Abstract: An assembly of two elements including a first element that has a cavity with an external opening connected to a first region defined by a sidewall that is a spherical segment and the diameter of the external opening is smaller than the diameter of the sidewall in the first region. Adjacent to the first region is a second region also defined by a second spheroid segment that is separated from the first region by an annular ring extending from the sidewalls towards a central axis of the cavity. The annular ring forms a passage having a diameter approximately the same as the external opening. An extension member on the second element is inserted into the cavity which has a first rod section terminating in a second spherically-shaped section.