A system for insertion of a cup implant into a patient's hip socket comprising: (i) a cup implant and a holding device (2, 31, 65, 78) that interacts with said cup implant or that interacts with an insert positioned in/on said cup implant, wherein the interaction between the cup implant and/or insert and the holding device creates a cavity with an initial volume within the resulting combination of said cup implant and/or insert and said holding device which cavity is at least partially filled with fluid, wherein the holding device comprises a cover assembly for placing into said cup implant and/or insert, said cover assembly comprising: a proximal surface, said proximal surface comprising a region that is able to interact with or receive an impaction device and at least one resilient and elastically deformable region (6, 7) such that when elastically deformed in a stressed state there is natural bias of the resilient and elastically deformable region to revert to its unstressed state; and a distal surface located away from the proximal surface of the cover assembly and arranged to fit in said cup implant and/or insert, said distal surface comprising at least a seal (12, 32, 68, 81); wherein the initial volume of the cavity is created when the holding device is inserted into the cup implant to a predetermined position in which the cover assembly interacts with the rim of the cup implant and characterised by (ii) an impaction device (1, 94) which is configured to be impacted in use by a surgeon with hammer blows required to achieve a jam fit of the cup implant into the hip socket, said impaction device comprising: a proximal end; a shaft; and a distal region that interacts with at least a part of said cover assembly, said shaft and/or distal region of said impaction device further comprising a mechanism that prevents additional fluid from entering the cavity. (i) a cup implant and a holding device (2, 31, 65, 78) that interacts with said cup implant or that interacts with an insert positioned in/on said cup implant, wherein the interaction between the cup implant and/or insert and the holding device creates a cavity with an initial volume within the resulting combination of said cup implant and/or insert and said holding device which cavity is at least partially filled with fluid, wherein the holding device comprises a cover assembly for placing into said cup implant and/or insert, said cover assembly comprising: a proximal surface, said proximal surface comprising a region that is able to interact with or receive an impaction device and at least one resilient and elastically deformable region (6, 7) such that when elastically deformed in a stressed state there is natural bias of the resilient and elastically deformable region to revert to its unstressed state; and a distal surface located away from the proximal surface of the cover assembly and arranged to fit in said cup implant and/or insert, said distal surface comprising at least a seal (12, 32, 68, 81); wherein the initial volume of the cavity is created when the holding device is inserted into the cup implant to a predetermined position in which the cover assembly interacts with the rim of the cup implant and characterised by a proximal surface, said proximal surface comprising a region that is able to interact with or receive an impaction device and at least one resilient and elastically deformable region (6, 7) such that when elastically deformed in a stressed state there is natural bias of the resilient and elastically deformable region to revert to its unstressed state; and a distal surface located away from the proximal surface of the cover assembly and arranged to fit in said cup implant and/or insert, said distal surface comprising at least a seal (12, 32, 68, 81); wherein the initial volume of the cavity is created when the holding device is inserted into the cup implant to a predetermined position in which the cover assembly interacts with the rim of the cup implant and characterised by (ii) an impaction device (1, 94) which is configured to be impacted in use by a surgeon with hammer blows required to achieve a jam fit of the cup implant into the hip socket, said impaction device comprising: a proximal end; a shaft; and a distal region that interacts with at least a part of said cover assembly, said shaft and/or distal region of said impaction device further comprising a mechanism that prevents additional fluid from entering the cavity. a proximal end; a shaft; and a distal region that interacts with at least a part of said cover assembly, said shaft and/or distal region of said impaction device further comprising a mechanism that prevents additional fluid from entering the cavity.