Comparison between intrathecal nalbuphine and butorphanol as adjuvants to isobaric ropivacaine in elective lower limb orthopedic surgeries: A prospective, randomized, double blind study - Nirmal, Akash; Singh, Yashpal; Mathur, Sharad Kumar; Patel, Satish (Anaesthesia, Pain & Intensive Care, 2020)