The First Metatarsophalangeal Joint (MTP Joint)
The joint at the ball of the foot under the big toe is the first metatarsophalangeal (MTP) joint, which connects the first metatarsal bone to the proximal phalanx of the great toe. 1, 2
Anatomical Structure
The first MTP joint comprises several key components:
Osseous elements: The metatarsal head articulates with the base of the proximal phalanx, with articular cartilage covering both surfaces 1, 2
Plantar plate complex: Unlike the lesser MTP joints, the first MTP joint does not have a single dominant fibrocartilaginous capsular thickening. Instead, it contains a complex fibrocartilaginous pad that invests the hallux sesamoids and is inseparable from the plantar capsule, the intersesamoid ligament, paired metatarsosesamoid and sesamoid phalangeal ligaments (SPLs), and musculotendinous structures 2
Supporting structures: The joint is stabilized by dorsal extensor tendons and collateral ligaments 2
Sesamoid bones: Two sesamoid bones are embedded within the plantar plate complex beneath the first metatarsal head 1, 2
Clinical Significance
The first MTP joint is vital to foot biomechanics and supports weight up to eight times heavier than body weight during athletic activities. 2
Common Pathologies
Osteoarthritis (hallux rigidus/limitus): This is a common and painful condition affecting the first MTP joint, characterized by degenerative changes and restricted motion 3, 4, 5
Turf toe injuries: Acute hyperextension injuries primarily load the distal sesamoid phalangeal ligaments, making them susceptible to damage. These injuries are best visualized in the sagittal plane on MRI 2, 6
Hallux valgus: Progressive deformity at the first MTP joint that can lead to disability if untreated 2
Gout: The first MTP joint is a classic site for gouty arthritis, with monoarticular involvement of this joint being a suggestive clinical feature 1
Imaging Considerations
Ultrasound: Can visualize the joint cavity, articular cartilage, extensor tendons, and detect synovitic lesions with high-frequency transducers (10 MHz or higher) 1
Radiography: Weightbearing AP, lateral, and sesamoid axial views are the best initial imaging for evaluating plantar plate disruption after MTP joint injury 1
MRI: Provides superior visualization of soft tissue injuries, including SPL tears, collateral ligament injuries, and osteochondral defects 1, 2
Clinical Examination
The first MTP joint is easily accessible to clinical examination, though ultrasound can detect minor synovitic lesions that may be missed clinically 1. The joint should be examined with dorsal longitudinal and transverse scans, with the metatarsal head, proximal phalanx, extensor tendon, joint cavity, and articular cartilage all visible on imaging 1