Leg length discrepancy - what's the latest?
This is one of the most controversial topics in sports medicine. Leg length discrepancy: what is it, does it matter, and if so, how much matters?
Limb length discrepancy, or anisomelia, is defined as a condition in which paired limbs are noticeably unequal. When the discrepancy is in the lower extremities, it is known as leg length discrepancy (LLD). LLD is a relatively common problem found in as many as 40  to 70% of the population. In a retrospective study, it was found that LLD of greater than 20 mm affects at least one in every 1000 people. The effects of LLD on function and the magnitude of LLD warranting treatment have been subjects of controversy for some time.
A very recent study found 7% of children aged 8-12 who were developing normally had a limb length difference of 2 cm or greater.
LLD can be subdivided into two etiological groups: a structural LLD (SLLD) defined as those associated with a shortening of bony structures, and a functional LLD (FLLD) defined as those that are a result of altered mechanics of the lower extremities.
In addition, persons with LLD can be classified into two categories, those who have had LLD since childhood, and those who developed LLD later in life. In terms of functional outcomes such as gait, persons who have developed a LLD later in life are more debilitated by LLD of the same magnitude when compared to persons who have had LLD since childhood.
Studies have investigated the effects of LLD on low back pain (LBP) , osteoarthritis (OA) of the hip , stress fractures, aseptic loosening of hip prostheses, standing balance, forces transmitted through the hip, running economy and associated running injuries. There is disagreement regarding the role LLD plays in musculoskeletal disorders and the acceptable amount of LLD necessary to warrant treatment. Some investigators have tried to quantify a significant LLD, accepting as much as 20 to 30 mm, while others define a significant discrepancy in terms of functional outcomes.
Perhaps the most controversial musculoskeletal disorder associated with LLD is LBP. Some authors have found a definite association between LLD and LBP while others found none.
The most recent study on this issue, concluded " that LLIs (limb length inequalities) >20 mm will lead to significant changes in the spinal posture of healthy test subjects. However, these changes were only found in frontal (surface rotation and lateral flexion) spinal parameters, but not in sagittal parameters." This study also found " significant correlation between different leg lengths and changes of the pelvic position. Further, females and males seem to react in the same way to LLIs."
A recent study Comparison of injury rates between cadets with limb length inequalities and matched control subjects over 1 year of military training and athletic participation, found no evidence to support any increased incidence of injuries in a young, healthy, athletic, military population with mild LLIs, compared with matched control subjects without LLIs, over 1 year. The included LLD was quite small however, at >0.5 mm.
This study Leg length discrepancies in elite track and field athletes with stress fractures from 2011 concluded "Leg length anisomelia in athletes seems to increase the possibility of stress fractures."
A 2010 study indicated there was a " strong correlation between a longer limb and unilateral plantar fasciitis pain."
In relation to gait, several authors have found that relatively small (20–30 mm), LLD created significant changes in gait such as increased ground reaction forces (GRF) increased energy consumption, and increased lower extremity kinetic energy while other authors have found that these parameters remain relatively unchanged until much larger LLD (60 mm) are realized.
It would be fair to say that even today the issue remains very murky indeed.
So what practical advice can we draw from the current body of knowledge?
Well, perhaps the most pressing question is "when to intervene?"
There is no easy answer to this question, because, as we have seen, there is no standard measurement indicating the LLD that will cause a problem. The consensus appears to sit at about 2 cm, but this by no means is cast in stone. I shall also now limit this discussion to a sporting context, and specifically running. The issue here is that in static stance, the body will of course compensate for a SLLD. But in running, when one leg only is in contact with the ground, the body will be unable to recognise the LLD, so how or why would it compensate? Now of course if we are discussing a 2cm+ LLD, then the recognition will be made, but many practitioners treat much smaller LLD's in the belief that they may be an issue.
I am not convinced of this, particularly if the LLD has been present since childhood. For the most part these LLD are compensated without issue.
Without even going into the issues of accurately and reproducibly measuring LLD (which are very substantial indeed), I like the concept of applying lower limb stiffness parameters to this issue. Dr. Simon Spooner from the U.K. recently made the following sensible comment: "My hypothesis is that the body has a preferred movement pathway for the centre of mass (COM) during running at a given velocity; running with a limb length discrepancy should tend to cause an asymmetrical displacement pathway for the COM; to compensate for this, the body will modulate the leg stiffness by either decreasing the stiffness of the longer limb and/or increasing the stiffness of the shorter limb. If the tissues are able to do this while functioning within their zones of optimal stress (ZOOS), injury may not ensue. However, if in order to maintain the displacement pathway of the COM, the tissues are forced to function outside of their ZOOS, then tissue injury may occur. If the contentions of Butler et al. are correct then a leg which is functioning too stiff for the environmental and task demands may be at greater risk of bony injury, while a leg which is functioning too compliant for the environmental and task demands may be more prone to soft-tissue injuries.
This is why I think asymmetrical density heel raises may be helpful in limb length discrepancy."
The interpretation of this is.. it all depends, and like so many things in the world of sports medicine, there is a lot of art to the science, and each case is different and individual.
My take home message is as follows:
- There is no standard compensation for a LLD
- There is no standard limit in mm for a LLD
- Just because there is a LLD, it does not mean there will neccessarily be a problem
- It is quite possible to create an injury by treating a perceived LLD where previously no injury exists. The human body is remarkably edept at compensating biomechanical anomolies.
- That said, IF there is a strong association between a LLD and symptoms, strong consideration should be given to treating the LLD with either a heel lift or a full sole raise.
- My clinical examination always includes investigation for LLD
- Be particularly suspicious of the combination of unilateral symptomatology and a measured LLD. This relaibly will benefit from intervention.
Now.. perhaps we should talk about LLD in cyclists.. another whole can of worms........