An orientation guide comprising a housing arranged to couple to a surgical instrument. In one embodiment the orientation guide further comprises first and second indicators (22, 26). The first indicator (22) is pivotally coupled to the housing (20) such that the first indicator (22) can rotate relative to the housing (20) about a first axis (24) to maintain its angular position about the first axis (24) relative to a vertical axis irrespective of movement of the housing (20). The second indicator (26) surrounds the first indicator (22) and is pivotally coupled to the housing (20) such that the second indicator (26) can rotate relative to the housing (20) about a second axis (32) orthogonal to the first axis (24) to maintain its angular position about the second axis (32) relative to the vertical axis irrespective of movement of the housing (20). The angular position of the housing (20) about the first and second axes (24,32) is indicated by the position of the first indicator (22) relative to the second indicator (26). In a second embodiment the housing (2) comprises a cavity filled with a fluid. An indicator (4) is supported within the fluid such that the indicator (4) can rotate relative to the housing (2) about a first axis to maintain its angular position about the first axis relative to a vertical axis irrespective of movement of the housing (2). The indicator (4) comprises a marking arranged to indicate the angular position of the indicator (4) relative to the housing (2) about the first axis. The housing (2) has at least one transparent portion for viewing the marking. A measuring element (8) is coupled to the housing (2) and arranged to slide relative to the housing (2) to be aligned with the indicator marking such that the angular position of the housing (2) about the first axis can be recorded.
