My mom is aware of her dementia, but not the degree of it. Today I told her about an appointment with the neurologist next week, we're lucky to get in so quickly. I told her it was to check in about her meds for Parkinson's and to see what he thinks about her getting a knee replacement, how this potentially can affect her brain. A few hours later she starts asking me about it, but is all confused as to what it is about, so I tell her again. I want to tell her her dementia is getting worse but not sure that is wise. She is 90 and very healthy other than the PD which is controlled by her medication. It is the only medication she takes besides OTC pain relievers and allergy relief. I know she can physically handle the knee replacement but unsure about her mind. She had the right knee replaced 3 years ago and did really well with that. This was before the PD diagnosis. Her left knee is getting really bad - could've been done the same time as the right, but she didn't want to at the time. It is the main source of pain and discomfort for her and is diminishing her quality of life quite a bit. She tries to joke about it saying it only hurts when she walks. She gets sad about her knee and that she can't really do anything anymore.
My mom had her knee replaced too, and she never walked real well, BUT eventually, at least it did not hurt her every time she put weight on it. She did seem worse right after the general anesthesia but not really much worse than she had been before - I do think she should have opted for regional and sedation as she'd had for her hip, but at that point they let her decide which she wanted. She got through some minor peripheral vascular procedures with local and sedation after that. I would spend some quality time talking to the anesthesia person to see what they can do to minimize the risk of her getting dramatically worse - there may be a good option and you'd hate to deny a procedure that would help with comfort and mobility.
Since there isn't anything that can be done about it, maybe calling it "mild memory issues" and choosing some healthy lifestyle activities (5 min of sitting exercise, a word game 10 min every day, "Name that tune", some of the fun online brain games, etc), may help her feels like she is contributing to her own well-being.
Regarding the knee. What I've observed is that General Anesthesia is VERY hard on the brains of the elderly, and even more so for those with dementia. Surgeons deal with the mechanical aspect of the body, so it is considered a "successful" surgery if the knee works, regardless of what condition her mind is in afterward.
Please look into some of the injection options (cocks comb, etc) if you haven't already done so. The pain in the knee is a problem, but not as much as drastic mental deterioration.