First metatarsophalangeal joint: Embryology, anatomy and biomechanics

Abstract

The first metatarsophalangeal (MTP) joint plays a crucial role in foot biomechanics, particularly in weight-bearing activities such as walking and running. It is frequently affected by conditions like hallux valgus (HV) and hallux rigidus, with HV impacting approximately 23%-35% of the population. This narrative review explores the embryology, anatomy, and biomechanics of the first MTP joint (MTPJ), highlighting its significance in maintaining foot stability and function. A comprehensive literature search was conducted using PubMed, Scopus, and Google Scholar, analyzing 50 relevant studies, including 12 clinical trials. The joint’s complex structure and mechanical demands make it susceptible to degenerative and structural disorders. Studies indicate that 25%-40% of individuals with HV experience significant pain and functional impairment, affecting mobility and quality of life. Biomechanical stress, abnormal gait patterns, and joint instability contribute to disease progression. Understanding the anatomical and biomechanical properties of the first MTPJ is essential for improving diagnostic and therapeutic approaches. Emerging surgical techniques, such as osteotomy and joint resurfacing, show promise in reducing recurrence rates and enhancing long-term outcomes. Further research is needed to refine minimally invasive interventions and optimize treatment strategies for first MTPJ disorders.

Key Words: Metatarsophalangeal joint; Anatomy; Biomechanics; Hallux valgus; Hallux varus; Hallux rigidus

Core Tip: This study explores the biomechanics, embryology, and anatomy of the first metatarsophalangeal joint (MTPJ), providing insights into its complex structure and function. The manuscript highlights recent advancements in surgical techniques for MTPJ disorders, emphasizing the importance of understanding its unique mechanical properties for successful intervention. By reviewing both historical and modern approaches, this article serves as a comprehensive resource for clinicians seeking to improve patient outcomes in MTPJ-related surgeries.

INTRODUCTION

The first metatarsophalangeal (MTP) joint plays a critical role in foot biomechanics, especially during weight-bearing activities like walking, running, and jumping. Located at the base of the big toe, it supports balance and forward movement through its complex structure of bones, ligaments, tendons, and cartilage. This joint enables essential motions, such as flexion and extension, but its high mechanical demands make it prone to disorders, particularly hallux valgus (HV) and hallux rigidus, which impair function and cause pain.

The embryological development, anatomy, and biomechanics of the first MTP joint (MTPJ) are key to diagnosing and managing its disorders. The joint forms early in gestation, with the lower limbs developing by the fourth week and the big toe’s digital rays appearing by the fifth week. It is a condyloid synovial joint, consisting of the first metatarsal, proximal phalanx, and sesamoid bones, stabilized by ligaments and tendons. The joint’s biomechanical function is vital for maintaining the medial longitudinal arch, aiding weight distribution and shock absorption. Disorders like HV often result from structural abnormalities and mechanical stress, making an understanding of the joint’s embryology, anatomy, and biomechanics crucial for improving patient outcomes.

EMBRYOLOGY

Embryology and development of the first MTPJ

The lower extremities experience several developmental and morphological alterations throughout different stages of gestation[1]. The embryo, two weeks post-fertilization, exhibits an uneven semicircular curvature and lacks visible indications of a lower limb bud in the caudal region[2]. At the third week, the earliest sign of lower limb development is observed like a protrusion through the lumbar and sacral myotomes. At the fourth week, this protrusion forms the lower limb bud. It undergoes growth, with distinct segments of the thigh, leg, and foot becoming distinguishable. Simultaneously, neurovascular and muscle progenitor cells begin migrating into the expanding limb buds to support further development[3,4]. The bud enlarges laterally from the trunk. It presents with a flat ventral surface and a rounded dorsal surface, connected by a convex margin[2]. By the fifth week, the foot disk becomes visible, with the future plantar surface oriented toward the embryo’s head[5]. Subsequently, an inward rotation occurs, positioning the future flexor surface to face obliquely toward the median sagittal plane of the trunk[2]. Eventually, digital rays begin to emerge, although digital notching is initially observed only in the big toe[5]. By the sixth week, the limb rotates internally by 90 degrees along its length, aligning with the transverse plane. This shift places the tibial side of the foot upward and the fibular side downward. The foot plate becomes aligned with the leg’s long axis, assuming an equinus position[3,6]. The digital rays are clearly defined, with interdigital notching also visible[2]. When the embryo reaches a crown-rump length of 21 mm to 23 mm, the foot plates transform into a more recognizable foot structure[2]. At the seventh week, the soles of the feet turn inward toward the midline, and the plantar pads start to form. In the eighth week, the legs remain externally rotated, with the knees pointing outward and the feet positioned together in a “praying” formation. As the pregnancy progresses into the fourth month, the feet gradually begin to pronate and dorsiflex and continue throughout the remainder of the fetal period[6].

Anatomy

The MTPJs are classified as condyloid synovial joints, facilitating flexion, extension, and limited adduction and abduction[7]. The first MTPJ is particularly susceptible to loading, bearing up to 90% of body weight[8]. Its anatomy is both differentiated and complex when compared to those of the smaller toes. This joint features a robust articular capsule that integrates with various tendinous, ligamentous, and osseous structures[9,10]. The capsuloligamentous complex of the first MTPJ consists of the joint capsule, several ligaments, the plantar plate, and supporting structures[11-13]. This complex serves as the primary support of the first MTPJ, and injuries to it can result in conditions such as “turf toe”[14]. For ease of approach we have considered the relevant anatomy under the heading’s bony anatomy, cartilage anatomy, ligaments anatomy, muscles and tendons anatomy and neurovascular anatomy.

Bony anatomy

The first MTPJ consists of four bones: First metatarsal bone (FMB), first proximal phalanx, lateral sesamoid, and medial sesamoid bones. Notably, the FMB is both the shortest and the widest among the other metatarsals[15]. The proximal phalanx of the first toe has a broad base with an oval-shaped concavity, allowing it to connect with the large metatarsal head. This structure supports numerous muscle and ligament attachments, enabling extensive flexion and extension while restricting side-to-side valgus and varus movements[16]. There are two sesamoid bones located beneath the first metatarsal’s head, commonly known as the lateral and medial sesamoids, although they are also referred to as the fibular and tibial sesamoids. Three key roles of the sesamoids have been recognized: Providing a mechanical advantage to the associated muscles, acting as shock absorbers, safeguarding the flexor hallucis longus (FHL) tendon, and changing the direction of action by which the FHL and flexor hallucis brevis (FHB) tendons operate, effectively increasing their moment arms as they move away from the metatarsal head[17]. Additionally, the sesamoids may elevate the first ray, allowing the first metatarsal to plantarflex during the extension of the hallux[18,19].

Cartilage anatomy

The MTPJ allows for a combination of sliding, rolling, and compression within physiological ranges of motion. However, when dorsiflexion exceeds normal limits, localized joint compression can occur. Severe turf toe injuries may be accompanied by damage to the articular cartilage and subchondral bone[20].

Ligaments anatomy

The first MTPJ lacks significant bony stability due to its shallow structure, relying instead on soft tissues like the plantar plate. This reinforced capsule segment, along with the ligaments and muscles, provides crucial support to maintain joint stability[21]. The capsuloligamentous complex of the first MTPJ consists of the plantar plate, collateral ligaments, and various supporting structures[22].

THE PLANTAR PLATE OF THE CAPSULOLIGAMENTOUS COMPLEX OF THE FIRST MTPJ

The plantar plate of the first MTPJ consists of a central portion connecting the metatarsal to the proximal phalanx, with additional lateral and medial extensions linking the sesamoids to the proximal phalanx, ensuring joint stability[23].

THE COLLATERAL LIGAMENTS OF THE CAPSULOLIGAMENTOUS COMPLEX OF THE FIRST MTPJ

The collateral ligaments consist of two main collateral ligaments and two accessory sesamoid ligaments. The medial ligament connects the inner side of the metatarsal head to the base of the proximal phalanx, while the lateral ligament links the outer side of the metatarsal head to the lateral aspect of the phalangeal base, supporting joint integrity (Figure 1)[22]. The accessory sesamoid ligaments originate at the same proximal site as the primary collateral ligaments, yet they extend outward to the edges of the sesamoid bone (Figure 2)[22].

Figure 1 A 33-year-old right foot specimen demonstrating the main lateral and medial collateral ligaments[22]. Citation: Wang JE, Bai RJ, Zhan HL, Li WT, Qian ZH, Wang NL, Yin Y. High-resolution 3T magnetic resonance imaging and histological analysis of capsuloligamentous complex of the first metatarsophalangeal joint. J Orthop Surg Res 2021; 16: 638. Copyright© The Author(s) 2021. Published by Springer Nature. This image is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution, and reproduction in any medium, provided the original author(s) and source are credited. License details: https://creativecommons.org/Licenses/by/4.0/. A: Diagrammatic illustration of the first metatarsophalangeal joint; B: Transverse anatomical section; C: T1-weighted magnetic resonance imaging of the foot; D: T2-weighted selective spectral attenuated inversion recovery magnetic resonance imaging of the foot. LMC: Main lateral collateral ligament (white arrowhead); MMC: Main medial collateral ligament (white arrow); MT: Metatarsal; P: Phalanx.

Figure 2 A 45-year-old right foot specimen revealing the capsuloligamentous complex of the first metatarsophalangeal joint[22]. Citation: Wang JE, Bai RJ, Zhan HL, Li WT, Qian ZH, Wang NL, Yin Y. High-resolution 3T magnetic resonance imaging and histological analysis of capsuloligamentous complex of the first metatarsophalangeal joint. J Orthop Surg Res 2021; 16: 638. Copyright© The Author(s) 2021. Published by Springer Nature. This image is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution, and reproduction in any medium, provided the original author(s) and source are credited. License details: https://creativecommons.org/Licenses/by/4.0/. A: Schematic coronal diagram through of first metatarsophalangeal joint and the sesamoid bones; B: Coronal anatomic section; C: and D: T1-weighted and T2-weighted spectral attenuated inversion recovery magnetic resonance imaging of the foot. LAS: Lateral accessory sesamoid ligament; Add: Adductor hallucis tendon; MT: Metatarsal; MAS: Medial accessory sesamoid ligamen; Adb: Abductor hallucis tendon; L: Lateral sesamoid; M: Medial sesamoid; FHB-L: Lateral head of flexor hallucis brevis tendon; IS: Intersesamoid ligament; FHL: Flexor hallucis longus tendon; FHB-M: Medial head of flexor hallucis brevis tendon; White triangle: The intersesamoid ligament; White pentagram: Tendon of flexor hallucis longus; White arrowheads: The accessory sesamoid ligaments; White short arrows: The flexor hallucis brevis tendons both medial and lateral heads; White long arrow; White curved arrow: The abductor hallucis and adductor hallucis tendons; White circle: The medial sesamoid phalangeal ligament.

THE SUPPORTING STRUCTURES OF THE CAPSULOLIGAMENTOUS COMPLEX OF THE FIRST MTPJ

The supporting structures of the capsuloligamentous complex of the first MTPJ include the lateral and medial heads of the FHB tendons, in addition to the abductor and adductor hallucis tendons[16].

Neurovascular anatomy

Understanding the blood supply of the first MTPJ is essential in planning surgical interventions especially in undergoing corrective procedures for HV deformities such as chevron’s, Lapidus’ and scarf’s osteotomies[24]. The first MTPJ receives both extraosseous and intraosseous vascular supply[25-27]. The FMB receives its extraosseous blood supply from various arteries, including the medial tarsal arteries, dorsalis pedis artery, deep plantar artery, deep plantar arch, first dorsal metatarsal artery, first plantar metatarsal artery, and both superficial and deep branches of the medial plantar artery. As these arteries travel, they generate several branches that nourish the FMB, eventually branching into smaller vessels that create a capillary network within the periosteum[28].

The intraosseous blood supply of the FMB is primarily sourced from a nutrient artery that stems from the first dorsal metatarsal artery. This artery consistently enters the bone at the junction between the distal third and fourth of the shaft[29,30], Additionally, a widespread network of delicate periosteal arteries encircles the diaphysis, accompanied by a system of metaphyseal and capital arteries, which collectively provide a significant blood supply to the metatarsal head[25].

Muscles and tendons anatomy

The first MTPJ is associated with six primary muscles and their tendons: FHL and FHB, extensor hallucis longus and extensor hallucis brevis, as well as abductor hallucis and adductor hallucis muscles[31]. The extensor hallucis longus shows significant anatomical variability, with the most common variation being the presence of an accessory tendon[32].

Biomechanics

The medial longitudinal arch serves as the main structural support of the foot, playing a crucial role in bearing and distributing weight during movement and standing[33,34] and depends on the motion of the first ray to ensure proper support and stability throughout the gait cycle[35]. The role of the first ray in foot mechanics is crucial because its metatarsocuneiform joint lies at the intersection of both the transverse and medial longitudinal arches, contributing to the foot’s structural integrity and dynamic function during movement[36].

A curved beam and a truss (Figure 3) are often utilized when modeling the medial arch[37-39]. Beams are engineered to endure bending under applied forces, while a truss consists of a triangular framework with two rigid supports connected at its base. At the onset of stance, the foot behaves like a beam due to the unsecured ends. As the body weight shifts forward, the calcaneus and metatarsal heads are pressed against the ground, with the arch acting as a truss. Therefore, the first ray functions as a pillar for the medial arch, making it a vital component in maintaining the structural integrity of the foot[35]. The medial longitudinal arch is essential for shock absorption and propulsion during walking[40,41]. The first MTP plays a vital role in athletic performance, particularly in running and making quick directional changes. The first toe supports over twice the weight of the other toes, with the highest plantar pressures centered around the first MTPJ during crucial movements like running, jumping, and pivoting in athletic performance[42-44]. First ray mobility is a vital consideration in routine clinical assessments as well as before and after HV surgery[45].

Figure 3 A curved beam and a truss are often utilized when modeling the medial arch[39]. Citation: Ghanem I, Massaad A, Assi A, Rizkallah M, Bizdikian AJ, El Abiad R, Seringe R, Mosca V, Wicart P. Understanding the foot's functional anatomy in physiological and pathological conditions: The calcaneopedal unit concept. J Child Orthop 2019; 13: 134-146. Copyright© The Author(s) 2021. Published by SAGE Publications. This image is distributed under the terms of the Creative Commons Attribution-NonCommercial 4.0 International License (CC BY-NC 4.0), which permits non-commercial use, reproduction, and distribution of the work without further permission, provided the original work is properly attributed. License details: https://creativecommons.org/Licenses/by-nc/4.0/. A: The longitudinal arch of the foot represented by a truss (F represent vertical descending load). The arrows represent the interior reaction forces; B: The windlass mechanism described by Hicks in 1954: 21 the arch raising movement and ray plantarflexion are synchronous; C: The coronal arch of the foot may be represented by a uni-or a multisegmental arcuate structure: The load produces a compression force at the convexity, and a traction force at the concavity.

The device that had the greatest impact on measuring first ray (FR) hypermobility is the “Klaue device” (Figure 4)[45]. It consists of an ankle-foot orthosis, positioning the ankle in a neutral stance. The examiner uses one hand to dorsiflex the FR to its maximum range, while blocking the second to fifth rays with the other hand. A micrometer is suspended from the orthosis frame, aligned above the first metatarsal head, with the ankle kept neutral and in line with the FR to measure mobility in the sagittal plane. The micrometer records the full dorsiflexion of the FR.

Figure 4 The examiner applies force to the plantar side of the first metatarsal head with one hand, immobilizing the second to fifth metatarsal bones with the other[45]. The patient was seated in a non-weightbearing position while the displacements were measured using the Klaue method. Citation: Biz C, Maso G, Malgarini E, Tagliapietra J, Ruggieri P. Hypermobility of the First Ray: The Cinderella of the measurements conventionally assessed for correction of Hallux Valgus. Acta Biomed 2020; 91: 47-59. Copyright© 2020 Acta Bio Medica Society of Medicine and Natural Sciences of Parma. This image is licensed under a Creative Commons Attribution 4.0 International License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author(s) and source are credited. License details: https://creativecommons.org/Licenses/by/4.0/. A: The force is applied initiall in a purely dorsal direction; B: At a 45-degree dorsomedial angle to the transverse plane.

The range of motion (ROM) of the first MTPJ varies widely according to both methodological differences[46] and individual variability[47]. Passive ROM is generally greater than active ROM (Table 1)[48], and even within the same person, the joint’s mobility can change over time[48]. The center of rotation in normal feet, according to Shereff et al[49] is situated in the metatarsal heads throughout both flexion and extension movements.

Table 1 First metatarsophalangeal joint’s passive range of motion[48].

Study	Plantar flexion	Dorsiflexion	Total range of motion
Joseph[47]	-	73°	-
Shereff et al[49]	34°	76°	111°
Buell et al[50]	17°	82°	97°
Nawoczenski et al[46]	37°	57°	94°

Citation: Frimenko RE, Lievers W, Coughlin MJ, Anderson RB, Crandall JR, Kent RW. Etiology and biomechanics of first metatarsophalangeal joint sprains (turf toe) in athletes. Crit Rev Biomed Eng 2012; 40: 43-61. Copyright© The Author(s) 2012. Published by Begell House Inc.

CONCLUSION

The first MTPJ is integral to foot function, facilitating mobility through complex anatomical and biomechanical mechanisms. Understanding the embryological development of this joint sheds light on its structural characteristics and vulnerabilities, which are essential for addressing the various disorders that can arise. By reviewing the interplay between anatomy and biomechanics, this paper emphasizes how these factors contribute to the joint function in activities such as walking and running. Such insights are crucial for clinicians and researchers alike, as they provide a foundation for developing more effective treatment strategies for conditions such as HV and hallux rigidus.

As the medical field continues to prioritize precision and minimally invasive treatment options, this comprehensive review serves as a valuable resource for practitioners aiming to improve their understanding of the first MTPJ. It highlights the importance of ongoing research to refine diagnostic methods and treatment protocols, addressing the multifaceted challenges posed by disorders of the joint. By enhancing clinical knowledge and fostering the development of innovative therapeutic approaches, we can ultimately improve patient outcomes and promote better management of conditions affecting the first MTPJ.