This is an interesting challenge, and at present, there are a lot of unstated variables. To quantify any change you obviously need a scale. Do you have access to similar photographic data prior to the swelling taking place? Or changes in the swelling? If so you have a comparison that will allow you to gauge the change.
I’d then suggest using a digital imaging program like Adobe Photoshop to set up a grid on medial and lateral images of the changed tissue. This grid might then offer a matrix within which to track change due to swelling.
I know nothing about medicine, and only a little about anatomy, but suspect that this approach might at least give you a starting framework which, with calibration, might offer the data you need.
You’ll need a computer and access to a program like Adobe Photoshop. If you need to seek assistance from someone who is familiar with the program.
Open the pre and post operative images in Photoshop. Make sure that the views and sizes of the images are comparable. Once the images are or comparable scale, go to the View command and scroll down to make the Rulers active. Once these appear go to the Preferences, and set the Units and Rulers preference to a measurement that suits, such as centimeters. Once this is done, use the cursor to click and drag from the Ruler around the edge of the image. This will drag on a guide that you can align with the units across the top and down the side. In this way you’ll create a regular one centimeters square graph on pre and opst operative images and on both medial and lateral views. This will give you a means of measuring the tissue distention due to swelling. It may not be extremely precise, but it will provide you with a reasonably accurate measurement.