Joint replacement spacers

A kit comprising (a) a spacer for replacing a removed portion of a first bone and articulating with a second bone, (b) a bone-cutting bit, and (c) a guide sized and shaped to guide the bone cutting bit, wherein:

• the spacer comprises:
• an articulating surface sized and shaped to articulate with an articular surface of the second bone;
• and
• a stabilizing surface sized and shaped to conform to a cut surface of the first bone, wherein:
• the spacer defines first axis and a second axis not parallel to the first axis;
• in a cross-section of the spacer perpendicular to the first axis, the stabilizing surface defines a first curve including:
• a first portion with a first radius of curvature;
• and
• a second portion with a second radius of curvature not equal to the first radius of curvature;
• in a cross-section of the spacer perpendicular to the second axis, the stabilizing surface defines a second curve including:
• a third portion with a third radius of curvature;
• and
• a fourth portion with a fourth radius of curvature not equal to the third radius of curvature;
• and
• the second and fourth radii of curvature are not equal;
• and
• the bone-cutting bit having a cutting blade, the cutting blade having a first portion with a first radius of curvature and a second portion with a second radius of curvature, the bone-cutting bit and the guide are sized and shaped such that, when the bone-cutting bit is guided by the guide, the cutting blade sweeps out a surface complementary to the stabilizing surface of the spacer.