A modest leg length discrepancy (LLD) is common and may be considered a normal variant. A study of 600 military recruits found an LLD of 0.5 to 1.5 cm in 32% of recruits, and 4% had a difference of over 1.5 cm.1
While a few small studies report an increased incidence of back or hip problems with modest LLD, most large series do not report an increased incidence of long-term problems with LLD of less than 1 inch. However, the long-term effect of LLD is not certain. Nor is there convincing evidence to support the position that an LLD less than 1 inch is acceptable while LLD larger than 1 inch warrants treatment. Clearly a 2-cm discrepancy in a 6-foot 10-inch NBA player may not be as much of an issue as it would be in a 5-foot 2-inch jockey. Gait analysis has shown that children with LLD less than 3% do not use compensatory strategies.2
A 3% LLD in an average adult is about 1 inch. Conversely and in some cases, a small LLD may actually be beneficial; in patients with a stiff ankle or a drop foot from hemiplegia, a leg that is 1 to 2 cm shorter will accommodate an ankle foot orthosis and be able to clear the toe better in swing phase.
Even though small to moderate LLD may not lead to functional issues; sometimes patients and parents are more concerned about the cosmetic appearance of a short limb gait and seek treatment for this issue. To stay out of trouble, it is important to recognize that equaling out the leg length in a child with cerebral palsy will not relieve the spastic components of their gait. Similarly, limb lengthening will not improve the Trendelenburg sway from a child with coxa vara from severe Perthes disease.
It is important to make an accurate diagnosis of LLD so you can to predict the eventual discrepancy at maturity and also rule out serious underlying causes, such as neurofibromatosis, bone or soft tissue tumors, and neurologic disorders that may require additional medical treatment. The first pitfall to recognize is determining which leg is abnormal. Is one leg too short, or the other one too long? While making this distinction sounds simple, this point is not just academic. For example, recognizing that the long limb is abnormal in an infant with idiopathic hemihypertrophy can be critical. If the patient has a variant of Beckwith-Wiedemann syndrome, regular ultrasounds of the abdomen at 3- to 6-month intervals for the first 6 to 8 years of life to rule out associated Wilm or other abdominal tumors are necessary. In some instances, the orthopaedic surgeon may be the first person to make this diagnosis.
Looking at the proportions of the legs to the body can be helpful in determining which limb is abnormal. Hint: pathologically short limbs often have associated musculoskeletal abnormalities such as coxa vara, bowing, absence of an anterior cruciate ligament (ACL), lateral femoral hypoplasia, fibular hemimelia, tumorous conditions such as fibrous dysplasia, etc. In contrast, pathologically long limbs are often associated with cutaneous or vascular anomalies as seen in Proteus syndrome (Fig. 28-1
), Klippel-Trenaunay syndrome, neurofibromatosis, or Beckwith-Wiedemann syndrome.
On physical examination, there are numerous pitfalls in the assessment of LLD. ORTHOPAEDICS 101: Any joint contracture will affect the functional discrepancy of the limb in gait even if there is no anatomic difference in the length of the bones.
An adducted hip makes the leg appear shorter, while an abducted hip makes the leg appear longer. In fact, an apparent discrepancy of 3 cm is created for each 10° of hip adduction/abduction.3
Similarly, a knee flexion contracture will functionally shorten the leg while an equinus contracture will make it functionally longer. Because of this, simply looking at the position of the feet of a supine patient is not accurate (Fig. 28-2
). Measuring leg lengths from the anterior superior iliac spine to the medial malleolus is subject to extreme variability and will not account for any difference in foot height that can be seen in children with congenital limb deficiency or overcorrected clubfoot (Fig. 28-3
). A good way to measure LLD on physical examination is to have the patient stand up, with knees straight, and place the examiner’s fingers on the iliac crest. Placing blocks of various sizes under the short leg until the pelvis is level provides a reasonable estimation of the LLD. This test is less accurate in the circumferentially challenged (obese) child, as the pelvis is more difficult to palpate. Beware that previous pelvic surgery, such as a Salter osteotomy in which part of the iliac crest has been removed, may make this test inaccurate. Another pitfall in evaluating for LLD by the standing method is that a chronically short leg often develops an equinus contracture. Make certain both feet are flat on the floor. Similarly children with significant LLDs will automatically bend the knee of the longer leg to level the pelvis—this is another pitfall.
Figure 28-1 A 13-year-old Honduran boy with likely Proteus syndrome showing asymmetric enlargement of his right thigh and left leg and variable macrodactyly. (Used with the permission of the University of Wisconsin Division of Pediatric Orthopaedics.)
Figure 28-2 Pelvic obliquity can cause an apparent leg length discrepancy; patient supine on examining table.
Figure 28-3 This 14-year-old boy presented with a 4-cm clinical leg length discrepancy (LLD). The scanogram revealed 2 cm at the tibial level. The remaining 2 cm is due to the loss in foot height as a result of pantalar release for clubfoot. Key point: Scanograms don’t identify all levels of LLD. (Used with the permission of the University of Wisconsin Division of Pediatric Orthopaedics.)
A good general musculoskeletal examination of the child is also critical. In children for whom lengthening may be considered, pay particular attention during the physical examination to evaluate joint stability of the hips, knees (is the ACL missing?), patella and ankles (valgus/varus). An unstable joint is best discovered and corrected before it dislocates during lengthening. Does the femur have rotational asymmetry that could be derotated after osteotomy? Are there scars or soft tissue contractures that would limit bone lengthening? Does the limb have normal vascular flow or is there altered vascular status? What is the neurologic function? Does the child have a drop foot that would be best accommodated with the limb being slightly shorter?
Plain radiography is the most common method used to quantitate LLD; yet it is still subject to a variety of errors, such as patient positioning and unrecognized joint contractures. A standing teleoroentgenogram (AP radiograph of the lower extremities including the hips, knees, and ankles on one film with the patella facing forward) has value as the initial screening examination for all patients with LLD. This study provides information about the entire lower extremity length from the pelvis to the bottom of the foot and allows each limb to undergo deformity analysis. This study can also detect any lesions within the bones of the lower extremities. Unfortunately, the X-ray beam of the teleoroentgenogram is at an angle to the hip and ankle and any subtle hip dysplasia or ankle abnormalities may be missed; the parallax can also introduce some magnification error. However, because the effect is symmetric to both limbs, a 10% magnification error does not likely effect clinical decision-making. It is a great tool for following small children that won’t stay still for the multiple exposures required during a scanogram (which is preferable to follow the LLD in children who can lie still).
Scanograms can give the clinician very accurate data on the length of the femur and tibia provided there are no hip or knee contractures. If contractures are present, computed tomography (CT) scanograms more accurately assess limb lengths in these children and can also quantitate differences in rotational profile. Because scanograms center on the joints, it also allows one to detect hip dysplasia or other joint abnormalities. One pitfall of the scanogram is that the foot is not included, so a foot deformity that contributes to the overall LLD will not be recognized by the scanogram (Fig. 28-4
). In these patients, standing lateral foot radiographs
are helpful to quantitate the deformity. A second pitfall of the scanogram that is used as an initial screening tool is that the diaphysis of the femur and tibia are not imaged, so a midshaft lesion, deformity, or other cause of the LLD may be missed.
Figure 28-4 This patient with fibular hemimelia has a loss of foot height (A) in comparison to the contralateral foot (B). In my experience, one can see up to 2 cm of leg length discrepancy in foot deformities, which has to be accounted for in treatment strategies. (Used with the permission of the University of Wisconsin Division of Pediatric Orthopaedics.)
TABLE 28-1 Useful Chart for Reading a Scanogram
Length of Femur
Length of Tibia
Length of Tibia + Femur
2.2 = LLD
When reading a scanogram, making a chart such as this helps to avoid errors, as the total leg length discrepancy (LLD) difference is calculated two ways. One is by subtracting the total leg lengths in the last column, and the other is by verifying the calculation by adding the differences of the femoral and tibial limb length in the last row.
When interpreting a scanogram, chart the lengths and differences of the femur and tibia individually (Table 28-1
), then confirm that the lengths and differences give the same total LLD for the entire limb independently. This can be written directly on the scanogram (Fig. 28-5
) or dictated in the chart, so the accuracy may be later verified. While this sounds elementary, most errors in LLD surgery are from miscalculations. Two studies looking at epiphysiodesis found high rates of failure due to errors in timing surgery. One review of 57 patients4
of patients had a final LLD of more than 1.5 cm. Another review of 67 patients5
found that 51% had a final LLD greater than 1.0 cm.
Figure 28-5 A quick way to double-check your calculated leg length discrepancy (LLD) from the scanogram: Measure the length from the top of the femur to the bottom of the tibia of both legs with a tape measure. The difference of these measurements should equal the formally calculated LLD.
Recently, low-dose standing biplanar radiographic imaging systems have been used for assessment of spinal deformity. Known as EOS, this technology has also been used to assess lower limb alignment and limb length discrepancy. It is expensive and is currently not available in most hospitals, but studies suggest that they have lower radiation exposure with greater accuracy then CT scanograms and standard radiographic measures. Another advantage is the ability to quantify limb alignment and deformity at the same time measurements of length are determined.
It is important to recognize that growth is not static, and thus, discrepancies in length in growing children can change over time. The family and the clinician should consider what the LLD will be at skeletal maturity, so having a few rules of growth is helpful in predicting what the ultimate LLD will be. For instance, a rule of thumb for prediction of a child’s final height is summation of parental height/2 +6.5 cm for boys and −6.5 cm for girls. A rough estimate of the growth of the physes of the lower extremity is proximal femur 4 mm/y, distal femur 10 mm/y, proximal tibia 6 mm/y, and the distal tibia 5 mm/y (Fig. 28-6
Figure 28-6 Estimate of growth of the physes of the femur and tibia: proximal femur 4 mm/y, distal femur 10 mm/y, proximal tibia 6 mm/y, and distal tibia 5 mm/y.
Lower extremity skeletal growth generally ends at age 14 years in girls and age 16 years in boys. Keep in mind that the age of growth cessation is also dependant on medical comorbidities. For example, patients with skeletal dysplasia, metabolic and nutritional disorders, and history of malignancy and chronic chemotherapeutic treatment may not follow the normal growth patterns. Also, beware that underlying disorders may affect maturation, for example, one third of children with spina bifida may have precocious puberty and that children with Blount disease often have an advanced skeletal age.
One of the biggest challenges in pediatric orthopaedics is to assess a child’s growth remaining. Qualitatively the experienced pediatric orthopaedist can gain some general insight by the radiologic appearance of physis, comparison of child’s height to the parents and their growth patterns (e.g., dad grew 4 inches in college), and comparison to siblings. Quantitatively we can use the patient’s chronologic age or the skeletal age using the Greulich-Pyle atlas. Bone age using the Greulich-Pyle method is subject to significant variability. In one study, 60 hand radiographs
were read by four radiologists, and 50% of the children were assigned a skeletal age that differed by more than 1 year between radiologists; 10% varied by more than 2 years.6
As skeletal age is subject to inaccuracies, one should consider comparing the skeletal with the chronologic age. When the chronologic age is the same as the skeletal age, they may give the most accurate picture and one can be reasonably confident in the skeletal age reading. NEWSFLASH! When the chronologic age and the bone age differ by more than 2 years, one should take pause in planning different limb equalization procedures like permanent epiphysiodesis that depend on years of growth remaining.
PREDICTING LEG LENGTH DISCREPANCY
At the risk of being too simple, remember the LLD we are treating is the LLD predicted at maturity, not the current LLD. The three traditional methods of predicting LLD are the arithmetic method, the growth-remaining method, and the straight-line method. These three all use data from one database by Anderson and Green. A potential pitfall in using only the growth-remaining method is that tall children obtain more correction than short children after an epiphysiodesis, because they will have more growth. Of these three methods, the Moseley straight-line method has several advantages to help minimize errors, such as using multiple measurements at different times and considering growth percentiles.7
While the Moseley straight-line method has been the gold standard, multiple measurements should be obtained to use it, and there are many potential sources of error. Remember to use the appropriate gender-specific Moseley growth chart. The multiplier method is an alternative method of assessing growth remaining. The multiplier method characterizes the pattern of normal human growth by using an age- and gender-specific coefficient. The multiplier for each age and gender is a measure of the percentage of growth remaining. It is, therefore, viewed as a universal method for predicting lower limb length because it is independent of percentile, regional, racial, ethnic, and generational differences in growth data. An advantage of the multiplier method is that it allows prediction of LLD with one measurement of limb lengths, without the need for bone age or graphing and is quite simple for predicting LLD in congenital discrepancies.8
Beware that one-time events can cause a bone to shorten by slowing or shutting down the growth of a bone, and thus, the above growth predication methods are unreliable. This is particularly important to remember in patients with a history of neonatal sepsis and multiple levels of old osteoarticular infections (Fig. 28-7
). In these patients, the growth plates may grow slower than the contralateral side and even shut down years before the other growth plate closes. Growth plates can also grow faster from juxtaphyseal inflammation (chronic juvenile inflammatory arthritis) infection, tumor, or traumatic events, and the degree and duration of growth stimulation is very unpredictable. Also remember that growth plates can increase growth in the face of acute malunion such as a femur fracture healing 3 cm short (Fig. 28-8
). To stay out of trouble, it’s important to remember that ultimate LLD in acquired cases is really only predictable in cases of complete physeal arrest. Because growth is constant in congenital limb deficiencies, one can use the Moseley growth chart and the multiplier method to predict LLD with good reliability. NEWSFLASH! The rate of growth in congenital conditions can be affected by treatment. Femoral lengthenings may increase growth rate of the distal femur while tibial lengthenings may slow the growth rate of the proximal tibia.9
Figure 28-7 This 12-year-old boy had a history of neonatal sepsis and likely multifocal joint infections. He had complete growth arrests in his left ankle (white arrow) and growth retardation at his right knee (red arrows). All of these problems led to a significant challenge in planning for the future as there was no way to predict final anatomic leg length discrepancy. Complicating matters further, his left hip never developed properly (yellow arrow) and his hip instability and Trendelenburg gait resulted in functional discrepancy during gait as well. (Used with the permission of the University of Wisconsin Division of Pediatric Orthopaedics.)