Locked knee is an orthopaedic condition requiring urgent treatment. Although the condition can be diagnosed via history, physical examination and imaging studies, the cause of the mechanical obstruction may only be apparent during arthroscopic examination of the knee joint. It is known that imaging plays a role in evaluating the integrity of intra-articular structures, however in some atypical cases, imaging cannot identify the definitive cause of locked knee. Here we report on two cases of locked knee, due to uncommon conditions which were unobservable via normal imaging studies.