Structure and Function of the Intermetatarsal Joints
The dorsal ligaments, plantar ligaments, and interosseous ligaments provide structural stability to the foot by tightly connecting the four lateral metatarsal bases. These anatomical contact points form three small, intricate synovial joints known as the intermetatarsal joints, which enhance the overall flexibility of the foot.
Although the metatarsal bases are interconnected by ligaments, the articulation between the first and second metatarsal bases is not a true joint. This anatomical distinction allows for greater relative mobility of the first metatarsal ray, similar to the thumb in the hand. However, unlike the hand, the distal ends of all five metatarsals are firmly bound together by deep transverse metatarsal ligaments, ensuring stability. The small but essential movements occurring within the intermetatarsal joints contribute to increased flexibility at the tarsometatarsal joints.
Anatomy of the Metatarsophalangeal Joints
The articular cartilage covering the distal ends of the metatarsal heads ensures smooth joint movement and optimal function (Figure 14-37). Structural stability is reinforced by pairs of collateral ligaments, which provide strong support for each MTP joint. These ligaments are closely integrated with the joint capsule, enhancing its durability. Similar to the hand, each collateral ligament courses obliquely from the dorsoproximal to the plantar-distal direction, forming two functionally significant portions:
- A thick cord-like portion
- A fan-shaped accessory portion
A particularly notable feature is the two sesamoid bones embedded within the tendons of the flexor hallucis brevis, which are strategically positioned over the plantar plate of the first MTP joint.
The four deep transverse metatarsal ligaments firmly interconnect the plantar plates of all five metatarsophalangeal joints, ensuring that the first ray remains in alignment with the other metatarsals. This design allows the foot to function as a weight-bearing and propulsion mechanism, rather than a fine manipulative structure like the hand. Unlike the deep transverse metacarpal ligament in the hand—which links only the four fingers while allowing the thumb to remain free for opposition—the deep transverse metatarsal ligaments unite all five metatarsals, reinforcing foot stability.
Joint Capsule and Dorsal Digital Expansion
The fibrous joint capsule surrounding each MTP joint is intricately connected to both the collateral ligaments and the plantar plate, further reinforcing stability. The dorsal aspect of the MTP joints is covered by an expansive dorsal digital expansion, a specialized structure resembling the extensor mechanism of the fingers. This thin connective tissue layer, which remains closely integrated with the dorsal joint capsule and extensor tendons, provides both stability and mobility to the joint.
Biomechanical Considerations and Functional Movements
Studies on MTP joint kinematics describe unique movement coupling phenomena between different planes of motion. One particularly significant finding is that:
- Plantarflexion of the MTP joint is accompanied by slight eversion
- Dorsiflexion is accompanied by slight inversion
These movements are most easily observed in non-weight-bearing conditions but do not strictly conform to the traditional definitions of pronation and supination. As a result, they are sometimes classified as atypical movements.
Despite this classification, these unique motion patterns provide functional advantages. For example, the combination of plantarflexion and eversion allows the medial foot column to adapt more effectively to uneven terrain. This movement pattern is biomechanically comparable to the thumb's motion when grasping a large spherical object.
Although ongoing research continues to explore these mechanisms, the precise functional role of these coupled movements within the overall biomechanics of foot motion remains incompletely understood.