Abstract:
A device for connecting a longitudinal support to a bone anchor having a rounded head includes a body defining a chamber for receiving the rounded head of the bone anchor and a first channel for receiving the longitudinal support. Further, a first sleeve is slidable over the body for compressing the chamber, a second sleeve is slidable over the body for biasing the longitudinal support against the first sleeve, and a fastener is operatively associated with the body for biasing the second sleeve toward the first sleeve. The forces exerted on the second sleeve by the fastener are transferred to the first sleeve in a plane perpendicular to the central axis. Preferably, the longitudinal support contacts the first sleeve at first and second contact points or zones and one of the sleeves includes at least one extended portion for contacting the other sleeve at at least one additional contact point or zone.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    This application is a continuation of the U.S. National Phase designation of co-pending International Patent Application No. PCT/CH98/00415, filed Sep.29, 1998, the content of which is expressly incorporated herein by reference thereto.  
         FIELD OF THE INVENTION  
         [0002]    The present invention relates to spinal fixation systems and, in particular, to a device for connecting a longitudinal support to a bone anchor.  
         BACKGROUND OF THE INVENTION  
         [0003]    Several devices for connecting pedicle screws to longitudinal supports for purposes of vertebral fixation are known in the art. These devices offer the advantage that the individual pedicle screws can be affixed to and removed from the longitudinal support at any time without requiring disassembly of the entire fixation system. In addition, the locked relationship between the head of the pedicle screw and the connection device can be maintained while adjusting the position of the connection device relative to the longitudinal support.  
           [0004]    One such connection device is disclosed in U.S. Pat. No. 5,584,834 to Errico et al. The Errico patent discloses a coupling element that provides a simple connection between a pedicle screw and a longitudinal support rod while maintaining considerable angular freedom between the two components. Essentially, the coupling element consists of a generally cylindrical body defining an interior chamber for receiving a head of a screw at its lower portion and an external thread on its upper portion. Slots are formed in the external surface of the lower portion so that the interior chamber may resiliently expand to receive the screw head. The intermediate portion of the cylindrical body includes a side receiving channel for receiving the longitudinal support. A locking collar and a securing sleeve both slide over the cylindrical body and a locking nut is threaded on the external thread of the cylindrical body. The longitudinal support may be inserted through the side receiving channel between the locking collar and the securing sleeve. To lock the coupling element, the locking nut is tightened to provide a downward force on the securing sleeve, longitudinal support rod, and ultimately the locking collar, which compresses the interior chamber and thereby locks the position of the screw.  
           [0005]    One disadvantage of the coupling element disclosed in the Errico patent is that the cylindrical body is bent upward when the locking nut is being tightened because of the geometry of the side receiving channel. Consequently, the tightening force applied to the longitudinal support is insufficient and the cylindrical body may elastically deform or the locking collar may warp because of uneven pressure distributed on it. When the cylindrical body bends upward, the longitudinal support tends to slip out of the side receiving channel and consequently the securing sleeve bends up too. This problem can only be solved by fitting the locking collar with a guide groove for partially receiving the longitudinal support. This groove, however, requires aligning the locking collar during assembly and creates a risk of assembly errors. Moreover, with regard to pressing the locking collar over the lower portion of the cylindrical body to lock the ball-head of the bone screw, there is a risk of warping the lower portion if the surfaces of the lower portion and the locking collar have imprecisely matching surfaces. Also, the geometry of the lower portion may change during tightening of the tightening nut and adversely affect the locking relationship between the coupling element and the ball-head of the screw.  
           [0006]    It is therefore desirable to provide a device for connecting a longitudinal support to a bone anchor that distributes locking forces over the device and longitudinal support in a plane perpendicular to a central axis of the locking collar, prevents warping of the cylindrical body and the locking collar, and prevents the longitudinal support from bending upward and slipping out of the side receiving channel.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention is directed to a device for connecting a longitudinal support to a bone anchor having a rounded head. According to one embodiment, the device includes a body defining a chamber for receiving the rounded head of the bone anchor and a first channel for receiving the longitudinal support. Further, a first sleeve is slidable over the body for compressing the chamber, a second sleeve is slidable over the body for biasing the longitudinal support against the first sleeve at first and second contact points or zones, and a fastener is operatively associated with the body for biasing the second sleeve toward the first sleeve. One of the sleeves includes at least one extended portion for contacting the other sleeve at at least one additional contact point or zone such that forces exerted on the second sleeve by the fastener are transferred to the first sleeve through the at least three contact points or zones. The contact points or zones may be radially displaced about a central axis of the body such that a resultant force exerted by the first sleeve and the longitudinal support on the second sleeve is applied substantially at the central axis.  
           [0008]    According to another embodiment, the portion of the body defining the chamber has an outer surface with a convex cross-section. For example, the portion of the body may have a substantially spherical outer surface. Contact between an inner wall of the first sleeve and the outer surface may be along a substantially circular contact zone.  
           [0009]    According to yet another embodiment, the body defines an annular recess, and the first sleeve includes a protrusion received in the annular recess. The protrusion limits sliding of the first sleeve on the body such that the chamber cannot expand sufficiently to allow the rounded head of the bone anchor to be removed from the chamber. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    The detailed description will be better understood in conjunction with the accompanying drawings, wherein like reference characters represent like elements, as follows:  
         [0011]    [0011]FIG. 1 is a longitudinal cross-section of one preferred embodiment of a connecting device of the present invention shown together with a longitudinal support rod and a bone screw having a ball-head;  
         [0012]    [0012]FIG. 2 is a longitudinal cross-section of a body and a lower sleeve of the connecting device of FIG. 1;  
         [0013]    [0013]FIG. 3 is an enlarged detail view of a contact zone between the body and the lower sleeve of FIG. 2;  
         [0014]    [0014]FIG. 4 is a bottom view of an upper sleeve of the connecting device of FIG. 1;  
         [0015]    [0015]FIG. 5 is an elevational view of the upper sleeve of FIG. 4; and  
         [0016]    [0016]FIG. 6 is a schematic representation of an applied force being distributed over a longitudinal support rod and the upper sleeve of FIG. 4. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]    Referring to FIG. 1, one embodiment of a connection device according to the present invention includes a body  3  defining a compressible chamber  34 , which may receive a ball-shaped head  21  of a bone anchor  2 . Chamber  34  is preferably spherical in shape with a downwardly open end, and the exterior surface of lower portion  72  of body  3  defines a substantially convex cross-section in a plane substantially parallel to central axis  33  of body  3 . Preferably, exterior surface of lower portion  72  is substantially spherical in the region of chamber  34 . The middle portion  71  of body  3 , located between the lower chamber  34  and upper portion  70 , includes a laterally open first channel  36  for receiving a longitudinal support, shown in FIG. 1 as longitudinal support rod  1 . First channel  36  is oriented transversely and eccentrically with respect to the central axis  33  of body  3 . An outer thread  35  is formed on an upper portion  70  of body  3 .  
         [0018]    The connection device further includes a lower sleeve  5  and an upper sleeve  6 , both mounted concentrically with body  3 , and a tightening nut  4 . In order to lock the longitudinal support rod  1  and a ball-head  21  of the bone anchor  2  to the connection device, the tightening nut  4  is threaded on the outer thread  35  and tightened. The upper sleeve  6  defines a downwardly open second channel  61  for receiving the longitudinal support rod  1 , which corresponds to first channel  36  in body  3 . Similar to first channel  36 , second channel  61  is transverse to the central axis  33  of body  3 . Second channel  61  encloses longitudinal support rod  1  from the top and sides, but not from below. Thus, when nut  4  is tightened, upper sleeve  6  is pressed downward and compresses longitudinal support rod  1 , inserted in first channel  36  and second channel  61 , and longitudinal support rod  1  in turn presses down lower sleeve  5  and slides it downward on lower portion  72  of body  3 . As a result, the longitudinal support rod  1  is clamped between the upper and lower sleeves  6 ,  5 , and chamber  34  is compressed over the ball-head  21  of the bone anchor  2 , locking its position.  
         [0019]    According to one embodiment, second channel  61  divides the upper sleeve  6  into first and second sections  62 ,  63 , which are of different heights. First section  62  serves to block first channel  36  and is of lesser height than diametrically opposed second section  63 . The larger height H of second section  63  may be implemented by providing an extended portion  80  projecting from lower surface  81  of upper sleeve  6 . As shown in FIG. 4, extended portion  80  extends through an angle β with respect to central axis  33 , which is preferably between 5° and about 20°. The height H of second section  63  in the area of extended portion  80  corresponds to the sum H=D+A, where D is the diameter of the longitudinal support rod  1  and A is the distance between the top  64  of second channel  61  and the uppermost surface  65  of upper sleeve  6 .  
         [0020]    The longitudinal axis  87  of second channel  61  is oriented eccentrically with respect to central axis  33  of body  3  such that first section  62  substantially closes first channel  36  of body  3 . Due to the eccentric location of second channel  61 , the first section  62  has a smaller cross-section perpendicular to central axis than does diametrically opposed second section  63 .  
         [0021]    The different heights of first and second sections  62 ,  63  are dimensioned such that when nut  4  is tightened, only the longitudinal support rod  1  and the extended portion  80  of second section  63  press against lower sleeve  5 . Thus, according to this embodiment, the clamping force exerted by nut  4  is distributed to three contact points or zones on lower sleeve  5 . The height difference between the first and second sections  62 ,  63  makes it possible that only the extended portion  80  of second section  63  and the longitudinal support rod  1  press on the lower sleeve  5  when nut  4  is tightened. Because the height H of the second section  63  is equal to the sum D+A, the extended portion  80  and the longitudinal support rod  1  apply uniform pressure to lower sleeve  5  in a plane substantially perpendicular to the central axis  33 , as a result of which lower sleeve  5  is maintained coaxially aligned over the spherical outer surface of the body  3  and uniformly compresses chamber  34 .  
         [0022]    By designing the second section  63  to have an extended portion  80  such that increased height H only extends through angle of about 5° to 20° with respect to central axis  33 , it is possible to adjust the distribution of forces along the longitudinal support rod  1  and the surface of extended portion  80 . The ratio of force division preferably is 1:3, and accordingly 75% of the compression generated by nut  4  passes through the longitudinal support rod  1  and 25% through the extended portion  80 . The clamping of longitudinal support rod  1  is caused by the force transmitted from upper sleeve  6  to lower sleeve  5 . The height of lower sleeve  5  may be selected in such a manner that when the longitudinal support rod  1  is inserted through channels  36  and  61 , rod  1  cannot contact the bottom of first channel  36  when nut  4  is tightened. Preferably, the height differential between the first and second sections  62 ,  63  is in the range of 0.3-1.0 mm.  
         [0023]    As shown in FIG. 2, chamber  34  has a diameter d and is in the form of a spherical segment of height x, which is preferably less than diameter d. The great-circle plane of the spherical segment perpendicular to central axis  33  is located within chamber  34  at a distance y&lt;x from the top  38  of chamber  34 . The exterior surface of lower portion  72  of body  3 , in the region of chamber  34 , also has a spherical shape. As shown in FIG. 3, the center  73  of shell-like lower portion  72  is located at a point on central axis  33  lower than the center  39  of the spherical segment of chamber  34 . Thus, the ball-head  21  of the bone anchor  2  can be resiliently snapped into chamber  34 , and is only removable therefrom upon applying a predetermined force. The configuration of chamber  34  allows the shank of a bone anchor  2  that has its ball-head  21  locked in chamber  34  to be locked at a range of angles from about −25° to about 25° relative to the central axis  33 . This range of motion substantially increases the applicability and versatility of the connection device of the present invention.  
         [0024]    According to one aspect of the present invention, both the ball-head  21  of the bone anchor  2  and the chamber  34  may receive a surface treatment to increase the friction between them. For example, the ball-head  21  and/or the chamber  34  may be knurled or sandblasted. Additionally, the ball-head  21  of the bone anchor  2  may include a section in the treated area of the chamber that is softer than the treated surface of the chamber, or vice versa. This hardness differential may be implemented by differential cold working, differential crystallization of identical materials, or by selecting different materials for the two parts. In the case of different materials, the hard material is preferably high-strained 1.4441 steel or a hard titanium alloy, and the soft material is preferably heat-treated 1.4441 steel or soft pure titanium. The hardness differential may also be implemented by applying a coating or by ion implantation. Corundum blasting of one or both of the contacting parts is also possible. Both the surface treatment and the hardness differential increase the non-rotational locking relationship of the ball-head  21  and chamber  34 .  
         [0025]    Referring back to FIG. 2, the lower portion  72  of body  3 , containing chamber  34 , is separated from the middle portion  71  of body  3  by an annular offset  66 , and lower sleeve  5  is fitted with projections  51  formed around its inner periphery. The inner diameter of the projections  51  is less than the diameter of the annular offset  66  and, as a result, the lower sleeve  5  may be placed over the upper end  32  of body  3  and the projections  51  resiliently snapped over the annular offset  66 . This makes it possible to use lower sleeve  5  as a clamping ring for chamber  34  while being prevented from sliding off of body  3 . In this manner, lower sleeve  5  also prevents chamber  34  from expanding sufficiently to allow the ball-head  21  of bone anchor  2  to pop out of chamber  34  when stressed. One of ordinary skill in the art will know and appreciate that projections  51  could alternatively be on the body  3  and that annular offset  66  could be on lower sleeve  5 . Further, one of ordinary skill in the art will know and appreciate that neither projections  51  nor annular offset  66  has to be continuous around the circumference of body  3 .  
         [0026]    According to one aspect of the present invention, the bone anchor  2  may be a pedicle screw. To facilitate turning of the pedicle screw into the bone, the screw may be provided with a hexagonal socket in its ball-head  21 . In addition, the body  3  of the connection device may include a continuous bore providing access to the socket in the ball-head  21 . This configuration allows the screw to be screwed into the bone independently of the connecting device or in conjunction with the connecting device, providing the advantage of turning-in or turning-out the connecting device at any time to adjust its height.  
         [0027]    [0027]FIG. 3 is a detail view of the lower portion  72  of body  3 , with the outside wall of chamber  34  defining slits  37  open toward the lower end  31  of body  3 . Slits  37  allow for the resilient expansion and compression of chamber  34 . The lower sleeve  5  acts as a clamping ring and is fitted with a conical inside wall  55  and is slidable over the outside surface of lower portion  72  to compress chamber  34 . When lower sleeve  5  is placed over body  3 , contact between the spherical outer surface of lower portion  72  and inside wall  55  is tangential to the spherical outer surface of lower portion  72 . Thus, contact between lower portion  72  and inside wall  55  is along a circle, defining a circular contact zone. This circular contact zone provides uniform compression of chamber  34  by lower sleeve  5 .  
         [0028]    Still referring to FIG. 3, lower sleeve  5  has a cylindrical wall section  76  on the inside of its lowermost end, and lower portion  72  has a cylindrical wall section  75  on the outer surface at its lowermost end  31 . The diameters, heights and relative positions of the cylindrical walls  75 ,  76  are matched such that when nut  4  is loosened and lower sleeve  5  slides upward on lower portion  72  until projections  51  engage annular offset  66 , cylindrical wall  76  prevents lower portion  72 , and ultimately chamber  34 , from expanding beyond a predetermined amount and prevents ball-head  21  from popping out of chamber  34 .  
         [0029]    When manipulating the connecting device and bone anchor  2  in the absence of inserted longitudinal support rod  1  (for instance, when moving the connecting device relative to the longitudinal support rod  1 ), the play between projections  51  and annular offset  66  only allows the lower sleeve  5  to slide upward on body  3  until cylindrical wall section  76  rests on the outer surface of lower portion  72 . This limits the expansion of lower portion  72  and chamber  34  and reduces the chances of ball-head  21  snapping out of chamber  34  when force is exerted on the bone anchor  2 .  
         [0030]    [0030]FIGS. 4 and 5 show the upper sleeve  6  and second channel  61  extending eccentrically with respect to central axis  33 , and the extended portion  80  projecting from the lower section  81 . Extended portion  80  has a height H, which extends through angle β, shown in FIG. 4.  
         [0031]    [0031]FIG. 6 shows the resulting forces acting on longitudinal support rod  1  and on upper sleeve  6  when nut  4  (not shown) is tightened. As shown therein, F M  schematically represents the resulting force exerted by nut  4 , F S  schematically represents the resulting force exerted by lower sleeve  5  on the longitudinal support rod  1 , and F H  schematically represents the resulting force exerted by the lower sleeve  5  on the extended portion  80 . As also shown in FIG. 6, a distance e is subtended on the diameter of upper sleeve  6  perpendicular to the longitudinal axis  87  of second channel  61  between the application point of resulting force F S  and the central axis  33 , and a distance f is subtended on the diameter perpendicular to longitudinal axis  87  between the application point of the resultant force F H  and the central axis  33 . The distance e also corresponds to the distance between the longitudinal axis  87  of second channel  61  and the central axis  33 . Based on equilibrium of forces and torques, F S =F M /[1+e/f]. For a ratio e/f of ⅓, which follows from designing an upper sleeve  6  with dimensions e=2 mm and f=6 mm, the ratio of F S /F M =0.75.  
         [0032]    The connection described above provides numerous advantages over the prior art. Due to the precise tangential contact between the inside wall  55  of lower sleeve  5  and the spherical outer surface of lower portion  72 , lower sleeve  5  provides a uniform clamping force on lower portion  72 , which in turn provides uniform clamping force between chamber  34  and ball-head  21 . In addition, the height difference between the first and second sections  62 ,  63  makes it possible to distribute the force exerted by nut  4  over longitudinal support rod  1  and extended portion  80 , thus transmitting this force to three or more contact points or zones on lower sleeve  5 . This assures that lower sleeve  5  remains accurately and coaxially aligned on body  3  when forced is applied by nut  4 . Furthermore, projections  51  on the inner periphery of the lower sleeve  5  and the corresponding annular offset  66  of the body  3  make it possible to have lower sleeve  5  slidable on body  3  while being prevented from sliding off the top of body  3 . Projections  51  thus prevent lower sleeve  5  from sliding upward far enough on body  3  for lower portion  72  to expand sufficiently to release ball-head  21  from chamber  34 . As a result, a larger amount of force may be applied from the connecting device to the spinal column without the possibility of body  3  unintentionally detaching from the ball-head  21 .  
         [0033]    While various descriptions of the present invention are described above, it should be understood that the various features can be used singly or in any combination thereof. Therefore, this invention is not to be limited to only the specifically preferred embodiments depicted herein.  
         [0034]    Further, it should be understood that variations and modifications within the spirit and scope of the invention may occur to those skilled in the art to which the invention pertains. Accordingly, all expedient modifications readily attainable by one versed in the art from the disclosure set forth herein that are within the scope and spirit of the present invention are to be included as further embodiments of the present invention. The scope of the present invention is accordingly defined as set forth in the appended claims.