Sickle Cell Disease with Avascular necrosis of Femoral head

Sickle cell anemia (SCA) is a hemolytic anemia characterize by abnormally shaped (sickled) red blood cells which are destroyed and removed from the circulation at an increase rate. The sickle RBCs also can cause vascular occlusion and lead to tissue ischemia and infarction. Sickle cell hemoglobin is formed when the amino acid valine is substituted for glutamic acid in sixth position of the B chain [1]. This abnormal sickle cell hemoglobin (HbS) when deoxygenated, becomes insoluble, and forms aggregates which deform the RBCs and impair the flow through blood vessels [1, 2]. Sickle cell anemia occurs in a patient who carries two sickle cell hemoglobin genes (Hb SS) who are homozygous. Sickle cell trait (HbSA) is the heterozygous form with one abnormal HbS gene and one normal HbA gene which is considered a benign condition with no propensity for vaso- occlusive complications [1]. The most common and earliest clinical presentation in SCA is acute painful vaso occlusive crises. Vaso occlusive crises usually manifest as dactylitis, a painful swelling of hands, fingers, feet and toes. Other complications of SCA are osteomyelitis, osteonecrosis, splenic infarcts, splenic sequestrations, acute chest syndrome, stroke, acute papillary necrosis, and renal insufficiency [1]. Manifestations of muscular skeletal system in sickle cell disease are categorized mainly as intramedullary marrow hyperplasia, extramedullary hematopoiesis, thrombosis and infarcts of bone, epiphyseal infarction, infection leading to osteomyelitis and septic arthritis and growth effects [2]. Epiphyseal ischemic necrosis is common in SCA and frequently involves femoral and humeral heads. It is more often bilateral than avascular necrosis seen in other diseases. The initial radiographs usually appear normal. Early findings in plain radiographs are lucency and sclerosis involving the epiphysis and subsequent development of crescent shape subchondral lucency and eventually depression of articular surface, collapse and fragmentation of femoral capital epiphysis. Changes may be seen involving the acetabulum with osteophyte formation [1]. MR imaging is the most sensitive method for detecting AVN of femoral head. The changes are commonly seen involving the posterosuperior aspect of the femoral head. A clear delineation of an otherwise normal femoral capital epiphysis with low signal intensity band on T1W sequences is the hallmark in early AVN. The low intensity band has a sharp inner face and a blurred outer margin. This inter face usually reaches the subchondral bone plate at a level beneath the acetabular margins or it can completely circumscribe a centrally located marrow area. T2W images show the characteristic double line sign which demonstrate an inner hyperintense band and an outer low signal intensity band [1]. The high signal intensity band correspond to live granulation tissue and the outer low signal intensity band thought to represent either peripheral fibrosis and sclerosis or it could be due to a potential chemical shift artifact. T1W contrast enhance studies show enhancement in the reactive interface [1]. In advance avascular necrosis shows inhomogeneous signal intensity. In majority of cases there is low signal intensity in T1W sequence and range from high to low on T2W sequences. Epiphyseal collapse usually occur in the posterosuperior aspect of femoral head and best demonstrated in saggital images. Subchondral fractures usually precede epiphyseal flattening. Fracture lines appear as low signal intensity bands running parallel to subchondral bone plate. They can be hardly recognized in T1W studies. Fractures are more often detected on T2W studies as a high signal intensity band within the intermediate or low intensity necrotic marrow [3]. When the involvement is limited to bone marrow space, bone scintigraphy studies with 99 Tc MDP bone scan may not reveal the extent of the lesion and the images could be within normal limits. As the healing begins the scan typically demonstrate increase uptake [4]. Spine gives a characteristic appearance due to end plate depression of vertebral bodies, which is called H shaped vertebrae. This H shaped vertebral deformity is caused by ischemia and infarction of central growth plate, leading to a central square shape depression. The H shape vertebra is considered pathognomonic for sickle cell disease although it is seen in only 10% of the time [1,5]. In infants and young children infarctions commonly involve diaphysis of small tubular bones of hands and feet. Infarctions at these sites are known as dactylitis or hand foot syndrome. Clinically they present with swollen hands and feet with loss of movements and fever. Radiographs demonstrate patchy areas of lucency, bone destruction and periosteal reaction [2]. Increase red cell destruction and consequent anemia leads to marrow expansion which causes widening of medullary cavity and thinning of cortex. Bone marrow expansion in skull leads to widening of diploic space, loss of trabecular pattern giving a ground glass appearance. The classical hair on end appearance is not commonly demonstrated in sickle cell anemia which is more commonly seen in thalassaemia [2, 5].