Sciatic Nerve Block Using the Subgluteal Space Approach: A Comparison With the Infragluteal Approach

A new ultrasound-guided "subgluteal space" approach to sciatic nerve block that has been recently described obviates the need to identify the sciatic nerve. The investigators hypothesize that the use of an ultrasound-guided subgluteal space injection to perform sciatic nerve block will increase the proportion of patients experiencing complete sensory block in both terminal branches of the Sciatic nerve in comparison to the same proportion in those who receive the block via ultrasound-guided infragluteal approach with circumferential injection in patients scheduled for unilateral TKA(total knee arthroplasty) surgeries.

The subgluteal space technique is performed by injecting local anesthetics in the space sandwiched between the gluteus maximus and biceps femoris muscle at the level of the greater trochanter and ischial tuberosity rather than circumferentially around the nerve. The bone landmarks are particularly easier to identify because of the high acoustic mismatch and attenuation of bone. The end point of this technique is the distention of the subgluteal space to injection through the block needle. This new approach may translate in benefits including reducing block procedure time, reducing number of needle passes, reducing side effects (vascular complications), and ultimately block success.

The philosophy of injecting local anesthetics in designated anatomic spaces or between the fascial planes where nerves travel, as in the fascia iliaca block for example, is a time tested approach and has become more popular with the introduction of ultrasound because of the ease of visualizing fascia and bone in comparison to nerves under ultrasound. The call for injecting between planes and to stay away from nerves being blocked has found supporting evidence in trials involving ultrasound-guided brachial plexus block at the interscalene level.

One important potential advantage of the subgluteal space approach may be an increased safety margin created when distance is maintained between injecting needles and target nerves, an advantage that seems to make these injection techniques progressively more popular and to expand into a variety of single and continuous nerve blocks.

The investigators aim to compare the newly described ultrasound-guided subgluteal space block technique of the sciatic nerve to the currently practiced ultrasound-guided infragluteal sciatic nerve block with circumferential perineural spread in patients undergoing unilateral TKA surgery.

Eligibility Criteria:
Inclusion Criteria:

* English speaking
* ASA(American Society of Anesthesiologists physical status classification system) I-III patients undergoing unilateral TKA under spinal anesthesia and nerve blocks
* Ages 18-85
* BMI ≤ 38 Kg/m2

Exclusion Criteria:

* Chronic pain disorders
* Significant pre-existing neurological deficits or peripheral neuropathy affecting the lower extremity
* Abuse of drugs or alcohol
* Allergies to any medication included in the study protocol
* Contraindication to spinal anesthesia
* History of significant psychiatric conditions that may affect patient assessment
* Pregnancy