What can a blunt force trauma to a knee with osteoarthritis do?

May break it. Osteoporosis weakens bone so any trauma will break this or weaken the area further and cause further damage.
Everything. Enough trauma can fully disrupt a normal or arthritic knee. The arthritis is usually in older persons with less mobility, muscle and ligament strength. So equal trauma may lead to even worse disruption and slower recovery. Medial and lateral collateral ligament cruciate ligaments meniscus are all at risk along with fractures of femur, tibia fibula patella.