A computer-assisted surgery system for planning/guiding alterations to a bone in surgery, comprises a trackable member adapted to be secured to the bone. The trackable member has a first inertial sensor unit producing orientation-based data for at least two degrees of freedom in orientation of the trackable member A positioning block is adapted to be secured to the bone, with at least an orientation of the positioning block being adjustable once the positioning block is secured to the bone to reach a selected orientation at which the positioning block is used to guide tools in altering the bone. The positioning block has a second inertial sensor unit producing orientation-based data for at least two degrees of freedom in orientation of the positioning block. A processing system providing an orientation reference associating the bone to the trackable member comprises a signal interpreter for determining an orientation of the trackable member and of the positioning block from the orientation-based data. A parameter calculator calculates alteration parameters related to an actual orientation of the positioning block with respect to the bone as a function of the orientation reference and of the orientation of the positioning block.