Abstract: A posterior stabilized/constrained reamer guide is provided that enables a surgeon to prepare a patient s distal femur to receive either a posterior stabilized or a constrained type femoral component. The instrument guide has an opening the largest anterior to posterior housing size. For example, a constrained housing has a longer anterior to posterior dimension than a posterior stabilized housing. The apparatus of the present invention provides an improved reamer guide that addresses differences in housing sizes and necessary bone removal. The apparatus provides a housing guide that accepts either a posterior stabilized collet or a constrained collet. Each of the collets accepts a rotary reamer or a box chisel. During surgery, the reamer is first used to form a cylindrically-shaped or oval shaped cut. The box chisel then removes additional bone to create a rectangular or squared opening. This insures that only the necessary bone is removed for each housing depending upon the one selected by the surgeon.