Buy an antique rack and pinion, sign a disclaimer and ask someone to use it on you.
(seriously: there are surgical procedures to lengthen the leg by inserting artificial plates in the shin and thighbone. Depending on your age hormone treatment can also be effective. The complexity of ligaments and cartilage mean that any such treatment is limited by the ability of your tendons and knee-mechanism to compensate. Your posture may also be adversely affected.) As with all such issues, consult your GP.