Acute postoperative pain after orthopaedic surgery is common and approximately 40% of all surgical patients experience moderate-severe acute postoperative pain. Treatment of acute pain after surgery is important to relieve patient suffering, expedite resumption of activities of daily living, mitigate the risk of postoperative complications, speed ambulation and rehabilitation and hospital discharge. Good pain treatment requires proper and sound pain assessment to guide analgesic therapy. Past studies have determined the MCID values for a variety of chronic pain disorders. There are however limited data on the MCID for acute pain.At the present time, most perioperative acute pain studies use MCID values that are extrapolated from chronic pain studies yet the validity of MCID extrapolation remains unknown. Procedure-specific MCID for pain after most major orthopedic surgery has not been reported.The MCID for a patient reported outcome can be determined using an anchor and a distribution based method.The anchor based method uses a subjective Global Rating Scale (GRS) scale to assess patient's perception of pain relief after treatment. This is a 15-point Likert scale that ranges from -7 to +7 .Another method to determine the MCID value in pain is the distribution based method using mathematical calculations to generate 3 MCID values - 1) 0.3 standard deviation (SD) of mean change in NRS scores, 2) the standard error of measurement (SEM), and 3) 5% of the instrument range of the mean change in the NRS scores after an analgesic intervention according to patient-rated GAR scale. The SEM is calculated as the SD multiplied by the square root of 1 minus the intraclass correlation coefficient. The final MCID value is the average of 4 values, 1 generated from the anchored and 3 generated from the distribution based methods.

The Patient Acceptable Symptom State (PASS) is another measure for patient-reported outcomes. It represents the threshold beyond which patients consider themselves well (satisfied) after an intervention. The PASS is an absolute value, not a change but this value will vary among patients. In essence, PASS indicates a state of wellbeing (feeling good) as opposed to MCID, a state of improvement (feeling better). The 2 concepts are complementary but contrary to MCID, the PASS is the outcome of interest, instead of the extent of improvement. To determine PASS, the patients are asked this question "In your opinion, do you consider your current pain state satisfactory after your operation?" Patients responding "yes" are considered having an acceptable pain state. The PASS is the 75th centile of the pain scale in those who rated their pain state as satisfactory. Higher baseline pain scores are often associated with higher PASS estimates. Little is known about PASS for acute pain after surgery thus a need to determine this benchmark to properly evaluate the clinical benefits of analgesic interventions.

Quality of Recovery (QoR) is another important PRO measure when evaluating the impact of novel intervention strategies. Quality of recovery can be assessed using the QoR-15 questionnaire (scores 0-150) which measures physical \& mental well-being as an indication of the quality of recovery after surgery and anaesthesia.

To determine MCID for QoR, both the anchor and distribution based methods will be used. The patient will be asked "How would you rate your overall recovery from surgery since yesterday?" Patients will use the same GRS (-7 to +7) to measure their response. With the anchored-based method, the MCID is the mean change in the QoR-15 score when the patient reports a GRS score of +2 or +3. The distribution-based method will generate 3 other MCID values: 1) 0.3 standard deviation (SD) of the mean change in the QoR score; 2) the standard error of measurement (SEM); and 3) 5% of the instrument range. Again, the final MCID value is the average of 4 values, 1 generated from the anchored and 3 generated from the distribution based methods.

To determine PASS for QoR, the patients will be asked, "In your opinion, have you made a good recovery from your operation?" with response options of yes, no, or unsure. Those who give a positive response is considered having made a good recovery. The PASS for QoR is the 75th centile of the QoR-15 score in those who rated their recovery as good. The only MCID and PASS study by Myles reported a QoR-15 score of 8 \& 118 (out of 150), respectively, in patients undergoing a broad range of surgical procedures. Procedure-specific MCID \& PASS for QoR-15 after major orthopaedic surgery are currently unknown. Our proposed observational study seeks to define what constitutes an MCID and a PASS in patients undergoing a variety of elective major orthopedic surgery.