Abstract:
A fixation system may comprise at least one carriage element being capable of receiving one or more bone fasteners. The carriage element is mountable on a plate, and the plate has an engaging portion for adjustably securing the carriage element to the main plate. The engaging portion of the carriage element allows translational movement of the carriage plate with respect to the plate in only one axial direction. In one embodiment, the plate includes arms for receiving the carriage element and the carriage element includes channels mountable on the arms, with the engaging portion disposed within the channels.

Description:
FIELD OF THE INVENTION 
     This invention relates to the field of fixation devices. More particularly, this invention relates to a fixation system for spines, the fixation system being capable of unidirectional translational adjustment. 
     BACKGROUND OF THE INVENTION 
     Orthopedic fixation devices such as plates are frequently coupled to bone with fasteners inserted through plate holes. It is known that securing such fasteners to the bone plate, for example through the use of expansion-head screws, can decrease the incidence of loosening of the fixation assembly post-operatively. It is also known that a bushing may be disposed in each plate hole to receive the fastener to permit polyaxial movement so that the fastener may be angulated at a surgeon-selected angle. However, polyaxial movement of fasteners through set plate hole locations only increases attachment alternatives of the fasteners themselves. The plate holes remain fixed in relation to each other and to the longitudinal axis of the plate. 
     Typically, a spinal fixation plate is applied to the anterior side of the affected vertebrae to span at least one affected disc space or vertebra (i.e. one in which at least a portion of the disc has been removed and a spinal fusion spacer has been inserted). The plate is fixed to the vertebrae using bone screws and acts to keep the vertebrae generally aligned during the initial period following fixation in which fusion of the spacer to the adjacent vertebrae occurs. The plate also acts to prevent the spacer from being expelled from the disc space during this initial period. 
     Where a spinal fusion spacer is implanted between a pair of vertebrae to be fused, the spacer rests on the endplates of the vertebrae. The outer circumference of the end plates comprises hard cortical bone and thus provides a the best surface upon which to seat the spacer. The center portion of the endplates comprises a thin cortical bone shell overlying a core of softer cancellous bone. Most, if not all, of the spacer contact surface, however, may be located in this center portion. 
     Subsequent to placement of the spacer, the surgeon typically compresses the disc space by pressing the adjacent vertebrae together. This compression ensures a good engagement between the spacer the endplates, increasing the chances that fusion will occur. Often in the period immediately following surgery, the spacer will subside slightly either into the under-portion of the endplates or due to graft resorption (in the case of allograft spacers). 
     Where a rigid fixation plate is used to connect the vertebrae, this subsidence may tend to shift more of the spinal load to the plate than is desirable. Such load shifting can also occur due to inaccuracies in installing the plate to the vertebrae. In extreme circumstances, this load shifting can result in non-fusion of the spacer to the vertebra, since firm compression between the spacer and the vertebrae is one factor contributing to successful fusion. 
     Accordingly, there exists a need for a fixation system which provides the desired support to the vertebrae to be fused, and which allows limited translation of the vertebrae with respect to at least a portion of the plate, thereby limiting the undesirable effects of load shielding by the plate due to graft subsidence caused by settling or normal forces experienced in the spinal column. Promoting fusion of the adjacent vertebrae is thus accomplished. 
     However, fasteners used with both rigid and translational plates have a tendency to back-out of their installed positions under the influence of force and movements of the spine. The back-out of the fasteners is undesirable, as the fixation assembly may shift post-operatively to an undesired location, or loosen to an undesirable level. 
     Therefore, there exists a need for a fixation system that provides stability and promotes fusion while preventing graft movement and enabling compression of the graft. There also exists a need for a fixation system that allows for continued post-operative compression of bone segments. There further exists a need for a fixation system that adjusts only in the direction of increased compression. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, a fixation system may comprise at least one carriage element being capable of receiving one or more bone fasteners, the carriage element being mountable on a plate, and a securing element for adjustably securing the carriage element to the plate, the securing element allowing translational movement of the carriage plate with respect to the plate in only one axial direction. In one embodiment, the plate includes arms for receiving the carriage element and the carriage element includes channels mountable on the arms, with the securing element disposed within each channel. The arms can include an engaging portion that engages with the securing element, the engaging portion comprising a series of ridges that can engage the securing element. In a preferred embodiment, the engaging portion and securing element are in the form of a ratchet and pawl construction. 
     A fixation system is described that may comprise a first carriage element capable of receiving one or more bone fasteners, and may have a first channel with a least one securing element disposed therein; a first plate may have a first arm insertable in a channel; wherein the first arm has a first engaging portion, the first engaging portion may have a series of ridges configured to engage a securing element; and wherein the first carriage element may be configured to axially translate in only one direction relative to the first plate. 
     The first carriage element may be allowed to translate in situ. The first plate may have a second arm having a second engaging portion insertable into the first carriage element. The first channel may have a groove configured to receive a portion of a securing element. The securing element may be an engaging clip. 
     The fixation system may further comprise a second carriage element. The first carriage element may be configured to be associated with a first bone segment, and the second carriage element may be configured to be associated with a second bone segment. The first and second bone segments may be adjacent vertebrae. 
     The securing element may be configured to expand and retract during translation. The ridges may be configured to provide progressive resistance. 
     The first carriage element may have at least one fastener hole for receiving a bone fastener. The first plate may have a body section having a longitudinal axis, and the first arm may extend substantially transverse to the longitudinal axis of the body section. The fixation system may further comprise a second plate. The first plate and first carriage element may be curved to accommodate a desired body site. 
     A method for fixating a plurality of bone segments is also described, comprising the steps of: (a) providing a fixation system comprising: first and second carriage elements capable of receiving one or more bone fasteners, and each having a channel with a least one securing element disposed therein; a first plate having arms insertable in a channel; wherein each arm has a first engaging portion, the first engaging portion having a series of ridges configured to engage a securing element; (b) positioning the assembly adjacent to a desired body site; (c) attaching the first carriage element to a first bone segment with at least one bone fastener, and the second carriage element to a second bone segment with at least one fastener; and (d) allowing the system to translate in situ. 
     The method may further comprise the step of manually compressing the fixation system. The system may be configured to axially translate in only one direction. The system may be configured to translate incrementally. 
     A fixation system kit is also described comprising a plurality of carriage elements capable of receiving one or more bone fasteners, each having a first channel with a least one securing element disposed therein; at least one plate having arms insertable in a channel; wherein each arm has a first engaging portion, the first engaging portion having a series of ridges configured to engage a securing element; and wherein at least two of the carriage elements are of a substantially different size. The kit may further comprise at least one bone fastener. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred features of the present invention are disclosed in the accompanying drawings, wherein similar reference characters denote similar elements throughout the several views, and wherein: 
         FIG. 1  is a perspective view of an embodiment of the fixation system of the present invention comprising a main plate and two carriage elements; 
         FIG. 2  is a top view of the spinal fixation system of  FIG. 1 ; 
         FIG. 3  is a cross-sectional view through line A-A of  FIG. 2 ; 
         FIG. 4  is a top view of a carriage element of the fixation system of  FIG. 1 ; 
         FIG. 5  is a front view of the carriage element shown in  FIG. 4 ; 
         FIG. 6  is a top view of the plate of the fixation system of  FIG. 1 ; 
         FIG. 7  is a front view of the plate of  FIG. 5 ; 
         FIG. 8  is a side view of the plate of  FIG. 5 ; 
         FIG. 9A  is an enlarged view of an engaging portion of  FIG. 5 ; 
         FIG. 9B  is an enlarged view of the engaging portion of  FIG. 9 ; 
         FIG. 10  is a top view of an engaging clip configured to assist in the unidirectional translational motion between a carriage element and the main plate; 
         FIG. 11  is a cross-sectional view of the engaging clip of  FIG. 10  taken along the line C-C; 
         FIG. 12  is a side view of the engaging clip of  FIG. 10 ; 
         FIG. 13  is a partial perspective view of an engaging clip shown in phantom lines as installed within a carriage element; 
         FIG. 14  is partial perspective view of an engaging clip as installed within a carriage element and engaging a main plate; 
         FIGS. 15A-15C  are enlarged views of the progression of an engaging clip translating along an engaging portion of a carriage element; and 
         FIG. 16  is a perspective view of an alternative embodiment of the fixation system of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The system described herein may be used in spinal fusion procedures in which a damaged or diseased disc (or part of a disc) is removed from between a pair of vertebrae and a spinal fusion spacer is placed between the vertebrae. The carriage elements may be applied to an anterior portion of the affected vertebrae to span the affected disc space, and may be fixed to the vertebrae using bone fasteners. The system may function to maintain the vertebrae aligned during the initial period following fixation in which fusion of the spacer to the adjacent vertebrae occurs. The system may also function to share some of the axial spinal load applied to the fusion spacer to prevent extreme subsidence of the spacer into the vertebral body, such as where the patient has poor bone quality. The system may also act to prevent the spacer from being expelled from the disc space during the initial post-operative period. 
     The system may be used for single level (i.e. one-disc) or multiple-level (i.e. multiple disc) fusion procedures. Some embodiments may be used for corpectomy procedures, in which at least a portion of a vertebral body is removed. Single level systems generally may have two pairs of bone fastener holes, while the multi-level systems generally may have three or more pairs of fastener holes. 
       FIG. 1  illustrates a perspective view of one embodiment of a fixation system  10  of the present invention comprising a main plate  30  and two carriage elements  12 . The fixation system  10  is shown in top view in  FIG. 2 , and in side view in  FIG. 3 . Fixation system  10  may have a longitudinal axis A-A, a length “L,” and may comprise at least one carriage element  12 , discussed in more detail in relation to  FIGS. 4-5 , infra. Carriage elements  12  may have at least one fastener hole  14  for receiving at least a portion of a bone fastener (not shown). Fixation system  10  may also comprise of a plate  30 , discussed in more detail in relation to  FIGS. 6-9B , infra. Plate  30  may have a body portion  32  and at least one arm  34  extending from the body portion  32 . Arms  34  may have an engaging portion  36  with ridges  37  for engaging a carriage element  12 . 
     Referring to  FIGS. 4-5 , each carriage element  12  may comprise at least one fastener hole  14  for receiving a bone fastener, which may allow the carriage element to be affixed to a bone segment. In the embodiment shown in  FIGS. 4-5 , each carriage element  12  comprises two fastener holes  14 , the bone fasteners being omitted from the illustration for purposes of clarity. However, it is expressly contemplated that a carriage element  12  may have one, three, or more fastener holes  14  as needed or desired. Fastener holes  14  may also be fitted with captive clips  15  to prevent bone fastener back-out, the details, materials, and methods of which are described in U.S. patent application Ser. No. 10/653,164 entitled “Bone Plate with Captive Clips”, by Duong, et al., filed Sep. 3, 2003, the entire disclosure of which application is expressly incorporated by reference herein. 
     Carriage elements  12  may further comprise at least one channel  16  for receiving at least a portion of plate  30 , the engagement of which is discussed in more detail below. Channel  16  may have openings  17 A,  17 B, and extend at least a portion of the way through the body of a carriage element  12 . Channel  16  may also have a cutaway portion  20  extending between openings  17 A,  17 B. Cutaway portion  20  may have a cutaway height H c . Each opening  17 A,  17 B may be generally circularly shaped to receive a similarly shaped arm  34  of plate  30 . Each opening  17 A,  17 B may also have an edge  21 A,  21 B. Channel  16  may also have an inner surface  18  and an outer surface  19  of the carriage element  12  disposed opposite the inner surface  18 . In the embodiments shown herein, each carriage element  12  has two channels  16 , with each channel  16  having two openings  17 . However, it is expressly contemplated that a carriage element may have one, three or more channels  16  with an varying number of openings as well. Such variations will be appreciated by those skilled in the art. 
     Carriage element  12  may also have an upper surface  13 A and a lower surface  13 B. One or both surfaces  13 A,  13 B may be generally curved along the length of carriage element  12 , as may be seen more clearly in  FIG. 5 . Such curvature may be advantageous to allow a carriage element  12  to more closely conform to a desired body site or bone segment. 
     An embodiment of a plate  30  is illustrated in  FIGS. 6-9B . A top view of plat  30  is shown in  FIG. 6 . A front view of plate  30  is shown in  FIG. 7 . A side view of plate  30  is shown in  FIG. 8 . Enlarged views of the engaging portion  36  of plate  30  are shown in  FIGS. 9A-9B . Main plate  30  may include a body portion  32  and at least one arm  34 , and a longitudinal axis B-B. Arms  34  may extend from body portion  32  transversely away from longitudinal axis B-B, and may be sized and dimensioned to be at least partially received within channels  16  of carriage elements  12 . Arms  34  may also have ends  35 . 
     Arms  34  may further have an engaging portion  36 , which may contain ridges  37 . As seen in more detail in  FIGS. 9A and 9B , each ridge  37  may include a forward surface  39 , a rear surface  38 , with an apex  33  disposed therebetween. The forward  39  and rear surfaces  38  may be arranged in an engaging portion  36  such that the surfaces  38 ,  39  alternate along the length of an engaging portion  36 . The resulting engaging portion may therefore have consecutive forward surfaces  39  from one perspective, and consecutive rear surfaces  38  from the opposite perspective. 
     Ridges  37  within an engaging portion  36  may be substantially the same size, or may be of varying sizes. The size of an engaging portion  36  may vary on different arms  34  of a single plate  30  and/or a single fixation system  10 . An arm  34  may have more than one engaging portion  36 . Each apex  33  may be in the shape of a point, or may be substantially rounded. For arms  34  of a generally circular cross-section, the maximum cross-sectional diameter of each such arm  34  may be at an apex  33 . 
     An embodiment of a engaging clip  40  is shown in  FIGS. 10-12 . A top view is seen in  FIG. 10 , a cross-sectional view in  FIG. 11 , and a side view in  FIG. 12 . Generally, each carriage element  12  may be provided with at least one engaging clip  40  for affecting secure engagement between arms  34  of plate  30  and channels  16  of carriage element  12 . In the embodiment illustrated in  FIGS. 10-12 , engaging clip  40  comprises two arcuate prongs  42 ,  44  having ends  43 ,  45  respectively, with a distance “D” between ends  43 ,  45 . Distance D may be substantially the same as cutaway height H c  of cutaway  20  of carriage element  12 . 
       FIG. 11  shows a cross-sectional view of the engaging clip  40  of  FIG. 10 , taken along the line C-C. As seen in this view, prongs  42  and  44  may have a polygonal cross-sectional shape having four surfaces  47 ,  48 ,  49 ,  50 . As described in more detail below, inner surface  47  may engage forward surfaces  39  of ridges  37 , and side surface  49  may engage rear surfaces  38  of ridges  37 . Moreover, the dimensions of inner surface  47  and side surface  49  may be substantially equivalent to the dimensions of forward surfaces  39  and rear surfaces  38 , respectively. 
     Engaging clip  40  may also have a midpoint MP. Engaging clip  40  may have an inner radius R l  and an outer radius R o . Inner radius R l  may be defined by the distance from the midpoint MP to the edge  60  between inner surface  47  and side surface  49 . Outer radius R o  may be defined by the distance from midpoint MP to the edge  61  between outer surface  48  and side surface  50 . 
     The engagement of engaging clip  40  to carriage element  12  is shown in  FIGS. 13-14 , which are partial, enlarged perspective views. As seen in  FIG. 13 , with carriage element  12  drawn in line form for clarity, an engaging clip  40  may engage carriage element  12  such that the engaging clip  40  is disposed along the inner surface  18  of a channel  16 . Channel  16  may have a groove  51  and notch (not shown) disposed along the inner surface  18  for receiving at least a portion of an engaging clip  40 . A portion of prongs  42 ,  44  may be inserted into the groove  51  and retaining element  41  may be inserted into the notch. The result may preferably be that engaging clip  40  is fixedly seated within carriage element  12 , and retaining element  41  is inserted into notch to prevent rotation of the engaging clip  40  relative to the carriage element  12 . The engagement of engaging clip  40  to carriage element  12  is also shown in  FIG. 14 , with a portion of the carriage element  12  for clarity. 
     Generally, at least one carriage element  12  engages a plate  30 , specifically at the engaging portion(s)  36  of plate  30 . Engaging clip  40  may engage ridges  37  of an engaging portion  36 . The ridges  37  may be arranged, and the engaging clip  40  may be dimensioned and sized, such that the clip  40  may only engage sequential ridges  37  in a single direction. The effect of such a relationship is that a carriage element  12  may be allowed to translate unidirectionally relative to a plate  30 . By way of example, the carriage element  12  and plate  30  shown in  FIG. 14  are engaged in such a way that engaging clip  40  may only engage subsequent ridges  37  to the left of where the clip  40  currently sits. As stated above, this unidirectional relationship may be achieved by sizing the surfaces  38 ,  39  of ridges  37  and clip prongs  42 ,  44  of engaging clip  40  so that once an engaging clip  40  translates from a first ridge  37 A to a second ridge  37 B, the engaging clip may not re-engage the first ridge  37 A—the clip  40  may only translate in the other direction via prongs  42 ,  44 . Ridges  37  may also be arranged in a “progressive resistance” format, the details and advantages of which, in addition to details of unidirectional translatable fixation, are described in U.S. patent application Ser. No. 11/001,902 entitled “Unidirectional Translation System for Bone Fixation”, by Barrall et al., filed Dec. 1, 2004, the entire disclosure of which application is expressly incorporated by reference herein. 
       FIGS. 15A-15C  show the progression of an engaging clip  40  translating from a first ridge  37 A to a second ridge  37 B. As shown in  FIG. 15A , engaging clip  40  engages ridge  37 A, with side surface  49  of clip  40  adjacent rear surface  38 A, and inner surface  47  of clip  40  adjacent forward surface  39 B. Upon a force “F” acting in the direction as shown, clip  40  may expand and translate in the direction of second ridge  37 B. More specifically, the expansion of clip  40  (which may be made of a resilient material, discussed below) may occur in the flexibility of prongs  42 ,  44  (see  FIG. 10 ), wherein the ends  43 ,  45  may be pushed outwards and away from midpoint MP. The expansion of clip  40  is shown in  FIG. 15B , wherein clip  40 , upon the impetus of force F, is translating away from apex  33 A and towards ridge  37 C. Inner surface  47  may slidingly engage a forward surface  39  during this step. If clip  40  is urged far enough such that the edge  60  moves past apex  33 B along forward surface  39 B, clip  40  may then snap into engagement with second ridge  37 B, as shown in  FIG. 15C . It is important to note, however, that if force F is sufficient to move clip  40  in the direction of second ridge  37 B, but insufficient to urge edge  60  past apex  33 B, then clip  40  may subsequently compress due to the resiliency of the material of clip  40 , and return to the position shown in  FIG. 15A . It is also important to note that the clip  40  as shown in  FIG. 15C  may not return to ridge  37 A absent surgeon intervention. Once clip  40  has moved to the location shown in  FIG. 15C , it may not translate back to its positions in  FIGS. 15A-15B  without surgeon intervention. 
     In use with spinal applications involving adjacent vertebrae at a body site, a surgeon may first remove at least a portion of a intervertebral disc. A spacer or other material may be inserted between the vertebrae. The surgeon then may place the system  10  adjacent to the body site, such that a first carriage element  12 A engages a first vertebrae and a second carriage element  12 B engages a second vertebrae. At any time before or after the step of placing the system  10  adjacent to a body site, a surgeon may manually compress or expand the system  10  to decrease or increase its overall length. Once the system  10  is in position, the surgeon may fixedly attach the carriage elements  12 A,  12 B to their respective vertebrae by inserted bone fasteners (not shown) through fastener holes  14  and into the vertebrae. The surgeon may also manually compress or expand the system  10  intra-operatively at this point as well. The surgeon may then close the incision site. 
     Post-operatively, a carriage element  12  may translate relative to a plate  30  in response to forces applied on the system  10  in situ. For example, when using intervertebral material that resorps or otherwise reduces in size or strength after implantation into the body, vertebrae may tend to shift and/or drift closer to one another. The system  10  described herein is capable of responding to such forces by allowing carriage elements  12  to axially translate by allowing engaging clip  40  to engage subsequent ridges on a plate  30 . The result may be that the system  10  is capable of compressing to a sufficient degree to respond to in situ, post-operative forces, and/or to maintain sufficient compression on intervertebral material to prevent expulsion and promote bone growth and/or fusion between vertebrae. 
     It is expressly contemplated that system  10  may be used in series or in combination with other devices. It is further contemplated that system  10  may be used with two, three or more vertebrae, either consecutive or non-consecutively. Other variations on the system, such as the relative size of the plate, carriage elements, and other components, will be appreciated by those skilled in the art. 
     While the previously-described method and advantages are described in reference to spinal application, it is also expressly contemplated that the system  10  described herein may be used with other parts of the body, such as joints, and long bone fractures. Further surgical uses will be appreciated by those skilled in the art. 
     It will be appreciated that engaging clip  40  may be constructed of a material that will allow the prongs  42 ,  44  to be separated in response to an applied force, but will also be resilient so as to return to their original configuration when the force is removed. A an example of a suitable material for engaging clip  40  is elgiloy. Captive clips  15  may be formed of the same material. 
     Each of the fasteners, plates, carriage elements, and other components disclosed herein may be formed of a titanium alloy such as titanium-aluminum-niobium, which may be anodized. One material for use with each of the plates and fasteners described herein is Ti-6Al-7Nb, with a density of about 4.52 gm/cc, a modulus of elasticity of about 105 GPa, an ultimate tensile strength of about 900 MPa, and a yield strength of about 800 MPa. Surfaces of the fasteners may also be burr free, with all sharp edges broken to a maximum of 0.1 mm. 
       FIG. 16  is a perspective view of an alternative embodiment  110  of a fixation system having longitudinal axis D-D. Carriage elements  112  may be substantially similar to carriage elements  12 , as previously described in relation to  FIGS. 1-15C . In this embodiment, plate  130  may include axially extending arms  134  having engaging portion  136  substantially similar to engaging portions  36  of arms  34 . An engaging clip  40  may also be disposed within the channels  116  of the carriage elements  112 . A significant difference between the embodiment shown in  FIG. 16  and the previous embodiments is that body portion  132 , rather than being in the form of an I-shaped bar, is in the form of an elongated plate configured to conform to a body surface. Body portion  132  may be provided with at least one window  150 , with three such windows  150  shown in the embodiment of  FIG. 16 . Each window  150  may be provided with an intermediate carriage block  160  that can slide longitudinally within window  150 . Each carriage block  160  may have a fastener hole  162 , with a captive clip  164  disposed therein for engaging at least a portion of a bone fastener (not shown). Contacting surfaces between the window  150  and the carriage block  160  are dovetailed to prevent the carriage  160  from coming loose once it is assembled in window  150 . The embodiment shown in  FIG. 16  may achieve the translatable fixation of more than two consecutive vertebrae along a spinal column. Further details, materials, and methods of intermediate carriage element, and multi-level fixation, are described in U.S. patent application Ser. No. 10/932,392 entitled “Track-Plate Carriage System”, by Suh et al., filed Sep. 2, 2004, the entire disclosure of which application is expressly incorporated by reference herein. 
     It should be noted that the aforementioned descriptions and illustrations have been provided as examples of the configurations of translation plates that may be designed and assembled using the principles of the invention. These examples will be understood to one of ordinary skill in the art as being non-limiting in that a fixation system employing one or more of the disclosed features may be produced as desired or required for a particular patient&#39;s need. Thus, the features disclosed are “modular” in nature. 
     This written description sets forth the best mode of the claimed invention, and describes the claimed invention to enable a person of ordinary skill in the art to make and use it, by presenting examples of the elements recited in the claims. The patentable scope of the invention is defined by the claims themselves, and may include other examples that occur to those skilled in the art. Such other examples, which may be available either before or after the application filing date, are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims. 
     While the invention has been shown and described herein with reference to particular embodiments, it is to be understood that the various additions, substitutions, or modifications of form, structure, arrangement, proportions, materials, and components and otherwise, used in the practice and which are particularly adapted to specific environments and operative requirements, may be made to the described embodiments without departing from the spirit and scope of the present invention. Accordingly, it should be understood that the embodiments disclosed herein are merely illustrative of the principles of the invention. Various other modifications may be made by those skilled in the art which will embody the principles of the invention and fall within the spirit and scope hereof.