Achievable depth is always limited by anatomy... first and foremost by size and shape of pelvis, secondary by available skin tissue. Another thing is dissection. If surgeon isnt able to create enough space, due to complicated dissection for reasons like inner scar tissue - adhesions, nothing else matters. What Thai surgeons do differently (with non-penile-inversion), is different arrangement of skin flaps, which are used on the outside. And use of skin graft(s) for neovaginal wall.
Dr. Suporn, in majority of patients, only uses scrotal skin graft for vaginal wall. He uses micro-fenestration in skin graft preparation, which increases the surface area. Even if a patient had small scrotum, it is likely enough for 6 inch vaginal depth. In cases where patients were 10, 20 or more years on HRT, or previously had orchiectomy, or skin damage by electrolysis, it might not be enough, and additional skin graft might be needed.
Dr. Chettawut and other Thai surgeons use normal full-thickness skin graft, which is sometimes enough for 7 or more inch (imagine sagging scrotum), and sometimes isnt. It comes down to your anatomy. If patient wants extra depth, but they only have scrotum skin available for 4-5 inch depth, additional graft (usually from groin area) can be used for additional depth.
