Tandem rod connectors and related methods

Abstract

Connectors are disclosed herein that can be used to attach a rod to a bone anchor assembly that is already occupied by a separate rod. Various ways of attaching the connector to the bone anchor assembly are disclosed, including arrangements in which the connector is locked to the bone anchor, arrangements in which the connector is constrained in one or more degrees of freedom relative to the bone anchor, arrangements in which the connector is adjustable in one or more degrees of freedom relative to the bone anchor, and arrangements that include a spherical articulation joint. In some embodiments, attachment of the connector to a bone anchor can be aided with the use of a positioner. The geometry of the connector can be selected to minimize the offset between a first rod received in the bone anchor assembly and a second rod received in the connector, for example using an angled or curved rod recess and/or a fastener, or set screw, that is offset from the center of the rod or angled relative to the bone anchor. The connector can be configured to align the first and second rods in a common coronal plane, or can be configured to position one rod more anterior or posterior than the other rod.

1 . A connector assembly, comprising:
 a connector having a proximal end and a distal end that define a proximal-distal axis, the connector including:
 a first portion configured to mate the connector to a receiver member of a bone anchor assembly, the bone anchor assembly having a first rod-receiving recess for receiving a first rod, and 
 a second portion in which a second rod-receiving recess is formed, the second rod-receiving recess being configured to receive a second rod; 
   a first fastener having a distal portion configured to engage the receiver member to lock the first rod to the receiver member and a proximal portion that extends through an opening formed in the first portion of the connector;   a nut mateable to the first fastener to secure the connector to the receiver member; and   a second fastener engaged with the second portion of the connector to lock the second rod to the connector;   wherein the second rod-receiving recess includes a proximal opening and a distal seat and wherein the proximal opening is offset from the distal seat in a direction perpendicular to the proximal-distal axis.   
     
     
         2 . The assembly of  claim 1 ,
 wherein the first and second rods have a rod diameter,   wherein the first and second rods have a center-to-center offset, when seated in the receiver member and the connector, respectively, and   wherein a ratio of the center-to-center offset to rod diameter is in the range of about 2:1 to about 4:1.   
     
     
         3 . The assembly of  claim 1 , wherein the first and second rods have a center-to-center offset, when seated in the receiver member and the connector, respectively, in the range of about 6 mm to about 16 mm. 
     
     
         4 . The assembly of  claim 1 , wherein the first and second rods have a center-to-center offset, when seated in the receiver member and the connector, respectively, of about 8 mm. 
     
     
         5 . The assembly of  claim 1 , wherein the first rod-receiving recess includes a first rod seat and the first rod seat is distal to the seat of the second rod-receiving recess. 
     
     
         6 . The assembly of  claim 1 , wherein the first rod-receiving recess includes a first rod seat and the first rod seat is proximal to the seat of the second rod-receiving recess. 
     
     
         7 . The assembly of  claim 1 , wherein the first rod-receiving recess includes a first rod seat and the first rod seat is at substantially the same proximal-distal height as the seat of the second rod-receiving recess. 
     
     
         8 . The assembly of  claim 1 , wherein the first portion of the connector includes a recess configured to receive a proximal end of the receiver member, the recess being cylindrical such that, when the receiver member is received in the recess, the receiver member is free to rotate relative to the connector about a central axis of the receiver member. 
     
     
         9 . The assembly of  claim 1 , wherein the first portion of the connector includes a recess configured to receive a proximal end of the receiver member, the recess including one or more flats that abut corresponding flats of the receiver member such that, when the receiver member is received in the recess, the receiver member is constrained to uniplanar motion relative to the connector. 
     
     
         10 . The assembly of  claim 1 , wherein the first portion of the connector includes a recess configured to receive a proximal end of the receiver member, the recess forming a substantial negative of the receiver member such that, when the receiver member is received in the recess, the receiver member cannot rotate or translate relative to the connector. 
     
     
         11 . The assembly of  claim 1 , wherein the first portion of the connector includes a domed distal-facing surface that bears against a proximal-facing surface of the receiver member in a gimbal interface. 
     
     
         12 . The assembly of  claim 1 , wherein the opening in the first portion of the connector is oversized relative to the first fastener to allow movement of the connector relative to the receiver member. 
     
     
         13 . The assembly of  claim 1 , further comprising:
 a positioner having a central opening in which the first fastener is received and distally-extending tabs that engage the receiver member to limit rotation between the positioner and the receiver member.   
     
     
         14 . The assembly of  claim 13 , wherein the positioner includes teeth configured to be selectively enmeshed with teeth of the connector to limit rotation between the connector and the receiver member. 
     
     
         15 . The assembly of  claim 13 , wherein the positioner includes a flat formed on an outer sidewall of the positioner and configured to selectively engage a flat formed on an inner sidewall of the connector to limit rotation between the connector and the receiver member. 
     
     
         16 . The assembly of  claim 13 , wherein tightening the nut is effective to clamp the tabs of the positioner onto the first fastener. 
     
     
         17 . The assembly of  claim 1 , wherein the nut has a spherical exterior surface received within a spherical interior surface of the opening formed in the first portion of the connector to allow the connector to move polyaxially relative to the receiver member. 
     
     
         18 . The assembly of  claim 1 , wherein the nut includes a conical distal-facing surface that contacts a conical proximal-facing surface of the connector. 
     
     
         19 . The assembly of  claim 18 , wherein the conical surfaces of the nut and the connector taper at angles that differ from one another. 
     
     
         20 . The assembly of  claim 1 , wherein the nut is a locking nut configured to expand within the opening to lock an orientation of the connector relative to the receiver member. 
     
     
         21 . The assembly of  claim 18 , wherein at least one of the first fastener and a throughbore of the locking nut has a tapered thread such that rotation of the locking nut relative to the first fastener is effective to radially expand the locking nut. 
     
     
         22 . The assembly of  claim 18 , wherein the locking nut has a castle drive feature with curved abutment surfaces. 
     
     
         23 . The assembly of  claim 18 , wherein a distal surface of the connector is configured to constrain movement of the connector relative to the receiver member to a direction parallel to the first rod. 
     
     
         24 . The assembly of  claim 18 , wherein a distal surface of the connector is configured to constrain movement of the connector relative to the receiver member to a direction perpendicular to the first rod. 
     
     
         25 . The assembly of  claim 1 , wherein the second rod is positionable in the second rod-receiving recess by moving the second rod distally relative to the connector. 
     
     
         26 . The assembly of  claim 1 , wherein the second rod-receiving recess follows a curved path between the proximal opening and the distal seat. 
     
     
         27 . The assembly of  claim 1 , wherein the second rod-receiving recess follows a path that is obliquely angled relative to the proximal-distal axis of the connector. 
     
     
         28 . The assembly of  claim 1 , wherein a rotation axis of the second fastener does not intersect with a central longitudinal axis of the second rod when the second rod is seated in the connector. 
     
     
         29 . The assembly of  claim 1 , wherein a rotation axis of the second fastener extends at an oblique angle with respect to the proximal-distal axis of the connector. 
     
     
         30 . The assembly of  claim 1 , wherein the second fastener is a set screw and is received within a set screw recess formed in the connector, the center of the set screw recess being offset in a direction perpendicular to the proximal-distal axis from a center of the proximal opening of the second rod-receiving recess. 
     
     
         31 . The assembly of  claim 1 , wherein the distal seat of the second rod-receiving recess defines a V-shape configured to receive rods of different diameters. 
     
     
         32 . The assembly of  claim 1 , further comprising the bone anchor assembly, the first rod, and the second rod. 
     
     
         33 . A connector assembly, comprising:
 a connector that includes an opening that defines a spherical interior surface and a rod-receiving recess laterally offset from the opening and configured to receive an elongate spinal rod;   a threaded fastener; and   a locking nut received within the opening of the connector, the locking nut having a spherical exterior surface and a threaded throughbore in which a proximal end of the fastener is received;   wherein the assembly has an unlocked configuration in which the connector is polyaxially movable relative to the fastener and a locked configuration in which an orientation of the connector relative to the fastener is fixed, the locking nut being radially expanded in the locked configuration as compared to the unlocked configuration.   
     
     
         34 . The assembly of  claim 33 , wherein the threads of the fastener or the threads of the locking nut are tapered such that rotation of the locking nut relative to the fastener is effective to radially expand the locking nut. 
     
     
         35 . The assembly of  claim 33 , wherein the locking nut has a castle drive feature with curved abutment surfaces. 
     
     
         36 . The assembly of  claim 33 , wherein a distal surface of the connector includes a first planar portion and a second planar portion oriented at an oblique angle with respect to the first planar portion to constrain movement of the connector relative to a receiver member in which the set screw is disposed. 
     
     
         37 . A spinal fixation method, comprising:
 implanting a bone anchor assembly in a vertebra of a patient, the bone anchor assembly including a receiver member having a first rod-receiving recess;   inserting a first rod into the first rod-receiving recess of the receiver member;   inserting a first fastener into the receiver member to secure the first rod in the receiver member;   positioning a connector over the receiver member such that a proximal portion of the first fastener extends through an opening formed in the connector;   inserting a second rod into a second rod-receiving recess formed in the connector;   inserting a second fastener into a fastener recess formed in the connector to secure the second rod in the connector;   adjusting an orientation of the connector relative to the receiver member to achieve a desired positioning of the first and second rods; and   tightening a nut on the first fastener to fix the orientation of the connector relative to the receiver member.   
     
     
         38 . The method of  claim 37 , further comprising positioning the first and second rods such that a ratio of a center-to-center offset, between the first and second rods, to a diameter of the first and second rods is in the range of about 2:1 to about 4:1. 
     
     
         39 . The method of  claim 37 , further comprising positioning the first and second rods such that a center-to-center offset between the first and second rods is in the range of about 6 mm to about 16 mm. 
     
     
         40 . The method of  claim 37 , further comprising placing a positioner between the connector and the receiver member, wherein the positioner restricts relative rotation between the connector and the receiver member about the first fastener after the nut is tightened. 
     
     
         41 . The method of  claim 37 , further comprising positioning the first and second rods such that the first rod is anterior to the second rod. 
     
     
         42 . The method of  claim 37 , further comprising positioning the first and second rods such that the first and second rods substantially lie in a common coronal plane. 
     
     
         43 . The method of  claim 37 , wherein inserting the second rod comprises moving the rod distally and laterally within a curved rod-receiving recess. 
     
     
         44 . The method of  claim 37 , wherein inserting the second rod comprises moving the rod distally and laterally within an obliquely angled rod-receiving recess. 
     
     
         45 . The method of  claim 37 , wherein inserting the second rod comprises top loading the rod into the connector. 
     
     
         46 . The method of  claim 37 , wherein adjusting the orientation comprises pivoting the connector about at least one of a uniplanar interface, a gimbal interface, and a spherical articulation joint.