Post-operatively adjustable spinal fixation devices
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 helps share the stress created on the spinal levels caused by the fixed levels below. In some embodiments, the soft zone is provided by terminating the construct with one of a flexible tether or a dampening rod.
1. A system for spinal fixation comprising:
a first bone anchor comprising a first bone fastener configured to affix to a first vertebra of a subject, and a first rod housing attached to the first bone fastener;
a second bone anchor comprising a second bone fastener configured to affix to a second vertebra of the subject, and a second rod housing attached to the second bone fastener;
a third bone anchor comprising a third bone fastener configured to affix to the first vertebra, and a third rod housing attached to the third bone anchor; and
a first spinal rod seated in the first and second rod housings;
a second spinal rod seated in the third rod housing;
a telescoping rod coupled to the first spinal rod and configured to be anchored to a third vertebra of the subject, the telescoping rod comprising:
a first elongate portion including: a first end, a second end opposite the first end, and a cavity therein,
a rod magnet proximate the first end and configured to rotate in response to an external force,
a lead screw coupled to the rod magnet and configured to rotate in response to rotation of the rod magnet, and
a second elongate portion configured to threadedly engage with the lead screw such that rotation of the lead screw causes the second elongate portion to translate relative to the first elongate portion along a longitudinal axis of the first elongate portion; and
a dampening rod coupled to the second spinal rod, the dampening rod comprising:
a first rod end,
a second rod end opposite the first rod end,
an elongate rigid portion extending from the first rod end; and
a flared portion proximate the second rod end and having a rod cavity sized to receive an end portion of the second spinal rod.
2. The system of claim 1 , wherein the cavity is sized to receive the lead screw and at least a portion of the second elongate portion therein.
3. The system of claim 1 , wherein the second elongate portion includes:
an internally threaded region configured to engage an externally threaded region of the lead screw, and
an externally threaded region configured to engage an internally threaded region of the first elongate portion proximate the second end.
4. The system of claim 1 , wherein a direction in which the second elongate portion translates relative to the first elongate portion along the longitudinal axis is dependent upon a rotational direction of the rod magnet.
5. The system of claim 1 , wherein the rod magnet is a cylindrical permanent magnet.
6. The system of claim 1 , wherein the external force comprises an electromagnetic signal or a spinning magnetic field.
7. The system of claim 1 , wherein the telescoping rod further comprises a seal that is disposed between the first and second elongate portions and configured to restrict entry of a bodily fluid into the telescoping rod.
8. The system of claim 1 , wherein at least one of the first bone anchor, the second bone anchor, the third bone anchor, the first spinal rod, the second spinal rod, or the telescoping rod comprises a non-absorbable biocompatible material.
9. The system of claim 1 , wherein the dampening rod further comprises a friction brake positioned adjacent the flared portion and configured to modulate friction between the second spinal rod and the dampening rod.
10. The system of claim 9 , wherein the friction brake comprises:
a string configured to exert a compressive force on the end portion of the second spinal rod, and a set screw configured to exert a force on the string.
11. The system of claim 10 , wherein the string is positioned adjacent to the end portion of the second spinal rod.
12. The system of claim 10 , wherein the set screw is positioned in a threaded channel of the friction brake and configured to rotate in response to an externally applied magnetic field, thereby varying the force exerted on the string and modulating the friction between the second spinal rod and the dampening rod.
13. The system of claim 1 , further comprising a cross connector unit fastened to the first and second spinal rods.
14. The system of claim 13 , wherein the cross connector unit is configured to rigidly engage the first and second spinal rods.
15. The system of claim 13 , wherein the cross connector unit comprises: a first rod connector configured to couple to the first spinal rod, and a second rod connector configured to couple to the second spinal rod.