Can microdermabrasion help fade a surgical scar? I have a pretty deep/large canthoplaxy scar near the corner of my eyelid.

Microdermabrasion is very unlikely to alter the appearance of scarring, particularly the kind of surgical scarring you’re referring to. Microdermabrasion is a non-surgical procedure intended to reveal a fresh, new layer of skin by gently removing skin that’s been damaged or dulled by dirt, oil or other pollutants, with the aim of creating a more youthful, rejuvenated look. If you’re worried about the scarring on your face, I’d recommend consulting with a board-certified cosmetic surgeon who has specific scar treatment experience for advice tailored to you individually.

Microdermabrasion is little more than what we used to call years ago "epidermabrasion," i.e. the kind of exfoliation of surface skin cells that might be obtained from the vigorous use of a Buf Puf, Loofah or even a stiff wash cloth. While I don't personally think it is worth the expense, it can impart a nice, healthy radiance to the skin and allow makeup to go on more smoothly and evenly. To your question, even if performed so vigorously that it begins to resemble a true dermabrasion, it is extremely unlikely to have much of an effect on scars. You would do well to save your money on a series of microdermabrasion treatments and instead consult with a board certified aesthetic physician with experience and expertise in all forms of scar treatments. Best of luck to you. 

A deep canthoplasty scar will not be improved by microdermabrasion. It is difficult to make specific treatment recommendations without being able to see and feel the scar. Possible treatment options include a filler to correct the depression of the scar, scar removal and re-suturing, fractionated CO2 laser resurfacing treatments or microneedling.  I recommend consulting with the surgeon who performed the canthoplasty, or a dermatologic surgeon who can offer you the above options.