Cord for Vertebral Fixation Having Multiple Stiffness Phases

Abstract

A dynamic stabilization system for stabilizing a vertebral segment of a spine. The system includes a first vertebral anchor, a second vertebral anchor and a cord. The system may also include a spacer positioned between the first and second vertebral anchor through which the cord extends. The first vertebral anchor is configured to be secured to a first vertebra and the second vertebral anchor is configured to be secured to a second vertebra. The cord is extendable from the first vertebral anchor to the second vertebral anchor. The cord has a variable stiffness through a range of displacement, wherein the stiffness of the cord increases as an applied tensile force on the cord is increased.

What is claimed is: 
     
         1 . A dynamic stabilization system for stabilizing a vertebral segment of a spine, the system comprising:
 a first vertebral anchor configured to be secured to a first vertebra;   a second vertebral anchor configured to be secured to a second vertebra; and   a cord extendable from the first vertebral anchor to the second vertebral anchor, the cord having a variable stiffness under tension through a range of displacement;   wherein the stiffness of the cord increases as an applied tensile force on the cord is increased;   wherein the cord has a first phase of stiffness under tension through a first range of displacement, a second phase of stiffness under tension through a second range of displacement, and a third phase of stiffness under tension through a third range of displacement;   wherein the stiffness in the cord in the first phase is less than the stiffness in the cord in the third phase, and the stiffness of the cord in the second phase progressively increases from the stiffness in the cord in the first phase to the stiffness in the cord in the third phase.   
     
     
         2 . The dynamic stabilization system of  claim 1 , wherein the first phase of stiffness is less than about 100 N/mm. 
     
     
         3 . The dynamic stabilization system of  claim 1 , wherein the second phase of stiffness is in the range of about 100 N/mm to about 250 N/mm. 
     
     
         4 . The dynamic stabilization system of  claim 1 , wherein the third phase of stiffness is greater than about 250 N/mm. 
     
     
         5 . The dynamic stabilization system of  claim 1 , wherein the cord includes a plurality of filaments, wherein a first subset of the plurality of filaments has a first tensile modulus of elasticity of less than 1 GPa and a second subset of the plurality of filaments has a second tensile modulus of elasticity of more than 1 GPa. 
     
     
         6 . The dynamic stabilization system of  claim 5 , wherein the first subset of filaments is intermingled with the second subset of filaments. 
     
     
         7 . The dynamic stabilization system of  claim 1 , wherein the cord includes an inner braided layer and an outer braided layer surrounding the inner braided layer. 
     
     
         8 . The dynamic stabilization system of  claim 7 , wherein the inner braided layer includes a first plurality of filaments having a first elasticity and a second plurality of filaments having a second elasticity different from the first elasticity. 
     
     
         9 . The dynamic stabilization system of  claim 8 , wherein the outer braided layer includes a third plurality of filaments having a third elasticity different from the first elasticity. 
     
     
         10 . A dynamic stabilization system for stabilizing a vertebral segment of a spine, the system comprising:
 a first vertebral anchor configured to be secured to a first vertebra;   a second vertebral anchor configured to be secured to a second vertebra; and   a cord extendable from the first vertebral anchor to the second vertebral anchor, the cord having a variable stiffness through a range of displacement;   wherein the stiffness of the cord increases as an applied tensile force on the cord is increased;   wherein the cord has a first phase of stiffness through a first range of displacement, a second phase of stiffness through a second range of displacement, and a third phase of stiffness through a third range of displacement;   wherein the stiffness in the cord is variable in each of the first phase, the second phase, and the third phase.   
     
     
         11 . The dynamic stabilization system of  claim 10 , wherein the stiffness in the cord is continuously variable in each of the first phase, the second phase, and the third phase. 
     
     
         12 . The dynamic stabilization system of  claim 10 , wherein the stiffness changes at a non-linear rate in each of the first phase, the second phase, and the third phase. 
     
     
         13 . The dynamic stabilization system of  claim 12 , wherein the stiffness increases at a non-linear rate in each of the first phase, the second phase, and the third phase. 
     
     
         14 . The dynamic stabilization system of  claim 10 , wherein second range of displacement is wider than the first range of displacement. 
     
     
         15 . A dynamic stabilization system for stabilizing a vertebral segment of a spine, the system comprising:
 a first vertebral anchor configured to be secured to a first vertebra;   a second vertebral anchor configured to be secured to a second vertebra; and   a cord extendable from the first vertebral anchor to the second vertebral anchor,   wherein the cord includes an inner braided layer including a first plurality of filaments having a first elasticity and a second plurality of filaments having a second elasticity different from the first elasticity, and an outer braided layer;   wherein the cord has a variable stiffness through a range of displacement;   wherein the stiffness of the cord increases as an applied tensile force on the cord is increased.   
     
     
         16 . The dynamic stabilization system of  claim 15 , wherein the cord has a first phase of stiffness through a first range of displacement, a second phase of stiffness through a second range of displacement, and a third phase of stiffness through a third range of displacement. 
     
     
         17 . The dynamic stabilization system of  claim 16 , wherein the stiffness in the cord in the first phase is less than the stiffness in the cord in the third phase, and the stiffness of the cord in the second phase progressively increases from the stiffness in the cord in the first phase to the stiffness in the cord in the third phase. 
     
     
         18 . The dynamic stabilization system of  claim 16 , wherein the stiffness of the cord is variable within each of the first phase, the second phase, and the third phase. 
     
     
         19 . The dynamic stabilization system of  claim 15 , wherein the cord includes a central core extending through the inner braided layer. 
     
     
         20 . The dynamic stabilization system of  claim 15 , wherein the first plurality of filaments has a first tensile modulus of elasticity of less than 1 GPa, and the second plurality of filaments has a second tensile modulus of elasticity greater than 1 GPa.