Abstract:
A long bone axis finder has a shaft extending along an axis and a first and second end. A tip portion is provided for releasable coupling to the shaft first end. A wedge element having an open side is provided for lateral engagement with said shaft. The wedge element is slidable along said shaft in a direction from the second to the first end. The axis finder is inserted into a long bone medullary canal until the tip portion engages a narrower portion of the canal. The wedge portion which is larger than the tip is laterally placed on the shaft and engages a portion of the canal closer to the shaft second end.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates to navigated instrumentation for the preparation of the proximal femur for receiving a prosthetic femoral component. More particularly, the invention relates to a femoral axis finder which can be used with an optical computer-aided navigation system to accurately locate the femoral axis utilizing a shaft with a proximal and distal wedge for insertion in the medullary canal of the femur. 
   Navigation systems are an interactive operative monitoring system designed to improve the surgical performance and clinical outcome of Total Hip Arthroplasty. A computer-aided, image-less guidance system provides accurate decision making for alignment and orientation of instruments, trials and ultimately implants. The system may provide surgeons pre-operative, intra-operative and post-operative assessments of the patient&#39;s joint kinematic measurements. Use of a computer-aided surgical navigation system results in decreased morbidity and shorter hospitalization. Such systems are described in U.S. Pat. Nos. 6,021,343, 6,385,475 and 6,434,415, the teachings of which are incorporated herein by reference. 
   In the past, the medullary canal of the femur has been opened by standard operative techniques. The surgeon would then digitize the distal and proximal points on a canal axis finder, which finder may be a shaft with a T-handle and a proximal plug for engaging the proximal femur. The shaft is digitized by touching two points on the shaft with a pointer optically coupled to the computer navigation system. Normally, the pointer would have multiple light emitting diodes which, upon activation, emit light which can be read by one or more cameras in the operating room which are connected to the computer-aided navigation system. Digitizing the distal and proximal points on the axis finder allows the navigation system to determine the axis along which a femoral broach has to be aligned to prepare the femoral canal. For patients with varus/valgus deformity, the shaft axis may deviate significantly from the anatomical axis of the femur. This deviation can be calculated by the navigation system and displayed on a CRT showing the proximal femur and the shaft axis. 
   Specifically, the shaft axis is digitized by inserting the optical tracker tip through the axis finder handle and digitizing a first distal point thereon and then inserting the tracker&#39;s tip through the handle to digitize a second point thereon located proximally of the first point on the axis finder. The navigation system checks the distance between the digitized distal and proximal points. Once the shaft axis has been digitized, the surgeon can proceed with preparing the femur. The surgeon removes the axis finder and can then insert a broach into the canal, which broach includes an optical tracker preferably on the broach handle. Thus, the navigation system can align the axis of the broach and the proper anteversion/retroversion angle in the proper frontal offset and varus/valgus angle of the broach. The axes of the broach is shown on the CRT display along with the axis found by the axis finder so that the surgeon can align the axes. 
   One drawback of this system is that it is difficult to get a snug fit both proximally and distally for all size femurs while using a single shaft axis finder. Thus, the present invention provides for a modular system having a plurality of distal and proximal spacers. 
   SUMMARY OF THE INVENTION 
   One aspect of the invention is to accurately locate the femoral axis and have the ability to translate that axis to a computer-aided navigation instrument device. 
   In the present invention, a “wedge” device is used to locate two spaced points within the canal that best represent a circular cross-section. These two points then represent the location and trajectory of the proximal femoral axis. The navigation “smart” instrument may be used in one of two ways. The first would be to use a navigation pointer to digitize the shaft of the instrument. The second would be to use one of the optical trackers mounted to a post on the instrument itself. 
   The axis finder instrument assembles and functions in the following manner. The first step is optional and reflects a method of removing the instrument and/or placing the femoral sleeve wedges into the canal. One first assembles an insertion sleeve over the instrument shaft until it butts up against the distal surface of the instrument handle at the shaft proximal end. The second step is to assemble the appropriate distal-sizer onto the distal end of the instrument shaft. 
   The third step is to plunge the distal end of the instrument into the proximal femoral canal until the distal sizer “wedges” and stops. This wedging occurs because the femoral canal gradually narrows on moving towards the isthmus of the canal. The fourth step is to assemble one of the proximal sizer wedge devices onto the shaft by side loading it onto the shaft. The proximal sizer wedge devices preferably come in at least four (4) different sizes, for example, ranging from 10-15 mm, 15-20 mm, 20-25 mm, and 25-30 mm. 
   The fifth step is to drive the proximal wedge device into the femoral canal. Optimally one would wedge the device at a level of about 20 mm below the midpoint of the lesser trochanter. This may be achieved by using the insertion sleeve which screws into the end of the proximal wedge device or by pushing the device into the canal with the fingers or forceps. Once both the distal sizer and the proximal wedge are located, a navigation instrument may be used to digitize the central axis. This may be done by putting the navigation pointer down the handle of the instrument in line with the femoral axis and digitizing at least two points. It may also be accomplished by using an optical navigation tracker that is mounted on the axis finder a fixed distance and angle from the instrument shaft. The digitized axis is then transmitted to the navigation unit through the navigation trackers and camera technology. 
   By accurately locating the axis of the femoral canal over a distance of 50-100 mm and being able to transfer that information to a navigation system, it allows the surgeon to know the precise location of the femoral canal during preparation and final implant insertion. Therefore, by using navigation in the preparation and final prosthesis insertion process, the surgeon can understand the relationship between the femoral axis and either the broach or the final prosthesis. Thus, the surgeon can judge varus/valgus angle of the broach and/or stem. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of the navigated femoral axis finder of the present invention; 
       FIG. 2  is a side view of the femoral axis finder of  FIG. 1 ; 
       FIG. 3  is an enlarged cross-sectional view of the femoral axis finder handle of  FIG. 1  along lines  3 - 3 ; 
       FIG. 4  is a perspective view of the insertion sleeve of the femoral axis finder of  FIG. 1 ; 
       FIG. 5  is a cross-sectional view of the insertion sleeve of  FIG. 4  along lines  5 - 5 ; 
       FIG. 6  is a perspective view of the shaft of the femoral axis finder of  FIG. 1 ; 
       FIG. 7  is a side view of the proximal wedge of the navigated femoral axis finder of  FIG. 1 ; 
       FIG. 8  is an end view of the proximal wedge of  FIG. 7 ; 
       FIG. 9  is a side view of the cylindrical distal section of the femoral axis finder of  FIG. 1 ; 
       FIG. 10  is a cross-sectional view of the cylindrical distal extension of  FIG. 9  along lines  10 - 10 ; 
       FIG. 11  is a cross-sectional view of the cylindrical distal extension along lines  11 - 11  of  FIG. 1 ; and 
       FIG. 12  is a view of the axis finder being used within a femur. 
   

   DETAILED DESCRIPTION 
   Referring to  FIGS. 1 and 2 , there is shown the navigated femoral axis finder instrument of the present invention generally denoted as  10 . While the axis finder of the present invention is described for use with a femur it could just as easily be used with other long bones such as the tibia and humerus. The axis finder  10  has a proximal handle portion  12 , a shaft  14 , a proximal wedge  16  and a generally cylindrical distal extension or tip  18 . An insertion, sleeve  20  is provided on shaft  14  for sliding engagement therewith. Insertion sleeve  20  slides on shaft  14  and contacts the proximal end  22  of the proximal sizer  16  to move the sizer into contact the bone in the proximal medullary canal after insertion of the femoral axis finder therein. Of course, the surgeon could use other means of moving proximal wedge  16  into a tight fit with the canal. 
   Referring to  FIG. 3 , there is shown a cross-sectional view of handle  12  along lines  3 - 3 . Handle  12  has a receptacle  30  at a distal end thereof for receiving the proximal end of shaft  14 . Handle  12  also includes a plurality of cross-bores  32  and  34  which are provided for receiving a tip an optical navigation system tracker as will be discussed in more detail below. In addition, handle  12  includes an axial aligned bore  36  which is co-axial with socket  30  and therefore co-axial with shaft  14  after assembly. In the preferred embodiment, handle  12  is made of a stainless steel material such as 17-4 stainless steel. 
   Referring to  FIGS. 4 and 5 , there is shown insertion sleeve  20  prior to its being mounted on shaft  14 . Insertion sleeve  20  has a hollow bore  40  adapted to slidingly engage the outer diameter of shaft  14 . At the proximal end of sleeve  20  is a gripping element  42  which can be grasped between the thumb and forefinger to slide sleeve  20  along shaft  14  in the distal direction. In the preferred embodiment, sleeve  20  includes a threaded end  44  which is adapted to threadably engage the proximal end of proximal wedge  16 . The threading is optional and the end  44  can be made non-threaded and still act to push wedge  16  in the distal direction. 
   Referring to  FIG. 6 , there is shown shaft  14  which includes a proximal end  46  and a preferably threaded distal end  48 . In the preferred embodiment, shaft  14  extends along axis  49  and has a first generally cylindrical section  50  and a second undulating section  52 . Undulating section  52  includes cylindrical portions  54  separated by concave portions  56 . Proximal end  46  may include a cross-bore  57  to enable shaft  14  to be pinned to handle  12 . 
   Referring to  FIGS. 7 and 8 , there is shown an enlarged view of proximal wedge  16  which, in the preferred embodiment, includes a first conically tapered end  60 , a proximal end  62  and a throughbore  64 . Throughbore  64  is preferably threaded at end  62  and includes a pair of convex surfaces  66  extending into bore  64  adjacent threaded end  62 . As best seen in  FIG. 8 , wedge  16  is open along one side  68  so that it may be inserted onto shaft  14  in a direction transverse to the axis  49  of shaft  14 . Once placed on shaft  14 , concave inwardly extending surfaces  66  engage any one of the concave surface segments  56  to allow wedge  16  to be held onto shaft  14  both laterally and in its longitudinal position along axis  49  in a plurality of different axial positions. Preferably, wedge  16  is made of a polymeric material and has sufficient flexibility to allow surfaces  66  to expand on contacting cylindrical surface  54  and to snap inwardly and hold on portions  56  of shaft  14 . Thus, portions  56  act as discrete stops along the shaft. In the preferred embodiment, wedge  16  includes threaded proximal bore  70  can engage the threaded section  44  of sleeve  20 . 
   Referring to  FIGS. 9 through 11 , there is shown the distal extension  18  of the present invention which may also be made of a stainless steel material. In the preferred embodiment, extension  18  has a generally cylindrical shape and includes five protrusions  71  symmetrically oriented about the outer circumference. The use of the protrusions avoids pressure building in the canal during insertion of the instrument. As best seen in  FIG. 11 , the proximal end of the distal extension  18  includes a bore  72  which, in the preferred embodiment, has a taper “A” to allow for the ease of insertion of the distal tip of shaft  14 . In the preferred embodiment, extension  18  includes a transverse bore  74 . 
   Referring to  FIG. 12 , there is shown the femoral axis finder  10  of the present invention inserted into the medullary canal  90  of a femur  92 . 
   Referring to  FIG. 12 , the preferred method of use of the navigated femoral axis finder involves sliding insertion sleeve  20  onto the proximal end of shaft  14  and attaching the distal extension  18  to the distal end of shaft  14 . Preferably, the handle  12  is fixedly attached to the proximal end of shaft  14  prior to use of the insertion sleeve. The proximal wedge  16  may then be inserted laterally onto the shaft  14 . The distal end of the assembly is then placed in the medullary canal of the femur and moved towards the isthmus  94  of the canal until the distal extension engages the isthmus  94 . To ensure engagement, a kit of distal extensions increasing in 1 mm increments from, in the preferred embodiment, 11 mm to 20 mm are provided to accommodate various size femurs. Once the distal extension is engaged in the isthmus, the surgeon moves the proximal wedge distally by grasping holding part  42  of insertion sleeve  20  moving the proximal wedge distally. This is done until the enlarged distal end  60  of proximal wedge  16  engages bone. To accommodate various size femurs, a kit of parts wherein the proximal wedge maximum diameter varies in 1 mm or 2 mm increments from between 10 mm and 30 mm is provided. If, intraoperatively, the surgeon is not able to seat the proximal wedge in a proper position within the femur, he can easily remove one size femoral wedge by moving it laterally off the shaft and substituting a larger size femoral wedge, again mounting it by moving it in a direction transverse to the shaft. The interaction of convex portion  66  and concave shaft portion  56  allows the surgeon to move the wedge distally and fix it on shaft  14  in an axial position where it contacts bone in the proximal femur. In addition, if necessary, the kit may include various lengths of shaft  14  to accommodate different size femurs. 
   Once the proximal wedge engages bone in the proximal femur, shaft  14  is closely aligned with the axis of the femur and then the handle portion  12  can be used to digitize the location of the axis  49 . In one embodiment, this is done by taking the standard tracker  100  shown in  FIG. 12  and inserting it either into the axial handle hole  36  or into one of the cross-bore holes  32  and  34  (as shown) which extend perpendicularly to the axis  49 . Once the tracker pointer has been properly positioned in handle  12 , its location is digitized and the axis calculated by the optical tracking system and computer system in the operating room and then stored in the computer. The axis orientation can be later used in aligning the femoral rasp broach or reamer to prepare the canal and also to align the femoral component for proper insertion and orientation within the canal. Once the navigation system tracks and stores the axis  49 , the navigated femoral axis finder can be removed. 
   Alternately, the optical tracker can be releasably mounted on the tracker handle  12  or on the proximal end of shaft  14 . 
   Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.