Keys To Recognizing And Treating Limb Length Discrepancy

Author(s): 
Joseph C. D’Amico, DPM, DABPO

Given the common presentation of limb length discrepancy, this author reviews the historical methods for diagnosis as well as an objective method for dynamic assessment. He also addresses the clinical significance of limb asymmetry and how computer-assisted gait analysis plays a key role in assessing treatment.

Human gait requires the controlled loss and regaining of the center of gravity as it shifts from one base of support to another. For efficient locomotion, a symmetrical, well-aligned musculoskeletal system is necessary. With symmetrical function and good alignment, there is decreased energy expenditure and increased muscular efficiency, resulting in decreased stress and fatigue.

   Unfortunately, most of us are not structurally or functionally symmetrical. A diversity of symptoms may occur as a result of limb length discrepancies, many of which proper treatment can alleviate.

   Limb length discrepancy (LLD) is a very common occurrence in the pediatric and adult population. In fact, the majority of individuals have some degree of limb inequality either in structure or function. The average discrepancy is less than 1.1 cm and usually patients easily compensate for this.1 In effect, these individuals lengthen or shorten their lower extremities to minimize their asymmetry.

   However, even seemingly insignificant discrepancies may become symptomatic in stress situations such as running. Larger discrepancies pose more of a problem and require more complex solutions. In any event, the underlying question is whether the LLD and its attendant compensatory pathomechanics are creating progressive pathologic alterations in structure and function.

   A survey of 376 patients conducted at the Growth Study Center of Children’s Hospital in Boston found that 95.5 percent of those tested have significant limb length discrepancies.2 Pearson’s classic 1951 study of 830 schoolchildren found that 93 percent had some lateral hip asymmetry.3 In the Children’s Hospital study as well as Klein and Buckley’s earlier study, discrepancies increased with age until full maturity.2,4 In the adult population, LLD is reportedly as high as 90 percent.5,6 Due to the widespread occurrence of LLD, physicians often consider this a “normal” finding although one should never confuse commonality with normalcy. “Normal” implies ideal and limb asymmetry, no matter how commonly it occurs, is never the ideal alignment for optimum musculoskeletal system function.7

A Review Of LLD Types And Etiology

There are three types of LLD: structural, functional and environmental.

   The structural or anatomic type is due to a difference in the actual length of the tibia or femur. This may be of congenital, post-trauma or post-surgery etiology as LLD commonly occurs following hip or knee replacement.

   The functional type is due to asymmetrical foot or limb function, which may have occurred from a variety of asymmetrical musculoskeletal findings. These include hip flexion or adduction contractures, flexion or hyperextension deformities of the knee or ankle, pelvic obliquity, genu varum, genu valgum, musculoskeletal injuries, asymmetrical pronation or supination, etc. Differentiation of these two types is not always straightforward since it is not unusual to have both occur together.

   The third type of LLD is referred to as environmental and is caused by the unevenness created by walking or running on crowned road surfaces, banked running tracks or along the beach. Excessive asymmetrical shoe wear may also create an environmental LLD. This third type may exist independently or be an additional pathologic influence into an existing functional and/or structural LLD.

Current Insights On Identifying LLD

The clinical identification of limb length discrepancy is often unreliable and confusing. There is poor to moderate correlation of direct method tape measurements to radiographic techniques in assessing LLDs.8,9 However, tape measurements, when taking the average of several measurements with steel tape, provide dependable clinical data.10

   Measure actual limb length from the anterior superior iliac spine to the distal aspect of the medial malleolus with the patient supine. Mark the malleolus point with a felt pen so the reference point is the same in each measurement. Measure apparent limb length from the umbilicus to the medial malleolus. Neither method takes into account asymmetrical functional or structural differences that take place distal to the malleolus (i.e. in the ankle mortise, subtalar joint and the calcaneus in relationship to its height, pitch and fat pad thickness). Combining several methods of clinical assessment may be necessary to accurately determine if a discrepancy is present.

   I learned a useful method to clinically assess weightbearing limb asymmetry from the late Richard O. Schuster, DPM, who routinely performed this method as part of his biomechanical examination. With the patient standing against a wall facing the examiner in subtalar neutral position, place a T-square along the superior brim of the right and then the left pelvis, and draw a mark on the wall for each side. Ask the patient to step away from the wall and measure and record the relative difference. Repeat this same procedure with the subtalar joint in a relaxed position. If there is a difference with the subtalar joint in relaxed position and none with the subtalar joint in neutral position, then there is a functional discrepancy. If there is a difference with the subtalar joint relaxed and in neutral position, then it is probably structural in nature.

   Palpation of the anterior or posterior superior iliac spine or the superior brim of the pelvis with the patient weightbearing may detect LLDs that are as small as 6 mm.1 Bailey and Beckwith supported the iliac crest drop reliability, reporting that 88 percent of their subjects had a short limb and ipsilateral iliac crest drop.12 Measurement of the relaxed calcaneal stance position should be another method of LLD assessment since differences of 3 degrees or more result in functional asymmetry.13

   In any event, measurement or clinical observation of pelvic level does not indicate the location of the discrepancy. Although clinicians primarily employ the Allis sign to detect developmental dysplasia of the hip in newborns and infants, it may help assess relative lengths of the femur and tibia in older individuals. This procedure, also referred to as Galeazzi’s sign, occurs with the patient supine and the knees flexed. In this position, one can easily see the relative height of the tibia (as represented by knee height) and femoral length, which is represented by one knee segment being anterior to the other. Asymmetric thigh folds and buttock creases are also helpful to examine, especially in younger children.

   Another method to determine if below-knee discrepancies are present is to have the patient seated with the foot and leg at a right angle and place a carpenter’s level across the knees. The side that the level dips to is the shorter below-knee extremity. Perform this procedure with the subtalar joint relaxed and then in neutral position. One can readily ascertain both structural and functional below-knee discrepancies in this manner.

   Some practitioners prefer radiographic assessment with the most accurate method being the scanogram in which a series of views examines the central ray aimed at the femoral head, tibial plateau and ankle mortise respectively.14 An alternative is to direct the central ray parallel to the femoral heads. No matter the chosen method, the subtalar joint must be neutral to prevent asymmetrical pronation from being considered a structural discrepancy. Additionally, one should evaluate the femoral neck angles since differences from left to right will result in limb asymmetry.

   Indirect assessment of limb length occurs by placing boards of a specific thickness under the foot until the pelvis is level. Please note that none of these methods assesses the pathomechanical impact of the LLD during ambulation or whether the individual is functioning asymmetrically.

Understanding The Pathomechanics

Whatever the cause of the discrepancy, there is an asynchronous placement of the foot during the gait cycle, resulting in pelvic position asymmetry. The body may compensate in a number of ways and the problems that arise are usually the sequelae of these compensatory mechanisms.

   Asymmetrical gait patterns are obvious in those with discrepancies greater than 2 cm.7 These individuals may exhibit plantarflexion at the ankle with secondary Achilles contracture on the shorter limb.15 When it comes to discrepancies from 1 to 1.5 cm, even though the shorter limb carries a greater risk of knee osteoarthritis, it is the longer limb that is subject to greater ground reactive forces and isometric torque.16 A study of military recruits revealed the occurrence of stress fractures to be 73 percent in the longer limb, 16 percent in the shorter limb and 11 percent in limbs of equal length.17 The hip on the longer side is also subject to increased stresses and resulting degenerative pathology. In a study of 100 patients presenting for hip replacement, 84 percent had osteoarthritis on the longer limb.18

   The foot on the longer side is relatively pronated to functionally decrease the length of the limb.19 This is evident in increased midstance, stance and single support phases of gait.15,19 The foot on the shorter limb is relatively supinated with increased lateral calcaneal pressure in an attempt to extend its length during the contact and adaptive phases of gait.15,19 Accompanying this longer limb pronation is anterior migration of the talus, forward displacement of the tibia and concomitant knee flexion.20,21 Additionally, on the longer limb, there is increased medial calcaneal pressure.19 If there is inadequate range of motion in the subtalar and midtarsal joints, then compensation occurs more proximally.15 At the genicular level, there may be compensatory flexion or hyperextension in the sagittal plane, and genu valgum or varum in the frontal plane in an attempt to shorten the longer extremity. In the frontal plane at the pelvis, there is a lowering to the shorter limb with secondary lumbosacral scoliotic compensation.1,11,14,15,20,21

   If the scoliosis is present with the spine flexed and extended, it indicates a structural deformity and any limb discrepancy would be secondary. If the scoliosis is present only when the patient is standing (spine extended) and disappears when seated (flexed), then it is a functional scoliosis secondary to the primary LLD.1,14 With the pelvis dropped toward the shorter limb, there is a compensatory tightening of the iliopsoas muscle contralateral to the scoliotic convexity in an attempt to counteract lateral spinal deviation.1 This process flexes the hip, consequently flexing the knee, internally rotating the femur and increasing subtalar pronation, all effectively shortening the longer limb.

   As a result of its anatomic configuration, the iliopsoas is incapable of effectively compensating for the lumbosacral scoliosis and may contribute to the formation of a proximal compensatory contralateral scoliotic curve. Continued spasm and adaptive iliopsoas shortening maintains and progressively magnifies this entire cycle of sequential internal derangement.

   Additionally, the femoral head on the longer limb is being wedged into the acetabulum, thereby forcing the anterior pelvis posteriorly and proximally, further complicating this situation.22,23 A typical compensatory mechanism for a short right limb, whether it is a structural or functional LLD, would be right pelvic drop, lumbar and cervical scoliosis with iliopsoas contracture, left shoulder drop and head tilt. Fingertips would be lower on the longer side. Variations on this theme can and do occur, and depend on the individual compensatory mechanisms that are available.

   Halliday and Karol examined the results of a three-dimensional kinematic and kinetic analysis along with Cybex strength testing of 35 children over 7 years of age with congenital or traumatic LLDs ranging from 0.6 cm to 11.1 cm.24 When it came to milder discrepancies, they found that the hip and knee of the longer limb was flexed to equalize limb lengths and level the pelvis and trunk.24 Some children decreased their normal hip and knee flexion, and vaulted over the longer limb while others laterally swung the longer limb during swing phase. Those with more severe discrepancies demonstrated equinus function on the shorter limb.

   Each child walked in a different way as the concept of mechanical efficiency during gait suggests, traveling a certain distance using the least amount of energy possible.24 With equal leg lengths, there is minimal vertical translation of the center of mass, which is 4 percent of the height of the individual. With LLDs in which no compensatory mechanisms are in place, large vertical translations in the vertical center of mass would be visible with secondary gait inefficiency and fatigue. Researchers observed inefficient gait patterns in nine of the 35 children tested. This group was comprised of toe walkers and those in whom the limb discrepancy was 5.5 percent or more.

   According to Subotnick, angle of gait variations in running result from limb length discrepancies.25 Short limb findings include external rotation for increased stability, posteroinferior pelvis imbalance and overstriding. The foot on the short limb is subject to a greater force for a shorter period of time in comparison with the contralateral extremity. Overstriding produces posterior central heel wear due to heel contact in front of the center of gravity, increasing the need for “braking” to avoid foot slap, thereby increasing anterior tibial activity with possible anterior tibial stress syndrome.25

   As a one-limb single plane deformity, limb length discrepancy is capable of producing a number of compensatory changes that are ineffective in correcting the original deformity and may magnify the problem throughout all body planes. The nature of compensation is individually unique and determined by age, site, severity, flexibility, body type, weight and other variables.

A Closer Look At The Clinical Significance Of Limb Asymmetry

Since limb length asymmetry is such a common situation, we often regard it as normal. However, nothing could be further from the truth.

   The musculoskeletal system is built to function most efficiently when neuromotor control is intact and there is adequate but not excessive joint ranges of motion in an aligned, symmetrical lower extremity complex. Since the foot represents the foundation for this complex, it is critical that it be properly aligned and function in the appropriate symmetrical, adaptive or stabilizing manner at each stage of the gait cycle.14,26,27 According to Von Baeyer’s 1898 “Closed Chain of Links,” postulate defects or deficiencies in one segment affect the entire chain.20

   Perhaps the most obvious effects of LLD are on posture. Limbs with length discrepancies are obviously less efficient during gait but there are increased demands on postural musculature in an imbalanced musculoskeletal system during stance as well.1

   Limb length discrepancy is reportedly the third most common cause for running injuries, most often occurring below the hip and appearing first on the longer side.28,29 Researchers have implicated asymmetrical pronation producing a functional limb length discrepancy as a causative factor in sciatica.30 As I previously noted, the longer limb is subject to osteoarthritis of the hip and increased risk of stress fractures while the shorter limb has a higher incidence of knee pain and degenerative joint disease.

Essential Insights On Symptomatology

The most common symptoms associated with limb length discrepancy are low back pain, which usually appears on the longer side first.31-33 Other complications include sacroiliac malalignment, functional scoliosis, sciatica, disc herniation, posterior vertebral facet impingement, hip and knee pain as well as degenerative joint disease, especially on the longer side. Additional findings include: piriformis and iliotibial band syndromes; greater trochanteric bursitis; plantar fasciitis; posterior tibial stress syndrome; patellofemoral pain syndrome; hallux valgus; altered gait patterns; pelvic obliquity toward the shorter limb; contracture of the Achilles tendon on the short limb; and short limb metatarsalgia.1,17,21,34-38

   In the sports participant, chronic unilateral overuse injuries that persist despite appropriate care sometimes diffuse and clinicians sometimes may falsely attribute them to LLD. Even “minor” discrepancies in runners may produce symptomatology due to the “rule of three” theory, which states that running forces average three times body weight — three times that required for normal walking — for which the body will compensate at the weakest link in the musculoskeletal chain.21

Who To Treat

The orthopedic literature is confusing regarding the amount of discrepancy adults can tolerate without treatment. Treatment often depends on whether symptoms are present and the degree of musculoskeletal malalignment present. However, an incidental finding of an LLD in an athlete might well be worth equalizing in order to improve symmetry, efficiency and performance as well as to prevent injury. Subotnick states that one should treat discrepancies of as little as 6 mm in an athlete.25

   Other authors feel that the commonly occurring 1 to 1.5 cm discrepancies do not lead to symptoms and may not need treatment.5 However, discrepancies as small as 1 cm have been associated with back pain and plantar fasciitis.32,39 Harvey and associates, in a study of 3,026 subjects with radiographically confirmed LLD, determined that those with 1 cm discrepancies were more likely to develop knee pain and osteoarthritis on the shorter limb.40 In those with 2 cm or greater discrepancies, pain and osteoarthritis were present bilaterally. Additionally, this study points out that LLDs as small as 5 mm may be associated with increased odds of prevalent symptomatic knee osteoarthritis. It is because of this and the documented prevalence of osteoarthritis in the hip on the longer limb that some authors contend that any LLD should receive treatment.40,41

   As I mentioned earlier, even minor discrepancies may result in major problems when the musculoskeletal system is placed in stress situations. A basic tenet in a biomechanics-based medical practice is that excessive loads, whether they are brief, high impact, loading or cumulative stresses, have the potential to cause structural damage to all facets of the musculoskeletal system. In any case, the ideal situation for all individuals is for both limbs to be of equal length and function symmetrically. Specifically, there should be normal application of pressure for the normal duration of time applied equally through each limb at each phase of the walking or running cycles.

What You Should Know About Functional Symmetry

A level pelvis does not ensure symmetrical function. Equal limb length does not ensure functional symmetry. An individual with a structural LLD may function symmetrically while some individuals with equal length of the extremities may function asymmetrically. Computer-assisted gait analysis may help determine this with in-shoe pressure transducers capable of measuring time, pressure and direction of force.19,42

   It is one thing to measure limb length either clinically or radiographically, and couple the findings with symptomatology and observational gait analysis. However, it is another story to be able to identify and assess symmetry where it matters most: in action. Life is movement and the locomotor system is designed just for that purpose (i.e. to transport the individual to a desired destination in the mot efficient fashion utilizing the least expenditure of energy). Symmetrical function lowers energy requirements and improves efficiency. With the population today living longer, more active lifestyles, it is imperative for patients to maintain the ability to walk without pain. This starts with optimum symmetrical lower extremity alignment and function.

How To Assess And Obtain Functional Symmetry

The first step in assessing and obtaining functional symmetry is to negate all untoward musculoskeletal influences and deficiencies. This includes identification and neutralization of all abnormal pronatory influences and malalignment. This may entail any or all of the following interventions: prescription of foot orthoses; identification and stretching of posterior group contractures (equinus influences); improving restricted ranges of motion, especially in the hip, knee, ankle and subtalar joints; strengthening of weak musculature; strengthening of joint stabilizers in individuals with ligamentous laxity; weight reduction (when appropriate); evaluation and remediation of inappropriate footwear, etc.

   After addressing these factors, one can assess symmetry functionally. In my early years in practice, I would equalize limb length based on clinical and radiographic measurements as well as sacral leveling. Computer-assisted gait analysis taught me that leveling the pelvis or equalizing limb length may in fact create pedal and limb imbalances during gait. This manifests via asymmetrical plantar pressures as well as temporal parameter disturbances.

   The goal of dynamic gait assessment via computer-assisted gait analysis is to identify and reduce torque and stress so the right and left pedal segments spend the same amount of time on the ground, generate the same normal pressure and move with the same speed. It is non-invasive, relevant, repeatable and reliable. It is capable of detecting locomotor events that one cannot observe with the naked eye or time with a stopwatch as well as forces too small or rapid to detect. Gait analysis quantifies and records weight distribution patterns and temporal parameters in a realistic environment (i.e. inside the shoe).

   Perform an initial computer-assisted gait analysis prior to orthotic prescription in order to gauge pre-treatment symmetry or asymmetry, weight distribution patterns, COF and phases of gait. When it comes to assessing symmetry, useful parameters include: stance, single support, propulsion, midstance, heel duration, heel pressure, time speed, center of pressure patterns, etc. Functional limb symmetry or asymmetry following the use of prescription foot orthoses is not predictable with certainty. Accordingly, one should only address this after orthotic realignment.

   After dispensing the orthotic devices and after a period of patient familiarization with their use, usually two weeks, perform a follow-up, computer-assisted gait analysis to assess symmetry and evaluate weight redistribution patterns. More often than not, individuals with functional discrepancies revealed by initial testing will exhibit functional symmetry after the orthotic intervention.

   In some cases, patients who did not exhibit functional symmetry initially now function asymmetrically and require a lift. This may be a result of treatment now rendering the musculoskeletal system unable to compensate pathologically for the discrepancy, in effect rendering it uncompensated. If symmetry is unachievable, this warrants osteopathic, physiatrist, physical therapist or chiropractic referral for further evaluation.

A Guide To The Methodology Of Assessing LLD

In a computer-assisted gait analysis study of 17 individuals with an identified limb length discrepancy and unilateral musculoskeletal symptomatology, the addition of a ¼-inch heel raise to the shorter limb resulted in 50 to 100 percent symptom improvement in all cases.19 Additionally, the average cadence of 48.2 steps per minute on the longer side and 52.3 steps per minute on the shorter side improved to 44.0 steps/minute on the longer limb and 45.0 steps per minute on the shorter limb.

   Customarily, one may place lifts of up to 5/8 inches inside the shoe. Discrepancies greater than ¾ inches will require a tapered extension to the forefoot of approximately half the amount of heel lift required. One may also reduce the heel or remove the shoe insole on the longer limb to achieve symmetry. One may address LLDs up to 7/8 inches in this manner without having to conspicuously alter the short limb shoe.

   To obtain functional symmetry, most individuals respond well to 1/8-inch, ¼-inch or 3/8-inch lifts. It is less common for ½-inch lifts or greater to be necessary to achieve symmetry. In those individuals who do not seem to be able to achieve functional symmetry, even with increasing lift heights, reassessment of sacroiliac and lumbosacral mobility is warranted.

   No matter what size lift is appropriate, begin with either a 1/8-inch or ¼-inch lift, and gradually increase the amount in 1/8-inch increments every few weeks. This is to ensure that adaptive contractures in the hip and low back slowly return to normal function. Physicians may adjunctively employ gentle stretching of this musculature.14

   Lifts are not forever. What is appropriate at the onset of treatment may not be what is required six months later. Periodic evaluation is necessary. Once the patient has achieved symmetry, perform a computer-assisted gait analysis in two to three months to reassess. It has been my observation that functional LLDs in most individuals tend to reduce over time with treatment. This may be due to assimilation of the lift action into the musculoskeletal functional framework, thereby in some cases obviating the need for a lift at all. If the need for the lift is still apparent at the time of the first follow-up computer-assisted gait analysis, reassess it in six months and periodically thereafter.

   The bottom line is when a patient states, “Fifteen years ago, my doctor told me my right leg is shorter so I have a lift built into all my shoes,” that lift is not doing the same thing now as it did at the initial prescription. Many times, I have found it is either too much, too little, not needed or on the wrong side.

In Summary

Limb length discrepancy is a common musculoskeletal deficiency with widespread untoward effects. Historic methods to statically assess its presence do not address or take into consideration its dynamic requirements. Computer-assisted gait analysis is a non-invasive, objective, relevant, reliable clinical method to identify, quantify and manage functional asymmetry.

   Dr. D’Amico is a Professor and Past Chairman in the Division of Orthopedics at the New York College of Podiatric Medicine. He is a Diplomate of the American Board of Podiatric Medicine, a Fellow of the American College of Foot and Ankle Orthopedics, and a Fellow of the American Academy of Podiatric Sports Medicine. Dr. D’Amico is in private practice in New York City.

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