US20060100501A1 - System for positioning on a patient an observation and/or intervention device - Google Patents

Abstract

The present invention relates to a system for positioning with respect to a patient's body an observation and/or intervention device having a portion penetrating into the patient's body comprising a base laid on the patient's body; a means for supporting the device formed of a first portion movably assembled on the base according to a connection with one degree of freedom, and of a second portion movably assembled on the first portion according to a connection with one degree of freedom and connected to the device; and means for actuating the first portion with respect to the base, and the second portion with respect to the first portion, in which the base surrounds at a distance at least partially the device, said device being detachably connected to the second portion to enable removal of the positioning system while leaving in place the device.

Description

• The present invention relates to a system for positioning on a patient an observation and/or intervention device having a portion penetrating into the patient's body. It for example applies to medical analysis systems, such as endoscopic systems, and it will more specifically be described in the case of a use of an endoscope in a laparoscopy.
• In such a type of operation, an endoscope formed of a thin optical tube is inserted into the patient's abdomen through a small incision performed at the level of the patient's abdomen. The optical tube is generally connected to an external video camera. Other incisions may be performed to introduce surgical instruments handled by a surgeon. The endoscope is used to visualize the patient's internal organs and the surgical instruments. Since the surgeon generally has both hands occupied by the surgical instruments, an assistant is necessary to maintain the endoscope in a desired position.
• Robotic systems have been developed to handle the endoscope instead of the assistant. Such systems are generally formed of massive, complex, and expensive robots comprising a base attached to the ground and an arm handling the endoscope. In the limited space of an operation table, the base of such a robot takes up a considerable place next to the patient. Further, the robot's arm maintaining the endoscope may hinder the access to the patient's abdomen.
• It is thus desirable to provide a system for positioning on a patient an observation and/or intervention instrument taking up a small volume to limit the space required in the operation theatre and clear the access to the patient as much as possible.
• International patent application WO0105319 assigned to the Universite Joseph Fourier describes, in an embodiment, a system for positioning an observation and/or intervention device, for example, an endoscope. The system comprises a mount placed on the patient's body, supporting a trocar in which the observation and/or intervention device slides, the trocar being likely to pivot with respect to the mount. An actuator assembly controls the trocar movement with respect to the mount. It may be formed of pneumatic or hydraulic actuators, each actuator being directly connected to the mount and to the trocar. It may also be formed of cables. Each cable then extends between the trocar and a guide attached on the mount, and is driven at its free end by an actuator placed at a distance from the patient.
• A disadvantage of such a system is the difficulty to impose significant movements to the trocar. Indeed, in the case where the actuators are placed directly on the mount, the volume taken up by the actuators limits the trocar movements. In the case where the actuators are placed at a distance from the patient, the trocar movements are obtained by the application of tractions of different amplitudes on the cables connected to the trocar. A difficulty then results, in large movements, from the high flexion angles imposed to the cables at the level of the guides. This may result in a fast deterioration of the cables. Further, with such a system, the forces exerted by the cables or the actuators on the trocar are the origin of a pressure exerted by the trocar on the patient's abdomen that may be undesirable. Another disadvantage of such a system is that it is generally not possible to remove the positioning system while leaving in place the trocar and the observation and/or intervention devices.
• The present invention provides a system, taking up a small volume, for positioning on a patient's body an observation and/or intervention device having a portion penetrating into the patient's body, enabling obtaining significant movements of the observation device and enabling removal of the positioning system while leaving in place the observation and/or intervention device.
• The present invention also provides a positioning system limiting the application of pressure forces on the patient's abdomen.
• To achieve these objects, the present invention provides a system for positioning with respect to a patient's body an observation and/or intervention device having a portion penetrating into the patient's body comprising a base laid on the patient's body; a means for supporting the device formed of a first portion movably assembled on the base according to a connection with one degree of freedom, and of a second portion movably assembled on the first portion according to a connection with one degree of freedom and connected to the device; and means for actuating the first portion with respect to the base, and the second portion with respect to the first portion, in which the base surrounds at a distance at least partially the device and in which the device is detachably connected to the second portion to enable removal of the positioning system with respect to the device while leaving in place the device with respect to the patient's body.
• According to an embodiment of the present invention, the base delimits a circular opening around the device and the first portion is a mobile ring with an axis substantially perpendicular to the patient's body and with an inner diameter substantially corresponding to the circular opening, the mobile ring being rotatably assembled on the base around its axis.
• According to an embodiment of the present invention, the second portion comprises a head connected to the device prolonging at least in an arm pivotally assembled on the mobile ring along an axis substantially tangent to the patient's body.
• According to an embodiment of the present invention, the first portion comprises a semi-circular rail having its ends pivotally assembled on the base along an axis substantially tangent to the patient's body.
• According to an embodiment of the present invention, the second portion comprises a carriage slidably assembled on the rail and connected to the device.
• According to an embodiment of the present invention, the device is movably assembled on the second portion according to a connection with one degree of freedom.
• According to an embodiment of the present invention, the motions of the first and second portions are controlled by cables driven by actuators located at a distance from the patient.
• According to an embodiment of the present invention, the rotating motion of the mobile ring with respect to the base is controlled by an actuator attached to the mobile ring, the actuator extending to be substantially parallel to the mobile ring axis.
• According to an embodiment of the present invention, the pivoting motion of the arm with respect to the mobile ring is controlled by an actuator attached to the second portion and extending to be substantially parallel to the pivoting axis of the arm with respect to the mobile ring.
• According to an embodiment of the present invention, the device extends substantially longitudinally along a direction perpendicular to the pivoting axis of the arm with respect to the mobile ring and the device is movably assembled with respect to the second portion, to slide along said direction, the device sliding with respect to the second portion being controlled by an actuator attached to the second portion and extending to be substantially parallel to the pivoting axis of the arm with respect to the mobile ring.
• The foregoing objects, features, and advantages, as well as others of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings, among which:
• FIG. 1A shows a perspective view of a first embodiment of a positioning system according to the present invention;
• FIG. 1B shows a partial top view with a partial cross-section view of the system ofFIG. 1A;
• FIG. 1C shows a partial lateral cross-section view of the system ofFIG. 1A;
• FIG. 2A shows a partial top view with a partial cross-section view of an alternative of the system according to the first embodiment;
• FIG. 2B shows a partial lateral cross-section view of the system ofFIG. 2A;
• FIG. 3A shows a very simplified side view of a second embodiment of the system according to the present invention;
• FIG. 3B shows a front view of the system ofFIG. 3A;
• FIG. 3C shows a top view of the system ofFIG. 3A;
• FIG. 4A shows a perspective view of an alternative of the first embodiment of a positioning system according to the present invention;
• FIGS. 4B and 4C shows side views of the system ofFIG. 4A at two different positions; and
• FIG. 4D shows a top view of the system ofFIG. 4C.
• FIGS. 1A to1C show a first embodiment of asystem10 for positioning anendoscope12 placed in atrocar14,endoscope12 andtrocar14 penetrating into a patient's abdomen through asmall incision16.Endoscope12 has the aspect of a cylindrical tube of axis Ω of a length of some forty centimeters and of a diameter of a few centimeters. InFIG. 1B, a portion only oftrocar14 is shown. InFIG. 1C, only axis Ω ofendoscope12 is shown. A camera, not shown, is attached to the end ofendoscope12 external to the abdomen.
• System10 comprises a substantiallyplanar base18 comprising a planar ring-shapedcentral portion20, surroundingincision16, from which fourarms21 extend. Acylindrical opening22 is formed at the end of eacharm21.Openings22 may be used to maintainbase18 on the patient's body via straps, cables, etc. attached to the table on which the patient is laid or directly attached to the patient.Base18 may also be glued on the patient's abdomen.
• Centralannular portion20 supports amobile ring24 of axis Δ substantially perpendicularly to the plane tangent to the abdomen at the level ofincision16.Mobile ring24 is rotatably assembled around axis Δ oncentral portion20 via abearing25.
• Astirrup26 is pivotally assembled onmobile ring24 according to an axis Γ substantially perpendicular to axis Δ, and substantially comprised in the plane tangent to the abdomen at the level ofincision16.Stirrup26 comprises ahead28 on which is attachedtrocar14 by any known attachment means, which enables simple and fast connection and separation oftrocar14 and ofhead28. Said means will for example beflexible tongues27. The inner diameters ofmobile ring24 and of ring-shapedcentral portion20 are selected to enable removal ofsystem10 during an intervention without displacingtrocar14, or to enable removal oftrocar14 from the patient's abdomen without displacingsystem10.Head28 prolongs in twoarms30,31, each comprising at one end a slidingsurface32, corresponding to a cylindrical portion, likely to slide on aprofile34,35 shaped as a half-cylinder of axis Γ. Eachprofile34,35 moves along with abase36,37 attached onring24, for example, by screwing. Eacharm30,31 comprises at its free end acap38,39 (not shown inFIG. 1C) which forms with slidingsurface32 of the associatedarm30,31 ahousing receiving profile34,35.
• Mobile ring24 comprises on its external cylindrical wall agroove42 receiving acable43. Aguide44 assembled onbase18 receives the ends of twosheaths45,46 in whichcable43 slides.Guide44 is arranged onbase18 so thatcable43, as it comes out of one ofsheaths45,46, is tangent to groove42 ofmobile ring44. The ends ofcable43 are connected to the ends of a rack (not shown) meshing in with the output shaft of an electric motor. The displacement ofcable43 by the motor rotatesring24 with respect to centralannular portion20 around axis Δ.
• Apulley sector47, substantially corresponding to a cylindrical sector, is attached to one ofarms31 ofstirrup26. A groove48 is formed on the lateral wall, corresponding to a cylinder portion ofpulley sector47. Groove48 receives twocables49 and50, eachcable49,50 being attached to an opposite end of groove48. The other ends ofcables49,50 are connected to the ends of a rack (not shown) meshing in with the output shaft of an electric motor. Two supports51,52 are attached tomobile ring24 and each receives the end of asheath53,54 in which one ofcables49,50 slides.Supports51,52 are arranged so thatcables49,50, as they come out ofsheaths53,54 are substantially tangent topulley sector47. When the electric motor displaces the rack, a traction is exerted on one ofcables49,50, and a thrust is exerted on theother cable49,50.Cables49,50 displacepulley sector47 and, accordingly,stirrup26. The sliding surfaces32 ofarms30,31 then slide onprofiles34,35 so thatstirrup26 generally pivots with respect to axis Γ.
• Endoscope12 is assembled to freely slide along axis Ω introcar14. Ahold element55 is attached toendoscope12. Acompression spring56 bears at one end againsthold element55 and at the opposite end againsttrocar14. Acable57 is connected at one end to holdelement55 and at its opposite end to a rack (not shown) meshing in with the output shaft of an electric motor. Aguide58 attached onhead28 ofstirrup26 receives the end of aprotection sheath59 in whichcable57 slides. A traction oncable57 makesendoscope12 slide introcar14 along axis Ω and compressesspring56. Whencable57 is released,spring56 relaxes and bringsendoscope12 back to an idle position. Holdelement55 can easily be removed fromendoscope12 to enable removal ofendoscope12 fromtrocar14, for example, to clean its lens, or to replace it with another endoscope having a different length or viewing cone.
• The three motors and the racks are for example arranged in a package distant from the patient. The motor control may be performed in any known fashion and will not be detailed hereafter. It may for example be a vocal control, a manual control by lever or buttons, a foot control, etc. The motor control may be performed in open or closed loop. For example,system10 may also comprise a location means to detect the position and the orientation ofincision16, ofendoscope12, and of an intervention instrument handled by the surgeon. The location means may be connected to a calculator capable of controlling motors. The endoscope is then displaced to, for example, transmit an image permanently following the end of the intervention instrument.
• The motors are preferably clutch releasable to enable the surgeon to manually displacepositioning system10.
• Cables43,57 andcable assembly49,50 each control a degree of freedom ofendoscope12.Cables43,49,50,57 are arranged not to undergo significant flexion angles. In particular,pulley sector47 has a sufficiently large radius, preferably greater than 50 mm, to limit the curvature ofcables49,50. This enables preventing fast deterioration ofcables43,49,50,57.
• Base18 andstirrup26 may be made of steel to increase their durability and ease their sterilization.Cables43,49,50,57 may be formed of a material covered with Teflon and placed inTeflon sheaths45,46,53,54,59.
• FIGS. 2A and 2B are similar, respectively, toFIGS. 1B and 1C and show an alternative ofpositioning system10 according to the first embodiment. Only the differences with respect to the first embodiment will be described.
• According to this alternative, the electric motors driving the actuating cables are directly arranged at the level ofpositioning system10. The motors are arranged to hinder as little as possible the movements ofendoscope12. For example, afirst motor60, comprising acylindrical package61 and anoutput shaft62, is assembled on asupport63 connected to base18 so thatoutput shaft62 substantially extends along an axis parallel to axis Δ.Output shaft62 rotates apulley64.Pulley64 is connected tomobile ring24 by acable65 to drivemobile ring24 similarly to the first embodiment.
• Asecond motor66, comprising acylindrical package67 and anoutput shaft68, is assembled on asupport69 connected tomobile ring24 so thatoutput shaft68 extends substantially along an axis perpendicular to axes Δ and Ω.Output shaft68 rotates apulley70.Pulley70 is connected topulley sector47 by acable71 to drivepulley sector47 similarly to the first embodiment. The supply and control means ofmotors60,66 are not shown.Motors60 and66 may be controlled by any conventional device. In particular, packages61,67 may comprise supply means and remote-control means. The sliding ofendoscope12 introcar14 may also be controlled by a third motor (not shown) directly placed at the level ofhold element55.
• The present alternative of the first embodiment enables completely freeing the patient's abdomen during the surgical intervention.
• FIGS. 3A to3C very schematically show a second embodiment ofsystem10 for positioningtrocar14 in which the endoscope (not shown) slides, the trocar and the endoscope penetrating into the patient's abdomen through incision89.
• According to the second embodiment,system10 comprises a “C”-shapedbase80 resting on the patient's abdomen. Asemi-circular rail82 is pivotally assembled onbase80 around an axis substantially tangent to the patient's abdomen. The two ends ofrail82 are substantially connected to the two ends ofbase80 by twopivotal connections84,86. Acarriage88 is slidably assembled onrail82.Carriage88supports trocar14.
• The means for controlling the sliding of the endoscope introcar14 are not shown inFIGS. 3A to3C and may be identical to those of the first embodiment.
• The sliding ofcarriage88 onrail82, and the pivoting ofrail82 with respect tobase80, may be controlled by cable driven by actuators placed at a distance from the patient as for the first embodiment, or by directly arranging the actuators at the level ofpositioning system10 as for the previously-described alternative of the first embodiment.
• According to an alternative of the present invention, the rotating motions ofendoscope12 introcar14 around its axis Ω may also be controlled by an actuator.
• According to the second embodiment, the removal of the positioning system is eased by the fact thatbase80 is “C” shaped.
• FIGS. 4A to4D show views of an alternative ofpositioning system10 according to the first embodiment. InFIGS. 4A to4D, the incision made on the patient for the placing ofendoscope12 has not been shown. InFIGS. 4B to4D, only axis Ω ofendoscope12 is shown.
• According to the present alternative of the first embodiment,trocar14 is maintained onstirrup26 by a mountingflange90, for example, screwed onstirrup26.Trocar14 comprises aprotrusion92 located on the side of mountingflange90 opposite to ring-shapedbase18.Protrusion92 may represent handles, tightness valves, connectors, etc. A fixedring95 is arranged between ring-shapedbase18 andmobile ring24. Fixedring95 forms one piece withbase18.Mobile ring24 is rotatably assembled on fixedring95. Fixedring95 comprises a peripheral toothing (not shown) on its external lateral surface.
• A firstelectric motor96, intended to rotatemobile ring24 with respect to fixedring95, is assembled to move along withmobile ring24 via astage98.First motor96 is controlled and supplied by means not shown, for example, by electric wires. Atoothed wheel100 is arranged at the end ofshaft102 offirst motor96.Shaft102 is substantially parallel to axis A ofmobile ring24.Toothed wheel100 meshes in with the toothing of fixedring95. Sincetoothed wheel100 cooperates with fixedring95, the rotating oftoothed wheel100 byfirst motor96 rotatesmobile ring24 with respect to fixedring95.Motor96 moves along withmobile ring24 and thus remains fixed with respect tomobile ring24 upon rotation thereof. Upon sole rotation ofmobile ring24, there thus is no risk fortrocar14 andfirst motor96 to collide. It is thus possible to pivotmobile ring24 by more than 360 degrees.
• A secondelectric motor104 is attached tostirrup26, on the side ofstirrup26 opposite to mountingflange90, via astage106. The shaft (not visible in the drawings) ofsecond motor104 is oriented along pivoting axis Γ ofstirrup26.Stage106 comprises an opening (not visible in the drawings) enabling passing of the shaft ofsecond motor104.Second motor104 is controlled and supplied by means not shown, for example, electric wires. Adrive element108 comprises acircular arc109 having its ends connected by arectilinear portion110 attached tomobile ring24.Drive element108 substantially extends in a plane perpendicular to the plane containingmobile ring24. The axis ofcircular arc109 corresponds to pivoting axis Γ ofstirrup26. The wall ofcircular arc109 opposite torectilinear portion110 comprises a toothing (not shown). The shaft ofsecond motor104 supports a toothed wheel (not shown) which cooperates with the toothing ofcircular arc109 so that, when the toothed wheel is rotated bysecond motor104,stirrup26 is pivoted with respect tomobile ring24. The toothing is provided inside ofdrive element108 for security reasons and to increase the compactness ofpositioning system10.
• The free end ofendoscope12 comprises acylindrical stop112 from which apin114 projects.Compression spring56 bears at one end againstcylindrical stop112 and at the opposite end againsttrocar14. A thirdelectric motor116 is attached tostirrup26, next tosecond motor104, via astage118.Stage118 comprises anopening120 enabling passing ofshaft122 ofthird motor116.Shaft122 ofthird motor116 is oriented along pivoting axis Γ ofstirrup26.Third motor116 is controlled and supplied by means not shown, for example, by electric wires. A windingcylinder124 is arranged at the free end ofshaft122. A helical threading (not shown) is formed on the outer surface of windingcylinder124. A cable125 (only shown inFIG. 4A) is connected at its ends to pin114 and tocylinder124 and wound aroundcylinder124. Thirdelectric motor116 is arranged betweensecond motor104 andprotrusion92 to avoid forcable125 to contactsecond motor104. Whenthird motor116 rotatesshaft122,cable125, guided by the threading ofcylinder124, winds aroundcylinder124 and bringscylindrical stop112 closer totrocar14.Endoscope12 then slides introcar14 along axis Ω and compressesspring56. Whenthird motor116 is no longer actuated,spring56 relaxes and bringsendoscope12 back to an idle position.Cylindrical stop112 may be disassembled fromendoscope12 to enable removal ofendoscope12 fromtrocar14.Cylindrical stop112 may move along withendoscope12. The cable end connected to pin114 can then be detached to releaseendoscope12 fromtrocar14.Stages106,118 may be directly integrated tostirrup26 and/or be provided on a same side ofstirrup26.
• The rotating motions ofendoscope12 introcar14 around its axis Ω may also be controlled by an actuator.
• InFIGS. 4A to4D, ring-shapedbase18 is maintained on the patient via fourcurved tongues126 radially extending from ring-shapedbase18. A support arm, oriented by the surgeon before the beginning of the operation, may be provided to supportpositioning system10 and avoid for the entire weight ofpositioning system10 to be applied on the patient.
• Anotch128 is provided at the level ofmobile ring24, substantially in diametrically opposite position with respect to firstelectric motor96.Notch128 is intended to receive a portion oftrocar14 when the latter is inclined to a maximum with respect to ring-shapedbase18, as shown inFIG. 4B. A maximum inclination angle greater than in the absence ofnotch128 has then been achieved.Notch128 may be replaced by a hollow print performed inmobile ring24 and the shape of which is complementary to the shape oftrocar14.
• InFIGS. 4C and 4D,trocar14 is shown as inclined with respect to the plane of ring-shapedbase18 to a position where the second andthird motors104,116 thrust againstfirst motor96.Trocar14 being attached tostirrup26 by mountingflange90 on the side opposite to second andthird motors104,106, it can then be easily released frompositioning system10.Stirrup26 being maintained in a thrust position againstfirst motor96, an obstacle-free region130 is freed betweenstirrup26 and ring-shapedbase18, enabling removal ofpositioning system10 while leaving inplace trocar14, the dimensions of the obstacle-free region being sufficient to enable removal ofpositioning system10 despite the presence ofprotuberance92 andcylindrical stop112.
• The system according to the present invention has many advantages.
• First, the positioning system according to the present invention takes up a small volume, is formed of a relatively simple structure, and has a small weight. As an example, the applicant has formed a positioning system according to the present invention comprised in a cylinder with a 70-mm diameter and with a 75-mm height, with a weight approximately smaller than 600 g (to which must be added the weights of the endoscope, of the trocar, and possibly of the camera). Several positioning systems according to the present invention may thus be placed simultaneously on a patient's body, each system enabling positioning an observation and/or intervention device.
• Second, the pivoting axis of the endoscope with respect to the patient's body is substantially tangent to the patient's body, which enables reducing to a minimum the dimensions of the incision to be performed for the introduction of the trocar and of the endoscope into the patient's body.
• Third, the positioning system according to the present invention enables obtaining movement angle of the endoscope between axes Ω and Δ greater than 80°, with a 0.5° accuracy and a pivoting axis on the order of 25°/s, and a sliding ofendoscope12 introcar14 along axis Ω greater than 200 mm, with a 5-mm accuracy and a sliding speed on the order of 25 mm/s.
• Fourth, the system enables applying no force on the patient's abdomen at the level of the incision through which the endoscope penetrates into the abdomen.
• Fifth, only three actuators are necessary to control the endoscope displacements. This enables simple and accurate control of the actuators.
• Sixth, the system according to the present invention may easily be arranged on the patient's body and removed from the patient's body while maintaining in place the trocar and the endoscope.
• Of course, the present invention is likely to have various alterations and modifications which will occur to those skilled in the art. In particular, the electric motors may be replaced with any type of actuators. Further, the system according to the present invention enables positioning devices other than an endoscope. It may be, for example, a separator, a clip, etc.

Claims (10)

1. A system (10) for positioning with respect to a patient's body an observation and/or intervention device (12,14) having a portion penetrating into the patient's body comprising:

a base (18,80) laid on the patient's body;

means for supporting the device formed of a first portion (24,82) movably assembled on the base according to a connection with one degree of freedom, and of a second portion (26,88) movably assembled on the first portion according to a connection with one degree of freedom and connected to the device; and

means (43,49,50,60,66) for actuating the first portion with respect to the base, and the second portion with respect to the first portion,

characterized in that the base surrounds at a distance at least partially the device and in that the device is detachably connected to the second portion to enable removal of the positioning system with respect to the device while leaving in place the device with respect to the patient's body.

2. The system ofclaim 1, in which the base (18) delimits a circular opening around the device (12,14) and in which the first portion (24) is a mobile ring with an axis (Δ) substantially perpendicular to the patient's body and with an inner diameter substantially corresponding to the circular opening, the mobile ring being rotatably assembled on the base (18) around its axis.

3. The system ofclaim 2, in which the second portion (26) comprises a head (90,28) connected to the device (12,14) prolonging at least in an arm (30,31) pivotally assembled on the mobile ring (24) along an axis (Γ) substantially tangent to the patient's body.

4. The system ofclaim 1, in which the first portion (82) comprises a semi-circular rail having its ends pivotally assembled on the base (80) along an axis substantially tangent to the patient's body.

5. The system ofclaim 4, in which the second portion (88) comprises a carriage slidably assembled on the rail (82) and connected to the device (12,14).

6. The system ofclaim 1, in which the device (12) is movably assembled on the second portion (28,88) according to a connection with one degree of freedom.

7. The system ofclaim 1, in which the motions of the first (24,82) and second (26,88) portions are controlled by cables (43,49,50) driven by actuators located at a distance from the patient.

8. The system ofclaim 2, in which the rotation motion of the mobile ring (24) with respect to the base (18) is controlled by an actuator (96) attached to the mobile ring, the actuator extending to be substantially parallel to the mobile ring axis (Δ).

9. The system ofclaim 3, in which the pivoting motion of the arm (30,31) with respect to the mobile ring (24) is controlled by an actuator (104) attached to the second portion (26) and extending to be substantially parallel to the pivoting axis (Γ) of the arm (30,31) with respect to the mobile ring (24).

10. The system ofclaim 3, in which the device (12) extends substantially longitudinally along a direction perpendicular to the pivoting axis (Γ) of the arm (30,31) with respect to the mobile ring (24) and in which the device is movably assembled with respect to the second portion (26), to slide along said direction, the device sliding with respect to the second portion being controlled by an actuator (116) attached to the second portion and extending to be substantially parallel to the pivoting axis (Γ) of the arm (30,31) with respect to the mobile ring (24).

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