Abstract:
A cam spreader having a substantially rigid body with a proximal end defining a handle and a distal end defining a cam. The cam has at least a first lobe, the first lobe having an outer surface in a first quadrant of the cam as viewed in cross section along a longitudinal central axis of the body. At least a portion of the outer surface, or profile, of the curve is mathematically driven and produces a constant incremental lift.

Description:
BACKGROUND 
     During medical procedures, including surgeries of various kinds, the need may arise to spread, or distract, bones or other anatomical parts away from one another. This may be done to allow access to an inner portion or cavity of the body, or to create proper clearance for insertion of additional instrumentation, or to create a better vantage point to observe tissues, organs, or systems. Often, the parts that are being spread or distracted are joined together by tightly bound tissue such as ligaments. This can lead to additional damage being done to the surrounding organs or tissues due to the distraction process. Additionally, further damage may be done because the surgeon may be unable to ascertain the additional stress that will be placed on the distracted body parts due to an additional degree of stress placed on the distractor being used. 
     One example of a surgical procedure in which distraction may be needed but where it may be difficult for the surgeon to gauge the incremental stress being placed upon adjacent distracted bones due to the additional load being placed on the distractor is intervertebral surgery. When discs within the spinal column are repaired or replaced, or during fusion techniques, the vertebrae adjacent to the damaged disc may need to be distracted to allow clearance to insert an implant or fusion device. The end plates of the vertebrae can be susceptible to breakage if too great a force is applied. Therefore what is needed is a device and method to address the above identified issues and similar issues involving the distraction of delicate tissues. 
     SUMMARY 
     The present disclosure introduces a cam spreader having a substantially rigid body with a proximal end defining a handle and a distal end defining a cam. The cam has a first lobe with an outer surface in a first quadrant of the cam as viewed in cross section along a longitudinal central axis of the body of the spreader. The first quadrant is defined by first and second orthogonal axes, with the first and second axes being orthogonal both to each other and to the central axis. The outer surface of the first lobe is defined by a first line segment intersecting the first axis at a first distance from the second axis, a second line segment intersecting the second axis at a second distance from the first axis. A segment of a curve connecting the two line segments is defined by a mathematical equation providing for a constant incremental lift. 
     The foregoing has outlined features of several embodiments so that those skilled in the art may better understand the detailed description that follows. Additional features will be described below that further form the subject of the claims herein. Those skilled in the art should appreciate that they can readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features may not be drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
         FIG. 1  is a top view of a constant lift cam spreader. 
         FIG. 2  is a side view of a constant lift cam spreader. 
         FIG. 3  is a cross sectional view of a constant lift cam spreader. 
         FIG. 4  is a plot from which one possible cam spreader shape may be derived. 
         FIG. 5  is a diagram of a constant lift cam spreader between two adjacent vertebrae in a minimum lift position. 
         FIG. 6  is a diagram of a constant lift cam spreader between two adjacent vertebrae in a partially lifted position. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention relates generally to the field of orthopedic surgery and more particularly to instrumentation for vertebral procedures. It is to be understood that the following disclosure provides many different embodiments, or examples, for implementing different features of various embodiments. Specific examples of components and arrangements are described below to simplify the present disclosure. These are merely examples and are not intended to be limiting. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. Moreover, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed interposing the first and second features, such that the first and second features may not be in direct contact. 
     Referring to  FIGS. 1-2  a top view and side view, respectively, of a constant lift cam spreader  100  is shown. A distal end  105  defining a series of cam lobes  110 ,  112 ,  114 ,  116 , flat surfaces  120 ,  122 ,  130 ,  132  and a blunt tip  140  is shown. The distal end  105  joins to an elongated body  150  which may attach to a proximal end  160  defining a handle  165 . The distal end  105 , elongated body  150 , and proximal end  160  may each be concentric to a single longitudinal axis  170 . 
     The distal end  105  may be integral to the body  150  or they may be a separate components coupled together. The distal end  105  may be formed from steel, iron, aluminum, or other suitable metals or alloys. The distal end  105  may also be formed from plastics, polymers, ceramics, or other materials. In one embodiment, the distal end  105  is made from surgical grade stainless steel. The depth, length, and width of the distal end  105  may vary according to the application of the spreader  100 . For example, a greater maximum available spread may require a greater width in the distal end  105 . 
     The surfaces of the distal end  105 , which define the cam lobes  110 ,  112 ,  114 ,  116 , flat surfaces  120 ,  122 ,  130 ,  132  and tip  140  may be created by machining, casting, forging, or some other method depending upon the application of the spreader  100  and the material composition of the distal end  105 . The surfaces  120 - 140  may have a brushed finish, a polished finish, or some other finish. In some embodiments, a separate coating (not shown) may be utilized to provide a desired surface texture. The distal end  105  may also comprise more or fewer surfaces than shown here and may also comprise multiple pieces that are coupled together to create the desired shape. 
     Shoulders or cam lobes  110 ,  112 ,  114 ,  116  may serve to provide an incremental lifting surface. Cam lobes  110 ,  112  join minimum lift flat surface  130  with maximum lift surfaces  120 ,  122  respectively. On the reverse side of the spreader  100 , cam lobes  114 ,  116  join minimum lift surface  132  with maximum lift surfaces  120 ,  122 , respectively. Thus a transition or incremental lift is provided between the minimum lift and the maximum lift provided by the spreader  100 . The shape of the lobes  110 ,  112 ,  114 ,  116  may be defined by a mathematically driven curve, as explained in greater detail below. 
     The flat surfaces  120 ,  122 ,  130 ,  132  may be substantially flat or slightly curved and may serve to provide a stable location at which maximum or minimum lift is achieved by the spreader  100 . The distance or spread between surfaces  130 ,  132  (the depth of the spreader  100 ) may be seen to be relatively smaller than the distance or spread between surfaces  120 ,  122  (the width of the spreader  100 ). By selecting the distance between surfaces  120  and  122 , the maximum spread or lift of the spreader  100  may be chosen according to the purpose of the spreader  100 . Similarly, by selecting the distance between the surfaces  130 ,  132 , the minimum spread or lift of the spreader  100  may also be chosen according to the purpose of the spreader. The change in lift between minimum and maximum, or vice versa, occurs on the cam lobes  110 ,  112 ,  114 ,  116 . A degree of lift that varies continuously between minimum and maximum lift may be provided on the cam lobes  110 ,  112 ,  114 ,  116 . Additionally, as explained in greater detail below, the varying lift of the cam lobes  110 ,  112 ,  114 ,  116  may be configured to provide a constant degree of incremental lift with the change in lift being based directly on the amount of rotation imparted to the tool about the longitudinal axis  170 . 
     The tip  140  of the spreader  150  may be sloped or tapered if needed. This may allow for easier insertion of the spreader  100  into an existing surgical opening or perforation. In some embodiments, the tip  140  may also be sharpened to a point. This may allow the spreader  100  to be used as a single tool that can both create an initial opening and then distract or enlarge the space or opening created. As with the other surfaces of the distal end  105 , the tip  140  may be a separate component coupled to the distal end  105  or may be integral with the distal end  105 . 
     The distal end  105  may also be integral to the body  150  of the spreader  100 . In other embodiments, the distal end  105  may be detachable from the body  150 . The body  150  may be formed from steel, iron, aluminum, or other suitable metals or alloys. The body  150  may also be formed from plastics, polymers, ceramics, or other materials according to need. In one embodiment, the body  150  is made from surgical grade stainless steel. The body  150  may be formed from casting, machining, or forging. The body  150  may have a surface finish corresponding to the application of the spreader  100 . For example a highly polished and nonporous finish may be utilized where the spreader  100  is designed for use in a sterile surgical environment. 
     The dimensions of the body  150  may also be chosen according to the application of the spreader  100 . For example, where the spreader  100  is needed for distraction deep within a surgical cavity, a longer body may be required than for distraction near the surface of a surgical incision. In cross section, the body  150  may be circular, but other shapes are also possible. 
     The body  150  may be integral with the proximal end  160 , or the body  150  and proximal end  160  may be formed as separate components and coupled together. The proximal end  160  may define a handle  165 . In  FIGS. 1 and 2 , the proximal end  160  is shown as defining a flat handle  165  corresponding to the general shape of the distal end  105 . Other embodiments may have handles of other shapes however. For example, a tee shaped handle may be utilized to provide additional leverage. Additionally, a textured surface or grip (not shown) may be provided in addition to, or instead of, the handle  165  depending upon the application of the spreader  100 . 
       FIG. 3  is cross sectional view of a constant lift cam spreader  100 . The view of  FIG. 3  is taken along the line  3 - 3  as shown in  FIG. 2 . The axis  170  is orthogonal to the plane of  FIG. 3  in the location shown. Axes X and Y serve to provide reference to the location of the central axis  170 . Minimum spread surfaces  130 ,  132  are seen to be a smaller distance apart than maximum spread surfaces  120 ,  122 . It can also be seen in this embodiment that the surfaces  130 ,  132  may be equidistant from the central axis  170 . Similarly, surfaces  120 , 122  may be equidistant from the central axis  170 . Thus, lift or spread may also be defined in terms of the distance of the flat surfaces  120 ,  122 ,  130 ,  132  from the central axis  170 . The shoulders, or cam lobes,  110 ,  112 ,  114 ,  116 , are seen to provide a transition from minimum to maximum lift or vice versa about the central axis  170 . The cam spreader may also be symmetric about the X axis and the Y axis. 
       FIG. 4  is a plot from which one possible cam spreader shape may be derived. With continued reference to  FIG. 3 , the plot of  FIG. 4  may be seen to correspond to the upper left quadrant of  FIG. 3 , rotated 90° to simplify explanation. The plot or diagram of  FIG. 4  illustrates a curve where the value of l increases in direct proportion to the angle θ. This describes the curve ACDE. Line BC is the vertical tangent to this curve and would form the side wall of a cam spreader tool or minimum lift surface  130 . DF represents the flat surface on the top of the spreader, or a portion of the maximum lift surface  120 . Here the maximum lift provided by the spreader  100  relative to the central axis  170  is represented by h, which is a constant. The symbol αrepresents the constant angle from the y axis  170  to the beginning of maximum lift surface  120 . Thus GBCDF represents one quadrant of the cross section of the spreader  100 . Here, the area bounded by GBCDF corresponds to the upper left quadrant of  FIG. 3 . Additionally, as explained in greater detail below, α is the y intercept of curve ACDE and is a variable parameter. 
     From  FIG. 4  it may be seen that
 
sin α= h/l   max 
 
∴ l   max   =h /sin α  (1)
 
In the case of a cam with constant lift, the lift may be expressed in terms of a change in lift relative to a degree to rotation of the spreader  100 , therefore
 
                     dl     d   ⁢           ⁢   θ       =   k           (   2   )               
where k is a constant lift which may be measured in mm/rad.
 
     
       
         
           
             
               
                 
                   
                     k 
                     = 
                     
                       lift 
                       angle 
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     k 
                     = 
                     
                       
                         
                           l 
                           max 
                         
                         - 
                         a 
                       
                       α 
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     k 
                     = 
                     
                       
                         ( 
                         
                           
                             h 
                             
                               sin 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               α 
                             
                           
                           - 
                           a 
                         
                         ) 
                       
                       α 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     Returning to equation (2): 
                 ⅆ   l       ⅆ           ⁢   θ       =   k         
Integrating with respect to θ yields:
 ∴ l=kθ+C    (4) 
where C is constant of integration. To determine the constant of integration, the condition where θ=α, l=l max  as seen in  FIG. 4  may be used, thus:
 
     
       
         
           
             
               
                 
                   
                     h 
                     
                       sin 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       α 
                     
                   
                   = 
                   
                     
                       
                         
                           k 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           α 
                         
                         + 
                         C 
                       
                       ⁢ 
                       
                         
 
                       
                       ∴ 
                       C 
                     
                     = 
                     
                       
                         h 
                         
                           sin 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           α 
                         
                       
                       - 
                       
                         k 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         α 
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     Returning to equation (4) and replacing C with its equivalent from equation (3) yields: 
                   l   =       k   ⁢           ⁢   θ     +     h     sin   ⁢           ⁢   α       -     k   ⁢           ⁢   α               (   5   )               
Similarly, substituting the value for k from equation (2) into equation (5) yields:
 
     
       
         
           
             
               
                 
                   
                     
                       l 
                       = 
                       
                         
                           
                             
                               ( 
                               
                                 
                                   h 
                                   
                                     sin 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     α 
                                   
                                 
                                 - 
                                 a 
                               
                               ) 
                             
                             α 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           θ 
                         
                         + 
                         
                           h 
                           
                             sin 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             α 
                           
                         
                         - 
                         
                           
                             
                               ( 
                               
                                 
                                   h 
                                   
                                     sin 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     α 
                                   
                                 
                                 - 
                                 a 
                               
                               ) 
                             
                             α 
                           
                           ⁢ 
                           α 
                         
                       
                     
                     , 
                     
                       which 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       may 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       be 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       simplified 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       to 
                       ⁢ 
                       
                         : 
                       
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     l 
                     = 
                     
                       
                         
                           
                             ( 
                             
                               
                                 h 
                                 
                                   sin 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   α 
                                 
                               
                               - 
                               a 
                             
                             ) 
                           
                           α 
                         
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         θ 
                       
                       + 
                       a 
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     Thus, as shown above, a single equation in polar form may be used to represent the curve ACDE of  FIG. 4 . However, as will be shown below, it may be useful to work with a parametric representation of the curve ACDE. Therefore:
 
x=l cos α
 
y=l sin α
 
     
       
         
           
             
               
                 
                   
                     ∴ 
                     x 
                   
                   = 
                   
                     
                       [ 
                       
                         
                           
                             
                               ( 
                               
                                 
                                   h 
                                   
                                     sin 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     α 
                                   
                                 
                                 - 
                                 a 
                               
                               ) 
                             
                             α 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           θ 
                         
                         + 
                         a 
                       
                       ] 
                     
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     cos 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     θ 
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
             
               
                 
                   
                     ∴ 
                     y 
                   
                   = 
                   
                     
                       [ 
                       
                         
                           
                             
                               ( 
                               
                                 
                                   h 
                                   
                                     sin 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     α 
                                   
                                 
                                 - 
                                 a 
                               
                               ) 
                             
                             α 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           θ 
                         
                         + 
                         a 
                       
                       ] 
                     
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     sin 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     θ 
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
     In both the radial form (equation (6)) and the parametric form (equations (7) and (8)) of the equation a value for the parameter a may be determined which will result in the curve ACDE being parallel to the Y axis at a distance from the Y axis corresponding to the chosen minimum lift provided by the spreader  100 . To determine the values of parameter αthat will satisfy this condition, it is necessary to take the first derivative of the equations in parametric form (equations (7) and (8)) with respect to θ. Thus: 
                       ⅆ   x       ⅆ           ⁢   θ       =           (       h     sin   ⁢           ⁢   α       -   a     )     α     ⁢           ⁢   cos   ⁢           ⁢   θ     -       [           (       h     sin   ⁢           ⁢   α       -   a     )     α     ⁢           ⁢   θ     +   a     ]     ⁢           ⁢   sin   ⁢           ⁢   θ               (   9   )                   ⅆ   y       ⅆ           ⁢   θ       =           (       h     sin   ⁢           ⁢   α       -   a     )     α     ⁢           ⁢   sin   ⁢           ⁢   θ     +       [           (       h     sin   ⁢           ⁢   α       -   a     )     α     ⁢           ⁢   θ     +   a     ]     ⁢           ⁢   cos   ⁢           ⁢   θ               (   10   )               
When
 
                 ⅆ   x       ⅆ   y       =   0         
the slope of the curve ACDE is infinite and the curve is vertical, or parallel to the Y axis. Thus:
 
                 ⅆ   x       ⅆ   y       =             ⅆ   x       ⅆ           ⁢   θ           ⅆ   y       ⅆ           ⁢   θ         ⁢           ⁢   when   ⁢           ⁢       ⅆ   x       ⅆ   y         =       0   ⁢           ⁢       ⅆ   x       ⅆ           ⁢   θ         =   0             
Therefore, from equation (9) the curve is vertical when
 
                             (       h     sin   ⁢           ⁢   α       -   a     )     ⁢             α     ⁢           ⁢   cos   ⁢           ⁢   θ     -       [           (       h     sin   ⁢           ⁢   α       -   a     )     α     ⁢   θ     +   a     ]     ⁢           ⁢   sin   ⁢           ⁢   θ       =   0           (   11   )               
The width of the curve ACDE may be given by the x coordinate from the parametric equation (7) at angle θ. Therefore the desired minimum lift of the spreader relative to the central axis  170  can be substituted for x, giving:
 
     
       
         
           
             
               
                 
                   
                     
                       [ 
                       
                         
                           
                             
                               ( 
                               
                                 
                                   h 
                                   
                                     sin 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     α 
                                   
                                 
                                 - 
                                 a 
                               
                               ) 
                             
                             α 
                           
                           · 
                           θ 
                         
                         + 
                         a 
                       
                       ] 
                     
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     cos 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     θ 
                   
                   = 
                   
                     minimum 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     lift 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     from 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     axis 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     170 
                   
                 
               
               
                 
                   ( 
                   12 
                   ) 
                 
               
             
           
         
       
     
     Equations 11 and 12 thus form a system of non linear simultaneous equations with 2 variables (θ and a). The value of h may be chosen to correspond to the desired maximum lift of the spreader  100  from the central axis  170 . A desired width of the flat surfaces corresponding to maximum lift (e.g., surfaces  120 ,  122 ) may also be chosen. The angle α corresponds to the angle from the central axis to an edge of the flat surface of maximum lift as may be seen in  FIG. 4 . Thus values for h, α, and minimum lift are known constants. Therefore equations 11 and 12 can be solved by numerical means for θ and a. Equations (11) and (12) may be solved numerically by the fixed point iteration method or the Newton-Raphson method, for example. The cam curve ACDE may then be plotted using either the polar form (equation (6)) or the parametric form (equations (7) and (8)). 
     Once the curve ACDE has been plotted along with the other components of the graph of  FIG. 4 , the area bounded by BCDFG may be used as a cross sectional pattern which defines a single quadrant of the distal end  105  of the spreader  100 . To create a full cross section of the distal end  105 , the area BCDFG may be reflected about the Y axis and the resulting graph reflected about the X axis. The resulting cross section may appear similar to  FIG. 3  but with all dimensions known. Using the full cross section as a pattern, the distal end  105  of the spreader  100  may be constructed according to known techniques and materials. In one embodiment, the resulting spreader will result in a maximum force or lift when rotated approximately 60 degrees from the position of minimum lift. 
       FIG. 5  is a diagram of a constant lift cam spreader  100  in a minimum lift position between two adjacent vertebrae  501 ,  505 .  FIG. 5  provides an example of one environment  500  in which one embodiment of the constant lift cam spreader  100  may operate. The surgical site  500  may be cleaned and an incision made that allows the spreader  100  access to the vertebrae  501 ,  505 . In a fusion or disc replacement procedure, the disc (not shown) interposing the vertebrae  501 ,  505  may be perforated or removed to allow insertion of the spreader  100 . As previously described, the spreader  100  may also be driven directly into tissue such as the spinal disc such that the initial perforation is made by the sharpened tip  140 . 
     The distal end  105  of the spreader  100  may be inserted between the vertebra  501 ,  505  in a position of minimum lift. Thus minimum lift surfaces  130 ,  132  may be in substantially solid contact with the endcaps  502 ,  507  of vertebrae  501 ,  505 , respectively. To effect distracting, or spreading, of the vertebrae  501 ,  505 , the spreader  100  may be rotated about the central axis  170 . The spreader  100  may be designed such that rotation in either direction results in the same distraction. Here the spreader is rotated counter clockwise in the direction of arrow B. 
       FIG. 6  is a diagram of a constant lift cam spreader  100  between two adjacent vertebrae  501 ,  505  in a partially lifted position. As the spreader  100  is rotated about axis  170  in the direction of arrow B, the minimum lift surfaces  130 ,  132  are replaced as the primary contact surface to the endcaps  502 ,  507  by cam lobes  110 ,  114 , respectively. Thus the distance between the vertebrae  501 ,  505  begins to increase as a result of the increasing lift provided by the spreader  100 . Here, the vertebrae  501 ,  505  may displace from one another in the general direction of arrows C and D, respectively. The displacement of the vertebrae may be more easily controlled and stress to the end plates  502 ,  507  minimized due to the constant rate of incremental lift relative to the amount of rotation of the tool  100 . 
     If so desired, the spreader  100  may provide increasing lift or displacement commensurate with the degree of rotation about the axis  170  until the endcaps contact  502 ,  507  come into contact with maximum lift surfaces  120 ,  122 , respectively. When the desired amount of lift has been achieved, the remaining steps of the surgical procedure may be carried out. For example, a spacer device, such as is described in U.S. Pat. Ser. No. 10/404,262, the disclosure of which is hereby incorporated by reference, can be inserted into the distracted disc space. The spreader  100  may also be rotated in an opposite direction (e.g., clockwise) to provide a proportionate decrease in lift relative to the amount of opposite rotation. The spreader  100  may be rotated completely back to the minimum lift position in preparation for retraction from between the vertebrae  501 ,  505 . 
     The foregoing has outlined features of several embodiments according to aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions and alterations herein without departing from the spirit and scope of the present disclosure.