A device for verifying a hip-knee-ankle angle (θ), the device being configured to be in communication with an inertial-based computer assisted surgery system during a knee replacement surgery, the device comprising: a mounting base having at least one planar abutting surface adapted for directly abutting against a resected surface on a distal femur, the mounting base having a bone anchor element to anchor the device to the femur, the bone anchor element being alignable with a distal mechanical axis reference point on the femur, the mounting base including at least a first inertial sensor configured to be in wireless communication with the computer assisted surgery system and operable to determine at least an orientation of the mounting base and to provide orientation data for at least two angular degrees of freedom to the computer assisted surgery system representative of said orientation of the mounting base, the mounting base being thereby operable to digitize at least a mechanical axis of a femur in combination with the computer assisted surgery system; a visual alignment guide element pivotally mounted to the mounting base such that the visual alignment guide element is pivotable about at least one axis of rotation (19,23) to adjust an angular position, and therefore an orientation thereof, the visual alignment guide element including at least a second inertial sensor configured to be in wireless communication with the computer assisted surgery system and operable to determine at least said orientation of the visual alignment guide element and to provide orientation data for at least two angular degrees of freedom to the computer assisted surgery system representative of the orientation of the visual alignment guide element, the visual alignment guide element being pivotable to be visually aligned with a mechanical axis of a tibia. a mounting base having at least one planar abutting surface adapted for directly abutting against a resected surface on a distal femur, the mounting base having a bone anchor element to anchor the device to the femur, the bone anchor element being alignable with a distal mechanical axis reference point on the femur, the mounting base including at least a first inertial sensor configured to be in wireless communication with the computer assisted surgery system and operable to determine at least an orientation of the mounting base and to provide orientation data for at least two angular degrees of freedom to the computer assisted surgery system representative of said orientation of the mounting base, the mounting base being thereby operable to digitize at least a mechanical axis of a femur in combination with the computer assisted surgery system; a visual alignment guide element pivotally mounted to the mounting base such that the visual alignment guide element is pivotable about at least one axis of rotation (19,23) to adjust an angular position, and therefore an orientation thereof, the visual alignment guide element including at least a second inertial sensor configured to be in wireless communication with the computer assisted surgery system and operable to determine at least said orientation of the visual alignment guide element and to provide orientation data for at least two angular degrees of freedom to the computer assisted surgery system representative of the orientation of the visual alignment guide element, the visual alignment guide element being pivotable to be visually aligned with a mechanical axis of a tibia.