reference is now made to fig1 to 8 , which is a series of schematic illustrations of a first exemplary implementation of the automatic dilators described in this disclosure , showing the sequential stages of insertion of the dilator . the device comprises a number of hollow dilator tubes having incrementally increasing diameters , such that the dilator tubes fit sequentially within each other to form a nest of dilator tubes of increasing diameter , starting with the innermost and finishing with the outermost . the outer diameter of any one of the set is a sliding fit within the inner diameter of the next largest dilator tube , such that the set covers a complete range of diameters from the inner diameter of the innermost dilator to the outer diameter of the outermost dilator . each of the dilator tubes has an internal screw thread over the whole length of its inner wall , and each of the dilator tubes , except the outermost one , has a section of screw thread formed on its outer wall at its proximal end . this section can be a short section covering typically only one or a few turns . although the short screw thread section is used as the example for this implementation , it is to be understood that the devices are not intended to be so limited , but that the section of screw thread can be replaced by one or more teeth or tabs , or any other elements protruding from the outer wall , that can mate with the internal thread of the inner spreader . as a result of this combination , when a particular dilator tube is rotated , the short external screw thread section at its proximal end riding within the internal screw thread of the dilator tube immediately external to it , causes the dilator tube external to the rotated tube to move longitudinally relative to the position of the dilator tube being rotated , the relative direction of motion depending on the handedness of the threads generating the motion . since it is conventional practice to positively activate rotary mechanical mechanisms by means of clockwise rotation of the actuating handle , and since the function of the auto - dilator is to drive the dilator tubes distally into the patient &# 39 ; s tissues , the screw threads should be left - handed so that clockwise rotation of the actuating handle of the auto - dilator results in the desired distal motion of each successive outer tube . in fig1 to 8 , there is shown an exemplary auto dilator using the above described arrangement of internal and external threads , for use in orthopedic applications , though it is to be understood that this is merely one exemplary implementation , and that dilators for use in other applications can also be constructed in the same manner . the exemplary device shown in fig1 to 8 comprises five separate dilator tubes or spreaders , 12 , 16 , 17 , 18 , 19 , though a different number can also be used . reference is now made to fig1 , which shows the device after the first stage of insertion has been performed . the auto dilator is first positioned on the patient &# 39 ; s skin 13 , and , using the drive handle 14 at the proximal end of the device , the innermost element , a spiked rod 10 , is driven into the subcutaneous tissue down to the bone surface 11 , to which the auto dilator is intended to prepare an access passageway . a guide wire ( not shown in the drawings ) can be optionally used in order to define the position of insertion of the innermost element . the distal end of the spiked rod is adapted to make firm or penetrating contact with the bone surface 11 . it may be fitted with a freely rotatable point , such that the rod can rotate freely even whilst its spiked point is firmly embedded in the bone , though this feature is not essential , since the spiked point may also be adapted to rotate within the bone surface without being dislodged from its intended position . the spiked rod has a number of external threads or protrusions 15 at its proximal end , so that as the rod is turned in a clockwise direction , the teeth of its external screw thread or the protrusions running within the groove of the internal thread of the spreader immediately external to it , causes that first spreader 12 to move distally down through the patient &# 39 ; s skin 13 , spreading the tissue as it proceeds . the chamfered distal end of the innermost spreader , with its leading edge sliding along the outer surface of the spiked rod , assists in enabling it to penetrate the patient &# 39 ; s tissue with minimal damage to the tissue . once the innermost spreader 12 reaches its destination position against the bone surface 11 , it can no longer move longitudinally into the patient &# 39 ; s tissue , and since the spiked rod is continuing to turn , the innermost spreader 12 locks itself to the rotating spiked rod , and rotates itself in unison with the spiked rod . once this action takes place , the commencement of the rotation of the few external threads 15 or the protrusions at the proximal end of the innermost spreader 12 , which are meshed with the internal thread of the next spreader 16 ( next in the sense of the radially outward direction from the innermost spreader ), cause that next spreader 16 to move distally down the innermost spreader 12 by means of the screw action of the two meshing thread forms . this action continues until , as shown in fig2 , the next spreader 16 also reaches its destination at the surface of the bone 11 , and its downward motion is stopped . at that point , it too begins to rotate in unison with the innermost spreader 12 and the spiked rod 10 , and by the same action as it has itself previously undergone , using the few external threads or the protrusions at the proximal end , it begins to pull the next outermost spreader , the third spreader 17 , downwards into the patient &# 39 ; s tissue . this process continues until the third spreader 17 reaches its most distal point , as shown in fig3 , when the fourth spreader 18 begins its distal motion , and so on in fig4 and 5 , in which the fourth spreader 18 , and the fifth spreader 19 are shown to have been respectively completely inserted . the chamfered leading edge of each spreader is the leading edge , which is that which slides along the outer surface of the previously deployed spreader , thus providing a spreading effect to the tissue it is passing through , rather than a cutting effect . fig5 now shows all five of the spreaders fully inserted , having spread the patient &# 39 ; s tissue to the required diameter for the access aperture desired . as seen in fig5 , the outermost spreader 19 — the fifth spreader in this example device — is longer than the previous , more internally disposed spreaders , and it is this outer spreader which constitutes the dilator tube , which will be used for providing access to the surgical site on the patient &# 39 ; s bone . however , the dilator is not yet usable since its entire internal volume is taken up with the previous spreaders and the spiked rod . in order to utilize the dilator , it is necessary to remove all of the internal spreader tubes 12 , 16 , 17 , 18 , and the spiked rod 10 , with their drive mechanism , leaving only the outermost spreader 19 . reference is now made to fig6 which illustrates how this is achieved . in fig6 , the actuating handle 14 is being rotated anticlockwise , and this action causes the nest of internal spreaders 12 , 16 , 17 , 18 , to rotate anticlockwise also , such that the outer screw thread on the fourth , penultimate spreader 18 meshing with the internal screw thread of the outermost spreader 19 , causes the entire internal nest of spreaders to move proximally upwards through the outermost spreader 19 , until they can be removed from the outermost spreader 19 . the spiked rod 10 can then also be removed proximally , as shown in fig7 , followed by the activation handle and its mechanism 14 , leaving just the outermost spreader tube 19 , as shown in fig8 , which can then be utilized as the dilator tube for the surgical procedure to be undertaken . reference is now made to fig9 , which is a schematic assembly drawing of a second exemplary implementation of the automatic dilators described in this disclosure . fig9 also includes a drawing of the assembled automatic dilator . this auto - dilator differs from that shown in fig1 to 8 , in that it incorporates positive mechanical mechanisms for ensuring that each successive spreader reaches its destined position and is locked into place , and to ensure that the spreader deployment is performed in the correct sequential manner . in addition this implementation , more readily enables the auto dilator to be used in soft tissue without a bone structure to position its destination point accurately . as in the previous implementation , this auto dilator operates on the principle of a nested series of spreaders , each , except the innermost one , having an external thread form which engages with an internal thread section on the spreader immediately outward of it . however , unlike the previous implementation , in this model , each spreader is inserted longitudinally by a conventional screw action , in that as a particular spreader is rotated , it descends distally through the neighboring spreader immediately outward of it and into the patient &# 39 ; s tissues , by conventional screw action of its external thread screwing into the internal thread of that outer spreader . in fig9 , there is shown an auto dilator with four nested spreaders , marked 91 , 92 , 93 and 94 in descending order of size . spreader 91 has a cover 116 , spreader 92 has a cover 96 , and spreader 93 has a cover 95 all of which are fixed onto the proximal ends of the spreaders . the cover of innermost spreader 94 is already attached to the top end of spreader 94 . a hollow splined rod 111 runs through all of these covers to the distal end of the spreaders , as shown in more detail in the blown - up drawing of the hollow splined rod running through cover 95 . the proximal end of the hollow splined rod 111 is attached to a rotating handle 112 , such that when handle 112 is rotated , the splined rod 111 rotates with it . a short section 100 of the proximal end of the splined rod has a reduced diameter , without splines . inside of the hollow splined rod 111 , there is a rod 98 having a spiked end , similar to that shown in the embodiment of fig1 to 8 . at the distal end of the spiked rod 98 , there is a pointed tip 120 , to enable insertion of the spiked rod with minimal tissue damage , and which defines the position to which the auto dilator is to be inserted . in addition , there is a mechanism 89 , 99 , 110 , at the distal end for locking the innermost spreader 94 to the spiked rod 98 . the spiked rod 98 extends proximally to an operating handle 119 used for its insertion into the patient &# 39 ; s tissue . between the rotating handle 112 and the cover 116 of the outermost spreader 91 , there is a collar - shaped cylindrical enclosure 114 , with a raised shoulder at its proximal end , and the rotating handle 112 fits over this raised shoulder and is attached to the cylindrical enclosure 114 by means of locking screws 113 which fit under the shoulder after the rotating handle has been mounted over the shoulder , but which do not clamp the cylindrical enclosure , such that they allow the handle 112 to rotate freely relative to the cylindrical enclosure 114 . the cylindrical enclosure 114 can be held stationary while the rotating handle 112 is rotated by means of a static handle 125 attached to the cylindrical enclosure . the cylindrical enclosure 114 has two spiral grooves 101 diametrically formed in its outer surface . a pair of positioning screws 117 , one passing through each of the spiral grooves , is screwed into the top cover 116 , and the position of the positioning screws 117 in the spiral grooves defines the longitudinal position of the cover 116 inside the collar shaped cylindrical enclosure 114 . three predefined positions are formed in the spiral grooves 101 , defining three different heights of the cover 116 within the collar shaped enclosure 114 , and hence also the longitudinal distance of the rotating handle 112 from the top of the spreader assembly , as defined by the cover 116 of the outermost spreader 91 . although in fig9 , the longitudinal height of the cover 116 inside the collar - shaped enclosure is shown as being determined by the position of two positioning screws 117 moving in two spiral slots 101 , it is to be understood that this is only one method by which the height of the cover 116 can be adjusted and set , and that any other mechanism which achieves this object can equally well be used . attached to the distal side of the rotating handle 112 , there is a slip ring bearing 115 , having two rotation prevention pins 118 extending in a distal direction into the covers of the spreaders . further details of the component parts will now be expounded in the following explanation showing how the auto dilator operates . reference is now made to fig1 to 24 in conjunction with fig9 , to illustrate how this exemplary auto - dilator operates , and to explain the sequential stages of insertion of the spreaders . each of fig1 to 24 illustrates a particular stage of insertion of the auto - dilator , generally in two views — a side elevation view and a cross sectional view showing the disposition of the internal parts . in fig1 , the auto dilator is shown ready for insertion . in this drawing , it is observed that the length of the spreaders are linearly staggered , such that the outermost spreader 91 is the longest and the succeeding spreaders 92 , 93 , 94 are successively shorter , each difference in length being equal to the incremental change in height of the cover 116 between each preset position of the positioning screws 117 . the differences in height are manifested at the top ends of the spreaders , such that the rotation prevention pins 118 protrude different lengths into the successive spreaders . additionally , the unsplined top section 100 of the splined rod 111 is of such a length that in the initial undeployed situation of fig1 , the splines only mesh with the innermost spreader 94 , which is the spreader with the most distally positioned proximal end . in fig1 , there is shown insertion of the spiked rod 98 into the patient &# 39 ; s tissue by means of pressure on , and also optionally rotation of the handle 119 . the other details are as numbered in fig1 , and are not repeated in this drawing or the following ones , if not necessary to explain operation of the auto - dilator . in fig1 , insertion of the innermost spreader 94 is commenced by the rotation of the handle 112 , such that the splined rod 111 rotates with it . because of the dense detail of the assembled dilator drawings in fig1 to 24 , reference should also be made to the assembly drawing of fig9 to see clearly how the insertion procedure is executed . in fig1 , as the splined rod rotates , its splines , being meshed with the inner splines in the cap of the innermost spreader 94 , also cause the innermost spreader 94 to rotate . the meshing of the outer thread of the innermost spreader 94 with the internal thread of spreader 93 , its immediately neighboring outwardly positioned spreader , causes spreader 94 to screw downwards into the patient &# 39 ; s tissue . because of the clear unsplined top section 100 of the splined rod 111 , rotation of the splined rod does not have any effect on the other spreaders , which are not rotated by rotation of the splined rod . the cylindrical enclosure 114 and all of the remaining components attached thereto are kept from rotating by means of the handle 125 . furthermore , the rotation prevention pins 118 attached to the rotating handle 112 , project into covers 95 and 96 of spreaders 93 and 92 respectively , holding them static . when spreader 94 reaches its bottommost position , as shown in fig1 , its distal end pushes an annular locking ring 99 ( see fig9 ) in a distal direction , thereby releasing the locking fingers 89 of the locking spring 110 , shaped like an omega , which can then expand in an output direction into an internal groove 126 formed on the end of innermost spreader 94 . this action locks innermost spreader 94 to the spiked rod 98 , which , since it is held in a static position within the patient &# 39 ; s tissue by means of handle 119 , also clamps innermost spreader 94 into position at its most distal position , with its end at the target position to which the auto - dilator is to be deployed in the patient &# 39 ; s tissue . furthermore , the innermost spreader 94 is also prevented from rotating further by means of this clamping mechanism . at this point , the innermost spreader 94 has thus been deployed into its correct within the patient &# 39 ; s tissue . the length of the splines on the shaft 111 are such that when the spreader 94 reaches its completely deployed position , its cap 99 has just slipped off the end of the splines on shaft 111 , which can therefore no longer continue to rotate spreader 94 . reference is now made to fig1 , which illustrates the following step , in which the next spreader 93 , in an outward direction , must now be deployed . in order to do so , two actions must be performed : ( i ) firstly , the splined drive shaft 111 must be released from engagement with innermost spreader 94 , and must be engaged with the next outer spreader 93 . ( ii ) secondly , the rotation prevention pins 118 must be retracted from spreader 93 to enable it to be turned freely by the splined driver shaft 111 . both of these results are achieved by the single action of freeing the positioning screws 117 in the cylindrical enclosure 114 , and by moving the enclosure 114 together with its attached rotary handle 112 in a proximal direction until the next screw position is reached , and by retightening the positioning screws 117 in that position , as can be observed by comparison of the position of the screws 117 in the elevation drawings of fig1 ( positioning screws 117 in top position ) and fig1 ( positioning screws 117 in middle position ). reference to the sectional drawing of fig1 now shows the operative result of this action . both the splined drive shaft 111 and the pin assembly 115 are attached to the rotary handle 112 . therefore , the motion of the rotary handle 112 in the proximal direction , away from the nest of spreader tubes partially inserted into the patient &# 39 ; s tissue , results in the motion proximally of the splined drive shaft 111 and the pin assembly 115 with its rotation prevention pins 118 away from the spreader tube assembly , as can be readily seen by the enlarged space 127 between the cover 116 of the outermost spreader tube 91 and the roof of the cylindrical enclosure 114 to which the rotary handle 112 is attached . this action essentially enables the two desired results mentioned above : ( i ) the splined drive shaft moves proximally by an amount which releases its distal end from engagement with the internal spline grooves of the innermost spreader tube 94 , and at the same time , the proximal end of the splines moves up and now engages the cover 95 of the second spreader tube 93 , where previously , in fig1 , that cover was positioned longitudinally in the unsplined top section of the splined shaft , so that it did not rotate with rotation of the splines . ( ii ) the rotation prevention pins 118 move proximally by an amount which frees their insertion into the cover 95 of spreader 93 , such that spreader 93 is no longer prevented from rotating , and can now be turned freely by the splined driver shaft 111 . reference is now made to fig1 , which illustrates the result of rotation of handle 112 , such that the splined rod 111 rotates with it and consequently also the second to innermost spreader 93 . its external threads propel it down the internal thread system of the next outward spreader 92 , further spreading the patient &# 39 ; s tissues , until it reaches its distal target position next to the innermost spreader , where the splines on the splined drive shaft 111 slip out of the internal splined grooves in its cover 95 , and thus no longer propel it distally downwards . at this point both of the innermost spreaders are deployed and reference is now made to fig1 , which illustrates how the device is adjusted again , so that the third from the innermost spreaders 92 can now be deployed . this is done in a similar manner to that shown in fig1 , but this time , the positioning screws 117 are shifted from the middle position in the cylindrical enclosure to the most distal position . the height of space 127 then increases even more from its height when pins 117 were in the intermediate position . pins 118 now only prevent cover 116 of outer spreader 91 from rotating . in this position , the proximal motion of the splined drive shaft 111 and the rotation prevention pin assembly 115 and its pins 118 away from the spreader tube assembly causes the splined drive shaft to release from engagement with the internal spline grooves of the cover 95 of spreader tube 93 , and to engage with the spline grooves of cover 96 of spreader tube 92 , and in addition , causes the rotation prevention pins 118 to move proximally to free their insertion in the cover 96 of spreader 92 , such that spreader 92 is no longer prevented from rotating , and can now be turned freely by the splined driver shaft 111 . rotation of handle 112 can then be used to deploy the third innermost spreader 92 , further spreading the patient &# 39 ; s tissue until it reaches the target destination alongside the first two spreaders , 93 , 94 . this position is shown in fig1 , where the presence is shown of the locking screws 113 , which lock the rotating handle 112 to the cylindrical enclosure body 114 . reference is now made to fig1 to illustrate the deployment of the final and outermost spreader 91 . since the outer spreader 91 cannot be inserted by screwing into any other element external to it , a somewhat different technique must be adopted . the locking screws 113 used to attach the rotating handle 112 to the cylindrical enclosure body 114 are removed , enabling the rotating handle 112 to be physically disconnected from the cylindrical enclosure body 114 . at this point , rotation of the cylindrical enclosure body 114 attached to the cover 116 of the outermost spreader 91 , enables the outermost spreader 91 to be screwed distally over the external threads of the penultimate spreader 92 , until it reaches its fully deployed position over the first three spreaders . a fine external thread on the outer surface of the outer spreader , and even on the outer surfaces of the other spreaders , may facilitate their insertion into the tissue of the subject , though such outer threads play no part in the dynamics of the insertion mechanism of the device . reference is now made to fig1 , which shows the outer spreader 91 fully deployed , and the rotating handle 112 at the top end of the splined shaft 111 on the unsplined part 100 , while the cylindrical enclosure body 114 has been screwed down with the outermost spreader towards the entry point of the device on the patient &# 39 ; s skin . all that remains now in order to leave the outermost spreader 91 lodged in the patient &# 39 ; s tissues , is for the nest of the three innermost spreaders 92 , 93 , 94 , to be removed from inside the outermost spreader 91 , together with the drive spline shaft , the spiked rod , and the cylindrical enclosure body 114 . this procedure is now shown in fig2 to 23 . referring first to fig2 , the positioning screws 117 are removed from the spiral groove in the cylindrical enclosure 114 . then in fig2 , there is shown that the cylindrical enclosure 114 , together with the top cover 116 of the outermost spreader 91 can be raised , leaving only the nest of spreaders and the splined drive shaft 111 , held in the patient &# 39 ; s tissues . reference is now made to fig2 , which shows how , by rotation of the splined drive shaft 111 , together with the spiked rod , in an anticlockwise direction , all of the spreaders 92 , 93 , 94 , and other components internal to or attached to the outermost spreader 91 , screw out of the outermost spreader 91 , leaving it alone penetrated through the patient &# 39 ; s skin 113 , and fixed in the patient &# 39 ; s tissue , as shown in fig2 . as previously mentioned , in this described implementation , all of the motion steps are described only as mechanical motion steps without reference to the manner in which these steps are performed . it is to be understood that the disclosure is intended only to explain the mechanical steps necessary in order to deploy the described auto dilator . in this respect , although manual operation may be the simplest way of using the device , it is to be understood that the disclosure is in no way intended to limit the device to such use . thus , if the device is to be completely automated , such that it could be used on a robotic surgical system , rotary and linear actuators could be used for each rotary or linear motion step , with the sequence being controlled and monitored by means of a robotic controller , and sensors or encoders incorporated on the various mechanical parts . for instance , the use of manually adjusted positioning screws 117 in a spiral groove 101 in the cylindrical enclosure 114 could readily be replaced by any other method of controlling and locking the longitudinal motion required . thus , for instance , a motorized linear motion drive could readily be used in fully robotic operation of the auto - dilator . the various rotary motions could also readily be replaced by at least some of servomotors with rotary encoders and position sensors . this second exemplary implementation of the automatic dilators described in this disclosure has a possible disadvantage or inconvenience in that the longitudinal position of the cylindrical enclosure 114 has to be shifted by means of the positioning screws 117 , or an alternative positioning mechanism , to enable deployment of successive spreaders . reference is now made to fig2 which illustrates schematically a third exemplary implementation of an automatic dilator as described in this disclosure , in which the complete insertion procedure is performed by one continuous screwing motion . fig2 to 27 illustrate how the dilator of fig2 operates through its various stages of insertion into the subject &# 39 ; s tissue . the exemplary dilator shown in fig2 to 27 has an internal dilation rod with only two spreaders external thereto , but it is to be understood that this is only one example , and that the device could have a larger number of spreaders to increase its expansion range . like the previously described examples , this implantation also depends upon the rotation of the thread form of an inner element meshed with the thread form of the spreader tube element immediately external to it , such that as the inner thread form is rotated , if the outer spreader tube is prevented from rotating , it will move longitudinally and into the subject &# 39 ; s tissue . this implementation differs from the previous ones in that the rotating inner thread takes the form of a set of radially positioned , spring - loaded , hinged pawl elements , each having a toothed protrusion which meshes with the inner thread of the spreader immediately external to it , driving it distally into the tissue as the toothed protrusions rotate . as soon as the immediately external spreader element reaches its distal deployed location , the toothed protrusions on the hinged pawl elements slip off the end of the internal thread on that first spreader element , and being spring - loaded in an outward direction , now mesh with the internal thread on the next outwardly positioned spreader element , and rotation of the innermost element now begins to move that next outwardly positioned spreader longitudinally into the tissue . in fig2 , there is shown an assembled and an exploded isometric view of an exemplary design of this implementation of the complete auto dilator 150 , showing its component parts . the innermost element of the device is the spiked rod 157 , which has a sharpened spike 177 at its distal end . this is the embodiment shown in the assembled drawing on the right hand side of fig2 . according to an alternative implementation , in order to ease insertion of the spiked rod into the patient &# 39 ; s tissue , a number of graded chamfers 166 can be provided to the spiked rod 157 , as shown in the exploded unassembled view on the left hand side of fig2 , to expand its diameter to the internal diameter of the first spreader tube 152 . in this case , the first spreader tube 152 should be allowed to extend only along the parallel section of the spiked rod , to avoid unnecessary trauma to the patient &# 39 ; s tissue as it is inserted . a handle 151 is provided at the proximal end of the spiked rod . the spiked rod is incorporated within an actuating tube 158 , which has an internal bore adapted to take the spiked rod . in the example shown in fig2 , the actuating tube 158 sits on a shoulder 159 on the spiked rod , so that the enlarged outer diameter of the spiked rod is contiguous with the outer diameter of the actuating tube 158 . in the construction shown in fig2 , the spiked rod 157 , with its distal spiked spreader point needs to be dismountable in order for it to be inserted into the actuating tube 158 . the actuating tube 158 has its own rotation handle 153 at its proximal end . at its distal end , the actuating tube has a number of grooved longitudinal indentations 160 , four in number in the example shown in fig2 , though it is to be understood that any other practical number of such grooved indentations could be provided , even a single indentation . in each of these grooved indentations 160 , there is fitted a spring loaded pawl element 156 , hinged on a pin 54 such that the pawl element 156 is enabled to swing in an outwards direction . the pawl elements 156 are fitted with an internal spring ( not shown ) which biases them in the outward direction . the pawl elements 156 each have an externally protruding tooth 161 at the end opposite to that at which they are hinged . the entire inner assembly comprising the spiked rod 157 and the actuating tube 158 with its spring - loaded pawl elements 156 , is located within the inner bore of the first spreader element 152 , with its chamfered distal end . this spreader element has an internal thread 162 and the externally protruding teeth 161 of the spring - loaded pawl elements 156 fit into the internal thread 162 . the externally protruding teeth 161 should be in axially staggered positions such that they are positioned to all fit correctly into the internal thread 162 , even though they contact the internal thread at different circumferential orientations . external and concentrically to the first spreader element 152 , there is located a second spreader element 155 , also having an internal thread form 164 , and although no further spreader elements are shown in the device of fig2 , it is to be understood that more spreaders could be incorporated external to second spreader element 155 . reference is now made to fig2 to 27 , which include assembled and cut away sections of the assembled auto dilator of fig2 , to illustrate how the device operates in use . in fig2 , section a - a shows the assembled auto dilator 150 , with the operative structure of one of the spring - loaded pawl elements 156 shown in blown - up detail . referring to this blown - up drawing , the spring - loaded pawl element 156 is shown with its protruding tooth 161 lodged in the internal thread 162 of the first spreader element 152 . in order to deploy the auto dilator , after the spiked rod 157 has been inserted into the tissue to the depth required , the rotation handle 153 of the first spreader element 152 is turned while the handle 151 of the spiked rod 157 is held stationary , in case the friction of the spiked rod 157 and its spike 177 stuck into the subject &# 39 ; s tissue does not hold it stationary . as the first spreader element 152 rotates , the static protruding tooth 161 riding in its internal thread , causes the first spreader element 152 to move distally over the spiked rod and into the tissue , widening the opening in the tissue . section b - b shows the device in axially directed cross section . reference is now made to fig2 , which shows the first spreader element 152 almost fully deployed , with the protruding teeth 161 of the spring - loaded pawl elements 156 still riding in the internal thread 162 of the first spreader element 152 . the blown - up detailed drawing shows this clearly . reference is now made to fig2 , which shows the situation when the first spreader element 152 has reached its most distal deployed position . the length of the spreader element 152 and hence of the internal thread 162 of the first spreader element , is adapted to be such that as the spreader element reaches its fully deployed position , the toothed protrusion 161 of the spring - loaded pawl 156 jumps out of the internal thread 162 in an outwards direction because of its spring loading , until it lodges within a groove at the distal end of the internal thread 164 of the second spreader element 155 , as shown in the blown - up detailed drawing . as the handle 153 continues to rotate , together with the toothed protrusion , the second spreader tube 155 now starts to move distally over the previously deployed first spreader element 152 , actuated by the protruding tooth 161 now riding in the internal thread 164 of the second spreader tube 155 . once the second spreader tube is fully deployed , further spreader tubes can be sequentially installed , with the toothed protrusions of the spring - loaded pawls jumping from one internal thread to the next in outward going order as each spreader tube completes its insertion and releases the protruding tooth element 161 from its own thread to spring further out to engage the internal thread in the next outwardly positioned spreader tube . although this third implementation has been described using a toothed projection on a hinged , spring loaded pawl as the element used for engaging the inner threads of the consecutive spreader tubes , it is to be understood that this is only one exemplary method by which to achieve engagement with the internal thread form . the invention is not intended to be limited thereto , but includes any alternative structure involving an externally protruding tooth , or teeth , which are rotated so as to cause the internal threaded element with which it or they engage to move distally . this implementation thus enables insertion of a sequence of spreader tubes by means of a continuous rotation action , without the need for any intermediate action in order to switch from one spreader tube to the next . this implementation is therefore suitable for a simple robotically actuated entry procedure . it is appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove . rather the scope of the present invention includes both combinations and subcombinations of various features described hereinabove as well as variations and modifications thereto which would occur to a person of skill in the art upon reading the above description and which are not in the prior art .
automatic dilator devices for generating minimally invasive access apertures for surgical procedures or endoscopic surveillance . the automatic dilators comprise a number of spreader tubes nested one inside the other . the devices deploy automatically by means of coupled mechanical mechanisms which insert one spreader after the other distally into the patient &# 39 ; s tissue . each spreader moves distally into the tissue by means of a screwing action , by which rotation is converted into linear motion of the neighboring spreader , immediately external to it , by means of interaction between a helical thread form on a surface of a spreader being engaged by a section of thread , or by one or more protrusions on the opposing face of the next spreader external to the rotating spreader . such a combination of helical thread and follower enables a rotatory mechanism to be used to deploy one nested spreader tube after the other , by continuous rotary motion .