An apparatus for holding a surgical instrument relative to a base is provided having a mounting portion configured and dimensioned to engage a portion of a base, a jaw assembly including first and second jaw members which define a retaining area therebetween configured and dimensioned to retain the shaft of a surgical instrument therein and thereby fix the length of the instrument shaft relative to the base and an operative site, and an instrument position adjustment mechanism which includes an adjustment member rotatably disposed in relative to the mounting portion to facilitate selective position adjustment of the jaw assembly with respect to the mounting portion.