Minimum Local Anesthetic Dose for Adductor Canal Block

The purpose of this study is to determine the minimum dose of ropivacaine 0.5%, required to produce pain relief without weakening the leg muscles.

The post-operative period after a Total Knee Arthroplasty (TKA) is known to be especially painful for the first 24 hours. Significant pain can persist up to 3 days in some cases. Successful management of pain post TKA is therefore regarded as essential to early recovery, rehabilitation and timely discharge.

Until now, multiple modes of analgesia have been employed including intravenous patient-controlled analgesia, continuous femoral nerve block and epidural analgesia. These are all effective alternatives but each is limited by side effects. Epidural analgesia provides excellent pain control and has been associated with early rehabilitation despite its negative impact on ambulation in the immediate peri-operative period. Additionally, an increased risk of spinal hematoma has been reported with epidural analgesia and peri-operative low molecular weight heparin prophylaxis. For this reason, epidural analgesia is not routinely offered to patients undergoing TKA today.

A multimodal analgesic approach centered on the use of continuous femoral nerve blocks has been more recently favoured, providing superior analgesia and less opioid-related side effects than a systemic opioid-based regimen. However, femoral nerve blockade is also associated with significant quadriceps muscle weakness, which can impair ambulation, delay physiotherapy and result in accidental fall.

Recent reports suggest that saphenous nerve blockade using an adductor canal approach is a novel technique with which to provide adequate analgesia for major knee surgery. Ultrasound-guided saphenous nerve block in the adductor canal is considered a technically simple and reliable block, providing consistent success. Although traditionally used to provide anesthesia and analgesia to the foot and ankle, recent reports suggest that saphenous nerve blockade in the adductor canal may provide adequate analgesia for major knee surgery. Taking into consideration the anatomy of the adductor canal, it appears possible to target not only the saphenous nerve but also multiple branches of the femoral and obturator nerve.

However, the optimal dose of local anesthetic required to establish knee analgesia without inducing quadriceps weakness has not yet been determined. Identifying an optimal dose would allow for maximum analgesic efficacy with minimal or no motor block, while minimizing other unwanted adverse effects.

This pilot study is designed to determine the optimal dose of ropivacaine 0.5% required to initiate sensory knee analgesia for the post-operative.

Eligibility Criteria:
Inclusion Criteria:

* ASA(American Society of Anesthesiologists)physical status classification system I-III
* 18-85 years of age, inclusive
* BMI 18 - 40
* Scheduled for elective total knee replacement under spinal anesthesia or general anesthesia.

Exclusion Criteria:

* Inability or refusal to provide informed consent
* Any contraindication to regional anesthesia (allergy to local anesthetics, bleeding diathesis, coagulopathy, malignancy or infection at the site of block)