Laboratory Studies
Aside from radiographic images, no studies are necessary unless concern exists regarding the diagnosis, in which case other studies (eg, a metabolic study to assess for rickets) may be ordered.
Imaging Studies
Radiography
The initial evaluation should include an anteroposterior (AP) radiograph of the lower extremity with a ruler to measure limb length and to assess the deformity. Hip dysplasia, though not normally seen in patients with this disorder, can be assessed at age 6-12 weeks. A lateral radiograph of both tibiae should be obtained. If the foot fails to correct by age 4-6 weeks, radiographs of the foot should be obtained, including AP and lateral views of the foot and a plantarflexion lateral view of the foot to assess for a vertical talus. (See the images below.)
Anteroposterior radiograph of a 1-year-old child with posteromedial tibial bowing.
Anteroposterior and lateral radiograph of a 9-year-old child with posteromedial tibial bowing. Note that the bowing has significantly improved.
Limb-length inequality at skeletal maturity must be assessed before any type of limb equalization procedure (epiphysiodesis, lengthening, or shortening) is performed. [22, 23] Typically, an epiphysiodesis is required at about age 11 years in females and age 13 years in males, but this varies, depending on the patient's skeletal age and the degree of the limb-length inequality. Epiphysiodesis can be indicated as early as age 8 years. As mentioned previously, if the degree of limb inequality is large (>5 cm) and the patient is not expected to be tall, a lengthening may be considered.
Limb-length inequality at skeletal maturity is most reliably predicted from a series of at least three radiographs taken at least 6 months apart. Various radiographic measures have been used to determine limb-length inequality, including the following:
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The teleoroentgenogram is a single-exposure AP radiograph of the lower extremity with a ruler; this study is subject to a magnification error of 5-10% at the outer border of the film but has the advantage of showing coronal (angular) deformities and is not subject to movement errors
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Orthoradiography incorporates three separate exposures (hip, knee, and ankle) in an effort to avoid magnification errors [24]
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Scanography (see the image below) uses a similar technique, but exposure size is reduced, and all three exposures are on a single film cassette
Scanogram of a patient with posteromedial tibial bowing and a limb-length inequality.
Both orthoradiography and scanography are subject to movement errors, and angular deformities cannot be assessed.
All of the techniques are inaccurate if the patient has knee or hip flexion contractures or if the patient is simply flexing the knee or hip asymmetrically at the time of exposure. If the knee appears as a tunnel view, there is undoubtedly a significant degree of knee flexion. Lateral radiographs or separate (prone) radiographs of the femur and tibia with a radiopaque ruler can be obtained to assess limb length in patients with knee flexion contractures.
Other modalities
The use of computed tomography (CT) to assess limb length has increased. CT uses less radiation and is more accurate than conventional radiographic techniques in patients with knee or hip flexion contractures. [25] EOS three-dimensional imaging has the ability to decrease the amount of radiation and provide data in both the AP and lateral planes. Ultrasonography (US) is used as well, primarily as a screening tool. [26]
Procedures
Once the current limb-length inequality has been measured, a prediction of the ultimate limb-length inequality at skeletal maturity is needed to determine treatment. [27, 28] This has generally been accomplished by using one of the following three methods [29] :
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Arithmetic method
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Growth-remaining curve
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Moseley straight-line graph
The simplest of these is the arithmetic method. This method assumes that the growth rate of the distal femur is 1 cm/y, that the growth rate of the proximal tibia is 0.6 cm/y, and that boys reach skeletal maturity at age 16 years and girls at age 14 years.
The growth-remaining curve relates chronologic age to limb length to determine a child's growth percentile. By using this, the remaining growth of the tibia or femur can be determined graphically.
The advantage of the Moseley straight-line graph, which combines information from both the arithmetic method and the growth-remaining curve, is that several measurements (preferably at least three, separated by 6 months) can be plotted on a single graph. The Moseley straight-line graph relies on determination of the bone age as estimated from a left hand/wrist film.
When these three techniques were evaluated, their accuracy rates showed little significant difference.
A fourth method, known as the multiplier method, uses an arithmetic formula to determine limb inequality at maturity, [30, 31, 32] simply taking the current limb-length inequality and multiplying it by a constant listed in a table by chronologic age. Timing of the epiphysiodesis can then be estimated by use of an arithmetic formula to determine limb-inequality at maturity. This method is as precise as the other three methods for determining limb length at maturity and can accurately estimate the timing for epiphysiodesis.
Histologic Findings
The histology of posteromedial bowing is unknown, but animal studies performed to model angular deformities demonstrate increased trabecular bone formation in the area of the apex of the angular deformity, with no new cartilage cells, and subepiphyseal bone condensation with subsequent thinning of the epiphyseal plate.
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Anteroposterior radiograph of a 1-year-old child with posteromedial tibial bowing.
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Lateral radiograph of a 1-year-old child with posteromedial tibial bowing.
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Anteroposterior and lateral radiograph of a 9-year-old child with posteromedial tibial bowing. Note that the bowing has significantly improved.
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Posteromedial tibial bowing. The Galeazzi test. Note the difference in the height of the flexed knees.
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Scanogram of a patient with posteromedial tibial bowing and a limb-length inequality.
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A hydroxyapatite-coated Schanz pin used to secure external fixator devices to the bone.
