Effect of Volume of Local Anesthetic for Adductor Canal Block on Quadriceps Muscle Function: A Dose Finding Study

Adductor canal block is commonly offered to provide pain relief following knee surgery with the hope that they cause less leg weakness than traditionally performed femoral nerve block. Infrequently, adductor canal blocks also result in leg weakness thereby potentially limiting the advantages of the technique. Investigators want to find out the effective dose for a 30% response (volume of local anesthetic which would result in clinically significant weakness of the leg)

This is a sequential design method of binary response variables for determination of the dose associated with the 50% point along the dose-response curve required to cause quadriceps weakness following adductor canal bock for knee arthroscopic procedures. It is a Dixon and Massey up-and-down study design.

The primary objective is finding the EV50 of 0.5% ropivacaine needed to cause a 30% reduction in quadriceps function 20 minutes post-procedure.

This study design was selected to allow the estimation of the target dose associated with the given outcome of quadriceps weakness. The study will be performed on 36 patients undergoing knee arthroscopic procedures. The quadriceps strength on both legs will be evaluated preoperatively by the physiotherapist.

The quadriceps function will be measured using a hand held dynamometer (HDD). For quadriceps muscle strength evaluation, investigators will place the patient in a seated position with the knees flexed 60 degrees. To avoid interrater reliability (when the strength of the subject overcomes the strength of the tester) investigators will fix the HHD for quadriceps evaluation. A non-elastic strap with Velcro closures will be used to fix the HHD. The Velcro strap will be attached to a chair leg and around the subject's ankle, perpendicular to the lower leg. The HHD will be placed under the Velcro strap, on the anterior surface of the tibia, 5 cm above the transmalleolar axis.

Subjects will be educated about the procedure before outcome assessments. They will be instructed to take 2 seconds to reach maximum effort, maintain this force for 3 seconds, and then relax. For each assessment, the subjects will perform 3 consecutive contractions, separated by a 30-second pause between each trial. Investigators will use the mean value at each time point for calculations, and calculated muscle strength as percent of baseline value. An independent investigator not involved in the performance of the block will assess the quadriceps muscle function before, 20 minutes and then 4 hours after the block.

Following initial motor assessment the patients will be taken to the block room for the blocks to be performed using standard monitors which will include NIBP and pulse oximetry.

Patients will be sedated with fentanyl and midazolam titrated to effect and supplemental oxygen will be administered.The first patient will be given 30ml of ropivacaine 0.5% in the adductor canal block.

Depending on post-operative quadriceps function, the dose for the subsequent patient will either be increased by 2ml or decreased by 2ml (of the same concentration).

The adductor canal block procedure will be standardised for each patients. All patients will receive ultrasound guided saphenous nerve block in the adductor canal using 0.5% ropivacaine. After preparing the skin with chlorhexidine in alcohol solution and draping the area, the puncture sites will be infiltrated with 2% lidocaine. A 22 gauge 90 mm peripheral nerve stimulation block needle will be introduced deep to the sartorius muscle using ultrasonography and a linear high frequency probe and 30 ml (initially) of 0.5% ropivacaine will be injected after negative aspiration for blood while observing the spread of injectate.

The injection will be done above the level where the descending genicular artery is taking off from the femoral artery. The cutaneous analgesia over the medial aspect of leg will be tested to evaluate the success of saphenous nerve block 20 minutes after injection. If the descending genicular artery is not visualised on ultrasound, then the block will be performed proximal to where the femoral artery enters the hiatus in adductor magnus.

Total duration of performance and the time for the onset of sensory block in saphenous distribution will be recorded after the performance of the block. The failure rate of saphenous nerve block will be determined by the performer. Patients with failure of saphenous nerve block will be excluded from the efficacy part of the study.

After performance of the blocks, the patient will be moved to the operating room for the surgery under general anesthesia according to the choice of the anesthesiologist.

The patients will be assessed for the severity of pain after arrival in the PACU for four hours. Patients quadriceps function will be measured at 4 hours post-operatively. Complications such as bruising and post-block neurological deficits will be prospectively collected.

Eligibility Criteria:
Inclusion Criteria:

* Male and females
* Age 16-60 years
* Scheduled surgery
* Knee arthroscopy
* ASA Class I - III

Exclusion Criteria:

* Ipsi- or contralateral leg weakness
* Preoperative neurological deficits
* Narcotic dependent (opioid intake more than 3 months)
* Chronic pain conditions
* Significant cardiac and respiratory disease
* Coexisting hematological disorder or with deranged coagulation parameters
* Pre-existing major organ dysfunction such as hepatic and renal failure.
* Psychiatric illnesses
* Emergency surgery
* Lack of informed consent
* Allergy to any of the drugs used in the study