Knee Pain of Neural Origin

Innervation of the Human Knee Joint

The innervation of the human knee joint is remarkably constant. On the medial aspect ( Fig. 39-1 ), the femoral nerve branch that innervates the vastus medialis continues past its motor point and exits deep and distal to the muscle. At this point, it lies deep within the medial retinaculum and becomes related to the medial recurrent geniculate artery and vein. This structure is termed the medial retinacular nerve . Both nerve and vessels continue directly adjacent to the vastus medialis and superficial to the synovium to enter the ligamentous structures of the medial aspect of the knee. These fibers also continue toward the midline of the knee to innervate the undersurface of the patella (see Fig. 39-1 ).

On the lateral aspect of the knee ( Fig. 39-2 ), a branch of the sciatic nerve leaves the popliteal fossa, travels laterally and anteriorly, emerges deep within the biceps tendon and enters the space beneath the lateral retinaculum. In this location, the nerve is adjacent to the superior lateral geniculate artery and vein. This structure is termed the lateral retinacular nerve . Both structures, the nerve and vessels, are immediately distal to the vastus lateralis and superficial to the synovium. The nerve enters the ligamentous structures of the lateral aspect of the knee (which have not yet been further defined histologically) and travels to the midline to innervate the undersurface of the patella.

Anteriorly, another source of innervation is derived from the femoral nerve innervation of the vastus intermedius. This nerve continues on the surface of the periosteum to innervate the tissues around the prepatellar bursa. Finally, posteriorly, branches from the sciatic nerve enter the posterior knee joint capsule to provide innervation (see Fig. 39-2 ).

Innervation of the Proximal Tibiofibular Joint

Although many patients complain of “knee pain,” they may be pointing or referring to the region distal and lateral to the knee, which is the articulation between the fibular head and the tibia. This joint space is likely to be injured from fractures of the fibula or lateral tibial plateau or from high tibial osteotomy or a Maquet operative procedure. It is innervated by the common peroneal nerve (see Fig. 39-2 ). As the common peroneal nerve travels from the popliteal fossa to the fibular head, it gives off a small (~1 mm in diameter) nerve that may enter the space posterior to the fibular head. The common peroneal nerve gives off a second innervating branch immediately as it curves anterior to the fibular head at the junction with the fibular neck. This branch is also approximately 1 mm in diameter. The next branch of the common peroneal nerve goes to the tibialis anterior, which is a critical motor branch. In the operating room, the only way to identify the branch to the joint is to stimulate it electrically. No motor function will be elicited when the branch is stimulated. This is a critical step. This small branch often looks like epineurium.

Common Peroneal Nerve

The common peroneal nerve is the lateral branch of the sciatic nerve. It can most often be identified as a distinct branch even to the sciatic notch. Therefore, in the thigh, there is clearly a common peroneal nerve even though it may be referred to as the sciatic nerve. In the distal thigh, the common peroneal nerve courses laterally, approaches the fibular head, and continues anteriorly to cross the fibular neck and enter the leg. As the common peroneal nerve approaches the fibular neck, it clearly divides into the superficial and the deep peroneal nerve, increasing in size as increased epineurium is required to ensheath these two branches ( Fig. 39-3 ). The critical anatomic sites that can cause compression have long been appreciated to occur where the common peroneal nerve crosses the fibular neck, requiring division of the fascia of the peroneus longus fascia. With increasing experience, it has become evident that three other anatomic factors must be considered. A fibrous band exists beneath the peroneus longus in 20% of cadavers, but this band is present (and ranges from 5 to 15 mm in width) in 80% of patients requiring neurolysis. The surgeon must identify the common peroneal nerve by elevating it from the fibular neck; determine whether or not fibrous thickening in the lateral gastrocnemius muscle fascia is present; and finally, be certain that the overall entrance into the anterior and lateral compartments is sufficiently large.

Deep Peroneal Nerve

The deep peroneal nerve arises over the fibular neck as a discrete fascicle. It can be dissected proximally by microtechnique if necessary. Immediately after crossing the fibular neck, it gives off tiny branches to the tibialis anterior and the toe extensor muscles. In so doing, the deep peroneal nerve effectively tethers the common peroneal and its distal branches from having too much excursion. This predisposes the common peroneal nerve and its branches to stretch/traction injuries. The entrapment site for the deep peroneal nerve is at the ankle, the “anterior tarsal tunnel,” which is a rare site for compression unless there has been a direct injury at this location. A more common site for compression is over the dorsum of the foot where the tendon of the extensor hallucis brevis represents the anatomic structure causing the compression. A portion of the tendon of the extensor hallucis brevis is excised in the region of the deep peroneal nerve (see Fig. 39-3 ).

Superficial Peroneal Nerve

The superficial peroneal nerve arises over the fibular neck from the common peroneal nerve. It can be dissected proximally by microdissection if required. The motor innervation to the foot everters arises within the first 1 to 2 cm distal to the neck of the fibula. The known site of entrapment for this nerve is the region where it transitions from deep fascia into the subcutaneous tissues in the distal third of the leg. The typical division into its dorsomedial and dorsolateral branches occurs just proximal to the ankle in approximately 75% of patients (see Fig. 39-3 ). In my experience, it is now clear that in at least 25% of cadavers and patients coming to surgery, this division can occur in the proximal third of the leg. This creates the situation in which the superficial peroneal nerve may be found in both the anterior and its most usually described location, the lateral compartment. Indeed, the superficial peroneal nerve may be present in just the anterior compartment or within the septum between the two compartments. The anatomic variability means that any surgical approach to this nerve must open both the anterior and the lateral compartments.

Proximal Tibial Nerve

Although the tibial nerve (like the common peroneal nerve) arises at the sciatic notch as the medial component of the sciatic nerve and is often clearly a distinct nerve in the thigh, the only well-described site for tibial nerve compression was at the medial ankle in the tarsal tunnel. It is now evident that the tibial nerve can be compressed in a site more proximal than the tarsal tunnel, and this site (just distal to the knee) is best described as the proximal tibial nerve, to distinguish it from the tarsal tunnel region. Tibial nerve compression in the popliteal fossa has been described related to the presence of compartment syndrome or space-occupying masses, such as a popliteal artery or a Baker cyst. Although anatomy texts clearly depict a fibrous arch or soleal sling from which the soleus muscle arises, only recently has there been an anatomic study describing the relationship of this sling to the proximal tibial nerve (unpublished data). This sling lies at a mean distance of 9.3 cm (range, 7-13 cm) from the middle of the popliteal fossa and causes a visible narrowing of the tibial nerve over a length of 1.5 cm in 55% of the cadavers, with a severe constriction being found in 2% of the specimens. With injury to the knee, especially one associated with postoperative bleeding into the popliteal fossa, tibial nerve compression at this proximal site must be considered as a source of symptoms of numbness in the plantar aspect of the foot, with proximal referral to the knee region.

Lateral Femoral Cutaneous Nerve

The lateral femoral cutaneous nerve has a great deal of anatomic variability related both to its location at the anterior superior iliac crest and to its innervated skin territory. If the location is within the inguinal ligament instead of inferior to it, the lateral femoral cutaneous nerve is at risk for compression or stretch/traction injury during falls, while wearing tight belts, during a seatbelt injury, or during manipulation of the knee with the patient on the operating room table. From Figure 39-4 , it can be noted that compression of the lateral femoral cutaneous nerve can produce symptoms that include numbness or pain about the lateral aspect of the knee. This nerve, or the femoral nerve itself, can be injured during regional anesthesia given for knee surgery.

Saphenous Nerve

The saphenous nerve is a branch that arises from the femoral nerve in the proximal thigh. Its cutaneous branches to the skin below the knee are well described and lie in a position to be injured directly from either a medial ankle arthroscopy portal or the midline incisions used for many surgical approaches to the knee. These branches can be directly injured by blunt trauma as well. Less well recognized is the nerve to the skin overlying the patellar itself. This is a branch of the femoral nerve, which may include sensory contribution from the obturator nerve joining within Hunter's canal. The medial cutaneous nerve of the thigh (see Fig. 39-1 ) approaches from the medial aspect of the knee compared with the vertical approach taken by the anterior femoral cutaneous nerves. Even less well appreciated is that the saphenous nerve can be compressed in the distal thigh within Hunter's canal. This is called adductor canal syndrome , which is rare in the absence of direct trauma to the region. Entrapment of the saphenous nerve in this location can present as medial knee pain.