Post-Operatively Adjustable Spinal Fixation Devices

Abstract

A system for spinal fixation with a non-rigid portion at least one of the caudal or cephalad terminus. Various devices and techniques are described for transition from a rigid fixation construct to a less rigid support structure applied to a “soft zone” that will help share the stress created on the spinal levels caused by the fixed levels below. In specific embodiments the soft zone is provided by terminating the construct with one of a flexible tether or a dampening rod.

The following is claimed: 
     
         1 . A system for spinal fixation, the system comprising:
 a first bone anchor, anchored to a first vertebra in a subject, the first bone anchor comprising a first bone fastener attached to a first rod housing;   a rigid spinal rod seated in the first rod housing to restrict translation of the rigid spinal rod relative to the first bone anchor;   a second bone anchor, anchored to a second vertebra in the subject, the second bone anchor comprising a second bone fastener attached to a second rod housing, wherein the rigid spinal rod is seated in the second rod housing to restrict translation of the rigid spinal rod relative to the second bone anchor; and   a compressible spinal connector, connected to the second bone anchor, and anchored to a third vertebra in the subject, the compressible spinal connector comprising a modulation mechanism for modulating at least one of:
 the tension on the compressible spinal connector or 
 the resistance to compression of the compressible spinal connector, 
   wherein said modulation occurs in response to a remote signal.   
     
     
         2 . The system of  claim 1 , wherein the remote signal is a spinning magnetic field. 
     
     
         3 . The system of  claim 1 , wherein at least one of the following is at least partially composed of a non-absorbable biocompatible material: the first bone anchor, the second bone anchor, the rigid spinal rod, and the compressible spinal connector. 
     
     
         4 . The system of  claim 1 , wherein the remote signal is a spinning magnetic field, and wherein at least one of the following is at least partially composed of a non-absorbable biocompatible material that is either non-magnetic or weakly magnetic: the first bone anchor, the second bone anchor, the rigid spinal rod, and the compressible spinal connector. 
     
     
         5 . The system of  claim 1 , wherein the compressible spinal connector is a tether assembly. 
     
     
         6 . The system of  claim 5 , wherein the modulation mechanism is an adjustable tensioner configured to vary the tension on a flexible tether between the second bone anchor and the third vertebra. 
     
     
         7 . The system of  claim 6 , wherein the adjustable tensioner is a turnbuckle comprising a threaded first end coupler, a second end coupler, and a rotatable magnet that rotates in response to a spinning magnetic field and that is connected to the threaded first end coupler to cause the threaded first end coupler to rotate about its longitudinal axis when the rotatable magnet rotates. 
     
     
         8 . The system of  claim 6 , wherein the adjustable tensioner comprises a spool about which the flexible tether is wound, and wherein rotation of a spool magnet drives rotation of the spool. 
     
     
         9 . The system of  claim 6 , wherein the adjustable tensioner comprises a locking mechanism configured to maintain tension on the flexible tether when engaged. 
     
     
         10 . The system of  claim 5 , wherein the flexible tether is constructed of a non-absorbable biocompatible material. 
     
     
         11 . The system of  claim 1 , wherein the compressible spinal connector is a dampening spinal rod that is compressible and expandable. 
     
     
         12 . The system of  claim 11 , the dampening spinal rod comprising:
 an elongate rigid portion for insertion into a bone anchor;   a flared portion for receiving a terminal end of a second spinal rod, the flared portion comprising a rod cavity of sufficient diameter to accept the second spinal rod.   
     
     
         13 . The system of  claim 11 , wherein the modulation mechanism is a friction brake configured to vary the resistance of the dampening rod to compression and tension. 
     
     
         14 . The system of  claim 13 , wherein the friction brake comprises a set screw in a threaded channel positioned to exert compressive force on a spring, said spring positioned to exert compressive force against both the compression and expansion of the dampening rod. 
     
     
         15 . The system of  claim 14 , wherein the set screw is magnetic and rotates in the threaded, channel in response to a spinning magnetic field. 
     
     
         16 . The system of  claim 1 , wherein the compressible spinal connector is a telescoping spinal rod positioned within the second rod housing. 
     
     
         17 . The system of  claim 16 , wherein the telescoping spinal rod comprises:
 a rod magnet configured to rotate when exposed to a spinning magnetic field and cause the telescoping spinal rod to either extend or collapse depending on the direction of the spinning magnetic field;   a first elongate element containing a cavity; and   a second elongate element dimensioned to at least partially fit within the cavity, and having an internally threaded region;   wherein the modulation mechanism comprises a lead screw coupled to rotate when the rod magnet rotates, and comprising an externally threaded region engaged to the internally threaded region of the second elongate element, such that rotation of the lead screw causes the second elongate element to translate relative to the first elongate element.   
     
     
         18 . The system of  claim 1 , wherein a second rigid spinal rod is seated in an additional rod housing of an additional bone anchor that is anchored in at least one of the first and second vertebrae. 
     
     
         19 . The system of  claim 18 , wherein a transverse connector is fastened to the first rigid spinal rod and the second rigid spinal rod. 
     
     
         20 . The system of  claim 1 , comprising:
 a third bone anchor comprising a third bone fastener and a third rod housing, anchored to the first vertebra;   a fourth bone anchor, comprising a fourth bone fastener and a fourth rod housing, anchored to the second vertebra;   a second rigid spinal rod seated in the third rod housing and the fourth rod housing; and   a first flexible tether at least partially wrapped around a structure of the third vertebra and connected to both of the first and second rigid spinal rods to exert tension between the third vertebra and the first and second rigid spinal rods;   
       wherein the compressible spinal connector is an adjustable tether assembly, the adjustable tether assembly comprising: a second flexible tether encircling the structure of the third vertebra and a spinous process of a fourth vertebra; and a tensioner connected to the first and second rigid spinal rods, the tensioner comprising a first magnet mounted to rotate in response to a spinning magnetic field, and a tether connection configured to increase or decrease then tension on the second flexible tether depending on the direction of rotation of the first magnet.