Abstract:
A support arm assembly for assisting in the performance of cardiac surgery has an articulated arm movable axially of a support base and rotatable relative to the support base. A distal end of the articulated arm receives a contact member and the contact member is moveable relative to the distal end. A surgeon may configure the support arm assembly to contact a desired portion of a heart and fix the articulated arm, its axial position, its rotated position and the orientation of the contact member with a single control.

Description:
RELATED APPLICATIONS  
       [0001]    This application claims priority from U.S. Provisional Patent Application Serial No. 60/351,986, filed Jan. 23, 2002, entitled “Support Arm for Cardiac Surgery.” 
     
    
     
       TECHNICAL FIELD  
         [0002]    The present invention relates generally to surgical tools including surgical tools for cardiac surgery, and more specifically to surgical tools used in conjunction with Coronary Artery Bypass surgery, both the on-pump and the off-pump variety.  
         BACKGROUND ART  
         [0003]    Off-Pump Coronary Artery Bypass (OPCAB) has evolved since about 1990, following the pioneering work done in North America by Ankeney in the period 1970-75, Akins et al. 1979, and Fanning et al. 1979-1992 in the U.S, and by Trapp &amp; Bisarya in the early 1970&#39;s in Canada. In South America other pioneering was done by Benetti (1978-85) in Argentina and Buffolo (1981-85) in Brazil. Generally the anastomotic site was immobilized with stay sutures, such as the technique described by Trapp &amp; Bisarya, who encircled the anastomotic area with sutures placed deep in the myocardium to incorporate enough muscle to suspend the heart yet prevent damage to the coronary artery. Later in the U.S. Phister (1985-90) and Gundry in 1990 (among others) performed OPCAB surgery but both had an assistant with a hand held instrument press on the surface of the heart near the anastomotic site to aid in epicardial immobilization.  
           [0004]    In the mid 1990&#39;s various epicardial stabilizing instrument that could be attached to sternal retractors were evolved. For example U.S. Pat. No. 5,836,311 described a vacuum epicardial stabilizer, U.S. Pat. No. 5,786,746 described an adhesive coated and vacuum epicardial stabilizer, and U.S. Pat. No. 6,213,941 described a mechanical foot that pressed on the myocardium to stabilize the anastomotic site.  
           [0005]    During the period of about five or six year following the mid 1990&#39;s OPCAB was increasingly used. Some surgeons reported that a great majority of their patients received OPCAB surgery. Initially, one driving force that brought this operative change included the potential to reduce the size of the surgical incision in the patient&#39;s chest; a second was the potential of reducing post-operative complications due to embolism or micro-embolism associated with an extracorporeal circulation and the use of the aortic cross-clamp. Perceived advantages were the potential of reducing patient post-operative pain; and the potential to shorten hospital and recovery time and hence reduce overall costs of the treatment procedure. Initially the procedure was called MIDCAB (Minimally Invasive Direct vision Coronary Artery Bypass) surgery (sometimes called “keyhole” surgery). It soon became apparent to many cardiac surgeons that a minimal incision (usually a thoracic as opposed to a sternal incision) was both surgically inadequate, and it resulted in increased post-operative pain as compared with a midline sternotomy. Hence, this procedure fell out of favor after a couple of years.  
           [0006]    Some results of OPCAB procedures demonstrated that post-operative embolic complications were reduced, while others did not. Generally, the incidence of postoperative embolic complications did not fall as dramatically as had been expected. However with OPCAB surgery blood loss, and the volume of blood perfused during and following surgery were shown to have been significantly reduced as compared to on-pump bypass surgery. In some studies overall hospital stay and hence treatment cost did fall, although in other studies this was not found to be the case. Operating time generally increased for OPCAB procedures, and this with the high cost of disposable epicardial stabilizers largely offset the cost savings of not using a cardio-pulmonary machine with its disposable blood handling circuit components.  
           [0007]    To more easily and accurately and speedily anastomose a bypass graft on a coronary artery the immediate surface of the beating heart surrounding the anastomotic site must be rendered relatively akinetic. Stabilization of this local area may be achieved by placing a stabilizing foot (attached to the distal end of a surgical arm) on the surface of the heart to lie on either side of the anastomotic site. The proximal arm is firmly attached to a sternal retractor, thus theoretically fixing and immobilizing the stabilization foot. However, in practice prior instrument have drawbacks that this invention overcomes. For example, most disposable instruments either have a rigid (straight or curved) metal arm, or a fully articulated plastic arm. While the rigid metal arms are fairly stiff, they are not suitable for accessing the proximal obtuse marginal branches of the circumflex coronary arteries on posterior part of the epicardium or the distal circumflex arteries. By contrast, many disposable arms have plastic articulated members. Because Young&#39;s Modulus of plastic is low compared to that of stainless steel, flexure of the plastic arm by forces applied by the beating heart to the stabilizing foot causes said arm to flex significantly. Clearly, high rigidity is necessary to minimize movement under load at the anastomotic site as the heart beats. A second important consideration concerns the force that can be applied to the distal portion of the arm before one or more of the nestling articulating joints slip. Obviously the longer the arm, the greater the force moment that causes arm flexure or slippage. If all parts of the heart are to be stabilized an arm of sufficient reach and versatility is required, thus low flexure and slippage under load is advantageous. Moreover, it is highly desirable to provide to the surgeon with multiple types, configurations and sizes of stabilizing, so that the surgeon can select the optimum for individual anatomies.  
           [0008]    The majority of the devices currently marketed for myocardial stabilization are only available as single patient use combined arm and feet, the arm and the foot being disposable. The currently marketed devices have some functional shortcomings, and do not offer a full “tool kit” to allow the surgeon to select a device configuration best suited to all surfaces of the heart. In addition, current devices do not provide sufficient adjustability to reach the entire surface of the heart. Furthermore, current devices lack the rigidity necessary to provide a stable support for remote portions of the heart. Finally, existing devices can be cumbersome to use and difficult to secure in a select orientation. The present invention is intended to overcome one or more of the problems discussed above.  
         SUMMARY OF THE INVENTION  
         [0009]    A first aspect of the present invention is a support assembly arm for assisting in the performance of cardiac surgery. The support arm assembly includes an articulated support arm made of a plurality of links having a socket housing at a distal end. A contact member for bearing on the surface of the heart has a ball connected thereto and the ball is received in the socket housing to enable the contact member to assume a select orientation relative to the support arm. A cable extends between the socket housing and a proximal end of the support arm. The cable is operatively associated with the links of the articulated support arm and the socket housing so that when the cable is tensioned it compresses the links to fix a select configuration of the support arm and compresses the socket housing against the ball to fix a select orientation of the contact member relative to the support arm. An apparatus for tensioning the cable is operatively associated with the cable and a clutch is operatively associated with the tensioning apparatus for preventing application of greater than a select tensile force to the cable.  
           [0010]    The tensioning apparatus preferably consists of a first end of the cable being fixedly attached to the socket housing and a hole in each link receiving the cable. A knob having a threaded receptacle is attached to a proximal end of the cable and is rotatable about a longitudinal axis of the support arm relative to the plurality of links. A threaded member is received in the threaded receptacle and fixed against rotation relative to the links. A second end of the cable is operatively associate with the threaded member, whereby as the knob is rotated in a first direction the threaded member is advanced within the receptacle to tension the cable and as the knob is rotated in a second direction the threaded member is withdrawn from the receptacle to slacken the cable. The clutch preferably includes at least one radial driver ramp which is fixed against rotation relative to the knob within the knob housing. An Acme nut is received within the knob housing to define the threaded receptacle. The Acme nut has an abutting end having a radial ramp nesting with the radial driver ramp. A spring compresses the radial driver ramp against the Acme nut with the radial driver ramp and the radial nut ramp nested. The radial driver ramp and the radial nut ramp will disengage if a tensile force on the cable exceeds a select amount as the knob is rotated in the first direction.  
           [0011]    The first aspect of the support arm assembly may further include a base including a clamp for fixed attachment to a support. An axial clamp extends from the base and axially receives the articulated support arm. The clamp has a locked position preventing axial movement of the support arm relative to the clamp and an unlocked position enabling axial movement of the support arm relative to the clamp. An actuator is associate with the cable for actuating the clamp to the locked position as the cable is tensioned. The base may further include a pivotal connection between the base and the axial clamp enabling rotation of the axial clamp relative to the base and an actuator operatively associated with the cable for fixing the pivotal connection with the clamp in a select rotated position relative to the base as the cable is tensioned.  
           [0012]    A second aspect of the present invention is a support arm assembly for assisting in the performance of cardiac surgery having a base including a clamp for fixed attachment to a support. An articulated support arm has a contact member for bearing on the surface of the heart operatively associated with its distal end. An axial clamp extends from the base and axially receives the articulated support arm. The axial clamp has a locked position preventing axial movement of the support arm relative to the axial clamp and an unlocked position enabling axial movement of the support arm relative to the axial clamp, whereby the effective length of the distal end of the support arm relative to the base can be varied. The articulated support may further include a ball and socket connection between the contact member and a distal end of the articulated support arm to enable the contact member to assume a select orientation relative to the articulated support arm. A fixing apparatus is provided operatively associated with the articulated support arm, the axial clamp and the ball and socket connection for fixing the support arm in the select configuration, for fixing the clamp in the locked position and for fixing the ball and socket connection with a select orientation relative to the articulated support arm by actuation of a single control. The support arm assembly may further include a pivotal connection between the base and the axial clamp, enabling rotation of the axial clamp relative to the base and the fixing apparatus then further includes a structure operatively associated with the pivotal connection for fixing the axial clamp in a select rotated position relative to the base by actuation of the single control.  
           [0013]    The fixing apparatus preferably includes a cable extending between the ball and socket connection and a proximal end of the articulated arm, with the cable being operatively associated with the articulated support arm, the clamp and the ball and socket connection so that as the cable is tensioned the support arm is fixed in the select configuration, the axial clamp is fixed in the locked position and the ball and socket connection is fixed with a select orientation relative to the support arm. The single control preferably consists of a knob rotatably attached to the proximal end of the support arm, with the cable being operatively associated with the knob and the knob being configured so that as it is rotated in a first direction any tension in the cable is increased and as the knob is rotated in a second direction any tension in the cable is decreased.  
           [0014]    Yet another aspect of the present invention is a method for performing a surgical procedure on a heart of a patient. The method includes making an incision in the patient&#39;s chest, inserting a retractor into the incision and securing the retractor in an open position to provide access to the heart. An articulated arm having a proximal and a distal end is provided. The articulated arm is attached to the retractor so that the articulated arm is movable axially relative to the retractor and the articulated arm is configured as desired to bring the contact member into contact with a desired portion of the heart. The articulated arm is fixed axially of the retractor and the configuration of the support arm is fixed to exert and maintain a stabilizing force on the desired portion of the heart while performing the surgical procedure. Preferably, fixing of the articulated arm axially of the retractor and fixing the configuration of the support arm is performed by actuation of a single control operatively associated with the proximal end of the articulated arm. Preferably, releasably attaching the interchangeable contact member further includes releasably attaching the interchangeable contact member in a manner allowing for movement of the contact member relative to the distal end of the articulated arm and the interchangeable contact member is oriented relative to the distal end of the articulated arm as desired. The orientation of the interchangeable contact member is then fixed relative to a distal end of the articulated arm. Preferably, the articulated arm is attached to the retractor in a manner allowing the articulated arm to rotate about an axis substantially vertical to the patient&#39;s chest and the articulated arm is rotated as desired and the rotational position is fixed in a desired position relative to the retractor.  
           [0015]    The method may further include providing a friction fit between the contact member and the distal end of the articulated arm for retaining the contact member connected to the distal end of the articulated arm followed by fixing the contact member to the distal end of the articulated arm in a select orientation.  
           [0016]    The surgical procedure may be a coronary artery bypass graft procedure and the desired portion of the heart may be any anastomotic site and the stabilizing force preferably provides surgical exposure to the anastomotic site. Alternatively, the surgical procedure may be a surgical procedure on a cardiac valve and the desired portion of the heart is any portion of the heart improving the surgical exposure to an atrium, aorta or pulmonary artery when the surgical procedure is performed. The support arm may also be used in other surgical procedures using the steps described in the methods above.  
           [0017]    The present invention is directed toward an improved platform or support arm from which to base coronary artery surgery, including beating heart stabilization. The invention is intended to provide the surgeon with a versatile, configurable, rigid base to which task specific accessories may be attached. Designed to be reusable, the present invention also offers a lower cost opportunity per procedure.  
           [0018]    The present invention incorporates a majority of reusable components, with only certain heart contact component used in coronary artery surgery being disposable. This arrangement combines disposable product economy with high quality and highly effective reusable devices, leading to significant cost reduction per procedure for the hospital. Multiple choice disposable heart contact members allows the surgeon to choose the optimum attachment to suit the procedure and anatomy. Furthermore, the extensive adjustability of the device, including the ability to vary the length of the articulated arm, vary the orientation of the contact member, rotate the contact member axially of the articulated arm axis, and rotate the articulated arm about the base, allows the surgeon access to the entire heart, including the entire external surface of the heart. A desired configuration of all the adjustable elements can then be fixed securely with a single control element, namely the knob at the proximal end of the articulated support arm. A clutch mechanism in the knob prevents over stressing of the components. The stainless steel construction of the device along with the roughened and hardened link interfaces, provides exceptional rigidity and a solid support for any portion of the heart.  
           [0019]    The instrument has utility in coronary artery bypass surgery carried out using cardio-pulmonary bypass. In such procedures the support arm for cardiac surgery, with a suitable stabilizing foot, may be used to retract the stationary heart to produce satisfactory surgical exposure of the anastomotic site without the necessity for a surgical assistant who otherwise be required to hold the heart in position, whether using hand held retractors or holding the heart directly. The elimination of this assistant has several advantages to the surgeon and hospital. For example, the surgeon has more freedom in scheduling the time of the operation, which otherwise, depends on the availability of an assistant. A second advantage is that the cost of assistant fee is saved, as well as various sundry hospital costs such as reduced laundry and disposable garment costs.  
           [0020]    In addition, the instrument has utility in cardiac valve surgery. In such procedures the support arm for cardiac surgery with a suitable retractor foot, may be used with a simple sternal retractor, to retract the aorta or left and right atrium to produce satisfactory surgical exposure of the cardiac valve without the necessity for an surgical assistant who otherwise be required to expose the valve use using hand held retractors. In such circumstances the use of this support arm for cardiac surgery has the potential for reducing operating room staff and otherwise saving costs. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    [0021]FIG. 1 is a perspective view of a support arm according to the present invention.  
         [0022]    [0022]FIG. 2 is an exploded view of the support arm of FIG. 1.  
         [0023]    [0023]FIG. 3 is a cross-sectional side view of the support arm of FIG. 1.  
         [0024]    [0024]FIG. 4A is an enlarged exploded view of the ball-receiver assembly of FIG. 1.  
         [0025]    [0025]FIG. 4B is an enlarged cross-sectional view of the ball-receiver assembly of FIG. 1 with a typical cardiac stabilizing member snapped into position but not locked.  
         [0026]    [0026]FIG. 4C is an enlarged cross-sectional view of the ball-receiver assembly of FIG. 1 with a typical cardiac stabilizing member locked into position.  
         [0027]    [0027]FIG. 5 is an exploded view of the torque-limiting knob system of FIG. 1.  
         [0028]    [0028]FIG. 6 is an enlarged exploded view of the clamp base and portion of the articulating arm it engages of FIG. 1.  
         [0029]    [0029]FIG. 7 is an enlarged exploded view of an alternative embodiment of the torque-limiting knob system and of the clamp base assembly of FIG. 1.  
         [0030]    [0030]FIG. 8 is a cross-sectional side view of the torque-limiting knob system and of the clamp base assembly of the alternative embodiments of FIG. 7.  
         [0031]    [0031]FIG. 9 is a plan view of the support arm according to the present invention mounted on the rack of a sternal retractor, with an epicardial stabilizer attached by the ball receiver. The arm is shown rotated about the clamp base.  
         [0032]    [0032]FIG. 10 is a side elevation of the support arm according to the present invention with the arm articulated and advanced. An epicardial stabilizer is shown mounted in the ball retainer.  
         [0033]    FIGS.  11 A- 11 C are perspective view of representative contact members having varying configurations. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0034]    A support arm assembly  100  is comprised of a clamp base  200  an articulated arm  300  and a locking mechanism assembly and torque limiting mechanism  400 .  
         [0035]    Referring to FIG. 2, the clamp  200  base provides a mechanism for attachment by clamping onto the arms or rack of common or specialized sternal retractors (as illustrated in FIG. 9). The clamp base  200  includes a foundation  205  having a pair of spaced grip fingers  206   207 , a transverse cylindrical recess  208  at right angles to fingers, not shown, and a vertical central through hole  209 . The upper face  210  has tapered recess  211 . Opposing clamp base foundation  205  is clamp hook  220  having a grip finger  222  which cooperates with fingers  206 ,  207  for gripping onto the retractor features. The clamp base  200  also includes a pivot  215  defining a pivotal axis  216  for the articulating arm to be radially positioned relative to clamp base  200 . In the present embodiment, the pivot axis is oriented vertically relative to the clamp base foundation  205  and the chest of a patient on whom a procedure is performed and it may be referred to herein as a vertical axis, but this is not intended to limit potential orientations. The pivot  215  has an annular head groove  217  at its upper end, a transverse through hole  218  near its lower end and a concentric countersunk axial hole  219  from its lower face to intersect transverse hole  218 . Clamp hook grip body  220  has a small side hole  221  in one side near a proximal face, and an annular groove  225  near a distal flange  226 .  
         [0036]    The clamp base  200  includes has a knob  228  and an internally threaded body  230  having a snout  231  at its distal end with a cross hole  232 . Knob  228  has cross holes  234  and is joined to threaded body  228  by a pin  235 . The pin  235  is free to rotate in hole  232  but is held in an interference fit in holes  234 . Thus knob is free to partially pivot about the axis of pin  235 , which is securely retained in knob  228 . A clamp shaft  238  has a threaded portion  239 , an unthreaded portion  240 , an axially extending slot  241  in the threaded portion  239  and a threaded hole  242  offset 90° from the slot  241  near a free end of the unthreaded portion  240 . The threaded body  230  threadably receives the threaded portion  239  of the shaft  238 . The clamp hook  220  also slidengly axially receives the threaded portion  239  of the shaft  238 . The flange  226  of the clamp hook  220  is received in an aperture  230 A in the distal end of internally threaded body  230  and secured therein by C clamp  250  received in an annular groove  251  in the aperture  230 A of the threaded body, as shown in FIG. 3. Pin  252  received in slot  241  and a corresponding hole  221  in the clamp hook  220  prevents the clamp hook  220  from rotating relative to clamp shaft  238 , but allows it to move axially. The pivot  215  is retained in clamp base foundation  205  by the plain proximal portion  240  of shaft  238  which is received in the transverse hole  218 . The shaft  238  is in turn retained in position by screw  253  received in hole  219  of pivot  215  which and threadably received in the threaded hole  242 , preventing axial or rotational movement of shaft  238  relative to the clamp base. Pivot  215  is concentrically received by a slide cradle  255  having a cylindrical concentric bore  256 , a partial elongated semi-circular recess  258  at one end with sides  259  to slideably retain slide loop  265  without undue clearance. It is important that the semi-circular recess  258  be elongated and sufficiently deep to provide clearance for a lower quadrant of slide loop  265 . Slide loop  265  has slot  266  in its lower quadrant. The slot has a flat uppermost recess  267  with vertical sides. The width of the slot is such that it will slide onto groove  217  at the upper end of pivot  215 . Slide loop  265  has an upper, inner quadrant designated  268 . Slide cradle  255  has a male taper  270  on its lower end that mates with female taper  211  on clamp base  205  to provide a self-releasing but high torsional friction interface.  
         [0037]    In conjunction with pivot  215 , slide loop  265  serves as an axial locking component, and prevents rotation of articulated arm  300  about the pivot axis  216  when the articulated arm  300  is fixed, as described below. The articulated arm  300  provides the versatility and flexibility to be translated and configured into proper configuration for optimal positioning, and then locked into configuration by the turn of a knob. The act of locking the arm also fixes the axial and radial position of the arm and the position of the installed end-accessory. The arm includes a socket housing or receiver  301  that accepts the attachment features of the attachable devices. The present embodiment contemplates this attachment feature to be a ball analogous in form to the ball of a common trailer hitch.  
         [0038]    The articulated arm  300  further includes a section of nested articulating links  307 , a rigid tubular section comprising a ramp body  360 , a machined ramp  370  and an internal hexagonal spacer  460 . The ramp body  360  and the machined ramp  370  interface with the clamp base  200 , and has a torque-limiting knob system  400  for tightening.  
         [0039]    [0039]FIG. 4A is an enlarged exploded view of the ball-receiver assembly  301  of FIG. 1. FIG. 4B is an enlarged cross-sectional view of the ball receiver assembly  301  of FIG. 1 with a typical disposable stainless steel stabilizing cardiac member  900  (having ball fixation pillar  902  and with non-shedding Velcro® high friction cardiac contact surface  901 ) snapped into loose retention (unlocked position). Referring to FIGS. 4A and 4B, socket housing or receiver  301  has a ball cavity  302  that snap-fit engages the ball feature  902  of the device to be attached. The snap-fit is accomplished by a plunger  305  having a ball-ended pin  303  which is received in a cylindrical cavity  301 A in the receiver  301 . The plunger  305 , including the pin  303 , moves axially against the biasing force of spring  304  within cylindrical cavity  301 A to allow the attachment to enter the ball cavity  302  and to thereafter hold the ball within the cavity. The receiver  301  is pinned through dowel holes  311  by dowel pin  315 , fixing it to a cable end  320 , such that the receiver  301  and cable end  320  always move together. The cable end  320  is crimped onto the cable  325 . The receiver  301  also has a partial-depth slot  306 , for visual reference opposite of the entry to the ball cavity  302 . A spring plunger  330  partially resides within the receiver  301  and is movable axially thereof.  
         [0040]    The cable end  320  fits axially within a bore of the spring plunger, and is located by dowel pin  315  through axially extending slots  337 , in the spring plunger, which allows for axial translation of the cable end  320  relative to the spring plunger  330  but constrains radial rotation. The spring plunger  330  has a snout  332  that protrudes through and internal bore of the ball receiver  301  into a ball cavity  302 . The snout may have an end a face  335  having a simple circular recess or the face may be highly textured. Both embodiments ensure secure incidence onto the ball feature of the attachment. Spring plunger  330  is preferably made of a corrosion resistant hardened stainless steel (such as 17/4 PH or 420 or 440 C) suitable heat treated and preferably passivated by electopolishing. In general it is desirable that the spring plunger  330  be harder than the “trailer hitch” stainless steel ball  902  used on the attachment. In use, with tension applied to the cable  325  the receiver  301  is drawn to the right, as shown in FIG. 4C. This causes the snout  332  to extend further into the ball cavity  302 , securing the ball therein. On the proximal end, the spring plunger  330  has a concave spherical surface  338  that mates to the convex spherical surface  340  of the first link  307  of a series of identical links.  
         [0041]    The links  307  are all identical components, preferably made of a 300 series stainless steel with a convex spherical shoulder  340  of approximately 0.312 inches radius of curvature, but this is not critical and other radii may be used with similar effect. Each link  307  has an axially offset annular concave spherical surface  338  opposite the convex shoulder  340 . The surface of the convex shoulder  340  of each link is roughened and hardened to induce a preferential friction relationship against the concave spherical surface  338  of the adjacent link. Alternatively, the concave spherical surface could be roughened and hardened. Roughening is readily accomplished by sand blasting using a suitable grit size. Alternatively spherical surfaces roughening may be by glass beading. A hard surface may be applied to the roughened surface by surface treatment such as the deposition of Titanium Nitride on the roughened surface. It is desirable to have the mating spherical surface smooth and uncoated. The number of links used may be varied depending upon the size of the link used and the overall desired length of the articulating arm. In the preferred embodiment of the invention  12 - 14  links are used, but the number is not critical. The ultimate link  307 A mates with arm-tube transition member  350  (see FIG. 2) which has a convex spherical surface  340  (of the same spherical radius as surface  338  of the links  307 ) to mesh with concave hemispherical surface  338  of the ultimate link. On its other end transition member  350  has a cylindrical recess  351  to accommodate ramp driver  352 , which is interference fit in a counter-sink within cylindrical recess  351 . An axial concentric hole  353  through arm-tube transition member  350  allows unimpeded cable movement. Likewise, an axial concentric hole  354  through ramp driver  352  allows unimpeded cable movement. Ramp body spring  356  is a clearance fit on ramp driver  352  and a clearance fit in recess  359  of ramp body  360 . The internal diameter of cylindrical recess  351  in arm-tube transition member  350  is such that it is a sliding fit on the outside diameter of ramp body  360 . A small hole  355  located in the lower quadrant of ramp driver  352  is to receive the plain end  357 a of machine ramp screw  357 .  
         [0042]    The ramp body  360  (shown in partial cut-away in FIG. 6 and shown in part cross-section for clarity in FIG. 2) has an uppermost surface designated  367 , an axial hole  361  for unimpeded cable movement, a cylindrical recess  362  to slidably accommodate ramp driver  352 , a second cylindrical recess  359  to accommodate spring  356  and a threaded nipple  368  (See FIG. 2). The ramp body also has a longitudinally machined slot  363 A approximately 0.250 inches in width. Ramp body  360  further has a forward machined incline plane  363  and a parallel rearward inclined plane  364  separated by a planar face  366  parallel to a longitudinal axis of the ramp body  360 .  
         [0043]    The machined ramp  370  has a lower surface  371  and an axial through hole  372  for unimpeded passage of cable  325 , and is of such width as to closely slidably fit in the slot  363   a  in ramp body  360 . A small tongue  373  with a threaded hole  374  protrudes from the front of machined ramp  370 . Machined ramp  370  has a forward incline plane  375  and a parallel rearward inclined plane  376  separated by a planar face  377  which is parallel to a longitudinal axis of the through hole  372  and lower face  371 . Machine ramp  370  moved by the ramp driver  352  by the plain section  357   a  of screw  357  (See FIG. 3).  
         [0044]    When the cable  325  becomes under tension the links  307  are brought into compression. As the cable tension is increased the mating spherical surfaces  340  and  338  are bound into a frictional lock. As the tension in the cable  325  is reduced, the absolute frictional force between the links is also reduced, and the links  307  will again move relative to each other. In use the cable tension can be adjusted so that the links  307  maintain a select position unless moved by the user and then can be locked into place by increasing the cable tension. The links have a clearance  341  cut into the belly to provide clearance for the shoulder  340  as the adjacent links translate with respect to one another. The cable  325  of the system runs through the tapered bore  345  of the links. The tapered bore  345  provides bending relief for the cable  325  between the adjacent links  307 .  
         [0045]    The ramp body  360 , machined ramp  370 , slide loop  265 , slide cradle  255 , pivot  215 , and clamp base  205  cooperate to define an axial clamp for the articulated arm as well as a vertical constraint system referenced on the clamp base  200 . The slide loop  265  and pivot  215  are tensile elements of this system, and the slide cradle  255 , machined ramp  370 , and ramp body  360  are compressive elements. At rest (no tension in cable  325 ), the system is freely movable and unlocked. As tension develops in the cable  325 , the arm-tube transition  350  and the ramp driver  352  and hence the inclines  375 ,  376  of the machined ramp  370 , are drawn against and along the ramp body  360  inclines  363 ,  364 .  
         [0046]    This effectively lengthens the chord  367  to  371  (the distance from the top of the ramp body  360  to the bottom of the machined ramp  367 ) in relation to the tension in the cable  325 . The complimentary chord from the top flat surface  257  of the slide cradle  255  to the upper inner quadrant  268  of the slide loop is held constant by the anchoring of the slide loop  265  onto the head groove  217  of the clamp base pivot  215  which is locked into the clamp  205  by the clamp shaft  238 . A binding compression develops through the stack of the ramp body  360 , machined ramp  370 , slide cradle  255  and clamp base  205  as the cable  325  is tightened locking the articulating arm assembly from sliding axially through the slide loop  265  and locking the arm/slide loop( 265 )/slide cradle( 255 ) assembly axially and from rotating around the vertical pivot  215 . The ramp driver  352  and arm tube transition  350  are predisposed to move away from the ramp body  360  by the ramp body spring  356 , such that the system unlocks when the tension on cable  325  is relieved.  
         [0047]    The articulating arm assembly is locked by imparting tension in cable  325 , creating the locking binding in the link and ramp body assemblies. This tension is imparted through the torque-limiting knob assembly  400  illustrated in FIGS. 3 and 5. The machined wing  405  has an internal bore  410  having a non-circular cross-section that axially receives a spring base  415  and a knob driver  418  each having complimentary recesses  421 ,  422  respectively in their outer-diameters (See FIG. 5) which mate with the internal bore  410  to allow for axial movement of these pieces relative to the machine wing  405 , but which fixes them radially relative to the machined wing  405 . A larger diameter distal position of the internal bore  410  similarly has such a non-circular cross-section  420  to similarly mate with a knob cover  430 . With this constraint, the wing  405 , knob cover  430 , spring base  415  and knob driver  418  always turn together. Also, the wing  405  and an Acme nut  445  are constrained axially, as the wing screws  402  fit into a stress relief c-ring  425  located in annular groove  448  in Acme nut  445 . The stress relief c-ring  425  distributes the axial load over about 65% of the circumference of the groove  448 , so that not all the load carried by the quadrant of the two wing screws  402 . Nut  445  utilizes an Acme thread for minimizing friction forces. However, other thread forms could also be used.  
         [0048]    Internal hexagonal spacer  460 , which has an outside diameter similar to that of ramp body  360 , has a fine pitch internal thread  461  that threadably engages with external fine thread  368  on ramp body  360  to tightly lock internal hexagonal spacer  460  to ramp body  360 . The internal hexagonal spacer  460  has an internal hexagonal bore  462  that slideably engages an external hexagonal portion  442  of Acme screw  440 .  
         [0049]    Acme screw  440  has an externally threaded portion  441  that threadably engages an internally threaded portion  446  of Acme nut  445 . As the wing  405  is tightened rotation of the Acme screw is prevented by slidable hexagonal interface  442 / 462 , thus the Acme screw  440  is drawn up into the Acme nut  445 . As a result, the cable  325  is tensioned by a thrust to the right exerted on the Boeing button  450  crimped onto the cable  325  being as shown in FIG. 3. This in turn imparts a force of the Acme nut  445  against the internal hexagonal spacer  460  at the end of the ramp body  360 , which in turn causes compression of the links  307  through the machine ramp  370 , ramp driver  352  and arm/tube transition  350  at the end of the tube assembly. The Acme nut  445  is radially constrained by Acme screw  440  having a hexagonal cross-section  442 , which is fit closely into the hexagonal bore  462  of the internal hexagonal spacer  460 . The internal hexagonal spacer  460  is fixed tight to the ramp body  360  by mating screw threads  368  and  461 .  
         [0050]    The knob driver  418  has at least two axially extending radial driver ramps  419  that nest with corresponding axially depressed radial nut ramps  447  in the Acme nut  445 . As the torque increases, the knob driver  418  tends to move up off of the Acme nut  445  due to the angle of the ramps. The force of the Belleville springs  470  against the knob driver  418  counters this tendency. When the force of the knob driver  418  moving up the ramps  447  allows enough translation for the knob driver  418  to run all of the way up the ramps  447 , it slips out of radial constraint with the Acme nut  445 , and the wing knob assembly simply turns, without further tightening the Acme nut  445 . The breakaway torque is adjusted by a set screw  480  in the wing  405 , which forces the spring base  415  against the Belleville springs  470 , increasing the preload in the springs.  
         [0051]    An alternative and preferred embodiment to clamp base  200 , ramp operating mechanism, articulating links and torque limiting mechanisms are shown in FIG. 7 (an exploded view of the clamp base, ramp and torque limiting assemblies) and FIG. 8 (a mid cross-sectional view of FIG. 7 including several articulating links  607 .) Identical elements will have the same reference numbers used above. In FIG. 7 wing  405  is shown rotated through 90° for clarity of illustration. Clamp foundation base  505  has a hexagon recess  560  concentric to opposing cylindrical recess  508 , and extending from short cylindrical recess  562  to intersect with vertical central through hole  509 . Clamp shaft  538  has a threaded portion  539  with a longitudinal slot  541 , a plain portion  540 , a hexagonal portion  580  and a short flanged head  582 . The threaded portion  539  may conveniently be a {fraction (1/4)} UNC thread, but this is not important and other screw sizes and thread pitches could be used. Hexagonal portions  580  and  560  are oriented such that slot  541  lies in a horizontal plain, thus ensuring that upon assembly when pin  252  in clamp hook  220  is engaged in slot  541  so that opposing finger  222  will lie in a vertical plain. Shaft  538  may conveniently be fixed in position by the application of a heat resistant thread locking compound such as LOCTITE®  272  to hexagonal portion  580 .  
         [0052]    With continued reference to FIGS. 7 and 8, in an alternate ramp assembly, machined ramp  670  has tongue  673  and rectangular pillar  680 . Arm tube transition member  650  has a cylindrical recess  656  which is a sliding fit on ramp body  360 . A through hole  653  in transition member  650  allows for unimpeded passage of cable  325 . A tapered recess  651  allows an adjacent articulating link  607  to partially rotate about a spherical convex surface  654  without binding of cable  325 . Articulating link  607  has a tapered annular buttress  610  to strengthen the link.  
         [0053]    Arm tube transition member  650  has protruding integral ramp driver  652  with a planar end  655  which contacts planar end  672  of machined ramp  670 . A through hole  662  in planar face  672  allows for unimpeded cable movement. Protruding integral ramp driver  652  has a Tee shaped slot  674  to receive tongue pillar  680  of machine ramp  670 . When the cable comes under tension planar end  655  of integral ramp driver presses against planar end  672  of machine ramp that causes chord distance  367  to  671  to increase, thus effectively widening locking ramp body  360  relative to clamp base foundation  505 . The ramp driver  652  and arm tube transition  650  are predisposed to move away from the ramp body  360  by the ramp body spring  356 , such that the system unlocks when the tension on cable  325  is relieved.  
         [0054]    Internal hexagonal spacer  760  has annular groove  745  with a rearmost face  747  to slideably retain stress relief c-ring  746 . The width of the annular groove is wider than that of the c-ring by a predetermined amount of approximately 0.010 inches. Two small set screws  750  threadably engage internal threaded holes  752  of knob cover  730 . As wing  405  is backed off, and approximately three complete turns after cable  325  becomes slack, c-ring  746  will contact rearmost portion  747  of annular groove  745 , thus preventing further counterclockwise rotation of wing  405  and the removal of the wing and torque locking member during routine use or cleaning. C-ring  746  distributes the axial load over approximately 65% of the circumference of the groove  745  of the hex spacer  760 , so that it is not all carried by the quadrant of the two set screws  750 . The knob driver  718  mates to the Acme nut  740  with two pairs of opposed radial ramps  747  and  719  in the same manner as discussed above with respect to  419 ,  447 . As the torque increases, the knob driver  718  tends to move up off of the Acme nut  740  due to the angle of the ramps. Acme nut  740  and knob driver  718  may be a hardened yet corrosion resistant stainless such as grade 17/4 PH hardened to 42 Rockville C hardness. Subsequent electropolishing produces further corrosion resistance. These member may also have a vacuum deposited hard surface finish, such as Titanium Nitride to prevent galling.  
         [0055]    [0055]FIG. 9 shows a plan view of the support arm for cardiac surgery  100  attached to a rack  801  of a sternal retractor  800 . A typical cardiac contact member  900  is shown locked in position.  
         [0056]    [0056]FIG. 10 illustrates the support arm for cardiac surgery with certain links articulated and a typical cardiac contact means  900  is shown locked in position.  
         [0057]    The support arm assembly is preferably manufactured of a corrosion resistant stainless steel, although other suitable metals, such as Titanium could be used. Alternatively, the instrument could be made of a suitable plastic of composite material that has sufficient hardness and durability and that could be sterilized in a steam autoclave or using a Ethylene Oxide gas as a sterilization means.  
         [0058]    The support arm assembly is useful for a variety of cardiac surgery techniques including coronary artery bypass surgery carried out using cardio-pulmonary bypass and cardiac valve surgery. Such procedures begin with making an incision in the patient&#39;s chest, more particularly at the patient&#39;s sternum. A sternal retractor  800  is inserted into the incision and opened to provided access to the heart. The support arm assembly  100  is attached to the rack  801  of the sternal retractor  800  as illustrated in FIG. 9. The knob  228  is tightened to bring the fingers  206 ,  208  and  222  into secure engagement with the rack  801 . The knob  400  is maintained loose to allow for adjustment and configuration of the articulated arm  300 . The physician then selects one of the interchangeable contact members depicted in FIGS.  11 A- 11 C and inserts the ball into the receiver where it is prevented from falling out by the spring biased finger  303 . The surgeon can then apply some tension to the cable  325  turning the wing  405  in a first direction. Sufficient tension is applied to allow for movement of the articulated arm  300  about its various adjustments as discussed above so that the surgeon can bring the contact member into contact with a desired portion of the heart. The articulated arm  300 , including the rigid portion of the articulated arm  300 , is long enough to provide access to any portion of the patient&#39;s heart. Once the surgeon has configured the articulated arm  300  as desired, oriented the contact member as desired, rotated the support arm about the base as desired and axially adjusted the support arm relative to the axial clamp as desired so as to contact the desired portion of the heart, all adjustments of the arm are rigidly fixed to exert and maintain a stabilizing force on the desired portion of the heart simply by turning the wing  405  further in the first direction to fully tension the cable  325 . As discussed above, the clutch mechanism prevents over tensioning of the cable so that the physician can concentrate on patient care without concern for damaging the support arm assembly. As should be readily apparent, where the surgical procedure is a coronary artery bypass graft procedure any anastomotic site on the heart may be accessed and exposed using the apparatus. Where the surgical procedure is one being performed on a cardiac valve, the support arm apparatus can be positioned anywhere on the heart as desired by the physician to improve exposure to an atrium, aorta or pulmonary artery as the surgical procedure is performed.