A tether assembly (12, 112, 250, 412, 550) of a medical device delivery system, the tether assembly comprising: a pull wire (34, 134, 284, 484, 584, 784) defining a proximal end (286, 486, 586) and a distal end; and a tether head assembly (18, 118) comprising: an inner retainer (36, 136) comprising a proximal portion (50, 150) and a distal portion (52, 152), wherein the inner retainer (36, 136) is coupled to and extends distally from the distal end of the pull wire (34, 134, 284, 484, 584, 784); an outer retainer comprising a proximal portion defining a channel configured to receive the inner retainer (36, 136) and a distal portion defining an aperture, the aperture comprising: a receptacle configured to receive an attachment member of a medical device; a passageway extending from a distal end defined by the outer retainer proximally to the receptacle, wherein the passageway is narrower than the receptacle; and a groove extending from the distal end of the outer retainer proximally at least to the receptacle, wherein the groove has a depth that is less than a thickness of the distal portion (52, 152) of the inner retainer (36, 136), wherein the inner retainer (36, 136) is movable between a first position wherein the distal portion (52, 152) of the inner retainer (36, 136) is partially received in the groove and extends into the passageway, thereby narrowing the passageway, and a second position wherein the distal portion (52, 152) of the inner retainer (36, 136) is positioned proximal to the passageway. a pull wire (34, 134, 284, 484, 584, 784) defining a proximal end (286, 486, 586) and a distal end; and a tether head assembly (18, 118) comprising: an inner retainer (36, 136) comprising a proximal portion (50, 150) and a distal portion (52, 152), wherein the inner retainer (36, 136) is coupled to and extends distally from the distal end of the pull wire (34, 134, 284, 484, 584, 784); an outer retainer comprising a proximal portion defining a channel configured to receive the inner retainer (36, 136) and a distal portion defining an aperture, the aperture comprising: a receptacle configured to receive an attachment member of a medical device; a passageway extending from a distal end defined by the outer retainer proximally to the receptacle, wherein the passageway is narrower than the receptacle; and a groove extending from the distal end of the outer retainer proximally at least to the receptacle, wherein the groove has a depth that is less than a thickness of the distal portion (52, 152) of the inner retainer (36, 136), wherein the inner retainer (36, 136) is movable between a first position wherein the distal portion (52, 152) of the inner retainer (36, 136) is partially received in the groove and extends into the passageway, thereby narrowing the passageway, and a second position wherein the distal portion (52, 152) of the inner retainer (36, 136) is positioned proximal to the passageway. an inner retainer (36, 136) comprising a proximal portion (50, 150) and a distal portion (52, 152), wherein the inner retainer (36, 136) is coupled to and extends distally from the distal end of the pull wire (34, 134, 284, 484, 584, 784); an outer retainer comprising a proximal portion defining a channel configured to receive the inner retainer (36, 136) and a distal portion defining an aperture, the aperture comprising: a receptacle configured to receive an attachment member of a medical device; a passageway extending from a distal end defined by the outer retainer proximally to the receptacle, wherein the passageway is narrower than the receptacle; and a groove extending from the distal end of the outer retainer proximally at least to the receptacle, wherein the groove has a depth that is less than a thickness of the distal portion (52, 152) of the inner retainer (36, 136), a receptacle configured to receive an attachment member of a medical device; a passageway extending from a distal end defined by the outer retainer proximally to the receptacle, wherein the passageway is narrower than the receptacle; and a groove extending from the distal end of the outer retainer proximally at least to the receptacle, wherein the groove has a depth that is less than a thickness of the distal portion (52, 152) of the inner retainer (36, 136), wherein the inner retainer (36, 136) is movable between a first position wherein the distal portion (52, 152) of the inner retainer (36, 136) is partially received in the groove and extends into the passageway, thereby narrowing the passageway, and a second position wherein the distal portion (52, 152) of the inner retainer (36, 136) is positioned proximal to the passageway.