1. Azienda Ospedaliero Universitaria Pisana, UO Pediatria Universitaria, 56126 Pisa, Italy
2. Section of Statistics, University Hospital of Pisa, 56126 Pisa, Italy
3. First Orthopedic and Traumatologic Clinic, University of Pisa, 56126 Pisa, Italy
4. Department of Clinical and Experimental Medicine, University of Pisa, 56126 Pisa, Italy