Patent Publication Number: US-8992577-B2

Title: Rod system for gradual dynamic spinal fixation

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 13/376,502, filed Dec. 6, 2011, which is the U.S. National Phase entry under 35 U.S.C. §371 of International Application PCT/CN2010/076195, filed Aug. 20, 2010, entitled “ROD SYSTEM FOR GRADUAL DYNAMIC SPINAL FIXATION,” the entirety of which is incorporated herein. 
    
    
     BACKGROUND 
     Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section. 
     During the 1960&#39;s, posterior pedicle fixation for vertebral body stabilization was commonly performed utilizing pedicle screws, hooks, and rods. This kind of rigid load bearing stabilization offered physicians the ability to address various patient morphologies. However, clinical experience suggested that rigid fixation does not comply with the dynamic nature of the spine and were leading to complications, including implant failure and accelerated degeneration of the adjacent levels. Subsequently dynamic semi-rigid fixation systems were introduced, such as those available from SCIENT&#39;X of Bretonneux, France. Nonetheless, these systems do not offer the rigid support needed immediately post-operation for vertebral body fracture. 
     SUMMARY 
     In one or more embodiments of the present disclosure, a rod system includes two rods each with a head and a shaft, two elastic members each fitted around one shaft against one head, and a case defining a first inner portion, a second inner portion, and a third inner portion in communication with the first and the second inner portions. A first rod with a first elastic member is seated in the first inner portion so the first elastic member abuts the bottom of first inner portion and the first head protrudes into the third inner portion. A second rod and a second elastic member is seated in the second portion so the second elastic member abuts the bottom of the second inner portion and the second head protrudes into the third inner portion and abuts the first head. 
     The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the drawings and the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an illustrative bone fixation system attached to vertebrae; 
         FIG. 2  is a perspective view of an illustrative dynamic rod system of the bone fixation system in  FIG. 1 ; 
         FIG. 3  is a perspective cross-sectional view of an illustrative rod system of  FIG. 2 ; 
         FIG. 4  is a partial side cross-sectional view of an illustrative rod system of  FIG. 2 ; 
         FIG. 5  is an exploded view of an illustrative rod system of  FIG. 2 ; and 
         FIG. 6  is a front view of an illustrative translation and rotation of the rod system of  FIG. 2 , all arranged in accordance with at least some embodiments described herein. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented here. It will be readily understood that the aspects of the present disclosure, as generally described herein, and illustrated in the figures, can be arranged, substituted, combined, and designed in a wide variety of different configurations, all of which are explicitly contemplated and make part of this disclosure. 
     This disclosure is drawn, inter alia, to manufacturing methods, apparatus, systems, and techniques related to a dynamic rod system for a bone fixation system. 
     Bone screws are used in spinal instrumentation to manage bone fractures and correct deformity. For example, pedicle screws may provide a means of gripping a spinal segment, where a screw acts as a firm anchor point in one vertebra that can be connected to other such anchor points in other vertebrae with a rod. With two or more consecutive vertebrae fixated by such a construct, motion between the vertebrae is prevented or limited. 
     In one or more embodiments of the present disclosure, a rod system includes two rods held against each other by elastic members within a cylindrical case. Each rod is initially immobilized by a corresponding stopper made of an absorbable material so the rod system is rigid. The two rods are connected to two bone screws attached to two vertebrae. As the absorbable material is decomposed by bodily fluid introduced through holes in the cylindrical case, the corresponding rod gains mobility and the rod system becomes dynamic. 
       FIG. 1  is a perspective view of an illustrative bone fixation system  100  attached to vertebrae in one or more embodiments of the present disclosure. The bone fixation system  100  includes a first bone screw  102  attached on a first bone  104 , a second bone screw  106  attached on a second bone  108 , and a rod system  110  connected at two ends to the bone screws  102  and  106 . The rod system  110  may initially form a rigid link between the bone screws  102  and  106  and then transition over time to a dynamic link between the bone screws  102  and  106 . The bone screws  102  and  106  may be pedicle screws, and bones  104  and  108  may be vertebrae. The configuration is mirrored on both sides of the vertebrae. 
       FIG. 2  is a perspective view of an illustrative rod system  110  of the bone fixation system  100  in  FIG. 1 . The rod system  110  includes a first rod  202  and a second rod  204  extending from two ends of a cylindrical case  206 . The first rod  202  is connected to the first bone screw  102  of  FIG. 1  and the second rod  204  is connected to the second bone screw  106  of  FIG. 1 . The cylindrical case  206  defines two sets of through holes  208  around the circumference for introducing bodily fluid into the cylindrical case  206 . 
     The details of an illustrative rod system  110  are now explained in reference to  FIGS. 3 ,  4 , and  5 , which show perspective cross-sectional view, a partial side cross-sectional view, and an exploded view of the rod system  110  in one or more embodiments of the present disclosure. The bone fixation system  100  includes the first rod  202 , the second rod  204 , first elastic members  302 , second elastic members  304 , a first retainer ring  306 , a second retainer ring  308 , a third retainer ring  310 , a fourth retainer ring  312 , a first stopper ring  314 , a second stopper ring  316 , a third stopper ring  307 , a fourth stopper ring  311 , and the cylindrical case  206 . The first rod  202 , the second rod  204 , the first retainer ring  306 , the second retainer ring  308 , the third retainer ring  310 , the fourth retainer ring  312 , the third stopper ring  307 , the fourth stopper ring  311 , and the cylindrical case  206  may be made from metal that resist biological corrosions, such as titanium alloy and stainless steel. 
     Referring to  FIGS. 4 and 5 , the cylindrical case  206  includes two symmetrical halves  401  each defining a first counterbore  402  at one end for receiving the first rod  202 , a second counterbore  404  at the other end for receiving the second rod  204 , and a mid-bore  406  in communication with the counterbores  402  and  404 . The first counterbore  402  includes an inner circumferential cutout  408  for receiving the first retainer ring  306 , and an outer circumferential cutout  410  for receiving the second retainer ring  308 . Similarly, the second counterbore  404  includes an inner circumferential cutout  412  for receiving the third retainer ring  310 , and an outer circumferential cutout  414  for receiving the fourth retainer ring  312 . 
     Referring generally to  FIGS. 3 to 5 , the first rod  202  includes a hemispheroid head  318 , a neck  320  extending from the head  318 , and a shaft  322  extending from the neck  320 . The neck  320  has a smaller radius than the head  318 , and the shaft  322  has a smaller radius than the neck  320 . 
     In an embodiment, the first elastic members  302  and the second elastic members  304  may each include one or more elastic washers. The first elastic members  302  have an outer radius substantially the same as the radius of the first counterbore  402 , and defines a hole having a radius substantially equal to the radius of the neck  320 . 
     The third stopper ring  307  has an outer radius substantially the same as the radius of the first counterbore  402 , and an inner radius substantially equal to the radius of the neck  320 . The third stopper ring  307  is configured to be a stopping base of the first elastic members  302  and to limit the motion of the second rod  204 . 
     The first retainer ring  306  has an outer radius substantially the same as the radius of the inner circumferential cutout  408 , and an inner radius larger than the radius of the neck  320 . 
     The first stopper ring  314  has an inner ring portion and an outer ring portion. In an embodiment, the inner ring portion has an outer radius substantially equal to the inner radius of the first retainer ring  306 , and an inner radius substantially equal to the radius of the neck  320 . The outer ring portion has an outer radius substantially equal to the radius of the first counterbore  402 , and an inner radius substantially equal to the radius of the shaft  322 . The first stopper ring  314  can be made of an absorbable material that is decomposed by bodily fluid, such as calcium phosphate and calcium sulphate. 
     The second retainer ring  308  has an outer radius substantially the same as the radius of the outer circumferential cutout  410 , and an inner radius larger than the radius of the shaft  322 . 
     The first rod  202  is inserted through the first elastic members  302 , the third stopper ring  307 , the first retainer ring  306 , the first stopper ring  314 , and the second retainer ring  308 . The first elastic members  302  abut against the bottom of the head  318 . The first retainer ring  306  fits around the inner ring portion of the first stopper ring  314 , and both abut against the bottom of the first elastic members  302 . The second retainer ring  308  abuts the bottom of the first stopper ring  314 . The assembly is placed in one half  401  of the cylindrical case  206  so the head  318  protrudes into the mid-bore  406 . The first elastic members  302  abut the bottom of the first counterbore  402 . The first retainer ring  306  fits in the inner circumferential cutout  408 , and the second retainer ring  308  fits in the outer circumferential cutout  410 . Note that one set of through holes  208  are defined in the cylindrical case  206  around the first stopper ring  314 . 
     Referring to  FIGS. 3 to 5 , the second rod  204  includes a hemispheroid head  324 , a neck  326  extending from the head  324 , and a shaft  328  extending from the neck  326 . The neck  326  has a smaller radius than the head  324 , and the shaft  328  has a smaller radius than the neck  326 . 
     The second elastic members  304  have an outer radius substantially the same as the radius of the second counterbore  404 , and defines a hole having a radius substantially equal to the radius of the neck  326 . 
     The fourth stopper ring  311  has an outer radius substantially the same as the radius of the second counterbore  404 , and an inner radius substantially equal to the radius of the neck  326 . The fourth stopper ring  311  is configured to be a stopping base of the second elastic members  304  and to limit the motion of the first rod  202 . 
     The third retainer ring  310  has an outer radius substantially the same as the radius of the inner circumferential cutout  412 , and an inner radius larger than the radius of the neck  326 . 
     The second stopper ring  316  has an inner ring portion and an outer ring portion. In an embodiment, the inner ring portion has an outer radius substantially equal to the inner radius of the third retainer ring  310 , and an inner radius substantially equal to the radius of the neck  326 . The outer ring portion has an outer radius substantially equal to the radius of the second counterbore  404 , and an inner radius substantially equal to the radius of the shaft  328 . The second stopper ring  316  is made of an absorbable material that is decomposed by bodily fluid, such as calcium phosphate and calcium sulphate. 
     The fourth retainer ring  312  has an outer radius substantially the same as the radius of the outer circumferential cutout  414 , and an inner radius larger than the radius of the shaft  328 . 
     The second rod  204  is inserted through the second elastic members  304 , the fourth stopper ring  311 , the third retainer ring  310 , the second stopper ring  316 , and the fourth retainer ring  312 . The second elastic members  304  abut against the bottom of the head  324 . The third retainer ring  310  fits around the inner ring portion of the second stopper ring  316 , and both abut against the bottom of the second elastic members  304 . The fourth retainer ring  312  abuts the bottom of the second stopper ring  316 . This assembly is placed in one half  401  of the cylindrical case  206  so the head  324  protrudes into the mid-bore  406  and abuts the head  318 , the second elastic members  304  abut the bottom of the second counterbore  404 , the third retainer ring  310  fits in the inner circumferential cutout  412 , and the fourth retainer ring  312  fits in the outer circumferential cutout  414 . Note that another set of through holes  208  are defined in the cylindrical case  206  around the second stopper ring  316 . The two halves  401  are then fixed by welding or other methods. 
     In an initial example configuration, the first stopper ring  314  is fixed by the first retainer ring  306  and the second retainer ring  308 . In turn, the first stopper ring  314  prevents the first rod  202  from translating outward and rotating. Similarly, the second stopper ring  316  is fixed by the third retainer ring  310  and the fourth retainer ring  312 . In turn, the second stopper ring  316  prevents the second rod  204  from translating outward and rotating. As the first rod  202  and the second rod  204  abut each other, they prevent each other from translating inward as well. 
     In an example operation, over time, bodily fluid enters through holes  208  and decomposes the first stopper ring  314  and the second stopper ring  316 . With the first stopper ring  314  removed, the first rod is able to translate outward and rotate against the first elastic members  302 . Similarly, with the second stopper ring  316  removed, the second rod  204  is able to translate outward and rotate against the second elastic members  304 . Note that if rigid fixation is not desired, rod system  110  may be constructed without the stopper rings  314  and  316 . 
     The two rods can translate outward independently of each other. One rod can also translate inward and push the other rod outward. The two rods can always rotate independently as they contact through their hemispheroid head  318  and  324 . Together they allow the rod system  110  to provide six degrees-of-freedom. 
     In an embodiment, the resilience of elastic members  302  and  304  is selected to provide predetermined translation of the first rod  202  and the second rod  204 . In one or more embodiments of the present disclosure, each rod may have an axial translation of about ±0.1 to 0.3 mm, such as ±0.2 mm, along each rod axis. The inner radii of retainer rings  308  and  312  are selected to larger than the radii of the shafts  322  and  328  provide a predetermined limit for frontal and the sagittal rotations of the first rod  202  and the second rod  204 . In one or more embodiments of the present disclosure, each rod may have frontal and sagittal rotations of about ±1 to 3°, such as ±2°.  FIG. 6  shows a front view of an illustrative translation and rotation of the rod system  110  of  FIG. 2 . 
     One skilled in the art will appreciate that, for this and other processes and methods disclosed herein, the functions performed in the processes and methods may be implemented in differing order. Furthermore, the outlined steps and operations are only provided as examples, and some of the steps and operations may be optional, combined into fewer steps and operations, or expanded into additional steps and operations without detracting from the essence of the disclosed embodiments. 
     With respect to the use of substantially any plural and/or singular terms herein, those having skill in the art can translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity. 
     It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims (e.g., bodies of the appended claims) are generally intended as “open” terms (e.g., the term “including” should be interpreted as “including but not limited to,” the term “having” should be interpreted as “having at least,” the term “includes” should be interpreted as “includes but is not limited to,” etc.). It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases “at least one” and “one or more” to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim recitation to inventions containing only one such recitation, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an” (e.g., “a” and/or “an” should typically be interpreted to mean “at least one” or “one or more”); the same holds true for the use of definite articles used to introduce claim recitations. In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should typically be interpreted to mean at least the recited number (e.g., the bare recitation of “two recitations,” without other modifiers, typically means at least two recitations, or two or more recitations). Furthermore, in those instances where a convention analogous to “at least one of A, B, and C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B, and C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). In those instances where a convention analogous to “at least one of A, B, or C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B, or C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). It will be further understood by those within the art that virtually any disjunctive word and/or phrase presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase “A or B” will be understood to include the possibilities of “A” or “B” or “A and B.” 
     From the foregoing, it will be appreciated that various embodiments of the present disclosure have been described herein for purposes of illustration, and that various modifications may be made without departing from the scope and spirit of the present disclosure. Accordingly, the various embodiments disclosed herein are not intended to be limiting, with the true scope and spirit being indicated by the following claims.