Excessive lateral pressure syndrome (ELPS) is a collection of symptoms and signs related to the edge of the kneecap being forced abnormally against the wall of the underlying groove.

Tight lateral structures or muscle imbalance may lead to tilt of the patella to the lateral side.

This may put strain on the medial side and tenderness in the region of the medial patellofemoral ligament (MPFL)

Cartilage damage may occur on the lateral side from the increased pressure
Cartilage damage from ELPS
Anterior knee pain is exerienced, which tends to worsen as contact pressure increases when the knee is flexed. Over time the adjacent lateral patella and lateral femoral condyle may show damage to the joint cartilage and underlying bone.
CLINICAL PUBLICATIONS
Diagnosis and Treatment of Lateral Patellar Compression Syndrom. Saper MG and Shneider DA. Arthrosc Tech. 2014 Oct; 3(5): e633–e638..
Hamstrings Loading Contributes to Lateral Patellofemoral Malalignment and Elevated Cartilage Pressures: An In Vitro Study. Elias JJ, Kirkpatrick MS, Saranathan A, Mani S, Smith LG and Tanaka MJ. Clin Biomech (Bristol, Avon). 2011 Oct; 26(8): 841–846.