A surgical navigation system comprising: a polyaxial bone screw (200, 400), comprising: a threaded shaft for insertion in bone; and a head (202, 402) connected to the shaft, the head (202, 402) being rotatable about multiple axes relative to the shaft about a centre of rotation (210, D) of the head (202, 402), the head (202, 402) being adapted to receive a bone rod; a navigation probe (300, 500, 600) for probing the position of the polyaxial bone screw (200, 400) when a bone rod is received in the head (202, 402), the navigation probe (300, 500, 600) having a probe axis; wherein the head (202, 402) comprises a first mating surface (211, 401) adapted to engage a second mating surface (301, 501) of the navigation probe (300, 500, 600) for probing the position of the bone screw (200, 400) when a bone rod is received in the head (202, 402); and wherein, the bone screw (200, 400) and the navigation probe (300, 500, 600) are configured such that, when the first (211, 401) and second (301, 501) mating surfaces are engaged, at all rotational positions of the head (202, 402) relative to the shaft: the probe axis of the navigation probe (300, 500, 600) has a fixed orientation relative to the head (202, 402) of the polyaxial bone screw (200, 400), and the second mating surface (301, 501) of the navigation probe (300, 500, 600) is at a constant distance from the centre of rotation (210, D) of the head (202, 402). a polyaxial bone screw (200, 400), comprising: a threaded shaft for insertion in bone; and a head (202, 402) connected to the shaft, the head (202, 402) being rotatable about multiple axes relative to the shaft about a centre of rotation (210, D) of the head (202, 402), the head (202, 402) being adapted to receive a bone rod; a threaded shaft for insertion in bone; and a head (202, 402) connected to the shaft, the head (202, 402) being rotatable about multiple axes relative to the shaft about a centre of rotation (210, D) of the head (202, 402), the head (202, 402) being adapted to receive a bone rod; a navigation probe (300, 500, 600) for probing the position of the polyaxial bone screw (200, 400) when a bone rod is received in the head (202, 402), the navigation probe (300, 500, 600) having a probe axis; wherein the head (202, 402) comprises a first mating surface (211, 401) adapted to engage a second mating surface (301, 501) of the navigation probe (300, 500, 600) for probing the position of the bone screw (200, 400) when a bone rod is received in the head (202, 402); and wherein, the bone screw (200, 400) and the navigation probe (300, 500, 600) are configured such that, when the first (211, 401) and second (301, 501) mating surfaces are engaged, at all rotational positions of the head (202, 402) relative to the shaft: the probe axis of the navigation probe (300, 500, 600) has a fixed orientation relative to the head (202, 402) of the polyaxial bone screw (200, 400), and the second mating surface (301, 501) of the navigation probe (300, 500, 600) is at a constant distance from the centre of rotation (210, D) of the head (202, 402). the probe axis of the navigation probe (300, 500, 600) has a fixed orientation relative to the head (202, 402) of the polyaxial bone screw (200, 400), and the second mating surface (301, 501) of the navigation probe (300, 500, 600) is at a constant distance from the centre of rotation (210, D) of the head (202, 402).