Haemarthosis is haemorrhage into a joint space and can be regarded as a subtype of a joint effusion.
Trauma is by far the most common cause of a haemarthosis. Other causes include bleeding disorders, neurological deficits, arthritis, tumours and vascular damage.
Lipohaemarthrosis is a result of an intra-articular fracture with mixing of fat and blood from bone marrow.
There is a wide overlap in the radiographic features of a haemarthosis and joint effusions of other aetiologies.
Haemarthosis displaces normal structures, for example in an elbow, anterior and posterior fat pads may be elevated or visible respectively. In the knee, there may be anterior displacement of the patella and quadriceps tendon.
Ultrasound of a haemarthrosis may appear as a joint effusion with or without internal echoes depending on age of the haemorrhage, and with layering of cells (haematocrit effect).
Management consists of removal of the underlying cause, e.g. correction of coagulation. Aspiration of haemarthrosis following trauma is controversial as needle insertion technically converts a closed fracture to an open one with increased infection risk