How can a spider bit cause someone to need a knee replacement?

Not likely : While certain spider bites can cause damage to the skin and underlying soft tissue, I have never heard of a case where a spider bite resulted in the the need for a total knee replacement.

They can't: While bites can cause infection and illness from the venom or developing infection, such bites cannot directly cause severe osteoarthritis and the need for replacement surgery. Very very rarely, an infection developing in a joint might lead to joint destruction and eventual need for joint replacement.