As somone who has suffered from knee problems his whole life, I have to give you a virtual <HUG> for what you're going through. My knee never has quite healed from my stumble in Hill Country SNA.
To answer your diagnostic question - Xrays and MRIs show the joint in different ways. Xrays show bone better than MRIs but are lousy at showing soft tissue & cartilage. MRIs are the exact opposite. If one physician ordered an MRI, likely he was suspecting a soft tissue or cartilage (in the knee this would be meniscus, like what I hurt) injury. The other Dr obviously was leaning more toward a bone injury.
As Toejam just said, you are young and healthy so I wouldn't worry overmuch.
In my less-than professional opinion (former RN student, current vet tech & fellow knee injury sufferer), if the knee has a tendency to "buckle", I'd be thinking soft tissue.
Incidentally, I did the same damn thing when moving 6 weeks ago. I was carrying an aquarium down the stairs from my apartment in Houston (a trip I've made several times a days for nearly 4 years) and just stepped out into space 2 steps from the bottom. I broke the last digit of my left hand middle finger and scraped both knees worse than I've ever scraped them in my life but I saved the aquarium!
