Congenital genu recurvatum, or knee hyperextension, is a rare condition that occurs once in every 100,000 live births.
People with this condition, which appears more often in women, are born with a deformity in their knee joints. Their kneecaps are dislocated to the side, causing them to bend backwards to varying degrees. While some are still able to walk upright, others are forced to use their hands as an extra pair of feet.
The first documented accounts of the deformity go back to the early 1800s. In 1886, a girl named Ella Harper was featured in a circus and called “The Camel Girl” because she walked on all fours.