A torque-delivery tool includes a cable and a distal coupling element. A tissue anchor includes an anchor head, which includes a shaft having a proximal coupling element. The distal and proximal coupling elements are shaped so as to define corresponding interlocking surfaces. An outer tether-securing element of the anchor is shaped so as to define a lateral opening through which a tether is disposed, and at least partially radially surrounds the shaft and a spring. In an unlocked state, a distal spring depressor restrains the spring in an axially-compressed state. In a locked state, the distal and proximal coupling elements are not coupled with one another, the distal spring depressor does not restrain the spring in the axially-compressed state, and the spring is in an axially-expanded state, in which the spring inhibits the sliding of the tether through the lateral opening by pressing the tether against the outer tether-securing element.