retention assemblies in accordance with the present invention can be placed directly within a burr hole or within a pre - positioned burr hole ring . whether a separate burr hole ring is used with a particular patient will depend on the specific circumstances involved in that patient &# 39 ; s case . further , as one might imagine , there are many possible sizes and shapes of burr hole rings . the selection of one burr hole ring over another will depend on numerous factors such as the size or shape of the burr hole , whether there is a need to hold multiple snap rings , etc . thus , it should be understood that the following description , by way of example , shows exemplary burr hole ring configurations which may be used in connection with the present invention . additionally , as noted previously herein , the device retaining mechanism may be formed as an integral portion of the burr hole ring , or it may be formed as a part of a separate member selectively attachable to the burr hole ring . such a separate burr hole ring would typically be pre - placed within the burr hole , with the retention mechanism carrying member insertable into the pre - placed burr hole ring . the exemplary configurations of retaining mechanisms disclosed herein are depicted as snap rings which would be independently engageable with a burr hole ring assembly . it should be already understood , however , that each of the disclosed retaining mechanisms may be integrally formed with the burr hole ring . burr hole ring 10 , shown in fig1 - 4 , is a burr hole ring comprising an upper flange portion 12 , lower portion 14 , and a receiver for receiving a snap ring . as shown in fig3 lower portion 14 has a snap ring receiver comprising a c - clip slot 16 . c - clip slot 16 is configured to receive snap ring assemblies such as the ones described herein . preferably , a snap ring released to assume its natural state will fit snugly within c - clip slot 16 so that no movement of the released snap ring relative to the burr hole ring 10 will occur . c - clip slot 16 further need not be located within the lower portion 14 of burr hole ring 10 . both the upper and lower portions 12 , 14 of burr hole ring 10 may have one or more of such slots or other snap ring receiving means . fig4 shows a snap ring 18 positioned within a c - clip slot 16 in the lower portion 14 of burr hole ring 10 . fig4 a shows an exemplary burr hole ring 11 having multiple snap ring receivers comprising c - clip slots , with snap rings 18 , 19 positioned within the two uppermost slots . in the depicted exemplary embodiment , burr hole ring 10 is itself a type of snap ring . burr hole ring 10 is a generally annular member , having a radial gap or opening 24 , and holes or pockets 26 adapted to receive forceps or another suitable device for squeezing burr hole ring 10 to a diameter slightly smaller than the nominal diameter of the burr hole ring when it is in a natural state . it is this squeezing or compression which permits placement of the burr hole ring within a burr hole , and the subsequent expansion which secures burr hole ring 10 within the burr hole . preferably , ridges or threads 28 situated at the outside diameter of lower portion 14 mechanically engage the skull within the burr hole , or engage another ring previously placed at the burr hole , and assist in preventing dislodgement of burr hole ring 10 . as noted earlier , use of a separate burr hole ring is optional depending upon the particular circumstances involved in a particular case . thus , a probe anchoring assembly in accordance with the present invention may comprise a snap ring adapted with an upper flange portion , a lower burr hole engaging portion , or both ; and an anchoring or gripping means coupled to the snap ring for fixing a probe in place . an exemplary embodiment of a snap ring 34 in accordance with the present invention is shown in fig5 - 8 . snap ring 34 is configured with a lead retaining mechanism including at least two arms 36 , 38 extending generally toward the center of the snap ring 34 . the arms 36 , 38 are preferably rigid bars adapted for engaging the lead 32 at any one of a plurality of positions along the lengths of the arms , with beach arm preferably equipped with opposed serrated edges or teeth 37 , 39 for grasping or holding the lead 32 , as shown in fig7 . holes or pockets 35 , preferably located as shown near the opening 33 of the snap ring 34 , are configured to receive forceps or another suitable device for spreading open the snap ring so that it can be placed about the lead 2 ( see fig5 ). when in place about lead 32 , the snap ring 34 then is squeezed together to create , i . e . open , a space between the opposed edges 37 , 39 for receiving lead 32 ( see fig6 ). the squeezing of snap ring 34 , in addition , reduces slightly its diameter , allowing snap ring 34 to be rotated and placed at a desired orientation within a pre - positioned burr hole ring . once snap ring 34 is suitably situated within the previously placed burr hole ring , the anchoring of lead 32 is accomplished by allowing snap ring 34 to assume its natural state , i . e . a condition in which snap ring 34 is neither spread open nor squeezed smaller significantly . under such circumstances , serrated edges 37 , 39 clamp about lead 32 , holding it in place . once the lead is secured in place within the area inside burr hole ring 10 , a flexible cap or plug may be installed to engage burr hole ring 10 to mechanically isolate the aperture and the exposed tissue . typically , the cap will be a silicon elastomer member configured to engage burr hole ring 10 . the lead ( or other device ) will typically be bent to the side . because of the securing of the lead at a location within the burr hole ring , any tendency of the distal portion of the lead to change in position in response to this bending is minimized . another exemplary embodiment of the present invention is shown in fig9 - 12 . a snap ring 40 having an opening 42 is again configured with holes or pockets 44 for receiving a compressing tool to place the snap ring within a previously positioned burr hole ring . in this embodiment , the mechanism for anchoring a lead , catheter , or other positioned device includes a first rigid bar 46 extending from snap ring 40 to within the circle formed by the snap ring ; and a second rigid bar 52 pivotally coupled to snap ring 40 so that rigid bars 46 and 52 form a scissors - like pair of jaws . as shown in the drawings , the pivotal coupling preferably comprises a post 50 extending from a proximate first end of second rigid bar 52 , and a socket 48 disposed in snap ring 40 near the point where rigid bar 46 joins snap ring 40 . socket 48 is adapted to receive post 50 , permitting rigid bar 52 to rotate about socket 48 . ( see fig1 ). post 50 may be secured within socket 48 by deformation , to operatively couple rigid bars 46 and 52 together . fixation points extend along the length of rigid bars 46 , 52 . in this embodiment , bars 46 , 52 will preferably have oppositely opposed serrated edges 54 , 56 which meet upon the rotation of bar 52 into bar 46 and together form a plurality of holes 57 for receiving a lead or catheter 55 . the dimensions and configuration of serrated edges 54 , 56 may be selected in reference to the size of the lead , catheter or other device being retained . a tab 58 preferably extends from the second end of bar 52 , and a hole 61 in bar 46 is adapted to receive tab 58 by an interference fit , so that when a positioned device is clamped between bars 46 and 52 the insertion of tab 58 into hole 61 comprises a locking mechanism securing the two bars together and preventing movement of bar 52 from about the positioned device . yet another exemplary embodiment of the present invention is shown in fig1 . a snap ring 60 having an opening 62 is adapted with holes or pockets 64 for receiving a device for spreading open the snap ring . at least one rigid bar 66 extends from snap ring 60 to within the circle formed by the ring . a plurality of holes 68 are located within bar 66 . the holes 68 are adapted to receive the catheter or lead to be placed . with this embodiment , the catheter or lead must be placed through one of the holes 68 prior to the positioning of the catheter 24 within the cranium as described above . selection of an appropriate hole 68 to receive the catheter or lead will depend upon the desired placement of the catheter or lead within the burr hole . the size and positioning of the holes may be selected in reference to the particular application , as may the size and shape of the bar itself . the orientation of the holes may be selected to facilitate optimal &# 34 ; coverage &# 34 ; of the dimension within the ring , through rotation . this can be generally determined through use of the stereotactic instrument . it is contemplated , therefore , that due to the shape of rigid bar 66 , the number of holes positioned within the bar , and the physician &# 39 ; s ability to rotate the snap ring / bar assembly in placing it within a patient &# 39 ; s skull ( or a burr ring or snap ring ), it will be possible to secure the catheter or lead at virtually any desired location within the burr hole . once the catheter or lead is satisfactorily positioned , more permanent fixation can be accomplished through the use of an adhesive , a grommet , frictional forces , or other suitable means . one such suitable means for fixing a catheter or lead to the rigid bar 66 of snap ring 60 is shown in fig1 a - b . an elongated , generally tubular outer carrier 70 is adapted on one end with means for engaging the bar 66 at the hole 68 selected to receive the catheter 72 to be positioned . preferably , the engaging means comprises a split , flanged collet 74 that locks within hole 68 . when an inner carrier 76 positioned about catheter 72 is placed within outer carrier 70 prior to locking , the subsequent locking of the assembly within the hole 68 creates a frictional or interference fit between the catheter 72 and inner carrier 76 ; between the inner carrier 76 and the outer carrier 70 ; and between the outer carrier 70 and the extension 66 , so that no substantial rotational or translational movement of the catheter 72 within the hole 68 may occur . as shown in fig1 a - b , depending upon the size of the catheter , to achieve a sufficient fit between the catheter 72 and inner carrier 76 , the inner surface 78 of inner carrier 76 may be adapted with one or more ridges 80 . the ridges 80 preferably are integrally formed with the inner carrier 76 , and made of a deformable , soft polymer such as polyurethane . of course , rings or seals also may be used within the annular space between the catheter 72 and inner carrier 76 to help hold a catheter or wire lead in place . again , the primary consideration is that upon the latching assembly being locked in place ( see fig1 b ) no undesirable movement of the catheter or lead may occur . fig1 a - b illustrate a modified burr ring / snap ring assembly in accordance with the present invention as holding in place a wire lead 82 . a snap ring 84 ( of the type depicted in fig1 ) is housed within the receiving slot of a modified burr ring . the wire lead 82 secured at a desired location 94 to extension 86 passes through the chamber 88 located beneath septum 90 and is situated within a channel 92 formed in the modified burr ring . because of the securing of the wire lead at the point 94 within the burr hole ring , any tendency of the distal portion of the lead situated within the patient &# 39 ; s body to change in response to bending of the proximate end of the wire lead is minimized . there are a variety of materials which can be used to construct the snap rings and burr hole rings discussed herein . preferably , the components are made of a durable , lightweight metals or polymers . examples of materials which might be used to construct the components disclosed herein include one or more of the following , either alone or in combination : metals , such as stainless steel , titanium , or other biocompatible alloys ; and polymers , such as polyurethane ( e . g . 75d ), polysulfone , or nylon . the burr hole plug preferably would be made of a softer polymer such as polyurethane ( e . g . 80a ) or silicone . of course , there are numerous options available for materials , and those listed above should be considered only as examples of materials which might be used . the selection of suitable materials will depend upon the circumstances surrounding the particular application involved . although the preferred embodiment of this invention has been described hereinabove in some detail , it should be appreciated that a variety of embodiments will be readily available to persons utilizing the invention for a specific end use . the description of the apparatus and method of this invention is not intended to be limiting on this invention , but is merely illustrative of the preferred embodiment of this invention . other apparatus and methods which incorporate modifications or changes to that which has been described herein are equally included within this application . additional objects , features and advantages of the present invention will become apparent by referring to the above description of the invention in connection with the accompanying drawings .