Leg length inequalities are quite common and are an often-overlooked source of back pain. Spinal stenosis may also occur as a result of differences in leg length. This is likely due to the effect of chronic torsion and postural deviation, such as a tilted pelvis, which can cause the body to attempt to stabilize the spine through new bone creation, calcified ligaments, or thicker musculature. Unfortunately, the spinal nerves may be collateral damage in this process as the small spinal spaces through which they travel can be further restricted by such changes. In addition to this, the twisted posture that occurs, however minor, when one leg is longer than the other can itself put pressure on the spinal nerves through spinal stenosis, although this is often reversible through postural reeducation and simple devices such as shoe lifts.
Detecting the effects of leg length inequalities (LLIs, also known as leg length discrepancy or LLD) is possible using rasterstereography, a technique described by Betsch, et al (2011). This is non-invasive, contact-free and is considered reliable as an indicator of the consequences for spinal posture and pelvic obliquity (tilt). Even minor lower limb length discrepancy can result in pelvic tilt and torsion, although lateral and frontal deviation of the spine are not thought to be as badly affected.
True Leg Length Discrepancy
Biomechanical dysfunction in the lumbopelvic region can result from both a ‘true’ leg length discrepancy, or structural discrepancy, where the femur or tibia are actually different lengths. This kind of difference in leg length is usually congenital and many people have slight differences of half a millimeter to an inch. Other causes of structural leg length discrepancies include surgery, accidents, or fractures. These structural LLDs do not usually require surgery as the less severe cases can be addressed through the use of lifts in the shoe. However, using heel lifts to correct a leg length discrepancy that is causing back pain is not always advisable unless the LLD is truly structural.
Functional Leg Length Discrepancy
A functional leg length discrepancy is one where the bones are actually the same (or very similar) lengths but postural problems have caused one leg to appear longer (or shorter) than the other. Tight muscles in the back can lead to functional leg length discrepancies, as can improper rotation of the pelvis (sometimes referred to as anterior innominate rotation). An examples is where a tight right quadratus lumborum muscle pulls the right side of the iliac crest (the hip) upwards. When a patient with this phenomenon is lying down it would look like the right leg was shorter although the bones may very well be of a similar length. There is such variation in the ways that the muscles, ligaments, and bones in the pelvis and lower back are arranged in individuals that simply using a visual leg length comparison is insufficient for accurately assessing leg length discrepancies.
Diagnosing Leg Length Discrepancy as a Source of Lower Back Pain
Clearly, identifying structural differences in leg lengths at an early age is preferable as this means that a child has less opportunity to develop abnormal posture of the lower limbs, pelvis, and lumbar spine. This common cause of spinal stenosis symptoms is, therefore, less likely where heel lifts and/or surgery are used to rectify the dysfunction. X-Rays allow for an accurate comparison of bone lengths in the legs and, where X-Rays are unavailable, using bony landmarks on each leg and taking measurements can also be helpful, although less accurate. Functional LLDs are more likely to be the cause of back pain and spinal stenosis symptoms than structural discrepancies but the cause of the dysfunction is often harder to identify.
Treating Leg Length Discrepancy for Back Pain
Most LLDs that result in back pain do so because of soft tissue problems rather than overt spinal stenosis. However, where massage, muscle relaxants, or postural reeducation fail to adequately address the problem it may be that chronic leg length discrepancy has actually created a structural problem through new bone growth or the calcification of ligaments in the spine. Spinal stenosis may continue to progress if the inequality is unaddressed and may actually worsen initially during treatment as the patient gradually relearns appropriate posture using devices such as heel lifts. Very few studies have been carried out to assess the efficacy of treatments such as heel lifts or surgery for lower back pain thought due to differences in leg length.
Chronic LLDs may cause symptoms that take many months, if not years, to resolve after treatment as the spine adjusts to the new ‘correct’ position. Where significant damage to the spine has occurred, it may be necessary to cut problem ligaments or shave off osteophytes that are causing pinched nerves. Unfortunately, there are few methods elucidated for assessing problems with leg length discrepancies, pelvic distortions, and back pain symptoms, with more research needed to improve diagnosis and, therefore, prognosis of patients with back pain due to leg length differences.
Betsch M, Wild M, Große B, Rapp W, Horstmann T., The effect of simulating leg length inequality on spinal posture and pelvic position: a dynamic rasterstereographic analysis.
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