Abstract
This chapter, reviews the basics of lower extremity innervation and the commonly performed regional anesthetic blocks. Through a case of lower extremity trauma, the author provides an overview of the available regional techniques with anatomic and ultrasound descriptions for their performance.
A family vacationing in the mountains experienced a motor vehicle accident (MVA) involving their six-year-old daughter and four-year-old son. Both were taken to the local hospital for treatment of non-life-threatening orthopedic injuries sustained in the MVA.
The six-year-old girl was complaining of bilateral leg pain for which X-rays demonstrated a right-sided femoral neck fracture.
Her orthopedic surgeon wishes to repair her fracture with surgical pinning and asks if there is a regional block that can be performed to assist in her postoperative pain relief.
What Is the Innervation to the Lower Extremity?
The entire lower extremity is innervated by the lumbar and sacral plexus and their corresponding terminal nerves. The lumbar plexus arises from the L1–L4 spinal nerves (with some patients adding T12) and innervates mostly the ventral aspect of the lower extremity while the sacral plexus arising from S1–S4 innervates mostly the dorsal aspect. Figures 52.1–52.3 and Table 52.1 demonstrate the lower extremity innervation.
Figure 52.1 Lumbar plexus nerve distribution.
Figure 52.2 Sacral plexus nerve distribution.
Table 52.1 Summary of the lumbo-sacral plexus terminal nerve functions. Reproduced with permission from Arbona FL, Khabiri B, Norton JA (eds.) 2011. Ultrasound-Guided Regional Anesthesia: A Practical Approach to Peripheral Nerve Blocks and Perineural Catheters. Cambridge, UK: Cambridge University Press.
| Lumbo-sacral plexus terminal branch | Cutaneous sensation | Joint sensation | Motor action |
|---|---|---|---|
| Iliohypogastric nerve (L1, +/–T12) | Inferior abdomen Anterior hip | None | Abdominal muscles (transverse abdominus and obliques) |
| Ilioinguinal nerve (L1) | Medial, proximal thigh Anterior scrotum/labia majora | None | Abdominal muscles (transverse abdominus and obliques) |
| Genitofemoral nerve (L1, L2) | Over femoral triangle Fascia and skin of scrotum/ labia majora | None | Elevation of scrotum (cremaster muscle) |
| Obturator nerve (L2–L4) | Medial aspect of thigh Medial aspect of knee | Anteromedial hip, flexion of thigh | Adduction of thigh Extension of thigh |
| Femoral nerve (L2–L4) | Anterior thigh Medial leg and ankle, +/– medial foot | Anterior hip, knee | Flexion and lateral rotation of thigh, extension of leg Flexion of leg |
| Lateral femoral cutaneous nerve (L2, L3) | Lateral thigh | None | None |
| Pudendal nerve (S2–S4) | Much of external genitalia | None | Muscles of perineum |
| Posterior cutaneous nerve of thigh (S1–S3) | Posterior thigh Posterior leg | None | None |
| Sciatic nerve (L4–S3) | See below for terminal branches | Posterior and posteromedial hip, Posterior knee, ankle, foot | Flexion of thigh, adduction of thigh Flexion of leg See below for terminal branches |
| Superficial peroneal nerve (L4–S2) | Lateral leg Dorsal foot | None | Eversion of foot Plantarflexion of foot |
| Deep peroneal nerve (L4–S2) | Webspace between first and second toes | Ankle | Inversion of foot Dorsiflexion of foot Extension of toes |
| Tibial nerve (L4–S3) | Plantar surface of foot | Ankle Foot | Flexion of leg, plantarflexion of foot, flexion of toes Adduction and abduction of toes |
| Sural nerve (S1) | Lateral foot and fifth toe | Lateral ankle | None |
What Are the Regional Anesthesia Options for This Procedure?
Femoral neck fractures are high enough on the femur that the typical injection site of a femoral nerve block at the inguinal crease will not provide blockade at the surgical site. A proximal femoral nerve block in combination with a lateral femoral cutaneous nerve block can be performed to cover the sensory area overlying the surgical site. Historically, the three-in-one femoral nerve block, where a large volume femoral nerve block injection in combination with applied distal pressure would cause the local anesthetic to spread proximally in a retrograde fashion and block the femoral, obturator, and lateral femoral cutaneous nerves, was thought to accomplish anesthesia of proximal femur/hip. Several studies have questioned the reliability of that technique. A fascia iliaca block could reliably block the lateral femoral cutaneous nerve as well as femoral nerve providing good coverage to the fracture. A lumbar plexus block where the femoral, lateral femoral cutaneous, and obturator nerves are blocked at their proximal plexus would also provide coverage. Lastly, a lumbar epidural block would likely provide excellent coverage at the expense of obtaining a bilateral block, and unnecessarily involving the unaffected side, and other possible side effects such as loss of motor control, and urinary retention.
How Is a Lumbar Plexus Block Performed?
Lumbar plexus blockade was traditionally done using anatomical landmarks to locate the plexus whose depth could be deep with vital organs in close proximity. With the patient in lateral position, a line was drawn connecting the iliac crest laterally and the spinous processes in the midline (at approximately the level of L3–L4). A spot is marked approximately 4–5 cm lateral to the midline on that line where the needle would be inserted in a perpendicular fashion. With a nerve stimulator set at 1.5 mA, a quadriceps twitch should be elicited. Once done, the current should be reduced to obtain stimulation between 0.5 and 1.0 mA.
The current standard of care is to use ultrasound to perform a lumbar plexus block as vital organs and adjacent structures can be visualized and avoided, such as the kidney, peritoneum, and intervertebral foramen. The most common views to visualize the lumbar plexus are the paramedian sagittal (Trident sign), the paramedian transverse oblique (Wave sign). and the transverse (Shamrock sign) scan. In the paramedian scan, the probe is placed longitudinally adjacent to the spinous processes (Figures 52.4–52.6). The resultant spinous processes can then be identified, including the desired L3 and L4 spinous processes. The view that results appears like a “trident” with the psoas muscle in between the transverse processes (spears of the trident). The lumbar plexus lies in the posterior third of the body of the psoas muscles which will appear to be the most superficial third of the muscle in this view.
Figure 52.5 Lumbar plexus block with patient and probe positioning.
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