**2.3 Diagnosis and classification of SCD and related secondary avascular necrosis**

To specifically diagnose SCD, cheap and widely available techniques such as hemoglobin electrophoresis or chromatography accurately determine the levels of HbA, HbF, HbS and HbC.

For secondary avascular necrosis and bony lesions due to SCD, standard X-ray, C-T Scan, Isotope Scan, and MRI are indicated in various stages for diagnosis and staging. However, the FICAT- ARLET classification is the must commonly used world wide. It is divided in four stages as below.

FICAT-ARLET classification of AVN (17): Stage I: MRI changes

necrosis that originated from Africa, the United States of America, the Indian subcontinent, the Persian Gulf and from Mediterranean countries. Multifocal secondary vascular necrosis

Herrick first described the characteristic sickle-shaped erythrocytes in 1910 and Pauling and colleagues identified the abnormal hemoglobin (HbS) and coined the term "molecular disease" in 1949 . Hemoglobin A is normally found in the adult and is composed of four globular protein subunits the α and β globins. A fetal form or hemoglobin F is also found, normally, in small proportion in the adult. In SCD, an inappropriate substitution of valine for glutamine at the sixth position of the β globin chain produces hemoglobin S that polymerizes at low oxygen tensions. This causes the red cell to sickle which increases viscosity in the microvasculature and leads recurrent episodes of vaso-occlusion. These recurring episodes of widespread infarcts in patients homozygous for the sickle cell gene (HbSS) cause life-threatening conditions such as renal failure, acute chest syndrome, autosplenectomy, immune deficiency and infection all leading to an early death. As far as heavy surgery and especially arthroplasty is concerned in SCD patients; the key points and the difference with non SCD patients will therefore be focused into the prevention, the monitoring, and if needed, the management of the above cited acute systemic

But SCD complications are not just acute; these repeated cycles of hypoxia and inflammation due to sickling cause chronic musculo-skeletal pain and finally secondary avascular necrosis of bone ends, especially, femoral and humeral heads, and less frequently, femoral condyles. This secondary avascular necrosis, at the end stages, causes severe chronic joints pain and functional impairment, for which very few solutions except of arthroplasties are currently available. In deed, secondary avascular necrosis is the leading indication of arthroplasty in SCD carriers. Secondary avascular necrosis doesn't occur only in homozygous HbSS patients; it is also common in heterozygous SCD clinically asymptomatic carriers with AS hemoglobin. In to the other hand, other types of abnormal hemoglobin such as HbC (Substitution of a lysine for glutamine at the 6th position of the β-globin chain) are also found in some patients with similar effects. Finally, secondary avasular necrosis due to SCD is therefore not exclusively observed in homozygous SS patients (HbSS); it should also be suspected in various other

**2.3 Diagnosis and classification of SCD and related secondary avascular necrosis**  To specifically diagnose SCD, cheap and widely available techniques such as hemoglobin electrophoresis or chromatography accurately determine the levels of HbA, HbF, HbS and

For secondary avascular necrosis and bony lesions due to SCD, standard X-ray, C-T Scan, Isotope Scan, and MRI are indicated in various stages for diagnosis and staging. However, the FICAT- ARLET classification is the must commonly used world wide. It is divided in

**2.2 Physiopathology of SCD & related secondary avascular necrosis** 

was found to occur in at least 64 percent of patients.

heterozygous forms; especially, HbAS and HbSC carriers.

complications.

HbC.

four stages as below.

FICAT-ARLET classification of AVN (17):

Stage I: MRI changes


### **2.4 Work-up and management of SCD systemic acute complications 2.4.1 General**

Preoperatively the HbS level should be of less than 30% of the circulating hemoglobin before major surgery such as Arthroplasty; however, VICHINSKY et al., have shown in a randomized controlled trial that, exchange transfusion may not be necessary to avoid complications. In all cases, it is prudent to take the preoperative hemoglobin concentration to 100 g/L and to keep it at this level in the early postoperative phase; this objective may be obtained by ordinary transfusion of normal red blood cells as it may be confirmed by post transfusion electrophoresis or chromatography. This will also reduce the risk of perioperative thromboembolic complications. The above target hemoglobin level may be rich by preoperative oral folic acid of few weeks, in those SCD patients with less than 30% of HbSS during the initial work-up; however, at least postoperatively, blood transfusion will be needed. It should be clear that, to the best of current literature, there is no place for autologous blood transfusion in SCD patients; these patients should always be managed with homologous bank blood products. In the other hand, Reduction of HbS concentrations may be obtained by the chronically use of hydroxycarbamide because this increases the concentration of fetal hemoglobin (HbF) which reduces hemolysis and prevents vasoocclusion. It is also well known that that hypothermia, acidosis, hypoxemia and dehydration should be avoided pre and postoperatively.
