Soft tissue anchors and implantation techniques

1. Apparatus comprising a tissue anchor, which comprises:

a proximal head at a proximal end of the tissue anchor; and

a helical tissue-coupling element disposed about a longitudinal axis thereof, extending to the proximal head, and having a distal tissue-penetrating tip, wherein the helical tissue-coupling element includes a first helical portion and a second helical portion, wherein the second helical portion is more distal than the first helical portion,

wherein the first and the second helical portions of the helical tissue-coupling element have a first axial stiffness and a second axial stiffness, respectively, and

wherein the second axial stiffness is greater than the first axial stiffness, such that if excessive tension is applied to the helical tissue-coupling element, the helical tissue-coupling element generally elongates along the first helical portion before along the second helical portion, such that the first helical portion serves as a mechanical fuse that reduces force applied to the second helical portion.

2. The apparatus according to claim 1 , wherein the first and the second helical portions of the helical tissue-coupling element have a first axial yield strength and a second axial yield strength, respectively, and wherein the second axial yield strength is greater than the first axial yield strength.

3. The apparatus according to claim 2 , wherein a third helical portion of the helical tissue-coupling element, more distal than the second helical portion, has a third axial yield strength that is less than the second axial yield strength.

4. The apparatus according to claim 1 ,

wherein the first and the second helical portions are shaftless helical portions of the helical tissue-coupling element, and

wherein the first and the second helical portions of the helical tissue-coupling element have a first axial thickness; and a second axial thickness, respectively, and wherein the second axial thickness is greater than the first axial thickness, the first and second axial thicknesses being measured along the axis.

5. The apparatus according to claim 3 , wherein a third helical portion of the helical tissue-coupling element, more distal than the second helical portion, has a third axial thickness that is less than the second axial thickness, the third axial thickness being measured along the axis.

6. The apparatus according to claim 1 , wherein the apparatus further comprises a flexible longitudinal member, which is coupled to the proximal head.

7. The apparatus according to claim 1 , wherein a third helical portion of the helical tissue-coupling element, more distal than the second helical portion, has a third axial stiffness that is less than the second axial stiffness.

8. A method comprising:

providing a tissue anchor, which includes a proximal head at a proximal end of the tissue anchor, and a helical tissue-coupling element disposed about a longitudinal axis thereof, extending to the proximal head, and having a distal tissue-penetrating tip, wherein the helical tissue-coupling element includes a first helical portion and a second helical portion, wherein the second helical portion is more distal than the first helical portion, wherein (a) the first and the second helical portions of the helical tissue-coupling element have a first axial stiffness and a second axial stiffness, and (b) the second axial stiffness is greater than the first axial stiffness, such that if excessive tension is applied to the helical tissue-coupling element, the helical tissue-coupling element generally elongates along the first helical portion before along the second helical portion, such that the first helical portion serves as a mechanical fuse that reduces force applied to the second helical portion; and

advancing the helical tissue-coupling element into soft tissue.

9. The method according to claim 8 , wherein the method further comprises:

applying tension to the proximal head of the tissue anchor; and

sensing elongation of the first helical portion while applying the tension.

10. The method according to claim 8 , further comprising applying tension to the proximal head of the tissue anchor.

11. The method according to claim 10 , wherein applying the tension comprises sensing elongation of the first helical portion while applying the tension.

12. The method according to claim 11 , wherein sensing the elongation comprises sensing the elongation using imaging.

13. The method according to claim 11 , wherein sensing the elongation comprises sensing the elongation using tactile feedback.

14. The method according to claim 10 , wherein applying the tension comprises pulling on a flexible longitudinal member that is coupled to the proximal head.

15. The method according to claim 8 , wherein advancing the helical tissue-coupling element into the soft tissue comprises advancing the second helical portion completely into the soft tissue, and leaving at least a portion of the first helical portion outside of the soft tissue.

16. The method according to claim 8 , wherein a third helical portion of the helical tissue-coupling element, more distal than the second helical portion, has a third axial stiffness that is less than the second axial stiffness.

17. The method according to claim 8 , wherein the first and second helical portions of the helical tissue-coupling element have a first axial yield strength and a second axial yield strength, respectively, and wherein the second axial yield strength is greater than the first axial yield strength.

18. The method according to claim 17 , wherein a third helical portion of the helical tissue-coupling element, more distal than the second helical portion, has a third axial yield strength that is less than the second axial yield strength.