Abstract:
A bone screw having a head and a shank. The shank having a ball thereon and the head having a socket for receiving the ball. The ball being externally threaded and the chamber communicating with the exterior of the head through a threaded bore. The ball being threadably received through the bore and being swivelable in the chamber after passing through the bore. The bone screw including at least one set screw that operably engages the ball to lock the head in position relative to the shank.

Description:
BACKGROUND OF THE INVENTION 
   The present invention is directed to a bone screw of the type wherein a head of the bone screw is swingable or can swivel about the shank of the bone screw until the surgeon is satisfied with the relative placement of the two parts and thereafter the head can be locked in position relative to the shank. 
   Bone screws are utilized in many types of spinal surgery in order to secure various implants to vertebrae along the spinal column. Bone screws of this type typically have a shank that is threaded and adapted to be implanted into a vertebral body of a vertebrae. The bone screw also includes a head which is designed to extend beyond the vertebrae and which has a channel to receive another implant. Typically the channel will receive a rod or a rod-like member. In bone screws of this type, the head may be open, in which case a closure must be used to close between opposite sides of the head once a rod-like implant is placed therein, or closed wherein a rod-like implant is threaded through the head of a bone screw. 
   When the head and shank of the bone screw are fixed in position relative to each other, it is not always possible to insert a bone screw in such a manner that the head will be in the best position for receiving other implants. Consequently, swivel head bone screws have been designed that allow the head of the bone screw to rotate or swivel about an upper end of the shank of the bone screw while the surgeon is positioning other implants and finding the best position for the bone screw head. However, once the surgeon has determined that the head is in the best position, it is then necessary to lock or fix the head relative to the shank and different types of structures have been previously developed for this purpose. Unfortunately, the prior art devices have a tendency to be bulky, slip under high loading or require many parts. 
   It is desirable to have a swivel head bone screw that can be captured by the shank prior to locking of the head, but that allows the head to freely swivel or pivot about a top of the shank prior to locking. It is then further desirable to have the head that can be fixably locked in a configuration or position relative to the shank where the head best fits with other elements of the overall spinal implant. 
   Furthermore, it is desirable to maintain the number of parts of the device at a minimum. Also, it is desirable to secure the various parts together in such a way, so that, if parts become loose under use for some reason, the device will not totally disassemble. 
   SUMMARY OF THE INVENTION 
   A bone screw for use in conjunction with spinal surgery and, in particular, for implanting into a bone and securing other medical implants to the bone. The bone screw includes a head and a shank. 
   The shank has a lower portion which is threaded and sized and shaped to be operably screwed into a vertebral body in the spine of a patient. The end of the shank opposite the threaded lower portion includes a ball having a partial external thread thereon that is coaxial with an axis of rotation of the shank. The top of the shank includes a bore, preferably having a hexagonal cross-section or the like that is adaptable to a tool such as an Allen wrench for driving the shank into the bone. 
   The head of the bone screw may be either an open or a closed type. Both types have a central channel that passes at least partially through the head. In the open type, the channel is open all the way to the top of the head and is closed after the placement of another implant into the channel by a closure. Where the head is closed, the channel that receives the implant is open on both ends, but encircled by the head. 
   The base of the head includes a chamber which is sized and shaped to fairly snugly receive the ball of the shank. In this manner the chamber and top of the shank function as a ball and socket arrangement that allows swiveling of the head relative to the shank until the head is locked relative to the shank. The chamber is connected to an exterior of the head by a threaded bore that is sized and shaped to threadably receive the ball from the shank with a minimum number of full rotations of the thread. That is, there are relatively few full 360° turns of the thread on the ball such that the ball very quickly can be screwed through the bore and into the chamber. Once within the chamber, the thread on the ball becomes disengaged from the thread on the bore and the ball is able to freely rotate within the chamber, until locked. 
   At least one additional bore extends from an exterior of the head so as to intersect with the chamber. The second bore is aligned to intersect with the ball and receives a first set screw, when the ball is in the chamber or socket, that preferably engages the ball somewhat tangentially or at an offset angle with respect to the radius of the ball. The first set screw is also designed to engage the thread on the ball and to urge the ball against the sidewall of the chamber so as to lock the ball in position relative to the head. This in turn locks the shank in a fixed position or configuration relative to the head. 
   Preferably, a third bore and second set screw are located opposite the second bore and first set screw and the second set screw also operably engages the ball in a non-radial manner so as to likewise urge the ball against the wall of the chamber and to fix the ball in position relative to the head. Each set screw also preferably includes a tip which is partially deformable upon engaging the ball so as to help secure the ball in position and to enlarge and resist inappropriate disengagement of the set screw from the bore within which it is received. Also, each of the set screws preferably includes a breakaway installation head that is designed to break off at a preselected torque so as to set the set screws at a desired uniform torque with each installation. 
   OBJECTS AND ADVANTAGES OF THE INVENTION 
   Therefore, the objects of the present invention are: to provide a bone screw for implantation into a vertebra of a patient wherein the head of the bone screw is swingable or swivelable about an end of a shank of the bone screw until a desired configuration is obtained after which the head is lockable in position relative to the shank; to provide such a bone screw wherein the shank includes a ball at one end thereof that is captured by the head, yet allows the head to remain swivelable after capture; to provide such a bone screw wherein the shank ball is threaded exteriorly and wherein the head includes a chamber that is spaced from the exterior by a threaded bore that threadably receives the head and allows the head to pass from the exterior by screwing into the chamber after which the threads of the ball and bore disengage so that the ball is free to rotate and swing about within the chamber; to provide such a bone screw wherein a second bore is provided that extends from the exterior of the head to intersect with the chamber and receive a set screw therein that abuts against the ball when the set screw is advanced within the second bore; to provide such a set screw wherein the set screw engages the thread of the ball in a non-radial and somewhat tangential manner so as to urge the ball against the sidewall of the chamber and to lock the ball in position so that the shank is in turn locked in position relative to the head; to provide such a bone screw wherein the set screw includes a tip that expands upon engaging the ball to better resist relative movement of the ball and to also resist unplanned withdrawal of the set screw from the second bore due to vibration or the like; to provide such a bone screw swivel head which is readily adaptable for both open-headed and closed-headed bone screws; to provide such a bone screw wherein a single size head may be utilized in conjunction with various shank lengths such that fewer sizes of comparatively expensive bone screw heads are necessary to be kept in inventory; and to provide such a bone screw which is relatively easy to use and especially well suited for the intended usage thereof. 
   Other objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention. 
   The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof. 

   
     BRIEF DESCRIPTION OF THE INVENTION 
       FIG. 1  is a perspective view of a bone screw in accordance with the present invention having a shank, a head and a closure therefor. 
       FIG. 2  is an enlarged and fragmentary cross-sectional view of the screw, taken along line  2 — 2  of  FIG. 1 , illustrating a step in the joining of the head to the shank. 
       FIG. 3  is an enlarged and fragmentary cross-sectional view of the bone screw, similar to  FIG. 2 , showing a ball of the shank being captured in a socket of the head. 
       FIG. 4  is a fragmentary and enlarged partial cross-sectional view of the bone screw shown in a vertebrae, similar to  FIG. 2 , with a rod captured in a channel of the head and by the closure and with a pair of set screws with breakaway heads being positioned to lock the shank relative to the head with one of the set screw heads shown as having broken away. 
       FIG. 5  is an enlarged and cross-sectional view of the bone screw, taken along line  5 — 5  of  FIG. 1 . 
       FIG. 6  is a top plan view of the bone screw located in a vertebra that is shown in cross section with the head of the bone screw receiving the rod and being locked in position relative to the shank. 
       FIG. 7  is a side elevational view of a pair of bone screws supporting a rod and positioned in vertebrae which are illustrated in the phantom. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which may be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. 
   The reference numeral  1  generally indicates a bone screw in accordance with the present invention. The bone screw  1  captures a rod  5  and is implanted in a vertebra  7  of a patient&#39;s spine  8 . The bone screw  1  includes a shank  10 , a head  11 , a closure  12  for the head and a pair of locking set screws  13  and  14 . 
   The shank  10  is sized and shaped to be screwed into one of the vertebra  7 . The shank  10  includes an external thread  19  that extends from an outward tip  20  to near a top  21  thereof. The top  21  includes an upper portion or ball shaped structure  22  that is truncated on a top  23  and bottom  24  thereof and has an external thread  25 . The ball shaped structure  22  has an external surface that is curved in vertical cross-section in the region of the thread  25 , whereas the thread  25  is cut on a cylindrical basis such that a depth associated with the thread  25  varies significantly vertically over the structure  22 . In particular, the thread  25  is deepest and extends out the furthest at the most laterally extending portions of the ball shaped structure  22  from an axis of rotation A of the shank  10 . 
   A bore  27  extends coaxially inward from the top of the ball shaped structure  22  and has a generally hexagonal cross-section in a plane perpendicular to the axis of rotation A of the shank  10 . The bore  27  is sized and shaped to receive a conventional installation tool (not shown), such as an Allen wrench or the like for driving the shank  10  into the vertebra  7 . 
   The head  11  is generally U-shaped having a body  30  with a pair of spaced but generally parallel arms  31  and  32  that form a channel  33  therebetween. Inside facing surfaces  36  and  37  of arms  31  and  32  respectively are threaded with discontinuous, but helically wound thread portions  39  and  40  respectively. The channel  33  has a lower seat  42  which is designed and shaped to snugly receive the rod  5  during use. 
   The head body  30  includes a chamber  45 . The chamber  45  is sized and shaped to fairly snugly, but slidingly, receive the shank ball shaped structure  22  therein. The chamber  45  communicates with and opens onto a body lower end  46  through a bore  47  that is generally circular in cross-section and includes an internal thread  48 . The thread  48  is sized and shaped to matingly receive the ball shaped structure thread  25  as the shank  10  is rotated. In this manner the ball shaped structure  22  is threadedly received through the bore  47  by rotation of the shank  10  until it passes through the bore  47  and into the chamber  45  upon which occurrence the threads  25  and  48  disengage. In this manner the inwardly extending thread  48  prevents the ball shaped structure  22  from being pulled from the chamber  45 , unless it is unscrewed therefrom again through the bore  47 , but allows the ball shaped structure  22  to freely rotate in the chamber  45  until locked in place as described below. In this manner the head  11  is able to swivel or swing about the upper end or top  21  of the shank  10  until subsequently locked in place. 
   An upper end of the chamber  45  also communicates with and opens into the channel  33  through an aperture  50 . The aperture  50  is positioned to be above the ball shaped structure  22  and especially the hex bore  27  when the ball shaped structure  22  is in the chamber  45  to allow access of a tool to the bore  27  for purposes of removal of the bone screw  1  from the vertebra  7 . 
   The head  11  also includes a pair of threaded bores  53  and  54  which extend inward from opposite outer surfaces of the arms  31  and  32  and intersect with the chamber  45  on opposite sides thereof respectively. The bores  53  and  54  are preferably at substantial angles with respect to the horizontal, such as is seen in  FIGS. 3 and 4  and also are angled to intersect with the chamber  45  so as to intersect with the ball shaped structure  22  slightly inward from a tangent thereof, when the ball shaped structure  22  is in the chamber  45 , as is seen in  FIG. 5 . 
   Threadably mounted in the bores  53  and  54  are the set screws  13  and  14  respectively, as is seen in  FIG. 4 . Each of the set screws  13  and  14  have a body  57  with an outer threaded surface  58  and a tip  59 . The tip  59  is elongate and generally triangular or trapezoidal in shape with a blunted or rounded distal surface. The tip  59  is designed to somewhat deform upon compression as will be discussed below. 
   Each of the set screws  13  and  14  also include a head  61  having an outer multifaceted surface  62  that is sized and shaped to receive a conventional socket type tool (not shown) for driving and rotating the head  61 . The head  61  is connected to the body  57  by a breakaway region  63 . As can be seen in  FIG. 4 , the set screw  14  on the right hand side has been torqued to a preselected torque and the head  61  has broken from the body  57 , leaving the body  57  in place in the bore  54 . Although a pair of set screws  13  and  14  are illustrated as being used herein with the present invention, it is foreseen that only a single set screw or alternatively a large number of set screws of the type of  13  and  14  could be utilized in conjunction with the invention. 
   The closure  12  has a generally cylindrically shaped body  66  with a radially outward threaded surface  67 . A thread  68  on the surface  67  is designed to matingly engage the thread portions  39  and  40  on the arms  31  and  32  respectively. The body also includes a lower surface  69  and an upper surface  70 . The lower surface  69  operably engages the rod  5  when the closure  12  is screwed between the arms  31  and  32 . 
   The body  66  also originally is secured to an installation or driving head  72 , seen in  FIG. 1 . The driving head  72  has a multifaceted polyhedral cross-sectioned outer surface  74  that is sized and shaped to receive a conventional socket type driving tool (not shown) during installation of the closure  12  into the head  11 . The driving head  72  is secured to the body  66  by a breakaway region  75 , such that the driving head  72  breaks from the body  66  when a preselected torque is applied to the closure head  72  and only the body  66  remains in the bone screw head  11  thereafter, as is seen in  FIG. 4 . The body  66  also includes four slots  78  that extend radially inward from the outer surface  58  and intersect with the upper surface  70 . The slots  78  are mateable with a tool (not shown) subsequent to installation of the closure  12  in the head  11  to allow counter-rotation of the closure  12  for purposes of removal, should such be necessary. 
   While a head  11  of the type that is generally referred to as an open headed bone screw head and that requires the closure  12  is illustrated herein, it is also seen that bone screw heads of the closed type may also be utilized in conjunction with the invention. A closed headed bone screw utilizes a channel that opens on two opposite sides of the head and does not open outwardly at the top, so that the head encircles a rod. That is, the head would be circular or oval shaped, as opposed to being U-shaped as in the illustrated embodiment. In such situations the rod  5  must be threaded through the channel in the closed head. Normally, a closed head would also incorporate a set screw that would come inwardly from the top and engage the rod  5 , so as to lock the rod  5  in place relative to the head. 
   Further, although a closure  12  of a particular type is shown herein, it is foreseen that various types of closures such as closures having retained heads for driving or removal may be utilized in conjunction with the invention. The closure  12  can also include a central bore that may be utilized for both attachment of a tool to be used during installation and/or for insertion of a set screw that is sized and shaped to abut against the rod  5  during use for locking the rod  5  in position relative to the head  11 . 
   In use the shank  10  is initially screwed into a vertebra  7 . The head  11  is then positioned such as is shown in  FIG. 2  and rotated so that the ball shaped structure thread  25  engages and mates with the bore thread  48  to allow the ball shaped structure  22  to pass through the bore  47 , as is seen in  FIG. 3 . In  FIG. 3  the threads  25  and  48  have disengaged and the ball shaped structure  22  is free to rotate or swivel relative to the chamber  45  so that the head  11  can swing or swivel on the top end of the shank  10 . 
   Once the surgeon has aligned and positioned the head  11  relative to the shank  10 , normally by insertion of a rod  5  and closure  12  for capturing the rod  5 , the set screws  13  and  14  are inserted in the bores  53  and  54  and torqued until the set screw heads  61  break therefrom. The set screw tips  59  are in this manner driven into the ball shaped structure  22 , such as is shown in  FIG. 4 . The tips  59  preferably somewhat deform as they engage the ball shaped structure  22  so as to resist against inadvertent removal of the set screws  13  and  14  from their respective bores  53  and  54  and to very tightly engage the ball shaped structure  22 . Preferably the tips  59  engage the ball shaped structure in the region of the thread  25  so as to use the thread  25  to help lock the shank  10  in position relative to the head  11 . Preferably, the ball shaped structure  22  is urged by the set screws  13  and  14  into tight engagement with the chamber  45  and/or locked between the set screws  13  and  14  themselves so as to lock the ball shaped structure  22  against swiveling, swinging or rotation relative to the head  11 . Before or after the head  12  is locked in place and after the surgeon is satisfied with the position of the rod  5 , the closure  12  is tightened by rotation thereof through the closure driving head  72  until the driving head  72  breaks away from the body  66  at a preselected torque (see  FIG. 4  which shows the body  66  without the driving head  72 ). 
   If it is necessary for some reason to remove part or all of the bone screw  1 , the closure  12  may be removed by use of a projecting pin tool mating with and received in the slots  78  to allow counterclockwise rotation of the closure  12 . Once the closure body  66  is removed, the rod  5  may be removed or readjusted. If it is necessary the bone screw may then also be removed by use of an Allen-type wrench inserted in the shank bore  27  that is exposed through the aperture  50  and the channel  33  and which is rotated counterclockwise to remove the shank  10  from the vertebra  7 . 
   It is to be understood that while certain forms of the present invention have been illustrated and described herein, it is not to be limited to the specific forms or arrangement of parts described and shown.