Background
The popliteal fossa has the semitendinosus and semimembranosus medially and the biceps femoris laterally. The sciatic nerve divides into the tibial and common peroneal nerve about 5–12 cm proximal to the popliteal crease. The sciatic nerve has a common epineural sheath that envelops the nerve trunks of the tibial and common peroneal nerve from their origin in the pelvis.
The sciatic nerve is formed by the union of the first 3 sacral spinal nerves and the fourth and fifth lumbar nerves (see the image below). It is the largest nerve supplying the leg. It leaves the pelvis through the greater sciatic foramen and runs toward the posterior aspect of the thigh between the greater trochanter and the ischial tuberosity. It separates into its terminal branches about 6 cm proximal to the popliteal crease into the tibial nerve and the common peroneal nerve.
The tibial nerve supplies the heel and the sole of the foot. The common peroneal, also known as the common fibular nerve, innervates the lateral aspect of the leg and dorsum of the foot. The medial aspect of the leg below the knee is the only area of the lower leg not innervated by one of the sciatic nerve branches; it is innervated by the saphenous nerve. The saphenous nerve is a cutaneous extension of the femoral nerve.
At the popliteal crease, the nerves are midway between skin and bone. They are lateral and superficial to the popliteal artery and vein in a separate sheath.
The tibial nerve supplies the heel and the sole of the foot. The common peroneal, also known as the common fibular nerve, innervates the lateral aspect of the leg and dorsum of the foot. The medial aspect of the leg below the knee is the only area of the lower leg not innervated by one of the sciatic nerve branches; it is innervated by the saphenous nerve. The saphenous nerve is a cutaneous extension of the femoral nerve.
At the popliteal crease, the nerves are midway between skin and bone. They are lateral and superficial to the popliteal artery and vein in a separate sheath.
The tibial nerve is the larger of the 2 divisions and runs in the middle of popliteal fossa passing inferiorly through the 2 heads of the gastrocnemius. The common peroneal nerve follows the tendon of the bicep femoris along the lateral margin of the popliteal fossa. It is more lateral and superficial than the tibial nerve.
Patients do experience moderate discomfort during a popliteal block because the needle traverses through the biceps femoris.
Indications
A popliteal nerve block is indicated for pain control perioperatively or postoperatively below the patella, the distal two thirds of the lower extremity especially for the ankle or foot but works well for the calf and Achilles tendon. This block provides great analgesia for a calf tourniquet as well. It does miss the medial aspect of the leg, which is innervated by the saphenous nerve, a cutaneous extension of the femoral nerve. A popiteal block in conjunction with an ankle block provides significantly better pain relief than does an ankle block alone for forefoot surgery.
Contraindications
Absolute contraindications to a popliteal nerve block include the following:
Patient not consenting
Allergy to local anesthetics
Infection at site of injection or if unable to insert needle or place probe at area needed because of a splint/cast/dressing
Relative contraindications are coagulopathy or systemic infection.
Technical Considerations
Complication Prevention
See the list below:
Infection is avoided by using strict aseptic technique.
Hematomas are avoided by preventing multiple passes of the needle into the biceps femoris or the vastus lateralis muscles. Avoid placing the needle too deep; the vascular sheath is medial and deeper to the sciatic nerve.
Vascular puncture: As mentioned above, don’t advance the needle too deep.
Nerve injury: Avoid epinephrine or placing a tourniquet at the injection site. This will decrease the chance of nerve ischemia. With nerve blocks, nerve injury is always a possibility. Avoid contact with the nerve bundles, and advance the needle slowly. Stop when a "pop" is felt entering the nerve sheath. Do not inject if high pressure is felt. Do not inject if patient experiences pain. Readjust the needle when the above scenarios are experienced. Most of the time, the needle needs to be withdrawn slightly. Inject slowly and without excessive pressure injecting. This is an extremely rare complication.
Necrosis: Necrosis of the heel has been reported. This is due to numbness at the heel, causing a pressure point when resting. Instruct the patient to use a heel pad or frequent repositioning.
Popliteal Nerve Block Periprocedural Care: Patient Education and Consent, Equipment, Patient Preparation.
Patient Education and Consent
Informed or written consent must be obtained. The patient should have the risk and benefits discussed with them as well as the positioning and what to expect.
An ultrasound machine with a probe of 10–12 MHz frequency.
An 80-mm 22 gauge needle, depth 3–4 cm.
Standard monitors: EKG, pulse oximeter, and blood pressure cuff.
Local anesthetic to inject and for the skin wheal.
Sterile prep, ChloraPrep preferred.
Ultrasound gel, not necessarily sterile since needle inserted away from probe.
Tegaderm to place on tip of probe.
Echogenic non stimulating needles.
An insulated stimulating needle (which can be connected to a nerve stimulator if wanted; these needles tend not to be very echogenic).
Nerve stimulator (if wanted in adjunct to ultrasound images or if an ultrasound is not available): A 21-gauge or 22-gauge insulated needle, nerve stimulator set at 1–1.5 mA, pulse frequency of 1 Hz, and pulse duration of 0.1 msec. Attach needle to nerve stimulator and place grounder on patient.
Local anesthetic (see Anesthesia in Patient Preparation): If a good image exists, 20 cc is all that’s necessary with the use of an ultrasound. Increasing the volume may increase the duration of the block but also increases the chance of complications. Maximum of 40 cc, also need to consider weight-based toxicity of local anesthetic used.
Rescusitation equipment and lipid emulsion must be readily available.
Patient Preparation
Anesthesia
For surgical anesthesia in adults, a single shot of 30 cc of 1.5% mepivacaine plain provides close to immediate (5 min) to 2–3 hrs of surgical analgesia. For longer surgical anesthesia up to 3–4 hrs, 1:400,000 epinephrine is added to the solution. If a longer block is needed, adding tetracaine at 2 mg/cc (0.2%) prolongs the block to 4–6 hrs. Also consider 15 ccl of 1.5% mepivicaine with 15 cc of 0.5% bupivicaine injected, this will give you immediate surgical anesthesia and longer-acting analgesia, over 12 hours.
For longer postoperative analgesia, 0.25% ropivacaine or bupivacaine is used and should provide more than 12 hours of pain relief. The onset is delayed from 5 minutes to about 20 minutes. This can be used for both perioperative and postoperative pain control. Once again, 20–40 cc depending on patient's weight and anatomy.
Positioning
For the popliteal block, the patient can be in 3 different positions: supine, lateral, and prone. The preferred position depends on patient comfort and the clinician’s experience.
Preprocedural Planning
Minimum of pulse oximetry and preferably 3- or 5-lead EKG, blood pressure cuff.
Sedation for the patient—versed and fentanyl, if patient will tolerate and if needed. Usually only needed if done before surgery.
Oxygen by nasal cannula if patient if being sedated.
Monitoring & Follow-up
Patients are usually monitored for at least 30 minutes after the block is done before being discharged.
Popliteal Nerve Block Technique: Prone Approach, Supine Approach, Nerve stimulation
Share
Prone Approach
Have patient lie prone (see image below); place ultrasound probe in the popliteal fossa in the crease. Look for the pulsation of the popliteal artery; adding color Doppler may help. Superficial and lateral to the artery is the tibial nerve. Angling the probe in different positions (toward and away from the clinician, as well as clockwise and counterclockwise) may help with getting the ideal view of the nerve. Once the tibial nerve is identified, move the probe slowly proximally, keeping the same rotation and angle of the probe looking for the common peroneal nerve.
Supine position
Supine position
Supine Approach
Supine is similar to lateral. The landmarks are best visualized with the knee bent; this can be accomplished by placing the leg on a stand and a pillow at the calf or just having the patient bend their leg and prop the foot up with blankets or pillows. Straightening the leg can help identify the nerves and make the block easier. [5]
Bent leg-supine approach.
Bent leg-supine approach.
The technique is the same in all 3 patient positions. Visualize the tibial and common peroneal nerve coming together as one moves proximally. The popliteal artery is deeper and soon can no longer be visualized. The idea of injecting the local anesthetic in the area around the nerves where the bifurcation occurs comes from the days of using the nerve stimulator. With the use of the ultrasound, which allows direct visualization of the nerves, one does not need to find the bifurcation, which can occur anywhere from the peroneal crease to the pelvis. With the ultrasound, the common peroneal nerve and the tibial nerve can be injected separately, making sure to get the local anesthetic circumferentially around each nerve. A single injection can cover both nerves if they are close enough together.
The preferred approach is to have the needle in plane so it can be visualized during advancement. Know which way the probe is oriented so you can see which way you are coming into the skin. Once the needle is in the correct location, slowly inject the local anesthetic (20–40 mL), aspirating every 3–5 mL to rule out intravascular injection. Watch for the spread of the local anesthetic around the nerve. Relocate, at least once, to ensure a good spread occurs.
Most of the popliteal blocks today are performed with the use of an ultrasound probe. Some facilities do not have the use of an ultrasound. Also, some providers that learned the blocks with a nerve stimulator still like using it as an adjunct to the ultrasound. Block needles are available that allow an attachment for the use of the nerve stimulator. Studies show no improvement in block completion is obtained with using both, even when the nerve stimulator is used to confirm that the nerve is what you are looking at by stimulating that area.
Nerve stimulation
Landmarks for the prone approach are the popliteal fossa, biceps femoris, and semitendinosus. Insert the needle 7–10 cm above the popliteal crease, 1 cm lateral to the point midway between the tendons of the semitendinosus and the biceps femoris.
For the lateral approach, the vastus lateralis and biceps femoris tendons are landmarks. The patient should be lateral, and the knee is flexed. The upper border of the patella is marked, and a line is extended laterally. The groove between the tendon of biceps femoris and vastus lateralis is marked. Having the patient flex the knee against the resistance on the calf muscle accentuates the groove. Place the needle at the junction of the groove and patellar line at about 7 cm proximal of knee crease. Advance the needle until it hits the femur, then withdraw and redirect 20–30° posteriorly while looking for foot twitch.
Basically, look for a foot or toe twitch at 0.3–0.5 mA. Start at about 1–1.5 mA and go down in intensity when a twitch develops. Look for inversion, eversion, plantar, or dorsiflexion. A plantarflexion response is more predictive of complete sensory blockade than a dorsiflexion response. A bit more local anesthetic is injected if it is not 100% clear if you are in the right location.
Approach Considerations
The tibial nerve after bifrucation from the sciatic moves down the back of the leg and wraps around the posterior medial malleolos. The common peroneal nerve passes posterior to the head of the fibula and winds anterior.
When performing the block make sure the nerves you are targeting are in the center of the screen.If the needle is difficult to visualize, make sure you are in plane and rock the probe until you get a better image.
Most practitioners use the ultrasound technique without nerve stimulation. Using ultrasound with neurostimulation has shown to be a better blockade than neurostimulation alone.
Popliteal Nerve Block Medication: Local anesthetics
Local anesthetics
Class Summary
Local anesthetics block the initiation and conduction of nerve impulses. Ephinephrine may be coadministered to prolong the duration of anesthetic effects.
Mepivacaine decreases permeability to sodium ions in neuronal membranes. This results in the inhibition of depolarization, blocking the transmission of nerve impulses.
Ropivacaine decreases permeability to sodium ions in neuronal membranes. This results in the inhibition of depolarization, blocking the transmission of nerve impulses. It is a longer-acting agent.
Bupivacaine decreases permeability to sodium ions in neuronal membranes. This results in the inhibition of depolarization, blocking the transmission of nerve impulses.It is a longer acting-agent. It is more cardiotoxic than ropivacaine.
Tetracaine decreases permeability to soduim ions in neuronal membranes. This results in the inhibition of depolarization, blocking the transmission of nerve impulses. This is an option for longer analgesia.