Genicular Peripheral Nerve Stimulation

Introduction

Peripheral nerve stimulation (PNS) is an effective neuromodulation modality for the treatment of chronic neuropathic pain in the distribution of specific peripheral nerves. Saphenous and genicular nerves play an important role in the treatment algorithm for chronic knee pain.

Clinical Presentation

Patients who are candidates for PNS of the knee may fall into two main categories:

• 1.

  Nociceptive pain that is chronic, leading to hypersensitization of peripheral nerves

• 2.

  Neuropraxic peripheral nerve pain, or peripheral nerve damage, resulting from neurotmesis, usually from trauma.

In all cases, the end result is peripheral nerve damage causing pain in the knee joint and surrounding tissue that can be exacerbated with weightbearing or passive and active range of motion or can be painful at rest in a nonweightbearing joint. The most common nerve distributions for posttraumatic knee pain are the infrapatellar saphenous (IPS) nerve and the anterior femoral cutaneous nerve. The peroneal nerve can also suffer injury.

Traumatic knee pain can be thought of in two main categories, where fractures, contusions, sprains, and strains or soft tissue disruptions form one group, and postoperative trauma from total knee arthroplasty, arthroscopic knee surgery, or open reduction and internal fixation form the other group. It has been estimated that in the postarthroplasty group alone up to 20% of patients can have postoperative knee pain, and it is seen most commonly if patellar resurfacing or osteotomy was performed. More generally, between 16% and 40% of patients overall after any knee surgery can develop chronic pain from any number of complications causing persistent somatic or neuropathic pain.

Physical findings in such patients may vary depending on the underlying cause of the neuropathic pain, but pain can cause impairments to joint range of motion, stability, sensation, and motor strength. Gait impairments are also common. Range of motion is often diminished, with end range of extension being impaired earlier than flexion. The resulting flexion movement at the knee resulting from the extension lag can cause a visible gait cycle impairment that is separate and distinct from an antalgic gait. Decreased range of motion may be primarily due to joint ankylosis, as is seen in advanced joint degeneration or fracture. It can also be seen secondarily after total knee arthroplasty, where preoperative or postoperative soft tissue contractures can restrict knee range of motion, as can prosthetic mispositioning.

Ligamentous instability is seen in individuals with varying underlying pathophysiologies. Previous history of traumatic ligament disruption may be present. In patients with a total knee arthroplasty, ligamentous instability can be present due to prosthesis sizing or traction injury to the remaining ligaments. Prior surgical history can result in both ligamentous instability and neurosensory alterations. The type of alterations seen are often consistent with the pathophysiology of nerve injury. Most frequently, there is decreased sensation around the operative incision, but there can be areas of hypoesthesia and hyperesthesia as well. These sensory changes are often seen after surgical trauma, including fracture open reduction and fixation, total joint arthroplasty and arthroscopic knee surgeries of either a diagnostic or therapeutic nature. Inferior medial portal entry sites used in the arthroscopic technique can cause neurotmesis of the IPS. Total knee arthroplasty and open reduction and internal fixation are known in certain cases to cause traction injury to the peroneal or genicular nerve branches, as well as neurotmesis of branches of the genicular nerves and IPS nerve. Similar neurosensory changes can also be seen after closed trauma to the joint, such as after falls or significant sprains and strains.

Postoperative pain can mimic infection, with increased edema/erythema and pain. Pain at rest and pain that increases at night and interferes with sleep often coexist with pain during activity. Allodynic symptoms are common along the dermatome, and affected patients may protect the part from clothing, shower water, and even light touch. Patients can sometimes develop complex regional pain syndrome, and although there are no direct motor disturbances, quadriceps weakness over time may occur from guarding of the knee in flexion and extension. Patients are exquisitely tender over the medial tibial fossa. Percussion over the nerve may elicit a paresthesia (Tinel’s sign), an additional diagnostic finding. Von Frey filaments can assess mechanical allodynia in the cutaneous distribution of the IPS, when comparing sensation thresholds to the nonaffected side.

An often-unrecognized etiology of persistent anterior knee pain is neuritis of the IPS, also known as the infrapatellar branch of the saphenous nerve. Primary entrapment of the saphenous nerve is rare in the absence of severe arthritis and most often results from postsurgical or traumatic entrapment, bursitis, or patellar dislocation. Neuropathic pain syndromes are a common complication of total knee replacements, arthroscopy, and other procedures involving the knee.

A diagnostic injection with 1 to 2 mL of local anesthetic may be extremely useful to confirm the diagnosis of IPS neuropathy/neuralgia, as it should demonstrate significant improvement in pain. Care should be taken to avoid infiltration into a large area because this may confound the evaluation.

Saphenous/IPS nerve entrapment may coexist with other pathologies, so that the complaints can easily be confused with arthrosis. Pain is the hallmark, frequently located diffusely at the anterior knee, and it may be severe. Electrophysiological study is usually not useful, other than to evaluate for alternative pathology. Magnetic resonance imaging (MRI) is usually unremarkable unless there is a space-occupying lesion, as the infrapatellar branch is too small for direct imaging. Clinical diagnosis is key, and it is particularly important to know the anatomy of this nerve to evaluate and treat saphenous/IPS neuritis correctly. But in most cases, the diagnosis of saphenous/IPSN can be accurately established based on clinical signs and symptoms.

Anatomy

The anterior and posterior knee are richly innervated regions ( Fig. 26.1 and Table 26.1 ). They receive branches from the saphenous, recurrent peroneal, common peroneal, superior medial genicular, inferior medial genicular, superior lateral genicular, and inferior lateral genicular nerves, as well as nerves to the vastus medialis, lateralis, and intermedius muscles.

The saphenous nerve is a branch of the posterior division of the femoral nerve, a purely sensory nerve composed of fibers from L3 and L4 nerve roots. It arises in the femoral triangle, pierces the fascia on the medial side of the knee, runs down in front of the great saphenous vein, and supplies sensation to the skin on the medial side of the leg and foot up to the ball of the big toe. As it passes the knee, the saphenous nerve gives off the IPS branch, which runs downwards and laterally and supplies the skin over the ligamentum patellae.

The IPS nerve tends to have significant anatomical variation, even between two limbs from the same body. Arthornthurasook et al. found the IPS to be superficial at the posterior border of the sartorius, crossing across to the front of the knee in 62% of the studied cadavers. Ackmann et al. studied 18 cadavers and completed 30 knee dissections to study the anatomy of the infrapatellar branch in relation to skin incisions and target for percutaneous cryodenervation. The IPS nerve was identified in four different patterns among the dissected knees:

• Anterior type: IPS pierces through anterior border of sartorius

• Posterior type: IPS pierces through posterior border of sartorius

• Penetrating type: IPS penetrates the sartorius

• Pes anserine type: IPS is in close proximity to pes anserine.

They found a medial prepatellar incision to have the highest risk (53.3%) of compromising the IPS nerve, followed by midline (46.7%) and the lateral parapatellar skin incision (30%).