The present invention relates to a first medical device (1, 100) configured to permit connection to a second medical device (3). The medical device (1, 100, 200) comprises a guiding track (12) for receiving a lock protrusion (4). The guiding track (12) comprises a surface (16) having a lock edge (15) extending between a first and a second level (H1, H2). The lock edge (15) extends in a smooth curvature between the first and the second level (H1, H2), the curvature of which is a function of at least of radius (Ra). The medical device (1, 100) may comprise a neck element (11) with at least one guiding track (12) for receiving a lock protrusion (4) of a second medical device (3) or optionally the medical device may comprise a sleeve member (202) which comprises the guiding track (12). The present invention provide for a connection site which enables a smooth lock but especially a smooth unlock motion of the second medical device being connected.