Popliteal Approach to Sciatic Nerve Block Is Not Inferior to Infragluteal Approach

Sciatic nerve block provides clinically significant analgesic benefits following total knee arthroplasty (TKA). These benefits include a reduction in pain scores and decrease analgesic requirements.

Despite these benefits, sciatic nerve block has remained among the least performed peripheral nerve block by anesthesiologists. Some impediments relating to single shot sciatic nerve block (SSNB) that may cause anesthesiologists to avoid it include, patient discomfort due to needle passage through dense gluteal or thigh adipose and musculature, and unreliable success because of difficulty in localizing the sciatic nerve (particularly in obese patients).

Even in the current era of US guidance, sciatic nerve block remains a challenge. Despite its helpfulness, ultrasound technology remains hindered by an intrinsic limitation: a trade off between depth of penetration and image resolution; therefore, it works well for superficial blocks but becomes less useful when deep structures are imaged-ironically where this guidance is needed most. The challenge of anatomical depth, as in the case of the sciatic nerve, is one remaining challenge that dictates practical restrictions on the patterns of practice of ultrasound-guided regional anesthesia.

Actually, the recommendations of the Joint Committee for Education and Training composed of the American Society of Regional Anesthesia and the European Society of Regional Anesthesia cite the depth of block resulting in degradation of both ultrasound and needle image as the first among other causes that increase the level of difficulty of a nerve block.

As the sciatic nerve travels caudally in the body, it becomes more superficial with less thickness of tissue separating it from skin surface, making distal sciatic nerve block an attractive alternative. Indeed, both articular branches which provide sensory innervation to the knee joint as well as its muscular branches which provide innervation to the muscles surrounding knee joint most commonly arise from the sciatic nerve either at the knee level, or just above the knee or within the popliteal fossa.

Blockade of the sciatic nerve at the level of the popliteal fossa, commonly termed a popliteal block, is technically easier to perform than gluteal sciatic block and may even be associated with less risk of intravascular injection and nerve injury. Indeed, popliteal block has been reported to provide good postoperative analgesia in total knee replacement and other major knee surgery. However, the small posterior cutaneous nerve of the thigh, which supplies only the skin on the back of the thigh and knee, separates from the sciatic nerve proximally and variably in the gluteal region, and will be spared in more distal approaches. It is for this reason why many practitioners are hesitant to perform distal sciatic nerve blocks for TKA. However, the relative importance of the posterior cutaneous nerve of the thigh for post-operative analgesia following TKA is unknown and maybe clinically insignificant.

Inclusion criteria

• English speaking
• American Society of Anesthesiologists (ASA) Physical Status Scale 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 or failure to institute spinal anesthesia after performing femoral and sciatic blocks
• Bilateral TKA surgeries
• History of significant psychiatric conditions that may affect patient assessment
• Pregnancy