A prosthesis is provided for implantation at a native semilunar valve of a native valve complex. The prosthesis includes a distal fixation member, configured to be positioned in a downstream artery, and to apply a first axial force directed toward a ventricle. The prosthesis further includes a proximal fixation member coupled to the distal fixation member, the proximal fixation member configured to be positioned at least partially on a ventricular side of the native valve complex, and to apply a second axial force directed toward the downstream artery, such that application of the first and second forces couples the prosthesis to the native valve complex by axially sandwiching the native valve complex from a downstream side and the ventricular side. A ratio of the first axial force to a radial force applied outwardly by the prosthesis against the native semilunar valve is greater than 1.5:1. Other embodiments are also described.