Ultrasound of the plantar foot: a guide for the assessment of plantar intrinsic muscles

Intrinsic muscles of the foot, particularly the plantar ones, can be considered the “foot core system”, since they play a pivotal role in the biomechanics of static posture and dynamic activities⁽¹⁾. Evidence in the literature suggests that the plantar intrinsic muscles (PIM) of the foot control the degree and velocity of longitudinal and transverse arch deformation, thus contributing to the functional integrity of the multiarticular structure of the foot during gait and balance control. Weakness of these muscles may contribute to the development of degenerative and overuse conditions such as pes planus, hallux valgus, increased navicular drop, osteoarthritis, and plantar fasciitis^(2–4). Foot deformities such as pes cavus varus, claw toes, and hammer toes are associated with the atrophy of the PIM resulting from neurological conditions such as inherited neuropathies, diabetic foot, and traumatic nerve injuries^(5,6). Nevertheless, pathological alterations of the intrinsic muscles are often overlooked. Their clinical evaluation is challenging, since they act as a unique functional unit and it is difficult to assess the strength of each muscle separately⁽⁶⁾. Consequently, a growing interest is given to the imaging evaluation of the PIM as an integration of clinical exams when approaching the musculoskeletal conditions of the foot. Different pathological conditions may cause specific morphostructural alterations in the plantar muscles. Furthermore, the muscle cross-sectional area measured with ultrasound (US) appears to be a good predictor of muscle strength and seems to correlate with the outcome of conditions such as diabetic foot and pes planus⁽⁷⁾. The purpose of this review is to provide a guide to disentangle the intricate PIM sonoanatomy and briefly summarize the main muscular pathologic findings that can be encountered in clinical practice.

The PIM are muscular structures that originate and insert in the foot, differently from the extrinsic muscles that originate in the leg and insert into the foot. In the human foot, two dorsal intrinsic muscles (extensor digitorum brevis and extensor hallucis brevis) and ten plantar intrinsic muscles exist. In this review, the focus is on the plantar intrinsic muscles, which are characterized by a more complex anatomy and function. The PIM are arranged in four layers and can be divided into three groups: medial, central and lateral.

The medial group encompasses two muscles, one superficial belonging to the first layer of the PIM, the abductor hallucis (AbdH), and one deeper, included in the third layer of the PIM, the flexor hallucis brevis (FHB) (Fig. 1). The AbdH originates from the tuberosity of the calcaneus, the flexor retinaculum, and the plantar aponeurosis. The muscle belly presents a larger and thicker origin, and it flattens moving distally. It inserts onto the medial aspect of the base of the first phalanx and medial sesamoid in a common tendon with the medial head of the FHB⁽⁸⁾. The FHB takes origin from the plantar medial aspect of the cuboid, the medial cuneiform, and partially from the tibialis posterior tendon. It consists of two heads, the medial one blended with the AbdH, and the lateral one (representing the first plantar interosseous) with the adductor hallucis. The two muscle heads insert respectively on the lateral and medial sides of the base of the first phalanx, and a sesamoid bone is embedded in each tendon at the level of the first metatarsal-phalangeal joint⁽⁹⁾. The flexor hallucis longus tendon runs between the two muscle bellies of the flexor hallucis brevis and then continues its course between the two sesamoids. The FHB and AbdH are innervated by the medial plantar nerve.

Fig. 1.

Scanning technique

The patient lies supine with the foot flexed dorsally, having the long axis of the foot perpendicular to the bench. A 15 to 18 MHz linear array probe should be used to have a good compromise between the spatial resolution to study smaller structures, such as nerves, and enough penetration to evaluate deeper muscular layers. To identify the AbdH, the probe should be placed over the medial aspect of the foot, perpendicular to its long axis, in correspondence with the palpable navicular tuberosity. The AbdH appears just inferiorly to the navicular tuberosity. Moving the probe toward the muscle origin, the medial and lateral plantar neurovascular bundles can be seen to cross the undersurface of AbdH. The best landmark to visualize the two bellies of the FHB is the flexor hallucis longus tendon. Placing the probe over the navicular tuberosity as above and moving toward the plantar aspect, the crossing point of the flexor hallucis longus and the flexor digitorum longus, also known as the master knot of Henry, can be spotted deep and lateral to the AbdH (Fig. 2). At this level, by passively flexing and extending the distal phalanx of the greater toe, the flexor hallucis longus can be easily identified. Following the flexor hallucis longus distally, the medial and lateral head of the FHB appears on the two sides of the tendon and can be traced toward the sesamoid and proximal phalanx insertions. The master knot of Henry is also a good landmark to identify the medial plantar nerve superficial to it.

Fig. 2.

In the central group, four layers of muscles can be detected. The flexor digitorum brevis (FDB) is the most superficial one, lying immediately underneath the central band of the plantar fascia (Fig. 3). The FDB originates from the inferomedial tubercle of the calcaneus, the plantar aponeurosis and the intermuscular septa between it, and the AbdH on the medial side and abductor digiti minimi on the lateral side. Distally, the FDB divides into four tendons, one for each of the four lesser toes. At the bases of the proximal phalanges, each tendon divides into two slips to allow the passage of the corresponding tendon of the flexor digitorum longus. Distal to the splits, the two tendon bundles reunite in a chiasma and divide again to insert into the mid-shaft of the intermediate phalanx⁽¹⁰⁾. Motor innervation is supplied by the medial plantar nerve. The second layer consists of the quadratus plantae (QP) muscle and the lumbrical muscles (Fig. 4, Fig. 5). These muscles are characterized by having at least one insertion on the flexor digitorum longus tendon. The standard anatomic configuration of the QP consists of two heads separated from each other by the long plantar ligament⁽¹¹⁾. The medial one is larger and fleshy. It originates from the medial concave surface of the calcaneus, below the groove that houses the tendon of the flexor hallucis longus tendon. The lateral head, flat and tendinous, arises from the lateral border of the inferior surface of the calcaneus and the long plantar ligament. The two heads coalesce into a flattened aponeurosis that inserts into the lateral margin, the upper and under surfaces of the flexor digitorum longus tendon⁽¹¹⁾. Distal to the QP insertion, the flexor digitorum longus tendon separates into four slips for II to V toes that give origin to the four lumbrical muscles. Each lumbrical springs from two adjacent tendons (bipennate), except the first (unipennate). After running inferiorly to the deep intermetatarsal ligament on the medial side of the respective flexor digitorum longus tendon slip, the lumbricals insert in the extensor hood of the phalanges. The second to fourth lumbricals and the QP are innervated by the lateral plantar nerve. The first lumbrical muscle is innervated by the medial plantar nerve.

Fig. 3.

Fig. 4.

Fig. 5.

The adductor hallucis (AddH) is located in the third layer and is comprised of two heads, oblique and transverse (Fig. 4). The oblique head is large, thick and fleshy, and crosses the foot obliquely occupying the hollow space under the metatarsals. It arises from the bases of the II, III, and IV metatarsals, the cuboid and lateral cuneiform undersurface, and the peroneus longus tendon sheath. The insertion is into the lateral side of the base of the first phalanx and the lateral sesamoid, together with the FHB (lateral head). The smaller transverse head (Fig. 5) originates from the plantar metatarsal-phalangeal joints of the II-V toes and the deep transvers metatarsal ligament. After running perpendicular to the metatarsal major axis, it inserts into the lateral side of the base of the first phalanx, blending with the fibers of the oblique head over the lateral sesamoids⁽¹²⁾. The lateral plantar nerve provides motor innervation.

Finally, the fourth layer includes three plantar interossei and four dorsal interossei muscles (Fig. 5).

The plantar interossei muscles originate from the bases and medial sides of the third to fifth metatarsals, and they insert onto the medial sides of the bases of the proximal phalanges of the third through fifth digits. The dorsal interossei are bipennate and emanate from the adjacent sides of the first to the fifth metatarsal. Both dorsal and plantar interossei have extensive soft tissue origins as the plantaris longus ligament and the peroneus longus tendon⁽¹³⁾. The first dorsal interosseous inserts on the medial side of the proximal phalanx of the second toe, while from the second to fourth interossei, the insertion is on the lateral side of the second to fourth proximal phalanx. They are innervated by the lateral plantar nerve.

The lateral group, similarly to the medial group, is made up of two muscles acting on the fifth toe. In the most superficial layer, the abductor digiti minimi (AbdV) arises from the inferior and lateral aspects of the calcaneal tuberosity, the plantar aponeurosis, and the intermuscular septum between it and the FDB (Fig. 6). Some authors describe two heads in the AbdV, the superficial and deep head. The first one is obliquely crossed on its ventral surface by the lateral band of the plantar fascia which inserts in the base of the fifth metatarsal. The deep head rests between the flexor digitorum brevis muscle (lateral) and the long plantar ligament (medial)⁽¹⁴⁾. The two bellies run distally along the outer border of the foot to end with a slender tendon inserted onto the lateral aspect of the base of the first phalanx of the fifth toe⁽¹⁴⁾. The flexor digiti minimi brevis (FDVB) runs from the base of the fifth metatarsal and the sheath of the peroneus longus to the lateral aspect of the base of the first phalanx of the fifth toe, where it inserts blending its tendon with the AbdV tendon (Fig. 6). The deeper fibers of the FDVB, inserted into the lateral part of the distal half of the fifth metatarsal bone, are described by some authors as a distinct muscle, the opponens digiti quinti. Along its course, the FDVB lies over the plantar surface of the fifth toe and is accompanied on the medial and superficial sides by the lateral branch of the lateral plantar nerve⁽⁹⁾. The motor function of both the AbdV and FDVB is supplied by the lateral plantar nerve. The anatomic situation of the motor branch innervating the AbdV muscle, namely the inferior calcaneal nerve, predisposes the latter to compression injuries causing a painful syndrome known as Baxter’s neuropathy⁽¹⁵⁾.

Fig. 6.

Scanning technique

The examination should start by placing the probe perpendicular to the long axis of the foot, over the lateral process of the calcaneal tuberosity. The smaller anisotropic lateral band of the plantar aponeurosis is a landmark used to identify the underlying AbdV. Moving distally, the lateral band of the plantar fascia is seen to insert onto the base of the fifth metatarsal, and at the same level, a new muscle appears on the medial side of the AdbV and the fifth metatarsal bone, the FDVB (Fig. 7). On the medial side of the FDVB, the lateral plantar neurovascular bundle is identified. The inferior calcaneal nerve can be visualized as a hypoechoic structure by placing the probe along the central band of the plantar fascia and looking at the area anterior to the medial process of the calcaneal tuberosity, deep to the FDB.

Fig. 7.

Ultrasound can identify dimensional and echotextural pathological alterations of the plantar intrinsic muscles related to a wide range of conditions. In the case of traumatic or compressive neuropathies, only the group of plantar muscles supplied by the damaged motor nerve shows the typical findings of denervation. An exemplificative condition is inferior calcaneal nerve neuropathy, also known as Baxter’s neuropathy, leading to isolated atrophy of the abductor digiti minimi. The nerve may be compressed between the AbdH and the medial plantar margin of the QP⁽¹⁶⁾ or deep to the FDB, where calcaneal plantar enthesophytes and/or soft tissue changes related to plantar fasciitis may occur (Fig. 8). Similarly, mononeuropathy of the medial plantar nerve, also known as jogger’s foot, may cause selective denervation of the FHB. In this condition, the medial plantar nerve can be injured at the level of the fascial sling between the spring ligament and the navicular attachment of the abductor hallucis onto the navicular bone, or at the level of the master knot of Henry⁽¹⁷⁾. In inherited or acquired polyneuropathies, the plantar intrinsic muscle of the foot shows diffuse signs of denervation. Patients affected by inherited Charcot-Marie-Tooth neuropathy commonly develop foot deformities such as cavovarus foot deformity, hammer or claw toes as a consequence of the intrinsic muscle loss of function and the ensuing imbalance between the extrinsic and intrinsic muscles^(18,19).

Fig. 8.

The ultrasound semiotics of muscle denervation depends on the timing of the nerve lesion. In the early phase of denervation (2–4 weeks after complete neurotmesis), the muscle size is preserved and slightly increased echogenicity can be noted as a consequence of intramuscular edema⁽²⁰⁾. As the denervation process evolves in subacute and chronic phases (>4 weeks), the muscle fibers are substituted by adipose and fibrotic tissue, leading to an increased echogenicity and reduced volume. In chronic conditions characterized by denervation and reinnervation processes, the reinnervated muscle bundles appear hypoechoic, interspersed in a hyperechoic background resulting in the “moth-eaten” pattern to the muscle.

Both acquired and inherited myopathies – such as muscle dystrophies - may involve the distal lower limb muscles and plantar intrinsic muscles, but the differences between the ultrasound features of primary myogenic muscle atrophy and neurogenic atrophy are subtle, when present. In muscle dystrophies, the muscle echotexture has been described as homogenously hyperechoic, with a groundglass appearance⁽²¹⁾. In the most advanced cases, the upper layer of the muscle reflects the majority of ultrasound waves, thus attenuating the beam and generating hypoechoic areas in the deeper layers. Degenerative conditions such as hallux valgus have been postulated to be related to the strength imbalance of the AbdH compared to the AddH. This hypothesis is supported by the histological findings of muscle atrophy in the AbdH⁽²²⁾, and the ultrasound findings of the decreased cross-sectional area of the AbdH and FHB⁽²³⁾. A similar pattern of PIM atrophy has also been found in flatfoot deformity⁽²⁴⁾, which is associated with compensatory hypertrophy of the flexor hallucis longus and the flexor digitorum longus muscles in the attempt to sustain the medial plantar arch (Fig. 9). With plantar fasciitis, the role of the PIM is not completely elucidated. A magnetic resonance-based study by Chang et al.⁽²⁵⁾ found decreased muscle diameters in the forefoot of patients with plantar fasciitis compared to healthy controls. A decreased strength in these muscles results in diminished support to the plantar arch, causing an overload of the plantar fascia. In fact, in patients with diabetes-related neuropathy, the decreased size of the plantar muscle correlates with the thickening of the plantar fascia and the claw toe deformity⁽²⁶⁾. US can be used to visualize other pathologic processes affecting the intrinsic plantar muscles beyond muscle atrophy. It is deemed to be useful in detecting complications of the diabetic foot such as soft tissue infections with involvement of the intrinsic muscles⁽²⁷⁾. In particular, in the presence of cutaneous ulcers, US may easily detect intra- or intermuscular fluid collections due to abscesses or deep fascial thickening related to necrotizing fasciitis. Finally, US is the first-line imaging modality for masses involving the PIM. US enables the identification of mass location (intramuscular, intermuscular or both) and the differentiation of “no touch lesions” (ganglion cyst, lipoma, vascular malformations, pseudo-mass as congenital muscle hypertrophy) from lesions requiring further imaging studies and histological characterization^(28,29)(Fig. 10).

Fig. 9.

Fig. 10.

The present review aimed to provide a guide for the sonographic assessment of the PIM by suggesting a scanning technique based on easy-to-spot anatomic landmarks and illustrating the most common pathologic findings affecting these muscles.