Abstract:
A base component for a tibial implant has a lateral compartment and a medial compartment spaced from the lateral compartment defining an open central section therebetween. A connecting portion connects the medial and lateral compartments at an anterior end of the base component which anterior end is located adjacent the anterior tibia. The open central section of the base component intermediate the spaced medial and lateral compartments is open to a posterior end of the base component. The medial and lateral compartments have a bone contacting surface and a superiorly facing surface. The connecting portion has a curved anteriorly facing surface and has a first angled surface extending at an angle from adjacent the anterior end of the base component at a bone contacting surface of the connecting portion superiorly to a posterior end of the connecting portion. The first angled surface defining an anterior end of the open central section.

Description:
BACKGROUND OF THE INVENTION 
     Bicruciate retaining baseplates are known and have been in use since at least the early 1970s. Their use allows the preservation of both cruciate ligaments, the anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL). Early designs were predominantly bicompartmental, i.e., the articular cartilage of the distal femur and the proximal tibia was replaced to alleviate pain and restore function, but not the patella and corresponding trochlear groove. However, starting in the late 1970s, the use of bicruciate retaining baseplates started to decline as the introduction of the tricompartmental knees gained popularity and are not generally used in the current day knee joint replacement. 
     There is a never ending quest to improve/restore natural knee kinematics following total knee replacement especially with the baby boomer generation getting surgery at a much younger age and still expecting a return to normal, active lifestyle activities such as golf, biking, hiking, skiing. However, modern day implants still have limitations in offering natural joint kinematics. For example, a common misnomer in what the surgical community presently refers to as a Cruciate Retaining (CR) knee, the PCL is preserved but not the ACL. Therefore, the best that this knee can ever be is equivalent to an ACL deficient knee. In a Posterior Stabilized (PS) knee, both cruciates are sacrificed and only a partial function of the PCL is restored through the use of a cam on the femoral component and a post on the tibial insert, well known and understood in the current art. Therefore, since knees with compromised or torn cruciates are intrinsically unstable, it is believed that preserving both cruciates would improve joint stability and function following Total Knee Arthroplasty. 
     Some drawbacks of prior art bicruciate retaining baseplate designs are avulsion (tearing away) of the tibial eminence, and less than optimal instrumentation making surgery more challenging. In all bicruciate retaining baseplates, the medial and lateral compartments are connected by an anterior bridge portion. The corresponding bone preparation for the bridged portion is made with a vertical cut anterior to the ACL insertion site and extends distally to intersect the resection plane thereby resulting in a 90° corner. In loaded conditions, the ACL is under increased tension thereby loading that corner and making it susceptible to avulsion of the eminence. This risk increases significantly if the corner is undercut during preparation thus creating a notch. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is intended to reduce the risk of avulsing the eminence. The distal surface of the bridged portion of the baseplate is angled. Correspondingly, the bone is prepared by making an angled cut that initiates on the anterior margin of the cortex and extends in a proximal-posterior direction finally exiting on the proximal surface of the eminence, just anterior to the ACL footprint. This angled cut eliminates stress risers especially prevalent in a loaded internal square corner and distributes stress more uniformly across the surface. Another benefit of the present invention is a second angle cut on the anterior-lateral corner of the eminence. The removal of bone in this area provides increased clearance for the saw blade during the transverse cut of the lateral plateau. Tibia preparation, especially the lateral side during cruciate retaining or posterior stabilized procedure is facilitated by subluxing, or displacing the tibia anteriorly to expose the lateral side. The absence of the ACL allows such maneuver. However, with an intact ACL, the joint is much tighter and subluxing the tibia is very limited. 
     A base component is provided for a tibial implant which component includes a lateral and a medial compartment. The medial compartment is spaced from the lateral compartment defining an opening therebetween straddling a central axis of the implant that is generally aligned with a sagittal plane through the center of the tibia. A bridge or connecting portion connects the medial and lateral compartments at an anterior end of the base component. The connecting portion is located adjacent the anterior tibia upon implantation. The open central section of the base component intermediate the spaced medial and lateral compartments is open to a posterior end of the base component as well as open in the proximal-distal direction. The medial and lateral compartments have a bone contacting surface and a superiorly facing surface adapted to receive an ultrahigh molecular polyethylene bearing component. The connecting portion has a curved anteriorly facing surface and has a first angled surface extending at an angle from adjacent the anterior end of the base component at the bone contacting surface of the connecting portion superiorly and posteriorly to a posterior end of the connecting portion. The first angled surface defines the anterior end of the open central section. 
     The anterior portion of the major connecting portion is curved in a sagittal plane as well as the normal transverse plane of the anterior surface of a tibial baseplate. The curve of the anterior facing surface of the connecting portion has a radius of curvature centered in a sagittal plane at a point posterior of the connecting portion. 
     The first angled surface is angled at about 20°-70° to the medial and lateral compartment bone contacting surfaces. The open central section is defined by parallel medially and laterally facing walls, which walls define the boundary of the medial and lateral compartments with regard to the central open section. The angled surface of the connecting portion extends between these parallel medially and laterally facing walls. The lateral wall may have a second angled surface extending at an angle from an anterior end of the first angled surface at the bone contacting surface laterally to the medially facing wall of the lateral compartment. This medially facing wall defines the boundary between the lateral compartment and the open central section. The bone contacting surface in this area extends from the end of the connecting portion nearest the lateral condylar surface laterally, at a second angle, from the lateral edge of the first angled surface to the medially facing wall of the lateral compartment. The superiorly facing surface of the connecting portion is generally planar and the bone contacting surface of the baseplate is also generally planar in the medial and lateral condylar area. The bridge or connecting portion may also have a small planar bone contacting surface area between the most anterior surface of the baseplate and the start of the angled surface posteriorly thereof. 
     A method for implanting the baseplate component for a tibial implant includes resecting the medial and lateral compartments of the proximal tibia to form two planar surfaces spaced from the anterior cruciate ligament and the posterior cruciate ligament. This resection leaves the central eminence of the tibia in place and utilizes various protectors to ensure that the oscillating saw making the medial and lateral proximal planar cuts on the tibia do not engage this central eminence. Another cutting guide is provided to allow the cutting of a medial and a lateral surface on the tibial eminence, which surface is perpendicular to the plane of the medial and lateral tibial cuts and engages the medially and laterally extending walls of the tibial baseplate component. An additional cutting guide is provided so that an angled surface can be cut on the eminence from the anterior surface thereof, adjacent the planar medial and lateral cuts, proximally and posteriorly at an angle matching the first angled surface on the posteriorly facing surface of the baseplate connection portion. Thus, when the implant is mounted on the proximal tibia, the eminence has an angled surface for contacting the first angled surface on the connecting portion of the baseplate and a second laterally extending surface for contacting the second angled surface on the tibial baseplate portion. 
     As used herein when referring to bones or other parts of the body, the term “proximal” means close to the heart and the term “distal” means more distant from the heart. The term “inferior” means toward the feet and the term “superior” means toward the head. The term “anterior” means toward the front part or the face and the term “posterior” means toward the back of the body. The term “medial” means toward the midline of the body and the term “lateral” means away from the midline of the body. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an anterior cross-section along a central sagittal plane of the tibia of a prior art bicruciate retaining tibial baseplate having a rectangular cross-section and a bone contacting surface engaging the proximal resected surface of the tibia; 
         FIG. 2  is a cross-section of the central portion of the bicruciate retaining tibial baseplate of the present invention; 
         FIG. 3  is a top view of the tibial baseplate of the present invention; 
         FIG. 3A  is a top view of an alternate tibial base plate; 
         FIG. 4  is a bottom view of the tibial baseplate of the present invention; 
         FIG. 4A  is a bottom view of the tibial base plate of  FIG. 3A  having an arcuate anterior recess extending proximally from the bone contacting surface of a tibial base plate; 
         FIG. 5  is a cross-sectional view along lines  5 - 5  of  FIG. 3 ; 
         FIG. 6  is an isometric bottom view of the tibial baseplate of  FIGS. 3 and 4 . 
         FIG. 7  is an isometric view showing a proximal tibia with a sagittal cutting guide in position to perform the medial and lateral sagittal cuts in the proximal tibia; 
         FIG. 8  shows a proximal tibia, including a tibial cutting guide, including an instrument designed to protect the medial eminence located in the previously prepared medial sagittal slot; 
         FIG. 9  is an isometric view of the proximal tibia with a cutting guide mounted thereon with a guide surface angled laterally so that a isolatory saw can be used to cut the anterior or lateral corner of the eminence; 
         FIG. 10  is an isometric view of a cutting mounted on the proximal tibia and including a lateral eminence protector mounted on the cutting guide; 
         FIG. 11  shows a top view of the instrument end for guiding an oscillating saw blade for resecting the lateral side of the tibia for forming a proximally facing surface thereon; 
         FIG. 12  is an isometric view of the cutting guide of  FIG. 11  used to make the shown lateral transverse cut in a manner that protects the eminence; 
         FIG. 13  is an isometric view of an anterior chamfer cutting guide mounted on the resected proximal surfaces of the tibia; 
         FIG. 14  is an isometric view of the proximal tibia with the anterior chamfer cutting guide mounted thereon after utilizing an oscillating saw to make a resection on the anterior of the eminence; 
         FIG. 15  is an isometric view of the proximal tibia showing the eminence resected in a manner to receive the tibial baseplate of  FIGS. 3 and 4 ; 
         FIG. 16  is a top isometric view of an alternate cutting instrument to be used with the tibial base plate shown in  FIGS. 3A and 4A ; 
         FIG. 17  is a bottom isometric view of the instrument shown in  FIG. 16  including various cutting surfaces thereon; and 
         FIG. 18  is an isometric view of the proximal tibia showing an eminence resected to receive the tibial baseplate of  FIGS. 3A and 4A . 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , there is shown a proximal tibia  10 , including a posterior cruciate ligament  12  and an anterior cruciate ligament  14 , including an eminence  16  located about a sagittal plane bisecting the tibia along its anatomic axis. A central cross-section of a prior art bicruciate retaining baseplate  18  is shown having a bone contacting surface  20  and a posterior distal corner  22 , which is an area of high stress. This prior art baseplate  18  has planar anterior and posterior surfaces  23  and  25 . 
       FIG. 2  shows proximal tibia  10 , including the posterior cruciate ligament  12  and the anterior cruciate ligament  14  and including eminence  16  and further including, at the anterior surface of eminence  16 , the implant of the present invention generally denoted as  24 , which is shown in cross-section. The cross-section is taken at a sagittal plane bisecting the medial lateral condyles of the tibia. Implant  24  includes a curved anterior surface  26  and an angled surface  28 , which faces generally posterior and inferior. The anterior surface could have a distal flat or straight portion intersecting the curved portion or could be entirely flat. 
     Referring to  FIG. 3 , there is shown a top view (looking at the superior surface when implanted) of the tibial baseplate or tray  24  of the present invention. Baseplate  24  includes lateral condylar portion  40  and medial condylar portion  42 . Portions  40 ,  42  are designed to receive a typical ultrahigh molecular weight polyethylene (UHMWPE) bearing component (not shown). Medial and lateral condylar portions are connected by a bridge section  44 , which, along the sagittal plane defined by axis  46  has a cross-section as shown in  FIG. 2 . Lateral and medial condylar portions  40 ,  42  are preferably recessed and surrounded by a raised wall portion  48  laterally and  50  medially to locate the UHMWPE bearing insert. Likewise, U-shaped open area  52  is defined by lateral wall  54  and medial wall  56 . Posterior walls  58  and  60  define the posterior ends of recesses  40  and  42 , respectively. As is known, undercut grooves may be located adjacent walls  58  and  60  at the superiorly facing surface of condylar portions  40  and  42 . This undercut along with tabs  62  and  64  allow the UHMWPE bearing components to be snapped into their respective condylar portions  40 ,  42 . 
     Referring to  FIG. 4 , there is shown a bottom view of tibial baseplate  24  including the bone contacting surface. As seen lateral portion  40  and medial portion  42  are separated by open area  52  and are connected only by bridge portion  44  at the anterior end of the baseplate  45 . Angled surface  28  in  FIG. 2  is shown extending from adjacent anterior end  45  proximally and posteriorly toward the posterior edge  29  of bridge  44 . The anterior-most portion  31  of angled surface  28  is the distal-most contact point with the anterior edge of the eminence  16  of the tibia with posterior edge  29  contacting eminence  16  in a more proximal and posterior position on the eminence. As will be discussed below, the eminence is prepared to receive angled surface  28  of bridge portion  44  in face-to-face contact. Condylar bone contacting surfaces of areas  40 ,  42  of baseplate  24  may each include a peg  70  and may include a porous coating to encourage bone ingrowth. 
     The bridge or connector portion  44 , at the bone contacting surface, includes a laterally extending edge portion  72 , which extends from anterior edge  31  of angled surface  28  to lateral wall  54  of opening  52 . In the preferred embodiment, this angled bone contacting surface extends at an angle of 10°-60° with respect to axis  46  in summary. 
     Referring to  FIG. 5 , there is shown a cross-section view of baseplate  24  along lines  5 - 5  of  FIG. 3 . As can be seen, the anterior surface  26  is curved with a radius of approximately one inch center is posterior about one inch. Bone contacting surface  33  extends from the most distal point of anterior surface  26  posteriorly to edge  31 , which forms the most anterior and distal point on angled wall  28 . Lateral wall  54  defines the lateral edge of opening  52  shown in  FIG. 4  and extends posteriorly to end wall  58  as shown in  FIG. 3 . Pegs  70  of  FIGS. 4 and 5  are shown angled posteriorly and are inserted into angled bores in the medial and lateral tibial condylar region generally formed after the proximal tibial cut is made. 
     Referring to  FIG. 6 , there is shown an isometric bottom view of the tibial baseplate  24  shown in  FIGS. 3-5 . 
     The surgical method of preparing the proximal tibia for receiving baseplate  24  will now be described. Referring to  FIG. 7 , there is shown a standard tibial cutting guide  100 , which is typically used to form a proximal resection of the tibia to receive condylar portions  40  and  42  of baseplate  24 . Typically, cutting guide  100  is mounted on the anterior surface of the proximal tibia by at least two bone pins  101 . Mounted on standard cutting guide  100  is a cutting guide  102  of the present invention adapted to perform a sagittal medial and lateral cut on either side of the tibial eminence  16 . The cutting guide  102  is fixedly attached to guide  100  by placing screws or bolts through holes  103  into threaded bores  130  (see  FIG. 8 ) on the proximally facing surface of cutting guide  100 . Alternatively, cutting guide  102  may contain dowel pins permanently assembled into holes  103  and releasably attached to cutting guide  100  by engaging dowel pins into precision machined mating bore  130  of cutting guide  100 . Guide  102  includes a pair of saw blade guide slots  104 ,  106  aligned to make lateral and medial resections  108  and  110  in the proximal tibia on either side of eminence  16 . As shown in  FIG. 7 , a saw blade is mounted in slot  106  for making a bone cut which extends distally generally parallel to a sagittal plane through the anatomic axis of the tibia  10 . Resections  108 ,  110  then separate condylar areas of the tibia  40 A and  42 A from the tibial eminence  16 . 
     Referring to  FIG. 8 , the proximal tibia is shown with cutting guide  102  removed and a medial eminence protector  120  placed on cutting block  100 . Protector  120  has lateral and medial arms  122  and  124 , respectively. Arm  124  sits within the previously prepared medial sagittal slot  110 . Arm  122  is angled laterally at an angle matching the angle of surface  72  of baseplate  24 . To ensure stability, a flange  126  extends anteriorly and sits flush on the saw blade guide proximal surface  128  of cutting block  100 . Also seen in  FIG. 8  are threaded bores  130  adapted to receive the screws which hold on cutting block  102  shown in  FIG. 7 . 
     Referring to  FIG. 9 , there is shown saw blade  112  engaging the lateral surface of arm  122  to form the lateral side surface on eminence  16 , which engages surface  72  on baseplate  24 . By reciprocating blade  112 , an oblique cut is made on the anterior-lateral corner of eminence  16 . An oscillating saw then can be used on surface  128  to resect the medial tibial condylar portion  42 . This leaves a planar surface  42   a  for receiving the bone contacting surface of condylar portion  42  of baseplate  24 . 
     Referring to  FIG. 10 , there is shown a lateral eminence protector  140  inserted within the slot formed by saw blade  112  using the lateral angled guide arm  122  of eminence protector  120 . Protector  140  allows the resection of the lateral condyle  40   b  of the proximal tibia using the lateral side of guide  100 . Arm  142  prevents inadvertent cutting of eminence  16  during the resection of the lateral condyle.  FIG. 11  shows a top view of this resection using an oscillating saw  144 . A patella  146  is shown displaced laterally to allow for the resection of the proximal tibia.  FIG. 12  shows both the lateral and medial condyles  40   b  and  42   b  resected to form a planar proximal surface on the tibia adapted to receive baseplate  24 . At this point, cutting guide  100  and protector  140  are removed from the proximal tibia. 
     Referring to  FIGS. 13 and 14 , with this instrumentation removed, an anterior chamfer cutting guide  170  is placed on the resected medial and lateral planar surface of the proximal tibia. Cutting guide  170  includes arms  172  and  174 , which engage the lateral and medial sides of eminence  16 , which sides remain after the medial and lateral planar cuts have been made with cutting guide  100 . Guide  170  includes an angled cutting guide  176  adapted to guide an oscillating saw blade along a proximal and posterior angled cutting path to resect the anterior edge  178  of eminence  16 . This resection is at an angle identical to the angle of angled surface  28  of bridge portion  44  of baseplate  24 . 
       FIG. 14  shows the anterior facing surface of the eminence  16  after this final resection has been accomplished. At this point the tibia has been fully prepared as shown in  FIG. 15  to receive the baseplate  24 . Also shown in  FIG. 15  are two angled bores  180  to receive pegs  70 . Bores  180  can be made in any standard manner. 
     Referring to  FIGS. 3A and 4A , there is shown an alternate tibial base plate generally denoted as  200  which, with the exception of the shape of the distally facing surface in the anterior area, is the same as that shown in  FIGS. 3 and 4 . 
     Plate  200  includes medial and lateral condylar portions  202  and  204  respectively. Portions  202  and  204  are again separated by an intercondylar space  206  having a curved anterior portion  208 . The outer anteriorly facing surface  210  is identical to that shown in  FIGS. 3 and 4 . 
     Posteriorly facing surface  208  at the anterior end of the opening  206  has a planer superior surface  212  which engages the intercondylar eminence compared during surgery. Curved surface  208  tapers posteriorly from its apex  214  posteriorly to a superior curved portion  216 . The central cross-section is identical to that shown in  FIG. 5 . 
     Referring to  FIGS. 16 and 17 , shows an instrument generally denoted as  250  which is designed to repair the anterior portion of the tibial eminence to receive the tibial implant in  FIGS. 3A and 4A . The top view of  FIG. 16  shows instrument  250  including a stop flange  252  connected to a curvate portion  254  which has an inwardly tapered portion  256 . The instrument includes first and second arms  258  and  260  which straddle the eminence formed as shown in  FIG. 7 . Thus the instrument  252  takes the place of the instrument shown in  FIGS. 8 ,  9  and  10  which would be utilized when utilizing the base plate of  FIGS. 3 and 4 . 
     Referring to  FIG. 7  there is shown a bottom view of instrument  250  which includes a sharp cutting edge  262  extending around the circumference of portion  256  and arms  258  and  260 . 
     In use, the instrument  250  is shown in the orientation of  FIG. 16  is placed on top of the tibial eminence prepared as shown in  FIG. 7 . Upper surface  264  of the instrument is then impacted such that cutting edge  262  of arcuate portion  256  is driven distally on the proximal tibia cutting a curved posteriorly tapered surface into the anterior portion of the eminence. Impaction is continued until surface  266 , shown in  FIG. 17 , contacts surface  128  of resection guide  100  thereby forming the curved anterior surface of the tibial eminence  16 . The implant of  FIGS. 13 and 14  is then attached and the planar cut  212 A of  FIG. 18  is made. Once this surface is formed the medial/lateral tibial plateaus are resected as shown in  FIG. 11 . 
     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.