what is it
There are two classifications of LLD, Functional, sometimes referred to as ‘apparent ‘ and Anatomical, sometimes referred to as a ‘true‘.
Anatomical or true leg-length discrepancies are when there are actual measurable skeletal differences in the shape and length of the leg bones, such as the femur, tibia and fibula. They can also include deformities of the foot and ankle bones, as well as the pelvis.
These bony differences are further subdivided into two categories; those that shorten a limb and those that lengthen it.
Congenital growth deficiencies, bone or joint infections, growth plate fractures or dysfunctions, among other things can cause lower limb shortening. Causes of lower limb lengthening are rarer and are caused by conditions such as hemihypertrophy.
Functional or apparent leg length discrepancies are where there are no bony differences and the legs are technically the same length, instead its other conditions such as spinal scoliosis or pelvic asymmetries that create the appearance of one leg to be longer or shorter than the other.
Other causes for apparent leg length discrepancies are believed to be from so excessive ankle pronation, knee genu varum or valgum, or even possibly due to soft tissue contractures or imbalances.
How to identify and measure LLD
There are many methods to measure LLD, some more reliable and validated than others. The gold standard is radiographically, with techniques called scanograms or orthoroentogenograms, these are a series of three X-rays taken in succession of the hip, knee and ankle joints, the bones can then be measured accurately. However, other imaging techniques such as CT scans and MRIs are also found to be useful in measuring LLD (source).
However, these tests aren’t infallible and more importantly they at not available to therapists in clinics. They are expensive and of course have radiation issues to consider.
So other clinical tests have to be used by therapists. The most commonly used method is by measuring the distance between bony landmarks, normally between the ASIS and the medial malleolus. However using this method does have issues. First is accurate location of the bony landmarks can be difficult in some overweight patients, and muscle girth differences can skew measurements as the tape runs down the leg.
Alternative methods of measuring from the umbilicus or even the xiphisterum have been suggested and some suggest that there is also a need to measure to the bottom of the heel so that any discrepancy in the ankle joint is included.
So what’s the reliability and accuracy of these tape measure methods? Well Woodfield found moderate reliability (58%) between two examiners within 1/8th of an inch difference (that’s 3.2mm to us metric users) and excellent reliability (92%) within 3/8th of an inch (roughly 1cm). Jamaluddin also found that tape measurements between the ASIS and medial malleolus between two trained assessors is reliable to within 5mm when compared to the gold standard CT scanogram measurements, as does this recent study. So tape measurements can be considered as actually pretty good at detecting leg length differences bigger 1cm, but not so good under this. Other clinical tests used to check for LLD are simple observational tests of the ASIS and PSIS bony landmarks to assess if there is any anterior or posterior rotation of the pelvis. Recently some instruments called digital inclinometers have been claimed to be more reliable and accurate to measure these angles, however there currently no evidence of this claim. However, even if you are using simple observational measurements or fancy digital inclinometers, the reliability of all these pelvic position tests is highly questionable (source). The normal variation in pelvic skeletal morphology makes tests that use bony landmarks completely invalid and useless. The position and heights of the ASIS and PSIS vary normally up to 23° with no robust correlation ever been found between increased or decreased angles and injury or pain in any conditions. So how anyone can determine if an ASIS is lower or higher than it should be due to misalignment or torsions. Another observational clinical test commonly used for LLD is where the patient is asked to lie supine, and perform a couple of bridging movements with their legs together, and knees bent at 90 degrees. An ‘eye ball’ check of the height of the top of the knees is taken to see if there is any difference in femoral length, and then the legs are extended and again another ‘eye ball’ scan is used to check the alignment of the medial malleoli to ascertain if any there is any tibial length differences. However yet again as this is only an observational test it’s accuracy is limited to either a ‘yes or no.
That’s simple, they are common, very common, very, very common.
In fact nearly all of us (up to 95%) have a LLD to some lesser or greater degree (source) with the average difference for most being approx. 5mm, with the right leg being most commonly the shorter than the left (source)
The more important question to ask is when does a leg length discrepancy become a significant and potential factor to consider in someone’s pain and pathology?
This is a lot harder, if not impossible to answer at times.
When and why LLD cause issues
When does LLD become significant isn’t truly clear, but the general consensus in most literature and by most clinicians is that anything over 1cm is probably worth considering (source)
However, the key word here is ‘considering’ not automatically blamed. Other factors such as the ability of the body and kinetic chain to compensate for the inequality have to be considered.
The mechanisms often explained as to why LLD causes pain and pathology is varied and sometimes complex.
To keep it simple most are thought to occur through excessive ground reaction forces, mainly in the shorter limb as it has further to travel to reach the ground (source).
These ‘excessive’ forces have to be absorbed into the kinetic chain and it is thought that they can adversely stress and strain joints, muscles and ligaments of the lower limb, pelvis, and lumbar spine.
Anyway, these excessive forces are thought to lead too and create pathology such as OA, tendinopathies, back pain etc.
other studies show a correlation with pathologies in the longer limb with LLD such as stress fractures and planta fasciitis.
Either way, there are many papers that may show a correlation between LLD and pains in certain populations in certain areas. For example this large prospective trail on over 3000 patients shows those with a LLD of more than 1cm may have a greater prevalence of knee joint osteoarthritis.
But conversely, there are also many papers that show no correlation between LLD and common musculoskeletal conditions such as this paper on lateral hip pain and this paper on chronic low back pain.
As always in research is important to remember that a correlation does NOT automatically imply a causation.
So in summary leg length differences are common, and finding them is easy! Very easy! However, deciding if they are a factor that is contributing to pain and if they need correcting is not so easy.
In my opinion leg length differences are mostly NOT a primary issue and many don’t need correcting. Our bodies have an amazing ability to adapt, and in most cases have already adapted to any leg length differences we may find, and if they haven’t, they can, and will do, given the opportunity!