Abstract:
During hip replacement surgery, a practitioner can attach a distal stem to a femur of the patient and can then fixedly attach a proximal body to the distal stem using a bolt. Prior to tightening, the bolt can be heated to an elevated temperature greater than average human core body temperature. The practitioner can tighten the bolt to a specified torque while the bolt is at the elevated temperature. After tightening, the bolt cools to average human core body temperature and experiences a tensile stress due to the effects of thermal expansion. The tensile stress in the bolt produces a compressive force between the distal stem and the proximal body. The compressive force can increase the attachment strength of the bolt to the distal stem and the proximal body, beyond what can be achieved by solely torquing the bolt to the specified level during surgery without first heating the bolt.

Description:
CLAIM OF PRIORITY 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/826,742, filed on May 23, 2013, the benefit of priority of which is claimed hereby, and which is incorporated by reference herein in its entirety. 
     
    
     BACKGROUND 
       [0002]    A human hip joint connects a femur (sometimes referred to as a thigh bone) to an acetabulum (sometimes referred to as a hip socket) of the pelvis. Hip joints support the weight of a human body, and are important for retaining balance. 
         [0003]    Some types of injury, disease, or degeneration can produce pain and/or restricted motion in a hip joint. One treatment for certain types of damage to a hip joint is surgery. For relatively mild hip damage, the hip can be surgically repaired. For more severe damage, the hip can be surgically replaced. 
       OVERVIEW 
       [0004]    In hip replacement surgery, a practitioner can remove the natural head and neck of a femur and replace them with a metallic hip prosthesis. This prosthesis can include two elements: a distal stem for fixation into the femur, and a proximal body for fixation in the metaphysis of the femur. In a modular hip stem, the proximal body and the distal stem can be adjustable with respect to each other. The prosthesis can also include a neck for replacing the natural femoral neck; this neck can be part of the proximal body, or can be a separate element that is adjustable with respect to the proximal body and the distal stem. 
         [0005]    The distal stem can be first attached to the femur. Once the distal stem is attached, the practitioner can adjust the orientation of the proximal body with respect to the distal stem. The prosthesis can include adjustments for anteversion (e.g., the degree to which the femur is rotated forward toward the front of the body or backward toward the back of the body), offset (e.g., the left-right distance of the femur from the centerline of the body), and height (e.g., the length of the femur), for example. Once the proximal body is suitably oriented with respect to the distal stem, the practitioner can fixedly attach the proximal body to the distal stem with a bolt. During surgery, the practitioner can tighten the bolt to a specified torque. Once tightened, the bolt remains in place beyond the end of the surgery, and continues to attach the proximal body to the distal stem for as long as the artificial hip is installed within the patient. 
         [0006]    It is important that the proximal body and the distal stem remain firmly attached over time. As such, in various instances, it can be desirable to increase the strength of the attachment provided by the bolt without over-torquing and causing damage to one or more elements within the modular hip stem. 
         [0007]    Prior to tightening, the bolt can be heated to an elevated temperature greater than average human core body temperature. The practitioner can tighten the bolt to the specified torque, while the bolt is at the elevated temperature. After tightening, the bolt cools to average human core body temperature. If the bolt were unattached during cooling, it would longitudinally shrink due to the effects of thermal expansion. Because the bolt is attached at an elevated temperature and cools in place, it experiences a tensile stress due to the effects of thermal expansion. The tensile stress in the bolt produces a compressive force between the distal stem and the proximal body. The compressive force can increase the attachment strength of the bolt to the distal stem and the proximal body, beyond what can be achieved by solely torquing the bolt to the specified level during surgery without first heating the bolt. 
         [0008]    This Overview is intended to provide examples of subject matter of the present patent document. It is not intended to provide an exclusive or exhaustive explanation of the invention. The Detailed Description below is included to provide further information about the present modular hip stems and the corresponding methods. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    In the drawings, which are not necessarily drawn to scale, like numerals can describe similar components in different views. The drawings illustrate generally, by way of example, but not by way of limitation, various embodiments discussed in the present patent document. 
           [0010]      FIG. 1  is a side-view drawing of an exemplary modular hip stem, including a distal stem connected to a proximal body using a bolt. The interface between the distal stem and the proximal body is cylindrical. 
           [0011]      FIG. 2  is a side-view drawing of another exemplary modular hip stem, including a distal stem connected to another proximal body using a bolt. The interface between the distal stem and the proximal body is tapered. 
           [0012]      FIG. 3  is a side-view drawing of an exemplary bolt. 
           [0013]      FIG. 4  is a plot of longitudinal length versus temperature, for the bolt of  FIG. 3 . 
           [0014]      FIG. 5  is a plot of bolt temperature versus time, for the bolt of  FIG. 3 , for an exemplary surgical procedure. 
           [0015]      FIG. 6  is a side-view drawing of an exemplary bolt having a hollow interior. 
           [0016]      FIG. 7  is a side-view drawing of an exemplary bolt having an electrical resistive heater. 
           [0017]      FIG. 8  is a flow chart of a method for assembling a modular hip stem. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    When surgically installing a modular hip stem, a practitioner can choose a distal stem and a proximal body from a kit. The kit can include various options for tapers, surface finishes, geometries, and lengths. In general, for a modular hip stem kit, the various configurations for the distal stem are compatible with the various configurations for the proximal body, and can include a common interface. For the example distal stems and proximal bodies described below and shown in the figures, it will be understood that other suitable options can also be used for tapers, surface finishes, geometries, and lengths. 
         [0019]      FIG. 1  is a side-view drawing of an exemplary modular hip stem  100 . The modular hip stem  100  includes an exemplary distal stem  110  connected to an exemplary proximal body  120  using a bolt  130 . 
         [0020]    The distal stem  110  includes an annular projection  112  at its proximal end. The annular projection  112  has an exterior surface  114  that is cylindrical in shape. The exterior surface  114  of the annular projection  112  is configured to contact a respective surface on the proximal body  120 . The annular projection  112  can have an interior surface  116  that is threaded. The female threads on the interior surface  116  are configured to mate with corresponding male threads on the bolt  130 . 
         [0021]    The proximal body  120  includes an annular projection  122  at its distal end. The annular projection  122  has an interior surface  124  that is cylindrical in shape. The interior surface  124  of the annular projection  122  is configured to contact the exterior surface  114  of the annular projection  112  of the distal stem  110 . During alignment of the proximal body  120  to the distal stem  110 , the interior surface  124  can slide past the exterior surface  114 , so that the proximal body  120  can rotate about a longitudinal axis (A) of the annular projection  112  without translating away from the longitudinal axis (A). The proximal body  120  includes a bore  126 , which is coaxial with the longitudinal axis (A). The most proximal portion of the bore  126  can be sized and shaped to accommodate a head of the bolt  130  so that, when installed, the bolt  130  extends fully into the bore  126 . The bore  126  can include an annular wall  128 , which is sized smaller than the head of the bolt  130 . 
         [0022]    The bolt  130  is elongated, with a head  132  at a proximal end and male threads  134  at or near a distal end. When the bolt  130  is installed, the threads  134  can engage the corresponding threads on the interior surface  116  of the annular projection  112  of the distal stem  110 . The head  132  can be tightened against the annular wall  128  in the bore  126  of the proximal body  120 . During and after installation, the bolt  130  is coaxial with the longitudinal axis (A). The bolt  130  can be tightened through the bore  126 , using a suitably sized wrench, hex key, or other suitable tool. 
         [0023]    In some examples, such as the modular hip stem  100  of  FIG. 1 , there is a cylindrical interface between the exterior surface  114  of the annular projection  112  of the distal stem  110  and the interior surface  124  of the annular projection  122  of the proximal body  120 . As such, the proximal body  120  and the distal stem  110  can be pivoted with respect to each other. The pivoting motion is rotationally symmetric about the longitudinal axis (A). In other examples, this interface can be not purely cylindrical, but can include an asymmetry with respect to the longitudinal axis (A). For instance, the interface can have an elongated or elliptical cross-section, when viewed in a slice taken perpendicular to the longitudinal axis (A). Such an asymmetric cross-section can prevent pivoting between the proximal body  120  and the distal stem  110 . In both the symmetric and asymmetric examples of  FIG. 1 , the interface has a cross-section taken in a slice perpendicular to the longitudinal axis (A), which can not vary for different locations along the longitudinal axis (A). In the example modular hip stem  100  of  FIG. 1 , the bolt  130  supplies a force that joins the proximal body  120  to the distal stem  110 . 
         [0024]      FIG. 2  is a side-view drawing of another exemplary modular hip stem  200 . The modular hip stem  200  includes a tapered interface between the exterior surface  214  of the annular projection  212  of the distal stem  210  and the interior surface  224  of the annular projection  222  of the proximal body  220 . Both surfaces  214  and  224  include a taper and, together, form a tapered junction. The tapered junction can increase in cross-sectional size over the length of the longitudinal axis (A), from the proximal end of the junction to the distal end of the junction. In some of these examples, the tapered junction supplies a portion of a frictional force that joins the proximal body  220  to the distal stem  210 ; the bolt  230  supplies the remainder of the joining force. An example of a suitable taper is a Morse taper; other suitable tapers can also be used. The interior surface  216  having female threads, the bore  226 , the annular wall  228 , the bolt head  232 , and the male threads  234  can be similar in structure and function to the similarly-numbered elements shown in  FIG. 1 . 
         [0025]    In the exemplary modular hip stems  100 ,  200  shown in  FIGS. 1 and 2 , respectively, the bolt  130 ,  230  supplies some or all of the joining force between the proximal body  120 ,  220  and the distal stem  110 ,  210 . 
         [0026]      FIG. 3  is a side-view drawing of an exemplary bolt  300  suitable for use in the modular hip stems  100 ,  200  of  FIGS. 1 and 2 . The bolt  300  includes a proximal end  320 , a distal end  340 , and a longitudinal axis (A) extending from the proximal end  320  to the distal end  340 . 
         [0027]    The bolt  300  can include a head  330  at or near its proximal end  320 . The head  330  can include a proximal portion  332  configured to be tightened by a suitable wrench, hex key, screwdriver or other tightening element that is removable from the bolt  300  and the hip stem. The tightening element can engage the head  330  of the bolt and can apply torque to the head  330  of the bolt  300  to rotate the bolt  300  around its longitudinal axis (A). In a cross-sectional slice taken perpendicular to the longitudinal axis (A), the proximal portion can have a polygonal shape, such as a triangle, a square, a pentagon, or a hexagon. Such a polygonal shape can be engaged by the opposing prongs of a wrench. Alternatively, the head  330  can include one of more depressions extending distally from the proximal end  320  of the bolt  300 . The depressions can be shaped to engage a hexagonal key, a flat-head screwdriver, a Phillips-head screwdriver, a Torx head screwdriver, or another suitable tightening element. A distal end of the head  330  of  FIG. 3  includes a distal-facing annular surface  334  that is pressed into contact with a corresponding surface on the proximal body. The annular surface  334  can radially extend farther from the longitudinal axis (A) than other portions of the bolt  300 . The bolt  300  can, in some examples, be rotationally symmetric about the longitudinal axis (A) except for the proximal portion  332  of the head  330 . 
         [0028]    The bolt  300  can include one or more helically-shaped male threads  350  at or near its distal end  340 . The threads  350  are configured to supply a frictional force against the corresponding female threads on the hip stem. The threads  350  can extend distally to the distal end  340  of the bolt  300  or can alternatively extend only to a location along the longitudinal axis proximal to the distal end  340  of the bolt  300 . The distal end  340  of the bolt  300  can be typically flat and perpendicular to the longitudinal axis (A) of the bolt  300 . The distal end  340  can optionally include a depression  360 , which can accommodate a smaller screw head or other element that can extend proximally from the hip stem into the interior of the bolt  300 . The distal end  340  can include a depression  360 , which aids in machining the bolt, and allows the bolt to be centered on a lathe during manufacturing. 
         [0029]    The bolt  300  can include a neck  310  that extends longitudinally from the head  330  to the threads  350 . In some examples, the neck  310  can have a smaller cross-sectional diameter than the threads  350  and the head  330 . In other examples, the neck  310  can have an equal or larger diameter than the threads and/or the head  330 . In some examples, the neck  310  is unthreaded. In other examples, the neck  310  is threaded. In still other examples, the neck  310  is absent, and the threads  350  extend to the head  330 . The shapes and configurations of the head  330 , the threads  350 , and the neck  310  shown in  FIG. 3  and described herein are examples, and other suitable shapes and configurations can also be used. In the configuration of  FIG. 3 , a longitudinal length (L) of the bolt extends from the annular surface  334  to the threads  350 . 
         [0030]    The bolt  300  can be formed from a material, or an alloy of materials, that has a characteristic thermal expansion coefficient, denoted as a. The thermal expansion coefficient α can measure a change in volume or a change in longitudinal length as a function of temperature. The thermal expansion coefficient α can be positive so that as the temperature of the bolt increases, its volume and/or longitudinal length also increases. The bolt can be formed from a biocompatible material, such as at least one of cobalt, chromium, titanium, titanium alloys, stainless steel, and stainless steel alloys. 
         [0031]      FIG. 4  is a plot of longitudinal length (L) versus temperature for the bolt  300  of  FIG. 3 . For a particular range of temperatures, such as a range that includes average human core body temperature, the length (L) can increase linearly or nearly linearly with temperature. The thermal expansion coefficient α of the bolt  300  describes the linear relationship as: 
         [0000]      α=(1/ L )( dL/dT ),
 
         [0000]    where L is the longitudinal length of the bolt  300 , and dL/dT is the rate of change of the longitudinal length per unit change in temperature. The bolt length L shown in the plot of  FIG. 4  assumes that the bolt is allowed to expand or contract freely as a function of temperature. 
         [0032]    A fastening scheme can use the properties of thermal expansion to increase the holding strength of the bolt. For instance, a bolt can be installed or tightened at a particular temperature. The bolt can then be cooled to a lower temperature while remaining installed. If the bolt were left unconstrained, the bolt would longitudinally contract due to the change in temperature. In an installed state, where the longitudinal length of the bolt is fixed or constrained, the cooling to a lower temperature produces tensile stress in the bolt. The tensile stress pulls the longitudinal ends of the bolt closer together, and therefore increases the fastening strength, between a distal stem of a modular hip stem and a proximal body of the modular hip stem, provided by the bolt. In some cases, the fastening strength achieved by tightening at one temperature, then cooling the bolt to a lower temperature, can exceed the fastening strength achieved by solely tightening the bolt at the temperature at which the bolt is used. 
         [0033]      FIG. 5  is a plot of bolt temperature (T) versus time for such an exemplary surgical procedure. In this procedure, the bolt is initially at room temperature, such as in a storage closet or cabinet. A practitioner then exposes the bolt to a heat source and heats the bolt to an elevated temperature that exceeds average human core body temperature (e.g., exceeding a temperature of 37° C., or 98.6° F.). The heat source can be outside the body of the patient, can be removable from the bolt, or can be made integral with the bolt. The practitioner tightens the bolt to a specified torque while the bolt is at the elevated temperature. In some examples, the bolt can be tightened while it is at a peak temperature; in other examples, the bolt can cool slightly from a peak temperature and can be tightened at the slightly cooled temperature, which still exceeds average human core body temperature. 
         [0034]    The practitioner turns off the heat source, removes the heat source from the bolt, or removes the bolt from the heat source. The heat source can be removed or turned off before, during, or after the bolt is tightened. The practitioner allows the bolt to cool. In a surgical procedure in which the bolt is to be implanted within the human body, such as hip replacement surgery, the bolt cools to average human core body temperature. The bolt, in use, can be under tensile stress. For hip replacement surgery, the tensile stress in the bolt can force the proximal body and the distal stem closer against each other, with more force than can be obtained by tightening the bolt to the specified torque at room temperature or at average human core body temperature. This increase in force can desirably increase the holding strength of the bolt. 
         [0035]    There are several ways to heat the bolt for the surgical procedure of  FIG. 5 . For instance, the bolt can be heated outside the body, such as in a warm liquid bath. The bolt can be heated in the bath to a specified elevated temperature, removed from the bath, then tightened in place while still at a temperature above the average human core body temperature. Alternatively, the bolt can include its own heating mechanism, so that it can be placed in the modular hip stem, can be heated while in place, and can be tightened at a suitable time at a suitable elevated temperature. Two exemplary heating mechanisms are shown in  FIGS. 6 and 7 . 
         [0036]      FIG. 6  is a side-view drawing of an exemplary bolt  600  having a hollow interior. An elongated hollow cavity  670  extends within the bolt  600 , generally parallel to the longitudinal axis (A), from a proximal end  620  of the bolt  600  toward a distal end  640  of the bolt  600 . An external heater can be placed into the cavity  670  to heat the bolt  600  to a desired elevated temperature, and can be removed from the cavity before or after the bolt  600  is tightened. The head  630 , neck  610 , threads  650 , and depression  660  can be similar in structure and function to corresponding similarly-numbered elements from  FIG. 3 . 
         [0037]      FIG. 7  is a side-view drawing of an exemplary bolt  700  having an electrical resistive heater  790 . An internal volume  770  extends within the bolt  700 , generally parallel to the longitudinal axis (A), from a proximal end  720  of the bolt  700  toward a distal end  740  of the bolt  700 . An electrical resistive heater  790  is included within the internal volume  770 . An electrically insulating, but thermally conductive, material  780  surrounds the electrical resistive heater  790  and fills the internal volume  770  around the electrical resistive heater  790 . 
         [0038]    One or more electrodes  794  are disposed outside the internal volume  770  and are configured to supply current to and from the electrical resistive heater  790 . In some examples, the bolt  700  includes exactly two electrodes  794  extending therefrom, and the electrical resistive heater  790  has an electrical path extending from one of the two electrodes  794  to the other of the two electrodes  794 . In some examples, at least one electrode  794  is detachable from the bolt  700  at a perforation  792 . For these examples, each electrode  794  can include a respective perforation  792  at or near the proximal end  720  of the bolt so that a practitioner can tear the electrode  794  from the bolt  700  once the bolt  700  has been heated and tightened. The electrical resistive heater  790 , the electrically insulating but thermally conductive material  780 , and the electrodes  794  can all be formed from suitable biocompatible materials. 
         [0039]      FIG. 8  is a flow chart of a method  800  for assembling a modular hip stem. The method  800  can be executed using the distal stem  110 ,  210 , the proximal body  120 ,  220 , and the bolt  300 ,  600 ,  700 . Step  802  includes heating a bolt to an elevated temperature greater than average human core body temperature. The heating step can occur before or after inserting the body into a lumen of the distal stem  110 ,  201  and the proximal body  120 ,  220 . Step  804  includes tightening the bolt to a specified torque while the bolt is at a temperature greater than average human core body temperature. The bolt attaches a proximal body of a modular hip stem to a distal stem of the modular hip stem. The modular hip stem is implantable within a hip of a patient. Step  806  includes allowing the bolt to cool to average human core body temperature. 
         [0040]    The above Detailed Description includes references to the accompanying drawings, which form a part of the Detailed Description. The drawings show, by way of illustration, specific embodiments in which the invention can be practiced. These embodiments are also referred to herein as “examples.” Such examples can include elements in addition to those shown or described. However, the present inventors also contemplate examples in which only those elements shown or described are provided. Moreover, the present inventor also contemplates examples using any combination or permutation of those elements shown or described (or one or more aspects thereof), either with respect to a particular example (or one or more aspects thereof), or with respect to other examples (or one or more aspects thereof) shown or described herein. 
         [0041]    In this document, the terms “a” or “an” are used, as is common in patent documents, to include one or more than one, independent of any other instances or usages of “at least one” or “one or more.” In this document, the term “or” is used to refer to a nonexclusive or, such that “A or B” includes “A but not B,” “B but not A,” and “A and B,” unless otherwise indicated. In this document, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Also, in the following claims, the terms “including” and “comprising” are open-ended, that is, a system, assembly, device, kit, article, or method that includes elements in addition to those listed after such a term in a claim are still deemed to fall within the scope of that claim. Moreover, in the following claims, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements on their objects. 
         [0042]    The above description is intended to be illustrative, and not restrictive. For example, the above-described examples (or one or more aspects thereof) can be used in combination with each other. Other embodiments can be used, such as by one of ordinary skill in the art upon reviewing the above description. The Abstract is provided to comply with 37 C.F.R. §1.72(b), to allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. Also, in the above Detailed Description, various features can be grouped together to streamline the disclosure. This should not be interpreted as intending that an unclaimed disclosed feature is essential to any claim. Rather, inventive subject matter can lie in less than all features of a particular disclosed embodiment. Thus, the following claims are hereby incorporated into the Detailed Description as examples or embodiments, with each claim standing on its own as a separate embodiment, and it is contemplated that such embodiments can be combined with each other in various combinations or permutations. The scope of the invention should be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.