The sliding genioplasty for chin augmentation is a multidimensional procedure. While it is commonly perceived as a horizontal chin movement, and this is what it was originally designed to do, it can create greater dimensional changes than just horizontal. It can just as easily create vertical chin lengthening (or less so vertical chin shortening) and to a limited degree transverse chin widening.
The ability to make the siding genioplasty a multidimensional procedure is plate fixation. While the operation was originally conceived as needing intimate bone contact to heal (hence the name sliding) that has been clinically proven to not be necessary. This is because the mobilized chin segment is a pedicled bone flap which survives based on its inferior soft tissue attachments. The exposed marrow space of the upper and lower chin segments allows bone to fill in enough of the gap created to heal with an ultimate bony union.
The step configuration is the most commonly used titanium plate for fixation of the bony genioplasty. While there are variations of it amongst different manufacturers, they all have a step shape configuration. While in its standard shape it provides a completely horizontal movement, the plate can be bent to also add a vertical length increase as well. The more the plate is bent the greater the vertical increase…..but at the sacrifice as some horizontal projection loss.
The relevant question is how much horizontal projection is lost as the vertical increase of the plate is increased? This is actually a common problem in sheet metal fabrication known as calculating the bend deduction. The bend allowance and the the bend deduction are two measures that calculate the bend length of a piece of sheet metal to the flat length. Therefore the bend allowance added to the flange lengths (the two bends at the upper and lower screws holes) is equal to the plate’s flat length. It is a fairly complex formula which must factor in metal thickness, bend angle and the K-factor. The simplified answer based on my calculations is that for every 1mm of vertical lengthening obtained the amount of horizontal projection lost is roughly .25mms. Thus for a 10mm step plate a 4mm vertical lengthening, for example, losses 1mm of horizontal projection. If the plate used had greater horizontal projection, vertical lengthening would have a more significant impact on horizontal projection loss.
This calculation helps in choosing the proper fixed plate length for the dimensions needed in a sliding genioplasty. As vertical lengthening requires that a longer horizontal plate length be chosen that needed to account for the vertical length increase.
Dr. Barry Eppley
Indianapolis, Indiana