A surprisingly large percentage of the population has a Leg Length Discrepancy (LLD)
Damn near everybody has one leg at least a little tiny bit shorter than the other. Less than 5 mm (0.2 in) is hard to see and usually needs no correction.
When a LLD causes problems, they usually take the form of low back pain, hip pain, or sometimes knee pain. The basic problem here is a non-level pelvis, which results in a lateral asymmetry in lumbar spinal loading. Intervertebral discs are designed to be asymmetrically “wedged” front to back in their normal lordotic curve, even while they are designed to be loaded symmetrically side-to-side. If the legs are of different lengths, they have not cooperated in this endeavour.
When there is a way to accurately measure leg length – and this is difficult when the discrepancy is small – perfectly symmetrical leg length becomes quite rare. Perfectly symmetrical anything is quite rare, because the processes that generate physical objects in our end of the universe are not uniform much above the molecular level.
Making the legs the same length restores the pelvis to a symmetrical position in the horizontal plane, and therefore restores lateral symmetry to the lumbar spine and any load it bears. Any residual asymmetries in any other plane usually correct themselves after the leg length difference has been corrected, the degree of correction depending on the length of time the discrepancy has been present (the age of the individual) and the severity of the uncorrected discrepancy. Read more…