Metatarsal bones

The metatarsal bones are a set of five elongated long bones located in the midfoot region of the human foot, positioned between the proximal tarsal bones and the distal phalanges of the toes.^[1] Numbered from I to V starting from the medial (big toe) side to the lateral side, they form the structural foundation of the forefoot and are essential for locomotion.^[2] Each metatarsal bone consists of three main parts: a proximal base that articulates with the tarsal bones, a central shaft that is typically triangular in cross-section and convex dorsally, and a distal head that connects to the proximal phalanx of the corresponding toe.^[3] The bases of the first, second, and third metatarsals articulate with the medial, intermediate, and lateral cuneiform bones, respectively, while the fourth and fifth articulate with the cuboid bone; additionally, adjacent metatarsals connect via intermetatarsal joints for stability.^[2] The first metatarsal is the shortest and thickest, providing robust support for the great toe, whereas the second is the longest, and the fifth is more mobile to facilitate foot inversion and eversion.^[1] Functionally, the metatarsals bear a significant portion of body weight during standing, walking, and running, distributing forces across the foot's longitudinal and transverse arches to absorb shock and maintain balance.^[2] Together with the tarsal bones, they form the foot's arches, which enhance propulsion and prevent excessive strain on the lower limbs.^[1] These bones also serve as key attachment sites for intrinsic foot muscles, tendons, and ligaments, contributing to movements such as toe flexion and foot stabilization.^[3] Clinically, the metatarsals are prone to fractures from trauma or stress, such as in athletes, and conditions like metatarsalgia highlight their role in weight distribution.^[2]

Anatomy

Location and general features

The metatarsal bones consist of five elongated long bones (numbered I through V) that form the intermediate segment of the foot skeleton, positioned distally to the tarsal bones and proximally to the phalanges of the toes.^[4] They occupy the forefoot region, extending anteriorly from the tarsus to support the overall structure of the foot.^[5] Each metatarsal exhibits a characteristic morphology of a proximal base, a central shaft, and a distal head, with the base wedge-shaped and the shaft pyramidal and slightly curved.^[5] The bases face posteriorly toward the tarsals, while the heads project anteriorly toward the toes, collectively forming the ball of the foot.^[4] These bones are oriented nearly parallel to one another, with a slight medial-to-lateral divergence, and are numbered from the medial (big toe) side to the lateral (little toe) side.^[2] The first metatarsal is the shortest and thickest, providing robust support on the medial side, while the second is typically the longest, and the fifth shows a lateral curvature.^[4] Their alignment contributes to a subtle transverse arch across the forefoot, which integrates with the foot's longitudinal arch for structural stability.^[5] In adults, average lengths vary by bone, sex, and population; for instance, the first metatarsal measures approximately 56 mm in males and 50 mm in females, while the second averages 68 mm in males and 62 mm in females, based on radiographic studies.^[6] These dimensions underscore the metatarsals' role in defining the forefoot's proportions and load distribution.^[5]

Characteristics of individual metatarsals

The first metatarsal is the shortest and thickest of the five metatarsals, featuring a robust prismoid shaft and a rounded base with a prominent tuberosity on its plantar surface for the insertion of the peroneus longus tendon.^[1][7] Its head includes two grooved facets to accommodate sesamoid bones. The second metatarsal is the longest among the metatarsals, characterized by a straight and slender shaft and a square-shaped base.^[1][8] The third metatarsal exhibits an intermediate length relative to the others, with a triangular base and a shaft similar in form to that of the second but slightly more tapered.^[1][9] The fourth metatarsal is more slender than the third, possessing a quadrangular base and a narrower shaft overall.^[1] The fifth metatarsal, distinguished by its lateral position and a prominent styloid process, or tuberosity, at the base for the attachment of the peroneus brevis tendon.^[1][8][10] In human anatomy, the comparative proportions of the metatarsals typically follow an "index-minus" configuration, where the second is the longest, followed by the first, third, fourth, and fifth in descending order (metatarsal formula: 2 > 1 > 3 > 4 > 5).^[8][11]

Articulations and ligaments

The bases of the metatarsal bones articulate proximally with the distal surfaces of the cuneiform and cuboid bones to form the tarsometatarsal joints, collectively known as the Lisfranc joint complex. Specifically, the base of the first metatarsal articulates with the medial cuneiform, the second metatarsal base with the intermediate cuneiform, the third with the lateral cuneiform, and the bases of the fourth and fifth metatarsals with the cuboid bone.^[12][13] These joints are classified as plane synovial joints, permitting limited gliding movements while maintaining midfoot stability.^[14] The Lisfranc ligament provides critical reinforcement to this complex, comprising three components: a dorsal ligament, an interosseous ligament, and a plantar ligament that collectively connect the medial cuneiform to the base of the second metatarsal.^[15] Additional stability arises from interosseous ligaments between adjacent metatarsal bases, as well as dorsal and plantar intermetatarsal ligaments that span the bases of metatarsals II–V.^[16][17] The intermetatarsal joints themselves are small synovial plane joints formed between the lateral aspects of adjacent metatarsal bases, bound by these strong fibrous connections consisting of transverse fibers.^[16][17] Distally, the rounded heads of the metatarsal bones articulate with the bases of the proximal phalanges to form the metatarsophalangeal (MTP) joints.^[12] These are condyloid synovial joints, characterized by the reception of the convex metatarsal heads into shallow cavities on the phalangeal bases, enabling flexion, extension, and limited abduction/adduction; the bases of the proximal phalanges are slightly concave to receive the convex metatarsal heads, enhancing joint congruence during motion.^[18][19] The heads of adjacent metatarsals are interconnected by the deep transverse metatarsal ligament, a narrow fibrocartilaginous band that runs across and blends with the plantar aspects of the MTP joint capsules, helping to maintain forefoot alignment.^[18][20]

Blood supply and innervation

Vascular anatomy

The arterial supply to the metatarsal bones derives primarily from the dorsalis pedis artery dorsally and the medial and lateral plantar arteries plantarily, ensuring comprehensive perfusion of the forefoot structures.^[21] The dorsalis pedis artery, a continuation of the anterior tibial artery, terminates by dividing into the first dorsal metatarsal artery, which supplies the first metatarsal bone, while its arcuate branch gives rise to the second, third, and fourth dorsal metatarsal arteries that course along the dorsum of the respective metatarsals II through IV.^[22] Plantarly, the lateral plantar artery, arising from the posterior tibial artery, anastomoses with the deep branch of the dorsalis pedis to form the deep plantar arch, from which the four plantar metatarsal arteries emerge to supply the interosseous spaces and the plantar aspects of the metatarsals.^[23] Specifically, the first metatarsal receives additional supply from branches of the medial plantar artery, while the second metatarsal is nourished by the deep plantar arch; these vessels form extensive anastomoses across the forefoot to maintain redundancy in circulation.^[24] Intraosseous vascularization of the metatarsals is provided by nutrient arteries that originate from the dorsal or plantar metatarsal arteries and enter the diaphyses primarily at the middle third of the shafts for the second through fourth metatarsals, typically via a single foramen directed obliquely proximally to supply the medullary cavity.^[25] For the second through fourth metatarsals, these nutrient arteries enter the lateral or medial plantar aspects just proximal to the midpoint, whereas in the fifth metatarsal, the nutrient artery arises from the fourth plantar metatarsal artery and inserts into the plantar medial diaphysis near the junction of the middle and proximal thirds.^[26] The first metatarsal's nutrient artery, stemming from the first dorsal metatarsal artery, enters the medial aspect at the distal third or the junction of the middle and distal thirds of the diaphysis, oriented obliquely from a proximal direction in the coronal plane.^[27] Venous drainage from the metatarsal bones follows the arterial pathways through dorsal and plantar venous plexuses that collect blood from the periosteum and intraosseous systems.^[28] The dorsal metatarsal veins converge into the dorsal venous arch, which drains medially into the great saphenous vein and laterally into the small saphenous vein, facilitating superficial return to the lower limb.^[22] Plantar metatarsal veins, accompanying the plantar arteries, form a deep plexus that empties into the posterior tibial vein, with perforating veins linking the dorsal and plantar systems for efficient overall drainage.^[28] Regional variations in vascularity emphasize greater density in the metatarsal heads to nourish the adjacent metatarsophalangeal joints, where dorsal and plantar branches form a rich periarticular plexus, particularly at the plantar-lateral neck of the first metatarsal head supplied by the first dorsal, first plantar, and medial plantar arteries.^[24] Anastomoses between dorsal and plantar metatarsal arteries occur via proximal and distal perforating branches in the intermetatarsal spaces, enhancing collateral flow across the forefoot.^[23] These patterns provide robust circulation adequate for bone maintenance and repair, with nutrient foramina consistently located in the middle third of the shafts in over 90% of cases.^[29]

Neural supply

The neural supply to the metatarsal bones primarily involves branches of the tibial and common fibular (peroneal) nerves, providing both sensory and motor innervation to the bones, surrounding soft tissues, and intrinsic foot muscles.^[30] Sensory innervation to the plantar aspects of the metatarsals and the overlying skin is derived from the medial and lateral plantar nerves, which are terminal branches of the tibial nerve, while the dorsal skin over the metatarsals receives supply from the superficial fibular nerve.^[31][32][33] Motor innervation targets the intrinsic foot muscles that attach to the metatarsals, predominantly via the lateral plantar nerve, which supplies the dorsal and plantar interossei, adductor hallucis, and lumbricals 2–4, enabling fine control of toe flexion and adduction.^[34][32] The medial plantar nerve contributes motor fibers to the abductor hallucis, flexor digitorum brevis, and the first lumbrical, supporting actions at the first metatarsal.^[31][35] Specific sensory distributions vary by metatarsal: the first metatarsal receives primary supply from the medial plantar nerve to its medial and plantar surfaces; the second and third metatarsals share innervation between the medial and lateral plantar nerves for their interdigital and plantar regions; and the fourth and fifth metatarsals are mainly innervated by the lateral plantar nerve laterally, with additional sural nerve contributions to the dorsal lateral aspects.^[36][33][37] The periosteum of the metatarsal bones contains dense sensory nerve fibers, including mechanosensitive endings that detect mechanical distortion, contributing to acute pain during fractures or trauma.^[38][39] Autonomic components include sympathetic fibers that travel via perivascular plexuses along arteries supplying the metatarsals, regulating vasomotor tone and blood flow to the bone and surrounding tissues.^[40][41]

Function and biomechanics

Role in weight-bearing and locomotion

The metatarsal bones play a crucial role in weight-bearing by forming the distal aspect of the foot's transverse arch, which helps distribute approximately 50% of the body weight across the forefoot during the stance phase of gait. The heads of the metatarsals create this arch, with the first and fifth metatarsals functioning as key pillars that provide stability and support the majority of the load, while the intermediate metatarsals (II-IV) contribute to even distribution. This structural arrangement allows the forefoot to adapt to ground contact and maintain balance under vertical loads.^[42][12] During locomotion, the metatarsals facilitate smooth transitions through the gait cycle, particularly via dorsiflexion at the metatarsophalangeal (MTP) joints, which typically ranges from 20 to 50 degrees to accommodate the foot's progression from heel strike to toe-off. The second metatarsal often experiences the peak pressure in normal walking, reflecting its central position in load transfer during mid-stance and propulsion phases. Integration of the transverse arch (formed by metatarsals II-IV) with the longitudinal arches enhances shock absorption by dissipating impact forces and enables efficient propulsion through controlled deformation and recoil.^[43][44][45] Biomechanically, the metatarsals endure ground reaction forces that can reach up to 1.5 times body weight during the push-off phase, where plantarflexion at the MTP joints generates forward momentum. This dynamic loading underscores the metatarsals' role in energy return and stability. In evolutionary terms, the robust configuration of human metatarsals represents an adaptation for bipedal locomotion, optimizing weight distribution and stride efficiency compared to quadrupedal ancestors.^[46][47]

Muscle attachments and dynamics

The metatarsal bones provide critical attachment sites for both extrinsic and intrinsic foot muscles, enabling precise control of toe movements and overall foot stability during locomotion. Extrinsic muscles, which originate proximal to the ankle, insert on the bones of the foot, including metatarsals and phalanges, to transmit forces from the leg to the forefoot. For instance, the tendons of the flexor hallucis longus and flexor hallucis brevis pass plantar to the first metatarsal head and insert on the phalanges of the hallux, facilitating flexion of the hallux essential for push-off. Similarly, the extensor digitorum longus tendons run along the dorsal shafts of the second through fifth metatarsals before inserting on the phalanges of the lateral toes, allowing extension of the lateral toes.^[12][48] Specific extrinsic muscle attachments occur at distinct bony prominences on the metatarsal bases. The peroneus longus tendon inserts on the medial tuberosity of the first metatarsal base, contributing to eversion and stabilization of the medial forefoot, while the peroneus brevis attaches to the lateral tuberosity of the fifth metatarsal base, supporting lateral stability. Additionally, the flexor digiti minimi brevis originates from the base of the fifth metatarsal, aiding in flexion of the fifth toe.^[2][49] Intrinsic muscles, arising within the foot, originate directly from the metatarsal shafts and bases, playing a key role in fine motor control and arch maintenance. The four dorsal interossei muscles originate from the adjacent sides of the metatarsal shafts (first to fifth), inserting on the proximal phalanges to abduct the toes relative to the second toe axis; they attach prominently to the second through fourth metatarsals. The three plantar interossei originate from the medial bases and shafts of the third through fifth metatarsals, inserting on the corresponding proximal phalanges to adduct the toes. The four lumbricals arise from the tendons of the flexor digitorum longus and attach to the medial aspects of the extensor hoods over the first through fifth metatarsal heads, functioning to flex the metatarsophalangeal joints while extending the interphalangeal joints.^[48][12] During dynamic activities such as walking, these muscle attachments generate coordinated forces that stabilize the metatarsals, particularly during the toe-off phase of gait. The interossei muscles contract to maintain the transverse arch integrity by countering lateral deviations and supporting the metatarsal heads against ground reaction forces. Tension transmitted across the metatarsal shafts by both extrinsic and intrinsic pulls enhances propulsion efficiency, with balanced muscular action distributing loads to prevent excessive stress on individual bones.^[50][2]

Development and variations

Embryological development

The metatarsal bones originate from the somatic layer of the lateral plate mesoderm, which migrates into the lower limb bud to form the mesenchymal core of the developing autopods.^[51] The lower limb bud emerges as a small protrusion on the lateral trunk wall around the fourth week of gestation (Carnegie stage 13), opposite the lumbar and sacral myotomes, with the foot plate becoming discernible by the sixth week (stage 17).^[52] Within this limb bud mesenchyme, the skeletal precursors of the metatarsals are preformed around the sixth week as condensations in the autopodal blastema, establishing the foundational pattern for the forefoot skeleton. Chondrification of the metatarsal anlagen begins in the seventh week (Carnegie stage 18), marking the transition from mesenchymal blastema to cartilaginous models.^[52] The process initiates in the central three metatarsals (second, third, and fourth), followed shortly by the first and fifth, within the flattening foot plate where digital rays emerge. This sequential chondrogenesis reflects the proximodistal maturation of the limb, with the metatarsal shafts forming as elongated cartilaginous rods by the end of the eighth week (stage 23).^[52] The specification of the five distinct metatarsal rays occurs through patterned gene expression in the limb bud mesenchyme, primarily driven by Hox genes (notably Hoxa and Hoxd clusters) and fibroblast growth factor (FGF) signaling pathways.^[53] These factors coordinate anterior-posterior identity and ray number, ensuring separation of the metatarsal condensations by intervening mesenchyme that inhibits coalescence and promotes individual bone formation.^[54] Proximal-distal outgrowth of the metatarsal precursors is regulated by FGF ligands secreted from the apical ectodermal ridge (AER), a thickened ectodermal structure at the limb bud's distal margin, which maintains mesenchymal proliferation and elongation until the full cartilaginous models are complete by the eighth week.^[55]

Anatomical variations and ossification

The metatarsal bones undergo endochondral ossification, beginning with primary ossification centers that form in the diaphysis during the 9th to 10th week of fetal development, with the centers for the second, third, and fourth metatarsals appearing slightly earlier than those of the first and fifth.^[56] Secondary ossification centers develop at the proximal epiphyses (bases) postnatally, typically between 15 months and 3 years after birth, with the first metatarsal's center emerging around 3 years and those of the second to fifth around 15 to 18 months; secondary ossification centers also develop at the distal epiphyses (heads) of all metatarsals around the fifth year of age, fusing with the shafts by late adolescence.^[57][58] These secondary centers fuse with the shafts by adolescence, with epiphyseal closure generally completing between 15 and 18 years of age, allowing longitudinal growth along the shafts through the proliferation of chondrocytes in the growth plates.^[58] Anatomical variations in metatarsal morphology include differences in relative lengths, classified by metatarsal formulas such as index minus (where the second metatarsal is longer than the first, prevalent in approximately 70% of populations), index plus (first metatarsal longer than the second), and index plus-minus (both equal).^[59] These length patterns influence forefoot biomechanics and load distribution. Accessory ossicles, such as the os vesalianum at the base of the fifth metatarsal, occur rarely with a prevalence of 0.1% to 5.9%, often arising from unfused secondary centers.^[60] Ethnic differences contribute to variations in metatarsal alignment, as populations exhibit distinct medial longitudinal arch heights; for instance, individuals of African descent typically have lower arch height indices compared to those of European descent, which can alter metatarsal angulation and forefoot stability.^[61] With aging, metatarsal bone mineral density peaks in the third decade of life (around the 20s) and subsequently undergoes resorption after age 50, primarily due to reduced osteoblastic activity and increased osteoclast function, leading to decreased structural integrity and heightened fracture risk.^[62] This age-related decline mirrors broader skeletal changes but is pronounced in weight-bearing foot bones like the metatarsals.^[63]

Clinical significance

Injuries and fractures

Metatarsal fractures are among the most common injuries to the foot, accounting for approximately 35% of all foot fractures and up to 5-6% of all skeletal injuries overall.^[64] These fractures can result from acute trauma or repetitive stress, with a higher incidence in athletes, where fifth metatarsal fractures represent up to 25% of foot injuries.^[65] In athletes, the risk is elevated due to repetitive loading and high-impact activities, such as running or jumping, making metatarsal fractures a significant concern in sports medicine.^[66] Common fractures include those of the fifth metatarsal, such as the Jones fracture at the base (zone 2, involving the metaphyseal-diaphyseal junction) and tuberosity avulsion fractures (zone 1, at the proximal tuberosity).^[67] The Jones fracture typically occurs from inversion of the foot with axial loading on the heel, while avulsion fractures result from sudden inversion or plantarflexion, pulling on the peroneus brevis tendon or lateral band of the plantar fascia.^[68] Stress fractures, often termed "march fractures," predominantly affect the second metatarsal due to its length and rigidity, arising from repetitive microtrauma during prolonged weight-bearing activities like marching or running.^[69] A variant of fifth metatarsal injury is the dancer's fracture, a spiral diaphyseal fracture in the distal shaft caused by twisting or rolling over the foot, commonly seen in ballet dancers during plantarflexed positions.^[68] Mechanisms of injury vary by metatarsal: direct trauma, such as crush injuries, commonly affects the central metatarsals (second through fourth); inversion sprains lead to fifth metatarsal fractures; and axial loading impacts the first metatarsal, often from hyperextension or falls.^[70] In pediatric cases, growth plate involvement may occur, classified using the Salter-Harris system, where type I and II fractures (through or above the physis) generally heal well without growth disturbance.^[71] For adults, the AO/OTA classification is used, categorizing fractures as extra-articular (type A, e.g., simple transverse or oblique patterns), partial articular (type B), or complete articular (type C), guiding surgical decisions based on displacement and pattern.^[72] Acute management focuses on immobilization for nondisplaced fractures, typically with a short leg cast or walking boot for 6-8 weeks, allowing union in most cases under protected weight-bearing.^[73] Displaced or unstable fractures, particularly Jones fractures, may require open reduction and internal fixation to promote alignment and healing.^[67] Non-union risk is notably higher in proximal fifth metatarsal fractures (up to 20%), attributed to relatively poor vascularity in zone 2, which can delay healing beyond 12 weeks despite conservative treatment.^[74]

Deformities and pathologies

Hallux valgus, commonly known as a bunion, is a prevalent forefoot deformity characterized by lateral deviation of the great toe at the first metatarsophalangeal (MTP) joint, often accompanied by medial deviation of the first metatarsal, termed metatarsus primus varus.^[75] This condition increases pressure on the second through fifth metatarsal heads, leading to secondary deformities such as hammertoes.^[75] It affects approximately 23% of adults aged 18 to 65 years and up to 36% of those older than 65, with a higher prevalence in females, estimated at around 30% in women over 50.^[75] Biomechanical factors, including prolonged use of high-heeled or narrow footwear, contribute to its development by altering forefoot loading and promoting medial metatarsal drift.^[76] Metatarsus adductus involves medial deviation of the metatarsals I through IV relative to the hindfoot at the tarsometatarsal (Lisfranc) joint, resulting in a curved forefoot appearance.^[77] This congenital or flexible deformity is the most common foot anomaly in newborns and typically resolves spontaneously, though rigid cases may persist and affect gait.^[78] Congenital brachymetatarsia presents as shortening of one or more metatarsals due to premature physeal closure, most frequently affecting the fourth metatarsal and leading to a shortened toe with potential cosmetic and functional concerns.^[79] In pes cavus, an elevated medial longitudinal arch results in prominent metatarsal heads on the plantar surface, increasing weight-bearing pressure and risk of calluses or stress under these heads.^[80] Freiberg's infraction is an osteochondrosis causing avascular necrosis of the second metatarsal head, predominantly in adolescents, particularly females during their second decade, and manifests as dorsal pain and swelling exacerbated by activity.^[81] Sesamoiditis involves inflammation of the sesamoid bones beneath the first metatarsal head, often from repetitive hyperextension or increased plantar pressure, resulting in localized pain at the ball of the foot.^[82] In rheumatoid arthritis, erosive synovitis at the MTP joints leads to bone erosions on the metatarsal heads and subsequent dorsal subluxation or dislocation of the proximal phalanges, affecting up to two-thirds of patients with chronic disease and causing significant forefoot deformity.^[83] Gouty tophi, deposits of monosodium urate crystals, can form on the shafts of metatarsal bones in chronic tophaceous gout, potentially weakening the bone and predisposing to pathologic fractures, as seen in cases involving the fifth metatarsal.^[84]