Abstract:
An expandable spinal fusion implant comprising first and second endplates coupled to an expansion member that sits within a housing. The expansion member is translated by a drive mechanism, whereby translation of the expansion member by the drive mechanism in a distal and proximal directions causes the distance between the endplates to increase and decrease, respectively.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of the filing date of U.S. Provisional Application No. 61/826,299, which was filed on May 22, 2013. The contents of U.S. Provisional Application No. 61/826,299 are incorporated by reference in their entirety as part of this application. 
    
    
     BACKGROUND 
     This application relates to an expandable spinal fusion implant for use in spinal surgery. 
     SUMMARY 
     To reduce risk of neural injury, the device will have the ability to be implanted to an intervertebral disc space in a collapsed state and expanded to a desired height. Expansion will be accomplished by translating an expansion mechanism mated to the inferior and superior endplates. In addition a large aperture at the proximal end of the device allows for post packing of bone graft material into the hollow interior of the device, which is in communication with a fusion aperture in each of the superior and inferior endplates. In order to have the large through aperture at the proximal end of the device, the drive mechanism is offset from the width centerline of the device. 
     The device includes a housing, expansion mechanism, support rails, superior endplate, inferior endplate, endplate retainer, endplate safety retainer, drive mechanism, and drive mechanism retainer. 
     The expansion mechanism rides on rails that are retained partially in both the housing and expansion mechanism. There is one rail on each of the two lateral sides of the device. The expansion mechanism has ramps that are on the superior and inferior sides at both the distal and proximal ends. The ramps on the superior side mate with the superior endplate and the ramps on the inferior side mate with the inferior endplate. The expansion member includes a hollow interior for receiving bone graft material and for allowing bone growth therethrough. The hollow interior of the expansion mechanism is in communication with fusion apertures in each of the superior and inferior endplates. 
     To achieve expansion and contraction the endplates must be fixed in the longitudinal direction during translation of the expansion mechanism. An endplate retainer housed within the distal end of the housing mates with both the superior and inferior endplates and prohibits translation of the endplates, but allows for expansion. 
     The expansion mechanism is translated by advancing the drive mechanism, which is retained within the proximal end of the housing and offset from the width centerline. This offset allows for the large through cannula and post packing of bone graft material. The drive mechanism is mated to the expansion mechanism with the drive mechanism retainer. Advancement of the drive mechanism toward the distal end of the device allows the endplates to expand, while the withdrawal of the drive mechanism toward the proximal end of the device results in contraction of the endplates. 
     An endplate safety retainer located in the expansion mechanism prohibits the removal of the superior and inferior endplates from the expansion mechanism. Superior and inferior in flat and lordotic configurations are contemplated for use with the device described herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an exemplary embodiment of an expandable spinal fusion implant in its collapsed state; 
         FIG. 2  is a perspective view of the expandable spinal fusion implant of  FIG. 1  in its expanded state; 
         FIG. 3  is a top view of the expandable spinal fusion implant of  FIG. 1 ; 
         FIG. 4  is a side view of the expandable spinal fusion implant of  FIG. 1  in its collapsed state; 
         FIG. 5  is a leading end view of the expandable spinal fusion implant of  FIG. 1  in its collapsed state; 
         FIG. 6  is a trailing end view of the expandable spinal fusion implant of  FIG. 1  in its collapsed state; 
         FIG. 7  is an exploded view of the expandable spinal fusion implant; 
         FIG. 8  is a cross sectional view of the leading end of expandable spinal fusion implant in its expanded state; 
         FIG. 9  is a cross sectional view of the trailing end of the expandable spinal fusion implant in its collapsed state; 
         FIG. 10  is a cross sectional view of the expandable spinal fusion implant; 
         FIG. 11  is a cross sectional view of the expandable spinal fusion implant; 
         FIG. 12  is a perspective view of the expandable spinal fusion implant after insertion into the disc space; and 
         FIG. 13  is a perspective view of the expandable spinal fusion implant in its fully expanded state in the disc space. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1-13  illustrate an expandable spinal fusion implant for use during spinal surgery for implantation to an intervertebral disc space. According to an exemplary embodiment, the device is dimensioned for posterior approach surgery, e.g. posterior lumber interbody fusion (PLIF) and transforaminal lumbar interbody fusion (TLIF) approaches. However, according to an alternative embodiment, the device may also be dimensioned for use in a lateral approach to the anterior column of the spine. To reduce the risk of neural injury, the expandable spinal fusion implant has the ability to be implanted in a collapsed state (see  FIG. 1 ) and expand to a height determined by the user (see  FIG. 2 ). Expansion is accomplished by translating a wedge shaped expansion mechanism that is mated to the inferior and superior endplates  14 ,  12 . As the expansion mechanism  26  is advanced towards the distal or leading end  16  of the implant  10  the endplates expand in height. To reduce the height of the implant or return the endplates back to their start position the expansion mechanism is advanced towards the proximal end of the device. In addition a large cannula at the trailing or proximal end of the device allows for post packing of hone graft material, i.e. filling the interior of the device with bone graft after the device has been inserted into the intervertebral space and expanded to the desired height. The ability to post pack improves the chances of a successful surgical outcome by allowing for insertion of a sufficient amount of bone graft in adequate contact with the vertebral body endplates adjacent the disc space to promote bone growth. 
     As shown in  FIGS. 1-13 , the expandable spinal fusion implant  10  has a top endplate  12  and a bottom endplate  14 . The endplates  12 ,  14  have substantially identical features as will be further described. Each endplate has a hone contacting surface  46  and an interior surface  48 . As shown in the exemplary embodiment, the bone contacting surfaces  46  may have anti-migration features  44 . The interior surfaces  48  of the endplates  12 ,  14  have ramped portions  36  that correspond to the angles of the ramps  34 ,  35  on the expansion mechanism  26 . The ramped portions  36  of the interior endplates also include a male dovetail feature  40  that mates with the female dovetail feature  38  on the ramps  34  of the expansion mechanism  26 . Each endplate  12 ,  14  has a central fusion aperture  38  to allow for bone growth through the implant  10  and with the endplates of the adjacent vertebral bodies. In order for each endplate  12 ,  14  to expand it must remain stationary in the longitudinal axis as the expansion mechanism  26  translates both proximally and distally. Both endplates  12 ,  14  further include a distal extension  70  to aid in retaining the endplates within the housing  20 . While the implant  10  according to an exemplary embodiment in  FIGS. 1-13  is shown with flat endplates, endplates having built in lordosis, i.e. having a distal height extending from the bone contacting surface to the interior surface that is greater than the proximal height, are also contemplated. 
     The expandable spinal fusion implant  10  includes an expansion mechanism  26  located between the top and bottom endplates  12 ,  14 . The expansion mechanism has two wedge portions  50 , each of which as a superior ramp  34  and an inferior ramp  35  that correspond to and mate with the ramped portions  36 ,  37  of the superior and inferior endplates, respectively. Each endplate  12 ,  14  mates to the expansion mechanism  26  by an undercut or dovetail connection, at both the proximal end and the distal end, that allows movement between the wedge  50  and the endplate  12 ,  14 . Each of the superior ramps  34  and inferior ramps  35  include a female dovetail feature  38  that mates with the male dovetail features  36  on the endplates  12 ,  14 . An endplate safety retainer is housed within the expansion mechanism to prohibit removal of the endplates once assembled. The expansion mechanism  26  has a recess  56  at its proximal end dimensioned to receive the drive mechanism retainer  24  therein. The expansion mechanism  26  has a hollow interior defining a central fusion aperture  39  that aligns with the central fusion aperture  38  of the top and bottom endplates  12 ,  14  to allow for bone growth therethrough. The distal wedge  50  of the expansion mechanism  26  includes an endplate safety retainer  32  extending therethrough to prevent the dislocation of the endplates  12 ,  14  from the expansion mechanism  26 . 
     As best shown in  FIG. 7 , the expandable spinal fusion implant  10  also includes a housing  20  dimensioned to house the expansion mechanism  26 . The expansion mechanism  26  is supported in the housing  20  by two support rails  60 . The housing  20  is defined by opposing lateral walls  21 , a distal wall  23  and a proximal wall  25 . The housing  20  has a longitudinal length that exceeds the longitudinal length of the endplates  12 ,  14 . The distal wall  23  of the housing is tapered to aid in insertion of the implant  10 . The distal wall  23  also includes recesses  58  for receiving the distal extensions  50  of the endplates  12 ,  14  to retain the endplates with in the housing  20 . As seen in  FIG. 6 , the proximal wall  25  of the housing  20  includes a cannula  52  for receiving bone graft material into the central fusion aperture  39  of the expansion mechanism  26  as well as a threaded drive mechanism aperture  54  for receiving the drive mechanism  22  therethrough. 
     According to the exemplary embodiment, the drive mechanism  22  has a head  62  at its proximal end for engaging an actuator tool (not shown) and a threaded shaft  64  extending from the head  62  and terminating at the distal end with a drive mechanism retainer  24  configured to anchor the drive mechanism  22  to the expansion mechanism  26 . The purpose of the drive mechanism  22  is to translate the expansion mechanism  26  both proximally and distally. The threaded shaft  64  of the drive mechanism  22  engages with the threaded aperture  54  of the housing  20  at the proximal end  25  and also mates with the recess  56  at the proximal end of the expansion mechanism  26  and is retained with the expansion mechanism  26  by a drive mechanism retainer  24 . As best seen in  FIG. 6 , the drive mechanism  22  is located at a position within the implant  10  that is offset from the central longitudinal axis of the implant  10  to allow for post packing of bone graft through the cannula  52  and into the central fusion aperture  39 . 
     According to the exemplary embodiment, the expandable spinal fusion implant  10  is implanted into a patient by first accessing the desired intervertebral disc space via lateral approach to the anterior spinal column or a posterior (e.g. PLIF or TLIF) approach. The implant  10  is inserted in its collapsed state into the intervertebral disc space and maneuvered into a desired position. Once the desired position is reached, a tool is engaged with the drive mechanism  22  to turn the drive mechanism  22  and thereby urge the expansion mechanism  26  in the distal direction and consequently increase the distance between the top and bottom endplates  12 ,  14 . The drive mechanism  22  can then be turned in the opposite direction to urge the expansion mechanism  26  in the proximal direction in order to decrease the distance between the endplates  12 ,  14  if necessary. Once the implant  10  has been set at the desired height, bone graft can be introduced through the cannula  52  in the proximal end  25  of the housing  20  to the interior of the implant  10 , into the central fusion apertures  38 ,  39  of the expansion mechanism  26  and endplates  12 ,  14 .