CHAPTER 91
Morton Neuroma
Sammy M. Lee, DPM; Robert J. Scardina, DPM
Definition
Morton neuroma is not a neoplasm; rather, it is a local, mechanically induced, degenerative enlargement of the third plantar intermetatarsal nerve with associated perineural fibrosis [1] caused by an accumulation of collagenous material within the sheath of Schwann and usually the result of repetitive trauma (Fig. 91.1). As such, it is more accurately defined as an intermetatarsal compression or entrapment neuropathy [2]. The exact etiology has not been clearly identified or proved conclusively, but the following have been postulated as contributing factors: flatfoot (pes planus); anterior splay foot; high-arch foot (pes cavus); equinus deformity [3]; ill-fitting (tight or high-heeled) shoes; abnormal proximity of neighboring metatarsal heads [4]; and associated forefoot deformities, including hallux abductus, bunion, and lesser hammer toes. Plantar intermetatarsal nerves of the foot are purely sensory at and distal to the level of the metatarsophalangeal (MTP) joints, as they course through a fibro-osseous canal composed of neighboring metatarsal heads and the overlying deep transverse intermetatarsal ligament [5]. Anatomic (cadaver) studies have identified the third intermetatarsal nerve as most commonly receiving proximal branches from both the medial plantar and lateral plantar nerves, each arising from the common posterior tibial nerve. Therefore, anatomically, the third intermetatarsal nerve is usually enlarged to some degree as it develops from proximal trunks of two separate nerve branches. This anatomic configuration may or may not be a causative factor. The classic Morton neuroma occurs in the third intermetatarsal space. Similar nerve compression neuropathies occur, but less commonly, in the second plantar intermetatarsal space (Hauser neuroma) and rarely in the first (Heuter neuroma) and fourth (Iselin neuroma) plantar intermetatarsal spaces [2]. Symptomatic plantar neuromas in neighboring intermetatarsal spaces may occur in the same foot, but uncommonly. They are all treated in a similar fashion.
Symptoms
Morton neuroma may be manifested symptomatically in a variety of ways: localized sharp, lancinating, or burning pain; paresthesias and dysesthesias; numbness and tingling; and toe cramping. Symptoms typically radiate distally, involving the opposing plantar sides of the third and fourth toes, but pain exclusively in the fourth toe is not uncommon. Unilateral presentation is most common, whereas bilateral is less so. Symptoms occur predominantly during weight-bearing activities, but residual non–weight-bearing or nocturnal pain is sometimes present. Not uncommonly, patients may experience symptoms while driving an automobile with the foot held in a slightly dorsiflexed position. A characteristic patient maneuver is to remove the shoe and massage or manipulate the plantar forefoot and MTP joints, producing transient relief of symptoms [6].
Physical Examination
On inspection, the foot may appear normal or may demonstrate a subtle divergence of the third and fourth toes, usually more pronounced with weight bearing. When it is present, the regional palpable pain is typically plantar. The lateral forefoot squeeze test may mimic a tight shoe, thereby reproducing symptoms. In long-standing cases, hypoesthesia or anesthesia may be noted in the third interdigital web space, distally on the opposing plantar sides of the involved toes (toe tip sensation deficit [7]) or plantar, distal to the third and fourth metatarsal heads. The most diagnostic and reliable clinical maneuver is the Mulder test [8], performed by alternating lateral compression of the forefoot with one hand and dorsal-plantar compression of the involved distal intermetatarsal space with the opposite forefinger and thumb (Fig. 91.2) [6]. A Mulder sign is considered present when symptoms are reproduced and a palpable and sometimes audible click is detected. In general, there are no signs of proximal nerve involvement (e.g., tarsal tunnel syndrome), vasomotor instability, or arterial insufficiency. Predisposing foot types (pes planus or pes cavus) or a tight Achilles tendon (equinus) [3] may be evident on clinical examination. Passive range of motion of the neighboring MTP joints is usually pain free without crepitus. Unilateral antalgic (pain-avoidance) gait may also be observed.
Functional Limitations
Functional limitations include difficulty with walking or running any significant distance and in performing other weight-bearing physical activities as well as the inability to wear dress shoes comfortably (particularly women’s high heels).
Diagnostic Studies
The diagnosis of Morton neuroma is generally made from history and clinical examination. However, other supportive diagnostic studies may be helpful in establishing a diagnosis, especially when surgical intervention is being considered or in the event of failed conservative measures. Ultrasonography is a relatively simple, inexpensive, and helpful diagnostic tool [9,10]. In the evaluation of a primary neuroma, a 5-mm or greater hypoechoic mass, visualized in the coronal (frontal) plane projection between the neighboring metatarsal heads, is considered a positive finding [11,12
