Abstract:
An instrument for assembling an implant on a neck. The implant and neck being coupled via a taper having a taper axis. The instrument includes a frame having a distal portion and a proximal portion, the distal portion of the frame including a retaining member adapted to engage a portion of the neck. The instrument also includes a force applier retained in the proximal portion of the frame. The force applier includes a screw having a screw axis that is coaxial with the taper axis, wherein the screw is adapted to engage the implant. As the screw is turned, the force applier applies a force on the implant while the neck is held in place by the frame, thereby causing the tapers of the implant and neck to engage.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority under 35 U.S.C. §119 to U.S. Patent App. Ser. No. 62/103,611 entitled “ASSEMBLY TOOL” which was filed Jan. 15, 2015 and is expressly incorporated herein by reference. 
         [0002]    Cross-reference is made to U.S. Patent App. Ser. No. 62/103,826 entitled “FEMORAL STEM INCLUDING AN ANCHOR TO FACILITATE ASSEMBLY AND IMPLANTATION,” which was filed Jan. 15, 2015 and is expressly incorporated herein by reference. 
     
    
     TECHNICAL FIELD OF THE INVENTION 
       [0003]    The present invention relates generally to the field of orthopedics, and, more particularly, to an instrument for assembling a head to a stem. 
       BACKGROUND 
       [0004]    A joint within the human body forms a juncture between two or more bones or other skeletal parts. The ankle, hip, knee, shoulder, elbow and wrist are just a few examples of the multitude of joints found within the body. As should be apparent from the above list of examples of joints, many of the joints permit relative motion between the bones. For example, the ankle permits a hinge movement, the knee allows for a combination of gliding and hinge movements and the shoulder and hip permit movement through a ball and socket arrangement. 
         [0005]    The joints in the body are stressed or can be damaged in a variety of ways. Gradual wear and tear is imposed on the joints through the continuous use of a joint over the years. The joints that permit motion have cartilage positioned between the bones providing lubrication to the motion and also absorbing some of the forces direct for the joint. Over time, the normal use of a joint may wear down the cartilage and bring the moving bones in a direct contact with each other. In contrast, in normal use, a trauma to a joint, such as the delivery of a large force from an automobile accident for example, may cause considerable damage to the bones, the cartilage or to other connective tissue such as tendons or ligaments. 
         [0006]    Arthropathy, a term referring to a disease of the joint, is another way in which a joint may become damaged. One form of joint disease is arthritis, which is generally referred to a disease or inflammation of a joint that results in pain, swelling, stiffness, instability, and often deformity. 
         [0007]    There are many different forms of arthritis, with osteoarthritis being the most common and resulting from the wear and tear of a cartilage within a joint. Another type of arthropathy is osteonecrosis, which is caused by the death of a part of the bone due to loss of blood supply and subsequent degeneration of the cartilage. Other types of arthritis are caused by trauma to the joint while others, such as rheumatoid arthritis, Lupus, and psoriatic arthritis destroy cartilage and are associated with the inflammation of the joint lining. 
         [0008]    The hip joint is one of the joints that is commonly afflicted. The hip joint is a ball and socket joint that joins the femur or thighbone with the pelvis. The pelvis has a hemispherical socket called the acetabulum for receiving the head of the femur. Both the head of the femur and the acetabulum are coated with cartilage for allowing the femur to articulate within the pelvis. Other joints commonly afflicted include those of the spine, knee, shoulder, elbow, carpals, metacarpals, and phalanges of the hand. One means to address this affliction is arthroplasty which commonly refers to the making of an artificial joint. In severe cases of arthritis or other forms of arthropathy, such as when pain is overwhelming or when a joint has a limited range of mobility, a partial or total replacement of the joint may be justified. The procedure for replacing the joint varies, of course, with the particular joint in question, but in general involves replacing a terminal portion of an afflicted bone with a prosthetic implant and inserting a member with structural support to serve as a substitute for the cartilage. 
         [0009]    The prosthetic implant is formed of a rigid material that becomes bonded with the bone and provides strength and rigidity to the joint and a bearing member chosen to allow for lubrication to the joint. Suitable materials for the implant include metals and composite materials such as titanium, cobalt chromium, stainless steel, ceramic and suitable materials for the bearing include polyethylene, metal and ceramics. A cement may also be used to secure the prosthetic implant to the host bone. 
         [0010]    Total hip replacement, for example, involves removing the ball shaped head of the femur and inserting a stemmed implant into the center of the bone, which is referred to as the medullary canal of the bone. The stem implant may be cemented into the medullary canal or may have a porous coated surface for allowing the bone to heal directly to the implant. The stemmed implant has a neck and a ball shaped head, which are intended to perform the same functions as the neck and head of a healthy femur. In some implants, the head is attached to the neck via a taper connection. It is important to assemble the head to the neck with enough force so as to limit micromotion between the head and neck. The acetabulum of the patient is reamed to receive a shell and liner. A polyethylene, metal or ceramic liner with a metal shell is inserted into the acetabulum and acts as socket for receiving the head on the stemmed implant. 
         [0011]    While performing the surgery, it would be beneficial to have an instrument that can easily and with certainty assemble the head to the neck. 
       SUMMARY OF THE INVENTION 
       [0012]    According to one embodiment of the present invention, an instrument for assembling an implant on a neck is provided. The implant and neck being coupled via a taper having a taper axis. The instrument includes a frame having a distal portion and a proximal portion, the distal portion of the frame including a retaining member adapted to engage a portion of the neck. The instrument also includes a force applier retained in the proximal portion of the frame. The force applier includes a screw having a screw axis that is coaxial with the taper axis, wherein the screw is adapted to engage the implant. As the screw is turned, the force applier applies a force on the implant while the neck is held in place by the frame, thereby causing the tapers of the implant and neck to engage. 
         [0013]    According to another embodiment of the present invention, a system for hip orthopaedic surgery is provided. The system includes a neck, an implant, and an instrument. One of the neck and the implant have a male taper and the other of the neck and the implant have a corresponding female taper, wherein when the male and female tapers are engaged, there is a taper axis. The instrument includes a frame having a distal portion and a proximal portion. The distal portion of the frame includes a retaining member adapted to engage a portion of the neck. The instrument further includes a force applier retained in the proximal portion of the frame, the force applier having an axis that is coaxial with the taper axis. As the force applier is activated, the force applier applies a force on the implant while the neck is held stationary by the frame, thereby causing the male and female tapers to engage. 
         [0014]    According to yet another embodiment, a method for assembling a taper between a neck and an implant is provided. The taper has a taper axis. The method includes using an assembly instrument. The assembly instrument includes a frame having a distal portion and a proximal portion. The distal portion of the frame includes a retaining member and a force applier retained in the proximal portion of the frame. The force applier has an axis that is coaxial with the taper axis. The method includes inserting the neck into the retaining member of the frame and inserting the implant into the proximal portion of the frame. The method further includes applying a force to the implant along the force applier axis while the retaining member of the frame holds the neck in place, thereby assembling the tapers. 
         [0015]    According to yet another embodiment of the present invention, an instrument for assembling an implant on a neck of a stem is provided. The implant and neck are coupled via a taper having a taper axis. The instrument includes a frame having a distal portion and a proximal portion. The distal portion of the frame includes a retaining member adapted to engage a portion of the neck. The retaining member includes a bracket extending from the distal portion of the frame, and the bracket adapted to engage opposing sides of the neck. A force applier is also included and is retained in the proximal portion of the frame. The force applier has an axis that is coaxial with the taper axis. The force applier is adapted to engage the implant. As the force applier is activated, the force applier applies a force on the implant while the neck is held in place by the frame, thereby causing the tapers of the implant and neck to engage. 
         [0016]    According to another embodiment of the present invention, a system for hip orthopaedic surgery is provided. The system includes a stem having a neck, a head, and an instrument. One of the neck and the head have a male taper and the other of the neck and the head have a corresponding female taper, wherein when the male and female tapers are engaged, there is a taper axis. The instrument is adapted to assemble the male and female tapers. The instrument includes a frame having a distal portion and a proximal portion. The distal portion of the frame includes a retaining member adapted to engage a portion of the neck, wherein the retaining member includes a bracket extending from the distal portion of the frame. The bracket is adapted to engage opposing sides of the neck. The instrument further including a force applier retained in the proximal portion of the frame, having an axis that is coaxial with the taper axis. The force applier is adapted to engage the implant. As the force applier is activated, the force applier applies a force on the implant while the neck is held in place by the frame, thereby causing the tapers of the implant and neck to engage. 
         [0017]    According to another embodiment of the present invention a method for assembling a taper between a neck and an implant is provided. The taper has a taper axis. The method includes using an assembly instrument, which has a frame having a distal portion and a proximal portion. The distal portion of the frame includes a retaining member. The retaining member includes a bracket extending from the distal portion of the frame. The bracket is adapted to engage opposing sides of the neck. The instrument further includes a force applier retained in the proximal portion of the frame, which has an axis that is coaxial with the taper axis. The method includes inserting the neck into the retaining member of the frame and inserting the implant into the proximal portion of the frame. The method also includes applying a force to the implant along the force applier axis while the frame holds the neck in place, thereby assembling the tapers. 
         [0018]    An instrument for assembling a head on a neck of a stem, the implant and neck being coupled via a taper having a taper axis. The instrument includes a frame having a distal portion and a proximal portion. The distal portion of the frame includes a retaining member adapted to engage a portion of the neck. The retaining member includes a pair of moveable arms extending from the distal portion of the frame and the pair of moveable arms adapted to engage a pair of recesses on opposing sides of the neck. The instrument also includes a force applier retained in the proximal portion of the frame, which has an axis that is coaxial with the taper axis. The force applier is adapted to engage the implant. As the force applier is activated, the force applier applies a force on the implant while the neck is held in place by the frame, thereby causing the tapers of the implant and neck to engage. 
         [0019]    According to another embodiment, a system for hip orthopaedic surgery is provided. The system includes a stem having a neck, a head, and an instrument. The neck has a pair of tool engagement features. One of the neck and the head have a male taper and the other of the neck and the head have a corresponding female taper, wherein when the male and female tapers are engaged, there is a taper axis. The instrument is for assembling the male and female tapers, and includes a frame having a distal portion and a proximal portion. The distal portion of the frame includes a retaining member adapted to engage the tool engagement features of the neck. The retaining member includes a pair of moveable arms, which are adapted to engage the pair of tool engagement features. The instrument further includes a force applier retained in the proximal portion of the frame, and has an axis that is coaxial with the taper axis. The force applier is adapted to engage the implant, wherein as the force applier is activated, the force applier applies a force on the implant while the neck is held in place by the frame, thereby causing the tapers of the implant and neck to engage. 
         [0020]    According to yet another embodiment of the present invention, a method for assembling a taper between a neck and an implant is provided. The taper has a taper axis. The method includes using an assembly instrument, which has a frame having a distal portion and a proximal portion. The distal portion of the frame includes a retaining member. The retaining member includes a pair of moveable arms extending from the distal portion of the frame. The pair of moveable arms are adapted to engage a pair of recesses on opposing sides of the neck. The instrument further includes a force applier retained in the proximal portion of the frame, which has an axis that is coaxial with the taper axis. The method also includes inserting the neck into the retaining member of the frame and inserting the implant into the proximal portion of the frame. A force is applied to the implant along the force applier axis while the retaining member of the frame holds the neck in place, thereby assembling the tapers. 
         [0021]    According to anther embodiment of the present invention, an instrument for assembling an implant on a neck of a stem is provided. The stem has a stem axis and the implant and neck being coupled via a taper having a taper axis. The instrument includes a frame having a distal portion, a middle portion having a body axis that is coaxial with the stem axis, and a proximal portion. The distal portion of the frame includes a retaining member adapted to engage a portion of the neck and a connector adapted to engage a corresponding feature of the stem. The instrument also includes a force applier retained in the proximal portion of the frame and having an axis that is coaxial with the taper axis. The force applier is adapted to engage the implant. The force applier axis is not coaxial or parallel with the stem access. As the force applier is activated, the force applier applies a force on the implant while the neck is held in place by the frame, thereby causing the tapers of the implant and neck to engage. 
         [0022]    According to yet another embodiment of the invention, a system for hip orthopaedic surgery is provided. They system includes a stem having a neck, a head, and an instrument. One of the neck and the head have a male taper and the other of the neck and the head have a corresponding female taper. When the male and female tapers are engaged, there is a taper axis. The instrument is for assembling the tapers and includes a frame having a distal portion and a proximal portion and a body axis. The distal portion of the frame includes a retaining member adapted to engage a portion of the neck. The instrument further includes a force applier retained in the proximal portion of the frame and has an axis that is coaxial with the taper axis but that is not coaxial or parallel with the stem axis. The force applier is adapted to engage the implant, and as the force applier is activated, the force applier applies a force on the implant while the neck is held in place by the frame, thereby causing the tapers of the implant and neck to engage. 
         [0023]    According to yet another embodiment of the present invention, a method for assembling a taper between a neck of a stem and an implant is provided. The taper has a taper axis and the stem having a stem axis. The method includes using an assembly instrument, which has a frame having a distal portion, proximal portion, and a middle portion having a body axis. The distal portion of the frame includes a retaining member. The instrument having a force applier, which has an axis that is coaxial with the taper axis, wherein the force applier axis is not coaxial or parallel with the stem axis. The neck is inserted into the retaining member of the frame and the implant is inserted into the proximal portion of the frame. A force is applied to the implant along the force applier axis while the retaining member of the frame holds the neck in place, thereby assembling the tapers. 
     
    
     
       BRIEF DESCRIPTION OF FIGURES 
         [0024]      FIG. 1  is a perspective view of a first embodiment of an assembly tool, shown as it may be used for assembling a head component and a stem component of a hip replacement prosthesis; 
           [0025]      FIG. 2  is a front view of the assembly tool of  FIG. 1 , shown as it may be used for assembling the head and stem components; 
           [0026]      FIG. 3  is a perspective view of a second embodiment of the assembly tool, shown as it may be used for assembling the head and stem components, while the stem is positioned in the femur of a surgical patient; 
           [0027]      FIG. 4  is a detailed, internal view of a portion of the assembly tool shown in  FIG. 3 , showing a quick release mechanism; 
           [0028]      FIG. 5  is a perspective view of a third embodiment of the assembly tool, shown as it may be used for assembling the head and stem components, while the stem is positioned in the femur of a surgical patient; 
           [0029]      FIG. 6  is a detailed, partial, sectional view of the assembly tool shown in  FIG. 5 , showing how a pair of opposing arms may be drawn together about the stem component; 
           [0030]      FIG. 7  is a perspective view of a fourth embodiment of the assembly tool, shown as it may be used for assembling the head and stem components, while the stem is positioned in the proximal femur of a surgical patient; 
           [0031]      FIG. 8  is a perspective view of a fifth embodiment of the assembly tool, shown as it may be used for assembling an intermediate component and a stem component of a hip replacement prosthesis, showing only the proximal portion of the stem component; 
           [0032]      FIG. 9  is a sectional view of a portion of the assembly tool shown in  FIG. 8 , showing the intermediate component assembled onto a taper of the stem component, and a first embodiment of a protective insert; and 
           [0033]      FIG. 10  is a sectional view of a portion of the assembly tool shown in  FIG. 8 , showing the head component assembled onto the taper of the stem component, and a second embodiment of the protective insert. 
           [0034]      FIG. 11  is a perspective view of a sixth embodiment of the assembly tool. 
       
    
    
     DETAILED DESCRIPTION 
       [0035]    Any one of the assembly tool embodiments next described may be used during a surgical, hip replacement procedure to assemble a head component to a stem component of a hip replacement prosthesis that is implanted in the proximal femur of a surgical patient. Each of the figures show at least partial views of a hip replacement prosthesis  30 , which has a head  2  and a stem  6  that defines a stem axis  18 . During a hip replacement surgical procedure, the surgeon implants the distal portion of stem  6  into the intramedullary canal of the proximal femur of the surgical patient. The proximal part of stem  6  includes a neck  8  having a taper  10  that defines a taper axis  20 , which is normally oriented relative to stem axis  18  at an angle of about 135 degrees, although this angle may vary. Head  2  includes a taper recess  4  for assembly to taper  10 , and the taper design may be a Morse taper or another taper design variation. Head  2  may be spherical or nearly spherical and may be formed from a metal such as, for example, a cobalt chromium alloy, or from a ceramic such as, for example, an aluminum oxide. The size, material, configuration, and surface treatment of each of head  2  and stem  6  depend on the patient anatomy and condition, type of hip replacement prosthesis used (primary or revision), surgical preferences, and other factors. Stem  6  may be formed from a metal such as, for example, a stainless steel or a titanium alloy. Stem  6  may also include a threaded bore  16  ( FIG. 2 ) located in the proximal portion of stem  6  and coaxial with stem axis  18 . Threaded bore  16  may be provided for use with conventional, insertion and/or extraction instruments, but may also be used for attachment of the assembly tool, as will be described. 
         [0036]    For some types of hip replacement prostheses, an intermediate component may be assembled between the head and the stem, for example, to provide a desired offset from the taper axis. Hip replacement prosthesis  30  of  FIG. 8  shows an example of such an intermediate component, an offset component  14  that has both a taper  24  for assembly to head  2  and a taper recess  34  for assembly to stem  6 . Each of the assembly tool embodiments herein may easily be adapted for assembling either one of the intermediate component or the head component to the stem component. Another type of intermediate component is a sleeve without an offset feature. A sleeve would also have both a taper for assembly to the head  2  and a tapered recess for assembly to the neck  8  of the stem  6 . These types of sleeves are well known in the art. 
         [0037]    A user, such as a surgeon or a surgical assistant, may use any one of the assembly tool embodiments next described to apply controllably a sufficient, quasistatic axial force to join the head component to the stem component. A quasistatic axial force shall be understood to be an axial force that is applied gradually, increasing from a low to a high, peak magnitude. Conversely, a quasidynamic axial force shall be understood to be a high axial force that is applied, more or less, instantaneously, such as like a hammer strike. 
         [0038]    For each of the following embodiments, the assembly tool generally includes a frame and a force applier. The frame has a proximal portion that supports the force applier for controllably applying a quasistatic force of a desired magnitude to the proximal side of the head, in a direction that is coaxial with taper axis  20 . The frame also has a distal portion that removably engages or connects to a proximal part of the stem, thereby providing an opposing, retention force distal to the taper connection, such that substantially no axial force is transmitted to the femur during actuation of the force applier. The assembly tool may be hand operable and can provide at least a peak axial force of about 4 kN. Briefly, the user first hand assembles the head and stem components, then positions and engages the assembly tool onto the head and stem components, and then actuates the assembly tool to complete the assembly of the head and stem components. Once the user has assembled the head and stem components, the user may reversely actuate the force applier and remove the assembly tool from the surgical site. 
         [0039]      FIG. 1  is a perspective view and  FIG. 2  is a front view of a first embodiment of an assembly tool  100 , shown as it may be used for assembling together head  2  and a stem  6  of hip replacement prosthesis  30 . Assembly tool  100  has a frame  150  with a proximal portion  106  supporting a force applier  152 , and a distal portion  104  that is removably attachable to the proximal part of stem  6 . Frame  150  is configured such that actuation of force applier  152  provides a quasistatic, axial force along taper axis  20  to the proximal side of head  2 , and distal portion  104  provides an opposing, retaining force distal to taper  10  of stem  6 , such that substantially no axial force generated by force applier  152  is transmitted to the femur. 
         [0040]    A retaining member having a bracket  114  extends from body element  140  in the distal portion  106  of frame  150 . Bracket  114  is U-shaped and retains a protective insert  136  that wraps around the opposing sides of neck  8  of stem  6  to stabilize and maintain alignment of frame  150  on stem  6  during actuation of force applier  152 . 
         [0041]    Force applier  152  has an adjustable screw  108  that defines a screw axis  120  (also referrred to as a force applier axis), an actuator  110  for turning screw  108  about force applier axis  120 , which is coaxial with taper axis  20 , and a foot  112  attached to the distal end of screw  108  for engaging head component  2  of hip replacement prosthesis  30 . Proximal portion  104  of frame  150  has an internally threaded throughbore that threadably retains screw  108 , such that rotation of screw  108  about screw axis  120  in a first direction advances screw  108  towards head  2  along taper axis  20 , and rotation of screw  108  about screw axis  120  in a second direction moves screw  108  away from head  2 . Frame  150  also has a middle portion  102  between proximal and distal portions  104 ,  106 . Middle portion  102  has an elongate body  140  defining a body axis  138 , which is coaxial with stem axis  18  when frame  150  is attached to stem  6 . Body element  140  is hollow and retains a connector, such as a threaded bolt  116  (hidden) that may be threadably inserted into threaded bore  16  of stem  6 . The user may rotate, about body axis  138 , a bolt knob  118  connected to the proximal end of threaded bolt  116  for threadable and removable attachment of frame  150  to stem  6 . 
         [0042]    Middle portion  102  also has a carriage  126  that is retained on and adjustably positionable on body  140  in a first direction, as indicated by arrow A. Carriage  126  slidably retains proximal portion  106 , which is adjustably positionable on carriage  126  in a second direction that is perpendicular to the first direction, as indicated by arrow B. Carriage  126  has a first screw clamp  128  and a second screw clamp  130  that may be loosened by the user to allow adjustment of proximal portion  106  in the first and second directions, respectively, to align foot  112  and screw axis  120  of force applier  152  with taper axis  20 . Once aligned, the user may tighten first and second screw clamps  128 ,  130  to fix the position of proximal portion  106 . 
         [0043]    A universal swivel joint  124  bipivotably attaches actuator  110  to screw  108 . Actuator  110  is sized and shaped for gripping by the user for applying sufficient torque to screw  108  to achieve the desired, peak axial force. 
         [0044]    Foot  112  may be formed from a non-metallic material such as a polymer or an elastomer to prevent damage to head  2 . Foot  112  may be rotatably attached to the end of screw  108  such that upon reaching a certain applied axial force, screw  108  turns about screw axis  120  in foot  112 . Alternately, foot  112  may be fixedly attached to the end of screw  108 , such that foot  112  and screw  108  always rotate together about screw axis  108  during actuation of force applier  152 . 
         [0045]      FIG. 3  is a perspective view of a second embodiment, assembly tool  200 , shown as it may be used for assembling head  2  onto stem  6  while stem  6  is positioned in the femur. Assembly tool  200  has a frame  250  with a proximal portion  206  supporting a force applier  252 , and a distal portion  204  that is removably attachable to threaded bore  16  (see  FIG. 2 ) of stem  6  using a bolt  216 . The force applier  252  may be adjustable to implant various head sizes and offsets. Frame  250  is configured such that actuation of force applier  252  provides a quasistatic, axial force along taper axis  20  to the proximal side of head  2 , and distal portion  204  provides an opposing, retaining force distal to taper  10  of stem  6 , such that substantially no axial force generated by force applier  252  is transmitted to the femur. 
         [0046]    Frame  250  also has a middle portion  202  between proximal portion  206  and distal portion  204 . Middle portion  202  has a body  240  defining a body axis  238 . Body  240  slidably retains proximal portion  206  for movement between a first position (shown in  FIG. 4 ) and a second position (shown in  FIG. 3 ). A hand grip  244  extends from the proximal end of body element  240  along body axis  238 . A quick-release actuator  242  pivotably attaches to the proximal end of body element  240 , such that the user may operate it using the same hand that is holding hand grip  244 . A linkage  246 , shown in  FIG. 4 , operationally connects quick-release actuator  242  to proximal portion  206 . In the second position, linkage  246  is in a locked orientation so that actuation of force applier  252  does not move proximal portion  206  away from head  2 . 
         [0047]    Proximal portion  206  has a threaded throughbore (hidden) and threadably retains a screw  208  that defines a screw axis  220 . A foot  212  is attached to the distal end of screw  208  and is similar to foot  112  of assembly tool  100 . An actuator  210  is a hex-drive screw head and may be driven by a conventional, surgical, hex-drive tool (not shown) to rotate screw  208  about screw axis  220  in a first direction to move foot  212  along screw axis  220  and towards head  2 , and in a second direction to move foot  212  along screw axis  220  and away from head  2 . The hex-drive tool may be a torque-limiting tool so that the desired, peak axial force may not be exceeded. 
         [0048]    A retaining member includes a bracket  214  with a protective insert  236  extends from distal portion  204  and is U-shaped to wrap around the sides of neck  8  of stem  6  to help stabilize and maintain alignment of frame  250  during actuation of force applier  252 . 
         [0049]      FIG. 5  is a perspective, partial view of a third embodiment, assembly tool  300 , shown as it may be used for assembling head  2  and stem  6  after stem  6  has been inserted into the proximal femur. Assembly tool  300  is similar to assembly tool  200  of  FIG. 3 , differing only by the attachment means to stem  6 . 
         [0050]    Assembly tool  300  has a frame  350  with a proximal portion  306 , a distal portion  304 , and a middle portion  302 . Proximal portion  306  threadably retains a force applier  352 , which has a screw  308  that defines a screw axis  320 , a foot  312 , and an actuator  310  (partially shown). Screw axis  320  is coaxial with taper axis  20 . Middle portion  302  has a body  340  that defines a body axis  338 , which is spaced apart from and parallel to screw axis  320 . Body  340  slideably retains proximal portion  306 , which is adjustably positionable on body element  340  between a first and a second position, as indicated by arrow E. 
         [0051]    Distal portion  304  has a retaining member having a slot  318  that retains a first arm  314  and an opposing, second arm  316 , each of which extend perpendicularly from body axis  338 . Each of arms  314 ,  316  is pivotably attached to distal portion  304 , such that arms  314 ,  316  can swing independently of each other within the same plane as shown in  FIG. 6 . Distal portion  304  threadably retains a locking screw  348 , which defines a lock screw axis  346  and which is oriented with respect to arms  314 ,  316 , such that when screw  348  is advanced along screw axis  346 , the tip (which may be conically shaped) of screw  348  drives between arms  314 ,  316  to draw arms  314 ,  316  together. Conversely, loosening screw  348  in the opposite direction allows arms  314 ,  316  to open. 
         [0052]    Assembly tool  300  may only be used with a stem component having compatible, tool engagement features. A first engagement recess  54  and a second engagement recess  56  are located on opposing sides of a neck  48  of a modified, stem  46 , such that taper axis  20  extends approximately between them. First and second engagement recesses,  54 ,  56 , may be formed into stem  46  to a relatively shallow depth, for example 1-3 mm, and have any one of a number of possible profiles, such as, for example, rectangular, circular, or D-shape. Each of a first tip  324  of arm  314  and a second tip  326  of second arm  316  may be configured to fit snuggly within either of recesses  54 ,  56  of stem  6 . To attach frame  350  to stem  46 , the user positions tips  324 ,  326  of arms  314 ,  316  over recesses  54 ,  56 , of neck  48 , respectively, when arms  314 ,  316  are open, while centering foot  312  over head  2 . The user then tightens arms  314 ,  316  using a conventional drive tool to advance screw  348  along screw axis  346 , so that tips  324 ,  326  tightly clamp into recesses  54 ,  56 , thereby forming a rigid connection between frame  350  and stem  46 . The user may then position foot  312  onto head  2  and actuate force applier  352  to complete the assembly of head  2  to stem  46 . The user may grip frame  350  with one hand to stabilize it while using the other hand to actuate force applier  352 . 
         [0053]      FIG. 7  is a perspective view of a fourth embodiment, assembly tool  400 , shown as it may be used for assembling head  2  onto stem  46  while stem  46  is positioned in the proximal femur. Assembly tool  400  has a frame  450  with a proximal portion  406 , a middle portion  402 , and a distal portion  404 . Proximal portion  406  retains a force applier  452 , which has a screw  408  defining a screw axis  420 , a foot  412 , and an actuator  410 . Proximal portion  406  has a body  440  with an internally threaded bore that threadably retains screw  408 . Body  440  defines a body axis  438 . Both body axis  438  and screw axis  420  are coaxial with taper axis  20 . The user may grip and turn actuator  410  in a first rotational direction about screw axis  420  to advance screw  408  towards head  2 , and in the opposite, second rotational direction to move screw  408  away from head  2 . 
         [0054]    Body  440  has a first channel  442  that slidably retains a first elongate arm  456  defining a first arm axis  468 . Body  440  also has a second channel  444  that slidably retains a second elongate arm  458  defining a second arm axis  470 . First and second arm axis  468 ,  470  are parallel to body axis  420 . The proximal ends (analagous to a person&#39;s “shoulders”) of first and second elongate arms  456 ,  458  are mounted on opposing sides of body  440 . First elongate arm  456  is positionable in a direction perpendicular to body axis  438 , as indicated by arrow F. Second elongate arm  458  is positionable in a direction perpendicular to body axis  438 , as indicated by arrow E. A first screw clamp  448  and a second screw clamp  446  allow the user to independently position and fix each of first and second elongate arms,  456 ,  458 , respectively, at a desired, spaced apart position, relative to body axis  438 . 
         [0055]    Assembly tool  400  may only be used with a stem component having compatible, tool engagement features, such as described for assembly tool  300 . First and second recesses,  54 ,  56  of stem  46  may be employed as anchoring locations for the distal ends (analagous to “hands”), of first and second elongate arms  456 ,  458 . The user may assemble head  2  onto stem  46  in a manner similar to that described for assembly tool  300 . 
         [0056]      FIG. 8  is a perspective view and  FIG. 9  is a partial view of a fifth embodiment, assembly tool  500 , shown as it may be used for assembling intermediate component  14  (shown in a sectional view) onto stem  6  of hip replacement prosthesis  30  (showing only the proximal portion). Assembly tool  500  has a frame  550  with a proximal portion  506 , a middle portion  502 , and a distal portion  504 . Proximal portion  506  retains a force applier  552  having a screw  508  defining a screw axis  520 , an actuator  510 , and a foot  512 . Middle portion has a body  540  that has a first arm  522  and a second arm  524 , each of which is fixed and equally spaced apart from a body axis  538 . Screw axis  520  and body axis  538  are coaxial with taper axis  20 . Distal portion  504  has a retaining member, which in this embodiment is a semicircular, annular groove  544  that retains a first embodiment, protective insert  536 . Frame  550  is rigid and configured (like half of an “open clamshell”) to allow positioning of assembly tool  500  onto hand-assembled, offset component  14  and neck  8  of stem  6 , even if stem  6  has already been fully inserted into the proximal femur. Protective insert  536  is sized and shaped to conform to a neck contour  40  that is immediately distal to taper  10  of neck  8 . Protective insert  536  may be formed from a non-metallic, biocompatible, sterilizable material, such as PEEK, UHMWPE, polycarbonate, or any of a number of available, engineering plastics. The user may use assembly tool  500  in a very similar manner as previously described for the other embodiments. 
         [0057]      FIG. 10  is a partial view of assembly tool  500 , shown as it may be used for assembling head  2  (shown in a sectional view) onto stem  6  (showing only the proximal portion). Assembly tool  500  is shown with a second embodiment, protective insert  566 , which is configured to extend partially into taper recess  4  of head  2 . Protective insert  566  provides for situations in which taper  10  is shorter than taper recess  4 , and upon assembly, head  2  “hangs” over neck  8  a small distance. 
         [0058]      FIG. 11  is a perspective view of a sixth embodiment, an assembly tool  600 , as a surgeon may position it to assemble head  2  onto stem  6 . Similar to the previous embodiments, assembly tool  600  has a frame  650  that is removably attachable to stem  6 , and a force applier  652  for the controllable application of a clamping force to head  2  along taper axis  20 . Force applier  652  has an elongate screw  608  defining a screw axis  620 , an actuator  610  attached to the proximal end of screw  608 , and a foot  612  attached to the distal end of screw  608 . 
         [0059]    Frame  650  has a distal portion  604 , a middle portion  602 , and a proximal portion  606 . Distal portion  604  has a connector  616  for the removable attachment of frame  650  to stem  6 . As previously described for assembly tool  100  ( FIG. 1 ), the proximal body of stem  6  may be provided with a threaded bore, coaxial with stem axis  16 , for the attachment of an insertion and/or extraction instrument. If stem  6  is provided with such a threaded bore, connector  616  may be a threaded bolt inserted through a counterbored through-hole  630  in distal portion  604 . Counterbored through-hole  630  defines a hole axis  624 . During the surgical procedure, the surgeon may use a conventional driver to tighten connector  616  when attaching frame  650  to stem  6 , and to loosen connector  616  when removing frame  650  from stem  6 . However, those skilled in the art will appreciate that connector  616  may be any one of numerous types of mechanical elements/mechanisms adapted to removably attach to any one of numerous types of slots, recesses, undercut holes, and the like, that may be provided in the proximal stem body, either specifically for use with assembly tool  600  or for alternate purposes, such as for the insertion and/or an extraction of the stem. For each of these arrangements, the surgeon may use connector  616  to achieve a secure, removable attachment of frame  650  to stem  6 , in order to provide a retention force that opposes the applied clamping force, thereby minimizing the transfer of force to the femur during actuation of force applier  652 . 
         [0060]    Distal portion  604  of frame  650  also includes a bracket  614  that helps to maintain the correct alignment of frame  650  with respect to taper axis  20  during the application of the clamping force using force applier  652 . As described for assembly tool  100  ( FIG. 1 ), bracket  614  may be provided with a protective insert  636  that helps prevent damage to the surface of the neck of stem  6 . Bracket  614  is U-shaped for easy positioning around the neck of stem  6  prior to attaching connector  616  to stem  6 , and for removal of assembly tool  600  from stem  6  after assembly of head  2  onto stem  6 . 
         [0061]    Proximal portion  606  of frame  650  has a body  640  defining a body axis  638 . Body  640  is generally tubular and has an internal screw thread (hidden in  FIG. 12 ) extending at least along a portion of the length of body  640 . Body  640  threadably retains screw  608  of force applier  652 , such that screw axis  620  is coaxial with taper axis  20  when assembly tool  600  is properly positioned for actuation of force applier  652 . 
         [0062]    To actuate force applier  652 , the surgeon rotates actuator  610  in a first direction (i.e., clockwise) to move foot  612  along taper axis  20  and towards head  2 . Conversely, to deactuate force applier  652 , the surgeon rotates actuator  610  in a second direction (i.e., counterclockwise) to loosen and remove force applier  652  from head  2 . Actuator  610  may have a T-bar configuration as shown in  FIG. 12 . The size and shape of actuator  610  may be based on ergonometric data known in the art, so that a person with average hand/arm strength could easily apply the necessary torque to achieve at least a predetermined, desired clamping force. Alternatively, actuator  610  may be provided with a torque-limiting mechanism, of which numerous types are well-known in the mechanical, hand tool industry, such that it would be impossible for the surgeon to apply a clamping force that exceeds the predetermined, desired force. The actual magnitude of the predetermined, desired force may be recommended by the implant manufacturer, and be based on the appropriate testing and analysis of the taper connection. 
         [0063]    Proximal portion  604  and middle portion  602  of frame  650  define an arm axis  624  that is parallel and offset from screw axis  620 . As shown in  FIG. 12 , middle portion  602  may be configured to be slidably retained in a channel  642  of body  640 . The surgeon may loosen a screw clamp  646  to adjust the offset of arm axis  624  relative to screw axis  620 , such as may be desirable for different sizes and configurations of stem and head components. The surgeon may then tighten screw clamp  646  to maintain a rigid attachment of middle portion  602  to body  640 . This ability to separate proximal portion  606  from middle portion  602  also may be advantageous for cleaning/sterilizing assembly tool  600  or for more efficiently storing assembly tool  600  in a surgical tray. Alternatively, middle portion  602  may be fixedly attached to body  640 , in which case foot  612  may be designed to accommodate various stem configurations and head sizes. In the latter arrangement, for example, foot  612  may be rigidly attached to screw  608  and have a flat face for interfacing against head  2 , thereby allowing slight misalignment of screw axis  620  and taper axis  20 . 
         [0064]    Each component of assembly tool  600  may be formed from any one of numerous types of stainless steels or biocompatible polymers commonly used for multiple-use and single-use, surgical instruments. Protective insert  636  and foot  612  may be formed from a biocompatible polymer, for example PEEK, which is softer than the implant materials, in order to prevent damage to the implant surfaces. Protective insert  636  may also be formed from a biocompatible elastomer, such as silicone rubber, that is retained on or bonded to bracket  614 . 
         [0065]    As those skilled in the art will appreciate, there are many variations of the assembly tool embodiments described herein. For example, any of them may have an actuator that incorporates a force limiting mechanism to prevent applying excessive torque while actuating the force applier. Also, any of them may have one or more additional grips or handles extending from or incorporated into the assembly tool frame to help maintain stability and alignment of the assembly tool while actuating the force applier. Also, each of the assembly tool embodiments may be designed to be either one of a single-use (disposable) or a multiple-use (reusable) device. Also, in addition to mechanical, screw-driven types of force appliers, other types of force appliers may include, for example, the following: mechanical, ratcheting types; hydraulically driven types; pneumatically driven types; expanding gas-driven types (incorporating, for example, a small, disposable cannister of carbon dioxide gas); electrical, motor-driven gear mechanism types; and mechanical, linkage driven types. An assembly tool having any one of these types of force appliers can provide the user with the ability to controllably apply a quasistatic, axial force that has the desired, peak magnitude (such as about 1000 pounds/5000 N) and that is directed along the taper axis of the head and stem components to fully join those components.