Patent Publication Number: US-11653955-B2

Title: Rod system including at least two rods and connector device for rods

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     The present application claims priority to and the benefit of U.S. Provisional Patent Application No. 63/055,100, filed Jul. 22, 2020, the contents of which are hereby incorporated by reference in their entirety, and claims priority from European Patent Application EP 20 187 183.7, filed Jul. 22, 2020, the contents of which are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     Field 
     The application relates to a rod system including at least two rods and a connector. Further, the application relates to a connector device for connecting at least two rods. The rod system is applicable in the treatment of spinal deformities, but can also be useful in degenerative spinal surgery, for example, in dynamic stabilization or hybrid constructs. 
     Description of Related Art 
     For the treatment of early onset scoliosis of the pediatric spine, use of growing rods is known. Such growing rods may be spinal implants fixed above and below an abnormally curved portion of the spine to correct the curvature in a first step to some extent. Thereafter, the rods are prolonged in further correction surgeries to adapt them to the growth of the spine. 
     Rod systems including at least two rods that are connected to each other and that are used in various applications are known in the prior art. 
     For example, U.S. Pat. No. 9,339,307 B2 describes a bone fixation device adapted to be coupled to bone anchors that allows movement of rods to permit a screw-rod construct to lengthen in response to bone growth, without necessitating post-surgical installation adjustment of the device. 
     Moreover, U.S. Pat. No. 10,610,262 B2, describes a spinal distraction system including a bearing connector fastened to a fixated rod and a sliding rod, wherein the sliding rod includes a spring and a stop ring. 
     US 2006/0233597 describes a connection member for coupling to one or more structural rods, including a coupler body with recesses to receive the rods and a cam screw which is configured to selectively vary a size of the rod receiving recesses to impart a frictional force on the rods. 
     SUMMARY 
     It is an object of the invention to provide a rod system including at least two rods and a connector device for the rods that provide an improved or alternative way of treating various spine disorders, conditions, and/or diseases, in particular spinal deformities, and more particularly deformities in the pediatric or juvenile spine, or in degenerative spinal surgery, in particular in dynamic stabilization or hybrid construct applications. 
     According to an embodiment, a rod system, in particular for the spine, includes a first rod, a second rod, a connector for connecting the first rod and the second rod, the connector including a main body having a first rod seat configured to accommodate a portion of the first rod, the first rod seat having a first longitudinal axis, and a second rod seat configured to accommodate a portion of the second rod, the second rod seat having a second longitudinal axis, wherein the connector is configured to permit the first rod to be fixed to the main body and configured to permit the second rod to be selectively fixable to the main body or slidably connectable to the main body. When the second rod is slidable in the second rod seat, it can move in the axial direction of the second longitudinal axis. Also, rotation of the rod in the rod seat is possible. 
     The rod system may be applied in spine surgery as a correction device and/or a stabilization device. 
     The connector of the rod system has a low profile. This renders it particularly suitable for applications where available space is reduced or limited, such as in pediatric applications. In addition, the connector has a small width, which renders it compact and lightweight. 
     In a further development, the connector also includes one, preferably two, fixation members, one of which is configured to fix the first rod to the connector and the other one is configured to fix the second rod to the connector. Alternatively, the second rod can be kept sliding relative to the connector main body. In a further development, a closure member is provided which can be used instead of the fixation member when the second rod is intended to remain slidable. Thus, the rod system forms a modular system that provides various methods of use. 
     According to an embodiment, a connector device for connecting at least two rods includes a main body having at least a first rod seat configured to accommodate a portion of a first rod, the first rod seat having a first longitudinal axis, a second rod seat configured to accommodate a portion of a second rod, the second rod seat having a second longitudinal axis, a fixation structure configured to fix the first rod to the main body, and an orifice configured to selectively receive one of a fixation member or a closure member to either fix an inserted second rod or to close the orifice while permitting the second rod to slide. 
     Such a connector device has a low profile, a compact shape, and provides a modular system with a plurality of options for use. 
     According to a still further embodiment, a connector device for connecting at least two rods includes a main body having at least a first rod seat configured to accommodate a portion of a first rod, the first rod seat having a first longitudinal axis, a second rod seat configured to accommodate a portion of a second rod, the second rod seat having a second longitudinal axis, an orifice located between the first rod seat and the second rod seat configured to receive a fixation member, the fixation member including a screw with a head, preferably a monolithic screw with a head, wherein the head has a lower side that is configured to act on at least one of the first rod or the second rod to immobilize the first rod and/or the second rod when the first rod and/or the second rod are in the respective first and second rod seats. Preferably the lower side of the head is tapered, for example, conically tapered. 
     The connector device has only a few parts and has a low profile. Also, it is possible to clamp or fix the two rods simultaneously with a single fixation member. Therefore, the width of the connector device can be made small. 
     In a further embodiment, at least one of the first rod seat or the second rod seat, preferably both rod seats, is/are configured to selectively receive pairs of rods having different diameters, and wherein each of the pairs of rods having different diameter is fixed in the rod seat through clamping with the lower side of the fixation member at at least three contact areas. 
     Such a connector device is configured to connect a first pair of rods each having a first diameter or a second pair of rods each having a second diameter different from the first diameter. 
     In a still further embodiment, the fixation member is configured to be tilted in the orifice. Such a connector device is configured to fix two rods having different diameters simultaneously with the same fixation member. 
     As used in the present specification and the appended claims, the term “rod” shall be understood as including any elongate member, regardless of the cross-sectional shape. Specifically, a spinal stabilization rod as used herein, may have, for example, a substantially circular, oval, or angular cross-section. Such cross-section may vary along the length of the rod. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features and advantages of the invention will become apparent from the description of embodiments by means of the accompanying drawings. In the drawings: 
         FIG.  1    shows a perspective exploded view of a rod system according to a first embodiment. 
         FIG.  2    shows a perspective view of the rod system of  FIG.  1    in an assembled state. 
         FIG.  3    shows a top view of the rod system of  FIGS.  1  and  2   . 
         FIG.  4    shows a cross-sectional view of the rod system of  FIGS.  1  to  3   , the cross-section taken along line A-A in  FIG.  3   . 
         FIG.  5    shows a cross-sectional view of the rod system of  FIGS.  1  to  3   , the cross-section taken along line B-B in  FIG.  3   . 
         FIG.  6    shows a cross-sectional view of the rod system of  FIGS.  1  to  3   , the cross-section taken along line C-C in  FIG.  3   . 
         FIG.  7    shows a perspective view from a top of a main body of a connector of the rod system of  FIGS.  1  to  6   . 
         FIG.  8    shows a perspective view from a bottom of the main body of  FIG.  7   . 
         FIG.  9    shows a front view of the main body of  FIGS.  7  and  8    in a direction of a longitudinal axis of one rod seat. 
         FIG.  10    shows a cross-sectional view of the main body of  FIGS.  7  to  9   , the cross-section taken along line D-D in  FIG.  9   . 
         FIG.  11    shows a top view of the main body of  FIGS.  7  to  10   . 
         FIG.  12    shows a cross-sectional view of the main body of  FIGS.  7  to  11   , the cross-section taken along line E-E in  FIG.  11   . 
         FIG.  13    shows a cross-sectional view of the main body of  FIGS.  7  to  11   , the cross-section taken along line F-F in  FIG.  11   . 
         FIG.  14    shows a perspective view of a fixation member of the rod system of  FIGS.  1  to  6   . 
         FIG.  15    shows a side-view of the fixation member of  FIG.  14   . 
         FIG.  16    shows a closure member of the rod system of  FIGS.  1  to  6   . 
         FIG.  17    shows a side-view of the closure member of  FIG.  16   . 
         FIG.  18    shows a perspective view from a top of a sliding member of the rod system of  FIGS.  1  to  6   . 
         FIG.  19    shows a perspective view from a bottom of the sliding member of  FIG.  18   . 
         FIG.  20    shows a top view of the sliding member of  FIGS.  18  and  19   . 
         FIG.  21    shows a cross-sectional view of the sliding member of  FIGS.  18  to  20   , the cross-section taken along line G-G in  FIG.  20   . 
         FIG.  22    shows a perspective exploded view of a rod system according to a second embodiment. 
         FIG.  23    shows a perspective view of the rod system of  FIG.  22    in an assembled state. 
         FIG.  24    shows a perspective view from a top of a main body of a connector of the rod system of  FIGS.  22  and  23   . 
         FIG.  25    shows a perspective view from a bottom of the main body of  FIG.  24   . 
         FIG.  26    shows a front view of the main body of  FIGS.  24  and  25    in a direction along a longitudinal axis of one rod seat. 
         FIG.  27    shows a cross-sectional view of the main body of  FIGS.  24  to  26   , the cross-section taken along line H-H in  FIG.  26   . 
         FIG.  28    shows a top view of the main body of  FIGS.  24  to  27   . 
         FIG.  29    shows a cross-sectional view of the main body of  FIGS.  24  to  28   , the cross-section taken along line I-I in  FIG.  28   . 
         FIG.  30    shows a perspective view of a rod system according to a third embodiment in an assembled state. 
         FIG.  31    shows a perspective view of a rod system including only one rod according to a further embodiment. 
         FIG.  32    shows a cross-sectional view of the rod system of  FIG.  31   , the cross-section taken in a plane perpendicular to a longitudinal axis of the rod and extending through a center of a fixation member. 
         FIG.  33    shows a perspective view of another embodiment of a rod system including only one rod. 
         FIG.  34    shows a cross-sectional view of the rod system of  FIG.  33   , the cross-section taken in a plane perpendicular to a longitudinal axis of the rod and extending through a center of a fixation member. 
         FIG.  35    shows a perspective view of a still further embodiment of a rod system. 
         FIG.  36    shows a cross-sectional view of the rod system of  FIG.  35   , the cross-section taken in a plane perpendicular to longitudinal axes of two inserted rods and extending through a center of a fixation member. 
         FIG.  37    shows a perspective view of a still further embodiment of a rod system in an assembled state. 
         FIG.  38    shows a cross-sectional view of the rod system of  FIG.  37   , the cross-section taken in a plane perpendicular to longitudinal axes of two inserted rods and extending through a center of a fixation member, wherein the rods each have a first diameter. 
         FIG.  39    shows a cross-sectional view of the rod system of  FIG.  37   , with two inserted rods each having a second diameter that is smaller than the first diameter of the rods shown in  FIG.  38   . 
         FIG.  40    shows a perspective view from a top of a main body of a connector of the rod system shown in  FIGS.  37  to  39   . 
         FIG.  41    shows a perspective view from a bottom of the main body of the connector of  FIG.  40   . 
         FIG.  42    shows a top view of the main body of the connector of  FIGS.  40  and  41   . 
         FIG.  43    shows a cross-sectional view of the main body of the connector of  FIGS.  40  to  42   , the cross-section taken along line J-J in  FIG.  42   . 
         FIG.  44    shows a perspective exploded view of a rod system of a still further embodiment. 
         FIG.  45    shows a perspective view of the rod system of  FIG.  44    in an assembled state. 
         FIG.  46    shows a cross-sectional view of the rod system of  FIGS.  44  to  45   , the cross-section taken in a plane perpendicular to longitudinal axes of two inserted rods and extending through a center of a fixation member, wherein the rods have different diameters. 
         FIG.  47    shows a cross-sectional view of the rod system of  FIGS.  44  to  45   , with two rods having a same diameter. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIGS.  1  and  2   , a rod system according to a first embodiment includes a connector including a main body  10  and at least a first rod  100  and a second rod  101  that are intended to be connected by the connector. The rod system further includes at least one, and optionally two fixation members  40 , which may be identical. One of the fixation members  40  is configured to fix the first rod  100  to the main body  10 . The other one of the fixation members  40  may be used to fix the second rod  101  to the main body, if desired. Further optionally, the rod system may include a closure member  50  that is configured to close an orifice in the main body  10  when the second rod  101  is intended to remain slidable. Sliding members  60  may also be part of the rod system that facilitate sliding of the second rod  101  in the main body  10 . 
     The first rod  100  and the second rod  101  may be substantially identical. They may have a circular cross-section and preferably a smooth surface. However, the rods are not limited to such a design. Usually, the first rod  100  and the second rod  101  are configured to be connected to bone anchors (not shown). Such bone anchors could be, for example, pedicle screws that are inserted into the pedicles of vertebrae. 
     The first rod  100  is configured to be connected to the main body  10  in such a manner that the rod is fixed, or in other words immobilized, with respect to the main body  10  by using the fixation member  40 . The second rod  101  can remain movable, specifically slidable, with respect to the main body  10 , for example within the sliding members  60 . The second rod  101  may alternatively be immobilized using a second fixation member  40 . Hence, the second rod  101  may be kept slidable or may be fixed depending on a particular desired application. If second rod  101  remains slidable, the closure member  50  may be used to close the respective orifice. 
     Referring in addition to  FIGS.  3  to  13   , the main body  10  will be described in greater detail. The main body  10  is preferably a monolithic part that has a substantially flat top  11  and a substantially flat bottom  12  that are substantially parallel to each other and that define a height h of the main body  10 . The top  11  and the bottom  12  are connected via, for example, outwardly bulged cylindrical sides  13 ,  14 , the distance between the outermost portions of which define a width w of the main body  10 , as shown in detail in  FIG.  4   . The main body  10  further has a front end  10   a  and a rear end  10   b  in a top view, as shown in  FIG.  3   , a distance between which defines a length I of the main body. By the height h, the width w, and the length I, a height direction, a width direction, and a length direction, respectively, are defined. 
     At a distance from each of the sides  13  and  14 , there are a first rod seat  15  and a second rod seat  16 , respectively, that each extend from the front end  10   a  to the rear end  10   b  completely through the main body  10 . The first rod seat  15  is configured to receive the first rod  100  and the second rod seat  16  is configured to receive the second rod  101 . More specifically, the first rod seat  15  is formed as a cylindrical channel with an inner diameter that substantially matches an outer diameter of the first rod  100  so that the first rod  100  fits therein, for example with friction. The first rod seat  15  defines a first longitudinal axis I 1  which may be located at the center of the main body  10  in the height direction. The second rod seat  16  is also a cylindrical channel defining a second longitudinal axis I 2  which may also be located at the center of the main body in the height direction. A central portion of the second rod seat  16  has an inner diameter that substantially matches an outer diameter of the second rod  101  so that the second rod  101  fits therein. Adjacent to the front end  10   a  and adjacent to the rear end  10   b , the second rod seat  16  has receiving sections  17  with a slightly greater inner diameter than the central portion of the rod seat for receiving the sliding members  60 , respectively. Circumferential inner grooves  18  are formed in the receiving sections at a distance from the front end  10   a  and the rear end  10   b , respectively, appropriate for receiving a corresponding projection of the sliding member  60 . Thereby, it can be ensured that the sliding members  60 , once inserted, are held in the receiving sections  17  and are prevented from falling out of the second the rod seat  16 . 
     The longitudinal axis I 1  of the first rod seat  15  and the longitudinal axis I 2  of the second rod seat  16  are arranged parallel to each other and at the same height with respect to the top  11  and/or the bottom  12 . As can be seen in the figures, the rod seats  15 ,  16  are substantially closed around an inserted rod. Thus, an inserted rod is circumferentially encompassed by the rod seat at least over a certain length except at positions where the fixation member  40  acts onto the rod. 
     Between the first rod seat  15  and the second rod seat  16 , a first orifice  19  and a second orifice  19 ′ are formed that serve for selectively receiving the fixation member  40  or the closure member  50 . The first orifice  19  defines a central axis c 1  as shown in  FIG.  5    that extends perpendicular to the first longitudinal axis I 1  of the first rod seat  15  and perpendicular to the second longitudinal axis I 2  of the second rod seat  16 . The first central axis C 1  of the first orifice  19  is located at a distance from the rear end  10   b  that is smaller than the distance between C 1  and the front end  10   a . Moreover, the first central axis C 1  is closer to the first rod seat  15  than to the second rod seat  16  in the width direction. 
     The first orifice  19  extends completely from the top  11  to the bottom  12 . Adjacent to the bottom  12 , the orifice  19  includes a threaded bore  20  for receiving a shaft of the fixation member  40  and adjacent to the top  11 , the orifice  19  has an enlarged space for receiving a head of the fixation member  40  in a countersink manner. The enlarged space has, adjacent to the threaded bore  20 , a tapered portion  21  that tapers towards the threaded bore  20 , and following the tapered portion  21 , a substantially cylindrical portion  22  which opens, preferably with a small bevel  23 , to the top  11 . The tapered portion  21  and the cylindrical portion  22  intersect the first rod seat  15  at the side facing the first rod seat  15 , such that a cutout  24  is formed that provides an opening between the first orifice  19  and the first rod seat  15 . Referring to  FIGS.  5  and  12    in particular, the cutout  24  is located above the center of the main body  10  in the height direction, so that the fixation member  40  can press from a position above the middle of the first rod  100  when the first rod  100  is inserted into the first rod seat  15 . On the opposite side of the cutout  24  in the width direction, the tapered portion  21  of the orifice  19  is spaced apart from the second rod seat  16 , so that an inserted fixation member does not contact a second rod  101  inserted into the second rod seat  16 . 
     Referring now in greater detail to  FIGS.  3  and  10   , the second orifice  19 ′ extends completely from the top  11  to the bottom  12  of the main body  10  and defines a second central axis C 2 . The second central axis C 2  of the second orifice  19 ′ extends perpendicular to the second longitudinal axis I 2  of the second rod seat  16  and also perpendicular to the first longitudinal axis I 1  of the first rod seat  15 . The second orifice  19 ′ is closer to the front end  10   a  than to the rear end  10   b  of the main body  10 . Moreover, the second orifice  19 ′ is closer to the second rod seat  16  in the width direction than it is to the first orifice. As a result, the central axes C 1 , C 2  of the first and the second orifice  19 ,  19 ′, respectively, are offset in the width direction. Similar to the first orifice  19 , the second orifice  19 ′ includes a threaded bore  20  adjacent to the bottom  12  and an upper portion that has the cylindrical portion  22  adjacent to the top  11  and an intermediate tapered portion  21  sandwiched between them. The second orifice  19 ′ is close to the second rod seat  16 , such that the tapered portion  21  and the cylindrical portion  22  intersect the second rod seat  16 , thereby forming a cutout  24 ′ or opening between the second orifice  19 ′ and the second rod seat  16 , as best seen in  FIGS.  11  and  13   . By means of the cutout  24 ′, a tapered portion  43  of the fixation member  40  is permitted to contact the second rod  101  when the second rod  101  is in the second rod seat  16 . The tapered portion  21  is above the second longitudinal axis I 2  of the second rod seat  16 . Hence, an inserted fixation member  40  is configured to press from an upper side of the second rod  101  on the second rod  101 . At the opposite side of the cutout  24 ′ in the width direction, the second orifice  19 ′ is spaced apart from the first rod seat  15 , so that an inserted fixation member  40  cannot act onto the first rod  100 . 
     Lastly, the front end  10   a  of the main body  10  includes a slanted portion  10   c  that may extend from a distance from the opening formed by the second rod seat  16  in the front end  10   a  to approximately a short distance behind or past the second central axis C 2  of the second orifice  19 ′ in the length direction of the main body  10 , as depicted in  FIG.  11   . By means of the slanted surface  10   c , material can be saved and the connector can be made more compact. 
     The main body has a low profile, since above and beneath the first rod seat  15  and the second rod seat  16 , only a minimum of necessary material is present. Also, because the fixation members are placed into the orifices which are arranged one after the other in the length direction, while the action zones  24 ,  24 ′ of the fixation members  40  with the first rod  100  and the second rod  101  have only a minimum necessary distance in the width direction, the main body is very slim in the width direction. Finally, by means of the slanted portion  10   c  at the front end  10   a , the main body can be constructed more lightweight. 
     Referring now in addition to  FIGS.  14  and  15   , the fixation member  40  will be described. The fixation member  40  is in the form of a screw having a threaded shaft  41  and a head  42 . The threaded shaft  41  is configured to cooperate with the threaded hole  20  of the first orifice  19  and the second orifice  19 ′, respectively. The head  42  has a shape that generally matches the shape of the upper portion of the first orifice  19  and the second orifice  19 ′, respectively. More specifically, the head  42  includes a tapered portion  43  adjacent the threaded shaft  41  that is configured to fit into the tapered portion  21  of the first and second orifice  19 ,  19 ′, respectively, and a cylindrical portion  44  adjacent the tapered portion  43  that is configured to fit into the cylindrical portion  22  of the first and second orifice  19 ,  19 ′, respectively. A beveled portion  45  may be provided that is between the cylindrical portion  44  and a free end surface  42   a  of the head  42 . In the free end surface  42   a , a tool engagement recess  46  may be formed. Various shapes of tool engagement recesses may be conceivable, such as a polygon recess, a torx-shaped recess, any type of longitudinal grooves, or any other engagement structure. A length of the fixation member  40  along its screw axis is such that the head  42  of the fixation member can be fully immersed or countersunk into the upper portion of the first and second orifice  19 ,  19 ′, respectively, and that preferably the threaded shaft  41  does not protrude substantially out of the threaded bore  20 . This further contributes to the low profile of the connector. 
     As shown in  FIG.  5   , when the fixation member is inserted into the first orifice  19 , the tapered portion  43 , which is preferably a conically-tapered portion, is permitted to contact the first rod  100  from an upper side thereof, i.e. from a side above the middle of the rod, when the first rod  100  is in the first rod seat  15 . Tightening the fixation member  40  increases the pressure onto the first rod  100 , so that the first rod  100  is immobilized in the first rod seat  15 . Similarly, when the fixation member  40  is inserted into the second orifice  19 ′, it can be fully immersed or countersunk therein, so that neither the head  42  nor the threaded shaft  41  protrude out of the top  11  or the bottom  12 , respectively. The tapered portion  43  is configured to contact the second rod  101  through the cutout  24 ′. Tightening the fixation member  40  in the threaded bore results in an increasing pressure onto the second rod  101  until the second rod  101  is immobilized or fixed in the second rod seat  16 . 
     Referring now to  FIGS.  16  and  17   , the optionally provided closure member  50  is in the form of a screw having a threaded shank  51  that is configured to cooperate with the threaded bore  20  in the first and second orifice  19 ,  19 ′, respectively, and a head  52 . The head  52  has a length in the axial direction of the screw such that the head can axially fit or be immersed into the upper portion of the first and second orifice  19 ,  19 ′, respectively, as can be seen in particular in  FIG.  6   . The head  52  is tapered between the shaft  51  and substantially the free end surface  52   a , wherein the taper may be steeper than that of the tapered portion  43  of the fixation member  40 . By means of this, the head  52  can be prevented from extending through the cutout  24 ,  24 ′ of the first or second orifice  19 ,  19 ′ and therefore prevented from contacting the first or second rod. A small bevel  53  adjacent the free end surface may also be formed. A length of the closure member  50  is such that the closure member can be immersed in the second orifice  19 ′ without portions extending out of the top  11  or the bottom  12 . In the free end surface  52   a  of the head  52 , a tool engagement recess  54  is cut out that may have any suitable tool engagement shape, such as a polygon, a torx-shape, or any longitudinal grooves or other suitable shape. 
     Referring now in greater detail to  FIGS.  18  to  21   , the sliding members  60  are preferably identical parts. Each sliding member  60  is formed as a sleeve with a first end  60   a , a second end  60   b , and a longitudinal slot  61  extending from the first end to the second end. By means of the slot  61 , the sliding member  60  is compressible such that it can be inserted into the receiving section  17  of the second rod seat  16 . In the compressed configuration, an inner diameter of the sliding member  60  is only slightly greater than an outer diameter of the second rod  101  such that the second rod  101  can slide within the sliding member  60 . An outer base diameter of the sliding member  60  corresponds to an inner diameter of the receiving section  17  when the sliding member is inserted therein. At a distance from the second end  60   b , a circumferential protrusion  62  is provided that fits into the groove  18  of the receiving section. By means of this, the sliding member  60  is axially fixed in the receiving section  17 . Adjacent the first end  60   a , a circumferential protrusion  63  is formed that tapers and narrows towards the first end  60   a . The circumferential protrusion  63  extends out of the second rod seat  16  at the front end  10   a  or the rear end  10   b  of the main body  10  once the sliding member is in the receiving section  17 . Thereby, further insertion of the sliding member into the second rod seat  16  is also prevented. The sliding members  60  are assembled with the main body  10  in such a manner that they are inserted into the second rod seat  16  with their second end  60   b  being the leading end. 
     The main body, the first and second rods, the fixation member or fixation members, and the closure member are made of a bio-compatible material, such as a bio-compatible metal or a metal alloy, or of a bio-compatible plastic material. Suitable materials may be titanium or stainless steel, NiTi alloys, for example Nitinol, or plastics like polyether ether ketone (PEEK) or poly-L-lactide acid (PLLA). The parts may be made of the same or of different materials from one another. 
     The sliding members may be specifically made of a plastic material that facilitates sliding, such as PEEK or polyurethane. The sliding members may also be made of the same material as the other components of the rod system, and can be coated with a coating that facilitates sliding of the second rod therein. 
     In use, the rod system is connected with bone anchors, for example, two bone anchors that are inserted in vertebrae, for example, of different motion segments of the spine such that the rod system bridges the respective motion segments. The bone anchors may be, for example, monoaxial or polyaxial pedicle screws inserted into the pedicles of vertebrae. In greater detail, the first rod  100  may be connected to a first bone anchor at the side of the front end  10   a , and the second rod  101  may be connected to a second bone anchor at the side of the rear end  10   b  of the main body  10 . The first rod  100  may be connected to the main body  10  in a way such that only a small portion of the first rod projects out of the rear end  10   b  of the main body  10 . Once fixed to the bone anchor, the first rod is immobilized relative to the main body  10  by inserting the fixation member  40  into the first hole  19  and tightening the fixation member. Thereby, the slanted portion  43  of the head  42  of the fixation member presses through the cutout  24  onto the first rod  100  to fix the same. 
     It shall be noted, that by means of adjusting the strength of tightening of the fixation member  40 , the frictional force holding the rod can be adjusted. For example, for the purpose of correction steps, a friction hold that can be manually overcome may be sufficient, whereas for final fixation the fixation member can be fully tightened. 
     The second rod  101  may be connected to the second bone anchor while remaining slidable with respect to the main body  10 . Due to the sliding members, the sliding is considerably facilitated. Hence, the second rod  101  allows the distance between the first and the second bone anchor to continuously vary corresponding to the sliding of the second rod with respect to the main body, and therefore, with respect to the fixed first rod  100 . In the second orifice  19 ′, the closure member  50  may be inserted to close the orifice so that the orifice can be maintained substantially free from vessels or tissue growing into it. Alternatively, instead of closing the second orifice  19 ′ with the closure member  50 , another fixation member  40  can be inserted into the second orifice  19 ′ to clamp and fix the second rod  101  at a desired axial position with respect to the main body  10 . When tightening the fixation member  40  , the tapered portion  43  presses through the cutout  24 ′ onto the second rod  101  until the second rod is immobilized. 
     In various examples of use, a surgeon can selectively use either another fixation member  40  to fix the second rod  101  or a closure member  50  to close the second orifice  19 ′. Keeping the second rod  101  slidable, the rod system can be used, for example, as a growing rod system. It shall be noted that alternatively, the second rod  101  can be fixed and the first rod  100  can be kept slidable or, alternatively both rods can be kept slidable. It may be advantageous to provide a stop at the sliding rod to prevent the sliding rod from falling out. Such a stop may be, for example, a clamping ring around the free end of the sliding rod which is close to the main body. 
     Thus, the stabilization system provides a modular system that permits a multitude of applications with sliding and/or fixed rods. 
     Referring now to  FIGS.  22  to  29   , a second embodiment of the rod system will be described. Parts and portions that are the same or similar to those of the first embodiment will be marked with the same reference numerals, and the descriptions thereof will not be repeated. The rod system according to the second embodiment differs from the rod system of the first embodiment mainly in that the first rod is permanently fixed to the main body of the connector using a pin  70 . Functionally corresponding to the first orifice in the first embodiment, a pin hole  27  for receiving the pin extends from the top  11  completely to the bottom  12  and is arranged offset from the middle of the main body  10 ′ in the width direction towards the first rod seat  15 . The pin hole  27  has a size and is positioned at a position such that the pin hole intersects the first rod seat  15 , as shown in particular in  FIG.  27   . In the length direction, the pin hole  27  is formed approximately at the center of the main body  10 ′. A central axis of the pin hole  27  extends perpendicular to the first longitudinal axis I 1  of the first rod seat  15 . 
     The first rod  100 ′ includes a substantially cylindrical recessed portion  100   a  that is at an axial position corresponding to the pin hole when the first rod  100 ′ is inserted into the first rod seat and projects only slightly out of the rear end  10   b . The pin  70  forms a first fixation member that is configured to be inserted into the pin hole  70 , thereby engaging the recessed portion  100   a  of the first rod  100 ′ when the first rod  100 ′ is in the first rod seat  15 . 
     A third orifice  119  is formed in the main body  10 ′ between the second orifice  19 ′ and the rear end  10   b . The third orifice  119  has a central axis that is perpendicular to the longitudinal axes I 1  and I 2  of the first and second rod seats  15 ,  16  and positioned in the width direction at the same position as the central axis C 2  of the second orifice  19 ′. The third orifice  119  is identical in shape to the second orifice  19 ′. Hence, there is a cutout  124  forming an opening between the third orifice  119  and the second rod seat  16 . The pin hole  27  may be arranged in the middle between the second orifice  19 ′ and the third orifice  119  on the side facing towards the first rod seat  15 . 
     In the rod system of the second embodiment, the first rod  100 ′ is fixed to the main body  10 ′ in a specific axial position. The fixation is effected by the pin  70  engaging the recess  100   a  of the first rod  100 ′. As the pin may be press-fit into the pin hole  27 , the axial and rotational fixation of the first rod  100 ′ is permanent during ordinary use of the rod system. Moreover, the rod  100  can also be welded to the main body  10 ′, so that the connection is non releasable. 
     In use, two fixation members  40  can be placed into the second and third orifice  19 ′,  119 , respectively, so that an enhanced fixation can be achieved by using two fixation members. Alternatively, one fixation member  40  and one closure member  50  may be used or two closure members  50  may be used for the second and third orifices  19 ′,  119  to keep the second rod  16  slidable. It shall be noted that the orifices  19 ′,  119  can also remain open, i.e. without inserting a closure member  50 . 
     In a modified embodiment, the first rod  100  and the main body  10 ′ can be a monolithic part. They may be, for example, machined out of one piece of material. In this modified embodiment, the pin is not needed. 
     Referring to  FIG.  30   , a third embodiment of the rod system will be described. Parts and portions that are identical or similar to those of the first or second embodiments are indicated with the same reference numerals, and the descriptions thereof will not be repeated. the first rod  100 ′ is identical to that of the second embodiment, i.e. the first rod has the recess  100   a  for engagement with the pin  70 . The main body  10 ″ lacks the third orifice  119  of the second embodiment. As a result, the main body  10 ″ only has the pin hole  27  for engagement with the pin  70  and one orifice  119  for receiving a fixation member  40  or a closure member  50 . The front end  10   a  lacks the slanted portion  10   c  so that the front end  10   a  is substantially parallel to the rear end  10   b  over the whole width. The first rod  100 ′ is fixed to the main body  10 ″ via the pin  70  which is press-fit into the pin hole  27 . Optionally, the first rod  100 ′ can be welded to the main body  10 ″. The second rod  101  can be fixed via fixation member  40  or can slide in the second rod seat  16 . 
     The rod system according to the third embodiment has an even more reduced size in the length direction. In a modified embodiment, the first rod  100 ′ and the main body  10 ″ can also be a monolithic part, so that the pin is not needed. 
     Referring to  FIGS.  31  and  32   , a still further embodiment of a rod system will be described. The rod system includes a rod  200  which may be identical to the rod  100  or  101  of the previous embodiments, and a connector including a main body  1000  and a fixation member  40 . The main body  1000  is a body with a top  11 ′, a bottom  12 ′, and sides  13 ′,  14 ′, wherein the sides may have a substantially cylindrical outer surface. It shall be noted, however, that the overall shape of the main body can be any shape, for example, cuboid. In the top  11 ′, a single rod seat  150  is formed which is substantially cylinder segment-shaped for a circular rod  200 . The rod seat is open, such that the circular rod can be inserted from the top  11 ′. Laterally offset from the rod seat  150 , an orifice  190  is formed, a central axis c of which extends perpendicular to the longitudinal axis I of the rod seat  150 . The orifice  190  has a threaded hole  191  that is configured to receive the threaded shaft  41  of the fixation member  40 . On the side facing away from the rod seat, the orifice  190  may have a tapered portion  192  for receiving the tapered portion  43  of the fixation member  40 . On the opposite side, the inserted rod is accessible by the head of the fixation member  40 . More specifically, the orifice  190  is at a position such that, when the fixation member  40  is inserted into the orifice  190  and tightened, the tapered portion  43  of the head  42  of the fixation member  40  contacts the rod  200  from above and presses the rod  200  into the rod seat  150 . As a result, the rod  200  is fixed or immobilized only by tightening the fixation member. The rod system includes only a few parts of simple structure. Moreover, the connector has a very low profile. 
     Referring to  FIGS.  33  to  34   , a still further embodiment of a rod system is shown. The rod system differs from the rod system in  FIGS.  31  and  32    mainly in that the main body  1000 ′ includes a rod seat in the form of a sleeve-like portion  150 ′ that may have a sleeve-like prolongation or extension  200  with a smaller outer diameter. The two portions  150 ′ and  200  form a rod extension in which a further rod (not shown) can be inserted. Hence the rod seat is substantially closed around an inserted rod. An outer contour of the main body  1000 ′ in the portion around the orifice  190  may be substantially rectangular, with one side attached to the sleeve-like portion forming the rod seat  150 ′. The upper portion of the orifice  190  intersects the rod seat portion  150 ′ such that a cutout  124 ′ forms an opening that permits the tapered portion  43  of the head  42  of the fixation member  40  to press on an upper portion of an inserted rod. A free end surface in the axial direction of the rod seat portion  150 ′ may be open or closed. In use, tightening the fixation member  40  against an inserted rod clamps and finally fixes the rod in the axial position. 
     Referring to  FIGS.  35  and  36   , a still further embodiment of a rod system includes two rods  100 ,  101  having a same diameter, a main body  1000 ″, and a fixation member  40 . The main body  1000 ″ has a top  11 ″, an opposite bottom  12 ″ which are substantially parallel, two outwardly bulged cylinder segment-shaped sides  13 ″,  14 ″ joining the top  11 ″ and the bottom  12 ″, a front end  1000 ″ a,  and a rear end (not visible in the Figures). In the top  11 ″, two cylinder segment-shaped rod seats  150 ″,  160 ″ are formed having longitudinal axes I 1  and I 2 , respectively, that are substantially parallel to each other. The rod seats  150 ″,  160 ″ are open so that the rods can be inserted from the top  11 ″. In the center of the main body between the rod seat  150 ″,  160 ″, an orifice  190 ″ is formed that has a threaded hole  191 ″. A length of the threaded hole  191 ″ is such that, when the fixation member  40  is inserted with the threaded shaft  41 , the fixation member can be tightened to press with the tapered portion  43  onto an upper side of the rods  100 ,  101  which are inserted into the first and second rod seats  150 ″,  160 ″. With the fixation member  40 , both rods can be fixed simultaneously. The rod system has few parts, a simple structure, and a low profile. 
     Referring to  FIGS.  37  to  43   , a still further embodiment of a rod system will be described. Parts and portions that are identical or similar to the previously described embodiments are marked with the same reference numerals, and the descriptions thereof will not be repeated. The rod system is configured to be used with various sets of rods, wherein the sets differ with respect to the diameter of the rods. The main body  1000 ″ is similar to the main body  1000 ″ of the previous embodiment, however, the rod seats  150 ′″,  160 ″ are configured to receive rods of various diameters. For example, as shown in FIG.  38 , the rod seats  150 ′″,  160 ′″ can receive a pair of rods  100 ,  101  of the same first diameter and can alternatively, as depicted in  FIG.  39   , receive a pair of rods  100 ′,  101 ′ of the same second diameter that is smaller than the first diameter. As depicted in  FIGS.  40  to  43   , the main body  1000 ′″ has a top  11 ′″, an opposite bottom  12 ′″, sides  13 ′″,  14 ′″ which may be cylindrically-shaped and connect the top and the bottom, a front end  1000 ″ a,  and a rear end  1000 ′″ b.  In the middle of the main body between the rod seats  150 ′″ and  160 ′″, an orifice  190 ″ is provided for receiving a fixation member  40 ′. The first rod seat  150 ″ includes a cylindrical portion  150 ′″ a  and a substantially V-shaped portion  150 ′″ b  (seen in cross-section), preferably with a rounded bottom that is connected to the cylindrical portion  150 ′″ a  in a continuous manner. The sidewalls of the V-shaped contour may form an angle between 45° and 135°, preferably around 90°. The cylindrical portion  150 ′″ a  is closer to the center of the main body or closer to the orifice in the width direction, whereas the V-shaped portion  150 ′″ b  is located closer to the side  13 ′″. Likewise, the second rod seat  160 ′″ includes a cylindrical portion  160 ′″ a  arranged closer to the center of the main body in the width direction, and V-shaped portion  160 ′″ b,  preferably with a rounded bottom that is connected to the cylindrical portion  160 ′″ a  in a continuous manner and that is located closer to the side  14 ′″ than the cylindrical portion  160 ′″ a  is to the side  14 ′″. The shape of the cylindrical section and of V-groove section of both rod seats may be identical and mirror symmetrical to a plane extending through the orifice  190 ′″ in the height direction. The orifice  190 ′″ includes a threaded portion  191 ′″ adjacent to the bottom  12 ′″ and a tapered portion  192 ′″ adjacent to the top  11 ′″. In greater detail, the orifice  190 ′″ is configured to receive the fixation member  40 ′ which differs from the fixation member  40  in that the head  42  has an enlarged tapered surface  43 ′ and may have a cylindrical portion  44 ′ with an axially reduced height. 
     In use, when a first pair of rods  100 ,  101  is used that has a first diameter which may correspond to the inner diameter of the cylindrical portions  150 ′″ a  and  160 ′″ a  of the rod seats  150 ′″,  160 ′″, respectively, the rods may each substantially fill the rod seat except a small portion in the ground region of the V-shaped portion. When the fixation member  40 ′ is inserted into the orifice  190 ′″, the fixation member touches with the tapered portion  43 ′ onto the upper side of both rods, while the threaded shaft  41  may not extend fully into the threaded portion  191 ′″, as depicted in  FIG.  38   . 
     When a second pair of rods  100 ′,  101 ′ with a smaller diameter than the first diameter is used, the rods  100 ′,  101 ′ do not fully fill the rod seats in the cylindrical portion  150 ′″ a,    160 ′″ a,  but rest on the flanks of the V-shaped portion  150 ′″ b,    160 ′″ b  along substantially two contact areas that are substantially parallel to the longitudinal axes I 1 , I 2  of the rod seats. Inserting and tightening the fixation member  40 ′ against the rods  100 ′,  101 ′ allows the fixation member  40 ′ to extend deeper into the threaded portion  191 ′″, so that the tapered portion  43 ′ of the head  42 ′ of the fixation member  40 ′ contacts the rods  100 ′,  101 ′ at a position farther outward in the radial direction of the head. The rods  100 ′,  101 ′ are firmly clamped along three contact areas, with two of the contact areas in the V-shaped portion of the rod seat and the third contact area at the tapered portion  43 ′ of the fixation member  40 ′. 
     Lastly, a still further embodiment of the rod system will be described with reference to  FIGS.  44  to  47   . 
     The rod system according to this embodiment differs from the rod system according to the previous embodiment in that a pair of rods with different diameters can be used and fixed simultaneously with a single fixation member. A first rod  100 ″ has a smaller outer diameter compared to a second rod  101 ″. The main body  1000 ″″ includes the first rod seat  150 ′″ and the second rod seat  160 ′″ as in the previous embodiment. The first rod  101 ″ is configured to be received in the first rod seat  150 ′″ and the second rod  101 ″ is configured to be received in the second rod seat  160 ′″. The orifice  190 ″″ between the first rod seat  150 ′″ and the second rod seat  160 ′″ is configured to receive a fixation member  40 ″ and a pivot sleeve  80 . The orifice  190 ″″ has, adjacent to the bottom  12 ′″, a spherical segment-shaped recess  193  which includes a region of the sphere with a greatest diameter, i.e. over the equator of the sphere. The spherical segment-shaped recess  193  is configured to receive the pivot sleeve  80 . The pivot sleeve  80  includes a spherical segment-shaped outer surface  81  and an inner cylindrical threaded passage  82  that is configured to cooperate with the threaded shank  41 ″ of the fixation member  40 ″. The pivot sleeve  80  further has an axial slot  83  that renders the pivot sleeve compressible and expandable. Adjacent to the spherical segment-shaped recess  193 , a tapered, preferably conically-tapered, portion  194  is formed in the orifice  190 ″″ that widens towards the top  11 ′″. The tapered portion  194  may have a smaller inner diameter adjacent the spherical segment-shaped recess  193  than an inner diameter of the spherical segment-shaped recess  193  adjacent the tapered portion. The tapered portion  194  permits the threaded shank  41 ″ to be tilted in the orifice  190 ″″. Similar to the fixation member  40  of the first embodiments, the head  42  of the fixation member  40 ″ includes a tapered section  43  and a cylindrical section  44 . 
     The pivot sleeve  80  may be inserted into the main body  1000 ″″ by compressing it so that it can snap into the recess  193 , where the pivot sleeve may be held by friction. Thereafter, the rods  100 ″ and  101 ″ may be placed in their respective rod seats  150 ′″,  160 ′″. Finally, the fixation member  40 ″ is inserted into the orifice such that its shaft  41 ″ engages the threaded inner side  82  of the pivot sleeve  80 . In this configuration, the fixation member  40 ″ can be tilted to some extent around its screw axis so that it can clamp both rods simultaneously, with the tapered portion  43  providing the third contact area for each of the rods that rest in the V-shaped portion of the rod seat, respectively. 
     In another mode of use, two rods  100 ,  101  of the same diameter are used, as shown in  FIG.  47   . In this case, the screw axis of the fixation member  40 ″ may be in the zero position, i.e. is perpendicular to an axis that extends along a width of the main body of the connector. 
     Modifications of the above described embodiments are also conceivable. The particular features, structures or characteristics of one embodiment may be combined with those of the other embodiments in any suitable manner to produce a multiplicity of further embodiments. Particular shapes of the elements are not limited to the specific shape shown in the drawings, but may vary. While straight rods are shown, the invention is not limited thereto. The rods may also have a curvature. Also, the rod seats and their respective longitudinal axes may have a curvature. Moreover, the rods can have another cross-section. The shape of the rod seat may be adapted to the cross-section of the rod. The orifices may also be closed at the bottom. Only one sliding member may be sufficient that may have any suitable shape. Also, the sliding members can be omitted. 
     For the bone anchoring elements that are not shown, all types of bone anchoring elements that are suitable for anchoring in a bone or a vertebra and configured to be connected to a rod may be used. 
     While the present invention has been described in connection with certain exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but is instead intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, and equivalents thereof.