A thickness gage including a transparent measurement tube configured to receive and be slidingly engaged over a tubular shaft of a retrograde reamer. The tubular shaft has linear graduation marks on its surface for identifying linear measures for use in measuring a thickness of a bridge bone gap. The linear graduation marks are located on the tubular shaft surface such that at least some of the linear graduation marks can be observed through the transparent measurement tube. Each linear graduation mark when aligned with a proximal end of the measurement tube provides a linear measure of the distance between a distal end of the measurement tube and a cutting member of the retrograde reamer positioned so that its central axis is disposed at a non zero angle relative to a longitudinal axis of the tubular shaft. Such a distance corresponds to the thickness of the bridge bone gap.
