Knee Pain of Neural Origin

Chapter 42 Knee Pain of Neural Origin





INTRODUCTION


Nerves related to the knee include not only those that go in the skin about the surface of the knee joint but also mixed (motor and sensory) nerves that pass the knee on the way to their targets distal to this joint. In addition to these well-known nerves are those that innervate the knee joint. Each of these nerves is at risk of injury whenever (1) there is blunt force directed at the knee, (2) there is a stretch/traction injury to the musculoskeletal system of the knee joint, (3) there is an inversion sprain to the ankle, and (4) open or arthroscopic knee surgery has been performed. Whereas it is intuitively clear that a joint is innervated, the exact pathways of this innervation are curiously omitted from the classic and even the newer anatomy texts. For the human knee, the innervation pattern was not found to be described until 1994 during the author’s search for an etiology of knee pain unrelated to musculoskeletal dysfunction.25 The approach to knee pain described in this chapter follows closely that developed for wrist pain,8,16,20 which preceded the author’s interest in knee pain and led to a subsequent similar approach to those for shoulder pain1,3 and ankle pain.6,13,29


The approach described in this chapter considers knee pain that exists after musculoskeletal treatment to be of neural origin. The physician caring for these patients must consider whether the pain is coming from direct damage to the cutaneous nerves about the knee, such as a neuroma of the infrapatellar branch of the saphenous nerve, or from an injury to one of the nerves arising within the knee joint structures themselves. A further source of knee pain due to nerve injury is an injury to a nerve far away from the knee, such as the lateral femoral cutaneous nerve, or a nerve that is simply going past the knee on its way to some other distal target. The concept of partial joint denervation is presented as an approach in which those joint afferents that transmit a pain message are interrupted, thereby stopping the pain and preserving the musculoskeletal components of the joint. A Charcot-type joint is not created because the deafferentation is partial instead of total.




PERIPHERAL NERVE ANATOMY RELATED TO THE KNEE



Innervation of the Human Knee Joint


The innervation of the human knee joint is remarkably constant.25 On the medial aspect (Fig. 42-1), the femoral nerve branch that innervates the vastus medialis continues past its motor point and exits deep and distal to the vastus medialis. At this point, it lies deep to the medial retinaculum and becomes related to the medial recurrent geniculate artery and vein. This structure is termed the medial retinacular nerve. These structures, 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. 42-1).



On the lateral aspect of the knee (Fig. 42-2), a branch of the sciatic nerve leaves the popliteal fossa, travels laterally and anteriorly, emerges deep to 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. These 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. 42-2).



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. This space is innervated by the common peroneal nerve (see Fig. 42-2). 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 this space posterior to the fibular head. The common peroneal nerve gives off a second branch to this space 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 to the joint 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. 42-3). The critical anatomic sites that can cause compression have long been appreciated to occur as the common peroneal nerve crosses the fibular neck, requiring division of the fascia of the peroneus longus fascia.35 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 5–15 mm in width) in 80% of patients requiring neurolysis.14 The surgeon must identify the common peroneal nerve by elevating it from the fibular neck, determine whether 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.






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.31 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.26,34 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.41 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 crest2 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 42-4, it can be appreciated that compression of the lateral femoral cutaneous nerve can produce symptoms that include numbness or pain about the lateral aspect of the knee.37 This nerve, or the femoral nerve itself, can be injured during regional anesthesia given for knee surgery.22





CLINICAL EVALUATION OF KNEE PAIN OF NEURAL ORIGIN


The history of knee pain is critical to obtain. Knee pain does not arise de novo, but due to a history of some type of sports event, overuse activity, new activity, or actual direct injury to the knee. The knee injury may be blunt or open. Surgery should also be considered as a source of trauma. Many patients with knee pain due to musculoskeletal problems awake from their reconstructive knee surgery to have a different pain or paralysis. These are known complications of knee surgery that should be recognized as such and referred for appropriate treatment in a timely manner. The determination of whether the pain is made worse by certain positions of the body or leg or by certain activities is important to arrive at a correct diagnosis. Certainly, most knee pain unrelated to structural impairment will be relieved by nonoperative treatment. But when pain persists beyond a reasonable timeframe of 3 to 6 months, the diagnosis is most consistent with pain of neural origin.


The physical examination is directed to identifying one or more sources of neural origin of the pain. First, the clinician should try to distinguish whether any skin territories are dysesthetic or painful when touched lightly. If so, these areas should be outlined. The most common distribution is that of the infrapatellar branch of the saphenous nerve; the second most common is the skin over the patella itself. These areas might be numb, indicating loss of sensation (Fig. 42-5). Once the pattern is identified, examine proximally along the course of the given nerve, observing for a trigger spot, which is either a true end-bulb neuroma or an in-continuity nerve lesion. The hypothesis is made that this nerve or these nerves are the source of the cutaneous pain; a diagnostic nerve block will be required to confirm this hypothesis.




The physical examination is then directed to the joint afferents. Palpation is done deeply to the spot located just distal to the vastus medialis muscle, through the medial retinaculum to elicit pain from the medial retinacular nerve. Palpation is next done deeply to the area located just distal to the vastus lateralis muscle, through the lateral retinaculum, to elicit pain from the lateral retinacular nerve (Fig. 42-5). The hypothesis is made that the knee joint pain is due to an injury to one or both of the joint afferents; a diagnostic nerve block will be required to confirm this hypothesis. The medial cutaneous nerve to the thigh and the medial retinacular nerve can be blocked at the same area. Ten minutes after the nerve blocks, the patient is instructed to walk in the hallway, climb and descend a few steps, and even to kneel on a padded chair (Fig. 42-6). A reduction of 5 points on a visual analog scale; for example, from 10 to 5, where 10 represents the worst pain, is confirmation that sufficient relief of pain has occurred to permit surgery for partial knee denervation and resection of cutaneous neuromas.



If the nerve blocks just described have not reduced the pain level to less than 5 points, or the blocks were not effective, the physical examination should proceed to include other more rare sources for the nerve pain. In the patient with a knee replacement, the remaining pain may stem from an anterior femoral cutaneous nerve found in the proximal portions of the incision or another branch of the infrapatellar saphenous nerve, which can have many branches, including more distal branches.


Finally, the groin should be examined to search for a Tinel sign over the lateral femoral cutaneous nerve. The patient will be noted to sit with the lower extremity extended at the hip joint (Fig. 42-7). The midthigh should be examined to search for a Tinel sign over Hunter’s adductor canal. This is most effectively done with the patient supine, and the effected leg externally rotated at the hip, leaving the knee bent. This stretches the adductor muscle group over the saphenous nerve. Then, gentle palpation over the canal produces a distal radiating painful response (Fig. 42-8).


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Jun 22, 2016 | Posted by in PHYSICAL MEDICINE & REHABILITATION | Comments Off on Knee Pain of Neural Origin
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