A delivery system comprising: a sheath; a push rod received in said sheath, said sheath being axially moveable relative to said push rod; and a handle provided at a proximal end of the push rod and comprising: a slide shaft fixedly coupled to the proximal end of the push rod and comprising a hollow tubular member having a threaded outer surface; and a hub assembly mounted around the slide shaft and fixedly coupled to a proximal end of said sheath for causing axial movement of the sheath relative to the push rod, said hub assembly comprising a housing within which is mounted an internal slider subassembly for selectively engaging and disengaging said hub assembly with said threaded outer surface of the slide shaft; said internal slider subassembly is axially movable relative to the external housing and comprises an outer sleeve provided with a thread tooth press member and slidably located around an inner slider on which is pivotally mounted a thread tooth (1804A, B) comprising a protruding tooth, said thread tooth being arranged to be pivoted by said thread tooth press member pressing on said thread tooth as the outer sleeve moves axially relative to the inner slider, to engage and disengage said hub assembly with said threaded outer surface of the slide shaft, wherein said protruding tooth is threadedly engaged with said threaded outer surface when said hub assembly is engaged with said threaded outer surface; and wherein said protruding tooth is spaced apart from said threaded outer surface when said hub assembly is disengaged from said threaded outer surface; and wherein characterized in that said outer sleeve comprises a thumb slider, wherein movement of said thumb slider selectively engages and disengages said hub assembly with said threaded outer surface. a sheath; a push rod received in said sheath, said sheath being axially moveable relative to said push rod; and a handle provided at a proximal end of the push rod and comprising: a slide shaft fixedly coupled to the proximal end of the push rod and comprising a hollow tubular member having a threaded outer surface; and a hub assembly mounted around the slide shaft and fixedly coupled to a proximal end of said sheath for causing axial movement of the sheath relative to the push rod, said hub assembly comprising a housing within which is mounted an internal slider subassembly for selectively engaging and disengaging said hub assembly with said threaded outer surface of the slide shaft; said internal slider subassembly is axially movable relative to the external housing and comprises an outer sleeve provided with a thread tooth press member and slidably located around an inner slider on which is pivotally mounted a thread tooth (1804A, B) comprising a protruding tooth, said thread tooth being arranged to be pivoted by said thread tooth press member pressing on said thread tooth as the outer sleeve moves axially relative to the inner slider, to engage and disengage said hub assembly with said threaded outer surface of the slide shaft, wherein a slide shaft fixedly coupled to the proximal end of the push rod and comprising a hollow tubular member having a threaded outer surface; and a hub assembly mounted around the slide shaft and fixedly coupled to a proximal end of said sheath for causing axial movement of the sheath relative to the push rod, said hub assembly comprising a housing within which is mounted an internal slider subassembly for selectively engaging and disengaging said hub assembly with said threaded outer surface of the slide shaft; said internal slider subassembly is axially movable relative to the external housing and comprises an outer sleeve provided with a thread tooth press member and slidably located around an inner slider on which is pivotally mounted a thread tooth (1804A, B) comprising a protruding tooth, said thread tooth being arranged to be pivoted by said thread tooth press member pressing on said thread tooth as the outer sleeve moves axially relative to the inner slider, to engage and disengage said hub assembly with said threaded outer surface of the slide shaft, wherein said protruding tooth is threadedly engaged with said threaded outer surface when said hub assembly is engaged with said threaded outer surface; and wherein said protruding tooth is spaced apart from said threaded outer surface when said hub assembly is disengaged from said threaded outer surface; and wherein characterized in that said outer sleeve comprises a thumb slider, wherein movement of said thumb slider selectively engages and disengages said hub assembly with said threaded outer surface.