Keys To Assessing And Managing Limb Length Discrepancy

Given that limb length differences are a common contributing factor in many conditions, this author offers pertinent insights on proper evaluation, diagnosis and treatment.

Limb length difference or discrepancy (LLD), which is also referred to as anisomelia, occurs when one leg is noticeably shorter than the other. Limb length discrepancy is reportedly so common that one author considers it a normal variant and other researchers cite prevalence estimates ranging anywhere between 40 to 90 percent of the population.^1-4 

The effects of limb length difference are associated with many painful orthopedic concerns. In the literature, researchers have noted a correlation with LLD either causing or exacerbating foot, knee, hip and back issues.^1-4 There are also significant kinematic and kinetic ramifications of LLD in regard to how it contributes to both static stance and dynamic gait asymmetry.^1-4 

Some orthopedic texts claim that any limb length discrepancy of less than one-half inch up to two inches is inconsequential.⁵ However, there is quite a bit of literature to support the likelihood that a LLD of less than five mm can indeed cause or exacerbate many musculoskeletal issues.^1-4 When I use pressure mapping analysis, I regularly see LLDs as little as three mm that affect gait symmetry. Therefore, LLD should be one of the primary issues podiatrists look for when performing a comprehensive biomechanical examination.

Accordingly, let us take a closer look at key differences with LLD and how to best identify this deformity, either with physical and/or radiographic examinations. It is also important to understand the direct impact of LLD on all levels of the kinetic chain and how we can seek to best address or mitigate LLD issues for our patients. 

Recognizing The Differences Between Structural And Functional LLDs

Limb length discrepancies are usually classified as either being structural or functional. Structural LLDs are due to bony abnormalities or asymmetry somewhere between the talus and the lumbar spine. These structural issues can be congenital but may also be due to a childhood growth plate injury, other trauma or result from joint replacement surgery later in life.^1-5 Functional LLDs usually result from physiologic changes in the body or biomechanical compensatory changes that occur anywhere from the foot to the lumbar spine.^1-4

Scoliosis, an abnormal lateral curvature of the spine, can be a cause of structural or functional LLD, depending on the type of deformity. Some patients can functionally compensate for a scoliotic LLD, which would then qualify as a functional LLD in most cases. Those patients who are unable to compensate for their scoliosis usually have a structural LLD.^6,7

How Does One Objectively Detect A Limb Length Discrepancy?

There are two different ways to determine if a patient has a LLD. One way is via clinical or physical examination. The other way is via radiographic imaging. One may accomplish this with a traditional full-length lower body standing X-ray, a three-part standing scanogram X-ray, magnetic resonance imaging (MRI) or computed tomography (CT). The latter two modalities require the patient to be in a supine position. 

Radiographic imaging is the most numerically reliable method for assessing LLD according to the literature.¹ In the author’s experience, though, rarely does the size of the deformity reported by imaging studies (either in centimeters or inches) compare to the size one measures via clinical examination. For example, several CT and MRI reports may state that patients have a one inch LLD, but the clinical exam shows their deformity is rarely above six to nine mm. The suggestion here is that the clinician should always perform a clinical assessment regardless of what the radiology report suggests about the size of a patient’s deformity.

Clinical examination for LLD can involve two different methods: direct and indirect. For the direct method, one utilizes a tape measure to determine the distance between the anterior superior iliac spine (ASIS) and the fibular malleolus. The clinician performs this measurement with the patient supine but can also do so with the patient standing. Patient body structure, weight and issues with unilateral muscle atrophy as well as clinician error can all impact the outcome of direct method measurement of LLD.²

Indirect method clinical measures occur with patients standing. The clinician will assess either the anterior superior iliac spine, the posterior superior iliac spine (PSIS) or both as well as the level of the pelvis to determine which leg is long and which leg is short. Studies have shown that the indirect clinical LLD measure is more accurate and reproducible between clinical users than the direct measure.²

The indirect method also often incorporates the use of different thickness blocks of wood, felt or ethylene-vinyl acetate (EVA) foam to assist in determining exactly how much of a measurable difference, in centimeters or inches, there is between the short and long limbs.² While one usually performs this method of measure statically, I will regularly combine the use of adhesive felt in three mm thicknesses and attach the felt to the patient’s heel(s) for a dynamic evaluation as the patient walks or runs. This may determine if there is mitigation of the usual clinical signs of LLD. 

There are other clinical signs of LLD the clinician should regularly assess beyond the aforementioned methods. For instance, one should compare shoulder height to pelvic level during gait. The high shoulder and low hip are considered to be on the short limb as opposed to the low shoulder and high hip on the longer limb. Checking for unilateral early heel-off during gait, even though it is not always easy to see in patients with more subtle LLDs, may be helpful. Standing hip hike testing is when the patient attempts to hike one leg and hip at a time up into their body without leaning medially or laterally away from the respective limb. The limb that he or she “feels” lifts higher towards their torso is considered to be the short-sided limb. Most of these tests have not been studied for their validity in evaluating LLD but they are worth mentioning as senior clinicians have taught these techniques at schools for years. 

What You Should Know About LLDs And Multi-Level Compensation

The real issue is LLDs create is the potential for multi-level compensation throughout the upper and lower extremities. Limb length discrepancies create asymmetrical structure and function throughout the kinetic chain. These issues can minimally or greatly affect static stance and/or dynamic gait, and can lead to or exacerbate the pain issues I mentioned previously. The interesting thing about LLDs and any other functional or structural issues of the lower extremity is that no matter where the primary deforming force may originate, the resulting pain will not always be in that same location. I hope that the explanations of the multi-level effects of LLD below will help clarify why this may be the case.

Pelvis. One of the first areas that most clinicians will seek to examine in patients whom they suspect of having a LLD is the hips or pelvic plateau. Pelvic obliquity, a tilting of the pelvis up toward the long side limb and down toward the short side limb, is common for clinicians to see when examining for LLD.^1-4,6-8 

Innominate bones. Single-sided sagittal plane rotation of the bilateral innominate bones (anterior superior iliac spine or posterior superior iliac spine) often occurs in patients with LLD. In most instances, the anterior superior iliac spine and the posterior superior iliac spine will be short on the short limb side and higher on the long limb side.^1-4,6-8 This can, over time, cause the sacral base to become unlevel and lead to a functional scoliosis. 

Hips. Researchers have stated that there can be significantly increased pressure and forces through the acetabulum of the long-sided hip joint.^1-4,6,8 This is because the foot and limb compensations, usually external rotation, can uncover the head of the femur in the acetabulum and lead to a much more concentrated area of force or pressure on the smaller functioning area of the femoral head cartilage.^1-4,6-8 

Knee. The knee on the long side may flex easily or hyperextend as a compensation for LLD. The short-sided limb will usually be fully extended in stance and will tend to flex less in stance phase gait than the long-sided knee.^1-4,6-8. Often in older patients, I have noticed that the knee that tends to flex easily is associated with weak quadriceps via muscle testing. Some older patients with knee osteoarthritis may present to the clinic with a fixed knee flexion. In these circumstances, the patient has developed a structural LLD due to the fixed knee flexion. Therefore, one should treat this deformity as the short limb in this instance.

Ankle. The ankle on the short limb side tends to plantarflex early to seek contact with the ground during swing phase and into the early stance phase of gait. This chronic swing phase plantarflexion can cause weakness of the tibialis anterior and often leads to a functional restriction of dorsiflexion on the short limb side ankle.^1-4,6-8

Foot. The foot is also considered to pronate on the long-sided limb while the short-sided limb’s foot will tend to remain more rectus or at times be more supinated.^1-4,6-9 Subotnick notes this is not always the case as does Blake’s study looking at the relationship between rearfoot valgus and LLD.^9,10 Blake states about 80 percent of his participants with a LLD had more calcaneal valgus on the long limb side but 20 percent had more calcaneal valgus on the short limb side.¹⁰

External rotation. The foot and lower leg on the long leg side often externally rotate and exhibit a valgus heel both in stance and in dynamic gait. In a caveat to this, Subotnick says the short limb and foot will at times externally rotate and have a valgus heel as much or more so than the the typical long sided foot or leg.⁹ It is quite possible that Blake’s study omitted any external rotation findings when looking at at rearfoot pronation and LLD. However, as Blake’s study showed, only about 20 percent of his patients had pronation findings on the short-sided limb.⁹

In my experience, I have often seen findings like this with other conflicting postural and kinematic elements during the physical examination. Basically, when one sees abnormal signs of LLD, like a high anterior superior iliac spine and low posterior superior iliac spine on the same side, or increased rearfoot valgus on the short limb foot, the clinician will often see that other expected signs of traditional LLD are not in alignment either.

Spine. Examination of the spine is crucial during the physical exam and when the patient is in stance. Simply tracing the spinal column as the patient faces away from you and as he or she begins to flex forward can give you a cursory but usually accurate idea if there are any scoliotic curvatures to take into account regarding a possible LLD.^6,7

Early Heel-Off During Gait: Is It Always A Surefire Sign Of LLD?

I have referred to this in previous blogs but when one uses pressure mapping to assess gait, the observation of an early unilateral heel-off is not always indicative of LLD.¹¹ There are many other factors in a patient’s biomechanics that can mimic an early heel-off during gait. 

In general, during gait, patients with LLD tend to “step down” onto the short limb and “vault” over the long-sided limb. One will also often see decreased stance times, step length and an increased cadence on the short-sided limb. When examining pressure mapping data, the accelerations on the short-sided limb are usually faster along with an early heel off and shorted stance phase.^1-4,6-10,13

One of the primary causes of an early heel-off not always associated with a short limb is limitation of dorsiflexion range of motion of the ankle joint. Once the ankle joint has hit its functional or structural limit of dorsiflexion, then the foot rarely has any other option but to start to off-weight the heel. The most reliable test for assessing ankle joint dorsiflexion range of motion is the ankle joint lunge test.¹⁴ Testing LLD on the table is highly unreliable between testers.² This does not mean we should not do it. However, we should understand that repeatability of tests does matter. Regarding ankle range of motion, it is best to make sure we always use at least two tests for reliability. 

Van Gheluwe, Dananberg and colleagues reference functional hallux limitus causing midfoot collapse, which can also cause an early heel-off.¹⁵ The functional restriction of the first MPJ’s ability to dorsiflex in a timely manner can cause a retrograde midfoot collapse, leading to an early heel off. Clinicians regularly see this deformity with the use of pressure mapping devices. Adding modifications to a foot orthotic and an accompanying early heel lift to alleviate the functional hallux limitus will usually also alleviate this midfoot collapse.

Another potential cause of early heel-off observed in pressure mapping data is when the patient has a chronically flexed knee position or weak quadriceps that may lead to the knee giving out early in the stance phase of gait.

All of these issues can show differing degrees of heel-off during gait. They can also occur with or without a unilateral functional or structural LLD. Many foot and ankle practitioners automatically assume that any detectable LLD is associated with functional LLD and that this is a result of the patient having more pronation in one foot than the other. They also wrongfully assume that simply correcting for the forefoot and rearfoot pronation via a custom orthotic prescription will take care of this issue. Subotnick has discussed this but there are no empirical studies to show that this approach works for all patients.⁹

In fact, a study by Cornwall and McPoil showed that a “slight to moderate limitation of ankle passive dorsiflexion range of motion significantly alters the timing, but not the magnitude, of frontal plane rearfoot motion during walking.”¹⁶ I feel that one should use heel lifts to treat the ankle equinus or ankle dorsiflexion range of motion restriction to alter the impaired timing. Then treat the heel position separately with varus posting. Do not assume that one will always affect the other as the studies don’t necessarily show that they will every time.

How Can We Best Utilize Heel Lifts In Treating LLDs?

Many studies on LLD discuss the basic use of heel lifts inside of shoes.^17,18 In general, heel lifts alone do not fulfill their potential for the patient. It is probably best in most instances to use a heel lift in conjunction with a foot orthotic, AFO and/or directly add it to a shoe to maximize treatment of the LLD and overall foot function during gait. 

In a 2018 article in Podiatry Today, Richie discussed the use of heel lifts.¹⁹ One of Dr. Richie’s final conclusions was that the literature strongly points to the combined use of heel lifts with a custom-molded, prescription foot orthotic for issues that require the use of a heel lift.

Ultimately, using heel lifts with custom foot orthotics will not change the actual measurable range of motion of any of the affected joints in the kinetic chain. However, the use of the custom foot orthotic with a heel lift will almost always lead to an increase in a joint’s functional range of motion, especially at the ankle joint, and to more symmetrical functioning gait.

Concluding Thoughts

Limb length discrepancies are associated with many different types of orthopedic pain issues from the foot to the low back. Accordingly, it is very important that clinicians regularly assess patients for LLD during their comprehensive biomechanical examination. Using the indirect method of clinical assessment has proven to be the most reliable approach, short of radiographic studies. 

Once you appreciate what type of LLD a patient is likely experiencing, you will then often have a greater appreciation for why he or she may be compensating up and down his or her entire kinetic chain. The kinetic and kinematic compensations may also explain other orthopedic complaints that your patients may be experiencing. Finally, it is best to treat these issues with a heel lift in conjunction with a custom-molded prescriptive foot orthotic device and/or directly add it onto a shoe. This method of treatment is the most likely to lead to the desired outcome for your patients.

Dr. Williams is a Past President and Fellow of the American Academy of Podiatric Sports Medicine. He is the Director of Breakthrough Sports Performance, LLC in Chicago. Dr. Williams has disclosed that is the Medical Director for Go 4-D and a consultant for HP FitStation.

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