Apparatus is provided for adjusting at least one dimension of an implant. The apparatus includes a rotatable structure having a channel extending between first and second openings of the rotatable structure. The channel is configured for passage therethrough of an elongate tool. The rotatable structure has a lower surface that is shaped to define a first coupling. The apparatus includes a mechanical element having a surface coupled to the lower surface of the rotatable structure. The mechanical element is shaped to provide a second coupling configured to engage the first coupling during a resting state of the mechanical element, in a manner that restricts rotation of the rotatable structure. The mechanical element has a depressible portion that is disposed in communication with the second opening, and is configured to disengage the first and second couplings in response to a force applied thereto by the elongate tool.