Abstract:
An orthopedic bone plate system includes a bone plate for placement adjacent one or more vertebral bodies. The system further includes a locking element including a compression member and a fixation member, the compression member having an aperture and a deformable portion. Also included is a bone fastener having a bone engaging portion and a stem portion. The stem is slidably receivable within the aperture of the compression member while the locking member is receivable within an aperture of the bone plate. As the compression member is brought into proximity of a vertebral body along the longitudinal axis of the bone fastener, an increasing force is exerted against the fixation member and translated to the bone plate. The resultant force is translated back to the fixation member to the compression member which causes the deformable portion of the compression member to clamp the bone fastener relative to the bone plate.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a divisional of U.S. Application Ser. No. 12/961,682, filed on Dec. 7, 2010, which is a divisional of U.S. application Ser. No. 11/175,426, filed on Jul. 6, 2005, the disclosures of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates generally to an orthopedic implant assembly system and in particular to a multi-axial bone plate system. 
         [0003]    Orthopedic implant assembly systems having a multi-axial bone plate are known in the art and include at least two pedicular screws anchored in adjacent vertebrae to be treated and a connecting plate designed to connect the screw heads together in a rigid manner. Previous references disclose a system of this kind in which each pedicular screw has a hexagonal section part for inserting the screw into the bone and on top of which is a threaded shank. The plate has a plurality of oblong openings through which the various threaded shanks can be inserted and an open groove on its bottom face to prevent rotation of the hexagonal part of each pedicular screw. 
         [0004]    Each pedicular screw is associated with a stirrup through which the threaded shank of the screw also passes and which straddles the top of the plate. Finally, a nut is screwed onto the threaded shank to trap and immobilize the plate with the stirrup on top of it, between it and the hexagonal portion of the screw. 
         [0005]    Further, known systems include at least two pedicular screws and a connecting plate for linking screws together in essentially a rigid manner. Each screw has a bone anchor threaded part, a non-circular section head, and a threaded end shank adapted to cooperate with a nut. The plate has at least one opening adapted to have the threaded end shank of the screw pass through it and be trapped between the screw head and the nut. Raised patterns are provided on the top face of the plate and on the bottom face of the stirrup to prevent longitudinal sliding of the plate relative to the screw. A locking member is also provided for preventing relative angular movement between the heads of the screws and the connecting plate. The locking member is adapted to be inserted between the plate and the screw head and includes a bar through which the threaded end shank of the screw passes. The locking member further includes a first locking cooperation of shapes with the screw head and a second cooperation of shapes with the plate. 
         [0006]    Although these systems are generally satisfactory, they nevertheless have certain drawbacks. Specifically, in certain systems, raised patterns have to be provided to prevent sliding because the plate and the stirrup cooperate via two plane faces in compression. Absence of the raised patterns would lead to the risk of entirely unacceptable relative movement of the vertebrae. The machining required to create these raised patterns significantly increases the mean cost of the plates and the stirrups. 
         [0007]    Additionally, the raised patterns can impede fine adjustment of the system. For example, there are only a particular number of discrete mutual positions of the plate and the stirrup, i.e., a particular number of discrete distances between the screws. Moreover, if the nut is overtightened before the final tightening, lateral sliding of the plate and the stirrup during adjustment may be impeded. 
         [0008]    Some of the drawbacks associated with other designs include that the first locking cooperation and second cooperation of shapes with the plate forbid and restrict plate movement in an anterior and posterior direction once the pedicle screw has been rested against the plate member. 
         [0009]    Additionally, most plate systems include a bone fastener with a threaded end extended from the vertebral body. In order to lock the screw relative to the plate, a nut must be used in combination with the screw. The problem associated with this design is that if the screw is not placed at the correct depth within the vertebral body, the assembly must be disassembled in order to either increase or decrease the depth of the screw in the vertebrae. An additional problem results from this action due to the fact that once the screw has been placed too deep within the vertebral body, the screw might be less securely locked within the vertebral body when the screw is backed out and placed in its correct position. 
         [0010]    Other shortcomings of known systems include angled orientations in the sagittal direction are not permitted due to the shape of the locking cooperation members. 
       SUMMARY OF THE INVENTION 
       [0011]    In accordance with one or more embodiments of the present invention, an orthopedic bone plating system includes a bone plate for placement adjacent one or more vertebral bodies. The bone plate has an aperture extending along a longitudinal axis of the bone plate. 
         [0012]    The assembly further includes a locking element having a compression member and a fixation member wherein the compression member has an aperture and at least one deformable portion. The assembly is connected to a vertebral body with the help of a bone fastener having a longitudinal axis. The bone fastener further includes a stem and a bone-engaging portion. The stem of the bone fastener may be slidably received within the aperture of the compression member. As the stem is being received by the compression member, the compression member may be brought into proximity of a vertebral body along the longitudinal axis of the bone fastener and create an increasing force which is exerted against the fixation member. A resulting force causes the fixation member to exert a pressure against the bone plate and translate a second resultant force back through the fixation member to the compression member causing the deformable portion of the compression member to clamp onto the bone fastener thereby locking the assembly together. 
         [0013]    The compression member and fixation member may be separate elements wherein the fixation member includes an aperture for receiving the compression member. Furthermore, the compression member and fixation member may have mateable threads which are screwed together as the compression member is brought into proximately of the vertebral body. 
         [0014]    The fixation member may further include a tapered aperture extending therethrough. Additionally, the bone fastener may be smooth. 
         [0015]    The orthopedic bone plate system may include a channel for receiving the fixation member, which permits the fixation member to slide relative to the bone plate. The channel being located within the bone plate. Furthermore, the fixation member may include at least one key and the bone plate may include at least one keyway wherein the key may be received by the keyway. In at least one embodiment, the keyway may have a height that is greater than the height of the key in order to permit an angled orientation of the fixation member relative to the bone plate. 
         [0016]    In an additional embodiment of the present invention, an orthopedic bone plate implant system is provided having a plate member which may be placed adjacent and along one or more of the vertebral bodies. The plate member includes an aperture extending therethrough. The assembly also includes a split sleeve having a central aperture and a slot extending from an exterior surface of the split sleeve to the aperture. The slot permits the expansion and contraction of the aperture of the split sleeve. The split sleeve is sized to be received within the aperture of the plate member. 
         [0017]    The channel portion may extend substantially the entire longitudinal direction of the plate and either come to a tapered end or abrupt end. Furthermore, the channel portion itself may also include a guide rail that receives a lateral pin extending from an exterior of the split sleeve. The lateral pin is placed and dimensioned on the split sleeve to be received by the guide rail when the split sleeve is placed in the channel portion of the plate member. 
         [0018]    The split sleeve may have two parallel flat walls and two opposite curved walls. 
         [0019]    In one embodiment of the present invention, the parallel flat walls may include the lateral pins of the split sleeve. 
         [0020]    The bone fastener may have an end portion that is either smooth, concaved inwards or outwards relative to the vertebral body. The bone fastener may further include a recess for cooperating with the tool. The recess may be in the form of a hexagon, a slot or various other recesses and projections which can be made into a tool. 
         [0021]    A bottom portion of the expansible screw may include at least one slit extending vertically to allow the compression or expansion of a portion of the expansible screw. 
         [0022]    Furthermore, the expansible screw may include a ridge which overhangs at least partly the split sleeve. The ridge may further be defined in that it has at least two flat surfaces on its sidewall. The split sleeve may include a threaded portion and the expansible screw may include a second threaded portion wherein the two threaded portions are able to be screwed to one another. 
         [0023]    In an alternate embodiment, the plate member of the orthopedic input may include slots or ridges. Additionally, the split sleeve may have a central aperture which has an interior with a greater circumference at the top of the interior as you get to the bottom of the interior. Furthermore, the expansible screw having an aperture within may include an exterior with a bottom portion angled so as to increase in size. 
         [0024]    The method of operation of the present invention may include the steps of providing a plate member, a bone fastener and a locking element as described herein. The bone fastener may first be engaged to a vertebral body and then the stem of the bone fastener slidably receiving the plate member and the locking element about its circumference so that the stem is located within the apertures of the plate and the locking element respectively. The locking element may include a compression member and a fixation member. As the compression member is brought into proximity of the vertebral body along the longitudinal axis of the bone fastener, the compression member causes an increasing force to be exerted against the fixation member which in response places pressure against the plate member until a resultant force is translated back through the two members and causes the deformable portion of the compression member to clamp the bone fastener relative to the bone plate. 
         [0025]    The method of fixing one or more vertebral bodies in a desired relationship may further include adjusting the position of the bone fastener after sliding the plate member over the stem of the bone fastener. 
         [0026]    A bone fastener is also included for connecting the plate member to a vertebral body. The fastener may include a first end portion engageable with the vertebral body and a second end portion capable of extending through the aperture of the split sleeve. 
         [0027]    A final component of the assembly is an expansible screw having an opening extending therethrough. The opening is sized to receive the second end portion of the fastener and further includes a top portion and a bottom portion. The bottom portion of the screw is capable of expanding and contracting about a central axis of the screw. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]      FIG. 1  is a schematic drawing illustrating an embodiment of the orthopedic implant of the present invention implanted in a vertebrae; 
           [0029]      FIG. 2  is an exploded perspective view of one embodiment of the system of the present invention; 
           [0030]      FIG. 3  is a sectional view of the embodiment illustrated in  FIG. 2  partly assembled and taken along line  3 - 3  in  FIG. 2  and viewed in direction of the arrows; 
           [0031]      FIG. 4   a  is a perspective view of an elongate plate used in one embodiment of the present invention; 
           [0032]      FIG. 4   b  is a cross-sectional view of a plate used in one embodiment of the present invention; 
           [0033]      FIG. 5  is a front perspective view of a bone fastener used in one embodiment of the present invention. 
           [0034]      FIG. 6   a  is a perspective view of a split sleeve used in one embodiment of the present invention; 
           [0035]      FIG. 6   b  is a cross-sectional view of the split sleeve of  FIG. 6   a ; 
           [0036]      FIG. 7   a  is a perspective view of an expandable screw used in one embodiment of the present invention; and 
           [0037]      FIG. 7   b  is a cross-sectional view of the expandable screw of  FIG. 7   a ; 
       
    
    
     DETAILED DESCRIPTION 
       [0038]    For the purposes of promoting and understanding the principles of the present invention, reference will now be made to the embodiment illustrated in the drawings and specification language will be used to describe the same. Nevertheless, by those skilled in the art, it will be understood that no limitation of the scope of the present invention is thereby intended, and further changes in the illustrated device may be made without deviating from the scope of the present invention. 
         [0039]    As shown in  FIG. 1 , the spinal implant system  10  of the present invention includes a plate  12 , an expansible locking screw  16  and a bone fastener  18 . As shown in the figures, bone fasteners  18  function to anchor plate  12  to vertebral bodies  2  and may be orientated at an angle with regard to the vertebral body. 
         [0040]      FIG. 2  details the assembly of the parts of  FIG. 1  employed in the spinal implant system  10 . In a preferred embodiment, the spinal implant system  10  includes an elongate member such as plate  12 , expansible screw  16 , bone fastener  18  and split sleeve  14 . Although the preferred embodiment illustrated in the figures is shown with only one plate  12  and bone fastener  18  and related elements, differing numbers of any of these elements may be utilized without departing from the scope of the present invention. For example, a plurality of plates  12  can be used in conjunction with each other, or a greater or lesser number of bone fasteners  18  may be used depending upon the configuration of the elongate member, the medical problem to be addressed and/or any other factors. The present invention contemplates having at least the same amount of split sleeves  14 , expansible screws  16  and bone fasteners  18 . Furthermore, the present invention contemplates each plate  12  will be used with at least a single bone screw  18  and related devices; however, each plate  12  is capable of receiving a plurality of split sleeves  14 , expansible screws  16  and bone fasteners  18  without deviating from the scope of the present invention. 
         [0041]    Referring now to  FIGS. 2 ,  3 ,  4   a  and  4   b , there is shown a preferred embodiment of plate  12 . Plate  12  preferably has a generally rounded-rectangular or oval shape, an upper surface  30 , a lower surface  32 , a proximal end  31  and a distal end  33 . Additionally, plate  12  has two opposing sides, right rail  13  and left rail  15 . Right rail  13  and left rail  15  may be substantially mirror images of one another. Plate  12  further includes elongate aperture  36  extending along the longitudinal axis  34  of plate  12  from upper surface  30  to lower surface  32  and from end  31  to end  33 . Elongate aperture has a generally open geometry enabling split sleeve  14 , expansible screw  16  and bone fastener  18  to be placed at any number of axial positions within elongate aperture  36 . Elongate aperture  36  generally has a non-uniform geometry about a central axis  35  which extends perpendicularly to the plane of plate  12 . 
         [0042]    In a preferred embodiment, right rail  13  and left rail  15  surround elongate aperture  36  of plate  12  and include an upper wall  38  adjacent to upper surface  30 . The geometry of left rail  13  and upper rail  15  of aperture  36  further include a key way or groove  40  adjacent to upper wall  38  composed of median ceiling  42 , median portion  44  and median ridge  46 . A lower portion  48  of rails  13  and  15 , adjacent to key way  40 , includes a lower surface  50  adjacent to lower wall  49  and sidewall  52  adjacent to lower ridge  50 , concludes the remaining exterior geometry of elongate aperture  36 . In one embodiment, elongate aperture  36  has substantially the same geometry extending from the proximal end  31  to the distal end  33  or the previously mentioned ridge portions and rails may either abruptly halt or in a preferred embodiment may linearly transition into a smooth geometry at the distal and proximal ends as shown in  FIG. 4   a . 
         [0043]    Plate  12  is of sufficient length to bridge more than one vertebrae, as shown in  FIG. 1 , for which stabilization is required, and it will be appreciated, various dimensions of the plate and its features exist, all within the scope of the present invention. For example, the plate  12  may be substantially planar as shown in  FIG. 2  or have a concave shape as shown in  FIG. 1 . 
         [0044]    The plate  12 , as well as the other elements of the assembly, is preferably made from a biologically inert material, for example, any metal customarily used for surgical devices and particularly those used for bone screws and pins, such as titanium or stainless steel. Other suitable materials include, but are not limited to, alloys, composite materials, ceramics or carbon fiber materials. 
         [0045]    With reference to  FIG. 5 , there is shown a preferred embodiment of a bone fastener  18 . Bone fastener  18  is in the shape of a pedicle screw; however, various other fasteners may be utilized including bone hooks. Preferably, bone fastener  18  has a threaded end  152  for anchoring the fastener into a vertebra or similar bone structure and an opposing stem end  150 . In a preferred embodiment, stem end  150  is smooth. Bone fastener  18  may further include recess  154  to enable bone fastener  18  to be screwed into a vertebrae using a screw driver, wrench, alien key or similar tool. Recess  154  may be in the form of a hexagon as shown in the figures or alternatively, may be a slot or other shapes which allow the tools previously mentioned to mate to the bone fastener  18  and screw the bone fastener  18  into a vertebral body. Smooth stem  150  is preferably sized to be able to be slidably received by expansible screw  16 , detailed in  FIGS. 7   a  and  7   b , specifically aperture  124  of expansible screw  16  as will be described below. Bone fastener  18  may also include upper end section  156 , which may have a substantially flat end surface area as shown in  FIG. 5 . However, in an alternate embodiment, upper section  156  may also have a spherical configuration either concave or convex relative to threaded end  152 . 
         [0046]    Referring now generally to  FIGS. 2 ,  3 ,  6   a  and  6   b , there is shown a preferred embodiment of a split sleeve  14 . Split sleeve  14  may have a generally rounded-square or oval shape. Split sleeve  14  may include a first flat side  60 , and opposing second flat side  62 , a first arcuate side  64  and an opposing second arcuate side  66 . Sides  64  and  66  could be of any shape, however, sides  60 ,  62  preferably permit split sleeve  14  to slide within aperture  36  without rotating. Split sleeve  14  may further include an upper surface  68  and a lower surface  70  along with aperture  72 , extending therebetween. Aperture  72  generally has a non-uniform geometry about central axis  74 . The first side  60  and second side  62  may include keys  76  and  78  which engage key ways  40  on rails  13  and  15  of the plate  12 , respectively. Although a description of only one key will be detailed with reference to key  76 , it should be realized that key  78  may be substantially similar to key  76  having all the same features. Key  76  protrudes from second side  62  and includes top ledge  80  and lower ledge  82  extending horizontally from second side  62 . Side  84  adjacent to both top ledge  80  and lower ledge  82  extends between the two ledges. Key  76  may be sized and located on second side  62  so when split sleeve  14  is placed into elongate aperture  36  of plate  12 , pin  76  is housed within key way  40  as defined by side  84 . Keys  76  and  78  may have a height slightly less than the height of key way  40  to permit split sleeve  14  to slide freely along key way  40  of plate  12 . Furthermore, the reduced height of key  76  enables split sleeve  14  to have an angled orientation relative to plate  12 , in the sagittal plane. In the preferred embodiment, keys  76 ,  78  extend only part way along sides  60 ,  62  but could extend the entire length. 
         [0047]    Split sleeve  14  further includes through-slot  102  vertically extending from upper surface  68  to lower surface  70 . Slot  102  is further defined in that it extends horizontally from the exterior of split sleeve  14  to aperture  72 . The outer circumference of split sleeve  14  is substantially continuous except for through-slot  102 . Through-slot  102  permits split sleeve  14  to expand and compress horizontally thereby increasing or decreasing the diameter of aperture  72 . Split sleeve  14  may be compressed about slot  102  to allow insertions of keys  76 ,  78  into key way  40 . 
         [0048]    As previously mentioned, aperture  72  has a non-uniform geometry extending between upper surface  68  to lower surface  70  of split sleeve  14 . Specifically, inner wall  90  is adjacent to upper surface  68 . Inner wall  90  may have a tapered design as shown in  FIG. 6B  to better receive expansible screw  16 , as will be described below. A first ridge  92  extends from inner wall  90  towards central axis  74 , with median wall  94  extending downward towards lower surface  70  adjacent to first ridge  92 . A second ridge  96  is adjacent to median wall  94  and extends toward central axis  74  while lower wall  98  is adjacent to second ridge  96 . Inner wall  90  includes threads  91  which may be mated to the threads of the expansible screw  16  described herein after. Lower wall  98  may include vertical wall  97  and tapered skirt  99  along with edge  101 . Inner wall  90 , first ridge  92 , median wall  94 , second ridge  96  and lower wall  98  all combine to define female cone  100 . 
         [0049]    Split sleeve  14  is placed within elongate aperture  36  of plate  12  when assembling the spinal implant assembly  10 . In this position, lower surface  70  of split sleeve  14  is rested on or slightly above lower ridge  50  of plate  12  and as previously mentioned, pins  76  and  78  are located within key way  40  at the right rail  13  and the left rail  15  of the plate  12 . Split sleeve  14  is sized to move freely along plate  12  and permit a possible angled orientation of the split sleeve with respect to plate  12  in the sagittal plane. Thus, preferably, the height of split sleeve  14  is less than the distance from the upper surface  30 , to lower ridge  50 . Preferably, in a relaxed state, split sleeve  14  has a width less than the distance between opposing rails  13  and  15  of aperture  36  of plate  12 . 
         [0050]    Referring now to  FIGS. 7   a  and  7   b , there is shown a preferred embodiment of the expansible screw  16 . The expansible screw  16  includes upper surface  120 , lower surface  122  and aperture  124  extending therebetween upper surface  120  and lower surface  122 . Expansible screw  16  further includes cap section  126  and deflectable male cone section  128 . Axially extending portion  130  includes threads  131  which are engageable with threads  91  on split sleeve  14 . Male cone section  128  further includes, extending downward from ridge  132 , individual fingers  134  with slits  136  therebetween each individual finger  134 . Preferably, fingers  134  are tapered inwards so as to be slidably received by lower wall  90  of split sleeve  14 . Male cone  128  of expansible screw  16  is designed so as to be able to receive fastener  18  and is geometrically constant throughout aperture  124 . However, as a force is exerted horizontally relative to central axis  138  of expansible screw  16 , male cone  128  is capable of compressing either inwardly or expanding outwardly in the same direction of the force applied. Cap section  126  of expansible screw  16  includes not only upper surface  120  but also grip surface  140  and overhang  142  and may also include opposing slots  139  for cooperating with a tool. Furthermore, aperture  124  also extends through cap section  126  thereby creating a continuous aperture through expansible screw  16 . 
         [0051]    In a method of use, once bone fastener  18  has been coupled with or fastened to a vertebral body, plate  12  along with split sleeve  14  is placed over smooth stem  150  of bone fastener  18 . First split sleeve  14  may be placed within aperture  36  of plate  12 . Then, by aligning upper section  156  of bone fastener  18  to aperture  72  of split sleeve  14 , split sleeve  14  and plate  12  are slidably received by stem  150 . Since bone fastener  18  is a substantially cylindrical body having a center axis, this alignment may be accomplished by aligning central axis  74  of split sleeve  14  to the center axis  158  of bone fastener  18 . Bone plate  12  and split sleeve  14  are then freely moveable in a vertical direction relative to bone fastener  18  and specifically an anterior-posterior direction. 
         [0052]    In order to lock the various components of the spinal implant assembly  10  together, expansible screw  16  must be screwed into split sleeve  14 . This is accomplished by first aligning aperture  124  of expansible screw  16  with upper section  156  of bone fastener  18 . Once again, since stem  150  of bone fastener  18  is substantially cylindrical and has a radial central axis  158 , central axis  138  of expansible screw  16  may be aligned with the radial central axis  158  of the bone fastener  18 . 
         [0053]    Aperture  72  of split sleeve  14  is sized so as to be able to accept expansible screw  16  and bone fastener  18  at opposing ends. Furthermore, expansible screw  16  engages threads  91  within aperture  72  of split sleeve  14  and the two are screwed together such that the tapered portions of each engage one another. This causes an outward horizontal force against split sleeve  14  and an inward force on deflectable fingers  134 . The fingers  134  grip the stem  156  of the bone fastener  18  as they are constricted inwards. This is due to the fact that male cone  128  of expansible screw  16  is wedged shaped, i.e., having a narrower lower portion as compared to its upper portion. Through-slot  102  of split sleeve  14  permits the expansion of split sleeve  14  in the horizontal direction. As split sleeve  14  is forced to expand, first side  60  begins to apply pressure against upper wall  38  and lower wall  49  of the plate  12 . Key  78  may contact median portion  44  of key way  40  located on plate  12 . Similarly, at the opposing end, second side  62  also applies pressure against upper wall and lower wall  49  as well as the fact that key  76  may contact median portion  44  of guide rail  40  of plate  12 . Thus, as a horizontal force in a direction away from central axis  74  of split sleeve  14  is placed on the interior of the split sleeve  14 , the split sleeve  14  expands horizontally increasing the size of through-slot  102  until the rails  13  and  15  of elongated aperture  36  of plate  12  apply a reactive force against the walls of split sleeve  14  thus locking the split sleeve in place. 
         [0054]    Once split sleeve  14  has reached its maximum expansion capability, further axial movement of expansible screw  16  causes it to lock on to screw  18 . Since expansible screw  16  is further translated downward, and split sleeve  14  can no longer expand, the reactive force caused by the plate  12  translated through split sleeve  14  begins to compress fingers  134  of expansible screw  16 . This causes fingers  134  to constrict about smooth stem  150  of bone fastener  18  locking the bone fastener relative to the plate. 
         [0055]    In a locked position, cap  126  of expansible screw  16 , specifically overhang  142  of expansible screw  16  may abut upper surface  68  of split sleeve  14 . Thus, male cone  128  is completely housed in aperture  72  of split sleeve  14 . 
         [0056]    The rotation of expansible screw  16  thus causes the plate to lock relative to the bone fastener  18 . The stem  150  of bone fastener  18  may have a smooth surface to allow easy adjustment after plate  12  has been positioned and assembled, thus overcoming the disadvantage of threaded end fasteners. The surgeon may easily change the bone screw position engaged with the vertebrae by loosening expansible screw  16  and sliding the assembly in either an anterior/posterior direction or inferior/superior direction. Afterwards, the assembly may be locked again by tightening expansible screw  16 . This avoids the problem of having to disassemble all the elements from one another in order to adjust only certain elements. 
         [0057]    Other embodiments not shown in the illustrated figures may include single elongated aperture  36  being divided into a plurality of apertures by cross-members extending between opposing rails  13  and  15  of plate  12  substantially perpendicular to longitudinal axis  34 . The cross-members would have to be sufficiently separated from one another as well as from proximal end  31  and distal end  33  to permit split sleeve  14 , expansible screw  16  and bone fastener  18  to be placed within the shortened apertures and still be locked together relative to plate  12 . 
         [0058]    Although the present invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.