Transient osteoporosis of the hip and spontaneous osteonecrosis of the knee: a common aetiology?

The aetiologies of transient osteoporosis of the hip (TOH) and spontaneous osteonecrosis of the knee (SONK) are still unknown. Both entities present with pain and extensive bone marrow oedema is seen on magnetic resonance imaging. We describe a patient who presented with TOH and later also developed SONK. Initial magnetic resonance imaging findings of both the hip and the knee showed extensive bone marrow oedema and a subchondral fracture line suggesting that subchondral fractures might be of importance to the aetiology of both conditions. To our knowledge, this is the first case report that describes the occurrence of TOH and SONK in the same patient and introduces the possibility of a common aetiology.