Abstract:
A surgical method and apparatus employs a trochanteric grip or bone plate having cable guides adapted to direct the necessary transitions in cable direction smoothly in different planes, thereby directing the cable in the direction of tension and preventing wear and concentration of stress in the cable.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates generally to surgical implants used in orthopedic surgery, and more specifically to an apparatus grip useful in the attachment of a greater trochanter. 
         [0003]    2. Description of the Related Art 
         [0004]    Hip replacement surgery has become commonplace. During a revision of a total hip replacement, the greater trochanter is commonly cut away from the femur and retracted, together with the abductor musculature to which the trochanter is attached. This greatly facilitates the surgical approach to the hip joint. A femoral stem is then replaced by new prosthetic implant. This technique is also employed in connection with trochanteric osteotomies and intra-operative fractures of the trochanter. 
         [0005]    After the implant is located, the greater trochanter is relocated and must be reattached. To properly heal, the greater trochanter must be secured in its proper position on the proximal femur and the position maintained for a time sufficient for the bone to heal. Maintaining the proper position is difficult because of the very substantial and dynamic forces applied to the trochanter and femur, both through the femur and from the attached abductor musculature, which tends to move the trochanter in relation to the femur. 
         [0006]    Various bone plates or grips have been introduced to secure the trochanter during healing. One such device is described in U.S. Pat. No. 6,066,141, for example. Other examples are described in U.S. Pat. Nos. 6,338,734; 5,993,452; 5,797,916; 5,665,088; 5,334,291; 4,889,110; and 4,269,180. A more recent example of a trochanteric cerclage plate is published in U.S. published application 2006058795. Typically the prior devices have a metallic body with one or more grooves or bores through which cables may be threaded. The cable is passed around the femur and fixed in tensioned loops, clamping the trochanter in place on the proximal femur. Some configurations require drilling holes through the femur, through which the cable is passed. 
         [0007]    These and other prior designs have cable retention features such as grooves or bores arranged in ways that require the cable to make abrupt bends or curves, in some cases crossing abrupt ledges or sharp corners. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention includes a surgical method and apparatus, employing a trochanteric grip having cable guides adapted to direct the necessary transitions in cable direction smoothly, directing the cable in the direction of tension, and preventing wear and concentration of stress in the cable. 
         [0009]    The apparatus of the invention is a bone plate for use in fixing a resected bony piece to a larger bone, suitable for fixing a greater trochanter to a femur. The apparatus includes: a body having a proximal end and a distal end disposed at opposite ends of a lengthwise dimension, said body also having an outer face and an inner face; said body having at least two cable guides, each of said cable guides including at least one pair of sloping ramps, converging at an obtuse angle to intersect, said ramps defining at least one cable-guide plane. Said cable guide planes are disposed generally transverse to the lengthwise dimension of said body. 
         [0010]    These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of preferred embodiments, taken together with the accompanying drawings, in which: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a perspective view of a bone grip in accordance with the invention; 
           [0012]      FIG. 2  is a top view of the bone grip of  FIG. 1 ; 
           [0013]      FIG. 3  is a side view of the bone grip of  FIGS. 1 and 2 ; 
           [0014]      FIG. 4  is a cross section taken along section line  4  in  FIG. 3 ; 
           [0015]      FIG. 5  is a cross section taken along Section line in  FIG. 3 ; 
           [0016]      FIG. 6  is a cross section taken along Section line  6  in  FIG. 3 ; 
           [0017]      FIG. 7  is a cross section taken along section line  7  in  FIG. 3 ; and 
           [0018]      FIG. 8  is a perspective view of the bone grip positioned in relation to a human femur, and showing cable attachments according to a method of using the device in reattachment of a greater trochanter. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    The terms “proximal” and “distal” as used herein are defined for convenience in relation to the intended anatomical orientation of the device when surgically fixed to a greater trochanter. However, in some applications the device might be reoriented without departing from the invention. Accordingly, the terms proximal and distal should be understood only as convenient labels for the purpose of description, and not as limiting the possibilities for reorientation. 
         [0020]    As shown in  FIGS. 1-3 , a bone grip in accordance with the invention has a solid, substantially rigid body  20  with a relatively wider proximal portion  22  and a relatively more narrow distal neck portion  24 . An inner face  26  of the body  20  is slightly concave in at least the proximal-distal direction, conforming to a generalized surface of a greater trochanter with its soft tissue attachments (or other bone, as required by each particular embodiment). An outer face  28  of the body  20  approximately follows the contour of the inner face on the opposite side of the thickness of the body. The overall shape of the body is thus generally shell-like, with its inner face cupped slightly inward to embrace a convex bony surface. 
         [0021]    The body  20  preferably has a length dimension longer than the width dimension, defining a lengthwise direction. The body  20  also preferably has generally bilateral symmetry so that an imaginary midplane or medial plane of symmetry is defined. The medial plane M of the body extends in the direction of the longer dimension (lengthwise) of the body and is disposed approximately midway across the width dimension. Because the body  20  is intended to be fixed to a femur with the long dimension extending from proximal to distal (in anatomical terms), the M plane will normally be fixed to extend in the proximal-distal direction in anatomical relation to the femur. The medial plane M intersects outer face  28  to define an imaginary midline. 
         [0022]    At the proximal end of Body  20  at least two proximal, hooked tines  30  are separated by a gap  32 . The tines are generally sharp, to securely engage into or over the proximal bony surface of a greater trochanter. Similarly, at the distal end of the body  20 , the distal end has at least two smaller hooked, distal tines  33  sharpened to engage a femur (or other similar bone). The tines are preferably disposed symmetrically. Thus, the medial plane M lies midway between the proximal tines  30 , midway between the distal tines  33 , and generally normal to the inner and outer faces  26  and  28 . 
         [0023]    Referring to  FIG. 2 , the body  20  has at least two, and preferably a larger number of oblique bores  34   a ,  34   b ,  36   a  and  36   b  (generally identified as oblique bores). The bores are aligned in convergent, opposed pairs. In the illustrated example, bores  34   a  and  34   b  converge at an apex near the medial plane M. Each bore-pair comprises a cable channel, as discussed in detail below. Preferably, at least a second set of convergent, opposed bores  36   a  and  36   b  are also provided; the second set together comprise a second cable channel. 
         [0024]    The oblique bores run generally transverse to the axis of the body  20 , entering from a side  40  and exiting the top surface  28 . It should be borne in mind that in a preferred embodiment with sides  40  and top surface nearly perpendicular, the bores will be non-perpendicular with either the top surface  28  or the sides  40 . As shown in  FIG. 2 , the intersection of bores  34   a ,  34   b ,  36   a  and  36   b  with the top surface  28  is acutely oblique, creating an aperture which appears elliptical (although it is not mathematically an exact ellipse). The intersection with the sides  40  is also non-perpendicular, yielding a non-circular side aperture  46  but with less eccentricity than the upper apertures where bores  34   a,b  and  36   a,b  exit the top surface  28 . 
         [0025]    Although only four lateral bores are visible from the side view of  FIG. 3 , it will be seen from the other views and from the approximately bilateral symmetry of the body that right and left bores are provided in sets, generally transverse to the medial plane M body  20  and lining up in pairs. Each pair of corresponding right and left bores together comprises a cable guide, having diameter complementary to a matching strand of cerclage cable. These cable guides are disposed to cooperate with surgical cerclage cables to allow fixation of the bone grip in the manner discussed below. 
         [0026]    The cross-section  4  shows that the left and right lateral bore pair  34   a  and  34   b  are generally transverse to the medial plane of the body  20 , but are neither parallel nor skew to one another. Rather, the bores are angled upwards, converging toward an intersection at or above an apex approximately at the medial plane of the body  22 . Preferably, the central axes of bores  34   a  and  34   b  intersect near the top surface  28 . The bores thus form complementary ramps sloping downward and outward from a central apex at  50 . In accordance with the invention, the axes of these ramps intersect at an obtuse angle θ. A curved transition between ramps is preferably provided at the apex  50 . 
         [0027]    Inasmuch as the axes of these bores  34   a  and  34   b  intersect at an obtuse angle, they define a plane (the “cable guide plane”). Two intersecting lines define a plane, as is well known (Euclid). Therefore, the cable guide comprising bores  34   a  and  34   b  lies generally on and defines a first cable guide plane  52 , as shown in  FIG. 3 . Each cable guide plane is transverse to the medial plane M; but multiple ones of the cable guide planes are not necessarily parallel to one another, as discussed further below and as seen in  FIG. 3 . 
         [0028]    It is preferred that the ridge separating bore  34   a  and  34   b  be slightly rounded to a saddle-like shape, to soften the transition for a cable running through  34   a  and  34   b , passing across the apex ridge. This can be manufactured, for example, by threading a strong, abrasive-impregnated cable through the cable guide comprising bore pair  34   a  and  34   b . The cable is then tensioned and pulled alternately back and forth through the channel while maintaining tension, to abrade the body and define a smooth saddle or cable groove. 
         [0029]    The ramps defined by  34   a  and  34   b  are preferably disposed at obtuse angles in relation to a desired direction of cable tension. The angle of cable tension is defined by the anatomy of the femur, and in particular the relationship between the greater and lesser trochanter. In consideration of this anatomical relationship, the arrangement of the invention tends to distribute cable stress by avoiding any acute or right-angle corners. Note that the angle φ between the left face  40  of the device and the bore  34   a  is preferably more than 90 degrees. 
         [0030]    The transition θbetween ramps again presents an obtuse angle; another obtuse angle is formed at the aperture of the right hand bore  34   b  in right face. The series of obtuse angles tends to distribute the contact stress across ramps  34   a  and  34   b  so that stress is distributed in a cable that is passed through the guides. 
         [0031]    Similarly,  FIG. 5  shows a second cable guide analogous to that of  FIG. 4 . However, cut plane  5  ( 54  in  FIG. 3 ) is not parallel with that of  FIG. 4  ( 52  in  FIG. 3 ). The Cut planes  54  and  52  are defined by the directions of their corresponding cable guides. As discussed above, each pair of bores ( 34   a,b ;  36   a,b ) are non-parallel, convergent and intersecting near an apex. Thus, each pair defines a plane. In accordance with the invention, planes  52  and  54  intersect as shown at a dihedral angle α which, in a preferred embodiment, is approximately three degrees. The actual angle in a give embodiment is determined by the desired anatomical fixation point at which the cables are intended to converge. Thus, the angle is predetermined such that the cable guide planes converge near an anatomical fixation point (for example, the lesser trochanter) when the plate is fixed on the greater trochanter. 
         [0032]    It should be noted that the angles θ′,φ in  FIG. 5  may vary slightly from corresponding angles θ, φ in  FIG. 4 , depending on anatomical geometry for the particular application, but preferably both will be obtuse, as discussed above. 
         [0033]    In the more distal neck  24  of the bone grip, at least one, and preferably at least two more directed cable guides  55  and  56  are provided. In one embodiment, bore pairs similar to  34   a,b  and  36   a,b  are provided at  55  and  56  in the neck  24 , each defining a different plane  58  and  60 . (differing from one another and from plane  4  and plane  5 ). Alternatively, through bores  55  and  56  can be used, as illustrated in the figures. The alternative arrangement is more easily fabricated in a body having a neck narrower than the proximal body, as shown. This alternative is illustrated because the arrangement of  34   a,b  and  36   a,b  has already been shown and described. 
         [0034]    Cross sections  6  and  8  show bores  55  and  56 , respectively. Although the bores are generally tranverse and pass through the body, each side is preferably counterbored at an oblique angle. The directions of the oblique counterbores provide in each bore a short ramp (shown at  62  and  64 ) which together with the bores defines cable guide planes  58  and  60 . Alternatively, the counterbore may simply be chamfered or rounded to avoid concentration of stress in the cable. The apertures of all bores should be smoothed as by abrasion to a finish, for example 32 Ra (microinches) rms roughness, to prevent abrasion of an elastic, polymer cable. 
         [0035]    Referring back to  FIG. 3 , one can see that the planes  58  and (optional)  60  are not parallel to one another or to either  52  or  54 . Preferably the dihedral angle β between the (innermost of the) proximal cable guide planes and the distal cable guide planes is approximately 22 degrees. More generally the angle could be in the range 10 to 50 degrees. This angle is, in any particular embodiment, determined by trochanter geometry more specifically, the angle is selected such that the cable guide planes converge at or near a predetermined fixation point (for example, the lesser trochanter). In this context, “near” is used to mean within 2 centimeters. The angle γ between planes of the two distal neck cable guides is approximately three degrees, but may vary in response to the relative locations of the greater trochanter and fixation point in a given anatomical context. Preferably, all of the planes defined by the multiple cable guides intersect at approximately the same line seen (pointing into the page) as  67 , which corresponds with an estimated attachment point (suitably at or below the lesser trochanter). This arrangement directs all the tension vectors in a cerclage cable in the most anatomically and mechanically desirable directions, and avoids unnecessary kinks and opportunity for cable abrasion. 
         [0036]      FIG. 8  shows an example of a method of use for the trochanteric grip of the invention. The device  20  is disposed in contact with a greater trochanter at  70 , with the concave inner face  26  disposed toward the bone, and the convex outer face  28  disposed outward and upward. The tines  33  and  30  are anatomically arranged so that the device naturally seats with the medial plane M running generally in the proximal-distal direction in relation to the patient&#39;s anatomy. Two lengths of elastic, polymer cable  72  are doubled to provide four strands  74 ,  76 ,  78  and  79 . Each strand is passed through a cable guide:  74  passes through the bore pair  34   a  and  34   b  (comprising guide  34 );  76  passes through guide  36 , and so on. The cable is passed around the femur, preferably passing through or below the lesser trochanter; and the free ends are secured under tension. 
         [0037]    One method of securing the cable is shown: The looped end and the free ends can be secured under tension by a pair of locking, wedged cable clamps  82 . An example of a suitable clamp is described in U.S. patent application Ser. No. 11/147,685 filed on Jun. 8, 2005 (allowed, pending issue). Other means could be employed for securing the cable under tension. Two such cable segments (four strands) are shown and are preferred. 
         [0038]    As shown in  FIG. 8 , the various strands of cable are not directed in parallel or in the same plane; rather, the loops tend to converge at  80  to engage the lesser trochanter. This arrangement is preferred by the surgeon for the security that it offers, and because it tends to direct the force vectors in the direction most preferred to maintain dynamic compression on the trochanter during post-surgical recovery period. This tends to promote proper healing and prevent post-operative displacement of the trochanter, which experiences large dynamic and static stresses from the muscle attachments (not shown, to maintain clarity of illustration). 
         [0039]    The cable guides of the present invention are preferably directed in convergent planes that follow the tension directed in a tensed, elastic cable looped through the trochanteric grip (at a first extreme of the loop) and secured around the femur at the lesser trochanter (at the opposite extreme of the loop). 
         [0040]    Numerous variations of the apparatus and method are possible. Optionally, a hole, notch, or other feature may be provided on the body  20  of the invention for engaging a complementary instrument for manually manipulating the body during surgery. For example, a threaded hole may optionally be provided in body  20 , for fitting to a complementary threaded shaft on an instrument. A shaft might alternatively be press fitted, or notches, projections, or recesses of various forms might be provided, depending on the specific design of the complementary handling instrument. In some instances of the method, holes may be drilled through the lesser trochanter and the cables threaded through; in other cases, it may be sufficiently secure to rely on the protrusion of the lesser trochanter to retain the cable loop (as shown in  FIG. 8 ). 
         [0041]    While several illustrative embodiments of the invention have been shown and described, numerous variations, additions, and alternate embodiments will occur to those skilled in the art. Such variations and alternate embodiments are contemplated, and can be made without departing from the spirit and scope of the invention as defined in the appended claims.