CROSS REFERENCE TO RELATED APPLICATIONS 
     This Patent Application is a continuation-in-part application of U.S. patent application Ser. No. 13/483,852, filed May 30, 2012, which is entitled “Expandable Interbody Spacer,” the entire contents of which are incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to devices and methods for treating one or more damaged, diseased, or traumatized portions of the spine, including intervertebral discs, to reduce or eliminate associated back pain. In one or more embodiments, the present invention relates to an expandable interbody spacer. 
     BACKGROUND OF THE INVENTION 
     The vertebrate spine is the axis of the skeleton providing structural support for the other body parts. In humans, the normal spine has seven cervical, twelve thoracic and five lumbar segments. The lumbar spine sits upon the sacrum, which then attaches to the pelvis, and in turn is supported by the hip and leg bones. The bony vertebral bodies of the spine are separated by intervertebral discs, which act as joints but allow known degrees of flexion, extension, lateral bending, and axial rotation. 
     The typical vertebra has a thick anterior bone mass called the vertebral body, with a neural (vertebral) arch that arises from the posterior surface of the vertebral body. The centra of adjacent vertebrae are supported by intervertebral discs. Each neural arch combines with the posterior surface of the vertebral body and encloses a vertebral foramen. The vertebral foramina of adjacent vertebrae are aligned to form a vertebral canal, through which the spinal sac, cord and nerve rootlets pass. The portion of the neural arch which extends posteriorly and acts to protect the spinal cord&#39;s posterior side is known as the lamina. Projecting from the posterior region of the neural arch is the spinous process. 
     The intervertebral disc primarily serves as a mechanical cushion permitting controlled motion between vertebral segments of the axial skeleton. The normal disc is a unique, mixed structure, comprised of three component tissues: the nucleus pulpous (“nucleus”), the annulus fibrosus (“annulus”) and two vertebral end plates. The two vertebral end plates are composed of thin cartilage overlying a thin layer of hard, cortical bone which attaches to the spongy, richly vascular, cancellous bone of the vertebral body. The end plates thus act to attach adjacent vertebrae to the disc. 
     The spinal disc and/or vertebral bodies may be displaced or damaged due to trauma, disease, degenerative defects, or wear over an extended period of time. One result of this displacement or damage to a spinal disc or vertebral body may be chronic back pain. A common procedure for treating damage or disease of the spinal disc or vertebral body may involve partial or complete removal of an intervertebral disc. An implant, which may be referred to as an interbody spacer, can be inserted into the cavity created where the intervertebral disc was removed to help maintain height of the spine and/or restore stability to the spine. An example of an interbody spacer that has been commonly used is a cage, which typically is packed with bone and/or bone-growth-inducing materials. However, there are drawbacks associated with conventional interbody spacers, such as cages and other designs. For instances, conventional interbody spacers may be too large and bulky for introduction into the disc space in a minimally invasive manner, such as may be utilized in a posterior approach. Further, these conventional interbody spacers may have inadequate surface area contact with the adjacent endplates if sized for introduction into the disc space in a minimally invasive manner. In addition, conventional interbody spacers designed for introduction into the disc space in a minimally invasive manner may lack sufficient space for packing of bone-growth-inducing material, thus potentially not promoting the desired graft between the adjacent endplates. 
     Therefore, a need exists for an interbody spacer that can be introduced in a minimally manner that provides a desired amount of surface area contact with the adjacent endplates and has an increased space for packing of bone-growth-inducing material. 
     SUMMARY OF THE INVENTION 
     The present invention relates to an expandable interbody spacer. The expandable interbody spacer may comprise a first jointed arm comprising a plurality of links pivotally coupled end to end. The expandable interbody spacer further may comprise a second jointed arm comprising a plurality of links pivotally coupled end to end. The first jointed arm and the second jointed arm may be interconnected at a proximal end of the expandable interbody spacer. The first jointed arm and the second jointed arm may be interconnected at a distal end of the expandable interbody spacer. The first jointed arm and the second jointed arm may each be configured to fold inward in opposite directions to place the expandable interbody spacer in an expanded position. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be more readily understood with reference to the embodiments thereof illustrated in the attached drawing figures, in which: 
         FIG. 1  is a top view of an expandable interbody spacer shown in a collapsed position in accordance with embodiments of the present invention; 
         FIG. 2  is a side view of the expandable interbody spacer of  FIG. 1  shown in a collapsed position; 
         FIG. 3  is a proximal end view of the expandable interbody spacer of  FIG. 1  shown in a collapsed position; 
         FIG. 4  is a distal end view of the expandable interbody spacer of  FIG. 1  shown in a collapsed position; 
         FIG. 5  is an exploded view of the expandable interbody spacer of  FIG. 1 ; 
         FIG. 6  is a top view of the expandable interbody spacer of  FIG. 1  shown in an expanded position; 
         FIG. 7  is a right side view of the expandable interbody spacer of  FIG. 1  shown in an expanded position; 
         FIG. 8  is a left side view of the expandable interbody spacer of  FIG. 1  shown in an expanded position; 
         FIG. 9  is a proximal end view of the expandable interbody spacer of  FIG. 1  shown in an expanded position; 
         FIG. 10  is a distal end view of the expandable interbody spacer of  FIG. 1  shown in an expanded position; 
         FIG. 11  is a view showing disc space between adjacent vertebrae in accordance with embodiments of the present invention; 
         FIG. 12  is a view of a tool for insertion of an expandable interbody spacer in accordance with embodiments of the present invention; 
         FIG. 13  is a view showing the tool of  FIG. 12  introducing an expandable interbody spacer into a disc space in a collapsed position in accordance with embodiments of the present invention; 
         FIG. 14  is a view showing the tool of  FIG. 12  expanding an expandable interbody spacer in a disc space in accordance with embodiments of the present invention; 
         FIG. 15  is a view showing a funnel for introduction of bone-growth-inducing material into a disc space in accordance with embodiments of the present invention; 
         FIG. 16  is an exploded view of another embodiment of an expandable interbody spacer; 
         FIG. 17  is a top view of another embodiment of an expandable interbody spacer shown in a collapsed position; 
         FIG. 18  is a top view of the expandable interbody spacer of  FIG. 17  shown in an expanded position; 
         FIG. 19  is an exploded view of the expandable interbody spacer of  FIG. 17 ; 
         FIG. 20  is an exploded view of a link of a jointed arm of the expandable interbody spacer of  FIG. 17 ; 
         FIG. 21  is a top view of another embodiment of an expandable interbody spacer shown in a collapsed position; 
         FIG. 22  is a top view of the expandable interbody spacer of  FIG. 21  shown in an expanded position; 
         FIG. 23  is a view of the expandable interbody spacer of  FIG. 21  shown in a disc space in a collapsed position; 
         FIG. 24  is a view of the expandable interbody spacer of  FIG. 21  shown in a disc space in an expanded position; 
         FIG. 25  is a top view of a tool shown engaging the expandable interbody spacer of  FIG. 21  in accordance with embodiments of the present invention; 
         FIG. 26  is a view showing the tool of  FIG. 24  expanding the expandable interbody spacer of  FIG. 24  in a disc space in accordance with embodiments of the present invention; 
         FIGS. 27A-27C  show different views of an expandable interbody spacer having an expandable containment bladder in accordance with embodiments of the present invention; 
         FIGS. 28A and 28B  show top views of an expandable spacer utilizing a shim member in accordance with embodiments of the present invention; 
         FIGS. 29A and 29B  show top perspective views of an expandable spacer utilizing a translation member in accordance with embodiments of the present invention; 
         FIGS. 30A and 30B  show top views of an expandable spacer including a sliding actuation member in accordance with embodiments of the present invention; 
         FIGS. 31A and 31B  show different views of an expandable spacer having slidable wings in accordance with embodiments of the present invention; 
         FIGS. 32A-32D  show an expandable spacer comprising an “I-beam” with multiple side slots for receiving complementary side members in accordance with embodiments of the present invention; 
         FIGS. 33A and 33B  show different views of a hinged expandable interbody spacer in accordance with embodiments of the present invention; 
         FIGS. 34A-34D  show different views of an alternate hinged expandable interbody spacer in accordance with embodiments of the present invention; 
         FIGS. 35 and 35B  show an expandable spacer including a flexible containment member in accordance with some embodiments; 
         FIG. 36  shows an expandable spacer including a rotating cam to actuate expandable wings in accordance with some embodiments; 
         FIGS. 37A and 37B  show an expandable spacer including four wings actuated by a gear mechanism in accordance with some embodiments; 
         FIGS. 38A and 38B  show an expandable spacer including deployable pins in accordance with some embodiments; 
         FIG. 39  shows an expandable spacer expandable via a guide wire in accordance with some embodiments; 
         FIG. 40  shows an expandable spacer including a add-on member in accordance with some embodiments; 
         FIG. 41  shows a buildable spacer that can be guided by tracks in a disc space to form a large footprint in a disc space in accordance with some embodiments; 
         FIGS. 42A-D  show a rotatable spacer capable of expansion following rotation in accordance with some embodiments; 
         FIGS. 43A-C  show an expandable spacer capable of outward folding in accordance with some embodiments; 
         FIGS. 44A and 44B  show a pair of expandable spacers having deployable arms; 
         FIGS. 45A-45C  show an expandable spacer having a rack and pinion actuator in accordance with some embodiments; 
         FIGS. 46A-46C  show an expandable spacer having an outer member with a slidable inner member therein; 
         FIGS. 47A and 47B  show an expandable spacer having upper and lower members separated by linking members in accordance with some embodiments; 
         FIGS. 48A and 48B  show an expandable spacer comprising a worm gear in accordance with some embodiments; 
         FIGS. 49A and 49B  show an expandable spacer having asymmetrical expansion in accordance with some embodiments. 
     
    
    
     Throughout the drawing figures, it should be understood that like numerals refer to like features and structures. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The preferred embodiments of the invention will now be described with reference to the attached drawing figures. The following detailed description of the invention is not intended to be illustrative of all embodiments. In describing preferred embodiments of the present invention, specific terminology is employed for the sake of clarity. However, the invention is not intended to be limited to the specific terminology so selected. It is to be understood that each specific element includes all technical equivalents that operate in a similar manner to accomplish a similar purpose. 
     Referring to  FIGS. 1-10 , an expandable interbody spacer  10  is shown in accordance with embodiments of the present invention. In the illustrated embodiment, the expandable interbody spacer  10  has a proximal end  20  and a distal end  30 . The expandable interbody spacer  10  may include a first jointed arm  40  and a second jointed arm  50  positioned on either side of longitudinal axis  15  of the spacer  10 . The first and second jointed arms  40 ,  50  may be interconnected at the proximal end  20 , for example, by a proximal connection member  60 . The first and second jointed arms  40 ,  50  may be interconnected at the distal end  30 , for example, by a distal connection member  70 . The first and second jointed arms  40 ,  50  The expandable interbody spacer  10  may be made from a number of materials, including titanium, stainless steel, titanium alloys, non-titanium alloys, polymeric materials, plastic composites, polyether ether ketone (“PEEK”) plastic material, ceramic, elastic materials, and combinations thereof. While the expandable interbody spacer  10  may be used with a posterior, anterior, lateral, or combined approach to the surgical site, the spacer  10  may be particularly suited with a posterior approach. 
     The first jointed arm  40  has a proximal end  80  and a distal end  90 . The proximal end  80  may be pivotally coupled to the proximal connection member  60 . The distal end  90  may be pivotally coupled to the distal connection member  70 . Any of a variety of different fasteners may be used to pivotally couple the proximal end  80  and the distal end  90  and the proximal connection member  60  and the distal connection member  70 , such as pins  100 , for example. In another embodiment (not illustrated), the connection may be a hinged connection. As illustrated, the first jointed arm  40  may comprise a plurality of links that are pivotally coupled to one another. In the illustrated embodiment, the first jointed arm  40  comprises first link  110 , second link  120 , and third link  130 . When the spacer  10  is in a collapsed position, the first link  110 , second link  120 , and third link may be generally axially aligned. As illustrated, the first link  110 , second link  120 , and third link  130  may be connected end to end. When the spacer  10  is in a collapsed position, the first link  110 , second link  120 , and third link  130  may be generally axially aligned. The first link  110  and the second link  120  may be pivotally coupled, and the second link  120  and the third link  130  may also be rotatably coupled. Any of a variety of different fasteners may be used to pivotally couple the links  110 ,  120 ,  130 , such as pins  100 , for example. In another embodiment (not illustrated), the coupling may be via a hinged connection. 
     As best seen in  FIGS. 1, 5-7, 9, and 10 , an upper surface  140  of the first jointed arm  40  may be defined by the links  110 ,  120 ,  130 . The upper surface  140  should allow for engagement of the first jointed arm  40  with one of the adjacent vertebral bodies. In some embodiments, the upper surface  140  may include texturing  150  to aid in gripping the adjacent vertebral bodies. Although not limited to the following, the texturing  150  can include teeth, ridges, friction-increasing elements, keels, or gripping or purchasing projections. 
     As best seen in  FIGS. 7, 9, and 10  a lower surface  160  of the first jointed arm  40  may be defined by the links  110 ,  120 ,  130 . The lower surface  160  should allow for engagement of the first jointed arm  40  with one of the adjacent vertebral bodies. In some embodiments, the lower surface  160  may include texturing  170  to aid in gripping the adjacent vertebral bodies. Although not limited to the following, the texturing  170  can include teeth, ridges, friction-increasing elements, keels, or gripping or purchasing projections. 
     The second jointed arm  50  has a proximal end  180  and a distal end  190 . The proximal end  180  may be pivotally coupled to the distal connection member  70 . The distal end  190  may be pivotally coupled to the distal connection member  70 . Any of a variety of different fasteners may be used to pivotally couple the proximal end  180  and the distal end  190  and the proximal connection member  60  and the distal connection member  70 , such as pins  100 , for example. In another embodiment (not illustrated), the connection may be a hinged connection. As illustrated, the second jointed arm  50  may comprise a plurality of links that are pivotally coupled to one another. In the illustrated embodiment, the second jointed arm  50  comprises first link  200 , second link  210 , and third link  220 . When the spacer  10  is in a collapsed position, the first link  200 , second link  210 , and third link  220  may be generally axially aligned. As illustrated, the first link  200 , second link  210 , and third link  220  may be connected end to end. The first link  200  and the second link  210  may be pivotally coupled, and the second link  210  and the third link  220  may also be pivotally coupled. Any of a variety of different fasteners may be used to pivotally couple the links  200 ,  210 ,  220 , such as pins  100 , for example. In another embodiment (not illustrated), the coupling may be via a hinged connection. 
     As best seen in  FIGS. 1, 2, 6, and 8-10 , an upper surface  230  of the second jointed arm  50  may be defined by the links  200 ,  210 ,  220 . The upper surface  230  should allow for engagement of the second jointed arm  50  with one of the adjacent vertebral bodies. In some embodiments, the upper surface  230  may include texturing  240  to aid in gripping the adjacent vertebral bodies. Although not limited to the following, the texturing  240  can include teeth, ridges, friction-increasing elements, keels, or gripping or purchasing projections. 
     As best seen in  FIGS. 8-10 , a lower surface  250  of the second jointed arm  50  may be defined by the links  200 ,  210 , and  220 . The lower surface  250  should allow for engagement of the second jointed arm  50  with one of the adjacent vertebral bodies. In some embodiments, the lower surface  250  may include texturing  260  to aid in gripping the adjacent vertebral bodies. Although not limited to the following, the texturing  260  can include teeth, ridges, friction-increasing elements, keels, or gripping or purchasing projections. 
     With reference now to  FIGS. 3, 5, and 9 , a bore  270  extends through proximal connection end  60 . The bore  270  may extend generally parallel to the longitudinal axis  12  (see  FIG. 1 ) of the spacer  10 . The first jointed arm  40  and the second jointed arm  50  may define a hollow interior portion (not shown) that extends axially through the spacer  10 . The bore  270  in the proximal connection end  60  may communicate with this hollow interior portion. As best shown on  FIG. 5 , the distal connection end  70  may include an opening  280 . As illustrated, the opening  280  may face inward and may not extend all the way through the distal connection  70 . In one embodiment, the opening  280  may be generally aligned with the bore  270  in the proximal connection end  60  such at a tool (e.g., tool  340  shown on  FIG. 12 ) inserted into the bore  270  may be received in the opening  280  for placement of the spacer  10  into a disc space and/or expansion of the spacer  10 . 
       FIGS. 1-4  illustrate the expandable interbody spacer  10  in a collapsed position. In accordance with present embodiments, the expandable interbody spacer  10  may be laterally expanded to an expanded position.  FIGS. 6-10  illustrate the expandable interbody spacer  10  in an expanded position. In the expanded position, the first arm  40  and the second arm  50  have each been folded inward in opposite directions. For example, the proximal end  80  and the distal end  90  of the first arm  40  may be folded closer together. The links  110 ,  120 ,  130  should pivot with respect to one another when the first arm  40  is folded inward. The proximal end  80  should pivot at the proximal connection end  60 , and the distal end  90  should pivot at the distal connection end  70 . By way of further example, the proximal end  180  and the distal end  190  of the second arm  50  may also be folded together. The links  200 ,  210 ,  220  should pivot with respect to another when the second arm is folded inward. The proximal end  180  should pivot at proximal connection end  60 , and the distal end  190  should pivot at the distal connection end  70 . After placement in the expanded position, the expandable interbody spacer  10  can be secured in the expanded position to prevent collapse of the expandable interbody spacer  10  upon application of spacer. Any of a variety of different techniques may be used to secure the expandable interbody spacer  10 , including pins or other suitable locking mechanism, for example. 
     As illustrated by  FIG. 6 , the first and second jointed arms  40 ,  50  define an interior cavity  290  when in an expanded position. The interior cavity  290  may be filled with a bone-growth-inducing material, such as bone material, bone-growth factors, or bone morphogenic proteins. As will be appreciated by those of ordinary skill in the art, the bone-growth-inducing material should induce the growth of bone material, thus promoting fusion of the adjacent vertebra. 
     The expandable interbody spacer  10  may be sized to accommodate different applications, different procedures, implantation into different regions of the spine, or size of disc space. For example, the expandable interbody spacer  10  may have a width W1 (as shown on  FIG. 1 ) prior to expansion of about 8 to about 22 and alternatively from about 10 to about 13. By way of further example, the expandable interbody spacer  10  may be expanded to a width W2 (as shown on  FIG. 6 ) in a range of about 26 to about 42 and alternatively from about 16 to about 32. It should be understood that the width W1 or W2 whether prior to, or after, expansion generally refers to the width of the expandable interbody spacer  10  extending transverse to the longitudinal axis  12  of the spacer  10 . In general, the width W2 of the expandable interbody spacer  10  after expansion should be greater than the width W1 of the expandable interbody spacer  10  prior to expansion. 
     In accordance with present embodiments, the expandable interbody spacer  10  may be used in the treatment of damage or disease of the vertebral column. In one embodiment, the expandable interbody spacer  10  may be inserted into a disc space between adjacent vertebrae in which the intervertebral disc has been partially or completely removed.  FIG. 11  illustrates a spinal segment  300  into which the expandable interbody spacer  10  (e.g.,  FIGS. 1-10 ) may be inserted. The spinal segment  300  includes adjacent vertebrae, identified by reference numbers  310  and  320 . Each of the adjacent vertebrae  310 ,  320  has a corresponding endplate  315 ,  325 . The disc space  330  is the space between the adjacent vertebrae  310 ,  320 .  FIG. 12  illustrates a tool  340  that may be used in the insertion of the expandable interbody spacer  10  into the disc space  330 . The tool  340  includes a shaft  350  having an elongated end portion  360  for coupling to the expandable interbody spacer  10 . The elongated end portion  360  has a distal tip  370 . 
       FIGS. 13 and 14  illustrate introduction of an expandable interbody spacer  10  into the disc space  330  using toot  340 . For illustrative purposes, the upper vertebra  330  shown on  FIG. 11  has been removed from  FIGS. 13 and 14 . As illustrated, the spacer  10  may be secured to the tool  340 . For example, the elongated end portion  360  of the tool  340  may be disposed through the bore  270  (e.g., see  FIG. 5 ) in the proximal connection end  60  with the distal tip  370  (e.g., see  FIG. 12 ) of the end portion  360  secured in the opening  280  (e.g., see  FIG. 5 ) in the distal connection end  70 . As illustrated by  FIG. 13 , the tool  340  may introduce the spacer  10  into the disc space  330  through an access cannula  380 . After introduction into the disc space  330 , the spacer  10  may be laterally expanded. In accordance with present embodiments, the spacer  10  can be laterally expanded by folding the first arm  40  and the second arm  50  inward. By expanding laterally, the spacer  10  has an increased surface area contact with the endplate  325 . In addition, the spacer  10  may engage harder bone around the apophyseal ring. As previously mentioned, an interior cavity  290  should be formed in the spacer  10  when in the expanded position. The tool  340  may then be detached from the spacer  10  and removed from the cannula  380 . As illustrated by  FIG. 15 , a funnel  390  may then be placed on the cannula  380 . Bone-growth inducing material may then be placed into the interior cavity  290  through the cannula  380 . Because the spacer  10  has been laterally expanded, the interior cavity  290  should have a desirable amount of space for packing of the bone-growth-inducing material. 
       FIG. 16  illustrates an expandable interbody spacer  10  in accordance with an alternative embodiment. In the illustrated embodiment, the expandable interbody spacer  10  comprises a first jointed arm  40  and a second jointed arm  50 . The first jointed arm  40  has a proximal end  80  and a distal end  90 . The first jointed arm  40  comprises a plurality of links  110 ,  120 ,  130  connected end to end, for example, by pins  100 . The first jointed arm  40  further may comprise washers  105  (e.g, PEEK washers) that may be disposed between the links  110 ,  120 ,  130  at their connections. The second jointed arm  50  has a proximal end  180  and a distal end  190 . The second jointed arm  50  comprises a plurality of links  200 ,  210 ,  220  connected end to end, for example, by pins  100 . The second jointed arm  50  further may comprise washers  105  (e.g, PEEK washers) that may be disposed between the links  200 ,  210 ,  220  at their connections. Washers  105  may also be disposed between the first arm  40  and the proximal connection member  60  and the distal connection member  70  at their respective connections. Washers  105  may also be disposed between the second arm  50  and the proximal connection member  60  and the distal connection member  70  at their respective connections. The washers  105  should have an interference fit to cause friction such that the spacer  10  may hold its shape in the entire range of the expanded implant. 
     The proximal ends  80 ,  180  may be pivotally coupled, for example, by pin  100 , as shown on  FIG. 19 . The distal ends  90 ,  180  may also be pivotally coupled, for example, by pin  100 , as shown on  FIG. 19 . The first jointed arm  40  comprises first link  110  and third link  130 , the first link  110  and the third link  130  being pivotally coupled. In contrast to the first jointed arm  40  of  FIGS. 1-10 , there 
     Referring now to  FIGS. 17-19 , an expandable interbody spacer  10  is illustrated in accordance with another embodiment of the present invention. In the illustrated embodiment, the expandable interbody spacer  10  comprises a first jointed arm  40  and a second jointed arm  50 . The first jointed arm  40  has a proximal end  80  and a distal end  90 . The second jointed arm  50  has a proximal end  180  and a distal end  190 . The proximal ends  80 ,  180  may be pivotally coupled, for example, by pin  100 , as shown on  FIG. 19 . The distal ends  90 ,  180  may also be pivotally coupled, for example, by pin  100 , as shown on  FIG. 19 . The first jointed arm  40  comprises first link  110  and third link  130 , the first link  110  and the third link  130  being pivotally coupled. In contrast to the first jointed arm  40  of  FIGS. 1-10 , there is no second link  120 . As shown by  FIG. 20 , the third link  130  may comprise a first link segment  400  and a second link segment  410 , which may be secured to one another by pins  420 , for example. First link segment  400  and second link segment  410  may also have a tongue-and-groove connection, for example a groove  430  in the first link segment  400  may receive a tongue  440  of the second link segment  410 . The second jointed arm comprises first link  200  and third link  220 , the first link  200  and the third link  220  being pivotally coupled. In contrast to the second joint arm  50  of  FIGS. 1-10 , there is no second link  210 . 
     In accordance with present embodiments, lateral expansion of the expandable interbody spacer  10  of  FIGS. 17-19  may include folding the first arm  40  and the second arm  50  inward. For example, the proximal end  80  and the distal end  90  of the first arm  40  may be folded together, and the proximal end  180  and the distal end  190  of the second arm  50  may also be folded together. 
     Referring now to  FIGS. 21 and 22 , an expandable interbody spacer  10  is illustrated in accordance with another embodiment of the present invention. In the illustrated embodiment, the expandable interbody spacer  10  has a proximal end  20  and a distal end  30 . The expandable interbody spacer  10  may include a first jointed arm  40  and a second jointed arm  50  positioned on either side of longitudinal axis  12  of the spacer  10 . As illustrated, the expandable interbody spacer  10  further may comprise an internal screw  450 . The internal screw  450  may comprise a head  460  and an elongated body  470 , which may extend generally parallel to the longitudinal axis  12  of the spacer  10 . In some embodiments, the internal screw  450  may extend from the proximal end  20  to the distal end  30  of the spacer  10 . In one embodiment, the elongated body  470  may be retractable. For example, the elongated body  470  may retract into the head  460 , as shown on  FIG. 22 . 
     As illustrated by  FIGS. 23 and 24 , the spacer  10  may be introduced into the disc space  330 , wherein the spacer  10  can be laterally expanded. In accordance with present embodiments, the spacer  10  can be laterally expanded by folding the first arm  40  and the second arm  50  inward. In some embodiments, the elongated body  470  may be retracted into the head  460  to cause folding of the first arm  40  and the second arm  50  inward, as the first arm  40  and the second arm  50  are secured to the distal end  480  of the internal screw  450 . 
       FIG. 25  shows attachment of a tool  490  to the expandable interbody spacer  10  of  FIGS. 22 and 23  in accordance with embodiments of the present invention. As illustrated, the tool  490  may have an attachment end  500 , which can be secured to the head  460  of the internal screw  450 . As shown by  FIG. 26 , the tool  40  can be used to introduce the spacer  10  into the disc space  330 , wherein the spacer  10  can be laterally expanded. 
     Additional embodiments of expandable interbody spacers are described herein.  FIGS. 27A and 27B  show top views of an expandable interbody spacer having an expandable containment bladder in accordance with embodiments of the present invention.  FIG. 27A  illustrates the spacer  610  in an unexpanded state, while  FIG. 27B  illustrates the spacer  610  in an expanded state. 
     As shown in  FIG. 27A , the spacer  610  comprises an outer body  615  and an inner bladder  618 . The inner bladder  618  can include an opening  620  through which an instrument can be inserted to deliver rods or beads that will result in expansion of the spacer  610 . In some embodiments, the spacer  610  comprises a convex longitudinal surface opposite a concave longitudinal surface. The spacer  610  can be expanded such that it maintains the convex longitudinal surface and concave longitudinal surface, as shown in  FIG. 27B . In other embodiments, expansion of the spacer  610  via rods or beads can result in a configuration that is different from the original shape. Advantageously, the spacer  610  is configured such that a surgeon can deliver rods or beads to thereby transform the spacer  610  into a desired shape to assist in implantation from a variety of different approaches. For example, the spacer  610  can be expanded such that it includes a “banana” type shape that is suitable for transforaminal delivery, or it can be a long, slender shape that is suitable for posterior delivery. In its unexpanded state, the spacer  610  can be easily delivered minimally invasively into a desired anatomical location. 
     As shown in  FIG. 27B , the spacer  610  can receive an instrument  690  through the opening  620  in the inner bladder  618 . The instrument  690  can deliver one or more rods or beads  688  that will cause expansion of the inner bladder  618 , as well as the overall spacer  610 . In some embodiments, the instrument  690  can be a curvable instrument that can deliver the beads  688  to desirable locations within the inner bladder  618 , thereby causing selective expansion of the spacer  610 . As shown in  FIG. 27B , in some embodiments, the spacer  610  can substantially maintain the same shape as in the unexpanded state; however, with the addition of the rods or beads  610 , the spacer  610  will be larger and have a much larger footprint than in the unexpanded state. In some embodiments, the overall footprint of the spacer  610  expands along its longitudinal length and/or width, while maintaining a substantially or the same height as the unexpanded spacer  610 . In other embodiments, the overall footprint of the spacer  610  expands along its longitudinal length and/or width, and the height of the spacer  610  also changes during expansion. 
       FIG. 27C  illustrates a third view of the spacer  610  with the expandable inner bladder  618  inserted between two adjacent vertebrae  310 ,  320 . The spacer  610  is configured to receive one or more rods or beads  688  via the delivery instrument  690 . As shown from this view, the delivery instrument  690  can comprise a tubular body that holds the rods or beads  688  in serial formation. The delivery instrument  690  can be accompanied by a pusher instrument  685  that can deliver the rods or beads  688  out in series. In some embodiments, the delivery instrument  690  can also include an automatic depositor such that multiple rods or beads  688  can be delivered in rapid fashion. 
       FIGS. 28A and 28B  show top views of an expandable spacer utilizing a shim member in accordance with embodiments of the present invention. The expandable spacer  710  comprises an outer body  715  having an opening  718 , as shown in  FIG. 28A . In some embodiments, the opening  718  is in communication with a channel  723  having opposing walls  724 ,  725  that extends along a longitudinal axis of the expandable spacer  710 . In some embodiments, the channel  723  extends along at least a majority of the length of the expandable spacer. When it is desired to expand the spacer  710 , a shim member  720  can be inserted through the opening  718  and into the channel  723 , as shown in  FIG. 28B . The addition of the shim member  720  causes the spacer  710  to expand by a distance as measured by the increase in distance between the opposing walls  724 ,  725  of the channel, thereby advantageously increasing the footprint of the spacer  710  once implanted in a desired location. In some embodiments, the shim member  720  is tapered such that the tapering facilitates ease of insertion in the channel  723 . 
       FIGS. 29A and 29B  show top perspective views of an expandable spacer utilizing a translation member in accordance with embodiments of the present invention.  FIG. 29A  illustrates the spacer  810  in a closed configuration, while  FIG. 29B  illustrates the spacer  810  in an open or expanded configuration. 
     The expandable spacer  810  comprises an upper endplate  812  and a lower endplate  814 . Each of the upper endplate  812  and the lower endplate  814  can include surface texturing  815  thereon to assist in engagement with an adjacent vertebra. In some embodiments, the surface texturing  815  comprises protrusions, teeth, ridges or ribbing. Each of the endplates  812 ,  814  is formed of two separate members that can be separated from one another laterally in a “v” configuration, as shown in  FIG. 29B . With reference to the upper endplate  812 , the upper endplate  812  includes a first endplate portion  822  and a second endplate portion  824  that can be separated from one another along a midline  805  that extends through the spacer  810 . In some embodiments, at least one of the first endplate portion  822  and the second endplate portion  824  can be connected via a hinge member  855  such that at least one of the endplate portions pivots away from one another. As shown in  FIG. 29B , the first endplate portion  822  and the second endplate portion  824  of the upper endplate  812  transition into corresponding members found along the lower endplate  814 . In some embodiments, the expandable spacer  810  comprises one or more side slots  828  that can be engaged by an installation instrument to assist in delivery of the spacer  810  to a desired anatomical location. 
     In order to expand the spacer  810 , the spacer  810  includes a translation member  830  and an actuation member  840 , as shown in  FIG. 29B . The translation member  830  can comprise one or more ramps that engage side ramps formed along inner sidewalls of the spacer  810 . As shown in  FIG. 29B , the spacer  810  can include at least a pair of ramps  832 ,  834  that engage with corresponding ramps formed along the inner sidewalls of the spacer  810 . As the translation member  830  is translated (e.g., in a first direction), the ramps  832 ,  834  slide along corresponding ramps formed along the inner sidewalls of the spacer  810 , thereby causing expansion of the implant. Translation of the translation member  830  in an opposite direction (e.g., in a second direction) causes contraction of the implant. In some embodiments, the spacer  810  includes more than just the ramps  832 ,  834 . For example, the ramps  832 ,  834  can be connected via a bridge member  836  to additional ramps along a longitudinal axis of the spacer  810 . In some embodiments, ramps  832 ,  834  are connected via a bridge member  836  to a second pair of ramps that can help with expansion of the spacer  810 . 
     In order to move the translation member  830 , in some embodiments, the translation member  830  is operably attached to an actuation member  840 . The actuation member  840  can comprise an actuation or set screw  840 . In some embodiments, the actuation member  840  includes an opening, such as a hex screw opening, for allowing rotation of the actuation member  840 . Rotation of the actuation member  840  in a first direction causes lateral translation of the translation member  830  in the first direction, thereby causing sliding engagement between the ramps  832 ,  834  of the translation member  830  and ramps of the inner sidewalls, and thus outward expansion of the first endplate portion and second endplate portion. Advantageously, as shown in  FIG. 29B , the first endplate portion  822  separates from the second endplate portion  824  in a v-shape, thereby enlarging the footprint of the implant. This advantageously creates an implant with greater load stability, as well as an increased region through which to deposit bone graft material. 
       FIGS. 30A and 30B  show top views of an expandable spacer including a sliding actuation member in accordance with some embodiments. The expandable spacer  910  includes a pair of upper wing members  912  and a pair of lower wing members  914 . As shown in  FIG. 30A , an upper wing member  912  and a lower wing member  914  is operably attached to sliding actuation member  920 .  FIG. 30A  illustrates the expandable spacer is an unexpanded configuration. When the spacer is ready for expansion, the sliding actuation member  920  can slide in between the upper wing member  912  and the lower wing member  914 , thereby causing the wing members to open outwardly, as shown in  FIG. 30B . In some embodiments, the wing members can expand from approximately 12 mm to 20 or more millimeters just by expansion of the wing members. 
       FIGS. 31A and 31B  show an alternative embodiment of an expandable spacer having slidable wings in accordance with embodiments of the present invention. The spacer  1010  can be composed of slidable wings  1012 ,  1013 ,  1014 ,  1015 . As shown in  FIG. 31A , slidable wings  1012 ,  1013  are on a left side of the spacer  1010 , while slidable wings  1014 ,  1015  are on a right side of the spacer  1010 . The spacer  1010  can be delivered to a disc space in a non-expanded, minimally invasive state, as shown in  FIG. 31A . Once in the disc space, the wings  1012 ,  1013 ,  1014 ,  1015  of the expandable spacer can be outwardly deployed, thereby causing expansion of the device. In some embodiments, the wings can be complimentary and symmetrical to one another prior to deployment. To deploy the wings, a pre-attached block member  1022 ,  1023  can be actuated to open the wings. As shown in  FIG. 31A , block member  1022  can operate wings  1012 ,  1013 , while block member  1023  can operate wings  1014 ,  1015 .  FIG. 31B  shows the wings separated and in an expanded state following actuation by the block members. 
       FIGS. 32A-32D  show an expandable spacer comprising an “I-beam” with multiple side slots for receiving complementary side members in accordance with embodiments of the present invention. The spacer  1110  can comprise a central I-beam  1111  with one or more side slots  1116  that receive protruding portions from adjacent side members  1112 ,  1114 . As shown in  FIG. 32A , the I-beam and its side members  1112 ,  1114  complement each other. The I-beam can include a slot  1118  for receiving an actuation member to outwardly expand the side members  1112 ,  1114 . In some embodiments, in a contracted configuration, the side slots  1116  of the I-beam receive the protruding portions  1119  of the adjacent side members  1112 ,  1114 . Upon expansion, the protruding portions  1119  of the adjacent side members will be offset with the side slots  1116  of the I-beam. To offset the protruding portions  1119  of the adjacent side members from the side slots  1116 , the I-beam can be slid in a first direction such that the protruding portions move away from the slots. In some embodiments, the protruding portions  1119  can be tapered to allow sliding between the I-beam and the protruding portions. In other embodiments, the actuation member can be any of the actuation components discussed herein for expanding and/or contracting the spacers. 
       FIGS. 33A and 33B  show different views of a hinged expandable interbody spacer in accordance with embodiments of the present invention.  FIG. 33A  illustrates the spacer  1210  in an unexpanded state and  FIG. 33B  illustrates the spacer  1210  in an expanded state within a vertebral space  3 . As shown in  FIG. 33A , the spacer  1210  can comprises two expandable portions  1212 ,  1214  that are connected to each either by a hinge joint  1210 . In the unexpanded state, the two expandable portions  1212 ,  1214  of the spacer  1210  can be positioned side-by-side or adjacent to one another. In some embodiments, inner facing side surfaces of the two expandable portions  1212 ,  1214  are in direct contact with one another in the unexpanded state. 
     To expand the spacer  1210 , a wedge member  1219  can be delivered in between the two expandable portions  1212 ,  1214 . The wedge member  1219  can be inserted where the inner side surfaces of the expandable portions  1212 ,  1214  meet, thereby separating the first expandable portion  1212  from the second expandable portion  1214 . As the first expandable portion  1212  and the second expandable portion  1214  are connected via a hinge  1215 , the spacer  1210  will assume an expanded v-shape upon expansion, as shown in  FIG. 33B . In some embodiments, the wedge member  1219  can comprise a triangular wedge member. As shown in  FIG. 33B , the wedge member  1219  can be placed substantially adjacent to or in contact with the hinge  1215  in some embodiments. In some embodiments, in the expanded configuration, the wedge member  1219  can advantageously remain embedded within the v-shape of the expanded spacer  1210 , thereby preventing closing or contraction of the expanded configuration. 
     In some embodiments, the wedge member  1219  can be accompanied by an insertion instrument  1223  to assist in delivery of the wedge member  1219 . The wedge member  1219  can comprise a proximal end and a distal end that is directly adjacent and/or in contact with the expandable portions  1212 ,  1214 . The insertion instrument  1223  includes a sleeve to guide the wedge member  1219  to a desired location between the hinged expandable portions  1212 ,  1214 . 
       FIGS. 34A-34D  show different embodiments of an alternative hinged spacer  1310  in accordance with some embodiments.  FIG. 34A  illustrates a hinged spacer  1310  in an unexpanded configuration, while  FIG. 34B  illustrates the hinged spacer  1310  in an expanded configuration. The hinged spacer  1310  comprises a first expandable portion  1312 , a second expandable portion  1313 , and a third expandable portion  1314  that are connected to one another via hinges  1315 ,  1316 . 
     In order to expand the hinged spacer  1310 , the spacer  1310  advantageously provides a holding point  1322  and a pushing point  1324  (as shown in  FIG. 34C ). The holding point  1322  is a point at which an insertion instrument can steadily hold the spacer  1310 . In some embodiments, an insertion instrument will hold the spacer  1310  by gripping a surface. In other embodiments, an insertion instrument can engage the spacer  1310  via one or more insertion surfaces (e.g., a threaded hole) that are formed in the holding point  1322 . While the spacer  1310  is being held at its holding point  1322 , the insertion instrument can further comprise a pusher that expands the spacer  1310  by applying a force on the surface of the pushing point  1324 . In some embodiments, a pushing instrument that is separate from the insertion instrument can be used (e.g., inserted through the insertion instrument) such that it causes expansion of the spacer  1310 . As shown in  FIGS. 34C and 34D , the expandable spacer  1310  can advantageously be expanded in situ. 
       FIGS. 35A and 35B  show an expandable spacer including a flexible containment member in accordance with some embodiments.  FIG. 35A  illustrates the spacer  1410  in an unexpanded state, while  FIG. 35B  illustrates the spacer  1410  in an expanded state within a disc space  3 . The expandable spacer  1410  can comprise a flexible containment structure  1411  that includes one or more channels  1413  for receiving blocks  1422 . Insertion of the blocks  1422  causes the channels to fill, thereby causing expansion of the spacer  1410 . Advantageously, the flexible containment structure  1411  can be inserted into a disc space with few if any blocks such that the spacer  1410  can be inserted through as small an incision as possible. After the spacer  1410  is placed in a desired position in a disc space, blocks  1422  can be added into the flexible containment structure  1411  to fill the channels, thereby causing expansion of the spacer  1410  in situ. 
     The flexible containment structure  1411  of the spacer  1410  can comprises one or more channels to accommodate the blocks. As shown in  FIG. 35B , the flexible containment structure  1411  can include a number of channels  1412 ,  1414 ,  1416 ,  1418 . In some embodiments, the channels are of a same size and shape, while in other embodiments, the channels can be of a different size and shape in order to more closely approximate the desired anatomical shape of the disc space. In some embodiments, the flexible containment structure  1411  can be comprised of a flexible material, such as a plastic, a rubber, or other elastomeric material. In some embodiments, the flexible containment structure  1411  can comprise a woven or braided member that expands with the addition of the blocks. 
     To assume their expanded configuration, the channels  1412 ,  1414 ,  1416 ,  1418  of the flexible containment structure  1411  are configured to receive one or blocks  1422  in each of the channels in order to for them to reach their maximum size. In some embodiments, the channels can each receive the same number of blocks, while in other embodiments (as shown in  FIG. 35B ), the channels can receive different numbers of blocks. Advantageously, by providing channels that accommodate a different number of blocks, a specific anatomical footprint can be achieved within the disc space that caters to different patients of different sizes. In some embodiments, the blocks  1422  can be formed of a polymeric material, such as PEEK. 
     In some embodiments, an instrument is capable of directing the blocks  1422  to individual channels in order to cause selective expansion of the implant  1410 . In other embodiments, the blocks  1422  fill the channels themselves without any specific directing by an instrument. The channels can be made of a distinct size such that upon filling, the blocks  1422  will fill other regions of the implant  1410 , without having to be directed by an insertion instrument. 
       FIG. 36  shows an expandable spacer  1510  including a rotating cam to actuate expandable wings in accordance with some embodiments. Rotation of the cam  1520  in a first direction causes wings  1522 ,  1524  to outwardly expand, wherein rotation of the cam  1520  in a second direction opposite the first causes wings  1522 ,  1524  to inwardly contract. 
       FIGS. 37A and 37B  show an expandable spacer including four wings actuated by a gear mechanism in accordance with some embodiments. The spacer  1610  comprises four wings  1622 ,  1624 ,  1626 ,  1628  that can be kept in a contracted state ( FIG. 37A ) and then expanded into an expanded state ( FIG. 37B ) using a gear mechanism  1630 . Advantageously, the gear mechanism, which can include levers, pivoting arms, etc. can control the expansion of the wings such that the wings need not be fully expanded. In other words, the expandable spacer  1610  can have a series of increased expansion widths, rather than just a single contracted state and a single expanded state. 
       FIGS. 38A and 38B  show an expandable spacer  1710  comprising deployable pins in accordance with some embodiments. In contrast to prior spacers that expand to provide a greater footprint in a disc space, the present spacer  1710  (via its pins  1722 ) expands in a superior and/or inferior direction in order to conform superior and inferior endplates  1712 ,  1714  of the spacer  1710  with adjacent vertebrae. 
       FIG. 38A  illustrates the spacer  1710  in an unexpanded state. The spacer  1710  comprises a superior endplate  1712  and an inferior endplate  1714  having a plurality of holes or openings  1721  therethrough. Within the openings  1721  are a plurality of deployable pins  1722  that can outwardly expand through the openings  1721  in order to increase the height of the spacer within the disc space. In some embodiments, the spacer  1710  body can comprise a port  1715  for receiving an expandable member  1718  (shown in  FIG. 38B ) that can outwardly deploy the pins to increase the height of the spacer  1710 . 
       FIG. 38B  illustrates the spacer  1710  in an expanded state. From this view, one can see an expandable member  1718  within the body of the spacer  1710 . Expansion of the expandable member  1718  within the body of the spacer  1710  causes the deployable pins  1722  to expand outwardly, thereby increasing the height of the spacer  1710 . In some embodiments, the expandable member  1718  can comprise a balloon member. In some embodiments, an expansion instrument is insertable through the port  1715 . The expansion instrument is capable of inflating or enlarging the expandable member  1718 . As the expandable member  1718  expands, exterior surfaces of the expandable member  1718  push against the deployable pins  1722 , thereby causing the pins  1722  to protrude outwardly and cause overall height expansion of the spacer  1710 . 
       FIG. 39  shows an expandable spacer expandable via a guide wire in accordance with some embodiments. The spacer  1810  comprises two or more linked members that can be fed into a disc space via a guide wire. Advantageously, the spacer  1810  can be inserted into a small incision that is about the width of a single linked member. The linked members can be attached to a guide wire or k-wire  1826  that extends through each of the linked members. As the spacer  1810  is fed into the disc space, the natural anatomy of the disc space causes the linked members to curve and expand to widen the footprint of the device. As shown in  FIG. 39 , the spacer  1810  can comprise at five linked members  1812 ,  1814 ,  1816 ,  1818 ,  1820 . In other embodiments, the spacer  1810  comprises less than five linked members or greater than five linked members. The linked members can be connected to adjacent members via a joint  1824  (such as a hinge joint). Each of the linked members can include an opening for receiving the k-wire  1826 . Following expansion of the implant in situ, the k-wire  1826  can be removed, thereby leaving the implant in place. The k-wire  1826  can be delivered by an instrument  1830 . 
       FIG. 40  shows an expandable spacer including an add-on member in accordance with some embodiments. This spacer  1910  includes a first member  1912  and a second member  1914  that can be inserted into a disc space  3  on their own. As shown in  FIG. 40 , the first member  1912  and the second member  1914  can be elongated members in the form of rods that are joined together at a hinge or joint  1922 . The first member  1912  and the second member  1914  can be inserted in a configuration whereby the two members are in contact with each other. Once the first member  1912  and the second member  1914  are inserted into the disc space  3 , the two members can be expanded into a V-shape configuration, such that they are ready to receive a third add-on member  1916 . 
     The third add-on member  1916  can be inserted into the disc space  3  and can be attached to the first member  1912  and second member  1914  at respective joints or hinges  1924 ,  1926 . In some embodiments, the third add-on member  1916  can be snap-fitted to the first two members. In other embodiments, the first member  1912  and the second member  1914  include openings near the joints  1924 ,  1926  for receiving the third add-on member  1916  easily therethrough. With the third add-on member  1916 , the implant can assume the shape of a triangle that advantageously has a large footprint within the disc space  3 . Bone graft material can be provided into the completed spacer  1910 , thereby helping to aid in a fusion process within the disc space. 
       FIG. 41  shows a buildable spacer  2010  that can be guided by tracks in a disc space to form a large footprint in a disc space in accordance with some embodiments. In this embodiment, multiple tracks  2022 ,  2024 ,  2026  can be formed within a disc space  3  to guide individual spacer members  2012 ,  2014  into desired positions within the disc space. The tracks  2022 ,  2024 ,  2026  can be pre-laid within a disc space prior to inserting the spacer members  2012 ,  2014 . In some embodiments, the tracks  2022 ,  2024 ,  2026  can compose tracks formed by the disc space itself (e.g., a surgeon can form the tracks out of the cut bone), while in other embodiments, the tracks  2022 ,  2024 ,  2026  can be formed by inserted materials within the disc space, such as metals, polymers or bone material. Once the tracks  2022 ,  2024 ,  2026  have been laid, individual spacer members  2012 ,  2014  in the form of elongated members or rods can be inserted and guided by the individual tracks, thereby creating a spacer with an expanded footprint in situ. Advantageously, in some embodiments, the spacer members  2012 ,  2014  can be inserted individually into the disc space, thereby requiring a small incision. As the spacer members  2012 ,  2014  are guided in the track, the spacer  2010  size is increased. In some embodiments, there are more tracks than spacer members, thereby advantageously providing multiple options for configuring the implant in situ. 
       FIGS. 42A-D  show a rotatable spacer capable of expansion following rotation in accordance with some embodiments. The spacer  2110  comprises a pair of expandable panels  2130 ,  2132  that are capable of expansion following rotation of the spacer  2110  in a disc space. The spacer  2110  includes a leading edge  2112 , a trailing edge  2114 , a bottom surface  2121  and a top surface  2123 . The spacer  2110  can be inserted in a first direction in a minimally invasive manner via its leading edge  2112 . Once within the disc space, the spacer  2110  can be rotated, such as 90 degrees. After rotation, the panels  2130 ,  2132  of the spacer  2110  can be advantageously expanded, thereby exposing a graft slot  2135  therein. In some embodiments, the footprint of the spacer  2110  can increase by at least 20-30 percent. For example, in some embodiments, the width of the spacer  2110  can expand from an initial 20 mm width to at least a 30 mm width, with a desirable volume in the middle of the spacer  2110  for receiving graft material. 
       FIGS. 43A-C  show an expandable spacer capable of outward folding in accordance with some embodiments. The spacer  2210  comprises a first section  2212  and a second section  2214  that are operably connected via a joint or hinge  2218 . As shown in  FIG. 43A , the spacer  2210  can have a minimally invasive configuration whereby the first section  2212  and the second section  2214  are inwardly folded together. 
     Once the spacer  2210  is inserted into a disc space, the spacer  2210  can be expanded whereby its first section  2212  and second section  2214  are outwardly folded. As shown in  FIG. 43B , the spacer  2210  in its expanded state can reveal surface protrusions or teeth  2220  along at least portion of the first and second sections  2212 ,  2214 . In some embodiments, the surface protrusions  2220  extend along a majority of the perimeter of each of the first and second sections  2212 ,  2214 . These surface protrusions  2220  advantageously provide a gripping surface to prevent extrusion of the spacer  2210  once it has been expanded within a disc space. 
       FIG. 43C  illustrates an alternative embodiment of the spacer  2210 . In some embodiments, the expanded spacer  2210  can reveal multiple embedded layers. The spacer  2210  can have first and second outer sections  2212   a ,  2214   a , first and second mid sections  2212   b ,  2214   b  and first and second inner sections  2212   c ,  2214   c . Each of these sections can include surface protrusions or teeth. With the multiple embedded layers, the spacer  2210  advantageously provides greater surface area for engagement with adjacent vertebrae and also a greater covered footprint for better loading distribution. 
       FIGS. 44A and 44B  show a pair of expandable spacers having deployable arms. The spacers  2310  comprise an elongated body  2312  having one or more arms  2314  extending from the body  2312 . In some embodiments, the arms  2314  are flexible members that can be bent along the length of the body  2312  prior to deployment, thereby providing for minimally invasive insertion. In other embodiments, the arms  2314  are more rigid members that can be deployed via an instrument that can be inserted through the body  2312  of the spacer  2310 . For example, when the arms  2314  are ready for deployment, an instrument can be inserted along the length of the body  2312  to release or outwardly rotate the deployable arms  2314 . In other embodiments, the arms  2314  can be inflatable, such as by adding an expandable medium into the arms. 
       FIG. 44A  shows the spacer  2310  in an unexpanded configuration, while  FIG. 44B  shows the spacer  2310  in an expanded configuration with the arms  2314  deployed. With the arms outwardly deployed, the spacer  2310  advantageously has a larger footprint in a disc space compared to when it is first inserted into the disc space. In addition, in some embodiments, one or more arms  2314  can include one or more ports  2320 . Advantageously, these ports  2320  can serve as graft windows, such that graft material can be delivered therein. While the illustrated embodiment shows the arms  2314  as having a single port in each arm, in other embodiments, two or more ports can reside on the arms. Moreover, in some embodiments, the elongated body  2312  can also include ports or graft windows for receiving bone graft material therein. 
       FIGS. 45A-45C  show an expandable spacer having a rack and pinion actuator in accordance with some embodiments. The spacer  2510  comprises two or more linking members  2512  that are joined together at joints or hinges. In some embodiments, the spacer  2510  can include a rack and pinion actuator that allows the spacer to be pulled in the direction  2518 . The rack and pinion actuator advantageously allows the spacer to expand incrementally, thereby allowing a surgeon to control the shape of the spacer within different types of patients. In some embodiments, the rack and pinion spacer will be controlled to sit on an apophyseal ring of the patient, thereby providing desirable load distribution when in use. In some embodiments, the spacer  2510  can include a graft window  2517  that can receive graft material therein. 
       FIGS. 46A-46C  show an expandable spacer having an outer member with a slidable inner member therein. The spacer  2610  comprises an outer member  2612  including an inner member  2614  capable of sliding in and out of the outer member  2612 . As shown in  FIG. 47A , the spacer  2610  can have a first, unexpanded configuration whereby the inner member  2614  is substantially within the body of the outer member  2612 . After being inserted into a disc space, the inner member  2614  can be slid outward from the outer member  2612 , thereby causing expansion of the spacer  2610  and a greater footprint. 
       FIG. 46C  shows a side view of the expandable spacer and a mechanism for sliding the inner member  2614  out of the outer member  2612  according to some embodiments. In some embodiments, in order to slide the inner member  2614  in and out of the outer member  2612 , the inner member  2614  can include pin members  2624  that ride in slots  2622  formed in the outer member  2612 , until a desired expansion of the inner member  2614  is reached. In some embodiments, the pin members  2624  can be locked at any point along the length of the slots, such as by rotating the pin members  2624 . In other embodiments, the pin members  2624  have designated unlocking/locking points, located at designated parts of the slots  2622 . 
       FIGS. 47A and 47B  show an expandable spacer having upper and lower members separated by linking members in accordance with some embodiments. The expandable spacer  2710  comprises an upper member  2712  and a lower member  2714 .  FIG. 47A  shows the upper member  2712  and the lower member  2714  in a first initial configuration whereby the lower member  2714  is positioned near or adjacent to the upper member  2712 . To separate the lower member  2714  from the upper member  2712  and form a larger footprint, the lower member  2714  can be moved away from the upper member  2712  via linking members  2722  and  2724 . In some embodiments, the linking members  2722 ,  2724  can be moved by moving respective pins  2732 ,  2734  along slots  2742 ,  2744  formed in the upper member  2712 . In other embodiments, the lower member  2714  can be moved away from the upper member  2712  via a gear mechanism, such as a gear drive (e.g., a worm gear). 
       FIGS. 48A and 48B  show an expandable spacer comprising a worm gear in accordance with some embodiments. The expandable spacer  2810  comprises six linking members  2812  that can expand via a worm gear  2840 . The worm gear  2840  can be engaged by an instrument  2850 , such as a worm drive. Rotation of the instrument  2850  causes actuation of the worm gear  2840 , thereby causing expansion of the linking members  2812 . As shown in  FIG. 48B , the expandable spacer  2810  can be expanded such that it forms a ring member having a larger footprint than its initial configuration. In some embodiments, the worm gear  2840  can be built into the spacer  2810 , while in other embodiments, the worm gear  2840  can be removably attached to the spacer  2810 . 
       FIGS. 49A and 49B  show an expandable spacer having asymmetrical expansion in accordance with some embodiments. The spacer  2910  includes five different linking members  2912 ,  2913 ,  2914 ,  2915  and  2916  that are connected to one another via a joint or hinge. In some embodiments, the linking members can be connected to one another via a click fit.  FIG. 49A  shows the spacer  2910  in its initial, non-expanded configuration and attached to an instrument  2930 . The instrument  2930  can deliver the spacer  2910  into a disc space, whereby the spacer  2910  can be pulled in the direction  2922 , thereby causing expansion of the spacer  2910 , as shown in  FIG. 49B . Advantageously, expansion of the spacer  2910  can be asymmetrical to accommodate a desirable footprint within a disc space. 
     The described embodiments are capable of insertion into a disc space, and subsequent expansion. In some embodiments, the implants will be expanded into a desirable lordotic form. In some embodiments, the implants will be expanded such that the footprint is increased. The implants can be expanded such that they rest on an apophyseal ring of a patient. While the above descriptions describe numerous embodiments, one skilled in the art will appreciate that any of the embodiments discussed above are unique and novel features that may be combinable with one another. 
     While the invention herein disclosed has been described by means of specific embodiments and applications thereof, numerous modifications and variations can be made thereto by those skilled in the art without departing from the scope of the invention as set forth in the claims.
The present invention relates to devices and methods for treating one or more damaged, diseased, or traumatized portions of the spine, including intervertebral discs, to reduce or eliminate associated back pain. In one or more embodiments, the present invention relates to an expandable interbody spacer. The expandable interbody spacer may comprise a first jointed arm comprising a plurality of links pivotally coupled end to end. The expandable interbody spacer further may comprise a second jointed arm comprising a plurality of links pivotally coupled end to end. The first jointed arm and the second jointed arm may be interconnected at a proximal end of the expandable interbody spacer. The first jointed arm and the second jointed arm may be interconnected at a distal end of the expandable interbody spacer.