My mom is needing a knee replacement. She is in the latter part of moderate dementia. Will this make her worse?

Beside all the other issues about major surgery for elders is the question of physical therapy. Would your mom be willing and able to complete the difficult PT following surgery? If there’s any doubt about this aspect it’s probably not a good idea to have the replacement surgery.

My father with vascular dementia did fine with a hip replacement. There is a risk the anesthesia will advance your mother's dementia and rehab is absolutely necessary. As a stop-gap measure, you may want to try gel pack wraps. I think the gentle pressure from the gel pack forming around the knee does as much good to remove inflammation as the icing.

https://www.amazon.com/Koo-Care-Flexible-Elastic-Strap-Therapy/dp/B019LOCWLU/ref=sr_1_25?crid=208GWRVHP2QEJ&keywords=ice+pack+wrap+for+knee&qid=1584534853&sprefix=ice+pack+wra%2Caps%2C386&sr=8-25

Other treatment options:

- nerve ablation (a minimally invasive procedure that cauterizes the nerve sending the pain). The only real down-side is that the nerve can sometimes regrow. I think there is no anesthesia required.

- SPRint PNS System (Peripheral Nerve Stimulation) Minimally invasive, no anesthesia. Requires having a very tiny lead (like a hair) inserted for up to 60 days which sends non-painful impulses and is controlled by a hand-held device. 74% of patients have no more pain after that. Could work sooner than 60 days. Lead is removed after treatment. No real down side. https://www.sprtherapeutics.com

- gel injections. This has worked on 1 knee for my mom but not the other. (My mom is 90 and still very active). You may need to have injections more than once. My mom didn't require more than once in her 1 knee.

- pain clinic. You can get topical anti-inflammatory and pain meds but you will need to administer these creams to your mom several times per day. Not systemic like taking pills which can affect stomach and liver, etc.

I hope you find a solution that works for you!

Anesthesia is not great for people with dementia.
I also have to ask these questions.
1. Can and will she follow direction?
2. Will she and the family follow through with rehab?
3. Does she REALLY need the knee replacement? If it is pain can that be managed with medication and ointments?
As the dementia progresses she may not walk in 6 months or a year. Ya never know.
And I am guessing if the surgery was scheduled to be done soon I am sure that is on hold. If not I certainly would not undergo surgery at this time. For a variety of reasons. But to be in a hospital now is not a good place to be, rehab facility is not a great place to be with the lock downs not to mention that after surgery she will be more susceptible to picking up any virus that she happens to be exposed to.
But basic answer is the recovery from anesthesia is slow and she may not ever get back to the baseline where she is now.

I can't and won't try to interpret what someone in a later stage could do, as this probably should be answered by the orthopedist who would perform the surgery, but I do think that the post op care might be challenging b/c of the need to exercise the lower leg.   I've seen some who manually exercise, and others who've used a device that automatically moves the lower leg back and forth.

The last time I saw this though was over a decade ago, so there may be new and better methods of post surgery rehab for knee replacement.

Another consideration:  I'm assuming she would rehab in a facility so she could get not only nursing but PT and OT.  Given the massive lockdowns of almost all commercial establishments, I wonder if she would be able to get a bed in a rehab facility at this time?   And would you and the ortho doctor want her to be exposed to a closed-in environment, given post surgery vulnerability?