The average fat 40-year old doesn't need an MRI when his knee hurts, swells, and feels like it is giving out. Usually he has exacerbated some arthritis, and the MRI will either be unrevealing or will lead the unwary down a path of acute meniscal pathology when they should be thinking degenerative (arthritis). If one's knee swells really big within 30 or so minutes of injury and/or he starts having painful pops and/or locking up short of full extension (straightening), an MRI may still be appropriate for someone in his demographic. The X-rays that a lot of people order before going on to MRI in such situations are typically "negative" because (1) The radiologist is looking for acute injury like a fracture instead of arthritis, (2) the degree of arthritis needed to cause such problems is not dramatic on X-ray, and (3) the X-rays are usually done in these cases without the patient's standing and bearing weight on the limb of interest. The signal change some radiologists are tempted to call contusion (which would be a logical acute change) is actually (at least partially, and possibly essentially completely) a chronic finding.
Here is an MRI image of someone with acute subchondral edema ("bone bruise"). Note the subtle vertically oriented band of white right in the middle of the picture.
Here is an MRI image of someone with arthritis. Ignore the arrow-- the finding of interest to us is the patchy white in the large, central, curved dark area.
MRI is significantly over-utilized in middle-aged people with acute knee pain. Not everyone is going to turn out merely to have "tweaked" a 40-year-old joint, but the overwhelming majority will. Disclaimer: I am not diagnosing or recommending a treatment plan for any specific person here.