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👣 Mechanism of Flatfoot Development: PTM and Navicular Bone

1. Role of the Posterior Tibial Muscle (PTM) and Navicular Attachment
PTM Function: The PTM, originating in the back of the lower leg, runs along the medial side of the ankle and is one of the most crucial tendons for creating and maintaining the foot's arch.

Navicular Attachment: The tendon of this muscle, the Posterior Tibial Tendon (PTT), attaches to the medial side of the Navicular bone.

Arch Maintenance Mechanism: When the PTM contracts, it inverts the rearfoot (hindfoot) and firmly supports and lifts the medial longitudinal arch of the foot, which is essential for maintaining the foot's arched shape.

2. Overpronation and Arch Collapse
Pronation Occurrence: When the foot exhibits excessive, abnormal inward rolling movement (known as overpronation), the PTM and its tendon are subjected to repeated damage and stress.

PTM Dysfunction: This continuous stress and excessive tension cause the PTM's function to weaken or the tendon to become inflamed, resulting in Posterior Tibial Tendinitis (PTT Tendinitis) or dysfunction.

Arch Collapse: As the PTM, the primary supporter of the arch, weakens, the medial support of the foot decreases. The arch gradually collapses, eventually leading to the development of Acquired Flatfoot Deformity.

3. Accessory Navicular Involvement and Exacerbation
Accessory Navicular (AN): The Accessory Navicular is a small bone found adjacent to the Navicular bone, present congenitally in about 10–14% of the population.

Abnormal Attachment: In patients with Type 2 Accessory Navicular Syndrome, the PTT (which should attach to the Navicular bone) is often found attached to the AN bone instead.

Weakened Function and Pain: When the PTT attaches to the AN, the line of pull (moment arm) of the tendon is altered, weakening its ability to support the arch and making it more likely to cause Acquired Flatfoot. If persistent irritation or trauma occurs at the AN attachment site, it causes pain, swelling, and inflammation, leading to Accessory Navicular Syndrome.

Relation to Flatfoot: The presence of the AN can negatively affect PTM function, leading to arch weakening, which is why it is closely associated with flatfoot.

In conclusion, the PTM supports the arch by attaching to the Navicular bone, but excessive overpronation causes inflammation (tendinitis) and weakens the tendon. This functional weakness leads to arch collapse and flatfoot. The congenitally present Accessory Navicular bone can further compromise the normal attachment and function of the PTM, exacerbating the flatfoot and PTT issues.