Dorsal Defect of the Patella! The dorsal defect of the patella is an uncommon developmental anomaly, seen only in 0.3 – 1% of all knee imaging studies. The pathophysiology in not well understood but it has been suggested to be sequelae from abnormal vastus lateralis traction and/or abnormal secondary ossification (bipartite patella forme fruste). It presents as a rounded well defined subchondral lesion at the superolateral aspect of the patella. It may show overlying cartilage irregularity but without surrounding marrow edema or inflammation. In the pediatric population, the main differential diagnosis would include osteochondritis dissecans (OD) and a chondroblastoma. OD is usually inferomedial in location and chondroblastoma usually shows prominent peri-lesional edema. In the adult the main differential diagnosis would be grade 4 chondromalacia. One of those cases - once you’ve see one, you’ve seem them all.
