Why would i have sharp knee pain on downhills or descending steps, but no pain going uphill?

Painful kneecap: You have described some on the classic symptoms of patella-femoral pain or chondromalacia patella. It is an inflammation under the kneecap. The pressure between the kneecap and femur is greatest when going downstairs or hills. See an orthopedic surgeon for evaluation. Often can be treated with just physical therapy alone, but may take unto 2-3 months to effect a cure.

Pain in knee: Descending stairs or downhill places more stress on the quad as the quad is expected to eccentrically "fire." often time are quads are stronger in the concentric (shortening) range of motion and not in the eccentric (lengthening) mode of motion. Work with a physical therapist to strengthen your quads eccentrically and you may experience less pain. E.G., walking slowly backwards on a treadmill.

Common complaint: Patients with knee problems commonly complain more going down than up stairs. The shearing movement may be partly responsible. People with osteoarthritis and meniscus tears have this similar complaint.