIG. 8 , this area  38  can be seen immediately superior to the femoral prosthesis component  50 . On  FIG. 7 , the rotary mill  2  has not been used and bone tissue remains superior to the prosthesis component  50 . This bone tissue will cause impingement of the meniscal component, as illustrated in  FIG. 1 . In contrast, and as illustrated in  FIG. 9 , when the mill has been used to remove bone over the desired area  38 , no impingement of the meniscal component is seen, even with the knee in full extension. 
     It is envisaged that the guide peg  24  may be integral with the guide body  22  or may be detachable from, or adjustable relative to, the guide body  22 , so as to allow limited adjustment of the angle of the guide peg  24  or of the height of the guide peg  24 . Such adjustment allows a degree of flexibility to the surgeon in tailoring the rotary mill  2  to the precise needs of individual patients. For example, if it is desired to remove less than the usual amount of bone, the guide peg  24  may be caused to protrude further from the surface of the condylar plate  26 . In this instance, the guide peg  24  also acts as a stop peg, the end surface  36  of the guide peg  24  contacting the base (not shown) of the bore  20  and preventing further movement. The guide peg may be caused to protrude to such an extent that it is engages as a stop peg before the stop surface  15  of the body portion  4  contacts the nodules  27 ,  29  of the guide body. It is also envisaged that the guide portion  6  of the rotary mill be provided as merely one of several available guide portions, each being of a different size to accommodate different sizes of knee. Thus, each size of prosthesis may have an associated guide portion  6  of the appropriate size. Each guide portion  6  will have a suitable guide peg, of a height and at an angle that is determined to be most appropriate for the associated prosthesis. 
     It will be appreciated that the guide portion  6  may be employed together with other embodiments of body portion  4 , including a range of rotary cutting devices. One embodiment of rotary cutting tool with which the guide portion  6  may be employed is illustrated in  FIGS. 10 and 11 . The rotary cutting tool  100  comprises a rotary body  102  and a guide shaft  104 . The guide shaft  104  is at least partially received within a blind axial bore  106  formed within the rotary body  102 . A closed proximal end (towards the left in the Figures) of the rotary body  102  terminates in a drive shaft  108 , operable to be received within the chuck of a surgical drill (not shown). An open distal end of the rotary body  102  flares outwards to terminate in an annular receiving plate  110  extending about the opening of the axial bore  106  and described in further detail below. 
     The guide shaft  104  comprises an inner portion  112  and an outer portion  114 . The inner portion  112  comprises a solid shaft a distal end  116  of which is telescopically received within a proximal end  118  of the outer portion  114 . The outer portion  114  comprises a substantially hollow shaft. A biasing spring  120  is mounted about the inner portion  112  of the guide shaft  104 . The spring  120  rests at one end on the annular end surface  122  of the proximal end  118  of the outer portion  114 . The other end of the spring  120  engages on an end cap  124  formed on a proximal end  126  of the inner portion  112 . In an alternative embodiment (not shown) the spring  120  may engage on the blind end  128  of the axial bore  106  in which the guide shaft  104  is received. 
     The guide shaft is received freely within the bore  106  of the rotary body  102 . An annular shoulder  130  is formed on the inner surface of the bore  106 , dividing the bore into a distal section and a proximal section, the distal section being of larger inside diameter than the proximal section. A corresponding annular shoulder  132  is formed on the outer surface of the outer portion  114  of the guide shaft, dividing the outer portion into proximal and distal sections, the distal section being of larger outside diameter than the proximal section. The corresponding annular shoulders  130 ,  132  function as a depth stop, preventing the outer portion  114  of the guide shaft  104  from being received into the rotary body  102  beyond a certain point. This position is illustrated in  FIG. 11 . The larger diameter distal section of the outer portion  114  of the guide shaft may also serve to centre the guide shaft within the bore  106  of the rotary body  102 . 
     Referring particularly to  FIG. 10 , the annular receiving plate  110  comprises a series of formations, operable to releasably engage an annular cutting plate (not shown). The formations comprise at least one screw head  134  and a plurality of magnets  136 , the magnets being recessed into the annular receiving plate so as to present a smooth surface. The annular cutting plate (not shown) comprises an annular cutting surface, similar to that described above with reference to the body portion  6 , and an opposed annular engaging surface. The annular engaging surface comprises corresponding recesses and magnetic elements enabling the cutting plate to be releasably yet securely attached to the receiving plate  110  of the rotary body. 
     In use, the rotary cutting tool  100  is first assembled and then placed over the guide peg  24  of the guide portion  6 . The guide peg  24  is received within the hollow outer portion  114  of the guide shaft until a distal end  138  of the outer portion  114  is seated against the surface  34  from which the guide peg  24  protrudes. The rotary body  102  is then connected to a surgical drill (not shown) via the drive shaft  108  and the rotary tool is guided to mill away the desired area of bone. During the cutting operation, the outer portion  114  of the guide shaft  104  remains seated in position over the guide peg  24 . Downward pressure applied to the rotary body engages the blind end  128  of the bore  106  against the end cap of the inner portion  112  of the guide shaft  104 , causing the inner portion  112  to be pushed further into the outer portion  114 . This action compresses the spring  120  acting between the inner and outer portions. In this manner, the spring  120  damps the downward motion of the rotary body, assisting the control of the surgeon and thus increasing the ease with which the tool may be employed. The inner portion  112  of the guide shaft  104  continues to slide further into the outer portion  114  until the annular shoulder  130  on the rotary body  102  engages the annular shoulder  132  on the outer portion  114  of the guide shaft. At this point the rotary body cannot travel any further towards the bone and the drilling action is ceased. In this manner, the annular shoulders act as a depth stop, preventing over reaming of the bone. 
     It will be appreciated that the present invention provides a means for accurately, predictably and repeatably removing a targeted area of bone from the femoral condyle. The amount of bone removed is determined by the precise angle and height of the guide peg  24 . These aspects of the guide peg  24  are determined when the guide portion  6  is initially formed and can thus be carefully assessed and fixed so as to guide milling of precisely the correct amount of bone for the associated prosthesis. The present invention is also bone conserving, requiring no additional drill hole for a guide rod, as is conventionally required for a guided mill. By fastening to the bone using the existing femoral peg holes, the guide portion  6  makes use of existing features, and requires no additional bone removal for fixation. The femoral peg holes in fact determine the eventual location of the milled bone, as they provide the location for the guide portion  6 . As these peg holes also provide location for the final prosthesis component, considerable time and development effort has been devoted to tools and techniques to ensure the accurate positioning of the peg holes in the femur. The present invention makes indirect use of these pre-existing tools and techniques in employing the femoral peg holes as the fixation means for the guide portion  6  of the rotary mill  2 . 
     The present invention additionally provides a rotary cutting tool optimised for use with the guide portion  6 , the action of which is damped or cushioned, improving ease of use for the surgeon.