Orthopedic surgeon here. Outpatient total joint replacements have taken off like a rocket in the last few years, in part due to advances in due to pain control (pre operative peripheral nerve blocks, etc), accelerated physical therapy, and all around better management and prevention of all the common postop complaints (nausea, opioid induced constipation, etc), but I would say the number one driver has been that insurance companies (including Medicare) will finally pay for it to be done outpatient. 3 years ago you had to keep people in the hospital at least overnight to even get reimbursed. The same phenomenon has happened with many of the spine procedures that can now be done outpatient. The actual surgical techniques have not really changed much over the last few decades other than popularizing a few approaches (anterior vs posterior) or bearing surfaces (metal on polyethylene vs ceramic on polyethylene, etc).