Aromatase Inhibitor in Bone Maturation, Children With Silver Russell or Prader-Willi Syndrome

Inclusion Criteria: * Patients with genetically proven SRS or PWS, under treatment with GH in the usual context of the disease, presenting with adrenarche (defined either by DHEAS levels as a function of age or by the appearance of pubic hair) associated with a bone age at least 6 months greater than chronological age and in the absence of the onset of central puberty (LH peak ≤ LH peak in prepubertal patients, according to the standards of the laboratory performing the GnRH stimulation test for LH and FSH, and dating back to less than 3 months). * Patients with medical coverage. * The lower age limit for inclusion is 5 years and the upper age limit is 10 complete years for girls and 12 complete years for boys. * The maximum body-mass index (BMI) Z-score for inclusion is +4 * Patients should be capable of swallowing pills of the same size as the experimental drug. Exclusion Criteria: * Renal insufficiency (creatinine clearance, calculated according to the Schwartz formula, lower than 70ml/min/l, 73 m²), * Hepatic insufficiency (prothrombin ratio \< 50% and factor V \< 50%), * Hepatic cytolysis (liver transaminases levels greater than twice the normal level for age), cholestasis (gamma-glutamyl transferase (GGT) levels greater than twice the normal level for age), * Contraindication to one of the components of Anastrozole or the placebo. * Patients with scoliosis requiring surgery.