Abstract:
A femoral slideway and femoral slideway/femur-size template combination is employed to create an installed knee prosthesis that creates a reduced amount of stress in the collateral ligaments relative to previous prosthesis. In one embodiment, the reduced stress is achieved by employing a femoral slideway/femur-size template combination which results in installation of a femoral slideway which has a smaller dorsal-ventral dimension than a corresponding dimension of the femur prior to resection.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This Application is a continuation application of U.S. application Ser. No. 10/616,102, filed on Jul. 9, 2003, which is a divisional application of U.S. patent application Ser. No. 09/517,674, filed on Mar. 2, 2000, now abandoned, the entire contents of which are incorporated herein by reference and should be considered a part of this specification. U.S. patent application Ser. No. 09/517,674 is based on and claims priority under 35 U.S.C. §119 to German Patent Applications DE 29906909.5, filed Apr. 16, 1999, and DE 29903766.5, filed on Mar. 2, 1999, the entire contents of which are incorporated by reference and should be considered a part of this specification. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a femoral slideway, and more specifically to a femoral slideway/femur-size template arrangement and a knee endoprosthesis system with such a femoral slideway. 
     2. Description of the Related Art 
     A femoral slideway of this kind is disclosed, for example, in the German patent DE 40 41 002 C2. In the surgical technique customarily used for knee-joint replacement by means of such a femoral slideway, an equal amount of bone is removed from the two condyles of the femur, so that the anterior or ventral cut is parallel to the posterior or dorsal cut. When the implant is in the position thus defined, the axis of rotation of the implant no longer coincides with the axis specified by the arrangement of the collateral ligaments, and this position is not anatomical inasmuch as when flexed, the implant is seated either too tightly on the medial side or too loosely on the lateral side. 
     The remedy that was recommended some time ago, namely an outward rotation of the cutting guide such that in the posterior region less bone is removed laterally than medially, whereas anterior-laterally more bone is removed than on the anterior-medial side, also presents disadvantages, which the construction of the femoral slideway proposed in DE 197 16 879 A1 of the applicant is designed to eliminate. The crux of this solution is to rotate the anterior or ventral cut in the transverse plane. 
     However, this more recent solution also requires improvement with respect to optimizing the joint function in cooperation with the collateral ligaments in particular, especially in order to reduce the load imposed thereon. 
     SUMMARY OF THE INVENTION 
     It is thus the object of the invention to disclose a femoral slideway with further improved function, as well as a knee endoprosthesis system that can be efficiently constructed and employed and has such a femoral slideway as its essential element, and finally an advantageous arrangement comprising femoral slideway and femur-size template. 
     This object is achieved in its first, foremost aspect by a femoral slideway with the features described in the present application. 
     The invention includes the essential idea that it is advantageous to prepare for a knee-joint replacement by resecting more bone from the femur dorsally than is replaced by the implant (the femoral slideway). The “diminution” of the femoral slideway thus brought about in the dorsal region, in comparison to the original dimensions of the (resected) condyles or to a femoral slideway fitted in the conventional manner, produces an effective reduction of the turning radius of the tibial plate belonging to the prosthesis system and hence reduces the load on the collateral ligaments. 
     The decrease in the dimensions of the femoral slideway measured between the outermost, dorsoventrally opposed points on the condyle shell surfaces, in comparison to the previously customary dimensioning, is preferably in the range between 2 and 5%. This is achieved by constructing the associated femoral slideway/femur-size template arrangement in such a way that the distance separating one or more pegs on the femoral slideway from its dorsal sliding surface is smaller by 5-15%, in particular by about 10%, than the corresponding distance by which bores in the femur-size template for positioning the pegs are separated from the contact surface that is to be apposed to the dorsal condyle surfaces of the femur. 
     The distance between the dorsal sliding surfaces and the one or more pegs on the inside of the femoral slideway is preferably in the range between 24 and 34 mm and in particular is 29 mm, the chosen value advantageously being kept constant in a knee endoprosthesis system for covering a relevant joint-size range. 
     As noted above, the femoral slideway/femur-size template arrangement is constructed in such a way that the distance between one or more pegs on the femoral slideway and its dorsal sliding surface is smaller than the corresponding distance between bores in the femur-size template and the contact surface of the template by 5-15%, and in particular about 10%. Stated differently, the corresponding distance between bores in the femur-size template and the contact surface of the template is larger by 5-15%, an in particular about 10%, than the distance between one or more pegs on the femoral slideway and its dorsal sliding surface. Accordingly, the distance between the bores in the femur-size template and the contact surface of the template can be between 26.4 mm and 37.4 mm (i.e., 10% larger than the range of 24 mm to 34 mm between the dorsal sliding surface and the one or more pegs of the femoral slideway). In another embodiment, the distance between the bores in the femur-size template and the contact surface of the template can be about 32 mm (i.e., about 10% larger than a distance of 29 mm between the dorsal sliding surface and the one or more pegs of the femoral slideway). In yet another embodiment, the distance between the bores in the femur-size template and the contact surface of the template can be between 30.45 mm and 33.35 mm (i.e., 5%-15% larger than a distance of 29 mm between the dorsal sliding surface and the one or more pegs of the femoral slideway). 
     Another distinguishing feature of the proposed femoral slideway is that particular dimensions maintain a largely constant relationship to one another, regardless of the size of the actual prosthesis. For instance, the ratio a:c between the maximal dorsoventral extent and the maximal lateral extent of the femoral slideway is about 0.9±0.02. The patellar pit formed between the condyle shells preferably has a depth “b”, measured from the dorsalmost point on the condyle shells, such that its ratio b:a to the maximal dorsoventral extent of the femoral slideway is in the range between 0.4 and 0.5, in particular is 0.44. 
     The patellar pit is thus lengthened in the dorsal direction, as a result of which the patella can be supported over a large area throughout its entire functional range of flexion. 
     This elongation of the patellar pit, which furthermore increases in accordance with the anatomy in implants of all sizes, allows for the fact that the patello-femoral contact surface in conventional femoral slideways has a relatively small bearing area. That is, in the region in which the patella leaves the trochlea and enters the intercondylar fossa, conventional femur components provide support only in the peripheral regions. 
     Furthermore, in the proposed femoral slideway the condyle shells are somewhat more strongly rounded in cross section (coronal section) than is the case in conventional femoral slideways. This modification was undertaken in the interest of improving the fit to the special tibia insert that belongs to a knee endoprosthesis system, but which is not within the scope of the invention. 
     The back surface of the femoral slideway, in one advantageous embodiment, bears a two-component Ti coating produced in a vacuum plasma procedure, consisting of a relatively thin, dense base layer and a several fold thicker, open-pored cover layer. The dense base layer allows the femoral slideway, which for example consists of CoCrMo, to become completely sealed to the bone and because it makes contact with the substrate over a large area, increases the stability of adhesion. 
     The open-pored and very rough surface of the cover layer provides ideal conditions for the growth of bony substance onto and into the carriage, producing a quasi “3-D interlocking” that can transmit pulling forces as well as pressure and transverse forces. 
     In accordance with one embodiment, a system for sizing and installing a femoral slideway implant is provided. The system comprises a femur size template comprising a base plate having at least one hole, at least one condyle engaging surface extending from the base plate, and a measurement tongue slidably mounted to the base plate, the tongue being movable between a plurality of discrete positions relative to the condyle engaging surface, wherein a first dimension is defined by a distance between the hole of the base plate and the condyle engaging surface. The system also comprises a femoral slideway implant comprising at least one condyle shell having an outer surface defining a dorsal sliding surface, a patellar shield located anterior to the condyle shell, and at least one peg on an inner surface of the implant and between the patellar shield and the condyle shell, wherein a second dimension is defined by a distance along a line extending perpendicularly between a plane tangent to the dorsal sliding surface and a longitudinal axis of the peg. The first dimension of the size template is larger than the second dimension of the implant by a predetermined amount. 
     In accordance with another embodiment, a system for installing a femoral slideway implant is provided. The system comprises a femur size template comprising at least one condyle engaging surface and a tongue configured to measure a maximal anterior posterior extent of the head of the femur. The system also comprises a femoral slideway implant comprising at least one condyle shell having an outer surface defining a dorsal sliding surface, and a patellar shield located anterior to the condyle shell, wherein a maximal anterior posterior extent of the condyle shell is less than the maximal anterior posterior extent of the femur. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Additional advantages and useful features of the invention will be apparent from the subordinate claims and the following description of an exemplary embodiment with reference to the figures, wherein 
         FIG. 1  is a view (from proximal) of a femoral slideway according to one embodiment of the invention, 
         FIG. 2  shows the femoral slideway according to  FIG. 1  in median section (sagittal section), 
         FIG. 3  is a plan view of an embodiment of a femur size template, 
         FIG. 4  is a side view of the latter, 
         FIGS. 5   a ,  5   b  show a conventional arrangement of a femoral slideway on a femur in comparison to an arrangement proposed here, and 
         FIGS. 6   a ,  6   b  show scanning electron micrographs of a cross section of a conventional layered structure and of an embodiment of the layered structure proposed here for the back-surface coating of a femoral slideway. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In  FIGS. 1 and 2  the femur component  10 , called a femoral slideway, of a knee endoprosthesis is shown. The femoral slideway  10  comprises two convexly curved condyle shells  11 ,  12  and a patellar shield  13 , which connects the two condyle shells  11 ,  12  rigidly to one another. 
     The condyle shells  11 ,  12  and the patellar shield  13  in their interiors define anterior and posterior fitting surfaces  14 ,  15  that correspond to a femoral ventral and dorsal cut, respectively, and are associated with a ventral and a dorsal saw-cut surface produced at the distal end of the femur when the latter was resected for fitting of the femoral slideway. The convex outer shape of the condyle shells  11 ,  12  specifies dorsal sliding surfaces  11   a ,  12   a  in the posterior region, over which the corresponding surfaces of the tibia insert slide when the knee endoprosthesis is flexed. The patellar shield  13 , which is recessed with respect to the convex outer surfaces of the condyle shells  11 ,  12 , defines a so-called patellar pit  16 , within which there is supported a patella component  17  of the knee endoprosthesis, which is indicated by a dashed outline in  FIG. 2  and does not belong to the femoral slideway  10 . 
     To assist anchoring and central placement of the femoral slideway  10  on the femur, on the inner surface of the femoral slideway two pegs  18 ,  19  are formed, the long axis of which is substantially parallel to the posterior fitting surface  15 . These pegs project into holes in the femur, which have been drilled in the appropriate positions with the aid of a corresponding drilling template (see below), and this engagement gives the attachment of femoral slideway to bone greater stability than is provided by the fitting surfaces alone. 
     To ensure that the femoral slideway will function optimally as a replacement for destroyed sliding surfaces on the femur, the construction must reflect as accurately as possible the anatomical arrangements and dimensions, but also within the scope of the invention includes a specific modification that will now be explained. 
     One of the relevant dimensions of the femoral slideway is the maximal anterior-posterior or dorsoventral extent of the condyle shells  11 ,  12 , a distance labeled “a” in  FIG. 1 . Another relevant dimension is the maximal lateral extent of the femoral slideway, i.e., the distance between the most lateral point on the lateral condyle shell  11  and the most medial point on the medial condyle shell  12 , which in  FIG. 1  is labeled “c”. Also significant is the distance from the outermost posterior point on the dorsal sliding surfaces  11   a ,  12   a  of the condyle shells  11 ,  12  to the posterior bounding edge of the patellar shield  13 , which in  FIG. 1  is labeled “b”. A final significant distance is that between the outermost posterior points on the dorsal sliding surfaces  11   a ,  12   a  and the long axis of the pegs  18 ,  19  (which lie in one and the same coronal plane), labeled “d” in  FIG. 2 . In the exemplary embodiment described here, the ratio a:c is 0.9 and the ratio b:a is 44. On grounds of biomechanics and surgical technique, it has proved useful to make the distance “d” (between sliding surface and peg axis) uniform for all sizes of femoral slideways used in a knee endoprosthesis system. In the present case, this distance is 29 mm. 
     To determine the correct femoral slideway size, a femur-size template  20  shown in  FIGS. 3 and 4  is used. This comprises a basic part  21  with two flanks  22  and  23 , each of which ends in a contact section  22   a ,  23   a  that is bent at a right angle and is apposed to the condyles of a femoral bone that is to be fitted with a femoral slideway ( FIGS. 1 and 2 ). 
     In the middle of the basic part  21  a measurement tongue  24 , bent at an angle in two places, is mounted so that it can be displaced in a direction perpendicular to the plane in which the contact sections  22   a ,  23   a  lie. The measurement tongue  24  is marked with a scale  25 , which indicates the maximal anterior-posterior extent of the head of the femur, i.e. the condyles, and thus indicates to the doctor the required size of the implant. In the basic part  21  of the femur-size template  20  two peg-hole bores  26 ,  27  are provided, which—in accordance with a supplementary drilling-template function of the femur-size template—assist the positioning of peg-holes in the femur so that they correspond to the pegs  18 ,  19  of the femoral slideway  10 , as shown in  FIG. 1 . The axes of the peg-hole bores  26 ,  27  are separated by a distance “e” from the contact surfaces of the contact sections  22   a ,  23   a.    
     This distance—along with the distance “d” between the sliding surfaces and pegs on the femoral slideway  10  itself (cf FIG.  2 )—is an additional relevant dimension in the concrete implementation of a knee endoprosthesis, for the following reason: 
     So that the above-mentioned peg-holes—which serve not only to position the implant but also to position the cutting guides used to produce the various saw cuts on the femur—can be drilled into the bone, drill bushes (not shown) are inserted into the peg-hole bores  26 ,  27 . 
     It has proved advantageous, in particular from the viewpoint of reducing the load on the collateral ligaments during flexion of the artificial knee joint, to resect more bone dorsally on the femur than will be replaced there by the thickness of the dorsal parts of the condyle shells. For this reason the distance “e” is made larger than the corresponding distance “d” ( FIG. 2 ). In the preferred embodiment, the relative distance reduction, i.e. the quantity (e−d)/d, is about 10%. 
     The effect thus achieved can be seen in  FIG. 5 , where a sketch representing the conventional way of attaching a femoral slideway  10 ′ to a femur F′, shown in  FIG. 5   a , is compared with the representation in  FIG. 5   b  of the arrangement proposed here. The anterior-posterior extent of the femoral slideway  10  in  FIG. 5   b , mounted on a femur F resected further in the dorsal region, is smaller by the amount (e−d) than in the conventional implant  10 ′. 
     Because the distance “e” is permanently specified by the femur-size template, which is used for all implants regardless of their size, and according to what has been stated above, the distance “d” in the embodiment of the femoral slideway is preferably kept constant for all implant sizes, the geometric relations will be slightly different for implants of different sizes. This is acceptable, however, in view of the advantages for manufacture and manipulation that such a system brings. 
       FIG. 6   b  shows—in comparison to a conventional femur carriage coating as shown in  FIG. 6   a —the appearance in the scanning electron microscope of a cross-section through a two-component titanium-coating construction consisting of a dense base layer G, about 50 μm thick, and an open-pored cover layer D averaging about 250 μm thick, on a CoCrMo substrate S. Although the thickness and average roughness of the coating according to  FIG. 6   b , which is applied by a vacuum plasma process, are comparable to those of the known, sprayed-on coating according to  FIG. 6   a , it should be emphasized that the former has a more open-pored structure and a considerably reduced number of interface defects (indicated in both pictures by vertical arrows). 
     Implementation of the invention is not limited to the exemplary embodiment described above, but can also incorporate modifications, which in particular include departures from the specified dimensions and ratio values. 
     LIST OF REFERENCE NUMERALS 
     
         
           10 ,  10 ′ Femoral slideway 
           11 ,  12  Condyle shells 
           11   a ,  12   a  Dorsal sliding surfaces 
           13  Patellar shield 
           14  Anterior fitting surface 
           15  Posterior fitting surface 
           16  Patellar pit 
           17  Patella component 
           18 ,  19  Peg 
           20  Femur-size template 
           21  Basic part 
           22 ,  23  Flanks 
           22   a ,  23   a  Contact sections 
           24  Measurement tongue 
           25  Scale markings 
           26 ,  27  Peg-hole bores 
         a, b, c, d, e Distances 
         A-A Plane of section 
         D Cover layer 
         F, F′ Femur (shaped) 
         G Base layer 
         S Substrate