Spinal fixation device with variable stiffness

An implantable orthopedic system for stabilizing first, second and third vertebrae of a spinal column, the system comprising:

• a cord;
• a first bone anchor configured to be secured to the first vertebra, a second bone anchor configured to be secured to the second vertebra, and a third bone anchor configured to be secured to the third vertebra, wherein each bone anchor comprises:
• a bone attachment portion, and
• a head portion attached to the bone attachment portion, the head portion including a first concave face and a second concave face oriented about 180 degrees from the first concave face, a cord receiving portion extending through the head portion from the first concave face to the second concave face, and a clamping mechanism adapted to secure the cord to the bone anchor;
• and
• first and second spacers each having a channel sized to receive the cord therethrough,
• wherein the first spacer is adapted to be positioned between the head portions of the first and second bone anchors and to maintain a predetermined spacing between the same, and the second spacer is adapted to be positioned between the head portions of the second and third bone anchors and to maintain a predetermined spacing between the same;
• wherein each spacer further comprises a first end surface disposed on a first end, a second end surface disposed on a second end and a peripheral surface extending between the first end surface and the second end surface, the first and second end surfaces configured so as to be multi-directionally pivotable relative to the head portions of the bone anchors;
• wherein the first end surface of the first spacer is substantially convex and configured to engage the first concave face of the head portion of the first bone anchor, and the second end surface of the first spacer is substantially convex and configured to engage the first concave face of the head portion of the second bone anchor;
• wherein the first end surface of the second spacer is substantially convex and configured to engage the second concave face of the head portion of the second bone anchor, and the second end surface of the second spacer is substantially convex and configured to engage the first concave face of the head portion of the third bone anchor;
• wherein a first portion of the cord extending through the channel of the first spacer and secured between the first bone anchor and the second bone anchor is tensioned a first amount while a second portion of the cord extending through the channel of the second spacer and secured between the second bone anchor and the third bone anchor is tensioned a second amount, the second amount being less than the first amount.

A method of stabilizing a portion of a spinal column of a patient comprising:

• implanting first, second and third bone anchors into respective first, second and third vertebrae, wherein each bone anchor includes a head portion including a cord receiving portion extending through the head portion from a first spherically concave end face to a second spherically concave end face;
• placing a first spacer between the head portions of the first and second bone anchors such that a first spherically convex end surface of the first spacer is engaged with the first spherically concave end face of the first bone anchor and a second spherically convex end surface of the first spacer is engaged with the first spherically concave end face of the second bone anchor, wherein the first spacer includes a channel adapted to receive a cord therethrough;
• placing a second spacer between the head portions of the second and third bone anchors such that a first spherically convex end surface of the second spacer is engaged with the second spherically concave end face of the second bone anchor and a second spherically convex end surface of the second spacer is engaged with the first spherically concave end face of the third bone anchor, wherein the second spacer includes a channel adapted to receive a cord therethrough;
• passing a cord through the cord receiving portions of the head portions of the first, second and third bone anchors and the channels of the first and second spacers;
• applying a first amount of tension to a first portion of the cord extending between the first and second bone anchors with the cable tensioning device such that the head portions of the first and second bone anchors exert a desired compressive force on the first spacer;
• applying a second amount of tension to a second portion of the cord extending between the second and third bone anchors while maintaining the first amount of tension in the first portion of the cord such that the head portions of the second and third bone anchors exert a desired compressive force on the second spacer, the second amount of tension being different from the first amount of tension.

A method of stabilizing a portion of a spinal column of a patient comprising:

• implanting first, second and third bone anchors into first, second and third vertebrae, respectively, wherein each bone anchor includes a head portion including a channel sized to receive a cord therethrough, wherein the channel extends through the head portion from a first end face to a second end face;
• placing a first spacer between the head portions of the first and second bone anchors wherein the first spacer includes a channel adapted to receive a cord therethrough;
• placing a second spacer between the head portions of the second and third bone anchors, wherein the second spacer includes a channel adapted to receive a cord therethrough;
• passing a cord through the cord receiving portions of the first, second and third bone anchors and the channels of the first and second spacers such that a first portion of the cord extends through the channel of the first spacer between the head portions of the first and second bone anchors and a second portion of the cord extends through the channel of the second spacer between the head portions of the second and third bone anchors;
• applying a tensile load to the cord such that the head portions of the bone anchors trimming the excess cord applying a first amount of tension to the first portion of the cord and a second amount of tension to the second portion of the cord, the second amount of tension being less than the first amount of tension;
• and
• securing the cord to the first, second and third bone anchors such that the first portion of the cord maintains the first amount of tension between the head portions of the first and second bone anchors and the second portion of the cord maintains the second amount of tension between the head portions of the second and third bone anchors such that fixation between the first and second vertebrae is more rigid than fixation between the second and third vertebrae.