With a leg length discrepancy the patient experiences one leg as being longer than the other. The experience is very real but on examination the legs may turn out to be the same length.
The main reason for legs of the same length being experienced as differnt is that the pelvis is tilted (pelvic obliquity). The leg on the raised side of the pelvis will be experienced as shorter.
And just to confuse matters more a person may experience their legs as being the same length when they're not. This happens when the pelvis has drops down on the side of the shorter leg to compensate.
Over time this obliquity becomes fixed. Now imagine that a few decades down the line the person needs a hip replacement. The surgeon will provide an implant that makes the shorter leg the same length as the other one. However the patient will experience the operated side as being longer because the pelvis hasn't adjusted.
Here's a summary.
You may experience you're legs as being different lengths because they are or because your pelvis is tilted making one seem longer.
You may experience your legs as being the same length because they are or because they are not and the pelvis has tilted to compensate.
Causes of leg length discrepancy
The most common cause of unplanned leg lengthening following hip replacement is that the femoral implant used is too long.
As well as making the patient feel unbalanced this will put strain on the soft tissues surrounding the joint.
In cases of unplanned shortening of the limb there is no additional strain on the soft tissues and the patient will manage well with shoe lifts.
If the femoral head has collapsed priory to surgery the muscles surrounding it adapt. They become shorter. When the hip is replaced the leg length will be longer but the shorter muscles will pull the pelvis down and the patient will experience the leg as too long. It takes time for the muscles to adapt.
In some cases the problem is caused by the positioning of the cup part of the joint.
How many people are affected?
It is estimated that 72% of all hip replacement patients will experience some lengthening on the operated side but the difference will be less than 10 mm. Such a small difference won't usually cause a problem as the surrounding soft tissues are not being significantly stretched. There may be some small effect on the spine but again not enough to be problematic.
A further 22% of hip replacement patients will experience a lengthening of more than 10mm and in a few cases there will be a shortening of the operated limb by more than 10mm (White 2002).
The most commonly reported symptoms (unsuprisingly) are uneven leg length and a feeling of being unbalanced. The patient may also experience pain in the groin and in the lower back. Confusingly this pain may be felt around the opposite hip.
It is hard to measure leg length accurately and the amount of discrepancy between the legs only has to be small to be problematic.
Measuring leg length on an X-ray is useful but not infallible.
Here's a good video of a physical therapist showing how he uses different methods to measure leg length discrepancy.
Treatment of leg length discrepancy
Apparent discrepancy If the leg length discrepancy is an apparent one it will probably disappear within three months of surgery. If the cause is a tilted pelvis this will slowly re-align itself. Physiotherapy is recommended and the therapist will advise if a temporary shoe lift should be used.
Real discrepancy If the leg length discrepancy is real, but small, most people will adapt to it quite quickly. If not then a raised shoe should be used. In some cases, fortunately rare, revision work will be needed during which either the femoral or the cup component will be changed.
The main issue of concern is what is happening to the soft tissues surrounding the joint and in the spine. The more tension there is on these the more need there is for corrective work.