Abstract:
A method of milling a calcar region of the femur. The method includes using a miller assembly including a cutter and a frame. The frame has a longitudinal axis and a cutter mount for mounting the cutter at a first angle with respect to the longitudinal axis of the frame. The cutter mount extends at the first angle from the longitudinal axis of the frame and receives a portion of the cutter and maintain the received cutter oriented at the first angle during rotation. The frame includes a handle that forms a portion of a drive joint for coupling the frame to a drill. The handle is coupled to the drill and the miller assembly is inserted into the femur such that the cutter is located in the calcar region. The drill is then operated, causing the cutter to rotate and mill bone in the calcar region.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a divisional of U.S. patent application Ser. No. 12/241,418 entitled “Minimally Invasive Bone Miller Apparatus” filed on Sep. 30, 2008. 
     
    
     TECHNICAL FIELD OF THE INVENTION 
       [0002]    This invention relates to the field of artificial joint prostheses and, in particular, to an improved instrument for machining a precise cavity in bone for receiving a prosthesis. 
       BACKGROUND OF THE INVENTION 
       [0003]    For implantation of prosthetic stems, such as hip stems, accurate preparation of the bone or intramedullary canal is extremely important in order to guarantee good contact between the prosthesis stem and the bone. The underlying concept behind precise preparation is that a precise bone envelope reduces the gaps between the implant (i.e. prosthesis or prosthetic component) and the bone, thereby improving the initial and long-term bone ingrowth/fixation. The bone canal is presently prepared for implantation of a prosthetic stem by drilling and reaming a resected end of a bone, such as a femur, and then preparing an area adjacent the drilled hole to provide a seat for the prosthetic stem or a proximal sleeve coupled to the stem of a modular prosthetic system. 
         [0004]    Modular prosthetic systems using proximal sleeves, stems, necks and heads, such as the S-ROM Modular Hip System, available from DePuy Orthopaedics, Warsaw, Ind., put more control in the hands of the surgeon, providing solutions for a variety of surgical scenarios, from primary total hip arthroplasty (THA) to the complex revision or DDH challenges. Such system provides such versatility because the provided plurality of stems, sleeves, necks and heads which can be assembled in a large number of configurations. 
         [0005]    Preparation of the area adjacent the drilled hole may be accomplished by broaching or by milling. Broaches or rasps, when used for bone preparation, have limitations. One such limitation is the risk of fracture during broaching. Since broaching is done by pounding the broach into the bone, the bone tends to fracture. Additionally, both broaches and rasps suffer from a tendency to be deflected by harder sections of bone so that they do not create as precise a triangular cavity as can be created by a miller system. In a study that compared an intimate fill with robotically machined femoral, Paul et al., found that broaching tore the trabecular bone, whereas femoral canal preparation with reamers was consistently more accurate. Paul, H. A., et al. “Development of a Surgical Robot for Cementless Total Hip Arthroplasty.” Clinical Orthopedics and Related Research 285 December 1992: 57-66. 
         [0006]    Thus, milling is currently the preferred method of bone preparation in many orthopaedic applications because it is an extremely precise method of bone preparation. A limitation of milling systems today is that they are typically formed so that the drive shaft extends at an angle relative to the remainder of the frame from the end of the miller cutter machining the bone. A fairly large incision must be made to accommodate such milling assemblies. A typical incision for preparing a femur for a total prosthetic hip replacement using a standard triangle miller system is eight to ten inches long. It is not uncommon for incisions as large as 12 inches to be used in a total hip replacement procedure. 
         [0007]    A standard triangle miller system typically includes a miller shell, a miller frame and a miller cutter having an end formed for coupling to a drill. A typical miller frame and miller cutter can be seen in U.S. Pat. No. 5,540,694 issued to DeCarlo, Jr. et al. on Jul. 30, 1996. This miller frame allows for precise machining of the triangular canal by a miller cutter held at an angle with respect to the shaft of the frame. The triangular canal facilitates an accurate fit of a proximal sleeve that distributes the load experienced by the prosthesis evenly and provides rotational stability. However, to accommodate this miller, it is necessary to make a fairly large incision which may be undesirable for cosmetic or other reasons. 
         [0008]    The large incision is required because the miller cutter includes a fixed input shaft for connecting to and/or receiving motive (i.e. rotary) power from a drill or similar instrument. As such, the prior reamer is able to accept rotary input power with respect to only one direction. Typically, this direction is at 0° (i.e. “straight on”) with respect to the reamer which is approximately thirty two degrees with respect to the shaft of the miller frame. Therefore, not only is the input power direction restricted, but this, in turn, restricts the angle at which the reamer may be used on a patient. Since the input shaft and the drill coupled thereto extend laterally beyond the edge of the miller frame an incision substantially larger than the width of the frame must be made to accommodate the reamer, frame and drill during surgery. The incision must be large enough to accommodate the reamer, frame, input shaft and drill without the input shaft engaging soft tissue. 
         [0009]    Recently, there have been some millers developed for minimally invasive surgery, however, they still require an incision of about three inches. However, as other instruments only require about one and a half to two inches, it is not ideal to use a miller that requires a larger incision. 
         [0010]    In view of the above, it would be desirable to have a bone miller or guided reamer that could fit into a smaller incision during a surgical process. 
       SUMMARY OF THE INVENTION 
       [0011]    According to one embodiment of the present invention, a miller assembly for creating a cavity in a bone is provided. The cavity has a cross section which has a generally triangular profile having a first side generally parallel with an axis of the bone and a second side forming an acute angle with the first side, and is contiguous with a pre-existing conical cavity in the bone. The assembly includes a cutter and a frame for carrying the cutter. The frame includes a connection portion having a longitudinal axis and a cutter mount for mounting the cutter at a first angle approximating the acute angle with respect to the longitudinal axis of the connection portion. The cutter mount extends at the first angle from the longitudinal axis of the connection portion and is configured to receive a portion of the cutter and maintain the received cutter oriented at the first angle during rotation. The frame further includes a handle configured to form a portion of a drive joint for coupling the frame to a drill, the handle having a longitudinal axis. The longitudinal axis of the handle is coincident with the longitudinal axis of the frame. 
         [0012]    According to another embodiment of the present invention, a miller assembly for creating a cavity in a bone is provided. The system includes a cutter and a frame for carrying the cutter. The frame has a longitudinal axis and a cutter mount for mounting the cutter at a first angle with respect to the longitudinal axis of the frame. The cutter mount extends at the first angle from the longitudinal axis of the frame and is configured to receive a portion of the cutter and to maintain the received cutter oriented at the first angle during rotation. The frame further includes a handle configured to form a portion of a drive joint for coupling the frame to a drill having a longitudinal axis. The handle has a longitudinal axis, such that the longitudinal axis of the handle is coincident with the longitudinal axis of the frame. The cutter is coupled to the connection portion and handle such that when the drill is activated, the cutter rotates. 
         [0013]    According to yet another embodiment of the present application, a method of milling a calcar region of the femur is provided. The method including providing a miller assembly including a cutter and a frame for carrying the cutter. The frame has a longitudinal axis and a cutter mount for mounting the cutter at a first angle with respect to the longitudinal axis of the frame. The cutter mount extends at the first angle from the longitudinal axis of the frame and is configured to receive a portion of the cutter and to maintain the received cutter oriented at the first angle during rotation. The frame further includes a handle configured to form a portion of a drive joint for coupling the frame to a drill, the handle having a longitudinal axis. The handle is coupled to the drill. The miller assembly is inserted into the femur such that the cutter is located in the calcar region. The drill is then operated, causing the cutter to rotate and mill bone in the calcar region 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in connection with the accompanying drawings, in which: 
           [0015]      FIG. 1  is a perspective view of a miller system according to one embodiment of the present invention; 
           [0016]      FIG. 2  is an exploded view of the miller system of  FIG. 1 ; 
           [0017]      FIG. 3  is a perspective view of the miller frame of  FIG. 1  illustrating the mating of the gears; 
           [0018]      FIG. 4  is a perspective view with parts broken away of the miller system of  FIG. 1  inserted into a resected femur of a patient; 
           [0019]      FIG. 5  is a flow chart describing a method of using the miller of  FIG. 1 ; and 
           [0020]      FIG. 6  is a perspective view of a miller system according to another embodiment of the present invention. 
       
    
    
       [0021]    Corresponding reference characters indicate corresponding parts throughout the several views. Like reference characters tend to indicate like parts throughout the several views. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    Embodiments of the present invention and the advantages thereof are best understood by referring to the following descriptions and drawings, wherein like numerals are used for like and corresponding parts of the drawings. 
         [0023]    The disclosed calcar miller assembly  10  allows a surgeon to machine (mill) bone through a smaller incision compared to existing surgical instruments. As shown in  FIG. 1 , the miller assembly  10  includes a frame  12  and a miller cutter  14  that is coupled to the frame  12  at both a proximal end  16  and a distal end  18 . The frame  12  includes a longitudinal axis  20 . The miller cutter  14  also has a longitudinal axis  22  that is offset at an angle α from the longitudinal axis  20  of the frame  12 . The angle α is greater than 0 degrees. In some embodiments, the angle α is between about 30 and about 60 degrees. In some embodiments, the angle α is between about 40 degrees and about 50 degrees. 
         [0024]    Turning now to  FIG. 2 , an exploded view of the calcar miller assembly  10  of  FIG. 1  is illustrated. As shown, the frame  12  includes three parts: a handle  24 , a connection portion  26 , and a pilot shaft  28 . The handle  24  includes a drill-connection end  30  and a connection-portion end  32 . The drill-connection end  30  includes a drill-connection feature  34  adapted to connect the handle to a drill (not shown), creating a drive joint. The drill will drive the miller cutter  14 . In this embodiment, the handle  24  also becomes the drive shaft, such that the longitudinal axis of the handle is coincident with the longitudinal axis of the drive shaft. 
         [0025]    The connection-portion end  32  of the handle  24  includes a gear  36  that will couple with a gear  38  of the connection portion  26 , as will be described in more detail below. The connection-portion end  32  of the handle  24  also includes a rod  40 , for connecting the handle  24  to the connection portion  26 . The rod  40  includes a locking mechanism  42  that corresponds to a locking mechanism  44  (shown in phantom) on the connection portion  26 . The rod and connection portion locking mechanisms  42 ,  44  may include a recess on one and an internal ridge on the other. Other locking mechanisms, including, but not limited to, threads, tapers, locking bulbs, and locking tabs, may be used. Also, the pieces may be welded together. 
         [0026]    Also, in other embodiments, the rod  40  may be located on the connection portion  26 . The rod  40  would engage a cavity in the handle  24 . 
         [0027]    The connection portion  26  includes a handle-connection end  46  and a shaft-connection end  48 . The handle-connection end  46  couples to the handle  24  as described above. The handle-connection end  46  includes an outwardly extending flange  50 . The flange  50  includes a recess  52  for receiving the gear  38 . The gear  38  couples to a lower gear  54  via a thread  56  on the gear  38  that matingly engages an internal thread  58  (as shown in phantom) on the lower gear  54 . The miller cutter  14  slides onto a cutter mount, or an outwardly extending pole  60 . The pole  60  extends at the desired angle α as described and shown above in reference to  FIG. 1 . The outwardly extending pole  60  includes a locking mechanism  62 , which in the illustrated embodiment, is a nut  62   a  ( FIG. 1 ) and a threads  62   b  on the pole  60 . Alternatively, other locking mechanisms such as taper locks, locking bulbs, locking tabs and other known locking mechanisms may be used. The miller cutter  14  also includes a gear  63  that will mate with the lower gear  54 . Also, the pieces may be welded together. 
         [0028]    The shaft-connection end  48  of the connection portion  26  includes a locking mechanism  64  for locking the connection portion  26  to the shaft  28 . As shown, the locking mechanism  64  includes a recess  66  which will engage an internal ridge  68  (shown in  FIG. 3 ) on the shaft  28 . Other locking mechanisms, including, but not limited to, threads, tapers, locking bulbs, and locking tabs, may be used. In other embodiments, the pieces may be welded together. 
         [0029]    In an alternative embodiment, the shaft  28  will be connected to a miller shell (not shown). The handle  24  and connection portion  26  will slide into the miller shell. The handle  24 /connection portion  26  can be moved up and down relative to the miller shell in order to adjust the depth of the drilling. 
         [0030]    Returning now to  FIG. 1 , a shroud  69  is shown in phantom. In some embodiments, the shroud  69  may be included to cover the gears  36 ,  38 ,  54 ,  63 . The shroud  69  would be to protect the gears  36 ,  38 ,  54 ,  63  during the surgery. Also, the shroud  69  could keep the gears  36 ,  38 ,  54 ,  63  from engaging the bone and/or soft tissue. 
         [0031]    Turning now to  FIG. 4 , a perspective view of the connection portion  26  with the miller cutter  14  placed on the pole  60  is shown. The connection portion  26  is shown in a femur  70 , with the miller cutter  14  located in the calcar region  72 . As shown, the handle gear  36  will rotate with the handle  24 . As the handle gear  36  is rotated, it will engage the top connection portion gear  38  to rotate, causing the bottom connection gear  54  to rotate. The bottom connection gear  54  is engaged with the miller cutter gear  63  such that when the bottom connection gear  54  rotates, so does the miller cutter gear  63 . As the miller cutter gear  63  rotates, so do the blades of the miller cutter  14  about the axis  22  of the rod  60 . The result is that the calcar region  72  is milled in preparation for receiving an implant. 
         [0032]    Turning now to  FIG. 5 , a flow chart describing the operation of the miller  10  will be described. First, at step s 100 , the miller will be inserted into the femur  70 , with the miller cutter  14  being inserted into the calcar region  72 . At step s 102 , the drill causes the handle  24  and handle gear  36  rotate. The rotation of the handle gear  36  causes the lower gear  54  to rotate (step s 104 ), which in turn drives the rotation of the miller cutter  14  (step s 106 ). The miller cutter  14  then mills the bone in the calcar region  72  of the femur  70 , preparing the calcar region for the insertion of the implant. 
         [0033]    In some embodiments, the miller  10  can be inserted into a miller shell (not shown). The miller shell is inserted into the femur and the miller  10  can be adjusted vertically so as to adjust the cutting depth. 
         [0034]    Turning now to  FIG. 6 , a second embodiment of the present invention is illustrated. In this embodiment, the handle  24  of the miller assembly  10  is coupled to the connection portion  26  via a belt  74 . As shown, the belt  74  winds around a cylinder  76  located on the handle  24  and a cylinder  78  on the flange  50 . The cylinder  78  on the flange is coupled to the bottom connection gear  54  in the same way that the top connection gear  38  is coupled to the bottom connection gear  54  in the above-described embodiment. As the handle  24  is rotated, the cylinder  76  rotates, causing the belt  74  to move. The movement of the belt  74  then causes the cylinder  78  of the flange to rotate, rotating the upper gear  38  and bottom connection gear  54 . Then, in the same manner as described above, the bottom connection gear  54  then couples with the cutter gear  63 . 
         [0035]    In some embodiments, the miller cutter assembly  10  is made of stainless steel or other biocompatible metal, such as titanium or cobalt chrome. Any other sterilizable metal may be used. In other embodiments, the handle  24 , connection portion  26 , and pilot shaft may be disposable and made of a biocompatible plastic such as polycarbonate, LEXAN®, ULTEM®, both manufactured by Sabic Innovation Plastics of Houston, Tex., CELCON®, manufactured by Ticona of Florence, Ky., UDEL®, RADEL®, ACUDEL®, MINDEL®, Epispire, Primospire, TORLON®, all manufactured by Solvay Plastics of Brussels, or any other biocompatible plastic, while the miller cutter  14  is made of stainless steel or other sterilizable metal. 
         [0036]    In the embodiments shown and described in reference to  FIGS. 1-6 , the width of the device is less than about 2 and one half inches, and in some embodiments, less than about 2 inches. By coupling the handle to the drive shaft directly, the surgical opening can be smaller than other current designs. 
         [0037]    In the above described embodiments, the frame  12  was in three parts. However, in alternative embodiments, the frame may be a single piece or may have more or less than three parts. 
         [0038]    Although specific embodiments of the invention have been described herein, other embodiments may be perceived by those skilled in the art without departing from the scope of the invention as defined by the following claims. For example, although the invention has been described in terms of the implantation of the femoral portion of a hip prosthesis, it can be used with prostheses for other joints such as the shoulder, knee, or elbow.