An intramedullary nail assembly, comprising: an intramedullary nail, and an insert sleeve, the intramedullary nail comprising at least one transverse throughhole and the insert sleeve comprising at least one transverse locking hole, wherein the positions and axes of a respective transverse locking hole and a respective transverse throughhole coincide in a locked position of the insert sleeve, wherein the intramedullary nail has a nail longitudinal axis and comprises a cavity being accessible from a nail proximal end, the cavity being confined by an nail inner wall, the cavity having a cavity longitudinal axis, wherein the nail inner wall is provided with a predominantly longitudinal groove and a predominantly transverse groove, the predominantly transverse groove and the predominantly longitudinal groove communicating with each other at a junction common to the predominantly longitudinal groove and the predominantly transverse groove, each of the grooves (31, 37) having a length, a width, and a depth, the predominantly longitudinal groove being accessible from a cavity proximal end, and wherein the insert sleeve is configured to be received within the cavity and comprises a cam, the cam radially protruding from the insert sleeve and sized to mate with each of the predominantly longitudinal and predominantly transverse grooves (31, 37) such that the cam is displaceable within the predominantly longitudinal groove from a predominantly longitudinal groove proximal end to the junction and is further displaceable towards a terminal end of the predominantly transverse groove into a locked position of the insert sleeve, wherein the predominantly transverse groove comprises a groove base, characterised in that the groove base, along a part of the length of the predominantly transverse groove, extends radially inwardly thus reducing the depth of the predominantly transverse groove such as to form a hump within the predominantly transverse groove. an intramedullary nail, and an insert sleeve, the intramedullary nail comprising at least one transverse throughhole and the insert sleeve comprising at least one transverse locking hole, wherein the positions and axes of a respective transverse locking hole and a respective transverse throughhole coincide in a locked position of the insert sleeve, wherein the intramedullary nail has a nail longitudinal axis and comprises a cavity being accessible from a nail proximal end, the cavity being confined by an nail inner wall, the cavity having a cavity longitudinal axis, wherein the nail inner wall is provided with a predominantly longitudinal groove and a predominantly transverse groove, the predominantly transverse groove and the predominantly longitudinal groove communicating with each other at a junction common to the predominantly longitudinal groove and the predominantly transverse groove, each of the grooves (31, 37) having a length, a width, and a depth, the predominantly longitudinal groove being accessible from a cavity proximal end, and wherein the insert sleeve is configured to be received within the cavity and comprises a cam, the cam radially protruding from the insert sleeve and sized to mate with each of the predominantly longitudinal and predominantly transverse grooves (31, 37) such that the cam is displaceable within the predominantly longitudinal groove from a predominantly longitudinal groove proximal end to the junction and is further displaceable towards a terminal end of the predominantly transverse groove into a locked position of the insert sleeve, wherein the predominantly transverse groove comprises a groove base, characterised in that the groove base, along a part of the length of the predominantly transverse groove, extends radially inwardly thus reducing the depth of the predominantly transverse groove such as to form a hump within the predominantly transverse groove.