A pedicle anchor assembly is provided. The pedicle screw assembly comprises:- a bone anchor (1) including a bone engaging portion (3) with a distal end and a proximal end, and a head (5) located at the proximal end of the bone engaging portion (3), where the head (5) includes two diametrically opposite sides with flat head faces (7) and has a first head width (d1) in a first direction extending between the flat head faces (7) and a second head width (d2) in a second direction perpendicular to the first direction, the second head width being larger than the first head width, and- a housing (13) extending along an axial direction and having an open axial end (14), where the housing (13) includes a bottom section (15) with a seat for the head (5) of the bone anchor (1) and with an axial bore (23) which allows the engaging portion (3) to extend from the head (5) through and out of the axial bore (23), the axial bore (23) having a bore width smaller than at least the second head portion width (d2), and where the housing (13) includes a jacket wall section (17) which extends in axial direction from ore close from the bottom section (15) to the open axial end (14) and which includes slits (19) extending axially in the jacket wall section (17) from the bottom section (15) to the open axial end (14), where the slits (19) are located on diametrically opposite sides of the jacket wall section (17),- a washer (125) with dimensions allowing it to be inserted into the housing (13) through the axial open end (14), the washer (125) including two axial extensions (131) which extend from a first side of the washer (125) in axial direction of the housing (13) when the washer (125) has been placed in the housing, the two axial extensions (131) each having an outer face (35) showing away from the respective other axial extension (131) and a flat inner face (33) showing towards the respective other axial extension (131), where the distance between the flat inner faces (33) of the axial e
