An assembly for positioning a tibia cut guide (10,110,210,310) includes a sensor bracket (314B) with a first inertial sensor (20A). The sensor bracket (314B) is mountable to the tibia (11) at a location thereon representative of a mechanical axis, and mountable to the tibia cut guide (10,110,210,310). An ankle clamp (316C) is mountable non-invasively about a skin (S) of an ankle (A) of a patient to be in fixed relation to the ankle (A) and has a second inertial sensor (20B). The ankle clamp (316C) is spaced apart from the sensor bracket (314B) along a length of a leg and free of interconnection therewith. The first and second inertial sensors (20A,20B) are displaceable with the tibia (11) to register the tibia (11) in a reference coordinate system defined by the first and second inertial sensors (20A,20B), and to calibrate the first inertial sensor (20A) with the mechanical axis.
