Open sore on head

<span style="-webkit-tap-highlight-color: rgba(0, 0, 0, 0);">Why is my female cat scratching her head open. I don’t see any evidence of mites in her ears, this has healed once and she scratched it open, the rest of her skin is clear from any dermatitis or allergies. </span>

Hello and I'm sorry for your cats' poor sore skin.   I trust that you know / can see that the sore area is being caused by scratching? - because there are plenty of non itchy things looking like that, including cancerous / polypoid growth, ringworm and so on.    Since you mention that it IS itchy, it could still be Ringworm or cancer - people do report that.  Its worth being aware that Ringworm is an infectious disease and can be passed on to humans, especially the immunocompromised, so if your vets think that it's possible, they may reccommend further steps to prevent that.   I'm not clear on what basis mites were ruled out, but you wouldn't necessary 'see' mites in the ear canal without a reasonable level of skill and experience, or an otoscope either.  Fleas are always a possibility and often don't live on the patient.  However, one of my biggest differentials for ear-scratching is actually pain, which can manifest in unexpected ways in a cat.  Cats seldom 'look in pain' in a way that their owners expect.  Tooth-ache, pancreatitis, cancers (of the skin or ear canal or upper nasal cavity, for example), foreign bodies eg a grass seed deep in the ear or skin, stress, sinus disease, arthritis of the angle of the jaw, even glaucoma, could all present via itching.  There could also be a bite abscess underneath that, or a neuropathy - it's difficult for me to tell out of context.   Self-trauma is a major expression of pain however - and however it started, that lesion looks potentially very painful now.  So we would strongly recommend presenting this to the vet again, in order that they can start ruling things out.  More symptomatic treatment may be needed (I wonder whether they might opt for pain releif?) and they might also propose tests in order to get to the route of this issue.  Wishing you the very best of luck - please would you see them as soon as you can (many vets will offer triage over the phone with a nurse) and let us know how you get on?