Hi,
I have read your query.
Knee replacement can only be advised by doing physically examining the patient,blood tests, x-rays.
Because, sometimes we advise only quadriceps strengthening exercise with painkillers+ muscle relaxants, as the patient is not fit for surgery or patient sometime does not need surgery.
You have not metion cardiac status of your dad , since he is 89 years old, which is quite old age.
For any operation cardiac fitness is must.
Get x-ray of knee standing AP view done and send us reporting or discuss with your doctor.
Only then further treatment can be advised.
Till then take painkillers+muscle relaxants by discussing it with your doctor.
Apply hot packs for 30 mins morning and evening.
Do quadriceps strengthening exercises as uploaded on you tube.
I hope you have get your answer.
Thanks.