Patent Publication Number: US-2022226123-A1

Title: Adjustable Implant

Description:
FIELD AND BACKGROUND OF THE INVENTION 
     The present invention relates to surgical implants and, in particular, it concerns an adjustable implant including a telescopic body. 
     It is known to employ adjustable implants which may be inserted into the body and then expand or otherwise change shape to assume a final shape. The present invention relates primarily, although not exclusively, to a subset of such devices in which the implant includes a telescopic body. i.e., where a body includes first and second portions which undergo relative motion towards and/or away from each other so that a length of the telescopic body can be varied. 
     SUMMARY OF THE INVENTION 
     The present invention is an adjustable implant. 
     According to the teachings of the present invention there is provided, an adjustable implant comprising: (a) a telescopic body comprising a first portion and a second portion, the first and second portions being in sliding engagement such that a length of the telescopic body is adjustable from a first length to a second length; and (b) a deflectable linkage comprising a first linking segment, an intermediate segment pivotally connected to the first linking segment about a first pivot axis, and a second linking segment pivotally connected to the intermediate segment about a second pivot axis, the first linking segment being pivotally connected to the first portion and the second linking segment being pivotally connected to the second portion such that adjustment of a length of the telescopic body causes a corresponding deflection of the deflectable linkage, wherein the first linking segment and the second linking segment are formed with projecting features configured to provide a partial gear engagement between the first and second linking segments such that, during adjustment of a length of the telescopic body and corresponding deflection of the deflectable linkage, pivotal motion of the first and second linking segments relative to the intermediate segment about the first and second pivot axes occurs in a fixed ratio defined by the partial gear engagement, and wherein the telescopic body and the deflectable linkage form at least part of an implant for deployment within a human body. 
     According to a further feature of an embodiment of the present invention, the projecting features define a gear tooth engaged in a complementary gear trough. 
     According to a further feature of an embodiment of the present invention, the partial gear engagement is configured such that pivotal motion of the first and second linking segments relative to the intermediate segment occurs equally and oppositely. 
     There is also provided according to the teachings of an embodiment of the present invention, an adjustable implant comprising: (a) a body having a length; (b) a deflectable linkage comprising a first linking segment, an intermediate segment pivotally connected to the first linking segment about a first pivot axis, and a second linking segment pivotally connected to the intermediate segment about a second pivot axis, the first linking segment being pivotally connected to the body about a third pivot axis and the second linking segment being pivotally connected to the body about a fourth pivot axis; and (c) a mechanism for adjusting a distance between the third and fourth pivot axes, thereby causing deflection of the deflectable linkage, wherein the first linking segment and the second linking segment are formed with projecting features configured to provide a partial gear engagement between the first and second linking segments such that, during adjustment of the distance between the third and fourth pivot axes, pivotal motion of the first and second linking segments relative to the intermediate segment about the first and second pivot axes occurs in a fixed ratio defined by the partial gear engagement, and wherein the body and the deflectable linkage form at least part of an implant for deployment within a human body. 
     According to a further feature of an embodiment of the present invention, the mechanism for adjusting a distance between the third and fourth pivot axes comprises a screw actuated mechanism for adjusting a length of the body. 
     According to a further feature of an embodiment of the present invention, the mechanism for adjusting a distance between the third and fourth pivot axes comprises a telescopic adjustment mechanism for adjusting a length of the body. 
     According to a further feature of an embodiment of the present invention, the projecting features define a gear tooth engaged in a complementary gear trough. 
     According to a further feature of an embodiment of the present invention, the partial gear engagement is configured such that pivotal motion of the first and second linking segments relative to the intermediate segment occurs equally and oppositely. 
     There is also provided according to the teachings of an embodiment of the present invention, an adjustable implant comprising: (a) a telescopic body comprising a first portion and a second portion, the first and second portions being in sliding engagement such that a length of the telescopic body is adjustable from a first length to a second length; and (b) a deflectable linkage comprising at least two interconnected segments including a first end segment and a second end segment, the first end segment being in articulated connection with the first portion and the second end segment being in articulated connection with the second portion such that adjustment of a length of the telescopic body causes a corresponding deflection of the deflectable linkage, wherein the first portion and the second end segment are formed with complementary cooperating surfaces shaped such that, during adjustment of a length of the telescopic body and corresponding deflection of the deflectable linkage, relative motion of the first portion and the second end segment maintains the cooperating surfaces in strain-limiting proximity, and wherein the telescopic body and the deflectable linkage form at least part of an implant for deployment within a human body. 
     According to a further feature of an embodiment of the present invention, the cooperating surfaces are configured to avoid contact in an unstressed form of the implant. 
     According to a further feature of an embodiment of the present invention, the cooperating surface of the second end segment includes a convexly curved bulge. 
     According to a further feature of an embodiment of the present invention, each of the first end segment and the second end segment is pivotally interconnected with an intermediate segment so as to be pivotable relative to the intermediate segment about respective first and second spaced-apart pivot axes. 
     According to a further feature of an embodiment of the present invention, the intermediate segment is formed with a tissue contact surface that extends along more than half a maximum length of the telescopic body. 
     According to a further feature of an embodiment of the present invention, the first end segment and the second end segment are interconnected so as to be relatively pivotable about a pivot axis. 
     According to a further feature of an embodiment of the present invention, the first end segment is formed with a tissue contact surface that extends along more than half a maximum length of the telescopic body. 
     There is also provided according to the teachings of an embodiment of the present invention, an adjustable implant comprising: (a) a telescopic body comprising a first portion and a second portion, the first and second portions being in sliding engagement such that a length of the telescopic body along an axis of the telescopic body is adjustable; and (b) a headless bolt deployed in a region of overlap between the first and second portions, the headless bolt having a threaded outer surface and having first end and second end abutment surfaces, wherein the first portion is formed with entrapment features configured to abut at least one of the first end and second end abutment surfaces of the bolt so as to prevent displacement of the bolt relative to the first portion in at least one direction along the axis, and wherein the second portion is formed with at least one elongated threaded surface deployed to engage the threaded outer surface of the bolt, the elongated threaded surface having a length greater than a length of the headless bolt, and wherein the telescopic body forms at least part of an implant for deployment within a human body. 
     According to a further feature of an embodiment of the present invention, a length of the headless bolt is less than the range of adjustment. 
     According to a further feature of an embodiment of the present invention, the length of the elongated threaded surface is sufficient to span a range of adjustment corresponding to a difference between a first length and a second length of the telescopic body. 
     According to a further feature of an embodiment of the present invention, the at least one elongated threaded surface is implemented as at least two elongated threaded surfaces deployed to engage spaced-apart regions of the threaded outer surface of the bolt. 
     According to a further feature of an embodiment of the present invention, the sliding engagement of the first and second portions is defined by sliding abutment surfaces of the first portion including two inward-facing walls, and sliding abutment surfaces of the second portion that are provided together with the clongatcd threaded surfaces by surfaces of two elongated projections, the elongated projections being shaped and sized to span a gap between the inward-facing walls and the threaded outer surface of the bolt. 
     According to a further feature of an embodiment of the present invention, the first portion includes two elongated projections carrying the entrapment features. 
     According to a further feature of an embodiment of the present invention, there is also provided a deflectable linkage comprising at least two interconnected segments including a first end segment and a second end segment, the first end segment being in articulated connection with the first portion and the second end segment being in articulated connection with the second portion such that adjustment of a length of the telescopic body causes a corresponding deflection of the deflectable linkage. 
     According to a further feature of an embodiment of the present invention, the deflectable linkage further comprises an intermediate segment, deflection of the deflectable linkage resulting in a change in spacing between the intermediate segment and the telescopic body. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: 
         FIGS. 1A-1C  are isometric views of an adjustable implant, constructed and operative according to a first embodiment of the present invention, shown in a collapsed state, a semi-deployed state, and a fully deployed state, respectively; 
         FIG. 2  is an exploded isometric view of the adjustable implant of  FIGS. 1A-1C ; 
         FIG. 3  is a partially assembled isometric view of the adjustable implant of  FIGS. 1A-1C ; 
         FIGS. 4A-4C  are side, top and bottom views, respectively, of the adjustable implant of  FIGS. 1A-1C ; 
         FIG. 4D  is a view similar to  FIG. 4A  showing a variant implementation of a partial gear engagement between two linking segments of the implant; 
         FIGS. 5A-5C  are cross-sectional views taken along the line A-A in  FIG. 4C , the adjustable implant being shown in a collapsed state, a semi-deployed state, and a fully deployed state, respectively; 
         FIGS. 6A-6D  are side views of the adjustable implant of  FIGS. 1A -IC in a collapsed state, two intermediate positions, and a fully deployed state, respectively, illustrating a strain-limiting configuration according to an aspect of the present invention; 
         FIG. 7A  is an isometric view of an alternative implementation of an adjustable implant, constructed and operative according to an embodiment of the present invention, shown in a semi-deployed state; 
         FIG. 7B  is a center-plane cross-sectional view taken through the adjustable implant of  FIG. 7A : 
         FIG. 8  is an exploded isometric view of the adjustable implant of  FIG. 7A ; 
         FIGS. 9A and 9B  are side views of a further implementation of an adjustable implant, constructed and operative according to an embodiment of the present invention, shown in a collapsed state and a deployed state, respectively; 
         FIG. 10  is an exploded isometric view of the adjustable implant of  FIGS. 9A and 9B ; and 
         FIGS. 11A and 11B  are isometric views of a further variant implementation of an adjustable implant, constructed and operative according to an embodiment of the present invention, shown in a collapsed state and a deployed state. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is an adjustable implant. 
     The principles and operation of adjustable implants according to the present invention may be better understood with reference to the drawings and the accompanying description. 
     By way of introduction before addressing the drawings in detail, the present invention encompasses a number of points of novelty which are exemplified by the various embodiments disclosed herein, but which each have utility independently in a range of other implementations and applications. For example, the embodiments of  FIGS. 1A-8  illustrate a first aspect of the present invention which provides a gear-like mechanical engagement between links of a four-pivot-axis adjustable implant structure to define the adjustment motion. The embodiments of  FIGS. 1A-6D and 9A-10  illustrate a second aspect of the present invention according to which complementary cooperating surfaces distinct from the articulating joints of an adjustable implant are shaped and positioned such that they provide additional support under conditions of loading of the implant to oppose the resulting strain. A third aspect of the present invention exemplified in the various illustrated embodiments relates to a mechanism for adjusting the length of a telescopic body, and hence adjusting the state of the implant. It should be noted that the first and second aspects of the invention may equally be implemented in adjustable implants which employ adjustment mechanisms other than the telescopic adjustment of the second aspect of the present invention, and that the telescopic body adjustment mechanism may be used in any or all implants which change their length, without requiring the features of the other aspects of the invention, and in some cases, without the presence of any deflectable linkage of the types to which the other aspects of the invention apply. 
     Referring now to the drawings.  FIGS. 1A-6D  illustrate a first embodiment of an adjustable implant, generally designated  10 , illustrative of certain aspects of the present invention. 
     Adjustable implant  10  has a telescopic body  12  including a first portion  14  and a second portion  16 . First and second portions  14  and  16  are in sliding engagement such that a length of the telescopic body is adjustable from a first length to a second length. In the embodiment shown, adjustable implant  10  also has a deflectable linkage including at least two, and in this case three, interconnected segments  18 ,  20  and  22  of which segment  18  is a first end segment (or “linking segment”) in articulated connection with first portion  14  and segment  22  is a second end segment (or “linking segment”) in articulated connection with second portion  16 . The structure is such that adjustment of a length of the telescopic body causes a corresponding deflection of the deflectable linkage, as best illustrated in  FIGS. 5A-5C  where progressive decrease in the length of telescopic body  12  from L 1  through L 2  to L 3  causes progressive deflection of the deflectable linkage, in this case corresponding to parallel motion of the intermediate segment  20  away from telescopic body  12 , to increase the height of implant  10  from an initial low-profile H 1  through H 2  to H 3 . 
     Implant  10  of this example has four distinct pivotal connections each having one or more pivot pin whose central axis defines a distinct pivot axis: a first pivot axis  24  between first end segment  18  and intermediate segment  20 ; a second pivot axis  26  between intermediate segment  20  and second end segment  22 ; a third pivot axis  28  between first end segment  18  and first portion  14 ; and a fourth pivot axis  30  between second end segment  22  and second portion  16 . (In the exploded view of  FIG. 2 , the corresponding openings for receiving the pivot pins are labeled with the corresponding numerals primed.) The distance between pivot axes  28  and  30  is adjustable, in this case by telescopic adjustment of telescopic body  12 , so as to adjust the shape of the implant. However, a structure with rigid links interconnected at four pivot axes does not inherently define a unique geometry, and could in principle allow a rocking motion of intermediate segment  20  relative to telescopic body  12 . A first aspect of the present invention addresses this issue by reducing the degrees of freedom of the four-pivot-axis so that rotation about two adjacent axes, in this case, axes  24  and  26  occurs in a fixed ratio, most preferably equal and opposite. This results in a stable and well-defined orientation of intermediate segment  20  at every stage of the adjustment process, providing, for example, parallel motion between telescopic body  12  and intermediate segment  20  as illustrated here. 
     Structurally, in the implementation illustrated here, first linking segment  18  and second linking segment  22  are formed with projecting features configured to provide a partial gear engagement between the segments. Specifically, in this case, first linking segment  18  is formed with a gear tooth  32  while second linking segment  22  is formed with projections defining therebetween a complementary tooth trough  34  for receiving gear tooth  32 . Tooth  32  and trough  34  are preferably partial profiles of corresponding virtual gear wheels centered at pivot axes  24  and  26 , respectively, and most preferably have standard involute gear geometry, as is well known in the field of gears. Given the limited range of angular motion of the implant from its collapsed state to its fully deployed state, in certain cases as illustrated here, it has been found sufficient to include a single gear tooth  32  and single corresponding inter-tooth trough  34 . However, alternative implementations employ two or more teeth and associated troughs on one or both of the linking segments, for example, to provide reliable engagement for an implant with a larger range of angles and/or using smaller size teeth. By way of one further non-limiting example,  FIG. 4D  shows schematically a variant implementation in which the gear configuration is implemented with a plurality of teeth, specifically, two teeth  32  and one interposed trough  34  as part of segment  18  and two troughs  34  with one interposed tooth  32  as part of segment  22 . 
     Although a conventional gear tooth shape is believed to be advantageous, it should be noted that alternative forms of “gear engagement” between the two linking segments may also be used. Various non-standard tooth shapes, or even a transverse pin (i.e., projecting parallel to the pivot axes) on one linking segment engaged in a slot extending radially from the pivot axis of the other linking segment, can provide a sufficiently good approximation to a fixed ratio of rotation between the two linking segments. 
     In the particular case illustrated here, the inter-axis lengths of linking segment  18  (between axes  24 ,  28 ) and linking segment  22  (between axes  26 ,  30 ) are equal, and the partial gear engagement employs tooth/trough profiles corresponding to virtual gears of the same sizes/angular pitch. As a result, the line between axes  24  and  26 , corresponding to the orientation of intermediate segment  20 , remains parallel to the length of telescopic body  12  (or a line joining axes  28  and  30 ). By employing linking segments of differing inter-axis lengths and/or by employing an asymmetric partial gear engagement, other desired geometries of motion can be achieved, such as various combinations of adjustable spacing and angle of inclination. In each case, however, the form of the motion is predefined by the structure, and the device is preferably stable against any rocking motion in each position over its range of motion. 
     This aspect of the present invention may be implemented in any implant in which the distance between axes  28  and  30  can be adjusted. Examples include any and all cases in which a screw (boll) is employed to adjust a length of a base/body between axes  28  and  30 , and all forms of telescopic bodies, whether using an internal adjustment mechanism such as that described herein or whether adjusted by an external/removable actuator mechanism. Additionally, this aspect of the present invention may be implemented in cases where a base of the implant is of fixed length, but where an adjustment mechanism displaces one or both of pivot axes  28  and  30  along the base so as to vary a distance between them. 
     It will be noted that the aforementioned pivot axes do not necessarily correspond to the extremities of the corresponding segments. Particularly notable is that intermediate segment  20  is formed with a tissue contact surface  21  which, in particularly preferred implementations, extends along more than half a maximum length of the telescopic body. This tissue contact surface together with the outward-facing lower surface(s) of telescopic body  12  provide opposing contact surfaces which, on adjustment of the implant, can be used to push apart tissues to achieve a desired extent of separation, distraction or height restoration, all according to the particular application. 
     Turning now to a second aspect of the invention, this is applicable in cases where relative motion of parts of the body/base cause deflection of a deflectable linkage, such as the three-segment linkage of adjustable implant  10  or a two-segment linkage such as that of adjustable implant  200  described below. The motion of the adjustable implants described herein is primarily defined by the geometry of the pivot axes and adjustment of the telescopic body, optionally further defined by a partial gear arrangement according to the first aspect of the present invention described above. However, under conditions of significant applied loading, it may be preferable that part of the load is borne by structures other than the pivot connections. 
     To this end, in the example of adjustable implant  10 , first portion  14  of telescopic body  12  and second end segment  22  of the deflectable linkage are formed with complementary cooperating surfaces shaped such that, during adjustment of a length of telescopic body  12  and corresponding deflection of the deflectable linkage, relative motion of first portion  14  and second end segment  22  maintains the cooperating surfaces in strain-limiting proximity. “Strain-limiting proximity” is used herein in the description and claims to refer to two components which are either in contact with each other or which are in proximity with each other to the extent that, when a load is applied to the implant sufficient to elastically deform the implant, the surfaces of the components come into contact within the range of elastic deformation. i.e., before the elastic limit of any of the components is exceeded. In certain particularly preferred embodiments, such contact occurs during the “initial stages” of elastic deformation which, for this purpose, may be defined as the first 50%, more preferably the first 10%, and most preferably the first 5%, of linear strain as a proportion of the maximum strain which would bring the implant to its elastic limit. In this manner, the complementary cooperating surfaces effectively serve as a “stop” to limit or resist elastic deformation of the implant, thereby improving the performance of the implant as a whole under conditions of loading. In certain alternative embodiments, an initial gap between the components in strain-limiting proximity may be chosen to be in excess of 50% of the maximum strain which would bring the implant to its elastic limit. This option may be particularly advantageous where it is desired to provide relatively high flexibility of the implant during routine loading while ensuring that the deformation “bottoms-out” and is stopped before approaching levels which might result in damage. 
     Structurally, this strain-limiting (deformation-limiting) feature is most preferably implemented as illustrated here by providing a small abutment surface or edge  36  at the extremity of first portion  14  and a convexly curved bulge  38  along a lower edge of second end segment  22 . The shape of the convexly curved bulge is calculated (or empirically derived) to match the path of relative motion between second end segment  22  and edge  36  as the implant is adjusted through its range of motion, thereby ensuring that the desired proximity is maintained throughout the range.  FIGS. 6A-61 ) illustrate a sequence of states spanning the range of adjustment, and illustrate a clearance  40  which is preferably maintained according to certain implementations of the present invention between edge  36  and the adjacent region of bulge  38 . This clearance as observed in the unstressed state of the implant is in certain preferred implementations small compared to the dimensions of the implant, typically amounting to no more than 10%, and more preferably 5%, of the overall dimension of the implant in the direction of the spacing, even in the collapsed state. This clearance preferably remains substantially constant over the range of adjustment of the implant, for example, most preferably varying by no more than ±20%. Provision of a small clearance in the unloaded state of the implant may be advantageous in that it ensures that the strain-limiting features do not add significant frictional resistance to a process of adjusting the implant shape, at least until a stage of deployment where significant loading is encountered. It may also provide a relatively high-flexibility state, as mentioned above. 
     Throughout the exemplary embodiments described herein, each of the “segments” described may be implemented as a unitary body, as a bilateral “forked” structure, or as a bilateral pair of separate elements which once assembled move in unison and are functionally equivalent to a single “segment”. In adjustable implant  10  as illustrated here, first and second end (linking) segments  18  and  22  are forked elements which extend bilaterally on either side of intermediate segment  20  which is disposed internally to those segments. In the context of the strain-limiting features, it will be appreciated that these features may be provided on only one side of the forked second end segment  22  or, more preferably, on both sides thereof, thereby maximizing the structural support provided by these configurations under conditions of loading. 
     The strain-limiting features according to this aspect of the present invention may be implemented in a range of different types of implant, and with a range of different adjustment mechanisms. By way of one further non-limiting example,  FIGS. 9A-10  illustrate implementation of these features in the context of an adjustable-angle implant  200  in which a first end segment  218  and a second end segment  222  are interconnected so as to be relatively pivotable about a pivot axis  224 . (Features of implant  200  that are analogous to features of implant  10  will be denoted by the same reference numerals with addition of 200 to the number.) First end segment  218  is preferably formed with a tissue contact surface  219  that extends along a major dimension having a length more than half a maximum length of a telescopic body formed by a first portion  214  and a second portion  216 . The result is an implant which has an adjustable angle between tissue contact surface  219  and an outwardly facing tissue contact surface  213  of the telescopic base. In this context, the preferred implementation of adjustable-angle implant  200  as shown also includes strain-limiting features, structurally and functionally equivalent to those described above, including abutment surface or edge  236  and convexly-curved bulge  238 . These and other features of implant  200  will be fully understood by analogy to the description of implant  10  above and from the further discussion of the structure and function of these implants described below. 
     Turning now to a third aspect of the present invention, the various embodiments of an adjustable implant illustrated herein also exemplify an adjustment mechanism for adjusting a length of a telescopic body which forms at least part of an implant. For conciseness of presentation, this aspect of the invention is illustrated herein in the context of an adjustable implant of implant  10 , where a change in length of the telescopic body effects a change to a height of a deflectable linkage. It should be noted however that this aspect of the present invention may be used to advantage in any implant with telescopic adjustment, even if no such deflectable linkage is present, and may be used to advantage in applications in which an initially minimum-length implant is to be extended once within the body as well as application where an initially maximum-length implant is to be shortened within the body. The mechanism may also be useful in application where unidirectional actuation (i.e., just elongation or just shortening) without reversibility is sufficient. 
     Referring now particularly to  FIGS. 2 and 3 , first and second portions  14  and  16  are here configured for sliding engagement such that a length of the telescopic body along an axis of the telescopic body is adjustable. A headless bolt  42 , having a threaded outer surface  44  between two end abutment surfaces  46 , is deployed in a region of overlap between the first and second portions. First portion  14  is formed with entrapment features configured to abut at least one, and preferably both, end abutment surfaces of bolt  42  so as to prevent displacement of bolt  42  relative to first portion  14  in at least one direction along the axis of the telescopic body. Second portion  16  is formed with at least one, and preferably two, elongated threaded surfaces  48   a  and  48   b , deployed to engage the threaded outer surface of the bolt. Elongated threaded surfaces  48   a  and  48   b  have a length greater than a length of headless bolt  42 . 
     In the non-limiting preferred example of implant  10 , the entrapment features of first portion  14  include the upper and lower edges of a slot  50  into which bolt  42  is inserted during assembly, such that the upper and lower edges of the slot abut end abutment surfaces  46  to prevent axial displacement of the bolt relative to first portion  14 . Rotation of bolt in that inserted position engages and draws inwards (or if reversed, forces outwards) elongated threaded surfaces  48   a  and  48   b , thereby adjusting a length of telescopic body  12 . 
     The term “headless bolt” is used herein to refer to a bolt which does not have a head of diameter greater than the threaded shaft of the bolt. In order to allow turning of bolt  42  to adjust the length of telescopic body  12 , one of end abutment surfaces  46  is preferably formed with a shaped recess, which may be a shallow recess or a through-bore passing along the length of the bolt, to receive a correspondingly shaped tool tip. 
     The use of a “trapped” headless bolt  42  within first portion  14  and relatively long threaded surfaces  48   a ,  48   b , allows the use of a bolt which in some embodiments is shorter than the range of adjustment of the device. This may provide certain advantages such as leaving a larger area available for openings through the implant, for example, to allow bone ingrowth through the implant. Where the bolt is relatively short, the range of adjustment of the telescopic body is typically defined primarily by the length of elongated threaded surfaces  48   a ,  48   b , which preferably span the range of adjustment corresponding to a difference between a first length L 1  and a second length L 3  ( FIGS. 5A-5C  of telescopic body  12 . 
     A range of implementations of the elongated threaded surface(s) may be used. In the particularly preferred non-limiting example of implant  10 , the elongated threaded surfaces are integrated with elongated projections  52  which also define sliding abutment surfaces as part of the sliding engagement with first portion  14 . Specifically, first portion  16  is here formed with two inward-facing walls  54 . Outer surfaces of elongated projections  52  are configured to move in sliding engagement with inward-facing walls  54  to define the sliding engagement between first and second portions  14  and  16 , while the inward-facing elongated threaded surfaces  48   a ,  48   b  engage threaded outer surface  44  of bolt  42 . Elongated projections  52  are preferably shaped and sized to span a gap between inward-facing walls  54  and threaded outer surface  44  of bolt  42 , such that contact between elongated projections  52  and walls  54  prevents any outward flexing of projections  52  and maintains reliable engagement of elongated threaded surfaces  48   a ,  48   b  with threaded outer surface  44  of bolt  42 . 
     The operation of the adjustment mechanism can be best understood from  FIGS. 5A-5C . In  FIG. 5A , bolt  42  is engaged with the extremities of threaded surfaces  48   a  (and  48   b ), corresponding to the longest state L 1  of telescopic body  12  which, in this embodiment, is also the lowest height H 1  configuration for insertion into the body. A suitable tool (not shown) is then inserted along an access channel from the right side of telescopic body  12  as shown, and is used to turn bolt  42 . As it turns, engagement of bolt  42  with threaded surfaces  48   a  (and  48   b ) draws second portion  16  towards first portion  14 , as seen in the successive positions of  FIGS. 5B and 5C . Longitudinal motion of bolt  42  relative to first portion  14  is prevented by the abutment of the ends of bolt  42  against the top and bottom edges of slot  50 . In this example, as the length progressively decreases through L 2  to L 3 , the deflectable linkage is progressively deflected to increasing heights of H 2  and H 3 , until the end of the range of motion is reached, typically defined either by reaching the end of the thread or by reaching mechanical abutment, for example, of second portion  16  against first portion  14 . 
     Turning now to  FIGS. 7A-8 , it should be noted that the trapped, headless bolt approach may be implemented using a wide range of different structures both for the entrapment features and for the elongated threaded surfaces. By way of one further non-limiting example,  FIGS. 7A-8  illustrate an alternative implementation of an adjustable implant, generally designated  100 , constructed and operative according to a further embodiment of the present invention. Implant  100  is generally analogous to implant  10  described above, and equivalent features are labeled with similar reference numerals incremented by 100. 
     In this case, first portion  114  includes two elongated projections  156  carrying entrapment features in the form of cut-outs  158  which provide surfaces abutting end abutment surfaces  146  of headless bolt  142 . Second portion  116  in this case is formed as a block with inward-facing elongated threaded surfaces  148  that are subdivided by slots  160  which are shaped to receive projection  156 . By positioning bolt  142  within cut-outs  158  and introducing the ends of elongated projections  156  into the beginnings of slots  160 , threaded outer surface  144  is brought up against the start of elongated threaded surfaces  148  such that rotation of bolt  142  by a suitable tool (not shown) causes bolt  142  to draw second portion  116  and first portion  114  together. The remaining structural features and function of implant  100  will be understood by analogy to that of implant  10  described above. 
     Turning now to  FIGS. 9A-10 , these illustrate variable-angle implant  200  which was described above in the context of the strain-limiting features. As best seen in the exploded view of  FIG. 10 , the telescopic adjustment mechanism employed in this example is essentially identical to that of implant  10  described above, with equivalent elements labeled similarly with addition of 200 to the reference numerals. 
     Turning now to  FIGS. 11A and 11B , these illustrate a further embodiment of a variable-angle implant, generally designated  300 . The structure and function of implant  300  are to a large extent analogous to that of implant  10  described above, and similar elements are designated by similar reference numerals with addition of 30) to the number. 
     Implant  300  is a variable-angle implant, functionally analogous to implant  200 , but differs from implant  200  primarily in that the first portion of the telescopic body is here implemented as two rigidly interconnected or integrally formed sections  314   a  and  314   b . (Projection  362  provides an anchor point for a delivery system (not shown) for the implant and is not a pivot.) The effect of this first portion structure is to position the pivotal connection  328  between first portion  314   b  and first end segment  318  in the mid-half (i.e., between 25% and 75%) of the maximum length of the implant, and in certain preferred implementations, at a relatively high position above an initial height of pivotal connection  324 . This geometry facilitates a range of angles of tissue contact surface  319  which spans from a “negative angle” as shown in  FIG. 11A  to a “positive angle” as shown in  FIG. 11B . The negative angle initial configuration may provide advantages in certain application during insertion of the implant into the body by providing a low profile leading end and a wedge-like overall profile for progressively separating tissues during insertion. Here too, the exemplary non-limiting adjustment mechanism for the telescopic body is essentially similar to that of implant  10  described above, and will not be described further. 
     All of the above exemplary embodiments are suitable for use, as is or with minor modifications that will be self-explanatory to a person of ordinary skill in the art, in a wide range of orthopedic applications, and especially in cases where a distance, spacing and/or angle between two tissue surfaces is to be increased or adjusted. One non-limiting field of particular relevance is spinal surgery, including devices for intra-body, inter-body placement within or between adjacent vertebral bodies. The devices are typically delivered in a low-profile form while held by an elongated holder (not shown), are inserted to the desired target location, and are then adjusted to provide the desired degree of tissue separation and/or angular correction. For intervertebral fusion applications, the devices may be used for any approach direction including, but not limited to, various posterior and lateral approach routes. Various bone-ingrowth openings (for example, as illustrated) are preferably provided to facilitate fusion and/or osteo-integration. 
     It should be noted that embodiments of the present invention which provide “parallel” adjustment of spacing between tissue contact surfaces do not necessarily, or even typically, have contact surfaces which are parallel per se. Thus, for example, an upper tissue contact surface  21  of implant  10  as best seen in  FIGS. 5A-5C  is shown with a preset incline to provide slight lordotic angle restoration in addition to an overall anatomically-rounded shape towards its extremities. Furthermore, the tissue contact surfaces are typically modified by various ridges, projection or other features to enhance mechanical anchoring against tissue surfaces, as well as openings as mentioned above. 
     The disclosed implants may be formed from any and all materials or combinations of materials known to be suitable for implementation of surgical implants. Examples include, but are not limited to, titanium, surgical stainless steel and polymers such as PEEK. 
     Except where stated otherwise, it should be noted that the various structures illustrated herein as being implemented on either a proximal or a distal end of an implant should be understood to be reversible. Thus, where features are described as being part of a first portion of the telescopic body or a first end segment of a deflectable linkage and are illustrated as being a proximal structure, these structures can readily be adapted for implementation at a distal end of the implant, and vice versa. 
     It will be appreciated that the above descriptions are intended only to serve as examples, and that many other embodiments are possible within the scope of the present invention as defined in the appended claims.