Pathophysiology of Neuroma Formation

In the peripheral nervous system, the response to division of a peripheral nerve has been well documented. This response involves degeneration of the distal axons, survival of the distal Schwann cells, sprouting of the proximal nerve fibers (because this is the physiologic response of the cell bodies located in the dorsal root ganglion), and production of nerve growth factors by the distal (denervated) Schwann cells. This response results in regeneration of the proximal axon sprouts distally toward the chemotactic gradient of nerve growth factors. These sprouts track along the basement, where fibronectin, type I collagen, and laminin also attract the sprouts.^7,40 No biologic tool is currently available to prevent this cascade of events. Without destruction of the dorsal root ganglion, neuroma formation always occurs.

One of the most common misconceptions about the management of a painful neuroma is that resection of the neuroma always results in another painful neuroma. Based on the neurophysiology just presented, it is evident that resection of a painful neuroma always results in an attempt by the nerve to regenerate. A recurrent painful neuroma results if that regeneration is allowed to occur into an area of movement, frequent contact, trauma, or scar. A recurrent painful neuroma does not occur if the proximal end is allowed to regenerate into a nonmoving, protected area (e.g., into innervated muscle away from an area of contact or trauma). Another suitable area into which the proximal end of the nerve can be put after neuroma resection is a large area containing fat, which is also away from contact points. A common example is seen when the sural nerve is harvested for nerve grafting and the proximal end of the nerve is allowed to lie in the popliteal fossa. A recurrent painful neuroma does not occur if the proximal end of the nerve is permitted to regenerate into an appropriate end organ. An example is the type of regeneration that occurs when a nerve is divided and then is reconstructed with a nerve repair or a nerve graft; the nerve regenerates into its own distal territory.

Treatment of a Painful Neuroma

A painful neuroma can be managed successfully if the following three steps are followed:

Studies in monkeys have shown that when the proximal nerve sprouts into an environment of innervated muscle, and when that muscle is chosen to have minimal excursion, classic end-bulb neuromas do not form.⁴² Clinically, this approach has proven successful for the upper and lower extremities—the radial sensory and lateral antebrachial cutaneous nerves into the brachioradialis muscle,³⁹ the palmar cutaneous branch of the median nerve into the pronator quadratus muscle,²⁰ the plantar digital nerves into the foot intrinsic muscles,⁸ and the superficial and deep peroneal nerves into the anterolateral compartment muscles.⁹ For neuromas related to nerves that innervate joints, resection of the neuroma, or the peripheral nerve in an area proximal to the joint, usually allows the resected proximal end of the nerve to lie in an internervous plane, and direct muscle implantation is not needed. Examples are the anterior and posterior interosseous nerves in the forearm.^6,7,13

History of Partial Joint Denervation in the Extremities

In 1966, Wilhelm⁵³ published in German his approach to the treatment of persistent wrist joint pain. This concept became available to non–German-reading surgeons in 1977, when Buck-Gramcko⁴ reviewed the German-speaking experience with 313 patients. With an 80% follow-up that exceeded 2 years, their findings included excellent results in 69% of 195 patients. This was a complex review that included results from many surgeons, with 10 separate nerves being resected circumferentially about the wrist during a total wrist denervation. Many patients had “incomplete” or partial denervation done based on local anesthetic blocks. The patient population included patients who had persistent pain after forearm or wrist fracture, advanced arthritis, carpal instability, or severe sprain. Poorest results were seen in patients with unstable wrists, who later required fusions attempts.

In 1978, Dellon and Seif¹⁷ published a description of the innervation of the dorsal wrist capsule by the terminal branch of the posterior interosseous nerve. This nerve could be identified easily over the distal dorsal forearm, proximal to the wrist. This publication implied that if wrist pain, regardless of its cause, could be isolated to neural transmission along this nerve pathway, that pathway alone could be interrupted, eliminating wrist pain—thus, this was a partial, rather than a total, approach to wrist denervation. This hypothesis was tested in 29 patients studied between July 1981 and June 1984.⁶ Pain in these patients was caused by wrist fracture (n = 8), carpal instability (n = 4), arthritis (n = 3), severe wrist sprain (n = 10), or dorsal ganglionectomy (n = 4). Range of wrist motion and grip strength were measured, an anesthetic block of the posterior interosseous nerve was performed, and range of motion and grip strength were then reassessed (Fig. 76-1). For the patient to be considered a surgical candidate, pain relief, increased range of motion, and increased grip strength had to be present. Results indicated that 90% of patients achieved pain relief and improved wrist function, and 83% returned to work. Failure occurred in three of the four patients with carpal instability, who required a subsequent wrist fusion attempt. Three patients had reflex sympathetic dystrophy after their wrist fracture; two of these patients had previously undergone Darrach procedures, and two had had a carpal tunnel decompression. Two patients with reflex sympathetic dystrophy had subjective improvement after partial dorsal wrist denervation and showed improved range of motion, but none of the three returned to work. Investigators concluded that partial joint denervation was effective in relieving pain and improving function in selected patients, that total joint denervation was not necessary, and that structural instability was a contraindication to partial joint denervation.

Figure 76-1 Local anesthetic nerve block. To identify the nerve that is the neural transmission route for the pain signal, it is essential to block a specific nerve with a local anesthetic. Relief of pain with a regional or an intra-articular block is not sufficient to identify the individual nerves that may be rejected in a partial joint denervation. This type of block is illustrated here for the wrist, where anterior or posterior interosseous nerves can be blocked. Many patients have a dorsal or a volar injury in the wrist, and a single nerve may be the source of pain. For some problems, such as a triangular fibrocartilage tear, it may be necessary to block both of these nerves to obtain pain relief. This same approach must be used in the knee to identify individual components of pain arising from the medial or the lateral retinacular nerve, or both.

Logically, a group of patients could be identified for whom anterior wrist pain could be treated by a partial volar wrist denervation procedure. In 1984, Dellon and colleagues,¹³ after basic anatomic dissections to identify the terminal branch of the anterior interosseous nerve distal to the pronator quadratus muscle, attempted partial volar wrist denervation in a small group of patients. From April 1982 through March 1984, 11 patients with persistent volar wrist pain were identified; 9 had work-related injuries and 2 were involved in motor vehicle accidents. Each patient had good relief of pain, increased wrist range of motion, and increased grip strength after a block of the anterior interosseous nerve (see Fig. 76-1). At a mean follow-up of 12.8 months (range, 4 to 24 months), all patients had good to excellent relief of their pain. Four patients had returned to their regular job, three returned to “light duty” jobs, and four were in vocational rehabilitation. Investigated concluded that partial joint denervation was effective in relieving pain and improving function in selected patients, and that total joint denervation was not necessary.

In 1993, Dellon and Horner¹⁰ reviewed the experience of 51 patients treated for wrist pain with partial denervation. Overall, a 98% improvement in pain was noted, with the visual analog scale level decreasing in all patients by between 6 and 8 points out of 10. Improved range of motion occurred in 60% of the whole group. No patients with carpal instability were included in this more recent group.

Currently, partial wrist denervation is an accepted procedure for patients (1) who have wrist pain persisting after at least 6 months of nonoperative management; (2) who have no carpal instability; and (3) who respond appropriately with decreased pain and increased motion after specific local anesthetic blocks of selected, identifiable peripheral nerves. Before the lessons learned in upper extremity joint denervation could be applied to the knee, it was essential to identify the specific nerves that mediate pain from this region.

Cutaneous Innervation of the Knee Region

Virtually all anatomy books agree on the presence of the saphenous nerve innervating the infrapatellar region. This nerve originates medially from the femoral nerve and descends into the medial thigh through the adductor canal to emerge around or through the sartorius muscle. The infrapatellar branch divides from the femoral nerve variably: in the proximal third of the thigh (17.6%), in the middle third of the thigh (58.8%), and in the distal third of the thigh (23.5%).²⁶ In 86% of people, two saphenous nerve branches are present in Hunter’s canal.²⁶ In this situation, the more anterior of the two branches becomes the infrapatellar branch of the saphenous nerve, which when it enters the medial knee region is about 4 mm wide and begins to branch as it approaches the anterior midline. It always crosses the leg below the patella and usually has branches from below the patella to below the tibial tuberosity. The infrapatellar branch of the saphenous nerve innervates the anterior midline of the region below the patella and the lateral region below the patella. It does not innervate the skin covering the patella (Figs. 76-2 and 76-3). It may send branches into the distal anterior knee joint capsule.²⁶

Figure 76-2 Anterior view of innervation of the knee region: medial cutaneous nerve of the thigh, medial retinacular nerve, infrapatellar branch of the saphenous nerve, lateral retinacular nerve, lateral femoral cutaneous nerve, anterior femoral cutaneous nerve, innervation of the prepatellar bursal structures, and innervation of the proximal tibiofibular joint. Dotted lines show innervation of the knee joint.

Figure 76-3 Lateral view of innervation of the knee region: the biceps femoris muscle and the iliotibial tract have been divided and reflected; the lateral retinacular nerve innervates the lateral knee joint structures after arising from the sciatic nerve; innervation of the proximal tibiofibular joint is derived from the common peroneal nerve.

The skin covering the patella is innervated by the medial cutaneous nerve of the thigh, which is a better name for this nerve than the anterior or medial femoral cutaneous nerve because this nerve almost always branches from the saphenous nerve and travels as a separate branch, superficial to (39.1%), through (30.4%), or deep to (30.5%) the sartorius muscle, to emerge in the region of the femoral condyle, lying in the superficial subcutaneous plane.²⁶ This nerve may range in size from 0.6 to 1.1 mm, and it may include more than one branch. Its branches often lie directly over the medial retinacular nerve, and a local anesthetic block in this region blocks both nerves (see Figs. 76-2 and 76-3).

It is unclear whether the obturator nerve contributes branches to one or both of the branches of the saphenous nerve. The sensory branches of the obturator nerve enter into the complex of nerves within the adductor canal and do not emerge as a single identifiable branch in this distal thigh or knee region. Branches of the obturator nerve do participate in the innervation of the posterior knee capsule (Fig. 76-4).

Figure 76-4 Posterior view of innervation of the knee joint: lateral retinacular nerve arising from the sciatic nerve. The posterior knee capsule is innervated from branches of the obturator nerve coming through the adductor canal and from branches of the sciatic nerve.

The distal saphenous nerve theoretically begins distal to the infrapatellar branch of the saphenous nerve. Usually it has a separate branch in the thigh, or it may be the continuation of the saphenous nerve after the infrapatellar branch. Most often, regardless of its origin, the distal saphenous nerve gives off one or more transverse branches in the region distal to the tibial tuberosity; these may become a source of pain from long anterior or medial incisions. The terminal skin innervated by the distal saphenous nerve includes the medial dorsum of the foot and a region posterior to the medial malleolus (see Figs. 76-2 and 76-3).

The anterior femoral cutaneous nerve originates from the femoral nerve in the groin region and terminates at the patellar region. This nerve has many small (<1 mm) branches in this region and does not usually have an identifiable neuroma (see Fig. 76-2).

The medial femoral cutaneous nerve probably does not exist as such. The territories shown for it are best considered from a surgical anatomy point of view as being supplied by the anterior femoral cutaneous nerve or the medial cutaneous nerve of the thigh.

The lateral femoral cutaneous nerve begins at the hip region as a continuation of the L1 and L2 nerve roots. It has a branch that innervates the lateral buttock and a branch that innervates the lateral thigh skin extending to the level of the lateral knee. At the level of the inguinal ligament, the lateral femoral cutaneous nerve measures 3 to 7 mm. At the lateral knee region, it has terminal branches only, which usually are too small to identify. Although pain can be referred to the knee from compression of the lateral femoral cutaneous nerve at the hip, direct injury to the knee or surgery in the knee region almost never causes a neuroma of this nerve. Lateral knee skin pain is almost always due to a neuroma of the medial cutaneous nerve of the thigh or the infrapatellar branch of the saphenous nerve (see Fig. 76-2).

Innervation of the Knee Joint

The innervation of the human knee joint has been described by Ruedinger (1857),⁵¹ Druner (1927),¹⁸ Jeletsky (1931),³⁰ Gardner (1948),²⁴ Wilhelm (1958),⁵⁴ Kennedy and coworkers (1982),³³ and Wojtys and associates (1990).⁵⁵ None of these descriptions were derived from a large number of fresh human cadavers, and none provided detailed anatomic drawings sufficient to reveal optimal surgical approaches to these nerves. Clarification of nomenclature was required. A study reported in 1994 by Horner and Dellon²⁶ served as the basis for the following description and nomenclature. In that study, 45 fresh cadaveric adult knees were dissected using loupe magnification. The following description is based on that study and has been supplemented by clinical and operative experience over the past 15 years.

The lateral retinacular nerve is consistently present. It arises directly from the sciatic nerve, proximal to the popliteal fossa, and continues laterally to go beneath the biceps femoris tendon to reach the lateral retinacular structures. The lateral retinacular nerve lies beneath the lateral retinaculum. At this level, it exists as two or three 1-mm branches entering into the deeper structures of the knee joint. It is always accompanied by the recurrent lateral geniculate vessel and always is just distal to the vastus lateralis muscle. It is immediately superficial to the synovial structures of the knee joint (Fig. 76-5; see Figs. 76-2 and 76-4).

Figure 76-5 Medial view of innervation of the knee region. The sartorius muscle (top) has been divided and reflected (bottom). The medial retinacular nerve continues distally after innervating the vastus medialis muscle to enter medial knee joint structures. The saphenous nerve and its branches are unlabeled.

The medial retinacular nerve is consistently present. It arises from the branch of the femoral nerve that innervates the vastus medialis muscle. It exits from beneath the distal posterior aspect of the vastus medialis muscle and travels anteriorly to innervate the medial retinacular structures in 90% of subjects, whereas in the other 10%, it exits through the muscle to innervate the medial retinacular structures.²⁶ The medial retinacular nerve lies beneath the medial retinaculum, and at this level exists as two or three 1-mm branches entering into the deeper structures of the knee joint. It is always accompanied by the recurrent medial geniculate vessels and always is just distal to the vastus medialis muscle. It is immediately superficial to the synovial structures of the knee joint (see Figs. 76-2 and 76-3).

Nerves to the prepatellar bursal structures arise from the terminal branches of femoral nerves that innervate the vastus intermedius muscle. These continue distal to the muscle, lying on the anteromedial side of the distal femur, to enter into the prepatellar bursa and its surrounding structures. No unique name has been given to these nerve branches, each of which is approximately 1 mm in diameter (see Fig. 76-2).

Nerves to the posterior knee capsule arise from the sciatic nerve over a 2-cm distance and widely innervate the posterior knee structures. No unique name has been given to these nerve branches, each of which measures less than 1 mm in diameter (see Fig. 76-4). Druner¹⁸ observed in 1927 that the posterior knee capsule received branches from the obturator nerve through Hunter’s canal. The confluence of these branches with the branches from the sciatic nerve was named the popliteal plexus by Ruedinger.⁵¹

Innervation of the Proximal Tibiofibular Joint

Although it is clear that the proximal tibiofibular joint is not part of the true knee joint, pain from the proximal tibiofibular joint may accompany knee pain as part of a patient’s complaints. It is crucial to be able to discern the difference between these two joints and to appreciate the separate sources of innervation of the proximal tibiofibular nerve and the true knee joint.

The common peroneal branch of the sciatic nerve provides branches that innervate the proximal tibiofibular joint. These branches do not have a unique name. One branch arises proximal to the fibular head, and one or two arise just distal to the fibular head. These branches are small, generally less than 1 mm in diameter, and require loupe magnification for identification. They travel into the structures between the fibular head and Gerdy’s tubercle of the tibia (see Figs. 76-2, 76-4, and 76-5).

Immediately adjacent to these two small nerves are motor branches that innervate the tibialis anterior muscle. They cannot be distinguished morphologically. It is crucial to use intraoperative electrical stimulation to identify the motor branches of the tibial nerve. To be most correct, it is crucial to stimulate the nerve fiber believed to innervate this joint and to ensure that it is not a motor nerve by showing that it does not cause a muscle contraction before it is cut.