What makes Hephaestus lame?

Hephaestus (or Hephaistos) is an Olympian Greek god, the divine smith, famed for inventions, who taught men glorious crafts. The fixed epithet for Hephaestus, used from the eighth century B.C. by Homer, Hesiod, and other ancient authors until the fifth century A.D., is "Amphiguéeis," i.e., with both feet crooked. He is also called "Kullopodíou," i.e., clubfooted. His body and his gait were described by Homer: "He spake, and from the anvil rose, a huge, panting bulk, halting the while, but beneath him his slender legs moved nimbly ... and with a sponge wiped his face and his two hands withal, and his mighty neck and shaggy breast, ... and grasped a stout staff, and went forth halting; but there moved swiftly to support their lord handmaidens wrought of gold in the semblance of living maids." His anomaly was congenital, as we learn from Hephaestus himself ("I was born misshapen") and from his mother Hera ("But my son Hephaestus whom I bare was weakly among all the blessed gods and shrivelled of foot."). Vase paintings of the sixth century B.C. depict Hephaestus' lameness, but his lameness is not emphasized in the fifth century and thereafter. It is most likely that bilateral congenital clubfeet made Hephaestus lame. Two sons of Hephaestus, Palaemonius and Periphetes, were also reported as having deformed feet.