Abstract:
A positioning system having a base, a manually movable end effector and a first joint interposed between the base and the end effector is disclosed. The first joint may comprise a proximal portion and a distal portion coupled together by magnetic attraction and configured to be intermittently separated by a pressurized gas cushion. The first joint may be configured to be changeable between a movable state and a fixed state. In the movable state, the proximal and distal portions are separated by the pressurized gas cushion and are movable relative to each other. In the fixed state, the proximal and distal portions contact each other and relative movement is thereby impeded. 
     Methods of precisely positioning an end effector may include providing a device having a base, a first joint located distally from the base, and an end effector located distally from the first joint. The first joint may have two portions separated by a gas cushion, the first joint allowing the end effector to be movable with respect to the base. The method may further comprise manually positioning the end effector, and removing the gas cushion to cause the two joint portions to contact each other, thereby locking the end effector in the precise location in which it was positioned.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention generally relates to precise positioning devices, in particular articulating devices that can be moved into a particular configuration and accurately locked in that orientation. 
       BACKGROUND OF THE INVENTION 
       [0002]    The need to precisely position a device occurs in many fields of endeavor. One such field is surgery, in which an instrument may need to be precisely positioned relative to a patient undergoing an operation or diagnostic procedure. For example, there is a need to enable an ear surgeon, such as an otologist, to precisely position and manipulate instruments in an around the structures of the ear, particularly the inner and middle ear. 
         [0003]    Other more general medical applications of precise positioning devices include those that enable medical personnel to adjustably position imaging, therapeutic, and other instruments at desired locations in and near a patient&#39;s body. Such positioning needs arise in ophthalmic, neurological, orthopedic and other medical fields. Even more generally, requirements for precise positioning of an object frequently occur in the field of optics, measurement and manufacturing. 
         [0004]    What is desirable, and not provided by prior art methods and devices, is a means for precisely positioning an object in a particular orientation with a support mechanism, and accurately fixing the object in that orientation. 
       SUMMARY OF THE INVENTION 
       [0005]    According to aspects of the present description, a support device may comprise one or more movable links interconnected by selectively lockable joints. The selectively lockable joints allow the linkage mechanism to be manually or otherwise moved to a desired configuration or position while the joints are unlocked, and then to become a partly or fully fixed configuration when some or all of the joints are locked. 
         [0006]    In some embodiments, a positioning system comprises a base, a manually movable end effector and a first joint interposed between the base and the end effector. The first joint may comprise a proximal portion and a distal portion coupled together generally by magnetic attraction and configured to be intermittently separated by a pressurized gas or other fluid cushion. The first joint may be configured to be changeable between a movable state and a fixed state. In the movable state, the proximal and distal portions are separated by the pressurized gas cushion and are movable relative to each other in at least one degree of freedom. In the fixed state, the proximal and distal portions contact each other and relative movement is thereby impeded. 
         [0007]    In some of the above embodiments, the proximal and distal portions of the first joint may rotate relative to each other in at least two rotational degrees of freedom. The first joint may be a spherical joint. The spherical joint may be capable of movement in three degrees of rotational freedom. In some of the embodiments, the system is configured without a prime mover to position the end effector. 
         [0008]    In some of the above embodiments, the system may further comprise a second joint interposed between the base and the end effector in series with the first joint. The second joint may have at least one degree of rotational freedom. Each of the first and second joints may comprise a separately interruptible pressurized gas cushion. In some embodiments, the second joint comprises a proximal portion and a distal portion, and the distal portion of the first joint is coupled to the proximal portion of the second joint by a rigid link. The rigid link may include an internal channel in fluid communication with both the distal portion of the first joint and the proximal portion of the second joint. The rigid link may comprise two or more ends and at least one orifice at each of the ends, wherein each of the orifices is in fluid communication with the internal channel and a gas cushion of one of the joints, and the internal channel may have a cross-sectional area that is larger than the cross-sectional area of each of the orifices in order to create a plenum chamber to store or damp gas flow. In some of these embodiments, the internal channel cross-sectional area is at least four times as large as the lateral cross-sectional area of each of the orifices. 
         [0009]    In some embodiments, the system may comprise at least a second joint interposed between the base and the end effector in parallel with the first joint. The first and/or second joints may be spherical, planar, cylindrical or other types of kinematic, joints. In some embodiments, a sphere, planar, cylindrical or other kinematic member serves as a common proximal portion for the first and second joints. 
         [0010]    In some embodiments, a positioning system includes at least two parallel links interposed between the base and the end effector. Each of the parallel links may include at least two joints, and each of the joints may be changeable between a movable state and a fixed state. 
         [0011]    In some of the above embodiments, the end effector is configured with a lumen for slidably receiving an instrument. In some embodiments, a miniature endoscope instrument may be provided that has a first portion configured to be precisely received within the end effector lumen and a second portion configured for entering a human cochlea. In some of the systems, the base is configured for attaching to a temporal bone. 
         [0012]    In some of the above embodiments, the pressurized gas cushion has a thickness of no more than about 50 microns. In other embodiments, the pressurized gas cushion has a thickness of no more than about 5 microns. In still other embodiments, the pressurized gas cushion has a thickness of no more than about 1 micron. 
         [0013]    According to aspects of the detailed description, methods of precisely positioning an end effector may include providing a device having a base, a first joint located distally from the base, and an end effector located distally from the first joint. The first joint may have two portions separated by a gas cushion, the first joint allowing the end effector to be movable with respect to the base. The method may further comprise manually positioning the end effector, and removing the gas cushion to cause the two joint portions to contact each other, thereby locking the end effector in the precise location in which it was positioned. 
         [0014]    In some of the above methods, the manual positioning step comprises moving the end effector in at least two degrees of rotational freedom. The two portions of the joint may be mutually attracted by a magnetic force. The first joint may be a spherical joint. In some of the methods, the device comprises a second joint located distally from the first joint and proximally from the end effector. 
         [0015]    In some of the above methods, the end effector may be positioned in a relatively coarse manner, the gas cushion may be removed from the first joint to lock in the relatively course position of the end effector, the end effector may then be positioned in a relatively fine manner, and then the gas cushion may be removed from the second joint to lock in the relatively fine position of the end effector. 
         [0016]    Some methods may include a step of removably attaching the base of the device to a bone of a patient. In some of the above methods, a miniature endoscope is moved relative to the end effector and into a cochlea. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The novel features of the invention are set forth with particularity in the claims that follow. A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description that sets forth illustrative embodiments, in which the principles of the invention are utilized, and the accompanying drawings of which. 
           [0018]      FIG. 1A  is a cross-sectional side view showing an exemplary positioning device constructed according to aspects of the Detailed Description and having an articulating joint in a locked or fixed state. 
           [0019]      FIG. 1B  is a cross-sectional side view showing the positioning device of  FIG. 1A  with the articulating joint in a movable state. 
           [0020]      FIG. 1C  is a partially broken away plan view showing the proximal portion of the articulating joint shown in  FIG. 1A . 
           [0021]      FIG. 1D  is a cross-sectional side view showing a variation of the proximal portion of the articulating joint shown in  FIG. 1A . 
           [0022]      FIG. 2A  is a side perspective view showing another embodiment of an articulating joint. 
           [0023]      FIG. 2B  is a bottom perspective view showing the concave portion of the articulating joint of  FIG. 2A . 
           [0024]      FIG. 2C  is a bottom view showing the concave portion of the articulating joint of  FIG. 2A . 
           [0025]      FIG. 2C  is a bottom view showing the concave portion of the articulating joint of  FIG. 2A . 
           [0026]      FIG. 2D  is a partially broken away top view showing the concave portion of the articulating joint of  FIG. 2A . 
           [0027]      FIG. 2E  is a top view showing a variation of the concave portion of the articulating joint of  FIG. 2A . 
           [0028]      FIGS. 3A-3D  are various perspective views showing alternative embodiments of positioning systems having one or more pairs of lockable articulating joints. 
           [0029]      FIGS. 4A-4C  are various perspective views showing an embodiment of a positioning system mounted on a templar bone and having joints in series and in parallel. 
           [0030]      FIG. 5A  is a cross-sectional side view showing an alternative embodiment of an articulating joint. 
           [0031]      FIG. 5B  is an exploded cross-sectional side view showing the articulating joint of  FIG. 5A . 
           [0032]      FIG. 5C  is an exploded perspective view showing the articulating joint of  FIG. 5A . 
       
    
    
     DETAILED DESCRIPTION 
       [0033]    Referring to  FIG. 1A , an exemplary positioning device  100  constructed according to aspects of the present invention is schematically shown. Device  100  includes an end effector  102  movably connected to base  104  by articulating links  106  and  108 . In this embodiment, end effector  102  includes a through-hole  110  for receiving an object to be positioned (not shown in  FIG. 1A ). Base  104  may rest on or be attached to a stationary reference surface. In alternative embodiments, base  104  may be coupled to a movable apparatus, such as for coarse positioning of device  100 . 
         [0034]    In the embodiment shown in  FIG. 1A , articulating link  106  is rigidly connected to end effector  102 , and articulating link  108  is rigidly connected to base  104 . Links  106  and  108  are movably coupled together by joint  112 . In this embodiment, joint  112  is a spherical joint comprising a proximal portion  114  located on link  108  and a distal portion  116  located on link  106 . Proximal portion  114  comprises an annular magnet  118  fixed within an axial bore of proximal portion  114 , such as by a press fit, adhesive, or molded therein. Distal portion  116  of joint  112  includes a spherical member  120 . Spherical member  120  comprises a ferrous and/or magnetic material such that it is attracted to magnet  118 . In this manner, proximal portion  114  and distal portion  116  of joint  112  are drawn towards each other by magnetic attraction. In this embodiment, proximal portion  114  includes a concave surface  121  having substantially the same radius of curvature as that of spherical member  120 , thereby creating intimate contact between substantially all of surface  121  and a portion of the outer surface of spherical member  120 . The combination of magnetic force and large surface area contact in this embodiment causes spherical member  120  to be in a locked position relative to proximal portion  114  when in this configuration. As a consequence of this fixed or locked configuration of joint  112 , end effector  102  is generally immovable relative to base  104 . 
         [0035]    Referring now to  FIG. 1B , proximal portion  114  of joint  112  further comprises an inlet port  122  connected to a compressed air supply (or other compressed gas or fluid supply) by valve  124 . Inlet port  122  is in fluid communication with channel  126  through the axial bore of proximal portion  114 . Channel  126  is located adjacent to spherical member  120 . When valve  124  is opened, compressed air flows from the supply through channel  126  with enough pressure to overcome the magnetic attraction between the proximal and distal portions of joint  112 , urging spherical member  120  apart from mating concave surface  121 . A thin air cushion  128  is thus formed between spherical member  120  and concave surface  121  as compresses air flows between the two and exits in the direction of the arrows labeled A. The farther that spherical member  120  moves away from concave surface  121 , the lower the resulting air pressure between the two surfaces, because the air is allowed to escape more easily. Therefore, the pressurized air will only move spherical member  120  a predetermined distance away from mating surface  121 , held in balance by the equal and opposite force of magnetic attraction between magnet  118  and spherical member  120 . The strength and location of magnet  118 , the pressure of the compressed air (or other gas), the surface area between spherical member  120  and mating surface  121 , surface roughnesses, and other parameters may be selected such that air cushion  128  may be kept to a minimal thickness yet allows spherical member  120  to move freely relative to mating surface  121 . In some embodiments, air cushion  128  has a thickness of about 5 microns or less. 
         [0036]    With the above arrangement, valve  124  may be opened to allow joint  112  to be changed from a fixed state to a movable state by creating an air cushion  128  between the proximal portion  114  and the distal portion  116 . When joint  112  is in the movable state, as shown in  FIG. 1B , end effector  102  may freely be moved by hand or other means to any desired position. Air cushion  128  allows end effector  102  to be moved with very little friction. Valve  124  may then be closed to remove air cushion  128  and cause proximal portion  114  and distal portion  116  to contact each other, thus changing joint  112  from the movable state to the fixed state, as shown in  FIG. 1A . Because the thickness of air cushion  128  may be very small in some embodiments, there is no appreciable movement when joint  112  is changed from the movable state to the fixed state. 
         [0037]    In some embodiments, joint  112  may be configured such that it is freely movable when valve  124  is sufficiently open, and movable with some resistance when valve  124  is opened to a lesser extend. In other words, when valve  124  is only partially open, the air cushion formed is sufficient to partially overcome the magnetic attraction between the proximal and distal portions of joint  112 , but is not so thick as to provide complete separation between the entirety of the mating surfaces. This arrangement may be desirable when some fixation force is desired to overcome gravity or other small disturbing forces, but still allow the position of end effector  102  to be adjusted before being locked. 
         [0038]    In the embodiment shown in  FIG. 1B , joint  112  (and therefore also end effector  102 ) may be moved in three degrees of rotational freedom when in the movable state. These degrees of freedom can be described as roll, pitch and yaw, as depicted by arrows labeled R, P and Y, respectively. In other embodiments (not shown), joint  112  may be constrained to just two or one degrees of freedom. 
         [0039]    To increase the holding force between spherical member  120  and mating surface  121  when joint  112  is in the fixed state, one or both of these surfaces may be roughened. This may be accomplished by choice of component material(s), coating the surface(s), and/or various finishing techniques such as sand blasting. In some embodiments, high-friction coatings or base materials may be used to increase friction without adding surface roughness, thereby increasing the precision of the joint. Porous surface materials may also be used to increase performance in both the locked and movable states of the joint. 
         [0040]    Referring to  FIG. 1C , a partially broken away axial view of proximal portion  114  is shown, illustrating magnet  118  within the body  130  of proximal portion  114 . 
         [0041]    Referring to  FIG. 1D , an alternate embodiment of proximal portion  114 ′ is shown. As depicted, two (or more) annular magnets  118  may be axially aligned to provide greater holding strength. Also, the upper magnet need not be embedded below concave surface  121  as in the embodiment shown in  FIGS. 1A-1C , but may be fabricated to form part or all of surface  121  itself. Along these lines, an alternate proximal portion (not shown) may be formed from a unitary piece of ferrous and/or magnetic material, such as by sintering, grinding, and/or other fabrication techniques known to those skilled in the art. 
         [0042]    Referring now to  FIGS. 2A-2D , various views of another embodiment are shown. In this embodiment, small cylindrically-shaped magnets  202  are arranged in an off-axis manner. Three magnets  202  may be equally spaced around the central axis of proximal portion  204  of spherical joint  206 . As can best be seen in  FIG. 2A , the individual axes of magnets  202  may be generally aligned with the center of spherical member  208 . The proximal end  210  of an air channel can be seen in  FIGS. 2A-2C , while the distal end  212  of the air channel can be seen in the middle of concave mating surface  214  in  FIG. 2D . 
         [0043]    Referring to  FIG. 2E , a variation of the embodiment of  FIG. 2A-2D  is shown. In this embodiment, magnets  202  protrude through concave spherical surface  214 ′ rather than being embedded below the surface. Magnets  202  may stand proud of surface  214 ′, be flush with it, or be slightly recessed. In some embodiments, the body of proximal portion  204 ′ may be fabricated by placing magnets  202  on spherical member  208  in a desired spacing, placing spherical member  208  with attached magnets  202  at least partially into a mold cavity, and filling the mold cavity with a hardenable substance around magnets  202 , such as with a thermoplastic, thermosetting plastic, resin or epoxy. Once the substance has hardened, spherical member  208  may then be separated from magnets  202  and the hardenable substance (a mold release may need to be applied to spherical member  208  before molding), leaving the concave mating surface  214  and magnets as shown in  FIG. 2E . 
         [0044]    In some embodiments (not shown), the distal end  212  of the air channel may be made larger, and/or a network of shallow grooves in fluid communication with the distal end  212  of the air channel may be provided along concave surface  214 . This arrangement provides a larger area of the pressurized air when it is first activated to move spherical member  208  away from magnets  202 . Accordingly, a lower pressure air supply may be used for a given magnet arrangement, and oscillations of the spherical member  208  when first separated from magnets  202  may be avoided. 
         [0045]    In some embodiments, fewer or more than the three magnets  202  shown in  FIGS. 2A-2E  are used. High energy magnets may be sued to allow for a greater magnetic attraction force in a smaller package. 
         [0046]    In some embodiments, the positions of the ball and socket members are reversed. In other words, spherical member  120  or  208  may be located on the proximal portion of the articulating link and mating concave surface  121  or  214  may be located on the distal portion, opposite of the arrangement shown in  FIGS. 1A and 1B . In some embodiments, the base may be located beside or above the end effector, mounted at an angle, and/or mounted on a movable platform. 
         [0047]    While only spherical articulating joints have been discussed up to this point, it is to be understood that other types of kinematic joints may be used to form bearing and braking mechanisms. For example, using the concepts described above, a revolute joint may be constructed (one degree of freedom), or a prismatic joint (one degree of freedom), a cylindrical joint (two degrees of freedom), a planar joint (two or three degrees of freedom), or a spherical joint (with up to three degrees of freedom). Combinations of these joints may also be used in series and/or parallel, as will be described in more detail below. In general, each joint may be formed by a pair of surfaces that have congruent areas of contact. 
         [0048]    Referring to  FIG. 3A , a positioning apparatus  300  is shown having two articulating joints  302  and  304  in series. Joint  304  is similar to those previously described, having a spherical member  306  forming its distal portion, and a mating concave member  308  with magnets  310  forming its proximal portion. Joint  302  has a similar construction, with a larger spherical member  312  forming its proximal portion, and a mating concave member  314  with magnets  310  forming its distal portion. A single air supply  316  may be used to simultaneously activate both joints  302  and  304 . When compressed air is introduced through common air supply  316 , a thin air cushion is formed between spherical member  306  and mating concave member  308 , and also between spherical member  312  and mating concave member  314 . Each of the two air cushions allows one of the articulating joints  302  and  304  to move in up to three degrees of freedom, thereby allowing the mechanism to move in up to six degrees of freedom. 
         [0049]    In the embodiment shown in  FIG. 3A , articulating joints  302  and  304  are separated by a rigid link  320 . An internal air channel (not shown) running longitudinally within link  320  delivers compressed air from common air supply  316  to each of the two joints  302 ,  304 . In some embodiments, this internal air channel is formed by a bore of constant diameter extending between an office in the concave member  308  of joint  302  and an orifice in the concave member  314  of joint  304 , and having the same diameter as the two orifices. In other embodiments, it may be desirable to maximize the diameter of the internal air channel, or to otherwise provide a plenum between joints  302  and  304 . Such arrangements can avoid oscillations that may otherwise occur in joints  302  and  304 . In some embodiments, the internal channel has a minimum lateral cross-sectional area that is at least four times as large as a lateral cross-sectional area of each of the orifices. In some embodiments, the internal channel has a maximum lateral cross-sectional area that is at least one-half as large as a minimum total lateral cross-sectional area of link  320 . 
         [0050]    Referring to  FIG. 3B , a positioning apparatus  300 ′ similar to positioning apparatus  300  shown in  FIG. 3A . Apparatus  300 ′ includes an instrument  322  that may be slidably received in a lumen within end effector  318 . Instrument  322  may be medical device, such as miniature endoscope having a first portion configured to be precisely received within the end effector lumen and a second portion configured for entering a human cochlea. 
         [0051]    In operation, compressed air may be supplied to common supply  316  as previously described, allowing joints  302  and  304  to freely articulate. It should be noted that the arrangement of positioning apparatus  300 ′ allows instrument  322  to remain in a particular orientation if desired as it is moved laterally and/or longitudinally in three dimensions. Once instrument  322  has been positioned and oriented as desired, the air supply to joints  302  and  304  may be interrupted. This allows magnets  310  to lock joints  302  and  304 , thereby holding instrument  322  precisely in place. In other embodiments, the air supply to joints  302  and  304  may be independently controlled, allowing only one joint to be locked while the other joint is still free to move. 
         [0052]    Referring to  FIG. 3C , another embodiment of a positioning system is shown. Positioning system  324  includes four articulating joints  326 ,  328 ,  330  and  332  connected in series between base  334  and end effector  318 . In this embodiment, each joint is a spherical joint that is changeable between a movable state and a fixed or locked state, similar to those previously described. Joints  326  and  328  share a first common compressed air supply line  336  within link  338 . Joints  330  and  332  share a second common compressed air supply line  340  within link  342 . Joints  328  and  330  are spaced apart by link  344 . 
         [0053]    First supply line  336  and second supply line  340  may be connected to a single control valve (not shown) such that all four joints  326 ,  328 ,  330  and  332  are either in a movable state or a locked state at the same time. Alternatively, the first and second supply lines  336  and  340  may be independently controlled. In this manner, the coarse positioning of instrument  322  may be obtained with all four joints, or at least joints  326  and  328  being in the movable state. Joints  326  and  328  may then be locked in position by turning off the air supply to first common supply line  336 . Joints  330  and  332  may be left in the movable state so that fine positioning of instrument  322  may be performed. The air supply to second common supply line  340  may then be turned off to fully lock the position of instrument  322 . To control one or both of the air supply lines  336  and  340 , manually or electrically actuated valve(s) may be used. To further control the valves, foot pedals, electronic switches, and/or electronic or mechanical controllers may be used. The foot pedal(s) and/or electronic switches may control the air supply in a binary fashion such that it is either fully on or fully off, or may allow variable control so that the air supply may be gradually turned on or off In other embodiments, two, three, four or more articulation joints may be independently controlled. 
         [0054]    Referring to  FIG. 3D , another embodiment of a positioning system is shown. Positioning system  350  includes four articulating joints  352 ,  354 ,  356 , and  358  connected in series between base  360  and end effector  362 . In this embodiment, the proximal portion of joint  352  includes a large spherical member  364  rigidly attached to base  360 . In other embodiments, spherical member  364  may form a movable and lockable joint with base  360 . 
         [0055]    Referring to  FIGS. 4A-4C , another embodiment of a positioning system is shown.  FIG. 4A  shows the base  400  of system  402 . Base  400  includes a large spherical member  404 , similar to base  360  shown in  FIG. 3D . Base  400  is shown attached to a temporal bone  406  adjacent to the external acoustic meatus  408  of the ear canal. Attachment may be made using one or more bone screws (not shown) passing through or formed in base  400  and temporarily extending into the temporal bone  406 . Such a mounting on bone provides a secure base from which to precisely position an instrument with system  402 , such as a miniature endoscope for insertion into the cochlea as previously described. 
         [0056]      FIGS. 4B and 4C  show the articulating joints of positioning system  402 . In this exemplary embodiment, system  402  includes a first series  410  of articulating joints and a second series  412  of articulating joints. The first series  410  includes joints  414 ,  416  and  418 . The second series includes joints  420 ,  422  and  424 . Spherical member  404  serves as a common proximal portion for both joints  414  and  420 . When in the movable state, the distal, concave portion of each of joints  414  and  420  may move across spherical member  404 . End effector  426  includes a section that serves as a common distal portion for both joints  418  and  424 . In this arrangement, the first and second series  410  and  412  of joints are arranged in parallel to connect end effector  426  with base  400 . With two series of joints arranged in parallel as shown in this example, end effector  426  may be held more stably while still being able to be positioned in a wide range of positions and orientations. 
         [0057]    End effector  426  also includes a section for receiving an instrument  428 , such as a miniature endoscope, and a handle section  430  for manually manipulating the position and orientation of instrument  428  when the articulating joints are in their movable state. 
         [0058]    Using the joint construction of the previously described exemplary embodiments, there is little movement of the joints when they are changed from there movable state to their fixed state. In many embodiments, this movement is less than 5 microns (the thickness of the air cushion between the parts of the joint.) This very small movement may become even more important when six or more joints are used in combination as shown. This inventive arrangement allows an instrument to be precisely positioned, and then locked firmly in place without significant movement occurring during the locking of the joints. 
         [0059]    In other embodiments, any number or type of articulating joints may be arranged in series, parallel or both to form lockable, precision positioning systems similar to those described above. 
         [0060]    In some embodiments (not shown), one or more joints may be formed with pairs of magnets aligned in such a way that the magnetic attraction between the opposing magnets provides a self centering effect. For example, instead of using a ferrous sphere as one portion of a lockable joint, a sphere having magnet(s) or ferrous portion(s) aligned with magnet(s) on the concave portion of the joint can be used, causing the joint to seek a particular orientation of the sphere relative to the concave portion. This type of arrangement can be used to at least partially overcome the effects of gravity or other disturbing forces when the joint is in the movable state. 
         [0061]    In some embodiments, an electromagnet can be provided in the articulating joint(s). The electromagnet may be energized to lock the joint without interrupting the air flow, or may be used to increase the frictional engagement of the joint when locked. 
         [0062]    In some embodiments, vacuum between the two portions of a joint may be used to further lock the joint from movement. 
         [0063]    In some embodiments, the joint can be configured to act as a mechanical fuse for overload protection. By properly selecting the characteristics of both sides of a joint, the joint can be designed to come apart when a predetermined load is reached. This can protect other parts of the positioning system from being damaged by excessive loading. 
         [0064]    Referring to  FIGS. 5A-5C , an alternative embodiment articulating joint  500  is shown. In this exemplary embodiment, joint  500  includes a plastic housing  502 , a ferromagnetic liner  504 , a permanent magnet  506 , and a steel ball  508 . As best seen in  FIG. 5A , liner  504  is cup shaped and is received within housing  502 . Magnet  506  is received within liner  504  and resides between the bottom of liner  504  and steel ball  508  when joint  500  is assembled. An axial lumen  510  is provided in housing  502  that extends through liner  504  and magnet  506  to allow compressed air to be supplied between ball  508  and a mating concave surface  512  formed in housing  502 . 
         [0065]    With the above arrangement, an effective connection can be made between both poles of magnet  506  (the top and bottom surfaces in this embodiment) and steel ball  508 . This completes the magnetic circuit between magnet  506  and steel ball  508  and significantly reduces the air gap between the two, thereby reducing the reluctance of the magnetic circuit. Such an arrangement can serve to increase the attractive force between magnet  506  and ball  508 . It can also minimize the magnetic influence between neighboring joints, thereby reducing undesirable attractive and/or repulsive forces from one joint on another. 
         [0066]    In other embodiments, a ferromagnetic body can be used in a similar manner to optimize magnetic flux. For example, in embodiments having planar joints (not shown), a ferromagnetic body with a particular configuration can be added to the joint to allow a magnetic circuit to be completed between both poles of a magnet in one side of the joint and a ferrous material located in the other side of the joint. 
         [0067]    In some embodiments, multiple instruments may be located at the distal end and/or at intermediate positions along the length of the articulating positioning system. For example, a surgical instrument may be located at the distal end, a suction device may be located at link proximal to the surgical instrument, and a lighting apparatus may be located at a link proximal to the suction device. In use, the more proximal link holding the lighting apparatus may be positioned and locked first, then the suction device link may be positioned and locked, and then the surgical instrument may be positioned and locked. 
         [0068]    While exemplary embodiments constructed according to aspects of the present invention have been shown and described herein, it will be obvious to those skilled in the art that such embodiments are provided by way of example only. Numerous variations, changes, and substitutions will now occur to those skilled in the art without departing from the invention. For example, other means of inducing attractive forces between bodies may be used including electromagnets, springs, capacitive and electrostatic forces, as well as other nuclear and inertial effects that give rise to force. Surface materials, textures, porosities and geometries may be selected to increase friction in the locked state. As an example of surface geometries that may be used, mating splines may be located on opposing portions of a joint to lock the joint in one of a series of discrete positions when the joint is changed to a fixed state. It should be understood that various alternatives to the embodiments of the invention described herein may be employed in practicing the invention.