holes are 7 mm from the edges a preferred slant angle for these edge holes is in the range of 50 60 degrees see fig5 to the front of this base 10 is attached a shelf 24 the front surface of which is shaped so as to have a curvature that is similar to that found on the top of the typical talus surface see fig6 measurements of a number of ankle joints suggests that the shelf 39 s radius of curvature should in the range of 15 5 20 5 mm the surgeon places his curved blade see fig7 against the shelf 39 s front surface to guide his cutting during the surgical procedure the shelf 24 is mounted so that it can be pivoted up and down about a pivot joint 26 which is mounted on the back of the shelf a set screw 27 allows the angular rotation of the pivot joint 26 to be fixed additionally the nature of this connection is such that the shelf can easily be detached from the shaft this allows for the opportunity to interchange the shelf that is attached to a shaft so that a shelf can be selected from a group of those having various degrees of surface curvature so as to utilize the shelf having the optimum degree of curvature from the shelf 39 s pivot joint there extends a shaft 28 which can slip into and out of a cavity that exists in the front of the base on the side.