Abstract:
An articulated arm for holding surgical instruments, having elongated arm members; rotational joints connecting the arm members end to end, such that members are pivotable about the joints and at least one of the joints is selectively lockable by fluid pressure; and a plurality of tubes communicating fluid pressure between two adjacent joints. When two adjacent members are pivoted relative to one another, the tubes connecting a particular pair of joints remains substantially stationary with respect to the arm member connecting that pair of joints.

Description:
FIELD OF THE INVENTION 
     This invention relates to an articulated arm for holding a tool and more specifically for holding a medical instrument during a surgical procedure. 
     BACKGROUND OF THE INVENTION 
     In the modern practice of medicine, surgeons and other medical personnel often use endoscopic scopes, tissue retractors, and other medical instruments that must be kept steady for extended periods of time during their use on a patient. Traditionally, it may be the job of a nurse or surgical assistant to hold the instrument. However, it can be quite difficult for a person to hold the instrument steady for extended periods of time due to fatigue. Also, many of these instruments, such as tissue retractors, require great amount of force to be applied, which can rapidly lead to extreme fatigue as well as make it very difficult to keep the instrument adequately steady. 
     For this reason, a number of mechanical support devices have been devised to accomplish these otherwise manual tasks. One such device, as disclosed in U.S. Pat. No. 3,858,578 to Milo, consists of an arm-like structure that is made up of a series of ball and socket members held together by a cable. One end of the arm is rigidly attached to a support structure, such as an operating table rail. The opposite end supports a surgical tissue retractor. Initially, the arm is flexible and can be positioned as necessary. When the interconnecting cable is tightened by applying a fluid pressure to a piston attached at one end, the arm becomes rigid until the cable is manually released by discharging the fluid. 
     There are, however, many drawbacks to this and other similar prior art devices. Generally, the locking force applied to these devices is minimal, making them suitable only for very lightweight medical applications. Those devices which are capable of withstanding greater forces are generally bulky, complex and cumbersome to operate. For example, see U.S. Pat. No. 4,863,133 to Bonnell and U.S. Pat. No. 5,184,601 to Putman. 
     Another disadvantage that applies specifically to prior art fluid operated devices is reliability. These devices are prone to leakage, which even in a small amount can cause the locking mechanism to give out over an extended period of use. This leakage is often caused by flexible tubing which has failed due to repeated flexing. Further, if fluid pressure is lost completely during a medical procedure, the device may disengage completely, without warning, potentially causing injury to the patient. To solve this problem, some such devices have been designed to lock by default and require positive fluid pressure to unlock. While this approach provides fail-safe locking and eliminates the problem of slow leakage altogether, if fluid pressure is lost completely, a surgeon may not be able to unlock the device when needed which may also lead to the injury of the patient. 
     Another problem with prior art devices is their method of actuation. Most of the prior art fluid powered devices have a foot-switch which allows for hands-free operation. However, a typical modern operating room will already have several foot switches associated with various pieces of equipment. Thus, it may be exceedingly difficult for a surgeon to quickly locate the correct switch and may lead to the inadvertent release of the device. However, many hand-operated or other types of switches used in the prior art tend to be impractical, since the surgeon generally requires one or more free hands to manipulate the device and to perform other tasks. 
     SUMMARY OF THE INVENTION 
     The present invention provides an articulated arm for holding surgical instruments which comprises a plurality of elongated arm members and a plurality of rotational joints connecting said arm members end to end. The members are pivotable about the joints and at least one of the joints is selectively lockable by positive fluid pressure. A plurality of tubes communicate fluid pressure between two adjacent joints. When two adjacent members are pivoted relative to one another, the plurality of tubes located on each of the members that connect two joints remains substantially stationary with respect to the arm member connecting the pair of joints, such that it is possible for the plurality of tubes to be constructed from a rigid material. 
     As another aspect of the present invention, the articulated arm is provided with a fluid pressure operated friction brake for locking at least one of the joints. The friction brake includes a substantially frustoconically shaped engaging surface. 
     As a further aspect of the present invention, the arm is provided with a check valve that prevents inadvertent unlocking of the joints in the event that a source of fluid pressure is interrupted. 
     As an even further aspect of the present invention, a plurality of isolated fluid paths is provided to each joint and each of the fluid paths within each joint is isolated from the others within the joint. These fluid paths allow fluid pressure to be communicated through the joint without the need for flexible tubing. 
     As a still further aspect of the present invention, the articulated arm is provided with a fluid switch located near a distal end of the arm for selectively controlling the fluid pressure within at least one of the joints, thereby unlocking the joint or joints. All of the plurality of joints may be unlocked by operating the single fluid switch. 
     As a yet further aspect of the present invention, wherein an arm member is pivotable with respect to at least one of the joints by greater than 360 degrees. 
     As a yet further still aspect of the present invention, a motion-limiting mechanism is provided to at least one of the joints to prevent the inadvertent collapse of the arm. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     It should be appreciated that the depiction of the present invention in the following described drawing figures may not be shown to scale and further may be partially schematic as necessary for the purpose of illustration. 
     FIG. 1 is a perspective view of an articulated arm for holding surgical instruments according to the present invention shown attached to an operating table and being manipulated by a surgeon; 
     FIG. 2 is an exploded perspective view of the articulated arm of FIG. 1; 
     FIG. 3 is an exploded view of a rotary shoulder joint of the articulated arm of FIG. 1; 
     FIG. 4 is a sectional view taken along a section line  4 — 4  shown in FIG. 6; 
     FIG. 5 is a sectional view taken along a section line  5 — 5  shown in FIG. 6; 
     FIG. 6 is a top view of the rotary shoulder joint of FIG. 3; 
     FIG. 7 is a side view of the rotary shoulder joint of FIG. 3; 
     FIG. 8 is an exploded view of a rotary base joint of the articulated arm of FIG. 1; 
     FIG. 9 is a sectional view taken along a section line  9 — 9  shown in FIG. 10; 
     FIG. 10 is a top view of the rotary base joint of FIG. 8; 
     FIG. 11 is a side view of the rotary base joint of FIG. 8; 
     FIG. 12 is a side view of a wrist assembly of the articulated arm of FIG. 1; 
     FIG. 13 is a sectional view taken along a section line  13 — 13  shown in FIG. 12; 
     FIG. 14 is a cut-away view of a portion of the articulated arm of FIG. 1 showing a counter balance mechanism; 
     FIG. 15 is a perspective view showing a shoulder joint of the articulated arm of FIG. 1 with an attached counter balance cable; and 
     FIG. 16 is an elevation showing the articulated arm of FIG. 1 in a folded position. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows an articulated arm  10  of the present invention as it might be used during a surgical procedure by a surgeon  12 . The arm  10  is shown secured to a side rail  14  of a standard operating table  16  using a universal rail clamp  18 . A fluid supply stem  20  is provided at a proximate end  22  of the arm  10 . The stem  20  serves both to support the arm  10  in the clamp  18  and to supply the arm  10  with fluid pressure that is used as a source of power. A quick-disconnect type connector  24  is provided on the stem  20  allowing for a connection to an external source of fluid pressure (not shown). In the present embodiment, a compressed gas, such as nitrogen, carbon dioxide or compressed air, is used as the source of fluid pressure to provide power to the arm  10 . A supply of such a compressed gas is readily available in most modern operating rooms, making it a convenient source of power. 
     As shown in FIGS. 1 and 2, the articulated arm  10  comprises a plurality of elongated articulating arm members  26 ,  28 ,  30 , including a torso  26 , an upper arm  28  and a forearm  30 . The arm  10  also comprises a plurality of rotational joints  32 ,  34 ,  36 , including a base joint  32 , a shoulder joint  34  and an elbow joint  36 , which connect the arm members  26 ,  28 ,  30  to one another, end to end. As a result, the arm members  26 ,  28 ,  30  are each pivotable about the joints  32 ,  34 , or  36  to which they are attached. 
     The torso  26  provides an advantage over prior art devices in that it provides a vertical offset to the shoulder joint  34 . In a surgical application, such as that shown in FIG. 1, the arm  10  would normally be secured to the rail  14  of the operating table  16 . The combined heights of a mattress placed on top of the table  16  and the patient&#39;s body act to raise the level of the table several inches or more beyond the rail  14  where the arm  10  is attached. The vertical offset of the shoulder joint  34  allows the arm  10  to easily clear the patient, providing for unobstructed movement of the arm  10 . Further, the design of the torso  26  provides some horizontal offset to the shoulder joint  34 . This horizontal offset may help the surgeon  12  to appreciate the range of movement that is available between the torso  26  and the upper arm  28  and the limits of a counter balance  188  (described in detail below), by providing directional orientation. In one sense, the offset portion of the torso  26  appears to be pointing in a direction that indicates a full extension of the upper arm  28  relative to the torso  26 . The horizontal offset may also further enhance patient clearance of the arm  10  as well as adding to the overall reach of the arm  10  without increasing the structural requirements of the shoulder joint  34 . 
     As shown in FIG. 16, the arm  10  can be folded to a compact size. This may be particularly useful for storage and transportation of the arm and may also allow it to be placed into an autoclaving chamber or other sterilization equipment. 
     The specific relative dimensions of the arm  10  and its subcomponents as disclosed in FIGS. 1-16 are shown only by way of example. It should be appreciated that the lengths of the arm members  26 - 30  can be altered to meet the demands of particular applications. 
     All of the rotational joints  32 ,  34 ,  36  are substantially similar, and thus only the shoulder joint  34  will be described in detail. Where appropriate, any differences between the shoulder joint  34  and the other joints  32 ,  36  will be explained. 
     As shown in FIGS. 1 and 2, the arm  10  is also provided with a wrist assembly  38  at a distal end  40  of the arm  10 . The wrist assembly  38  comprises a distal fluid switch  42 , a ball joint  44  and an instrument stem  46 . 
     The rotational joints  32 ,  34 ,  36  and the ball joint  44  allow the arm  10  to be selectively positioned in an infinite number of poses within a given field of range. Each joint  32 ,  34 ,  36 ,  44  is releasably locked in response to pressure from the external source of fluid pressure. Thus, the surgeon  12  can position the instrument stem  46  in a given location within the operating field and the arm  10  will remain locked in place, resistant to movement by external forces. The instrument stem  46  is provided with a quick-disconnect type connector  48  to allow different surgical instruments, such as a tissue retractor or an endoscope holder, to be attached. 
     In a typical scenario, such as that illustrated in FIG. 1, a surgical instrument is attached to the connector  48  by the surgeon  12 . The surgeon then unlocks all of the joints  32 ,  34 ,  36 ,  44  by pressing and holding a push-button  50  provided on the distal switch  42 . After positioning the instrument in a desired location and orientation by manipulating the articulated arm  10 , the surgeon simply releases the push-button  50  and all of the joints  32 ,  34 ,  36 ,  44  are locked by the fluid pressure. The instrument may be repositioned at any time by pressing, holding and subsequently releasing the push-button  50 . Since the push-button  50  is located at the distal end  40  of the arm  10 , it is possible for the surgeon  12  to unlock and position the arm  10  using only one hand. This may allow the surgeon  12  to keep other hand free for tasks such as positioning or operating the surgical instrument. 
     As previously mentioned, the joints  32 ,  34 ,  36 ,  44  are locked by positive fluid pressure provided by the external fluid pressure source. Each joint  32 ,  34 ,  36 ,  44  is subsequently unlocked when the fluid.pressure is sufficiently reduced by the discharge of fluid. As shown in FIGS. 12 and 13, for this purpose, the distal switch  42  is provided at the distal end  40  of the arm  10 . The rotational joints  32 ,  34 ,  36  are all connected as a closed-loop in series with the external fluid supply and a fluid valve  52  of the distal switch  42 . The valve  52  is operated by the push-button  50  and is spring-biased to a normally open position. In this open position, the valve  52  transmits fluid pressure from the external source to the rotational joints  32 ,  34 ,  36  and also transmits fluid pressure to the ball joint  44 . When the push-button  50  is pressed, the valve  52  is caused to simultaneously block the fluid pressure source and to vent all of the joints  32 ,  34 ,  36 ,  44  to a lower pressure environment to discharge some of the fluid. In the absence of positive fluid pressure, the joints  32 ,  34 ,  36 ,  44  unlock. 
     As will be described in detail hereafter, in the present embodiment, a series of fluid carrying tubes and passages communicate fluid pressure throughout the arm  10 . The tubes primarily carry fluid between the joints  32 ,  34 ,  36 ,  44 . The passages are provided, in part, to carry fluid from one tube, through one of the joints  32 ,  34 ,  36 ,  44 , to another tube. As a result, the need for flexible tubing used in prior art devices that carries fluid around a joint, and is repeatedly flexed as a result of joint rotation, has been eliminated. The tubes that are provided in the present invention remain substantially stationary during operation and do not substantially interfere with the rotation of the joints  32 ,  34 ,  36 . 
     When the external source of fluid pressure is connected to the fluid supply stem  20  at the connector  24 , fluid travels through the stem  20  and into a first fluid inlet  54  of the base joint  32  (see FIG.  9 ). A first fluid passage  56  is provided in the base joint  32  which carries the fluid to a fluid outlet  58  (see FIG.  9 ). 
     The fluid then passes from the fluid outlet  58  of the base joint  32  into a first fluid supply tube  60  that extends through the torso  26  and then into a first fluid inlet  62  of the shoulder joint  34  (see FIG.  4 ). A first fluid passage  64  is also provided to the shoulder joint  34  which carries the fluid to a first fluid outlet  66  (see FIG.  4 ). 
     The fluid then passes from the first fluid outlet  66  of the shoulder joint  34  into a second fluid supply tube  68  that extends through the upper arm  28  and then into a first fluid inlet (not shown) of the elbow joint  36  (see FIGS.  2  and  4 ). A first fluid passage (not shown) provided to the elbow joint  36  carries the fluid to a first fluid outlet  74  (see FIG.  2 ). As noted above, details of the elbow joint  36  that are not shown are substantially the same as those of the shoulder joint  34 . 
     The fluid then passes from the first fluid outlet  74  of the elbow joint  36  into a third fluid supply tube  76  that extends through the forearm  30 , through a one-way check valve  78  (shown schematically) and then into a fluid inlet  80  of the distal switch  42  in the wrist assembly  38  (see FIGS.  12  and  13 ). The check valve  78  acts as a fail safe, preventing fluid pressure to the joints  32 ,  34 ,  36 ,  44  from being lost if the supply pressure is inadvertently interrupted. However, as will be described below, the check valve  78  is positioned directly in front of the distal switch  42 , which allows the joints  32 ,  34 ,  36 ,  44  to be unlocked during a situation involving a loss of source pressure (see FIGS.  12  and  13 ). Although a particular location of the check valve  78  has been disclosed, it should be appreciated that the check valve  78  could be alternatively positioned at any point between the proximate end  22  of the arm  10  and the distal switch  42  and still function as desired. 
     As best shown in FIGS. 12 and 13, when the push-button  50  of the distal switch is not depressed, the valve  52  is in an open position and fluid pressure is transmitted from a first fluid outlet  82  of the distal switch  42  to a fluid inlet  84  of the ball joint  44 . Also, when the valve  52  is in an open position, fluid is supplied from a second fluid outlet  86  of the distal switch  42  into a first fluid return tube  88  that extends through the forearm  30  and then into a second fluid inlet  90  of the elbow joint  36  (see FIG.  2 ). A second fluid passage (not shown) is provided to the elbow joint  36  which carries fluid through a friction brake assembly (not shown) and to a second fluid outlet (not shown). Fluid pressure operates the brake (not shown) which, in turn, locks the elbow joint  36 , as will be explained in detail with regard the operation of the shoulder joint  34  below. 
     The fluid then passes from the second fluid outlet (not shown) of the elbow joint  36 , and as best shown in FIG. 5, into a second fluid return tube  98  that extends through the upper arm  28  and then into a second fluid inlet  100  of the shoulder joint  34 . A second fluid passage  102  is provided to the shoulder joint  34  which carries fluid through a friction brake assembly  104  and to a second fluid outlet  106 . Fluid pressure operates the brake assembly  104  which, in turn, locks the shoulder joint  34 , as will be explained in detail below. 
     The fluid then passes from the second fluid outlet  106  of the shoulder joint  34  into a third fluid return tube  108  that extends through the torso  26  and then, as best shown in FIG. 9, into a second fluid inlet  110  of the base joint  32 . A second fluid passage  112  is provided to the base joint  32  which carries fluid to a friction brake assembly  114 . Fluid pressure operates the brake assembly  114  which, in turn, locks the base joint  32 , as will be explained in detail below. 
     As shown in FIG.  12  and described above, when the valve  52  is in an open position, fluid is also transmitted from the first fluid outlet  82  of the distal switch  42  to a fluid inlet  84  of the ball joint  44 . This fluid is carried from the fluid inlet  84  into a friction brake assembly  116  of the ball joint  44  when operates the brake assembly  116  and, in turn, locks the ball joint  44 , as will be explained in detail below. 
     When the push-button  50  of the distal switch  42  is depressed, causing the valve  52  to be in a closed position, the fluid inlet  80  of the distal switch  42  is blocked preventing any fluid from flowing beyond the valve  52 . In this closed position, the valve  52  also connects the first fluid outlet  82  and the second fluid outlet  86  with a fluid discharge port  118 . The discharge port  118  is in communication with an interior cavity  120  of the forearm  30 . Thus, when the valve  52  is closed, much of the fluid in the brake assemblies  104 ,  114 ,  116  (including the brake assembly of the elbow joint  36 , not shown) is discharged into the interior cavity  120  and the joints  32 ,  34 ,  36 ,  44  are thereby unlocked. 
     Since both the supplied fluid and the interior of the arm  10  may not be sterile, the fluid discharged may contain contaminants. Thus, discharging the fluid into the interior cavity  120  of the forearm  30  instead of directly into the surrounding atmosphere may help to prevent contamination of the sterile field around the operating table. As a further compliment, additional tubing establishing a third fluid path (.not shown) that is directed either around or through each of the joints  32 ,  34 ,  36  could be provided to carry the potentially contaminated fluid out of the proximate end  22  of the arm  10  and away from the sterile field of the operating table  16 . 
     As shown in FIGS. 3-7, the shoulder joint  34  comprises a housing  122  and a face plate  124 . The face plate  124  rotates freely within the housing  122  and is held in place by a retaining ring  126 . The retaining ring  126  is secured to the housing  122  by four bolts  127 . Two thrust washers  128 , comprising TEFLON or another suitable material, are provided above and below a flange  129  of the faceplate  124  to reduce friction between the retaining ring  126 , faceplate  124  and the housing  122 . 
     As best shown in FIGS. 3-5, the friction brake assembly  104  consists of a disc brake  130  that slides freely along a shaft  132  of the face plate  124  and is prevented from rotation relative to the face plate  124  by at least one guide pin  134 . The brake  130  has a frustoconical engaging surface  136  which, when forced downward, engages a complementary frustoconical surface  138  on the inside of the housing. A brake biasing spring  140  which biases the brake  130  upward and normally keeps the frustoconical surfaces  136 ,  138  separated. Multiple brake biasing springs (not shown) could also be used in place of the single spring  140 . 
     By using frustoconically shaped surfaces, as opposed to traditional flat disk-shaped engaging surfaces, the braking power of the joints is significantly increased without increasing the overall size of the joints. The presently illustrated embodiment of the arm  10  is designed to hold a minimum of eight pounds of force applied at the distal end  40  while the arm is extended thirty inches in a horizontal direction. Approximately 115 psi of fluid pressure is required to provide sufficient staying power to the joints  32 ,  34 ,  36 ,  44 . This performance is superior to many prior art devices that support only a few pounds of force. 
     As best shown in FIG. 4, the first fluid passage  64  terminates at one end into the first fluid inlet  62  in the housing  122  and at the other end into the first fluid outlet  66  in the face plate  124 . The intermediate portion of the first fluid passage  64  is defined by a horizontal bore  142  and a vertical bore  144  through the face plate  124  and a horizontal bore  146  through the housing  122 . The first fluid passage  64  is isolated from the second fluid passage  102  by a brake o-ring  148  retained by the brake  130  and two shaft o-rings  150  retained by the face plate shaft  132 . 
     As best shown in FIG. 5, the second fluid passage  102  terminates at one end into the second fluid inlet  100  in the face plate  124  and at the other end into a second fluid outlet  106  in the housing  122 . The intermediate portion of the second fluid passage  102  is defined by a horizontal bore  152  and a vertical bore  154  in the face plate  124 , the space between the face plate  124  and the brake  130 , and a notch  156 , a vertical bore  158  and a horizontal bore  160  in housing  122 . In the current embodiment, the housing  122  is machined from a solid piece of metal. Consequently, the vertical bore  158  is machined by drilling through the exterior of the housing  122 , and thus a plug  162  is provided to isolate the bore  158  from the atmosphere. Alternately, the housing  122  could be cast and the bore  158  would be cast in place, thus eliminating the need for the plug  162 . The second fluid passage  102  is further isolated from the atmosphere by a face plate o-ring  164  retained by the face plate  124 . As previously mentioned, the second fluid passage  102  is isolated from the first fluid passage  64  by a brake o-ring  148  retained by the brake  130  and two shaft o-rings  150  retained by the face plate shaft  132 . 
     When fluid pressure is present in the second fluid passage  102  (i.e. when an external fluid pressure source is connected to the fluid supply stem  20  and the valve  52  is in an open position), the fluid forces the brake  130  to slide downward along the shaft  132  of the face plate  124  causing the frustoconical surface  136  to engage with the complementary surface  138  of the housing  122  (see FIG.  5 ). As mentioned above, a frustoconical shape was chosen to enhance the holding power of the joint  34 . The resulting angle of the surfaces  136 ,  138  amplifies the normal force. 
     The construction and operation of the elbow joint  36  is substantially identical to that of the shoulder joint  34 . 
     As shown in FIGS. 8-11, the base joint  32  varies from that of the shoulder joint  34  primarily in that the second fluid outlet has been eliminated and that the first fluid inlet  54  is positioned vertically in the bottom center of a base joint housing  166  to allow for attachment of the fluid supply stem  20  to the first fluid inlet  54 . As a result, the first fluid passage  56  of the base joint  32  is defined by the first fluid inlet  54  in the housing  166  and a vertical bore  168  and the fluid outlet  58  in a face plate  170 . The second fluid passage  112  is defined by the second fluid inlet  110  and a vertical bore  172  in the face plate  170  and the space between the face plate  170  and a disc brake  174 . Other details of the construction and operation of the base joint  32  are substantially identical to those described above with regard to the shoulder joint  34 . 
     As shown in FIGS. 12 and 13, the ball joint  44  comprises a ball  176 , a ball joint sleeve  178  and the friction brake assembly  116 . The sleeve  178  is threaded onto the wrist assembly  38  and retains the ball  176  between the friction brake assembly  116  and an opening  180  in the sleeve  178 . The friction brake assembly  116  comprises a brake  182  and a biasing spring  184 . The brake  182  has an engaging surface  186  that is complementary to a portion of the ball  176 . The biasing spring  184  pushes the brake  182  toward the ball  176  so that the engaging surface  186  and the ball  176  are always in at least minimal contact. This prevents the instrument stem  46  from moving until the surgeon  12  applies force. 
     When the valve  52  is open, the additional force of the fluid provided at the fluid inlet  84  against the brake assembly  116  causes the brake  182  to engage the ball  176  firmly, locking the ball joint  44 . When the valve  52  is closed by the push-button  50 , the fluid is discharged and the ball  176  can be moved by the surgeon by gripping and applying a moderate force to the instrument stem  46 . 
     As shown in FIGS. 14 and 15, the counter balance mechanism  188  is provided to the shoulder joint  34 , in part, to prevent the weight of the upper arm  28 , forearm  30  and wrist assembly  38  from inadvertently collapsing the arm onto the patient when the shoulder joint  34  is unlocked. The counter balance  188  also makes it easier for the surgeon  12  to position instruments, since it reduces the perceived weight of the arm  10 . As shown in FIG. 2, a hard stop or motion-limiting mechanism  190  and a cooperating rib  191  are provided on the torso  26  and upper arm  28  respectively, to limit the range of motion of the shoulder joint  34 . The base joint  32  is permitted to spin infinitely (greater than 360 degrees). 
     Referring again to FIGS. 14 and 15, the counter balance  188  comprises an extension spring  192  and a cable  194 . A first end  196  of the cable  194  is secured to the retaining ring  126  of the shoulder joint  34  using a screw  198 . A hook-shaped first end  200  of the extension spring  192  is secured to a looped second end  202  of the cable  194 . A hook-shaped second end  204  of the extension spring  192  is secured to the upper arm  28  by a pin  206 . 
     FIG. 14 is a cutaway view of the upper arm  28  with the shoulder joint  34  partially removed. Only the retaining ring  126  of the shoulder joint  34  is shown. The retaining ring is in fixed connection with the torso  26  and is rotationally associated with the upper arm  28 . The position of the upper arm  28  reletive to the retaining ring  126  of the shoulder joint  34  shown in FIG. 14, such that the cable  194  is fully extended toward the spring  192 , indicates that upper limit of the counter balance mechanism  188 . This position represents the full extension of the upper arm  28  reletive to the torso  26 . In this position the tension in the spring  192  is minimal so that it just keeps the cable  194  taut. 
     As best shown by both FIGS. 14 and 15, as the upper arm  28  is moved down, such that the rib  191  of the upper arm  28  approaches the stop  190 , the cable  194  is wound around an annular groove  208  in the retaining ring  126 . This winding of the cable  192  shortens the length of the cable  194  extending from the shoulder joint  34  and extending the spring  192 . The resulting tension of the spring  192  offsets at least a portion of the weight of the upper arm  28 , the forearm  30  and the wrist assembly  38 . FIG. 15 shows the cable  192  partially wound around the ring  126 , such that it rests within and is kept aligned by the annular groove  208 . 
     Alternatively, the stop mechanisms and counter-balances of the above-described embodiment could be eliminated, allowing unlimited pivoting (greater than 360 degrees) of the upper arm  28  about the shoulder joint  34 . 
     Although a particular embodiment of the invention has been described in detail, it is understood that the invention is not limited correspondingly in scope, but includes all changes and modifications coming within the spirit and terms of the claims appended hereto.