https://orcid.org/0000-0002-8279-1621
Abstract
Peripheral nerve stimulation (PNS) of the lower extremity has progressed significantly over the last decade. From the proof of concept that ultrasound-guided, percutaneous implantation was possible to advances in waveforms, the field has been rapidly evolving. While most nerves in the lower extremity can be PNS targets, consideration must be given to the ergonomics of pulse generator placement, patient comfort, and avoidance of lead migration. For this paper, we examine some of the conditions amenable to lower extremity PNS, review the evidence and history behind PNS for these conditions, and describe approaches for the tibial, sural, and superficial peroneal nerves.
A literature search was conducted using PubMed. Search terms used were “peripheral nerve stimulation,” “lower extremity entrapment neuropathies,” “sural nerve,” “superficial peroneal nerve,” “tibial nerve,” and “tarsal tunnel syndrome.” Emphasis was placed on randomized controlled studies, anatomical dissections, and comprehensive review articles. Approaches to nerves and ultrasound images were based on anecdotal PNS cases from an experienced implanter (SP).
The development of ultrasound as a viable method of image guidance for percutaneous peripheral nerve stimulation has led to an exponential growth in the field. Lower extremity percutaneous lead placement is both feasible and an appropriate treatment modality for certain pain conditions.
Relevant Conditions
Tarsal tunnel syndrome is a multifaceted compression neuropathy that is caused by a compression of the tibial nerve or its associated branches as it passes under the flexor retinaculum of the ankle [1]. The diagnosis is still considered controversial due to poor diagnostic sensitivity, its actual contribution to foot pain, and which treatment approaches are most appropriate [2]. Although the tarsal tunnel is the most common site for tibial nerve entrapment, there are multiple locations along the course of the nerve where entrapment can occur. If entrapment occurs at the fibrous sling between the tibia and fibula (at the origin of the soleus muscle), the patient may develop “soleal sling syndrome.” Proximal tibial neuropathy may be confused for and should be differentiated from chronic posterior compartment syndrome and popliteal artery entrapment syndrome [3, 4].
Postoperative nerve entrapment is a common cause of chronic pain. Isolated sural neuropathies are rare, but a well-documented cause of sural nerve injury is Achilles tendon repair (especially with percutaneous repair) [5]. Because of its superficial anatomic course down the distal leg and lateral ankle, the nerve is more susceptible to injury. The superficial peroneal nerve (SPN) is a commonly injured nerve during ankle surgery (especially after ankle arthroscopy, with an incidence of 1.92–5.4%) [6], after an ankle fracture, or after injury as a result of participation in sports [3]. Entrapment typically occurs at the area where the nerve exits the crural fascia because of a thickened fascial tunnel, a defect in the fascia with muscle herniation, or due to soft tissue mass [7, 8]. Similar to the sural nerve, the superficial course of the SPN makes it susceptible to injury, and it may be compressed at the lateral calf or ankle [8].
Approaches to Nerves
When considering PNS for the lower extremity nerves, implanters have several factors to consider. It is critical that PNS leads do not cross large joints, including the hip or the knee, because of the risk of lead migration. Ergonomics and wearability of the PNS pulse generator are also factors, and avoiding placement in certain areas (posterior thigh or pelvic area in particular) should be considered. Wearing the pulse generator in these areas can interfere with sitting and exercise. Commercially available percutaneous PNS systems can be permanent or temporary (intentionally reversible), and therefore one must consider the trajectory of the pain condition before placing the device.
The sural and SPN are located in most cases <1 cm under the skin in the lower leg, making them good targets for PNS. The challenge lies in the fact that current permanent percutaneous PNS systems require tunneling of relatively longer leads (≥15 cm) in the subcutaneous space. For temporary devices, insertion must be accomplished in such a manner to allow for at least 3–4 cm of lead to be in the body before exiting at the skin to avoid inadvertent migration or even removal. The approach to these superficial nerves often necessitates an out-of-plane, proximal-to-distal, or distal-to-proximal approach with ultrasound to allow for an optimal length lead to be inserted, thus avoiding migration or traversing other vital structures including adjacent nerves or tendons.
The sural nerve is a fusion of the medial sural cutaneous nerve (MSCN), which is a terminal branch of the tibial nerve and the lateral sural cutaneous nerve (LSCN), which is one of the terminal branches of the common peroneal nerve. The point where the MSCN and LSCN join to form the sural nerve is highly variable, but in most cases, this occurs around the mid-calf. The sural nerve then continues down the posterolateral leg and courses posterior to the lateral malleolus and anterior to the Achilles (Figure 1). The nerve can be seen traveling adjacent to the small saphenous vein distally, and this is an optimal area to place PNS leads. Given the superficial nature of the sural nerve, a parallel, out-of-plane approach is often utilized.
![The variable anatomy of the lateral sural cutaneous nerve (LSCN) and medial sural cutaneous nerve (MSCN) down the posterior leg. The LSCN and MSCN join to form the sural nerve, which then traverses posterior to the lateral malleolus and supplies sensation to the lateral foot [9].](https://oup.silverchair-cdn.com/oup/backfile/Content_public/Journal/painmedicine/21/Supplement_1/10.1093_pm_pnaa202/1/m_pnaa202f1.jpeg?Expires=1749116117&Signature=P-T9~rrqKMaGbpGIYpAjgegwG2mskSnX4I6hK7JO1I9siN2NmIxtJaGjVWXsfH-bmbJhDpXr2kVOd1o9mMyVvVDJoIqRuh3UO7C-6lJTmekCR9eojm5y7fwK8IyLdS0NQ7h~i15vwrXbOaVPcyqfjd-j~JpcR0CYkrUT3EwyhNILLytdgDY~SoHtI0r47fJ-uYuYd-7RpJqgdt4CJSQBa-fQznz~pa~3o18-ur0FX4LMG~~g2sauqkE0UhEoICLs7-UXuzNrhY4z9CwDxC6dog4QBUiSjuwIx9Zjq~ObkIZ7OiYIKCy4gRFaKKtcXyy6IPL-0Gd9TxrQYvAnAc-Jfw__&Key-Pair-Id=APKAIE5G5CRDK6RD3PGA)
The variable anatomy of the lateral sural cutaneous nerve (LSCN) and medial sural cutaneous nerve (MSCN) down the posterior leg. The LSCN and MSCN join to form the sural nerve, which then traverses posterior to the lateral malleolus and supplies sensation to the lateral foot [9].
A common area to target the SPN is the distal leg, just proximal to the lateral malleolus. The nerve runs in a predictable manner between the anterior and lateral compartments. As it travels distally toward the foot, it becomes more superficial and pierces the crural fascia. In many cases, it is beneficial to place PNS leads proximal to this point to avoid placing leads in the immediate subcutaneous space. The pulse generator can be worn comfortably on the lateral lower leg (Figure 2).
Superficial peroneal peripheral nerve stimulation lead placement in the lower leg. The nerve (*) can be visualized between the extensor digitorum longus and the peroneus brevis muscles and superficial to the fibula. The lead (red dotted line) was placed parallel to the nerve via an out-of-plane approach.
The tibial nerve is frequently targeted proximal to the tarsal tunnel in cases of foot and medial ankle neuropathic pain. At this level, the tibial nerve is easy to visualize with ultrasound, and leads are typically placed parallel to the nerve given the minimal amount of tissue in this area and the proximity of the tibia and Achilles tendon. More proximally, the nerve travels deep in the soleus and gastrocnemius muscles with the posterior tibial artery (Figure 3).
Placement of a permanent, percutaneous peripheral nerve stimulation lead targeting the tibial nerve (*) above the tarsal tunnel. An out-of-plane approach is employed with ultrasound. The lead (red line) is placed parallel to the nerve proximally to distally and subsequently tunneled laterally.
As an alternative approach to targeting the terminal nerve branches in the leg and foot, PNS leads can also be placed proximally at or above the popliteal fossa. The sciatic nerve can be targeted at the popliteal fossa just as it starts to bifurcate or even above this level. Depending on the patient’s pattern of pain, optimal stimulation may be achieved by placing leads along the lateral aspect of the sciatic nerve (for peroneal or SPN-related nerve pain) or the medial aspect of the sciatic nerve (for tibial-related nerve pain). The benefits of placing leads at this more proximal level include a convenient placement of the pulse generator for the patient, the ability to stimulate nerves reliably in cases of distal anatomic variability, and fewer technical challenges with tunneling for the implanter due the presence of more muscle and fat in this area of the body (Figure 4).
Temporary peripheral nerve stimulation lead placement for postsurgical dorsal foot pain following excision of a ganglion cyst. Superficial peroneal nerve injury was suspected, but due to allodynia and patient discomfort in the lower leg, the common peroneal nerve (*) was targeted at the popliteal fossa just distal to the sciatic bifurcation. The lead trajectory (red dotted line) is shown beneath and perpendicular to the nerve.
Conclusions
The development of new percutaneous PNS systems and use of ultrasound for lead implantation have rapidly expanded the availability and applications of peripheral nerve neuromodulation. In this paper, we discussed some of the more common lower extremity pain conditions that may be amenable to PNS therapies and reviewed placement approach considerations. Although challenges remain as the field develops, the interest from clinicians to incorporate PNS in their practice shows both the economic potential and the importance of appropriate placement and utilization of the therapy.
Funding sources: None.
Conflicts of interest: Dr. Pritzlaff is a paid consultant for Bioness, SPR Therapeutics and Nalu Medical. Dr. Hanyu-Deutmeyer has no conflicts to report.
Supplement sponsorship: This article appears as part of the supplement entitled “Peripheral Nerve Stimulation: Update for the 21st Century” sponsored by Bioness and by SPR Therapeutics, Inc.
