Abstract:
This invention concerns a device, and instruments for its insertion, that aligns two sections of bone and fixates the two sections to one another. The alignment feature and the fixation feature are typically independent, but they are incorporated into one device. The device is particularly well adapted to the alignment and fixation of a fragment of cranial bone with the remainder of the cranium. The device can be applied to a cranial bone fragment, and it allows the bone fragment to be aligned with the outer cortex of the cranium; prevents the bone fragment from entering the cranial cavity; and if desired, fixates the bone fragment to the cranium. The device may take the form of a clip having a tab to surface engage one section of bone, two projections to respectively engage edge portions of two bone sections, and an S-shaped flange integral with the tab and at least one of the projections.

Description:
This application is a continuation-in-part of prior pending U.S. patent application Ser. No. 09/435,646, filed Nov. 9, 1999 now U.S. Pat. No. 6,190,389. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to the alignment and fixation of bone segments as required for appropriate bone healing, for example after fracture or surgical intervention, and specifically to a device, and the tools needed to install the said device, for the alignment and fixation of cranial bone fragments. 
     In cases of bone fragmentation where bone fixation is desired, the appropriate alignment of the bone is also a desired result. This is especially true in the cranium, where bone fragmentation can occur as a result of trauma, congenital deformity, or of surgical intervention. In the field of neurosurgery, cranial bone fragments are frequently cut and removed to create defects to allow for access into the cranial cavity and the brain. 
     The bony cranium is generally regarded to have two surfaces: the outer surface which is characterized by the outer cortex of the bone and is adjacent to the scalp and soft tissue; and the inner surface which is characterized by the inner cortex of the bone and which is adjacent to the cranial cavity and the brain. Between the inner cortex and the outer cortex, which are dense layers of bone, lies the diploe which generally consists of soft bone and bone marrow. When a bone fragment is created, a cut between the bone fragment (the primary bone zone) and the remainder of the cranium (the secondary bone zone) is present. 
     Several methods of alignment and fixation of primary and secondary bone zones are known. Traditional techniques involve the use of several pieces of filament, such as wire, that are tied after being threaded through holes drilled obliquely through the outer cortex to the cut surface of both bone zones. Precise alignment of the two zones can be difficult and the technique can be cumbersome. 
     Commonly, the zones of bone can be aligned and fixated with a system of plates and screws (U.S. Pat. Nos. 5,372,598; 5,413,577; and 5,578,036). A plate made of metal or other substance can be fixated to the outer cortex of the primary bone zone with screws whose penetration of the bone can be limited to the outer cortex. With three of more plates attached to the primary bone in such a way that the plates protrude beyond the edges of the primary bone zone, the primary bone zone can be introduced into a defect and aligned to the outer cortex of the secondary bone zone without danger of the primary bone zone falling too deeply into the defect in the secondary bone zone and exerting pressure on the underlying tissue such as the brain. Fixation can then be achieved by employing additional screws fixating the plates to the outer cortex of the secondary bone zone. Plates and screws systems allow for the alignment and fixation of the zones, while preventing the primary bone zone from falling below the level of the secondary bone zone without actually introducing a component of the device below the secondary bone zone. A plate with a spring clip extension has been described (U.S. Pat. No. 5,916,217). Plate and screw systems can be expensive and time consuming to use. 
     Devices that align the two bone zones by way of compressing them between the two disks positioned along the inner and outer cortex have been described. (Foreign Patents: DE 19603887C2, DE 19634699C1, DE 29812988U1, EP 0787466A1.) A pin connects the two disks aligning and securing two bone zones. These devices introduce foreign material that is left below the inner cortex, and they do not protect the underlying tissue from compression during the installation procedure. 
     Devices that fixate bone zones using friction forces created by a cam without a component that extends below the inner cortex are known and described (Patent DE 19634697C1). These devices also do not protect the brain from compression during the installation procedure. 
     Intramedulary pins are well known in the orthopedic fields for alignment of long bones. Such pins have also been described for cranial fixation (U.S. Pat. No. 5,501,685); however, the bone zones can not be aligned in three dimensions with this technique. 
     There is a need for an alignment and fixation device that is simple and rapid to use, versatile, and ultimately cost effective. 
     OBJECTS OF THE INVENTION 
     The object of the invention is to provide a device and instruments for its use and installation that aligns one cortex of a primary zone with one cortex of a secondary bone zone without extending to the opposing cortex, and which accurately fixates the bone zones to each other. When used in the field of neurosurgery, the device is applied to the primary bone zone and it aligns the outer cortex of the primary bone zone with the outer cortex of the secondary bone zone; it prevents the primary bone zone from entering the cranial cavity; and it provides fixation of the two bone zones. The alignment feature can be used independently from the fixation feature. An example of the use of the alignment feature is in the replacement of a cranial bone fragment which will be held in place by the tissue forces of the scalp, which allows for the bone fragment to be elevated away from the cranial cavity in cases where brain swelling occurs. Fixation can also be applied to attach the alignment device to the bone, using elements alone or in combination such as filaments, screws, rivets, pins, clips, cams, friction or adhesives. The alignment aspect of the invention can also be applied to situations where it is desired to offset the alignment of the bone fragment to the adjacent bone such as where the object is to create a more prominent chin by cutting the bone of the chin and advancing the bone fragment. 
     The fixation feature of the invention is likewise independent from the alignment feature. The fixation feature of the device relies on the principle that the device is fixated to the primary bone zone and the fixation feature grips the secondary bone zone by means of spring loaded tab or hook elements engaging the soft areas of the medullary space, irregularities along the cut surface, or a slot cut into the cut surface of the secondary bone zone. 
     SUMMARY OF THE INVENTION 
     The invention provides an improved clip meeting the above need or needs. 
     As will be seen the preferred clip is configured to interconnect primary and secondary bone zones having edges spaced apart by a gap, the clip comprising 
     a) a tab such as a small plate to extend over and generally parallel to a surface of the secondary bone zone, and above a first level defined by that surface, and 
     b) a first projection carried by the tab and configured to penetrate the primary bone zone at the edge thereof, and below said surface level. 
     As will be seen, a second projection may be provided and carried by the tab and configured to engage the secondary bone zone at the edge thereof, and below said surface level. 
     It is another object to provide an extension of the tab projecting below said first level. That extension may carry the first projection, and may carry the second projection, if it is provided. In this regard, the second projection is typically located beneath the tab; and the first projection extends generally parallel to the tab and forwardly from a part of the tab extension below said surface level, and it preferably has a sharp terminal to enable penetration of diploe. 
     A further object is to provide the second projection to have a sharp terminal, and to extend at an angle toward the tab, in order to resist removal relative to the secondary bone zone. 
     Yet another object is to provide another second projection carried by the tab in sidewardly spaced relation to the first mentioned second projection, and configured to engage the secondary bone zone at the edge thereof, and below said surface level. 
     An additional object is to provide a tab extension as referred to, but having S-shape or configuration, whereby enhanced spring support of one or both projections is realized; and also the S-shape of the extension facilitates its formation or manufacture. 
     An additional object is to provide a plate or flap defining the primary bone zone, and to provide multiple of the clips having their first projections penetrating the primary bone zone at different edges thereof, below a surface defined by the plate or flap. 
     The method of using the clip as referred to includes the steps 
     i) advancing the first projection to penetrate the primary bone zone, 
     ii) and locating the tab to extend over the surface of the secondary bone zone, and attaching the tab to that surface. 
     As will be seen, the step i) preferably includes pushing the clip toward the primary bone zone to effect push-in penetration of the first projection into the primary bone zone. The method may further include providing a second projection carried by the tab and configured to engage the secondary bone zone at the edge thereof, and below its top surface level, the method including displacing the clip and said second projection to engage the secondary bone zone at the edge thereof, below said surface level. An additional step includes displacing the clip in a direction to effect scraping of the edge of the secondary bone zone by the second projection, the second projection oriented to resist reverse displacement of the clip in an upward or opposite direction relative to the secondary bone zone. In this regard, the method may include effecting penetration of the edge of the secondary bone zone by the second projection in an angular direction toward the tab. The bowed or S-shape of the extension provides enhanced spring effect to aid in effecting such penetration. 
     These and other objects and advantages of the invention, as well as the details of an illustrative embodiment, will be more fully understood from the following specification and drawings, in which: 
    
    
     DRAWING DESCRIPTION 
     FIG. 1 is a perspective view showing a bone flap fixated on a skull, employing fixation clips; 
     FIG. 2 is an enlarged perspective view showing a clip employing the invention; 
     FIG. 3 is a top plan view of the FIG. 2 clip; 
     FIG. 4 is an end view of the clip taken on lines  4 — 4  of FIG. 3; 
     FIG. 5 is a side elevational view taken on lines  5 — 5  of FIG. 3; 
     FIG. 6 is a plan view of a clip blank in one plane, prior to deformation to FIG. 2 configuration; 
     FIG. 7 is a section showing the FIG. 1 clip attached to primary and secondary bone zones; 
     FIG. 8 is a perspective view showing clip attachment to a primary bone zone; 
     FIG. 9 shows multiple clips attached to opposite edges of a bone flap defining primary bone zones; 
     FIG. 10 shows the clips installed in a skull opening; 
     FIG. 11 is a perspective view like FIG. 2, showing a modified clip; 
     FIG. 12 is a perspective view like FIG. 11 showing another modified clip; 
     FIG. 13 is a perspective view similar to FIG. 8, and showing attachment of a clip of FIG. 11 form, to a primary bone zone, such as a bone flap; 
     FIG. 14 is a section taken through a cranial bone flap, having two FIG. 11 type clips attached at opposite edges, and positioned for clip attachment to secondary bone zone sections; 
     FIG. 15 is a perspective view showing a cranial bone flap having four FIG. 11 type clips attached, at its four edges; 
     FIG. 16 is a perspective view of a further modified clip, of FIG. 2 type; 
     FIG. 17 is a perspective view of a tool usable in conjunction with the FIG. 16 clip, to effect penetration of a clip projection into a primary bone zone; 
     FIG. 18 shows use of a barb; 
     FIG. 19 is a top plan view of a further modified clip using a barb; 
     FIG. 20 is an end view taken on lines  20 — 20  of FIG. 19; 
     FIG. 21 is a side elevational view taken on lines  21 — 21  of FIG. 19; 
     FIG. 22 is a plan view of a clip blank in one plane, prior to deformation to FIG. 19 configuration; 
     FIG. 23 is a perspective view of a modified clip, of the type shown in FIG. 16; 
     FIG. 24 is a top plan view of the FIG. 23 clip; 
     FIG. 25 is a front elevation view of the FIG. 24 clip, and taken on lines  25 — 25  of FIG. 24; 
     FIG. 26 is a right side elevation taken on lines  26 — 26  of FIG. 24; 
     FIG. 27 is a view like FIG. 26, but showing use of the clip of FIGS. 23-26, in the manner of FIG. 21; 
     FIG. 28 is a view like FIG. 24, but showing a further modification; and 
     FIG. 29 is a front elevation of the FIG. 28 modified clip, taken on lines  29 — 29  of FIG.  28 . 
    
    
     DETAILED DESCRIPTION 
     Referring to FIGS. 2-5 and  7 , the illustrated clip  10  is configured to interconnect primary and secondary bone zones  11  and  12  having opposed and spaced apart edges  11   c  and  12   c.  A cut or gap  13  is formed between the opposed edges of the primary and secondary bone zones. Diploe is shown at  15  between the top and bottom surfaces  11   a  and  11   b  of zone  11 ; and at  16  between the top and bottom surfaces  12   a  and  12   b  of zone  12 . As also seen in FIG. 1, primary bone zones  11  may be defined by bone flap  17 ; and secondary bone zones  12  may be defined by skull  18  and its zone extents at  12  opposing zones  11 . In the adult, cranial bone or skull averages 7 mm in thickness, but varies between 3 and 12 mm. 
     The clip  10 , which is preferably metallic includes the following 
     a) a tab  20  to extend over a surface  12   a  of the secondary bone zone  12 , above surface level and generally parallel to surface  12   a;    
     b) a first projection  21  carried by the tab and configured to penetrate the exposed diploe of the primary bone zone  11  at the edge  11   c  of that zone (and typically into diploe  15 ); 
     c) and at least one second projection  22  carried by the tab and configured to engage (for example gouge into) the exposed diploe of the secondary bone zone  12  at its edge  12   c,  below the level of surface  12   a.    
     In the example, two such second projections are provided, as is clear from FIGS. 4 and 6, and they are located at opposite sides of a lengthwise plane  23  bisecting the clip, including projection  21 . See FIGS. 4 and 6. Such projections are equally spaced from plane  23 , as indicated by dimensions D 1 , seen in FIG.  4 . The projections  21  and  22  have legs  21   a  and  22   a,  and their terminals are sharpened at  21   b  and  22   b,  to facilitate penetration of the diploe zones, as seen in FIG.  7 . Leg  21   a  and projection  21  extend forwardly in direction  24  from a tab downward extension  20   a;  and projection  22  extends back upwardly at an approximate angle of 30° toward the underside of the tab  20 . Note that leg  22   a  extends from tab extension  20   a  and is U-shaped. A bend is formed at  22   d.  Projections  22  may also diverge laterally oppositely, as seen in FIG. 4, to provide greater stability of the plate or flap  17 , as in FIG. 10 installed condition. Four edges  11   c  of that flap are seen in FIGS. 1 and 9, and corresponding four edges  12   c  of the skull face the flap edges and receive penetration of the stabilizing clip projections  22 , as described. 
     The method of use of the clip or clips includes the following steps: 
     i) causing the first projection or projections  21  to penetrate the primary bone zone or zones; 
     ii) and then causing the second projection or projections  22  to grip the secondary bone zone at the edge thereof. 
     Step i) includes pushing the clip  10  relatively toward the edge  11   c  of the primary bone zone  11 , as in direction  30  seen in FIG.  8 . This effects push-in penetration of the first projection  21  into the bone zone  11 , as for example into diploe  15 . Push-in is typically completed when bent down tab extension  20   a  closely approaches and/or engages edge  11   c  of the primary bone zone  11  defined by the plate or flap  17 . Four such pushed-in clips are seen in FIG. 9, the clips located in opposed pair positions, at four sides of the flap  17 . Each tab  20  has a through hole  40  drilled or formed therein to receive a fastener such as a retention screw, indicated at  41  in FIG. 5, to penetrate and attach to the skull proximate the secondary bone regions. 
     The step ii) preferably also includes displacing the clip in a direction (typically relatively downwardly toward the skull to bring  21 ,  22 , and  20   a  into gap  13  as seen in FIG. 7) to effect scraping of the edge  12   c  of the secondary bone zone  12  by the tip of the angled second projection. That projection is oriented, i.e. angled, to resist displacement of the clip in an upward or opposite direction, relative to bone zone  12 . For example, attempted upward and outward displacement would increase the “gouge-in” movement of the second projection, into the diploe  16 . 
     Note further that the installed spacing d 2  of the bone zone edges  11   c  and  12   c  is slightly less than the spacing d 3  as measured from the sharp terminal of the projection  22  to the surface  32  of the tab extension facing the edge  11   c.  The width d 2  of gap  13  between  11   c  and  12   c  is slightly less than the dimension d 3 , i.e. 
     
       
         d 2 &lt;d 3 , 
       
     
     to provide a desirably tight installation of plate  17  into the corresponding skull opening. 
     In FIG. 3 note angularity β of the sharpened taper of projection  21 , where β is approximately 67°, and the through opening  43  in tab extension  20   a  to receive a fastener  44  (if used to attach extension  20   a  to  11 . 
     Projections  22  can resiliently deflect, slightly to accommodate the multiple clips to the gaps  13  between  11  and  12 , as during plate or tab downward installation, as seen in FIG.  10 . 
     Reference is now made to the modified clip  110  of FIGS. 11 and 14. It includes: 
     a) a tab or plate  120  to extend over a surface  112   a  of secondary bone zone or zones  112  (see FIG.  14 ), above a level defined by that surface; and 
     b) a first projection  121  carried by the tab  120 , and configured to penetrate the edge of exposed diploe  111   a  of primary bone zone  111 , below the levels of tab  120  and surface  112   a.    
     The projection or tang  121  has a leg  121   a,  and its forward terminal is sharpened at  121   b  to facilitate penetration into the bone marrow zone, as seen in FIG.  14 . Leg  121   a  extends forwardly from a tab downward extension  120   a  in the form of a flange. The method of use of the clip  110  includes the following steps: 
     i) causing the projection  121  to penetrate the primary bone zone, such as into diploe, (see FIG.  13 ); and 
     ii) locating the tab  120  to extend over the surface  112   a  of the secondary bone zone, as in FIG. 14 for example, and attaching the tab to that surface, one mode of attachment being by use of a screw seen at  150  in FIG. 14, to penetrate through a hole  151  in tab  120 , and into secondary bone zone  112 . 
     FIG. 14 shows two such clips  110  attached to opposite edges  111   b  and  111   c  of a primary bone zone  111 , such as a flap removed from the skull. When the flap is attached to the skull, as into opening  130 , the tabs  120  are attached to the upper surfaces  112   a  of the skull, at opposite sides of the opening. FIG. 15 shows four such clips attached to the flap  111 , at four edges  111   b-   111   e.    
     FIG. 12 illustrates a modified clip  210 , having elements  220 ,  220   a,    221 ,  221   a  and  221   b,  like corresponding elements of clip  110 . Extension  120   a  in FIG. 11 has two laterally spaced legs  120   a′  and  120   a″  that extend downwardly below the level of projection  121 , and projection  121  has flat upper and lower surfaces; whereas in FIG. 12 the extension lower extent  220   a′  is laterally continuous, and projection  221  is cylindrical, and tapers at its forward end. 
     A further modified clip  250  is shown in FIG. 16, and has elements like those of clip  10 , as viewed in FIGS. 2-7. Such corresponding elements are given the same numbers. Also, the clip downward extension  20   a  has left and right wings  20   a′  and  20   a′.    
     FIG. 17 shows a hand tool  80  to receive the FIG. 16 clip in position for forward, push-in attachment to bone zone  11 , as described. Tool  80  has a body  81 , with a top recess  82  to fit the tab  20 . Forward facing surface  83  engages and positions the clip downward extensions  20   a  and its two wings  20   a′  and  20   b′.  Tool pins  84  and  85  closely fit into holes  86  and  87  in those two wings, for alignment. Aligners in the form of alignment bars  87  and  88  projecting forwardly from body  81  ride onto the top surface  11   a  of the flap  11 , prior to penetration of the projection  21   a  into the marrow  15 , so that the proper level of the projection  21  relative to top surface  11   a  is selectively established by operation of the aligners. A tool handle appears at  89 , and facilitates forward pushing of the tool and clip, and retraction of the tool, off the clip after its push-in assembly to the flap. In this way, accurate assembly is rapidly achieved. 
     The clips as disclosed herein may consist of metal or plastic (synthetic resin) material. One desirable metal is titanium. 
     Clips  10 ,  110 , and  210  may be inverted, for alternate installations relative to the bone zones. 
     Referring to FIGS. 19-22, the illustrated views of modified clip  200  correspond to views  3 - 6  of clip  10 . 
     The clip  200 , which is preferably metallic, includes the following: 
     a) a tab  220  to extend over a surface  212   a  of the secondary bone zone  212  above surface level; 
     b) a first projection  221  carried by the tab and configured to penetrate the exposed diploe of the primary bone zone  211  at the edge  11   c  of that zone (and typically into diploe  215 ); 
     c) and at least one second projection such as barb  222  carried by the tab and configured to engage (for example gouge into) the exposed diploe  216  of the secondary bone zone  212 , below the level of surface  212   a.    
     In the example, two such second projections or barbs  222  are provided, as is clear from FIGS. 20 and 22, and they are located at opposite sides of a lengthwise plane  223  bisecting the clip, including projection  221 . One such barb is seen in FIG.  18 . Such projections are equally spaced from plane  223 , and are formed in lower portions  222   a  of  220   a,  with adjacent through openings  222   a′.  The projection  221  has a leg  221   a,  and its terminal is sharpened at  221   b,  to facilitate penetration of the bone zone  215 , as seen in FIG.  21 . Leg  221   a  and projection  221  extend forwardly from a tab downward extension  220   a;  and projection or barb  222  extends back upwardly at an acute toward the underside of the tab  220 . Note that projection  222  extends from tab lower extension  220   a  and is U-shaped. A bend is formed at  222   d.    
     Side wings  240  and  241  integral with downward extension  220   a  contain through openings  243  to receive fasteners (if used) to attach to  221 . 
     Referring to FIGS. 23-27, the illustrated and preferred clip  310  is configured to interconnect primary and secondary bone zones  311  and  312  having opposed and spaced apart edges  311   c  and  312   c.  A cut or gap  313  is formed between the opposed edges of the primary and secondary bone zones. Diploe is shown at  315  between the top and bottom surfaces  311   a  and  311   b  of zone  311 ; and at  316  between the top and bottom surfaces  312   a  and  312   b  of zone  312 . As also seen in FIG. 1, primary bone zones  11  may be defined by bone flap  17 ; and secondary bone zones  12  may be defined by skull  18  and its zone extents at  12  opposing zones  11 . In the adult, cranial bone or skull averages 7 mm in thickness, but varies between 3 and 12 mm. 
     The clip  310 , which is preferably metallic includes the following 
     a) a tab  320  to extend over and generally parallel to a surface  312   a  of the secondary bone zone  312 , above surface level; 
     b) a first projection or tang  321  directly or indirectly carried by the tab and configured to penetrate the exposed diploe of the primary bone zone  311  at the edge  311   c  of that zone (and typically into diploe  315 ); and wherein the tang  321  may have barbed edges at  321   d  and  321   e;    
     c) and at least one second projection  322  carried by the tab and configured to engage (for example gouge into) the exposed diploe of the secondary bone zone  312  at its edge  312   c,  below the level of surface  312   a.    
     In the example, two such second projections are provided, as is clear from FIGS. 23-25, and they are located at opposite sides of a lengthwise plane  323  bisecting the clip, including projection  321 . Such projections are equally spaced from plane  323 , as indicated by dimensions D 1 , seen in FIG.  24 . The projections  321  and  322  have legs  321   a  and  322   a,  and their terminals are sharpened at  321   b  and  322   b,  to facilitate penetration of the diploe zones, as seen in FIG.  7 . Leg  321   a  and projection  321  extend forwardly in direction  324  from a tab downward extension  320   a;  and projections  322  extend back upwardly at an angle γ between 25° and 45° toward the underside of the tab  320 . Note that each projection  322  extends from tab ring-shaped extension  320   a  and is U-shaped. A bend is formed at  322   d.    
     Four edges  11   c  of the flap  17  are seen in FIGS. 1 and 9, and corresponding four edges  12   c  of the skull face the flap edges and receive penetration of the stabilizing clip projections  22  or  322 , as described. 
     It will be noted that the generally upright extension  320   a  is bowed to produce an enhanced spring effect for urging one or more of the projections, and also to facilitate ease of manufacture. See extension sections  380  and  381 , the former bowed frontwardly in the direction of projection  321 ; and the latter section  381  bowed in the general direction of the projections  322  extents. Section  380  is curved at  380   a  to merge with the tab. Projection  321  is carried by section  380 , and projections  322  are carried by section or sections  381 , whereby movements of the projections  322  are isolated from movements of the projection  321 , enhancing completeness and permanence of fastening to bone. See for example FIG. 25, showing such isolation. 
     The method of use of the clip or clips includes the following steps: 
     i) causing the first projection or projections  321  to penetrate the primary bone zone or zones; 
     ii) and causing the second projection or projections  322  to grip the secondary bone zone at the edge thereof. 
     Step i) includes pushing the clip  310  relatively toward the edge  311   c  of the primary bone zone  311 , as in direction  324  seen in FIG.  27 . This effects push-in penetration of the first projection  321  into the bone zone  11 , as for example into diploe  15 . Push-in is typically completed when bent-down and bowed tab extension  380  closely approaches and/or engages edge  311   c  of the primary bone zone  311  defined by the plate or flap  17  (or bone zone  311 ). As described above, four pushed-in clips are seen in FIG. 9, the clips located in opposed pair positions, at four sides of the flap  17 . Each tab  320  has a through hole  340  drilled or formed therein to receive a fastener such as a retention screw, indicated at  41  in FIG.  5  and also in FIG. 27, to penetrate and attach to the skull proximate the secondary bone regions. 
     The step ii) preferably also includes displacing the clip in a direction (typically relatively downwardly toward the skull to bring  322 , and  320   a  into gap  313  as seen in FIG. 27) to effect scraping of the edge  312   c  of the secondary bone zone  312  by the tip or tips of the angled second projection or projections. Projection or projections  322  is or are oriented, i.e. angled, to resist displacement of the clip in an upward or opposite direction, relative to bone zone  12 . For example, attempted upward and outward displacement would increase the “gouge-in” movement of the second projection, into the diploe  16 . 
     As described above, the installed spacing d 2  of the bone zone edges  11   c  and  12   c  is slightly less than the spacing d 3  as measured from the sharp terminal of the projection  322  to the surface  332  of the tab extension facing the edge  311   c.  The width d 2  of gap  13  between  311   c  and  312   c  is slightly less than the dimension d 3 , i.e. 
     
       
         d 2 &lt;d 3 , 
       
     
     to provide a desirably tight installation of plate  17  into the corresponding skull opening. 
     Projections  322  can resiliently deflect, as by spring bending of sections  381 , to accommodate the multiple clips to the gaps  13  between  11  and  12 , as during plate or tab downward installation, as seen in FIG.  10 . In FIG. 25, the lateral spacing of bowed sections  381  enhances clip installed stability. 
     Reference is now made to FIGS. 28 and 29 showing a clip  350  like clip  310 ; except that projection  321  is carried by an L-shaped arm  370  having horizontal and vertical extents  370   a  and  370   b.  The former extent  370   a  is integral with the tab at  372 , horizontally spaced from  370   b , the tab being relieved at horizontal slits  373  along the edge lengths of  370   a,  providing for greater or enhanced bending movement of the arm  370 , as during penetration of the projection  321  into bone tissue. Projection  321  is integral with the vertical extent  370   b  of the arm. Projections  322  are carried and formed as in FIGS. 23-27. 
     The clips as referred to above are metallic, and preferably consist essentially of one of the following: 
     i) titanium 
     ii) titanium alloy 
     iii) an alloy consisting essentially of titanium, aluminum and vanadium 
     iv) an alloy consisting essentially of: 
     about 90% by weight of titanium 
     about 6% by weight of aluminum 
     about 4% by weight of vanadium.