Balancing is important whether or not you are doing a primary or a revision.  Restoring the joint line is key.  From that point you need to balance your extension and flexion gaps.  Although a primary TKA the main variables are soft tissue releases or bone cuts, in a revision you have those options but more commonly you will use implant size, augments and offset to restore balancing.  Rotation is another important factor, especially in fixed bearing knees.  I balance initially with a CR to get a "feel" of the knee.  I then balance with a stabilized insert to identify the appropriate rotation of the tibia while the post is engaged with the femoral component.

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