This study was a prospective, two arms, parallel-group, randomized controlled trial conducted in a single second-level hospital in Spain. The ethics committee of the institution approved the study. Every patient received complete information and signed written consent. A total of 44 patients were randomized in two groups assigned to receive PI or placebo. The perioperative protocol was the same for both groups. The principal outcome was pain measured at 8 and 24 hours by a visual analog scale (VAS). The pain was registered in the knee and the proximal donor site. Pain scores were also assessed to determine whether the VAS improvement would reach the threshold values reported for the minimal clinically significant difference. The secondary outcome was the need for opioid rescue medication.

The surgical procedure was a single bundle transtibial mono tunnel technique with autologous hamstring under limb ischemia.

Patients allocated to the intervention group received a periarticular infiltration with a solution containing 200 mg of ropivacaine hydrochloride (10mg/ml Kabi Fresenius Kabi Norway AS),150 mg if the patient weight was below 65kg, 5 mg of morphine hydrochloride (10mg/ml Braun Medical SA Spain), 30 mg of ketorolac tromethamine (Normon Spain), 300 micrograms of adrenaline (epinephrine Braun Medical SA Spain), and saline 0.9% till reaching 50 ml of volume. The surgical operation nurse prepared the mixture under sterile conditions at the end of the procedure. The 50 ml syringe was connected to a female probe. The surgeon introduced the female probe inside the closed tendon stripper to use it as a guide. The tendon stripper and female probe were inserted through the distal wound to the proximal stump of the hamstring muscles. Half of the solution was injected into this area. The assistant extended the knee, and the sartorius fascia was sutured to minimize the leaking of the mixture. The surgical team closed the wounds and injected the other half of the intervention into the subcutaneous tissue in the distal wound and portals. There was not any medication introduced inside the articulation. Patients of the placebo group were treated in the same way, but the solution injected was 50 ml of saline 0.9%.

The primary outcome was pain registered in the knee and the posterior proximal thigh at 8 and 24 hours from the surgery. The pain was rated using a 100 mm horizontal line, for which 0 represented no pain and 100 maximum pain. In addition to statistical significance, differences in patient-reported outcomes should be assessed for clinical importance or relevance. The secondary outcome was the need for analgesic rescue medication in the post-anesthesia care unit and during the stay in the orthopedics ward. Complications throughout the hospitalization and in the first month were also registered.

All statistical analyses were performed using IBM SPSS 25.0 Statistics software, and the level of significance was set at α \< 0.05. Data are presented as mean (standard deviation-SD) for continuous variables and as counts and percentages for categorical variables. Characteristics of participants were compared IP group vs. control group using the Chi-square test for categorical variables and Student's t-test for continuous variables.