Patent Publication Number: US-2021186566-A1

Title: Minimally invasive screw extension assembly

Description:
RELATED APPLICATION INFORMATION 
     This application is a continuation of U.S. application Ser. No. 16/051,670, filed Aug. 1, 2018, which is a divisional of U.S. application Ser. No. 15/236,036, filed Aug. 12, 2016, now U.S. Pat. No. 10,064,662, the disclosure of each of which is incorporated by reference herein in its entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a minimally invasive screw extension assembly for seating a fixation rod into a spinal fixation rod receiving implant and securing the seated fixation rod as part of a vertebral corrective surgery. 
     BACKGROUND OF THE INVENTION 
     Spinal surgeons are required to implant a variety of rods, screws and plates into the bony skeletal structure of the spine to correct a variety of misalignments and repair damage that exist between the vertebral bodies. A particularly useful procedure involves the placement of rod receiving spinal implants with pedicle screws into the vertebrae. These rod receiving implants commonly have a slotted “U” shaped body with a pedicle screw extending from the base of the slotted body. When the surgeon implants these devices in the bones along the portion of the spine to be corrected, he must then connect two or more of these implants using fixation rods. The fixation rods are typically solid round cylindrical metal devices that can be straight or curved. The rods must be driven inwardly to be seated to fit between the “U” shaped slotted body. Once in a seated position, the rod can be fixed rigidly into the rod receiving implant by tightening a set screw into the threaded legs of the slotted body clamping the fixation rod securely to the slotted body. 
     One difficulty for the surgeon is aligning the fixation rods with the slotted rod receiving implants and moving the rod inwardly toward the slot. This is particularly difficult when the implants need to be positioned to correct a preexisting misalignment. This aspect of positioning the rod is called reduction or reducing and a variety of elongated tools have been developed to facilitate the proper placement of fixation rods. 
     The present invention, as described hereinafter, is an improved tool assembly that can easily be clipped onto a rod receiving spinal implant and reduce a fixation rod, and while clipped in place, deliver a set screw to fix the rod in a proper position to achieve the corrective spinal alignment and support for the particular surgery. 
     These and other objectives are achieved by the invention as described hereinafter. 
     SUMMARY OF THE INVENTION 
     A screw extension assembly for use in minimally invasive spinal surgery, the assembly has an inner slotted shaft and an outer shaft, a rod reducer and a removable nut. The combination when assembled is configured to move a spinal fixation rod into a slotted rod receiving spinal implant where it is seated and affixed thereto. The inner slotted shaft is an elongated hollow tubular shaft having a proximal end, a distal end and a slotted distal end portion having a pair of slots open through the distal end defining a pair of deflectable leg extensions. Each leg extension has a projection configured to engage a groove on an outer surface of a slotted rod receiving implant thereby coupling the inner shaft to the slotted rod receiving implant adjacent slots at the distal end of the leg extensions having a plurality longitudinal edges keyed to abut proximal walls of the slotted rod receiving implant to prevent rotation. The outer shaft is configured to receive and pass over the inner shaft. The outer shaft has a pair of slots open through a distal end with longitudinal rails adjacent edges of each slot of the outer shaft to slide along each edge of the slots of the leg extensions of the inner shaft along at least a portion of the slots. The outer shaft when moved inwardly toward the distal end of the inner shaft into an engaged position locks the deflectable leg extensions in a coupled position to the grooves of the slotted rod receiving implant. The outer shaft at the distal end has two or more prongs extending from the distal end, the two or more prongs when the outer shaft is moved engages an inner side of the slotted rod receiving implant in the engaged position. 
     A proximal portion adjacent the proximal end of the inner shaft has a threaded portion wherein the threads of the threaded portion have longitudinal extending grooves aligned to receive the longitudinal rails of the outer shaft. The grooves are aligned with the edges of the slots forming a keyed pathway allowing the longitudinal rails to slide relative to the inner shaft toward the distal end on assembly. 
     The screw extension assembly further has a locking knob rotationally coupled to a proximal end of the outer shaft, wherein the inner shaft has one or more cam grooves and the locking knob has a pin extending into and guided by said cam groove causing the outer shaft to translate longitudinally upon rotation of the locking knob relative to the inner shaft toward an engaged position locking the deflectable legs in the coupled position to the slotted rod receiving implant. The one or more cam grooves has a cam over pocket feature in a distal end of the cam groove, the cam over pocket locks the locking knob at or past the engaged position. The cam over pocket at the distal end of the cam groove rotationally moves the locking knob in the absence of forward longitudinal translation of the outer shaft to the inner shaft at the locked position. 
     On assembly of the inner and outer shafts, the pair of slots in the inner shaft and the pair of slots in the outer shaft are aligned to pass rods therethrough. The screw extension assembly further has the rod reducer. The rod reducer is a hollow tube with a distal end to push a rod received in the aligned slots of the assembly when the rod reducer is placed over the inner and outer shaft. The screw extension assembly further has the nut. The nut has female threads for engaging the threaded proximal end of the inner shaft and wherein the nut when tightened abuts a proximal end of the reducer and pushes the reducer inwardly to move a rod received in the aligned slots of the assembly into the slotted rod receiving implant. 
     In one embodiment, the threads of the proximal end of the inner shaft has a pitch sufficient to allow the nut to auto-rotate, solely due to the weight of the nut, to abut the proximal end of the reducer and further rotation pushes the reducer inwardly to seat a rod in a rod seating position. The nut is preferably removable and separate from the outer shaft, this allows the rod reducer to fit easily over the assembled inner and outer shafts. The outer shaft has flats in the proximal portion and the locking knob has flats configured to align with the flats of the outer shaft when the locking knob is rotated to a locked position allowing the reducer with complimentary internal flats to non-rotationally move relative to the assembly as the nut pushes the reducer. 
     A kit for use in minimally invasive spinal surgery can be assembled using these components. The kit would have a slotted rod receiving implant, the rod receiving implant having a proximal slotted rod holding element and a threaded bone fastener extending distal from the rod holding element; an inner slotted shaft, the inner slotted shaft being an elongated hollow tubular shaft having a proximal end, a distal end and a slotted distal end portion having a pair of slots open through the distal end defining a pair of deflectable leg extensions, each leg extension having a projection configured to engage a groove on an outer surface of the slotted rod receiving implant thereby coupling the inner shaft to the slotted rod receiving implant adjacent slots at the distal end of the leg extensions having a plurality longitudinal edges keyed to abut proximal walls of the slotted rod receiving implant to prevent rotation; an outer shaft configured to receive and pass over the inner shaft; the outer shaft having a pair of slots open through a distal end with longitudinal rails adjacent edges of each slot of the outer shaft to slide along each edge of the slots of the leg extensions of the inner shaft along at least a portion of the slots; and wherein the outer shaft when moved inwardly relative to translate toward the distal end of the inner shaft into an engaged position locks the deflectable legs in a coupled position to the grooves of the slotted rod receiving implant. 
     The kit may further include a threaded set screw for attachment to internal threads of the rod holding element configured to hold a rod when tightened and a fastener driver tool to pass through the inner shaft to fasten and tighten the set screw; a rod reducer, the rod reducer having a distal end for pushing a rod and an opposing proximal end. A proximal portion adjacent the proximal end of the inner shaft has a threaded portion and the kit further may include a nut for engaging the threads of the proximal end of the inner shaft and to abut at the proximal end of the rod reducer, the handle when tightened pushes a rod into a seated position in the rod receiving element. 
     The kit further may include a locking knob rotationally coupled to a proximal end of the outer shaft wherein the inner shaft has one or more cam grooves and the locking knob is pinned to said cam groove causing the outer shaft to translate longitudinally upon rotation of the locking knob relative to the inner shaft toward an engaged position locking the deflectable legs in the coupled position to the slotted rod receiving implant, wherein the one or more cam grooves has a cam over pocket feature in an end of the cam groove, the cam over pocket locks the locking knob at or past the engaged position and wherein the cam over pocket at the end of the cam groove rotationally moves the locking knob in the absence of forward longitudinal translation of the outer shaft to the inner shaft at the locked position. 
     The kit wherein the inner shaft and outer shaft have aligned slots to pass rods therethrough. The kit further has a nut having female threads for engaging the threaded proximal end of the inner shaft and wherein the nut when tightened pushes the reducer inwardly to move a rod into the slotted rod receiving implant wherein the nut is removable and separate from the outer shaft wherein the threads of the proximal end of the inner shaft have a pitch sufficient to allow the nut to auto-rotate, solely due to the weight of the nut, to abut the proximal end of the reducer and further rotation pushes the reducer inwardly to a rod seating position. The kit wherein the outer shaft has flats in the proximal portion and the locking knob has flats configured to align with the flats of the outer shaft when the locking knob is rotated to a locked position allowing the reducer with complimentary internal flats to non-rotationally move relative to the assembly as the nut pushes the reducer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described by way of example and with reference to the accompanying drawings in which: 
         FIG. 1  is a perspective view of the screw extension assembly of the present invention showing the deflectable leg extensions of the inner slotted shaft attached to a slotted rod receiving implant with the outer shaft not yet in the locked position. 
         FIG. 1A  is a more extreme perspective end view showing the inner shaft and rod receiving implant coupled and the outer shaft separated. 
         FIG. 2  is a side perspective view taken from  FIG. 1 . 
         FIG. 2A  is an enlarged view of the distal end of the inner shaft taken from  FIG. 2 . 
         FIG. 2B  is a further increased enlarged view of the inner shaft coupled to the rod holding element as the outer shaft translates distally. 
         FIG. 2C  is the view taken from  FIG. 2B  showing the outer shaft engaging internal portions of the rod holding element just prior to being fully engaged and locked. 
         FIG. 3  is a side perspective view showing the outer shaft in a fully engaged and locked position. 
         FIG. 3A  shows a partial cross-section of the rod holding element locked and held by the inner shaft. 
         FIG. 3B  shows the locking knob prior to being moved to a locked position. 
         FIG. 3C  shows the locking knob rotated 90 degrees toward a locked position. 
         FIG. 3D  shows a side view of the locking knob fully moved to the locked position. 
         FIGS. 4A, 4B and 4C  show the locking knob in phantom with a fixed pin in the one or more cam grooves, wherein the pin is shown moving in the cam groove in  FIG. 4B  as the knob is rotated to the engaged position of  FIG. 4C  to the fully locked position of  FIG. 4D . 
         FIG. 5  is an exploded view of the inner and outer shafts assembled and coupled to a rod receiving implant with a rod reducer tube being aligned to pass onto the assembly. 
         FIG. 6  shows the rod reducer onto the assembly. 
         FIG. 7  shows the rod reducer tube being pushed distally toward a rod seated position by the rotation of the abutting removable nut. 
         FIG. 8  is a perspective view of an exemplary set screw for fixing a rod. 
         FIG. 9  is a view of an exemplary set screw driver. 
         FIG. 10  is s view of an exemplary fixation rod. 
         FIG. 11  is a perspective view of an exemplary spine with two of the rod holding elements and bone screws prepositioned to have a fixation rod installed. 
         FIGS. 12A-12M  are views of the screw extension assembly and reducer being used to seat and fix a spinal fixation rod into the pair of prepositioned slotted rod receiving implants shown in  FIG. 11 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to  FIG. 1 , a screw extension assembly  10  is illustrated. The screw extension assembly  10  has an outer shaft  20  and an inner shaft  40 . As shown, at a distal end of the screw extension assembly  10  is a rod receiving implant  100 . The rod receiving implant  100  has a rod holding element  101  and a bone or pedicle screw  120  extending from the base of the rod holding element  101 . The rod holding element  101  is slotted and configured to receive a spinal rod. As shown in  FIGS. 1 and 1A , the inner shaft  40  has slots  44  defining deflectable leg extensions  47 ,  48 . The deflectable leg extensions  47 ,  48  have a distal end  45  configured to deflect and engage the rod holding element  101 . The rod holding element  101  has a groove  109  configured to engage a projection  49  on the distal end  45  of the deflectable legs  47 ,  48 , best shown in  FIG. 3A . 
     With further reference to  FIG. 1A , the outer shaft  20  has a proximal end with a rotatable locking knob  30  affixed thereto. The outer shaft  20  has a slotted opening  24  on each side defining legs  27 ,  28  on the outer shaft  20 . The leading edge of the outer shaft  20  has a chamfered end  26 . The chamfered end  26  along edges of the slotted openings  24  has rails  21  that project and extend along the length of the outer shaft  20 . These rails  21  are configured to fit in grooves  41  that run longitudinally relative the inner shaft  40 . The inner shaft  40  additionally has threads  42  at a proximal end and a groove  33  configured to receive a pin  32  as shown in  FIG. 2  on the rotatable locking knob  30 . 
     As shown in  FIG. 2 , when the outer shaft  20  is slid over the inner shaft  40 , the rails  21  engage the grooves  41  so that the two are coupled together in such a fashion that they are non-rotatable relative to the other. As shown in  FIG. 2 , as the distal end of the outer shaft  20  slides toward the distal end  45  of the inner shaft  40  the slots  44  and  24  start to overlap. With reference to  FIG. 2A , the rod holding element  101  has internal threads  102 . The deflectable legs  47 ,  48  at the distal end  45  are shown engaged with the rod holding element  101 . The outer shaft  20  with its chamfered end  26  is shown proximally spaced from the distal end  45  in  FIG. 2A . As the outer shaft  20  is moved forward, as shown in  FIG. 2B , the chamfered end  26  starts to engage the distal end  45  of the inner shaft  40 . As shown, the rod holding element  101  has a groove  109 , shown in  FIG. 2B , in which the distal end  45  has a projection  49  that engages this groove  109  clipping the deflectable legs  47 ,  48  onto the rod holding element  101 . The rod holding element  101  further has a cutaway section  106  which is encircled by the end  46  such that the projection  49  is locked between a portion of the rod holding element  101  on an outer surface and extends outwardly to where it is exposed as shown in  FIG. 2B . 
     As shown in  FIG. 2C , as the outer shaft  20  moves further inwardly into an engaged position. The chamfered end  26  covers that portion of the rod holding element  101  along an inner surface of the slot defining the rod holding element  101 . When this occurs, the rod holding element  101  is held externally by the deflectable legs  47 ,  48  and internally by the end  46  and also locked from rotation movement as the chamfered end  26  engages edges of the rod holding element  101 . 
     With reference to  FIG. 3 , the screw extension assembly  10  is shown in the fully engaged and locked position. To better appreciate how this occurs, a cross-sectional view,  FIG. 3A , is illustrated showing the rod receiving spinal implant  100  and rod holding element  101  fully locked into the distal end of the deflectable legs  47 ,  48  and locked rigidly into place by the outer shaft  20 . 
     With reference to  FIG. 3B , when the outer shaft  20  and inner shaft  40  are engaged with the spinal implant  100 , the knob  30  is positioned 90 degrees off its final locked position. A counter clockwise 90 degree rotation of the knob  30  will cause the assembly to lock into a locked position from this engaged positon shown in  FIG. 3B . 
       FIG. 3C  shows this rotation. When this rotation is accomplished, the spinal implant  100  is fully locked onto the screw extension assembly  10 . 
     With reference to  FIG. 3D , the fully engaged position is shown. When this occurs, the locking knob  30  has moved upwardly through a cam groove  33  guided by one or more pins  32 . 
     This rotation and movement is best seen in  FIGS. 4A-4C . In  FIG. 4A , the pin  32  is shown in an aft position with the locking knob  30  illustrated in phantom. As the knob  30  is being moved toward the distal end as it is being rotated counter clockwise,  FIG. 4B  shows the pin  32  following the groove  33 . Once the groove  33  has moved from the positon in  4 B, the pin  32  hits a section  34  of the groove  33  defined as a cam over projection  34  shown in  FIG. 4C . When this occurs, there is a noticeable locking of the knob  30  so it cannot be moved unless physically moved out of the cam over position. 
     Once the screw extension assembly  10  is fitted over the rod holding element  101 , a reducer tube  60  can be positioned over the assembly  10 , as shown in  FIGS. 5 and 6 . A separate nut  70  can be provided. The nut  70  abuts the proximal end of the tube  60 . The nut  70  can alternatively be captively held to the reducer tube  60 , but free to rotate relative to the reducer tube, if so desired. The tube  60  has a distal end  61  which has a cavity for engaging a spinal fixation rod and to reduce that rod when the reducer tube  60  is positioned over the assembly  10  as shown in  FIG. 6 . When this occurs, end  61  is positioned upwards of the slots  24 ,  44  which are coaligned. As the reducer  60  is moved forward by rotation of the nut  70 , the reducer tube  60  translates forward. As shown, the reducer tube  60  has flats  65  that align with the external flats  25  on the outer shaft  20 . These coaligned flats prevent the reducer tube  60  from rotating about the assembly and guide the reducer tube  60  down the shaft  20  as the nut  70  translates. Interestingly, the nut  70  is shown as a separate component, so that it can easily be removed leaving the reducer tube  60  in position, if so desired. The nut  70  has internal threads  72  that engage threads  42  of the inner shaft  40 . 
     With reference to  FIG. 8 , an exemplary set screw  80  is shown. The set screw has a driving recess  81  and external threads  82 . The external threads  82  are configured to engage internal threads  102  of the rod holding element  101 . 
     With reference to  FIG. 9 , an exemplary set screw driver  90  is shown with the handle removed. The set screw driver  90 , as shown, has a threaded end  92  for receiving a handle and a driving end  91  for engaging a set screw. 
     With reference to  FIG. 10 , an exemplary spinal fixation rod  12  is illustrated with a cylindrical, solid body designed for spinal fixation. 
     With reference to  FIG. 11 , a perspective view of a portion of a spine  2  is illustrated. The spine  2 , as shown, has a rod holding element  101  with a bone screw  120  embedded into a vertebrae. A pair of these assemblies are shown in adjacent vertebrae and are prepositioned to receive a spinal fixation rod. 
     With reference to  FIGS. 12A-12M , the fixation of the spinal rod using the screw extension assembly  10  for use in minimally invasive spinal surgery is illustrated showing a step by step procedure of using this assembly  10  of the present invention. 
     With reference to  FIG. 12A , the assembly  10  with the inner shaft  40  and outer shaft  20  is prepositioned to align with one of the rod holding elements  101 . With reference to  FIG. 12B , the assembly  10  is shown with the deflectable leg extensions  47 ,  48  positioned onto the groove  109  of the rod holding element  101 . To accomplish this, the surgeon just pushes down on the assembly  10  and the legs  47 ,  48  automatically deflect outwardly and the projections  49  will then engage the rod holding element groove or recess  109 . With reference to  FIG. 12C , two of the screw extension assemblies  10  are shown, one prepositioned on each of the rod holding elements  101 . As shown in  FIG. 12C , the outer shaft has been moved directly inwardly covered thereby encircling the distal end  45  of the leg extensions and the locking knob  30  has been rotated into the fully engaged position, as illustrated. With reference to  FIG. 12D , a rod delivery tool  600  is shown. The rod delivery tool  600  has an arm  602  and a shaft  604  adapted to receive and hold an end of a spinal fixation rod  12 . As shown, the fixation rod  12  is being aligned in  FIG. 12D  such that the slotted openings  24 ,  44  of the inner and outer shafts of each assembly  10  are aligned so as to receive the fixation rod  12 . With reference to  FIG. 12E , the surgeon will now have moved the rod delivery tool  600  in such a fashion that the fixation rod  12  is now positioned in the slotted openings  24 ,  44  of each assembly  10 . In this position, the rod  12  is positioned to be seated. With reference to  FIG. 12F , the reducer tube  60  can now be positioned over one of the assemblies  10  and the rod delivery tool  600  is still shown holding the rod  12  in position at this point. Once the reducer tube  60  is positioned over the assembly  10 , a removable nut  70  as shown in  FIG. 12G  is positioned above the threads  42  of the inner shaft  40 . The removable nut  70 , when tightened, as shown in  FIG. 12H , drives the reducer tube  60  distally pushing against the fixation rod  12 . When the nut  70  is fully turned and the fixation rod  12  is fully seated in the rod receiving implant  100 , then as shown in  FIG. 12I , a set screw  80  shown attached to a set screw driver  90  can be passed through the nut  70  and the assembly  10  inwardly towards the distal end to engage the internal threads  102  of the rod holding element  101 , as shown in  FIG. 12I . Once this is accomplished, the set screw  80  is securely holding the fixation rod  12  in position, as shown in  FIG. 12J . When this occurs, as shown in  FIGS. 12K and 12L , a second set screw  80  can be attached to the driver  90  and it can be delivered to the second assembly  10  to fix the opposite end of the fixation rod  12  into the second rod receiving implant  100 . It is noted that the reducer tube  60  has not been used on the second rod holding element  101  because it is believed that seating the rod  12  in the first rod holding element  101  will deliver and hold the rod  12  tightly and securely in position sufficiently low that the set screw  80  can engage the threads  102  of the second rod holding element  101 . If however, it is determined that the rod  12  needs to be reduced on the second rod receiving implant  100 , the reducer tube  60  and nut  70  can be employed as previously discussed in  12 F- 12 H. This however, should not be necessary when the rod  12  is securely positioned in one of the rod receiving implants  100  as it should have lowered the entire rod  100  into seated positions in both implants  100 . Once the set screw  80  is delivered to the second assembly  10  and tightened and engaged to the internal threads  102  of the rod holding element  101 , the set screw driver  90  can be removed. Once the set screw driver  90  has been removed, similarly the nut  70  can be removed from the threaded portion of the inner shaft  40  and the reducer tube  60  can be removed. When the reducer tube  60  is removed, the locking knob  30  can be rotated and the outer shaft  20  can be pulled back distally, this releases the deflectable legs  47 ,  48  of the inner shaft  40  from the locked position and at this point with the outer shaft  20  pulled proximally away from the distal end  45 , the deflectable legs  47 ,  48  can be simply deflected off of the fixed rod  12  and spinal rod receiving implant  100 , as shown in  FIG. 12M . Finally, not illustrated, the second assembly  10  can be similarly removed simply by turning the locking knob  30  into the unlocked position and proximally moving the outer shaft  20  to disengage from the locking positon so the deflectable legs  47 ,  48  of the inner shaft  40  of the second assembly  10  can be freely deflected to disengage the second spinal rod receiving implant  100 . Once this is accomplished, the assembly of the fixation rod  12  will be complete between the two spinal rod receiving implants  100 . 
     Variations in the present invention are possible in light of the description of it as provided herein. While certain representative embodiments and details have been shown for the purpose of illustrating the subject invention, it will be apparent to those skilled in this art that various changes and modifications can be made therein without departing from the scope of the subject invention. It is, therefore, to be understood that changes can be made in the particular embodiments described, which will be within the full intended scope of the invention as defined by the following appended claims.