I had very similar knee pains a number of years ago when I first took up hiking and started doing a lot of hikes. The pain was very sharp right under the knee cap on steep descents. My doctor did not find any tears in anything on the MRI. There did not seem to be a definitive diagnosis. I went to physical therapy to strengthen the muscles around the knee which seemed to do the trick. After therapy I slowly got back on the trails and built up my elevation gain. I haven't had any problems since then so I think it was really rest and muscle strengthening that was needed.