Abstract:
An intramedullary nail has a dynamization opening that accommodates and utilizes a dynamization window. The dynamization window is press-fit into the dynamization opening. The dynamization window is oval shaped and includes first and second protrusions that extend from respective surfaces of the dynamization window. The dynamization opening is sized to receive the dynamization window but compress the protrusions/window during insertion. The dynamization opening includes channels or grooves that are sized to accommodate the protrusions and allow the protrusions/window to return to an uncompressed state when situated therein. The dynamization opening and window cooperate to allow a press fit of the window into the dynamization opening with an elastic snap-fit and audible sound.

Description:
[0001]    The present U.S. non-provisional patent application is a continuation-in-part and thus claims priority to and/or the benefit of co-pending U.S. patent application Ser. No. 09/575,764, filed May 22, 2000 and entitled Non-Metal Spacers For Intramedullary Nail, which is a continuation of PCT application number PCT/US00/09582, filed Apr. 10, 2000. 
     
    
     
       TECHNICAL FIELD OF THE INVENTION  
         [0002]    The present invention relates generally to intramedullary nails used for treatment of a fracture of a bone having a medullary canal extending longitudinally within the bone and, more particularly, to the structure of the intramedullary nail and/or spacer and methods for anchoring the intramedullary nail and spacers with respect to one or more fragments of the fractured bone.  
         BACKGROUND OF THE INVENTION  
         [0003]    Intramedullary nails are used by orthopaedic surgeons to treat fractures involving long bones such as the femur, humerus, tibia, fibula, and others. In such treatment, the medullary canal of the various fragments or pieces of the fractured bone is drilled out, reamed, or otherwise opened from one end. The intramedullary nail is then longitudinally placed in the medullary canal to contact at least two of the fragments, i.e. such that the nail extends on both sides of the fracture. As used herein, the term “fragment” refers to a portion of a fractured bone regardless of whether the fracture is complete. When implanted, the intramedullary nail supports and strengthens fragments of the fractured bone during healing of the fracture.  
           [0004]    Various types of intramedullary nails are well known within the medical device arts, and several different methods have been used to attach the intramedullary nail to and within the bone. As an example, in U.S. Pat. No. 4,338,926 to Kummer et al., an intramedullary nail is shown that provides a compressive force radially outward on an interior wall of the cortex structure surrounding the intramedullary nail. The compressive force secures the Kummer intramedullary nail within the medullary canal of the fragments. Similarly, in U.S. Pat. No. 5,514,137 to Coutts, cement is injected through a cannula in an intramedullary nail to the bone. Other intramedullary nails employ a more secure and mechanically positive attachment to the bone, such as through the use of one or more bone fasteners that extend generally transversely to the longitudinal axis of the intramedullary nail and through the cortex of the bone. The bone fasteners is received within a receiving recess or through-hole within the intramedullary nail to secure the intramedullary nail relative to the bone fastener. In a transverse attachment, the receiving opening of the intramedullary nail defines an axis that is at an angle to the longitudinal axis of the nail (commonly at 450 and 900 angles), and the bone fastener is advance on this receiving opening axis. U.S. Pat. No. 4,733,654 to Marino, U.S. Pat. No. 5,057,110 to Kranz et al., U.S. Pat. No. 5,127,913 to Thomas, Jr., U.S. Pat. No. 5,514,137 to Coutts (proximal end) and others disclose such a transverse bone fastener attachment in a bicortical attachment. U.S. Pat. No. 5,484,438 to Pennig shows a nail design with a recess that permits only unicortical attachment. The subject invention particularly pertains to intramedullary nails that utilize bone fasteners transversely through the cortex for attachment.  
           [0005]    Problems may arise when attaching an intramedullary nail to a fragment with a bone fastener. It is occasionally difficult for the surgeon to properly align the bone fastener and/or a hole for the bone fastener with the receiving opening in the intramedullary nail. Part of the alignment problems is simply due to difficulty in aligning the bone fastener with the receiving opening when the receiving opening is within the bone. Additionally, the intramedullary nail may be slightly bent or distorted during insertion thereof into the medullary canal. Such bending and/or distortion may be desired in some instances so that the shape of the inserted intramedullary nail better matches the particular shape of the medullary canal for a particular patient.  
           [0006]    Regardless of whether intended or unintended, such bending or distortion of the intramedullary nail creates further alignment errors between the bone fastener and/or a hole for the bone fastener and the receiving opening on the nail. Four types of alignment errors can be identified: (a) in transverse displacement (e.g., when the axis of the bone fastener is in the same transverse plane as the receiving opening in the intramedullary nail but does not intersect the axis of the intramedullary nail); (b) in longitudinal displacement (i.e., when the bone fastener is at a different longitudinal location than the receiving opening in the intramedullary nail); (c) in longitudinal angular misalignment (i.e., when the axis of the receiving opening and the axis of the bone fastener are at different angles relative to the longitudinal axis of the intramedullary nail); and (d) in transverse angular misalignment (i.e., when the axis of the receiving opening and the axis of the bone fastener are in the same transverse plane but at different radial positions relative to the intramedullary nail.  
           [0007]    In view of the above, various types of jigs have been proposed to reduce alignment errors, such as shown in U.S. Pat. No. 4,733,654 to Marino and U.S. Pat. No. 5,776,194 to Mikol et al. Such jigs may be temporarily attached to the proximal end of the intramedullary nail to help align the bone fastener and/or the drill to the receiving opening in the intramedullary nail. While such jigs are helpful, the jigs become less reliable as distance from the proximal end of the intramedullary nail increases, particularly if any bending of the intramedullary nail has occurred. Additional solutions are needed, especially for attaching the distal end of the intramedullary nail to a distal fragment.  
           [0008]    Another method to reduce such alignment problems is to locate the receiving opening(s) in-situ, such as through an x-ray or the use of magnets as taught in U.S. Pat. No. 5,127,913 to Thomas, Jr. Such methods are not typically preferred by surgeons in as much as the methods require significant additional time and effort during the orthopaedic surgery, to the detriment of the patient.  
           [0009]    A third method to reduce such alignment problems is to drill the receiving opening into the intramedullary nail only after the intramedullary nail is placed into the bone, allowing the receiving opening to be formed at a range of locations. Such in-situ drilling is disclosed in U.S. Pat. No. 5,057,110 to Krans et al., wherein a tip section of the intramedullary nail is formed of a bioresorbable material. Bioresorbable materials, however, are not as strong as metals, leading to an intramedullary nail that is weaker than desired and has a weaker attachment than desired.  
           [0010]    Further problems with intramedullary nails occur during placement of the intramedullary nail. For minimal damage to cortical tissue of the bone and most beneficial to healing, both the hole that is drilled in the medullary canal for the intramedullary nail and then the intramedullary nail itself, need to be precisely located and secured with respect to the medullary canal.  
           [0011]    Additional problems with intramedullary nails occur due to the healing requirements of a bone with respect to the strength and rigidity of the intramedullary nail. U.S. Pat. No. 4,756,307 to Crowninshield and U.S. Pat. No. 4,338,926 to Kummer et al. disclose intramedullary nails with bioresorbable portions to weaken the mail relative to the bone over time. These intramedullary nails, however, forsake the use of a transverse bone fastener to achieve the benefit of the bioresorbable portions.  
           [0012]    It would thus be advantageous to provide an intramedullary nail that overcomes the above-noted shortcomings. It would be further advantageous to provide an intramedullary nail and related portions and/or components that overcome the above-noted and other shortcomings.  
         SUMMARY OF THE INVENTION  
         [0013]    The subject invention is an intramedullary nail for treatment of a fracture of a bone by placement of the intramedullary nail within the medullary canal of the fractured bone. The intramedullary nail is formed with at least one opening in an exterior side that is adapted to receive a spacer or window of a non-metal material. The opening and window have cooperating structures that provide a spring type elastic fit, snap-fit, and/or interference fit of the window within the opening. The nail is used with a bone fastener such as a bone screw that is advanced transversely through the bone and into the spacer, preferably in a bicortical attachment with the bone.  
           [0014]    The bone fastener is smaller across than the window so that the window spaces the bone fastener relative to the metal structure of the intramedullary nail. The opening may have a longitudinal length that is different from its width, while the bone fastener has a circular length. Because the bone fastener is smaller across than the opening and the window, a larger error in placement of the bone fastener is permissible.  
           [0015]    In one form, there is provided an intramedullary nail for treatment of a fracture of a bone having a medullary canal extending longitudinally. The intramedullary nail includes a nail structure and a window. The nail structure has a longitudinal nail body defining a distal end and a proximate end. The nail body has a dynamization opening in one of the distal end and the proximate end. The dynamization opening has a first interference receptor disposed in a wall of the dynamization opening and a second interference receptor disposed in the wall. The window is formed of a non-metal material and is adapted to be inserted into the dynamization opening. The window has a window body having a first interference extending from a first surface of the window body and a second interference extending from a second surface of the window body. The first and second interferences are adapted to be received in the first and second interference receptors of the dynamization opening respectively upon insertion of the window into the dynamization opening.  
           [0016]    In another form, there is provided an intramedullary nail for treatment of a fracture of a bone having a medullary canal extending longitudinally. The intramedullary nail is defined by a longitudinal nail body. The longitudinal nail body defines a distal end and a proximate end, with the nail body having a dynamization opening extending through the nail body at one of the distal end and the proximate end essentially transverse to a longitudinal axis of the nail body. The dynamization opening is defined by an oval wall with the oval wall having a first groove positioned essentially parallel to the longitudinal axis and a second groove disposed diametrically opposite the first groove and essentially parallel to the longitudinal axis. The dynamization window is formed of a non-metal material and is adapted to be inserted into the dynamization opening. The window has an oval window body having a first rail extending a first distance from a first surface of the window body and a second rail extending a second distance from a second surface of the window body. The first and second rails are adapted to be received in the first and second grooves of the dynamization opening respectively upon insertion of the dynamization window into the dynamization opening.  
           [0017]    In yet another form, there is provided a window for a dynamization opening of an intramedullary nail. The window has an oval body defining a top and bottom surface, a first and second side surface, and a first and second curved side. A first rail is disposed on the first side and extends a first distance therefrom. A second rail is disposed on the second side and extends a second distance therefrom.  
           [0018]    In still another form, there is provided a method of retaining bone fragments. The method includes the steps of: (a) placing a dynamization window in a dynamization opening of an intramedullary nail, the intramedullary nail having a longitudinal nail body defining a distal end and a proximate end, the dynamization opening extending through the nail body at one of the distal end and the proximate end essentially transverse to a longitudinal axis of the nail body, the dynamization opening defined by an oval wall, the oval wall having a first groove positioned essentially parallel to the longitudinal axis and a second groove disposed diametrically opposite the first groove and essentially parallel to the longitudinal axis, the dynamization window formed of a non-metal material and adapted to be inserted into the dynamization opening, the window having an oval window body having a first rail extending a first distance from a first surface of the window body and a second rail extending a second distance from a second surface of the window body, said first and second rails adapted to be received in the first and second grooves of the dynamization opening respectively upon insertion of the dynamization window into the dynamization opening; (b) placing the intramedullary nail through the bone fragments, and (c) inserting a bone fastener through the dynamization window.  
           [0019]    In a yet further form, there is provided a bone support assembly for treatment of a bone. The bone support assembly includes a bone support implant formed of a non-resorbable material, and a first window formed of a non-metal material. The bone support implant has a first opening defined therein. The first opening has a first opening shape including a narrow portion adjacent an outer surface of the bone support implant and a wider portion more interior the bone support implant. The first window has outer dimensions that correspond to the first opening. The first window has a first section that mates with the narrow portion of the first opening. The first window has a second section wider than the first section that mates with the wider portion of the first opening more interior of the bone support implant. In this manner, the second section of the first window is wider than the narrow portion of the first opening and serves to positively secure the first window in the first opening with an interference fit. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    [0020]FIG. 1 is a perspective view of an exemplary intramedullary nail in accordance with the principles of the subject invention;  
         [0021]    [0021]FIG. 2 is a perspective view of the exemplary intramedullary nail of FIG. 1 in partial sectional;  
         [0022]    [0022]FIG. 3 is an enlarged fragmentary sectional view of the intramedullary nail of FIG. 2;  
         [0023]    [0023]FIGS. 4A and 4B are an enlarged sectional views of the intramedullary nail of FIG. 3 taken along line  4 A- 4 A thereof;  
         [0024]    [0024]FIG. 5 is an enlarged top plan view of an exemplary window for the intramedullary nail;  
         [0025]    [0025]FIG. 6 is an enlarged front plan view of the window of FIG. 5;  
         [0026]    [0026]FIG. 7 is a fragmentary view of the window of FIG. 5;  
         [0027]    [0027]FIG. 8 is an enlarged side plan view of the window of FIG. 5;  
         [0028]    [0028]FIG. 9 is a perspective view of an exemplary intramedullary nail receiving an exemplary window in the dynamization opening of the intramedullary nail;  
         [0029]    [0029]FIG. 10 is an enlarged sectional view of the intramedullary nail of FIG. 4B with the exemplary window being initially inserted into a dynamization opening of the intramedullary nail;  
         [0030]    [0030]FIG. 11 is an enlarged sectional view of the intramedullary nail of FIG. 4B with the exemplary window being further inserted into the dynamization opening of the intramedullary nail;  
         [0031]    [0031]FIG. 12 is a front plan view of a fractured bone in sectional with an intramedullary nail inserted therein;  
         [0032]    [0032]FIG. 13 is a longitudinal sectional view of two bone fragments of a fractured bone having an intramedullary nail and window structure inserted therein, particularly with bone fasteners extending through the bone fragments and the respective windows;  
         [0033]    [0033]FIG. 14 is a sectional view of an exemplary bone plate or washer accommodating an exemplary window in accordance with the principles of the subject invention; and  
         [0034]    [0034]FIG. 15 is a sectional view of an exemplary bone plate or washer accommodating an exemplary window in accordance with the principles of the subject invention.  
         [0035]    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  
       [0036]    While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and will herein by described in detail. It should be understood, however, that there is no intent to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. As well, reference should be made to the parent case of the present case (i.e. U.S. non-provisional patent application Ser. No. 09/575,764, filed May 22, 2000 and entitled Non-Metal Spacers For Intramedullary Nail, which is a continuation of PCT application number PCT/US00/09582, filed Apr. 10, 2000), the specification of which is hereby specifically incorporated by reference. Additionally, reference should be made to U.S. patent application Ser. No. 09/289,324, filed Apr. 9, 1999, entitled Intramedullary Nail With Non-Metal Spacers, now issued as U.S. Pat. No. 6,296,645, the specification of which is hereby specifically incorporated by reference.  
         [0037]    Referring now to FIGS. 1 and 2 there is depicted an exemplary intramedullary nail generally designated  20 , in accordance with the principles of the subject invention. The intramedullary nail  20  has a body or shaft  22  that defines a proximal end  24  and a distal end  26 , with “proximal” and “distal” being defined in accordance with the direction the nail  20  is intended to be inserted into a bone. As known in the art, the dimensions of the proximal end  24 , the distal end  26 , and the body  22  may be selected based on the required strength of the intramedullary nail  20  and the intended use of the intramedullary nail  20 . The intramedullary nail  20  depicted in the various figures is generally sized and shaped for treating a fracture toward the middle of an otherwise healthy adult human femur. If desired, the intramedullary nail  20  may be included in a kit having various sizes of intramedullary nails to accommodate the femurs of variously sized patients, and/or having various sizes of intramedullary nails to fit various types of femoral bone conditions or various types of femoral fractures, and/or further having various sizes of intramedullary nails to fit various other bones. For instance, the length of the intramedullary nail  20  may be selected as needed between about 10 to 20 inches.  
         [0038]    The distal end  26  may include a tip  30  having, for instance, a conical or partially conical profile. The conical profile of the tip  30  aids in inserting the intramedullary nail  20  into the medullary canal of a bone. The body may be generally of constant diameter. The proximal end  24  may include a portion of larger diameter than the body  22 .  
         [0039]    As known in the art, the intramedullary nail  20  has an overall cross-sectional shape selected based on the intended use. For the femoral intramedullary nail  20 , the cross-sectional shape may be generally circular to match the shape of the medullary canal of a healthy femur. For instance, the body  22  may be generally formed with an outside diameter of 0.394 inches.  
         [0040]    A cannula  34  preferably extends the internal length of the intramedullary nail  20 . The cannula  34  facilitates insertion and alignment of the intramedullary nail  20  within the medullary canal. The cannula  34  may be formed in various manners, but is typically formed by drilling longitudinally the length of the flail  20  through a lateral opening  35 . The cannula  34  receives a guide wire (not shown) during insertion of the intramedullary nail  20  into the medullary canal. The guide wire has to be thick enough to provide the requisite strength and rigidity for placement into the bone, and the cannula  34  must be large enough to receive the guide wire and permit longitudinal travel of the intramedullary nail  20  along the guide wire. Conversely, because a larger cannula  34  detracts from the strength of the intramedullary nail  20 , the cannula  34  should be as small as required for travel over the guide wire. The preferred guide wire is circular in cross-section, as well as the cannula  34 . For instance, the cannula  34  may be about 0.156 inches in diameter. With a body  22  of 0.394 inch (10 mm) diameter, the cannula  34  leaves a wall thickness for the body  22  of about 0.118 inches.  
         [0041]    The preferred intramedullary nail  20  includes a radius bend  38  in the body  22 , generally intended to match the anterior-posterior bend of a healthy femur. The bend  38  may have a large radius in relation to the length of the intramedullary nail  20 , such as a bend with a radius of 2 to 10 times the length of the intramedullary nail  20 , or may be a small radius in relation to the length of the intramedullary nail  20 , such as a bend with a radius of less than 2 times the length of the intramedullary nail  20 .  
         [0042]    Other than the cannula  34  being open from only one side of the shaft  28 , the intramedullary nail  20  is preferably symmetrical about a bisecting anterior-posterior plane. This allows the intramedullary nail  20  to be used in either the right or left femur while still maintaining the bend  38  appropriate for the curvature of the femur.  
         [0043]    The body  22  is formed of a structurally strong biocompatible material as known in the art. For instance, the body  22  can be formed of a single piece of metal, with the preferred metal being titanium, such as a Ti-6AL-4V ELI titanium per ASTM F-136.  
         [0044]    The distal end  26  is preferably formed with one or more through-holes  40  to facilitate attachment to a distal bone fragment. For instance, the distal end  26  may include two through-holes  40  each having a central axis that is positioned perpendicular to the cannula  34 . While not shown, the through-holes  40  may extend at an angle relative to a longitudinal axis of the body  22 . The through-holes  40  allow attachment to a bone fragment such as a femoral fragment by bicortical attachment. Alternatively, or in conjunction with the through-holes  40 , an annular recess  42  is provided in the distal end  26  to permit attachment to the distal end of a bone.  
         [0045]    The proximal end  24  of the intramedullary nail  20  is formed with a through-hole  46  to facilitate attachment to a proximal bone fragment. The through-hole  46  has a central axis that is perpendicular to a longitudinal axis of the proximal end  24  as the longitudinal axis of the proximal end  24  is angled relative to the longitudinal axis of the body  22 . The proximal end  24  may further include structure to facilitate attachment of a drilling or alignment jig (not shown) as known in the art for placement of bone fasteners relative to the intramedullary nail  20 . For instance, a proximal opening  44  aligned along the longitudinal axis of the proximal end  24  may be used to receive an end of a jig in a mating relationship. Those skilled in the art will appreciate that numerous other structures could be equivalently used to temporarily hold the jig relative to the intramedullary nail  20 .  
         [0046]    The proximal end  24  of the body  22  includes a dynamization opening  48  that extends therethrough. The opening  48  is referred to as a “dynamization” opening because, when used in conjunction with a properly dimensioned bone fastener (see FIG. 13) and with a window or spacer  50  (see e.g. FIGS.  5 - 8 ), formed of a polymer and/or bioresorbable material, the proportion of stress carried by the intramedullary nail  20  relative to stress carried by the healing bone across the fracture site dynamically changes as a function of time.  
         [0047]    It should be appreciated that the single dynamization opening  48  permits only unicortical attachment. Therefore, two dynamization openings may be provided (see FIG. 13) to permit bi-cortical attachment. The two dynamization openings would be provided on opposite sides of the body  22 . Of course, additional dynamization openings or sets of dynamization openings may be provided. In all cases, the structure and function of each dynamization opening is the same in accordance with the subject principles.  
         [0048]    Referring now to FIG. 3, the proximal end  24  of the intramedullary nail  20  is shown in sectional particularly showing the dynamization opening  48 . An oval wall  50  that extends through the body  22  defines the dynamization opening  48 . A first channel or groove  52  is formed in a first elongated wall portion  53  of the oval wall  50 . The first channel  52  preferably extends an entire length of the elongated wall portion  53 , but not necessarily. A second channel or groove  54  is formed in a second elongated wall portion  55  of the oval wall  50 . The second channel  54  preferably extends an entire length of the elongated wall portion  55 , but not necessarily. Additionally, the first and second channels  52  and  54  are preferably the same length and overall dimension. If the dynamization opening intersects (or cuts through) the cannula of the intramedullary nail (which is not the case in FIG. 3), the first and second channels  52  and  54  each preferably extend respectively along the first and second elongated wall portions; from where the cannula intersects the wall portion of the dynamization opening to the opposite side of the wall portion where the cannula intersects the dynamization opening  48 . It should be appreciated that the designations first and second are arbitrary. As such, the first may be the second and the second may be the first. Other designations may be used as appropriate.  
         [0049]    Referring to FIG. 4A, there is depicted a cross section of the intramedullary nail  20  taken along the middle of the dynamization opening  48 , particularly along line  4 A- 4 A of FIG. 3. Since FIG. 4A only shows half of the cross-section, reference is also made to FIG. 4B which shows a full cross section of the intramedullary nail  20  taken along the middle of the dynamization opening  48 . The first channel  52 , in one form, is defined by an elongated wall  56  bounded on one side by a first side wall  57  and on another side by a second side wall  58 . The walls  56 ,  57 , and  58  define a rectangular shape for the first channel  52 . It should be appreciated that the first channel  52  may take shapes other than rectangular such as rounded or arcuate, square, or the like. The second channel  54 , in one form, is defined by an elongated wall  60  bounded on one side by a first side wall  61  and on another side by a second side wall  62 . The walls  60 ,  61 , and  62  define a rectangular shape for the second channel  54 . It should be appreciated that the second channel  54  may take shapes other than rectangular such as rounded or arcuate, square, or the like. Preferably, the first and second channels  52  and  54  have the same shape, which is rectangular.  
         [0050]    Access to the dynamization opening, represented by the arrows (which also represent the direction of insertion of a dynamization window  70 , see e.g. FIGS.  5 - 8 , into the dynamization opening  48 ) is characterized by an angled, chamfered, or tapered wall  64  on one side of the dynamization opening  48  and by an angled, chamfered, or tapered wall  66  on another side of the dynamization opening  48 . The angled walls  64  and  66  each extend about the entire periphery of the respective sides of the dynamization opening  48 . As explained more fully below, the angled walls  64  and  66  assist in receiving or introducing the window  70  (depending on the side of insertion of the window  70 ) into the dynamization opening  48 .  
         [0051]    Referring now to FIGS.  5 - 8  there is depicted an exemplary window in accordance with the principles of the subject invention that is generally designated  70 . The window  70  is configured to be received in the dynamization opening  48  and particularly any and all dynamization openings in the intramedullary nail. During use of the intramedullary nail  20  as shown in FIGS. 12 and 13, and explained more fully below, a bone fastener is positioned into the opening  48  through the window  70 . The window  70  spaces the bone fastener relative to the intramedullary nail  20  (and the dynamization opening  48 ). Force is transmitted between the intramedullary nail  20  and the bone fastener primarily as a compressive load on a portion of the window  70 .  
         [0052]    The window  70  is formed of a non-metal material, preferably a bioresorbable material. The term “bioresorbable” as used herein refers to any biocompatible material that dissolves or degrades over time after implantation into the human body. Among others, possible bioresorbable materials includes polymers and copolymers, glycolic acid, lactic acid, aminocaproic, acid, lactides, desoxazon, hydroxybutric acid, hydroxyvaleric acid, hydroxymethacrylate, peptides, polyesters of succinic acid and cross-linked hyaluronic acid, or even a biologically absorbable hydroxyapatite or tricalcium phosphate. The preferred bioresorbable material is a polylactic acid (“PLA”), that provides a strong material for the window  70 . The compressibility of the PLA material shows little change over the first few weeks of implantation, but then increases linearly over the next few months until resorption to the point where the material will no longer support a load. With the preferred PLA material, full resorption will typically occur within about two to five years. If no bioresorption is desired, the non-metal material may be any other polymer commonly used in medical implants, such as a preferred non-metal non-resorbable material of a ultra-high molecular weight polyethylene (“UHMWPE”).  
         [0053]    The dynamization opening  48  and the dynamization window  70  are shaped and/or configured based on the required strength and the desired dynamization characteristics for the intramedullary nail  20 . In the preferred embodiment as shown in the figures, both the dynamization opening  48  and the window  70  are oval shaped. Particularly, both the dynamization opening  48  and the window  70  are oval shaped with elongated sides.  
         [0054]    With respect to the window  70 , the window  70  is defined by a body  72  of an appropriate material as explained above. The body  72  includes a top surface or wall  74  and a bottom surface or wall  76 . The top and bottom surfaces  74  and  76  are essentially flat and define planes that are essentially parallel. It should be appreciated that the designation or nomenclature of top and bottom are arbitrary. The body  72  also includes a front side surface or wall  79  and a back side surface or wall  81  disposed between the top and bottom surfaces  74  and  76 . The front and back side surfaces  79  and  81  essentially define planes that are essentially parallel. Again, it should be appreciated that the designation or nomenclature of front and back are arbitrary. A first rounded end surface or wall  78  is disposed between the top and bottom surfaces  74  and  76 , and the front and back surfaces  79  and  81 , at one end of the body  72 , while a second rounded end surface or wall  80  is disposed between the top and bottom surfaces  74  and  76 , and the front and back surfaces  79  and  81 , at another end of the body  72 .  
         [0055]    A cannula  82  extends through the body  72  from one end surface  78  to the other end surface  80 . The cannula  82  is preferably centrally disposed in the body  72  and thus aligns with a cannula of the intramedullary nail  20  when the intramedullary nail  20  has a cannula and the dynamization window intersects or cuts through the cannula. The cannula  82  is preferably sized to the dimensions of the cannula of the nail.  
         [0056]    The front surface  79  includes a protrusion, rail, ledge, or the like  84  that preferably, but not necessarily, extends from the first end surface  78  to the second end surface  80 . The rail  84  is raised relative to the front surface  79  and joins the front surface  79  at a top end by a chamfer, taper, or angled surface  85 , and at a bottom end by a chamfer, taper, or angled surface  86 . As best seen in FIG. 8, the rear surface  81  also includes a protrusion, rail, ledge, or the like  88  that preferably, but not necessarily, extends from the first end surface  78  to the second end surface  80 . The rail  88  is raised relative to the rear surface  81  and joins the rear surface  81  at a top end by a chamfer, taper, or angled surface  89 , and at a bottom end by a chamfer, taper, or angled surface  90 . In FIG. 7, it is best shown how the rail  84 , and particularly the angled surface  85  joins the end surface  80 . This is exemplary of both rails  84  and  88  and each of their respective angled surfaces  85 ,  86  and  89 ,  90 .  
         [0057]    In accordance with an aspect of the subject invention, the rail  84  is raised a distance (rh 1  or rail height  1 ) relative to the front surface  79 . In particular, the end surface of the rail  84  extends a distance, rh 1 , from or relative to the front surface  79 . In like manner, the rail  88  is raised a distance (rh 2  or rail height  2 ) relative to the rear surface  81 . In particular, the end surface of the rail  88  extends a distance, rh 2 , from or relative to the rear surface  81 . The window material determines the distances (or interferences) rh 1  and rh 2  and, more particularly, determines a range of distances for each rail height rh 1  and rh 2 . With the material PLA, the distance rh 1  is preferably a minimum of 0.003 inches (0.003″) to a maximum of 0.005 inches (0.005″) relative to the front surface  79 , while the distance rh 2  is preferably a minimum of 0.003 inches (0.003″) to a maximum of 0.005 inches (0.005″) relative to the rear surface  81 . In this case, the overall distance that the rails  84  and  88  extend from the surfaces  79  and  81  ranges from 0.006 inches (0.006″) to 0.010 inches (0.010″). Depending on the composition of the window, the individual rail height and the total rail height will be variable for optimum compression and uncompression of the rails during insertion of the window into the dynamization opening. In all cases, however, the total rail height allows the compression thereof during insertion of the window into the dynamization opening and the uncompressing thereof when reaching the dynamization opening channels (interference receptors). Such distance may be considered an interference distance or simply interference. As explained more fully below and in accordance with an aspect of the subject invention, such interference interacts with the dimensions of the dynamization opening  48  including the first and second channels  52  and  54  thereof.  
         [0058]    Referring to FIG. 9, the window  70  is shown being inserted into the intramedullary nail  20 . Particularly, the window  70  is shown being inserted into the dynamization opening  48  of the intramedullary nail  20 . This is accomplished before the intramedullary nail  20  is implanted into a patient. Also, it should be appreciated that while the intramedullary nail  20  has only one dynamization opening  48 , the intramedullary nail  20  may have multiple dynamization openings both at the same end (proximal or distal), or both ends (proximal and distal).  
         [0059]    Referring to FIGS. 10 and 11, the insertion of the window  70  into the dynamization opening  48  is depicted in greater detail. As the window  70  is initially placed into the dynamization opening  48  the angled surface  66  (or the angled surface  64  depending on the direction of insertion) guides the window  70  into the dynamization opening  48 . The width of the window  70  (taken between the front surface  79  and the rear surface  81 ) is slightly less than the width of the dynamization opening  48 . Such dimensions provide easy centering of the window  70  in the dynamization opening  48  with little to no canting of the window  70 . Particularly, the window  70  slip fits into the dynamization opening  48  almost half way. This positions the window  70  true to the dynamization opening  48  (and the intramedullary nail  20 ) for final insertion.  
         [0060]    The interference of the rails  84  and  88  lend itself to a spring action of the window  70  relative to the dynamization opening  48 . Particularly, when angled surfaces  86  and  90  of the respective rails  84  and  88  reach the angled surface  66  of the dynamization opening  48  as shown in FIG. 11, the rails  84  and  88  (and thus the body  72 ) are/is thereafter compressed during further insertion into the dynamization opening  48 . When the window  70  is in the position shown in FIG. 11, the angled surfaces  66  of the dynamization opening  48 ,  86  of the rail  84 , and  90  of the rail  88 , holds the window  70  in place until further insertion.  
         [0061]    Further insertion of the window  70  seats the rails  84  and  88  in the respective channels  54  and  52 . The square shoulders provided by the side walls  62 ,  61  and  57 , 58  of the channels  54  and  52  respectively, allow the interference of the rails  84  and  88  to snap or expand into the respective channels  54  and  52 . The square shoulders also help prevent the window  70  from being inserted past the center point of the dynamization opening  48 .  
         [0062]    Tests of the subject invention indicate an insertion force for the window  70  of approximately 200 pounds have been measured and are typical. A removal force, measured in tests of the subject invention, indicate a force of approximately 300 pounds are typical. While not shown, a tool may be used for insertion and removal of the window  70  after the intramedullary nail  20  is placed in the patient.  
         [0063]    Because of the configuration of the window  70 , the window  70  provides a spring type elastic fit in the dynamization opening  48 . Such a spring type elastic fit provides an audio response and a snap feel when inserted. As well, the present configuration allows the window  70  to be press fit into the dynamization opening  48 , is self-centering (of the window relative to the window opening), and provides increased resistance to removal.  
         [0064]    The window  70  may be configured to provide a line-to-line interference fit with the dynamization opening  48  of the intramedullary nail  100 . In this case, the opening  48  is configured with a first opening shape having a narrow portion adjacent an outer surface of the intramedullary nail  100 , and a wider portion more interior the intramedullary nail  100 . The window  70  has outer dimensions that correspond to the opening  48 . Particularly, the window  70  has a first section that mates with the narrow portion of the opening  48 , and a second section that is wider than the first section. The second section mates with the wider portion of the opening  48 . The second section of the window is wider than the narrow portion of the opening  48  which serves to positively secure the window in the opening. Interference of the window may be on the order or 0.010 to 0.030 per side.  
         [0065]    It should be appreciated that the subject invention may be utilized in various other bone holding/mending devices. Particularly, a dynamization window in accordance with the principles of the subject invention may be utilized with a bone holding/mending device that has a dynamization opening in accordance with the principles of the subject invention. In this regard, reference is made to FIG. 14. FIG. 14 depicts a bone plate, washer, or the like generally designated  130 . The bone plate  130  may be made of a metal, a polymer, or other suitable material such as in known in the art for bone holding/mending devices.  
         [0066]    The bone plate  130  has a dynamization opening  132  that is shaped in accordance with the principles presented herein, particularly such as that described above. A dynamization window  134  is received in the dynamization opening  132  in the same manner as that described above. The window  134  may be made from a bioresorbable or absorbable material or a non-resorbable/absorbable material such as that mentioned above and includes a longitudinal bore or cannula  139  and a lateral bore  138 . The lateral bore  138  may be used to guide and/or allow a suture to extend therethrough. The bone plate  130  may be used in a manner known in the art.  
         [0067]    Referring to FIG. 15, there is depicted another bone plate, washer, or the like generally designated  140 . The bone plate  140  may be made of a metal, a polymer, or other suitable material such as in known in the art for bone holding/mending devices. The bone plate  140  has a dynamization opening  142  that is shaped in accordance with the principles presented herein, particularly such as that described above. A dynamization window  144  is received in the dynamization opening  132  in the same manner as that described above.  
         [0068]    The window  144  may be made from a bioresorbable or absorbable material or a non-resorbable/absorbable material such as that mentioned above. The window  144  includes two longitudinal bores or cannulas  146  and  147 . As well, the window  144  has a first suture bore  150  and a second suture bore  151  each of which is laterally situated relative to the cannulas  146 ,  147 . A suture  152  (rather than a bone fastener) is shown looped through the suture bores  150  and  151 . The suture  152  would thereafter be used in a known manner. The preferably polymer material of the window  144  provides a cushion for the suture  152 . Insertion and removal of the window  144  is accomplished as described above.  
         [0069]    Use of the Subject Invention  
         [0070]    A use of the subject invention will now be described. It should be appreciated, however, that the below-described use of the subject invention is only exemplary of one manner of use. Other manners of use not specifically described herein are contemplated. Referring to FIG. 12, there is depicted a cross section of a tibia that has been fractured into four bone fragments  102 ,  103 ,  104 , and  105 . An intramedullary nail  100  has been inserted in the medullary canal of the bone fragments  102 ,  103 ,  104 , and  105 . The intramedullary nail  100  thus holds the various bone fragments  102 ,  103 ,  104 , and  105  in alignment for healing to take place. The intramedullary nail  100  has a first dynamization opening  110  in the distal end thereof and a second dynamization opening  112  in the proximate end thereof. The first and second dynamization openings  110  and  112  are configured as described above. The bone fragments  102 ,  103 ,  104 , and  105  are aligned and drilled according to known procedures. The intramedullary nail  100  is inserted into the bone fragments  102 , 103 ,  104 , and  105  according to known procedures.  
         [0071]    Referring to FIG. 13, the intramedullary nail  100  is shown in cross section after insertion through the various bone fragments  102 ,  103 ,  104 , and  105 . As well, only the proximal and distal ends of the intramedullary nail  100  are shown. Additionally, only the bone fragments  102  and  105  are shown. Before the placement of the intramedullary nail  100  through the bone fragments  102 ,  103 ,  104 , and  105 , a first dynamization window  120  is placed in the dynamization opening  110 . The particular material of the window  120  is chosen by the surgeon. The window  120  has a cannula  121  that is aligned with a cannula  114  of the intramedullary nail  100 . A second dynamization window  124  is placed in the dynamization opening  112 . The particular material of the window  124  is chosen by the surgeon. The window  124  has a cannula  125  that is aligned with the cannula  114  of the intramedullary nail  100 . It should be appreciated that the sequence of placement of the first and second windows  120  and  124  is arbitrary.  
         [0072]    After placement of the first and second windows  120  and  124  into the intramedullary nail  100 , and the placement of the intramedullary nail  100  into the bones/bone fragments, bone fasteners or screws  122  and  126  are driven through the respective bone fragment  102 ,  105 , and into and through the respective windows  120 ,  124 . The angle of placement of the bone fastener relative to its respective window and thus the longitudinal axis of the intramedullary nail  100 , is variable and selected by the surgeon accordingly. In FIG. 13, the bone screw  122  is essentially perpendicular (90°) to the longitudinal axis of the window  120  and the intramedullary nail  100 . The bone screw  126  is at an approximately 45° angle relative to the longitudinal axis of the window  124  and the intramedullary nail  100 . The oval shape of the windows allows more room for cross screw placement and targeting of the screw. The placement of the screws as shown in FIG. 13 allows the bone fragments to shift during healing. Various deflections and/or stresses in the walls of the intramedullary nail and/or the window is limited through a design in accordance with the principles of the subject invention such as those set forth herein.  
         [0073]    There are a plurality of advantages of the subject invention arising from the various features of the intramedullary nail and/or spacers therefor described herein. It will be noted that alternative embodiments of the intramedullary nail and/or spacer(s) of the subject invention may not include all of the features described yet still benefit from at least some of the advantages of such features. Those of ordinary skill in the art may readily devise their own implementations of an intramedullary nail that incorporate one or more of the features of the subject invention and fall within the sprit and scope of the subject invention.