Patent Publication Number: US-2010131009-A1

Title: Spinous process implant spacer and method of use therefor

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is related to, and claims priority from, European Patent Application Serial No. 08 020 138.7, filed Nov. 19, 2008, the entire contents of which is incorporated herein fully by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a spacer for facilitating a spinous process implant. More specifically, the present invention relates to a spinous process implant and spacer that can be implanted easily and is usable in many anatomical situations 
     2. Description of the Related Art 
     Degeneration of the vertebral column, which occurs particularly with increasing age, frequently leads to lumbar spinal canal stenosis. This is a narrowing of the spinal canal and of the intervertebral foramina (foramina intervertebralia), generally as a consequence of spinal disk degeneration. Typical symptoms are nerve pains in the back and in the legs, and in severe cases paralysis symptoms in the legs. 
     In simple cases, treatment with drugs and physiotherapy can be successful. In severe cases, distraction of the spine causes a decompression of the nerve structure. For long-term therapy, it is possible in this case to use implants between the spinous processes of the affected vertebrae. These spinous process implants have in common the use of a spacer which is inserted between the spinous process of the upper cranial vertebra, and the spinous process of the adjacent lower caudal vertebra. The spacer is introduced here laterally between the spinous processes, where its longitudinal axis is arranged so it is substantially perpendicular to the median sagittal plane, i.e., to the middle plane of the body. 
     From U.S. Pat. No. 5,860,977, a spinous process implant is known in which the spacer is substantially a cylindrical body, which is inserted between the spinous processes, and protected against lateral shifting in its longitudinal axis by wings arranged on its two lateral ends. A unilateral insertion of the implant is not possible here, because the spacer with a wing is inserted from one side, and then the second wing has to be placed on the other side of the spinous process. 
     The company Kyphon sells a unilaterally insertable implant under the name of “Aperius.” This implant presents a tubular spacer with two thinned zones at a separation. The spacer is introduced unilaterally between the spinous processes, and then braced in the longitudinal direction, so that the thinning zones are deformed to radially protruding wings, which keep the spacer positioned on both sides of the spinous processes. 
     What is not appreciated by the prior art is that these spinous process implants share the feature that the spacer presents a predetermined diameter. Therefore, depending on the anatomical circumstances, spacers with different diameters have to be used. Consequently, on the one hand, the interspinal space has to be measured in a first surgical step, to be able to choose the fitting spacer. In addition, spacers in different sizes have to be available. 
     In EP 1 330 987 B1, a spinous process implant is described in which, as spacer, a closed spring band is used, which has essentially the shape of a figure eight that is symmetric with respect to the sagittal plane. The implant is secured with securing clips to the respective spinous processes. The implant thus fixes the spinous processes of consecutive vertebrae with respect to each other, so that bending of the vertebral column is now possible only within the narrow limits of the elastic deformability of the implant. Here too, the implant must be chosen in each case in a size that fits the anatomical requirements. 
     Accordingly, there is a need for an improved spinous process implant that can be implanted easily and is usable in many anatomical situations. 
     ASPECTS AND SUMMARY OF THE INVENTION 
     An aspect of the present invention is to provide a spinous process implant that can be implanted easily and is usable in many anatomical situations. 
     The present invention relates to a spinous process implant that can be positioned with its longitudinal axis substantially perpendicular to the median sagittal plane of the patient, between an upper and an adjacent lower spinous process. The implant presents, as spacers, upper support means, and lower support means, which, in each case, are applied with a recess saddle against the upper or lower spinous process. The support means can be spread apart in each case by means of clamping means, which are arranged between the support means in the longitudinal direction. 
     The essential idea of the invention is to use, as spacer that is inserted between the spinous processes, not a body with a predetermined, fixed diameter, but a spacer which presents two support means that can be spread apart from each other. In an introduction position, the support means are positioned one against the other with a small separation, so that the spacer presents only a small diameter. After the introduction between the spinous processes, the support means are spread apart, so that their separation becomes larger. As a result, the upper support means are applied against the upper cranial spinous process, and the lower support means against the adjacent lower caudal spinous process. In this way, the spacer is able to adapt to the given anatomical circumstances, and there is no need for different spacers for different dimensions of the interspinal space. 
     Clamping means are used here to spread the support means apart. They are arranged between the support means in the longitudinal axis of the implant, i.e., in the axis that is perpendicular to the median sagittal plane, and they engage on both lateral ends of the support means. As a result, a unilateral introduction of the implant is possible. The implant can be introduced unilaterally through a percutaneous incision between the spinous processes, and then it is spread apart by the unilateral actuation of the clamping means, and positioned and fixed between the spinous processes. The support means present, in each case, a recessed saddle, which forms a receptacle for the given spinous process. When the support means are applied, in the spread state, against the respective spinous process, then they are positioned with this saddle over a peripheral area against the periphery of the spinous process, which results in securing the support means, and thus the entire implant, with positive locking against lateral shifting, i.e., against shifting in the longitudinal axis of the implant, perpendicularly to the sagittal plane. The implant therefore does not require additional wings or other measures to protect against lateral shifting with respect to the spinous processes. This simplifies the construction and facilitates particularly the surgical technique during the introduction of the implant. 
     In another embodiment of the present invention, the support means are designed as spring leaves whose lateral ends are pulled together by the clamping means, so that the spring leaves form an upward or downward convex bulge, and spread apart. 
     In another embodiment, the support means are designed as support plates, which, during the actuation of the clamping means, are swiveled out of their introductory position, upward or downward, so that their separation becomes larger. In a simple embodiment, it is possible here to connect by articulation in each case an upper and a lower support plate to the two lateral ends, whose mutually facing free ends are swiveled away from the longitudinal axis upward or downward. 
     In an embodiment that allows for easy handling, the clamping means present a telescopic design. By telescopically pushing the lateral ends together, they can be moved toward each other, to spread apart the support means. The telescopic pushing together can here occur unilaterally by means of a self-inhibiting thread. The clamping means can also be designed so they can be moved freely into each other telescopically, where the two telescope parts become latched in the respective spread position, against the counterpressure of the spinous processes. In a variant of the invention, the telescope parts can be shifted further into each other by means of a resetting force, to spread the support means further apart, for the purpose of adapting the implant to the increasing separation of the spinous processes, for example, in the case of bone resorption. 
     The above, and other aspects, features and advantages of the present invention will become apparent from the following description read in conduction with the accompanying drawings, in which like reference numerals designate the same elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a side view of a first embodiment of the spinous process implant in the introduction position; 
         FIG. 2  shows a perspective view of the implant in the spread position; 
         FIG. 3  shows an additional perspective view of this implant; 
         FIG. 4  shows a longitudinal section of the first embodiment of the implant; 
         FIG. 5  shows a second embodiment of the spinous process implant in the introduction position; 
         FIG. 6  shows the second embodiment in the spread position; 
         FIG. 7  shows a perspective view of the second embodiment, where the support plates have been omitted; 
         FIG. 8  shows a partial view of  FIG. 7 ; 
         FIG. 9  shows an additional partial view, in which a support plate is shown for illustration; and 
         FIG. 10  shows a detail view of an embodiment of the clamping means. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to several embodiments of the invention that are illustrated in the accompanying drawings. Wherever possible, same or similar reference numerals are used in the drawings and the description to refer to the same or like parts or steps. The drawings are in simplified form and are not to precise scale. For purposes of convenience and clarity only, directional terms, such as top, bottom, up, down, over, above, and below may be used with respect to the drawings. These and similar directional terms should not be construed to limit the scope of the invention in any manner. The words “connect,” “couple,” and similar terms with their inflectional morphemes do not necessarily denote direct and immediate connections, but also include connections through mediate elements or devices. 
     The purpose of the spinous process implant according to the invention is to keep the spinous process (processus spinosus) of two consecutive vertebrae, in most cases two lumbar vertebrae, separated vertically. For this purpose, the implant is introduced laterally between the spinous processes in the median direction, and positioned in such a way that the longitudinal axis of the implant is arranged substantially perpendicularly to the median sagittal plane of the patient, i.e., to the middle plane of the body. To simplify the description, the following indications refer to the implanted position of the implant. Thus, the terms “upper” or “lower” refer to the cranially or caudally directed sides of the implanted implant. The terms “lateral” and “right” or “left” accordingly denote in each case areas in the implanted implant that are located laterally to the median sagittal plane. 
       FIGS. 1-4  show a first embodiment of the spinous process implant. The implant presents upper and lower support means that are designed as a spring leaf  10 . The upper spring leaf  10 . 1  and the lower spring leaf  10 . 2  here are parts with identical shape that, however, are arranged with mutual twisting by 180° about the longitudinal middle axis of the implant. The spring leaves  10  have the shape of a longitudinally stretched, flat band that is shape elastic, i.e., it maintains its shape, although it is deformable when exposed to a relatively strong elastic resetting force. The spring leaves  10  are manufactured for this purpose from an appropriate biocompatible material, for example, from a metal, particularly a titanium alloy, or from an appropriate plastic, for example PEEK. The spring leaves  10  present, for example, a length of 15-30 mm, preferably 20-25 mm, a width of 4-8 mm, and a thickness of 0.8-1.2 mm. 
     The spring leaves  10  form a convex bulge, upward or downward, in their longitudinal direction, as a flat arc. In the middle area of their longitudinal extent, the spring leaves  10  here present a concave recess with opposite bulge forming a saddle  12 . At its two lateral end edges, the spring leaves  10  present notches, so that axially protruding teeth  14  are formed. When the two spring leaves  10 . 1  and  10 . 2  are assembled with mutual twisting by 180°, the teeth  14  of the corresponding lateral ends of the two spring leaves  10 . 1  and  10 . 2  mutually engage with interdigitation, so that the lateral ends of the two spring leaves  10 . 1  and  10 . 2  cross over each other, as can be seen in  FIGS. 2 and 3 . 
     Clamping means are inserted between the two spring leaves  10 . 1  and  10 . 2 , in the middle longitudinal axis of the implant. The clamping means consist of an outer tube  16  and an inner bolt  18  that engages telescopically in the outer tube  16 . The outer tube  16  and the inner bolt  18  in each case engage through central axial notches  20  of the lateral end edges of the spring leaves  10 . On the mutually facing lateral ends of the outer tube  16  and of the inner bolt  18 , an end piece  22  or  24  is arranged in each case. The end pieces  22  and  24  are in the shape of a roller whose axis runs perpendicularly to the axis of the outer tube  16  or of the inner bolt  18 . The end pieces  22  and  24  are positioned in each case laterally from outside in the fork of the mutually crossing, interdigitating, lateral ends of the leave springs  10 . 1  and  10 . 2 . 
     In an introduction position of the implant, as shown in  FIG. 1 , the outer tube  16  and the inner bolt  18  are moved apart from each other laterally outward with the end pieces  22  or  24  in the middle axis of the implant. The spring leaves  10 . 1  and  10 . 2  are, as a result, stretched flat due to their elastic shape stability, so that they are applied with the mutually facing inner surfaces of the saddle  12  against the clamping means, i.e., particularly against the outer tube  16 . The entire implant presents, in this introduction position, a height of only approximately 5-7 mm. In this introduction position, the implant is inserted between the spinous processes of two consecutive vertebrae. For this purpose, with the patient in an appropriate position, a percutaneous incision is made unilaterally and parallel to the middle plane of the patient, and the interspinal space between the spinous processes is made accessible. Using an appropriate introduction instrument, the implant is introduced laterally in the medial direction between the spinous processes, and positioned in the interspinal space, as close as possible to the vertebral arch. The implant is positioned here in such a way that the middle axis, i.e., the axis of the clamping means, namely of the outer tube  16  and of the inner bolt  18 , runs substantially perpendicularly to the median sagittal plane. The upper spring leaf  10 . 1  is directed with its saddle  12  against the spinous process of the upper vertebra, and the lower spring leaf  10 . 2  with its saddle  12  against the spinous process of the lower vertebra. 
     Then, the clamping means are actuated, by moving the inner bolt  18  and the outer tube  16  coaxially into each other, and as a result the end pieces  22  and  24  are pulled toward each other in the lateral direction. By means of the end pieces  22  and  24  which are to be moved toward each other, the interdigitating lateral ends of the spring leaves  10 . 1  and  10 . 2  are pulled toward each other, so that the spring leaves  10 . 1  and  10 . 2  form a convex bulge, upward or downward, and are spread apart in the vertical direction. This spread position is shown in  FIG. 2 . As a result of the spreading apart of the spring leaves  10 . 1  and  10 . 2 , the latter are pressed against the corresponding spinous processes, with the result of distracting them in the desired way. Because the spinous processes are located in each case in the saddle  12  of the upper or of the lower spring leaf  10 , the implant is protected against shifting in the lateral direction substantially by positive locking. In the median sagittal plane, i.e., in the plane that is perpendicular with respect to the clamping means, the implant is secured by the anatomic concavity of the spinous processes. 
     The clamping of the clamping means can be achieved in different ways. In an embodiment, as shown in  FIG. 4 , the inner bolt  18  engages with a thread  26  in an inner thread of the outer tube  16 . The design of the threads  26  is self-inhibiting here. The end piece  22  can here be connected firmly to the outer tube  16 , while the inner bolt  18  passes through the end piece  24  in a way that allows free rotation. The inner bolt  18  is braced axially with a screw head against the end piece  24 , and it can be rotated by means of this screw head from the lateral end side to clamp the clamping means. The self-inhibition of the thread  26  is ensured here by the fact that the implanted implant remains in the clamped and spread position. If surgical removal of the implant becomes necessary, then the inner bolt  18  can be rotated out of the outer tube  16 , so that the implant returns to the introduction position. Moreover, the implant can be clamped again in a minimally invasive intervention by means of the screw head, if the separation of the spinous processes has increased, for example, as a result of bone loss. 
       FIGS. 5-9  represent a second embodiment of the spinous process implant. 
     In this embodiment, the support means are designed as support plates  32 . On the upper side, a left support plate  32 . 1  and a right support plate  32 . 2  are provided, and, accordingly, on the lower side, a left support plate  32 . 3  and a right support plate  32 . 4  are provided. The support plates  32 . 1 ,  32 . 2 ,  32 . 3  and  32 . 4  all have the same shape. The support plates  32  consist of an appropriate biocompatible, dimensionally stable material, for example, metal, particularly a titanium alloy or preferably an appropriate plastic, particularly PEEK. The fact that the shape of the support plates  32 . 1 ,  32 . 2 ,  32 . 3  and  32 . 4  is the same is advantageous with regard to the manufacturing costs. 
     The support plates  32  present the shape of a plate which is stretched in the longitudinal direction, with a length of approximately 15-30 mm, and a width of approximately 4-8 mm. The thickness of the material is approximately 1-2 mm. The support plates  32  are in each case connected by articulation to lateral end pieces  34  and  36 , where the end pieces  34  or  36  can be pulled toward each other in the lateral direction by means of clamping means. The end pieces  34  and  36  here present substantially the same shape of a block. 
     For the swivelable attachment of the support plates  32  on the end pieces  34  or  36 , these end pieces  34 ,  36  present in each case recesses  38 , on their mutually facing sides, on the upper side and on the lower side. In each case, an end of a support plate  32  is inserted in these recesses  38 , and attached swivelably about a swivel axis  40  which passes through the recess  38 , in each case parallel to the upper side or lower side of the implant, and perpendicularly with respect to its middle axis, and which inserted in the bores  41 . In this way, the upper left support plate  32 . 1  is attached swivelably with its left end in the upper recess  38  of the left end piece  34 , where the other free end of this support plate  32 . 1  is directed to the right. Correspondingly, the right upper support plate  32 . 2  is attached swivelably in an upper recess  38  of the right end piece  36 , the left lower support plate  32 . 3  in a lower recess  38  of the left end piece  34 , and the right lower support plate  32 . 4  in a lower recess  38  of the right end piece  36 . The support plates  32  present, at their respective end located in the recess  38 , a width that fills the recess  38 . The support plates  32  present a greater width at their other free end. In their middle area in the longitudinal direction, the support plates  32  present, in each case on one of their side edges, a lateral cutout  42  which engages up to approximately half of the width of the support plates  32  into the latter. On the end that faces the swivel axis  40 , the cutout  42  presents a sliding edge  44  which runs parallel to the swivel axis  40  and thus perpendicularly to the longitudinal extent of the support plate  32 . The shape of the support plates  32  can be seen best in  FIG. 9 . The support plates  32 . 1  and  32 . 2  on the upper side, and the support plates  32 . 3  and  32 . 4  on the lower side, are in each case mutually twisted by 180° and attached swivelably on the corresponding end pieces  34  or  36 . As a result, the left support plate  32 . 1  and the right support plate  32 . 2  engage into each other on the upper side, while the left support plate  32 . 3  and the right support plate  32 . 4  similarly engage into each other on the lower side, in each case with their cutouts  42 . The left support plate  32 . 1  is positioned with its small middle area in the cutout  42  of the right support plate  32 . 2 , and on the sliding edge  44  of this right support plate  32 . 2 . Conversely, the right support plate  32 . 2  engages with its middle area in the cutout  42  of the left support plate  32 . 1 , and it is positioned on its sliding edge  44 . The lower support plates  32 . 3  and  32 . 4  present a corresponding arrangement. 
     In this second embodiment, the clamping means can also be telescopic clamping means which present an outer tube  16  arranged at one end piece  34 , and an inner bolt  18  arranged at the other end piece  36 . The clamping means with the outer tube  16  and the inner bolt  18  are arranged in the middle axis of the implant. In the horizontal plane, which is perpendicular to the median sagittal plane, on both sides of the clamping means, guidance means are arranged, which prevent a mutual twisting of the end pieces  34  and  36 , and of the support plates  32  attached to the latter, about the middle axis of the clamping means. The guidance means consist, in the embodiment example, in each case of a guidance tube  46  which is arranged on one end piece  34 , and a guidance rod  48  which is arranged on the other end piece  36 . The guidance rods  48  are guided in each case with axial sliding into the guidance tubes  46 . The guidance means formed from the guidance tubes  46  and the guidance rods  48  are parallel to the axis of the clamping means, and arranged at the same separation from the latter. 
     In the introduction position of the implant, which is shown in  FIG. 5 , the end pieces  34  and  36  are moved laterally apart from each other. The upper support plates  32 . 1  and  32 . 2 , which mutually interdigitate via their cutouts  42 , and also the lower support plates  32 . 3  and  32 . 4  which are positioned with interdigitation above their cutouts  42 , lie on each other in this introduction position, so that the entire height of the implant in the vertical direction is small, for example, 5-7 mm. In this introduction position, the implant can be introduced unilaterally into the interspinal space between the spinous processes of two consecutive vertebrae, as described above. The implant is positioned in the interspinal space in such a way that the implant is in the lateral direction symmetric with respect to the median sagittal plane, and the upper support plates  32 . 1  and  32 . 2  face the upper spinous process, while the lower support plates  32 . 3  and  32 . 4  face the lower spinous process. 
     Subsequently, the clamping means are actuated, by moving, for example, the inner bolt  18  and the external bolt  16  telescopically toward each other. As a result, the end pieces  34  and  36  are pulled together in the lateral direction. The upper support plate  32 . 1  and  32 . 2 , and accordingly the lower support plates  32 . 3  and  32 . 4 , are also moved against each other, as a result. In the process, the upper support plates  32 . 1  and  32 . 2  slide on the sliding edge  44  of the other support plates  32 . 2  and  32 . 1  in each case, and likewise the lower support plates  32 . 3  and  32 . 4  slide against each other. In this sliding motion, the free end of the left support plate  32 . 1  is swiveled upward by the right end piece  36 , while, conversely, the free end of the right support plate  32 . 2  is swiveled upward by the left end piece  34 . The free ends of the lower support plate  32 . 3  and  32 . 4  are swiveled downward correspondingly by the respective end pieces  36  or  34 . This spread position is shown in  FIG. 6 . As a result of the upward spreading of the free ends of the support plates  32 . 1  and  32 . 2  or  32 . 3  and  32 . 4 , a central recessed saddle  12  forms between these two ends of the support plates. As the implant is spread apart, the support plates  32  that have been swiveled upward are applied against the corresponding spinous processes, distracting the latter. The spinous processes here lie in each case in the saddle  12  formed between the free ends of the upper or of the lower support plates  32 , so that the implant is protected against lateral shifting with positive locking on the spinous processes. 
     The clamping of the clamping means can also occur in the same way in the second embodiment, as explained in reference to the first embodiment of  FIGS. 1-4 . Accordingly, the inner bolt  18  can be rotated into the outer tube  16  with a self-inhibiting thread. 
     An alternative embodiment of the clamping means is shown in  FIG. 10 . In this embodiment, the inner bolt  18  is designed with latch teeth  50  on its periphery, which engage in the latching receptacles  52  on the inner periphery of the outer tube  16 . The latch teeth  50  and the latching receptacles  52  are designed so that, for the purpose of clamping the clamping means, the inner bolt  18  can be pushed into the outer tube  16 , while the latched position prevents the inner bolt  18  from being pulled out of the outer tube  16 . The tooth partition of the latch teeth  50  and of the latching receptacles  52  here determines the corresponding spread position of the implant. Naturally, this latch design of the clamping means can also be used with the first embodiment of the implant, shown in  FIGS. 1-4 . 
     An embodiment of the clamping means in which the latter are latched in successive clamped positions, as shown, for example, in  FIG. 10 , allows clamping the implant again, for example, in case of bone loss and bone resorption, by means of a resetting force that pushes the inner bolt  18  axially into the outer tube  16 , to set the spread position to the next latch position. This resetting force can naturally also be introduced mechanically in a minimally invasive way, as in the case of a resetting by means of a thread. It is also possible to introduce the resetting force by injection, hydraulically or pneumatically or inductively-electrically. An automatic resetting can be achieved by means of an integrated resetting spring force. Such a resetting spring force is applied to the clamping means in the clamping direction, i.e., in the direction in which the support means are spread apart from each other. As long as the support means are applied against the spinous processes, the spinous processes brace the support means against this additional resetting spring force. If the spinous processes yield as a result of bone loss, then the resetting spring force becomes effective, and actuates the clamping means in such a way that the support means are spread apart again, and are applied again against the spinous processes. 
     The resetting spring force can be achieved, for example, by spring means that are integrated in the clamping means, or also possibly in the guidance means in the embodiment of  FIGS. 5-9 . Such spring means can consist, for example, of a traction spring which is inserted into the outer tube  16  or the guidance tubes  46 , and engages on the inner bolt  18  or the guidance rods  48 , pulling the inner bolt  18  into the outer tube  16  or the guidance rods  48  into the guidance tubes  46 . Alternatively, pressure springs can be provided, which press the inner bolt  18  into the outer tube  16  or vice versa the outer tube  16  over the inner bolt  18 , or the guidance rods  48  and the guidance tubes  46  press into each other in a corresponding way. 
     Such spring means can consist particularly of a shape memory alloy. Here, the spring means can be kept at a low temperature during the introduction of the implant, at which they are unstressed, and not applied to the clamping means. It is only when the implant has been heated after insertion to the body temperature of the patient that the shape memory spring means go into the state in which they apply the resetting spring force. 
     Additionally, it should be noted that the surface of the saddle  12 , where the upper spring leaf  10 . 1  is directed with its saddle  12  against the spinous process of the upper vertebra, and the lower spring leaf  10 . 2  with its saddle  12  against the spinous process of the lower vertebra, presents a surface to the vertebrae which can be adapted to the specific needs of the implant, or be adapted for mass production. The surface can be modified so as to present a coated or cushioned aspect to the biomass to be supported; or, the surface can be manufactured so as to present a non-smooth aspect to the biomass so as to reduce slippage. 
     Those of skill in the art having studied the current disclosure and appreciated the invention therein will additionally understand that the particular spring forces and spring means elements described herein may be provided by alternative constructions, shapes, and embodiments without departing from the scope and spirit of the present invention. For example the smoothly undulating shape disclosed may be provided by a non-smoothly undulating shape, or one with a series of joined plane portions (not shown) that are effective to result in the purpose achieved by the proposed preferred embodiments. As a result, the present disclosure should be understood to present illustrations for the proposed claims, but the invention is not limited to such illustrations but fully includes the entire scope and spirit of the present invention. 
     In the claims, means or step-plus-function clauses are intended to cover the structures described or suggested herein as performing the recited function and not only structural equivalents but also equivalent structures. Thus, for example, although a nail, a screw, and a bolt may not be structural equivalents in that a nail relies on friction between a wooden part and a cylindrical surface, a screw&#39;s helical surface positively engages the wooden part, and a bolt&#39;s head and nut compress opposite sides of a wooden part, in the environment of fastening wooden parts, a nail, a screw, and a bolt may be readily understood by those skilled in the art as equivalent structures. 
     Having described at least one of the preferred embodiments of the present invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes, modifications, and adaptations may be effected therein by one skilled in the art without departing from the scope or spirit of the invention as defined in the appended claims.