Rebound pain is a well-recognized phenomenon after the effects of a nerve block wear off. Severe rebound pain can happen after outpatient surgery, with patients often needing to come back to the hospital or use other health care resources.

People who are younger, female or have more pain before surgery are at higher risk of having rebound pain. Bone surgeries and upper limb surgeries also increase the chances of having rebound pain after surgery. Patients who have a tourniquet used during leg surgery often have worse pain after surgery compared to patients who get surgery without the use of a tourniquet.

Things that may reduce the chances of getting rebound pain are the use of nerve block catheters, certain additives in nerve blocks and using multiple types of pain killers before the block wears off. Using nerve block catheters can be expensive, so a targeted approach of giving these catheters to patients who have the highest chances of getting rebound pain may be the best way to allocate resources and help patients.

The aim of this study is to create and test a risk model for severe rebound pain after lower limb surgery where patients are getting a sciatic nerve block.

Rebound pain is a well-recognized phenomenon after the resolution of peripheral nerve blocks. Severe rebound pain is prevalent after ambulatory surgery, with potential resultant increased health-care utilization and cost. Risk factors for severe rebound pain may include younger age, female sex, high preoperative pain score, bone surgeries, and upper limb surgeries. Use of tourniquet for lower limb procedures is also associated with worse postoperative pain compared to without. Protective factors may include continuous perineural catheter, regional anesthesia adjuncts, and multimodal oral pain management prior to resolution of peripheral nerve blockade. However, placement of continuous peripheral nerve block catheters utilizes additional health care resources; hence, a targeted approach where continuous perineural catheters are offered to patients at the highest risk of severe rebound pain could potentially offer the biggest benefit.

The aim of the study is to derive and validate a multivariable prediction model for severe rebound pain after lower limb surgery involving popliteal blockade, to assist with risk stratification and shared decision making.

The primary model endpoint is severe rebound pain, defined by transition from well-controlled pain (numerical rating scale [NRS] 3 or less or patient report of satisfactory pain control) in the post anesthetic care unit (PACU) while the block is working to severe pain (NRS pain score 7 or greater) within 48 h of block performance. This definition is modified from that of Barry et al., 2020 as clinically, the investigators have noted some patients having sensory blocks lasting more than 24 hours with the use of adjuncts.

Inclusion Criteria:

• Patients who underwent lower limb surgery and received a single-shot popliteal sciatic nerve block pre-operatively at St. Paul's Hospital from January 4, 2016 to November 1, 2019

Exclusion Criteria:

• Patients who had uncontrolled pain (defined as NRS greater than 3 and/or nursing note documentation of uncontrolled or severe pain and/or undocumented pain status) in the post anesthesia care unit (post-operatively)
• Patients who received a popliteal sciatic nerve catheter

All patients who received a single-shot popliteal sciatic nerve block at St. Paul's Hospital in the study period.