Abstract:
A malleable shaft device for use in surgical procedures, having a tension fiber therein, whereby the malleable shaft transitions from a malleable state to a rigid state when a force is applied to the tension fiber, has a variable-pitch cam operatively connected to the tension fiber, and a lever operatively connected to the variable pitch cam for applying force. The mechanical advantage of the cam changes with displacement to accommodate increasing load, and to increase the rate of travel while loads are light. The force profile may provide for a constant input force to accomplish the full travel of the cam.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of Invention  
           [0002]    The invention relates to the field of tension- stiffening devices and surgical instrumentation. More specifically, it relates to a variable-pitch cam mechanism for use in a malleable-shaft surgical retractor, particularly those that have a fiber running through their length and stiffen upon the tensioning of that fiber.  
           [0003]    2. Description of Related Art  
           [0004]    In the field of medical instrumentation, malleable shaft stabilization devices are known in the art. Particularly in Coronary Artery Bypass Graft (CABG) procedures, such instruments are used to stabilize the surface of the beating heart muscle at the site where the anastomosis will be formed. The instruments will have a malleable shaft of a certain length, with a stabilizer foot at a distal end. The shaft can be positioned as required, then a lead screw at a proximal end is turned by some manner of handle. The turning of the lead screw applies tension to a fiber, such as a metal wire or braided cable. The tensioning of the cable compresses the malleable shaft, and secures it rigidly into position. These devices are considered an improvement over other stabilizers, because the length of the shaft can be positioned away from the surgical field, as compared to fixed-arm stabilizers that may impede access and vision. With a malleable shaft stabilizer, the surgeon&#39;s hands are free to perform another task, and/or the surgical field is clear of obstruction.  
           [0005]    These devices are not without drawbacks, however. Among these are the difficulties with the tensioning method. When tensioning the fiber, the forces on the fiber are relatively light as tension is first applied, but comparatively higher at the end of the range, typically approximately 150 lbs. or more. However, the nature of a lead screw is that it has a constant mechanical advantage at all times. The lead screw must then be designed with a great mechanical advantage to accommodate the high forces at the end of the range. Consequently, the pull per turn is reduced. Several turns of the screw are therefore required. It is not uncommon for between 4 and 6 full turns of the screw to be required in order to transition the device from a malleable state to a rigid state. This process is tedious, tiring, and time-consuming.  
           [0006]    Moreover, during the time required to transition the malleable shaft into a rigid state, the position of the stabilizer positioned on an end of the malleable shaft may drift on the surface of the heart. To overcome this, it is also not uncommon for the surgeon to position the retractor and hold it in place, and then instruct an assistant to turn the handle the several turns required to tension the shaft.  
         BRIEF SUMMARY OF THE INVENTION  
         [0007]    In order to overcome these and other problems in the prior art, it would be desirable to have a cam mechanism that can accomplish the transition from malleable to rigid states in a smaller motion, while comfortably handling the accompanying loads. It would also be desirable to accomplish the transition while maintaining a constant input force over the range of motion of the actuator.  
           [0008]    Therefore, provided by the present invention is a malleable shaft device for use in surgical procedures, having a tension fiber therein, whereby the malleable shaft transitions from a malleable state to a rigid state when a force is applied to the tension fiber, has a variable-pitch cam operatively connected to the tension fiber, and a lever operatively connected to the variable pitch cam for applying force. The mechanical advantage of the cam changes with displacement to accommodate increasing load, and to increase the rate of travel while loads are light. The force profile may provide for a constant input force to accomplish the full travel of the cam. The force profile can provide for the full travel of the cam in a limited range of motion, for example one full turn, one-half turn, or within the range of motion of the longitudinal turning of a human forearm for some portion of the population.  
           [0009]    Moreover, according to the present invention, the cam may be transformed into any of several actuation modalities by transforming the cam profile, for example from a cylindrical cam to a linear cam. Also provided by the present invention is a plural-stage device that has varying mechanical advantages among the plural stages. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    These and other features, benefits and advantages of the present invention will become apparent with reference to the following specification and accompanying drawing, in which like reference numerals indicate like features across the several views.  
         [0011]    [0011]FIG. 1 illustrates a malleable shaft surgical device including a variable pitch cam according to the present invention;  
         [0012]    [0012]FIG. 2 illustrates a first embodiment of the variable pitch cam according to the present invention;  
         [0013]    [0013]FIGS. 3A through 3C illustrate a second embodiment of the present invention in three mutually orthogonal views;  
         [0014]    [0014]FIG. 4 illustrates yet another embodiment of the present invention having a linear, rather than rotating cam;  
         [0015]    [0015]FIGS. 4A and 4B illustrate alternate configurations of the linear cam plate for use with the embodiment of FIG. 4;  
         [0016]    [0016]FIG. 5 illustrates various displacement profiles for cams used in tension devices, including two according to the present invention;  
         [0017]    [0017]FIG. 6 illustrates a two-stage embodiment of the present invention in an exploded assembly view;  
         [0018]    [0018]FIG. 6A shows an underside of the upper portion of the embodiment illustrated in FIG. 6; and  
         [0019]    [0019]FIG. 7 illustrates the dimensions of rotation of the human forearm taken into account in the design and practice of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    Referring now to FIG. 1, shown is a first embodiment of a malleable shaft device  10 . In this case, the device is attached to a rib retractor  12 , also commonly used in thoracic surgeries. The malleable shaft device  10  has a base  14 , which can include a slot  16  for attachment to another device such as retractor  12 . Extending distally from the base  14  is a malleable shaft  18 . The malleable shaft  18  will typically have an operative tool such as a stabilizer foot  19 , alternately a retractor blade, or other attachment at a distal end thereof. Internal to the malleable shaft  18  is a tension fiber (not shown) that may be a single- or multi-strand wire, braided cable, or other suitable element and material. Extending proximally from the base  14  is a handle  20  for turning the variable-pitch cam  22 , located generally internal to the base  14 . The variable-pitch cam  22  is operatively connected to the tension fiber.  
         [0021]    Referring now to FIG. 2, shown is a cylindrical cam  22  according to a first embodiment of the present invention. The cylindrical cam  22  has a cam slot  24  formed along its circumference. Cam  22  may also include a keyway  26  at a proximal end  30 . The keyway can be provided for the attachment of a torque-applying element, such as handle  20  (See, FIG. 1). The tension fiber (not shown) is operatively connected at a distal end  32  of the cam  22 .  
         [0022]    The cam profile of the present invention can be defined in one of a variety of ways. Beginning with the geometric relationships  
              S          l       =     tan                 α             l   =       D   2        θ                           
 
         [0023]    where S is tension fiber displacement at a proximal end, 1 is the circumference movement of the cam  22 , a is the angle of the cam grove measured perpendicular to the direction of S, and D is the diameter of cam  22 . Cable force F c  is proportional to the proximal cable displacement S as F c =β·S, where β is a constant determined by the design of the malleable shaft portion  18 .  
         [0024]    In one embodiment, the goal is to produce a constant actuating force regardless of the tension required. In the case of a rotatable cam, the actuating force F t  is tangent to the cable force F c , and is given by  
         F   t     =         F   c                   tan                 α     =         F   c               S          l         =     β                 S             S          l                                   
 
         [0025]    Where F t  is constant, the equation yields  
         β                 S           S       =       F   t             l                 S   2     =           2        F   t       β        l     =           F   t        D     β        θ                             
 
         [0026]    In this case, S=0 when 1=0, so  
         l   =       β                   S   2         2        F   t           ;     θ   =       β                   S   2           F   t        D                               
 
         [0027]    A further object of the present invention is to accomplish the full travel, S max , of the tension fiber in a much smaller angular displacement than currently required by the prior art. Preferably, the full travel of the tension fiber is accomplished in less that one full turn of the cam (2π). More preferably, the cam will accomplish the full travel of the tension fiber in less than one half turn (π). Alternately, the cam will accomplish the full travel of the tension fiber within the typical range of motion of the longitudinal turning of a human forearm.  
         [0028]    Taking the case of accomplishing the full travel of the tension fiber in less than one half turn,  
           S=S   max ;θ≦π 
         [0029]    which requires  
           F   t     ≥       β                   S   max   2         2      l         =       β                   S   max   2         D                 θ             and           tan                 α     =            S          l       =         F   t       β                 S       =         S     2      l                     or                 tan                 α     =            S          l       =         F   t        D       β                 SD                           S   2     =           2        F   t       β        l     =       λ                 l     =       γθ                 where                 λ     =         2        F   t       β     =     const   .               ;             γ   =           F   t        D     β     =       const   .     
          S        (   l   )         =           F   t        l     β             ;       S        (   θ   )       =           F   t        D                 θ     β                               
 
         [0030]    Referring now to FIGS. 3A, 3B and  3 C, a second embodiment  110  of the present invention is shown in three orthogonal views, respectively. In this second embodiment  110 , the activation lever  34  is operatively connected along the length of cam  22 , though it may also be positioned at a proximal end thereof, similar to the first embodiment, supra. This embodiment is particularly advantageous when used in combination with the cam profile described above, wherein the full travel (S max ) of the tension fiber is accomplished in one-half full turn (π) of the cam  22 .  
         [0031]    Alternately, the cam can be designed to accomplish the full travel (S max ) of the tension fiber within the range of motion of the longitudinal turning of a human forearm. It would be advantageous for a surgeon to be able to transition the malleable shaft to a rigid state in one motion. Data quoted from  Human Factors Design Handbook,  2d Ed., Woodson, et al. (McGraw Hill, 1992) by the National Institute of Science and Technology indicates that among male Air Force personnel, the average range of forearm supination and pronation (See FIG. 7) are 113 and 77 degrees, respectively, with standard deviations of 22 and 24 degrees, respectively. Therefore, one can expect half the population to be able to accomplish at least 190 degrees of rotation about the long axis of the forearm in a single motion.  
         [0032]    Data such as that quoted can be used to design the cam so that the full travel (S max ) of the tension fiber is accomplished within the range of motion of the longitudinal turning of a human forearm of some portion of the population. Some allowance can be made to accommodate the expected variance among the human population to accommodate a greater percentage of users. For example, from statistical principles, choosing to full travel to be one standard deviation below the mean, 144° in this case, will allow nearly 85% of users to accomplish the full deflection in a single motion.  
         [0033]    An elegant feature of the present invention is that the cam profile may easily be mapped to different activation modalities by a simply transforming the coordinates to the corresponding coordinate system, for example, cylindrical, polar, Cartesian, etc. Referring now to yet another embodiment  210  shown in FIG. 4, shown in partial cross-section. In this embodiment  210 , the displacement of the tension fiber (not shown) is accomplished in the same plane as the fiber through a lever action, rather than by a rotating cylindrical cam.  
         [0034]    Lever  36  is pivotally attached to the base  14  at a pivot  38 . Attached to lever  36  is a follower pin  40 , which fits into a cam slot  42  of the cam plate  44 . As handle  36  is rotated around pivot  38  in the direction of arrow  46 , follower pin  40  moves proximally and downward in slot  42 , moving the cam plate  44  proximally, in the direction of arrow  48 . Cam plate  44  also has a clearance slot  50  to allow it to move proximally without interfering with pivot  38 .  
         [0035]    In this embodiment  210 , cam slot  42  is a straight slot. In that case, the mechanical advantage applied will vary as L/I cosθ, where L is the length of lever  36  measured from pivot  38  to its free end  54 ,  1  is the length  52  between pivot  38  and follower pin  40 , and e is measured at angle  56  between line  58  perpendicular to length  52  and a vertical line  60 . Those skilled in the art will recognize that the advantage increases significantly as the lever approaches the horizontal. This arrangement has shown promise as a simple approximation to substitute for the constant force paradigm discussed above.  
         [0036]    Referring now to FIGS. 4A and 4B, alternate cam plates  144 ,  244  are shown as a further refinement of the embodiment  210 . In cam plate  144 , cam slot  142  is cut on an angle, which alters the mechanical advantage only to the extent that the reference line  160  from which θ is measured remains parallel to the long axis of the straight slot  144 . In cam plate  244 , cam slot  242  varies in angle along its length according to a geometric transformation of the constant force equations discussed above, in a manner apparent to those skilled in the art. Alternately, the profile of cam slot  242  can vary according to another force profile as deemed suitable.  
         [0037]    Referring now to FIG. 5, a further refinement of the cylindrical cam profile is described. It must be acknowledged that despite advances in numerical control machining techniques, that the profile of cam slot  24  according to a constant force paradigm will present manufacturing challenges, in terms of accuracy and repeatability. Among the reasons for this is that the angle α is continuously changing over the length of the cam slot  24 . Therefore, it would be advantageous to simplify the cam profile.  
         [0038]    [0038]FIG. 5 illustrates the displacement profiles of the prior art, as compared to various embodiments of the present invention. For example, profile  501  illustrates the known prior art, namely a simple lead screw. It will be seen that with on full turn (2π) of the lead screw, the cable displacement does not even approach S max , the required tension fiber displacement for full rigidity of the malleable shaft  18 . By contrast, profile  503  illustrates a profile that, like the constant force profile, is continuously variable. Profile  505  describes the further refinement. Profile  505  is divided into at least three areas. Area  505   a  represents a low advantage profile, which can quickly take up tensioning fiber length while forces are low. The cam then enters a transition area  505   b , which smoothly transitions from area  505   a  to area  505   c . Finally, area  505   c  is highly advantaged, to allow the user to comfortably apply the necessary force to complete the transition of the malleable shaft  18  to a rigid state.  
         [0039]    The profile  505  improves the ease of manufacturing because area  505   a  and  505   c  both exhibit constant screw angles α. Only transitional area  505   b  requires a change. Further, having the force to the user increase at the end of the range presents other advantages as well. For example, it simulates the traditional end-of-range feel that users of the prior art may be accustomed to when securing those devices.  
         [0040]    Referring now to FIG. 6, yet another embodiment, generally  310 , of the present invention is shown. In this embodiment, full tensioning of the malleable shaft is accomplished with a two-stage carriage  320 . The two-stage carriage  320  has first stage  322  with an exterior thread  324 , preferably having a high mechanical advantage. The exterior thread  324  of the first stage  322  mates with a complementary interior thread (not shown) formed within the block  315 . Block  315  is operatively connected to the tension fiber and with flanges  317  is constrained to move only axially by ribs  319  in the base  314 , and complementary ribs  321  formed in an upper section  314   a  (See FIG. 6A). Ribs  321  are made shorter, and end at wall  313  to allow the flange  323  of first stage  322  to rotate when the appropriate displacement is achieved.  
         [0041]    Carried within the first stage  322  is a second stage  330 . The second stage  330  is operatively connected with the tension fiber of the malleable shaft  18 . Second stage  330  is formed with an exterior thread  332 , preferably having a low mechanical advantage. Exterior thread  332  mates with a complementary interior thread (not shown) formed in the first stage  322 . Each of the first stage  322  and second stage  330  are operatively connected to lever  350 , respectively, for applying torque to the stages. Alternately, each stage may have an independent lever for actuation.  
         [0042]    In operation, the second stage  330  will be torqued with lever  350 , whereby the low advantage thread will take up a relatively high portion of the tension fiber displacement quickly, while forces are relatively low. In the turning of second stage  330 , first stage  322  and block  315  are pulled axially. When thread  332  of the second stage  330  reaches the end of its travel, preferably within approximately one-half rotation (n) of the second stage  330 , torque will be applied to the first stage  322 . The pin  360 , carried within hole  362  of flange  323 , will interface with the wall  313 , and against the urging of spring  362 , will press through to interface with hole  366  in the second stage  330  as the pin and the hole align. The two stages will then turn together to exercise the travel of the first stage  322 .  
         [0043]    At the end of travel of the thread  324  of the first stage  322 , preferably within approximately one-half rotation (π) of the second stage  330 , the transition of the malleable shaft  18  to a rigid state will be complete, and the combined linear displacement of the first stage  322  and second stage  330  will equal S max .  
         [0044]    Alternately, the respective threads can be chosen to accomplish the full travel of each stage within the range of motion of the longitudinal turning of a human forearm of some portion of the population. Some allowance can be made to accommodate the expected variance among the human population to accommodate a greater percentage of users. Alternately, the respective threads can be chosen to accomplish the full travel of each stage within the one-quarter full rotation (π/2), or one half of the range of motion of the longitudinal turning of a human forearm of some portion of the population.  
         [0045]    The present invention has been described herein with respect to certain embodiments. Certain modifications or alterations may be apparent to those skilled in the art without departing from the scope of the invention. The exemplary embodiments are meant to be illustrative, not limiting of the scope of the invention, which is defined by the following claims.