1. Department of Orthopaedic Surgery, Duke University Medical Center, Durham, North Carolina, USA.
2. Department of Anesthesiology-Regional Division, Duke University Medical Center, Durham, North Carolina, USA.
3. Department of Biostatistics & Bioinformatics, Duke University Medical Center, Durham, North Carolina, USA.
4. Duke CTSI Biostatistics, Epidemiology and Research Design (BERD) Methods Core, Durham, North Carolina, USA.