At least once a day I hear or see a therapist discussing leg length discrepancy (LLD) or to give it it’s technical term ‘Anisomelia’. I hear therapists often explaining how LLD is a cause of pain and blame it for a whole host of movement dysfunctions and muscle imbalances, and then go on to use various treatments to correct it.
I question LLD. I question why it gets blamed so often, I question how it’s assessed, but more importantly I question does it actually cause any significant adverse biomechancial dysfunctions so often in so many.
What is LLD?
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 called ‘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).
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 acurate 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).
As I’ve discussed before in my SIJ posts, 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 a so called ‘misalignment’ or ‘torsion’ is beyond me and the evidence, it’s merely guess work.
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’.
How common are LLD?
That’s simple, they are common, very common, very, very common.
In fact nealy 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 does 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. How this is done, is even more unclear, and from my experience and observations of other therapists who ‘do’ this it tends to go on nothing more than their ‘intuition’.
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. In fact some blame LLD for pains felt as far as the jaw TMJ, which I think is pushing the boundaries of interregional dependance and plausibility a little too far!
Anyway, these excessive forces are thought to lead too and create pathology such as OA, tendinopathies, back pain etc.
However just to confuse matters, other studies show a correlation with pathogies 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 prevelance of knee joint osteoarthritis.
As always in research is important to remember that a correlation does NOT automatically imply a causation.
As with most things in the evidence base, I am sure if we looked hard enough, digged deep enough, and ran enough data through some clever statistical skulduggery we could find that LLD is correlated with most things, or conversely that it isn’t!
Too quickly and too often blamed?
In my opinion LLD is a nice, easy, and convenient scapegoat for many therapists to blame as a cause or source of pain, often with most of those blaming LLD not really taking into account that humans are asymmetrical and the remarkable and amazing capacity for us to adapt to asymmetry.
This ability to adapt to asymmetry is neatly emphasised by this study which induced a LLD, up to 3cm, and measured joint kinematics and forces and found no significant differences between limbs.
In my opinion therapists who rush to blame a LLD as a problem usually have a vested interest in doing so, normally a sales opportunity for some fancy gadget or gizmo designed to identify or correct the ‘wonkyness’.
However, some therapists simply have misinterpreted or cherry picked the research to support the notion of leg length differences, and some just don’t have a good enough understanding of the other factors that influence biomechanics and of course can produce pain.
Some therapists have also been lead astray having been taught the assessments and treatments of LLD by a charismatic or influential ‘guru’ again usually with a vested interest in finding LLD.
Simply put, blaming LLD for a lot of patients pain presentations and issues just doesn’t make any rationale sense to me, nor does it fit with any sound clinical reasoning.
For example it’s not as if people wake up one morning, falling over as they get out of bed because one of their legs suddenly became shorter over night, LLD doesn’t happen quickly, most of the time, unless it’s been created traumatically or surgically as in after a hip replacements, most LLD are congenital.
The traumatic causes of LLD are a different ‘kettle of fish’ and yes they may indeed need correcting for some. But most LLD don’t happen suddenly, they happen gradually, over time, over a long time, and so the body slowly and quietly adjusts and adapts.
So a therapist who suddenly finds a LLD in a patient one Wednesday afternoon, that’s been with the patient all their life, and blames it as the cause of their back, knee, hip pain just doesn’t make any sense to me.
The simpler explanation?
The inconvenient truth for those that blame LLD is that are far more simple, rational and evidence based reasons for most common pains and problems that patients come to see us for, such as joint or tissue overload.
However it is these simple diagnosis that I find that many therapists do not want to give, or feel uncomfortable giving to patients. Many therapists feel that they have to justify and demonstrate their ‘skill’ and training by spotting and identifying subtle biomechanical abnormalities such as a LLD.
This they think gives an appearance of skill to their patients, demonstrating that they have found a plausible scientifically sounding diagnosis, and so reassures the patient that they are dealing with a true professional.
Now, before I sound like I am trashing all therapists again and tarring them all with the same brush, and completely rubbishing and ignoring LLD, I’m not.
I do accept that a LLD may amplify and contribute to certain overload problems, in certain conditions for some people, and I do check for LLD in some instances, and if I do find one greater than 1cm I will see if correct it, with a simple off the shelf insole or heel lift, reduces the sensations of pain or discomfort, just like a symptoms modification test.
If it helps great they keep the insole or heel raise in but usually only temporarily to see if it reduces pain and keeps them moving. I then get most to wean off any device and begin the most important part of the rehab, a progressive strengthening and loading of the joint and tissues to restore is capacity to withstand the stresses and strains of the movement or task it couldn’t do before it started to breakdown and hurt.
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!
As always thanks for reading!
Happy ‘wonky’ exercising