The tibialis posterior tendon has a complex fan-like insertion with multiple slips spreading across the medial and plantar midfoot: the primary navicular tuberosity slip (largest, most medial); secondary slips to all three cuneiforms (medial, intermediate, lateral); additional slips to the cuboid, the sustentaculum tali, and the plantar aspects of the 2nd-4th metatarsal bases. This broad insertion across the medial midfoot makes the TPT the primary dynamic stabiliser of the medial longitudinal arch, distributing load across the entire midfoot.
The complex multiplanar insertion of the tibialis posterior is the basis for its arch support function across the medial and plantar midfoot. In PTTD, the primary navicular slip fails first (producing medial midfoot pain and navicular tuberosity tenderness), followed by progressive arch collapse as the secondary slips also fail. FDL tendon transfer to the navicular tuberosity replaces the primary slip function while the secondary slips may partially maintain midfoot stability. The secondary slips are relevant in spring ligament repair, as the spring ligament and TPT secondary slips work synergistically to support the talar head from below.
Acute TPT avulsion from the navicular tuberosity — less common than the degenerative elongation pattern of PTTD — produces sudden medial foot pain and medial arch collapse; MRI demonstrates the avulsed primary navicular slip and surgical reattachment with suture anchors to the navicular tuberosity restores the primary arch support function.
