Just wanted to chime in... I rarely post here, but do use it as a resource. Wanted to reassure you that it is doable even with a wound of that size.
I have a hen that had a wound of that size under her wing and around most of her leg... full skin was gone & all muscle was showing + some of the lower internal organs were exposed (i'll post you a photo of 2 weeks in later tonight... I actually didn't take or post any earlier ones as I thought people would be appalled I was trying to heal a wound of this size). This happened around 3/1 and she is is almost healed now.
For the first two weeks, I bandaged her with vet wrap and sterile gauze, changing the dressing twice a day with tons of triple antibiotic ointment and vetericyn spray first + moistening the bandage with it. I cut back the dead skin and cut the extra feathers that hung into it. I also injected antiobiotics penicillin in the breast for 7 days in the recommended amounts found online here. I kept it clean and moist, slightly loose with the ability to drain out the sides (and drain it did!)... the smell was not good, but luckily there was no infection. I found that with all that muscle, if it dried too much, that was terrible for the muscle. A heat lamp and extra nourishment was very important to treat the shock, moisture loss through thte wound and healing. The greenish/dark brown areas that formed scared the heck out of me, but it appeared to be related to the muscle drying/getting damaged and did not spread.
After 12-14 days or so, she would no longer tolerate the dressing ( at first, she left it alone and was quiet, not minding the dressing, but after 2 weeks she literally got more energetic and would pick at it/pull it off, so I decided to go with what she was telling me) and a thin protective layer had formed so I stuck to vetericyn three times daily. She picked at it herself a bit, but seemed to actually remove mostly the dead flesh
She was in a dog crate with food water and treats for over 1.5 months. Loneliness was a big issue and she seemed super depressed, so I had to bring in another chicken at times to even get her to eat. I spent a lot of time just being around her. Also battled a secondary infection of the skin that must;ve been related to her weakened system... a very weird condition that caused her to scale and peel with a thick yellow crust that actually crusted over the face/eyes, this has gradually gone away on it's own, but I'm not sure what it was and had to treat that as well.
The skin closed, pulled in on itself, covered the area and her feathers have actually begun to come back AMAZING!!! There is only a small half dollar sized area that I wonder if it will ever feather up.
Today, she is back out with the flock, but in a wire crate as they have rejected her since she was gone for so long. ..
My advice... it is a long and expensive process (lots of vetericyn and dressings) to do it right. very time consuming if you are cleaning and dressing it adequately. But if you have the time, $, and patience, he will heal. .
