Patent Publication Number: US-2007123903-A1

Title: Medical Device installation tool and methods of use

Description:
FIELD OF THE INVENTION  
      The invention relates broadly to a tool for inserting a prosthesis within a body, and more particularly to a tool for inserting prostheses, such as artificial discs or other implants within an intervertebral space.  
     BACKGROUND OF THE INVENTION  
      Spinal surgery involves many challenges as the long-term health and mobility of the patient often depends on the surgeon&#39;s technique and precision. One type of spinal surgery involves the removal of the natural disc tissue that is located between adjacent vertebral bodies. Procedures are known in which the natural, damaged disc tissue is replaced with an interbody cage or fusion device, or with a disc prosthesis.  
      The insertion of an article, such as an artificial disc prosthesis, presents the surgeon with several challenges. The adjacent vertebral bodies collapse upon each other once the natural disc tissue is removed. These bodies must be separated to an extent sufficient to enable the placement of the prosthesis. However, if the vertebral bodies are separated, or distracted, to beyond a certain degree, further injury can occur. The disc prosthesis must also be properly positioned between the adjacent vertebral bodies. Over-insertion or under-insertion of the prosthesis can lead to pain, postural problems and/or limited mobility or freedom of movement.  
      Specialized tools have been developed to facilitate the placement of devices, such as disc prostheses, between adjacent vertebral bodies of a patient&#39;s spine. Among the known tools for performing such procedures are separate spinal distractors and insertion devices. The use of separate tools to distract the vertebral bodies and insert a disc prosthesis or graft can prove cumbersome. Further, the use of some distractors can cause over-distraction of the vertebral bodies.  
      Despite existing tools and technologies, there remains a need to provide a device to facilitate the proper and convenient insertion of an object, such as a disc prosthesis, between adjacent vertebral bodies while minimizing the risk of further injury to the patient.  
     SUMMARY OF THE INVENTION  
      The present invention generally provides methods and devices for facilitating the proper and convenient insertion of an object, such as a disc prosthesis, between adjacent vertebral bodies. In one embodiment, a medical device installation tool can include a housing, a pair of opposed levers, and a prosthesis positioning mechanism at least a portion of which is disposed between the pair of opposed levers. The opposed levers can each have a proximal end and a distal end, the proximal end of each lever being moveably coupled to a portion of the housing. The prosthesis positioning mechanism can be selectively configured such that at least a portion of the prosthesis positioning mechanism translates along a longitudinal axis of the installation tool while maintaining a substantially fixed length of the installation tool.  
      In yet another embodiment, a medical device installation tool can include a housing, a shaft coupled to the housing and a pair of opposed levers, each having a proximal end and a distal end wherein the proximal end of each lever can be pivotably coupled to a portion of the housing such that the distal ends are configured to separate in response to the movement of one or more objects between the levers in the proximal to distal direction. The tool can be selectively configured such that the shaft will translate along a longitudinal axis of the installation tool or will rotate about the longitudinal axis of the installation tool as a result of manipulation of a single driver. For example, the medical device installation tool can include an actuator that can be configured in a first position that allows the driver to effect translation of the shaft along the longitudinal axis of the installation tool, and a second position that allows the driver to effect rotation of the shaft about the longitudinal axis of the installation tool.  
      Methods for implanting a prosthetic device are also provided. In one embodiment, the method can include disposing portions of opposed, pivotable levers of an installation tool between vertebral bodies. The method can further include linearly translating a shaft along a longitudinal axis of the installation tool to move a pusher block and/or a prosthetic device between the opposed levers toward the vertebral bodies while causing distal ends of the opposed levers to separate and distract the vertebral bodies to implant the prosthetic device between the distracted vertebral bodies while maintaining the overall length of the tool. When the implant reaches its final position, continued translation of the shaft draws the opposed levers from the disc space leaving only the implant in the disc space. If the shaft is connected directly to a prosthesis, the method can further include rotating the shaft about its longitudinal axis to decouple the installation tool from the prosthetic device and linearly translating the shaft along the longitudinal axis of the installation tool to cause the levers to retract from the vertebral bodies. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention will be more fully understood from the following detailed description taken in conjunction with the accompanying drawings, in which:  
       FIG. 1  is a perspective view of one embodiment of an installation tool;  
       FIG. 1A  is a perspective view of another embodiment of an installation tool;  
       FIG. 2  is an assembly view of the installation tool of  FIG. 1 ;  
       FIG. 3  is a cross-sectional view of a prosthesis positioning mechanism according to one embodiment of installation tool showing;  
       FIG. 4  illustrates a sectional view of a shaft and housing of the installation tool of  FIG. 3  taken along section  4 - 4 ;  
       FIG. 5  illustrates an embodiment of an installation tool that provides linear translation and rotational motion of a shaft of the tool;  
       FIG. 6  illustrates a sectional view of an interface between an actuator and the shaft of the installation tool of  FIG. 5  taken along section  6 - 6 ;  
       FIG. 7  illustrates another embodiment of an installation tool that provides linear translation and rotational motion of a shaft of the tool;  
       FIG. 7A  illustrates a sectional view of an interface between an actuator and the shaft of the installation tool of  FIG. 7  taken along section  7 A- 7 A;  
       FIG. 8  illustrates an embodiment of the installation tool in use during an initial stage of inserting a prosthesis between adjacent vertebrae;  
       FIG. 9  illustrates the installation tool of  FIG. 8  in use to insert a prosthesis between adjacent vertebrae, distracting the adjacent vertebrae;  
       FIG. 10  illustrates the installation tool of  FIG. 8  during a further stage of inserting a prosthetic device between the adjacent vertebrae;  
       FIG. 11  illustrates decoupling a shaft of the installation tool of  FIG. 8  from the prosthetic device after inserting a prosthesis between adjacent vertebrae; and  
       FIG. 12  illustrates the installation tool of  FIG. 8  being withdrawn from between the adjacent vertebrae. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Certain exemplary embodiments will now be described to provide an overall understanding of the principles, structure, function, manufacture, and use of the devices and methods disclosed herein. One or more examples of these embodiments are illustrated in the accompanying drawings. Those skilled in the art will understand that the devices and methods specifically described herein and illustrated in the accompanying drawings are non-limiting exemplary embodiments and that the scope of the present invention is defined solely by the claims. The features illustrated or described in connection with one exemplary embodiment may be combined with features of other embodiments. Such modifications and variations are intended to be included within the scope of the present invention.  
      The present invention provides a medical device installation tool for implanting a prosthetic device, such as a spinal implant, between adjacent vertebral bodies. In general, the installation tool includes a proximal housing from which a pair of opposed levers extend distally. The installation tool also includes a shaft that is at least partially disposed within the housing and a movable handle, which is or forms part of a driver, connected to the shaft. In one aspect a pusher block is coupled to or able to be coupled to a distal end of the shaft. The pusher block is, in turn, adapted to be disposed between the levers, and distal movement of the pusher block between the levers causes separation of the levers by the pusher block and/or the prosthesis acting on the levers. Alternatively, the distal end of the shaft is attached directly to a prosthesis, which is adapted to be positioned between the levers, and distal movement of the prosthesis between the levers causes separation of the levers. The installation tool can be configured such that movement (e.g., rotational movement) of the handle causes either rotation of the shaft about its longitudinal axis or translation of the shaft along the longitudinal axis of the installation tool. Among the advantages of the installation tool is that the overall length of the device does not change during use, regardless of whether the tool is used in the shaft rotation of shaft translation modes.  
      The installation tool can be provided as a kit having modular components which allow the surgeon to select from among a variety of components to assemble an installation tool that is optimized for its intended use. Although the invention is described primarily with reference to use of the tool to install an artificial disc between adjacent vertebral bodies, it is understood that the installation tool of the invention can be used to place other elements between vertebral bodies, or in other locations within a patient&#39;s body. Exemplary elements that can be placed between vertebral bodies include, but are not limited to interbody cages, fusion devices, spacers, grafts, and the like.  
       FIGS. 1-2  illustrate one embodiment of an installation tool  10  having a housing  12  which can facilitate grasping and manipulation of the tool  10 , and a pair of opposed levers  14 ,  15  that extend distally from the housing  12 . The installation tool  10  also includes a movable (e.g., rotatable) handle  18  at a proximal end of the housing  12  and a shaft  16 , coupled to the handle  18  by way of a drive shaft  64  and at least partially disposed within the housing  12 . In one embodiment, shown in  FIGS. 1 and 2 , a distal end  44  of the shaft  16  extends from the housing  12  and is coupled to a pusher block  20 . As discussed below, the pusher block  20  can be attached to or disposed adjacent to an implant during use of the installation tool  10 . In another embodiment, shown in  FIG. 1A , the distal end  44  of shaft  16  is adapted to connect directly to an implant  100  without an intervening pusher block. One skilled in the art will appreciate that the installation tool  10  can be provided as modular kit that will enable a user to attach or remove the pusher block, or to use pusher blocks of different shapes and sizes, as required by a given application.  
      The opposed first and second levers  14 ,  15 , each have a proximal end  14 A,  15 A and a distal end  14 B,  15 B, respectively. The proximal ends  14 A,  15 A of each lever  14 ,  15  can be pivotably coupled to the housing  12  of the installation tool  10  to allow each of the levers  14 ,  15  to pivot about its attachment point. For example, the proximal end  14 A of the first lever  14  and the proximal end  15 A of the second lever  15  can each include a bore  21 A,  21 B, which seats pivot pins  26  to pivotally mount each lever to the housing. As the levers  14 ,  15  pivot about pins  26 , the distal ends  14 B,  15 B of the levers  14 ,  15  separate to facilitate distraction or separation of adjacent vertebral bodies as explained below. One skilled in the art will appreciate that the coupling of the levers  14 ,  15  to the housing  12  can be done in such a way as to allow some play (e.g., linear movement) to facilitate convenient use and to accommodate anatomical features or irregularities. For example, the levers  14 ,  15  can each include a slot which seats about the pivot pins  26  to allow some linear translation of the levers  14 ,  15  relative to the housing  12 . One skilled in the art will also appreciate that the levers  14 ,  15  can be detachably coupled to the housing  12  to allow attachment of various types of levers to the housing, such as levers having varying geometries.  
      The distal ends  14 B,  15 B of the levers  14 ,  15  can include blade tips  28 A,  28 B sized and configured to facilitate their placement between vertebral bodies. The blade tips  28 A,  28 B include outwardly facing surfaces  30 A,  30 B that can be beveled or radiused. In one embodiment, outwardly facing surfaces  30 A,  30 B can be substantially curved or angled in a superior or inferior direction to facilitate placement of the blade tips  28 A,  28  between adjacent vertebrae.  
      The distal ends  14 B,  15 B of the levers  14 ,  15  can include stop surfaces  32 A,  32 B disposed adjacent to the blade tips  28 A,  28 B. The stop surfaces  32 A,  32 B can be configured to abut a vertebral body during a surgical procedure for installing a prosthesis, such as an artificial disc, between adjacent vertebral bodies. The stop surfaces  32 A,  324 B can have a variety of geometric configurations. In one embodiment, the stop surfaces  32 A,  32 B can have a substantially concave profile when viewed in the vertical plane.  
      The facing surfaces of levers  14 ,  15  are adapted and configured to allow a prosthetic device to be positioned and guided therebetween. For example, in one embodiment the facing surfaces of levers  14 ,  15  can include substantially planar surfaces that can guide and/or support the prosthetic device as it moves distally along the levers  14 ,  15 . In another embodiment, the facing surfaces of levers  14 ,  15  can be configured to support a portion of a prosthesis positioning mechanism, such as a pusher block  20 . For example, the pusher block  20  can be coupled to the facing surfaces of levers  14 ,  15 , or to other portions of the levers  14 ,  15 , to minimize rotational motion of the pusher block  20  about the longitudinal axis  22  of the insertion tool  10 .  
      The shaft  16  serves as part of a prosthesis positioning mechanism, and the tool can be configured so that shaft  16  is capable of rotational movement or translational movement (e.g., to position a prosthetic device between adjacent vertebral bodies) while maintaining a substantially fixed overall length of the installation tool  10 . While the shaft  16  can be configured in a variety of ways, in one embodiment it is a generally elongate member such as a rod. One skilled in the art will appreciate that other geometries can be used as well. As illustrated in  FIGS. 1-3 , a proximal end of the shaft is disposed within the housing  12  and a distal end  44  ( FIG. 2 ) can extend from the housing  12  and be disposed between the levers  14 ,  15 . As noted above, the shaft  16  can be adapted for translational movement along the longitudinal axis  22  of the installation tool  10  to position a prosthetic device between adjacent vertebral bodies. During such translation at least a portion of the shaft  16  remains disposed within the housing  12  and no portion of the shaft  16  extends proximally from the handle  18  or substantially beyond the distal portion of the levers  14 ,  15 . Accordingly, the installation tool  10  substantially maintains its overall length during use of the tool  10 .  
      With further reference to  FIGS. 1-2 , the distal end  44  of the shaft  16  may include a coupling mechanism, such as threaded tip  46 , that can be coupled to a prosthetic device  100  ( FIG. 8 ) and/or to pusher block  20 . The coupling mechanism  46  can attach to a corresponding coupling mechanism carried by the pusher block  20  and/or a prosthetic device. For example, the prosthetic device or pusher block  20  can include a threaded bore matable with the threaded end  46  of the shaft  16 . With such a coupling, forward and rearward motion of the shaft  16  will effect corresponding motion of the distal end of the shaft  16  along longitudinal axis  22  and any prosthesis and/or pusher block  20  attached thereto.  
      As noted above, the installation tool  10  is designed such that linear translation of a pusher block and/or prosthetic device along the levers  14 ,  15  in a proximal to distal direction causes the opposed levers  14 ,  15  to separate. Such separation will enable the levers  14 ,  15  to distract two adjacent bodies during an installation procedure as discussed below.  
      In one embodiment, illustrated in  FIG. 1 , the installation tool  10  includes a pusher block  20  that can also form part of a prosthesis positioning mechanism. The pusher block  20  can be coupled to the distal end  44  of the shaft  16  and disposed between the levers  14 ,  15 . Linear translation of the shaft  16  can cause the pusher block  20  to move between the levers  14 ,  15  in a proximal to distal direction. As the pusher block  20  (and any attached prosthesis) moves distally, such movement will cause the levers  14 ,  15  to pivot about their respective pivot pins  26  and separate the distal ends  14 B,  15 B and blade tips  28 A,  28 B of the levers  14 ,  15  from each other. For example, in a closed or at-rest state, the pusher block  20  (and any attached prosthesis) can be positioned in proximity to the proximal ends  14 A,  15 A of the levers such that the proximal ends  14 A,  15 A are separated by a distance D 1  and the blade tips  28 A,  28 B are separated by a distance D 2 , where D 2 &lt;D 1  as shown in  FIG. 1 . As the pusher block  20  moves from the proximal end to the distal end of the levers  14 ,  15 , the pusher block  20  (and any attached prosthesis) separates the blade tips  28 A,  28 B of the installation tool  10 , thereby increasing the distance D 2  between the blade tips  28 A,  28 B.  
      In one embodiment, the size (e.g., height) of the prosthetic device can determine the amount of separation required between the blade tips  28 A,  28 B, and thus the amount of distraction required of the vertebral bodies to implant a prosthesis. That is, a relatively larger prosthetic device can require greater amount of separation between the blade tips  28 A,  28 B and a corresponding amount of distraction of the vertebral bodies. As a result, the pusher block  20  and/or prosthesis can be configured to have various heights (H), depending upon the amount of separation required between the blade tips  28 A,  28 B. One skilled in the art will appreciate that the adjacent vertebrae should only be distracted by an amount sufficient to insert a prosthesis therebetween. Thus, the pusher block and/or prosthesis should be selected to cause only the minimum amount of distraction necessary to implant a prosthesis. To this end, the tool  10  can be provided with multiple, interchangeable pusher blocks  20  having different sizes and shapes. By way of example, while the pusher block  20  can have a variety of configurations, shapes, and sizes, in one embodiment, the height (H) of the pusher block  20  is in the range of about 8.0 mm to 14.0 mm.  
      In one embodiment, the pusher block  20  can be configured to guide a prosthetic device through the installation tool  10  into the disc space. For example, as shown in  FIG. 2 , the pusher block  20  can include a leading face  39  configured to contact a prosthetic device. As the pusher block  20  moves distally between the levers the prosthetic device also moves distally. As a result of such movement, the pusher block  20  and/or the prosthetic device cause the levers  14 ,  15  to separate as they move distally between the levers  14 ,  15 .  
      The pusher block  20  can also be configured to allow connection of the distal end  44  of the shaft  16  to the prosthetic device. In one embodiment, illustrated in  FIG. 2  the pusher block  20  can include a bore  37  extending therethrough. The shaft  16  can extend through the bore  37  such that the shaft  16  is coupled to the pusher block  20  and such that at least a portion of the coupling mechanism  46  of the shaft  16  extends past face  39  of the pusher block  20 . In this embodiment, the coupling mechanism  46  can mate directly to the prosthetic device, or it can mate to a connector element which, in turn, can mate to the prosthetic device.  
      While the pusher block  20  can be configured to allow connection of the distal end  44  of the shaft  16  to the prosthetic device, the pusher block  20  can have other configurations as well. In one embodiment, the pusher block  20  can include a connection mechanism, such as disposed along the face  39  of the pusher block  20 , that enables the pusher block  20  to couple directly to the prosthesis device. By way of non-limiting example, the connection mechanism of the pusher block  20  can include a threaded connection, a dovetail connection, a snap-on connection or a taper lock connection.  
      In another embodiment, illustrated in  FIG. 1A , there is no need for a pusher block  20 . Instead, the shaft  16  has a distal portion  44  with a coupling mechanism, such as a threaded tip  46 . The distal end of the shaft  16  can thus couple directly to a prosthesis, and the prosthesis causes separation of the levers as it travels distally therebetween.  
      As indicated above, the prosthesis positioning mechanism can translate along a longitudinal axis  22  of the installation tool  10  while maintaining a substantially fixed length of the installation tool  10 . In one embodiment, the installation tool  10  can include a driver mechanism that includes handle  18  configured to effect linear translate the prosthesis positioning mechanism along a longitudinal axis of the installation tool  10  while maintaining the substantially fixed length of the tool  10 . For example, the handle  18  and the shaft  16  of the prosthesis positioning mechanism can be configured such that rotation of the handle  18  about the longitudinal axis  22  of the insertion tool  10  adjusts a linear position of the shaft  16  and any attached components.  
       FIG. 3  illustrates one embodiment in which rotation of handle  18  causes only linear translation of the shaft  16 . In this embodiment the handle  18  is part of a driver that includes a drive shaft  64 . As shown, the handle  18  can be disposed at a proximal end of the housing  12  and it can be configured to receive a rotational force or torque  76 . The drive shaft  64  can be disposed within the housing  12  and can be threadably coupled to the proximal end of the shaft  16 . In one embodiment, the drive shaft  64  is annular, having internal threads  66  configured to mate with threads  65  disposed about an external surface of the proximal end of the shaft  16 .  
      A portion of the shaft  16  can be rotationally constrained within the housing  12  such that rotation of the threaded drive shaft  64  by the handle  18  can cause linear translation of the shaft  16  along the longitudinal axis  22  of the installation tool  10 . For example, a portion of the distal end  44  of the shaft  16  can be “keyed” relative to the housing  12  such that engagement of the housing  12  and the shaft  16  prevents rotation of the shaft  16  when a rotational force is applied to handle  18 , thus transferring the rotational force to linear movement of the shaft  16 . By way of one example, shown in  FIG. 4 , the distal end  44  of the shaft  16  can have a cross section with an irregular shape, such as including a flattened surface  70 , which fits within a portion of the housing  12  that has a complementary shape, such as a corresponding flattened surface  74 . As the threaded drive shaft  64 , is rotated, such as by handle  18 , constrainment of the shaft  16  by the flattened surface  72  of the shaft  16 , prevents rotation of the shaft  16 , thereby allowing the shaft  16  to translate along the longitudinal axis  22  of the installation tool  10 .  
      In another embodiment, the installation tool  10  enables a user to select a mode of operation in which rotation of a driver, such as handle  18 , causes either linear translation of the shaft  16  or rotation of the shaft  16 . Such a design is desirable because linear translation can be useful to implant a prosthesis while rotation of the shaft  16  is useful to couple or decouple the tool  10  and a prosthetic device.  FIGS. 5-7A  illustrate embodiments of an installation tool that enable both linear translation and rotational movement of the shaft, thereby allowing the tool to both install a prosthetic device and couple to or decouple from a prosthetic device.  
      One skilled in the art will appreciate that a variety of designs can be implemented to enable the installation tool to be selectively configured to effect linear translation of the shaft  16  or rotation of the shaft  16  upon applying a rotational force to a driver, such as through a handle  18 . Generally, a tool with selective linear translation and rotational modes of operation can be provided by rotationally constraining the shaft  16  when a rotational force is applied to a driver, thus enabling the installation tool to operate in a linear translation mode. To effect a rotational mode of operation, the shaft  16  is rotationally unconstrained such that the rotational force applied to a handle  18  effects rotation of the shaft  16 .  
       FIGS. 5 and 6  illustrate a portion of one embodiment of an installation tool  10 ′ that can be selectively configured between linear translation and rotational modes of operation of the shaft  16 ′. As shown, the installation tool  10 ′ has a housing  12 ′, a shaft  16 ′ disposed within the housing  12 ′, a handle  18 ′ threadably coupled to the shaft  16 ′, and an actuator  80  coupled to the housing  12 ′. The actuator  80  can be selectively positioned in a first position A that allows linear motion of the shaft  16 ′ along the longitudinal axis  22 ′ and a second position B that allows or rotational motion of the shaft  16 ′ relative to the longitudinal axis  22 ′.  
      When the actuator  80  is in position A, the tool is configured for a mode of operation in which the shaft  16 ′ is rotationally constrained, thereby enabling linear translation of the shaft  16 ′. As illustrated in  FIG. 5 , with the actuator  80  in the first position A, the handle coupling portion  88  of the actuator  80  is seated within the first, distal set of detents  90  formed in the handle  18 ′ and the housing coupling portion  86  of the actuator  80  is mated within the openings  89  formed within the housing  12 ′. In this configuration the housing coupling portion  86  and the housing  12 ′ rotationally constrain the shaft  16 ′ relative to the housing  12 ′.  FIG. 6  illustrates that in the embodiment of  FIG. 5 , the actuator  80  has a shaft coupling portion  84  that mates within a notch or groove  85  formed in the shaft  16 ′. As a rotational force  87  is applied to the handle  18 ′ and drive shaft  64 ′, interaction between the shaft coupling portion  84  and the notch  85  of the shaft  16 ′ prevents any rotation of the shaft  16 ′ and the actuator  80 . Thus, the rotational force applied to the handle  18 ′ will cause the drive shaft  64 ′ to rotate such that threads  66  of the drive shaft  64 ′ rotate relative to the threads of the shaft  16 ′, thereby causing the shaft  16 ′ to translate along the longitudinal axis  22 ′ of the installation tool  10 ′.  
      With the actuator  80  in the second position B, rotational movement of the shaft  16 ′ is permitted. The actuator  80  is placed in position B by raising the actuator  80  such that the handle coupling portion  88  of the actuator  80  mates within the second, proximal set of detents  92  formed in the handle  18 ′, thereby securing the actuator  80  to the handle  18 ′. At the same time, the housing coupling portion  86  is disengaged from the openings  89  to decouple the actuator  80  and the shaft  16 ′ from the housing  12 ′. When a rotational force  87  is applied to the handle  18 ′, the drive shaft  64 ′ will rotate, causing both the shaft  16 ′ and the actuator  80 ′ to likewise rotate relative to the housing  12 ′.  
       FIGS. 7 and 7 A illustrate another embodiment of an installation tool  10 ″ that can be selectively configured between linear translation and rotational modes of operation of the shaft. As shown, the installation tool  10 ″ has a housing  12 ″, a shaft  16 ″ disposed within the housing  12 ″, a handle  18 ″ threadably coupled to the shaft  16 ″ by way of a drive shaft  64 ″, and an actuator  120 . The actuator  120  is selectively moveable between a first position A that allows rotational motion of the shaft  16 ″ and a second position B that rotationally constrains the shaft  16 ″ and allows linear motion of the shaft  16 ″ along the longitudinal axis  22 ″. In this embodiment, as shown in  FIG. 7A , the actuator can include a shaft coupling portion  124  that mates within a notch or groove  122  within the shaft  16 ″. Thus, the shaft  16 ″ and the actuator  120  are coupled together such that one is not able to rotate independent of the other.  
      The actuator  120  can include a mechanism, such as a switch  121  to control the positioning of the actuator  120  in position A (rotational mode) or position B (linear translation mode). When the actuator  120  is in the first position A, a first, proximal face  128  of the actuator  120  is coupled to the handle  18 ″, such as by a mechanical coupling or an interference fit between the actuator  120  and a distal portion of the drive shaft  64 ″. The coupling of the actuator  120  to the shaft  16 ″ enables rotation of the shaft upon the application of a rotational force to handle  18 ″. As a rotational force is applied to the handle  18 ″, the drive shaft  64 ′ will rotate, causing both the shaft  16 ″ and the actuator  80 ′ to rotate.  
      When the actuator  120  is moved to the second position B, such as by distal movement of the actuator  120 , which may result from movement of switch  121 , the first, proximal face  128  is detached from its mating connection to the handle  18 ″. A second, distal face  126  of the actuator  120  is then coupled to a proximal surface  130  on a stationary housing block  132 . The coupling of the actuator  120  to the shaft  16 ″ via the shaft coupling portion  124 , as noted above, causes the shaft  16 ″ to be rotationally constrained. That is, since the actuator  120  and the shaft  16 ″ are keyed to one another, when the distal face  126  of the actuator  120  is coupled to the stationary housing block  132  any rotation of the handle  18 ″ and the drive shaft  64 ″ is not able to cause rotation of the actuator  120  or the shaft  16 ″. In this configuration, when a rotational force is applied to the handle  18 ″, the drive shaft  64 ″ will rotate but the shaft  16 ″ will not. As a result, the rotational motion of the drive shaft  64 ″ will be converted to linear motion of the shaft  16 ″ along the longitudinal axis  22 ″ of the installation tool  10 ″.  
       FIGS. 8-12  sequentially illustrate the use of an installation tool  10  for the implantation of a prosthetic device  100 , such as a vertebral disc, between adjacent vertebral bodies  102 ,  104 . As illustrated in  FIG. 8 , the tool  10  can be assembled in one embodiment with the threaded portion  46  of the shaft  16  extending through the bore  39  of the pusher block  20  and coupled to the prosthetic device  100 . For example, the tool can be configured in a shaft rotation mode in which rotation of handle  18  ( FIG. 1 ) will cause the shaft to rotate so that it can be threaded onto prosthetic device  100 . In an initial state, the pusher block  20  can be positioned in proximity to a proximal end of the levers  14 ,  15  such that the blade tips  28 A,  28 B are in a closed or non-distracted state. The blade tips  28 A,  28 B can then be inserted or wedged between adjacent vertebral bodies  102 ,  104  to effect slight separation between the vertebral bodies  102 ,  104 . Although not illustrated, one skilled in the art will appreciate that tool  10  can be manipulated such that the blade tips  30 A,  30 B are fully inserted between the vertebral bodies such that the stop surfaces  32 A,  32 B of the levers  14 ,  15  can abut a surface of the vertebral bodies.  
      As illustrated in  FIG. 9 , the shaft  16  and pusher block  20  can then be advanced distally along the longitudinal axis  22  of the installation tool  10 . For example, with the tool  10  in a shaft translation mode, rotation of a handle  18  ( FIG. 1 ) of the tool  10  will cause the shaft  16  to translate along the longitudinal axis  22  and advance the pusher block  20  and prosthetic device  100  and the prosthetic device  100  toward the vertebral bodies  102 ,  104 . As a result, the distal movement of the pusher block  20  and the prosthetic device  100  between the levers  14 ,  15  will cause the blade tips  28 A,  28 B to distract which, in turn, causes distraction of the vertebral bodies  102 ,  104 . Advancement of the pusher block  20  continues until, as shown in  FIG. 10 , the prosthetic device  100  is properly installed between the adjacent vertebral bodies  102 ,  104 . When the implant reaches its final position, continued translation of the shaft draws the opposed levers from the disc space leaving only the implant in the disc space.  FIGS. 8-12  illustrate that at all times separation of the vertebral bodies is only effected to the extent necessary to insert the prosthetic device. Excessive distraction or separation of the vertebral bodies does not occur because the separation of vertebral bodies is caused by the height of the pusher block and/or the prosthetic device.  
      Following insertion of the prosthetic device  100 , as shown in  FIG. 11 , if the shaft is connected directly to a prosthesis, the tool can be reconfigured in a shaft rotation mode of operation to detach the shaft  16  from the prosthetic device  100 . In this manner, rotation of the handle  18  ( FIG. 1 ) will cause the shaft  16  to rotate  108  about the longitudinal axis  22  of the installation tool  10  to decouple the threaded portion  46  of the shaft  16  from the prosthetic device  100 . Once the shaft  16  has been disconnected from the prosthetic device  100 , the insertion tool  10  can be removed from between the adjacent vertebral bodies  102 ,  104 . For example, the tool can be reconfigured in a shaft translation mode of operation such that further linear translation of the shaft  16  toward the vertebral bodies  102 ,  104  will cause the pusher block  20  to apply a force to the vertebral bodies  102 ,  104  which, in turn, will cause the blade tips  28 A,  28 B to retract from between the vertebral bodies  102 ,  104  leaving only the prosthetic device  100  in the disc space.  
      The installation tool of the present invention can also be provided as a kit having modular components which allow the surgeon to select from among a variety of components to assemble an installation tool that is optimized for its intended use. The kit preferably includes several different shafts, pusher blocks, and other elements, each adapted to be used with a particular type or size of implant. For example, the kit can include different types of pusher blocks, each adapted to mate with a particular prosthesis. A person skilled in the art will appreciate that the installation tool can include a variety of components having a combination of different features. Moreover, the components can be adapted for use with particular types of prosthesis, or for use with other components.  
      One skilled in the art will appreciate further features and advantages of the invention based on the above-described embodiments. Accordingly, the invention is not to be limited by what has been particularly shown and described, except as indicated by the appended claims. All publications and references cited herein are expressly incorporated herein by reference in their entirety.