In metatarsus primus varus, the first metatarsal bone, which connects to the proximal phalanx bone of the big toe, is rotated and angled away from the second metatarsal bone. The rotation of the first metatarsal bone causes the head of the bone to push outward, creating a bump on the side of the foot.

A hallux valgus (plural: halluces valgi) is a fixed abduction of the first metatarsophalangeal joint of the great toe. It is usually due to metatarsus primus varus which is a medial deviation or adduction of the first metatarsal with an increased first-second metatarsal angle. In many cases it is acquired but rarely can be congenital.

Intermetatarsal angle – the angle between the first and second metatarsals (the bones that lead up to the bases of the big toe and second toe). If this angle is wider than nine degrees (9º), it is abnormal and referred to as metatarsus primus varus.

The Metatarsus Varus is an assemblage of bones in the central section of the foot. Each foot has five metatarsal bones, each conjugated to the phalanges of the toes. Metatarsus primus varus refers to a condition where the metatarsal bones are spun toward the middle of the body. This causes an apparent defect, and both feet are frequently affected.