Medial compartment insufficiency
The problem is due to early deterioration of the normal 'joint spacers' on the medial side - that is, the medial meniscus and/or the medial joint cartilage, or after a meniscectomy when the meniscus has been surgically removed.
Medial knee loading is altered in subjects with early osteoarthritis during gait but not during step-up-and-over task. Meireles S, Wesseling M, Smith CR, Thelen DG, Verschueren S and Jonkers I. PLoS One. 2017; 12(11): e0187583.