It will be appreciated that depth rings 150 may comprise a series of indentations, or grooves, for example V-grooves, or may even comprise protrusions in the outer surface 120 of the main body portion 105 for running an instrument, such as a scalpel or other surgical device along an edge, therein such that the series of indentations, or grooves may act as a guide to score, cut and/or trim the tibi