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Blount Disease: Imaging

Author: Jugesh Cheema, MD, Consulting Staff, Department of Radiology, Brigham and Women's Hospital
Coauthor(s): H Theodore Harcke, MD, Chief of Imaging Research, Department of Medical Imaging, Alfred I DuPont Hospital for Children; Professor, Departments of Radiology and Pediatrics, Jefferson Medical College
Contributor Information and Disclosures

Updated: Nov 29, 2007

Radiography

Findings

Radiography is the primary modality used to diagnose tibia vara.

Radiographic findings primarily involve the posteromedial parts of the proximal tibial epiphysis, growth plate, and metaphysis. A standing anteroposterior radiograph of both legs is used to demonstrate bowing and abnormality at the medial aspect of the proximal tibia. In more advanced cases, bowing is seen at both ends of the tibia. On lateral knee radiographs, a posteriorly directed projection at the proximal tibial metaphyseal level is seen.

Different radiologic measurements have been used in an attempt to confirm the presence of Blount disease. The femoral-tibial angle helps confirm the varus position of the leg, but it can be misleading secondary to the rotation of the leg, which may be positional or due to a coexisting rotational abnormality.

The metaphyseal-diaphyseal angle has been suggested to provide more precise indications of Blount disease than the femoral-tibial angle (see Image 2). The metaphyseal-diaphyseal angle is obtained by measuring the angle formed between a line drawn parallel to the top of the proximal tibial metaphysis and another line drawn perpendicular to the long axis of the shaft of the tibia. Overlap may be found in measurements between patients with and without tibia vara. Angle measurements are 9º ± 3.9º in cases of physiologic bowing and 19º ± 5.7º in patients with Blount disease. Reportedly, angles greater than 20º confirm true tibia vara in children, whereas angles of 15-20º may or may not indicate tibia vara.

Another angle used is the tibial metaphyseal-metaphyseal angle. This angle is larger than the metaphyseal-diaphyseal angle in children with the most marked bowing and indicates distal tibial bowing in severe cases.

In 1952, Langenskiold first proposed a 6-stage classification of radiographic changes. This remains the most commonly used system (see Image 1).^6,7,8 This classification was not intended for use in determining the prognosis or the most desirable type of treatment, and the author cautioned against such use. However, the fact remains that surgical treatment commonly is needed for any child with stage 3-6 changes (see Image 6).

Degree of Confidence

In the most severe cases, the diagnosis can be made with a high degree of confidence in the presence of a tibial metaphyseal-diaphyseal angle measurement of 20 º or more. However, in less-severe cases, measurements may not be confirmatory, and differentiating tibia vara from physiologic bowing is difficult. In such patients, 6 months of follow-up observation is recommended (see Image 2).

False Positives/Negatives

Extreme physiologic bowing may cause false-positive results. Early or less-severe Blount disease may be misdiagnosed as physiologic bowing of the legs when measurements and medial tibial changes are not confirmatory. Some authors have suggested that children with a metaphyseal-diaphyseal angle greater than 11º eventually develop tibia vara, whereas those with measurements less than 11º have physiologic bowing. Other authors have found standard deviations of ± 2.6º and ± 4.6º. Still others have recommended 6 months of follow-up observation to better differentiate the 2 conditions.

Computed Tomography

Findings

Computed tomography (CT) has no defined role in the evaluation of Blount disease.

Magnetic Resonance Imaging

Findings

Although radiographic findings in Blount disease usually are diagnostic, MRI has the advantage of direct depiction of the epiphysis and the growth plate. How MRI can aid in evaluation and treatment of patients with Blount disease is debatable. MRI has a distinct advantage in a subset of patients with advanced or recurrent tibia vara. In these patients, MRI can demonstrate the extent of the physeal bar to quantify the percentage of physeal involvement. On a T2-weighted image, an open physis is bright and the physeal bar appears black. Early physeal fusion of the medial proximal tibial and, less frequently, medial distal femoral physis can occur from the injury of chronic weight bearing. This injury can lead to progressive genu varus from medial tethering of the growth plates. Removal of the physis medially may help restore normal growth.^9,10,11

An article about MRI changes in bowleg deformities of early infancy suggested a possible role for MRI in differentiating physiologic bowing from Blount disease.¹² Children who eventually had Blount disease were found to have a depression of the medial physis and abnormal signal intensity in the metaphysis in addition to the lesion in the epiphysis. In comparison, children with physiologic bowing were found to have high signal intensity only in the epiphyseal cartilage. However, most patients with combined changes did not develop Blount disease (see Image 5).

Degree of Confidence

MRI does not yet have a well-established role in the evaluation of Blount disease. MRI can be useful to the orthopedist who wishes to know which portion of the medial knee (epiphysis, physis, metaphysis) is injured and what corrective steps must be undertaken. MRI is also useful in the assessment of possible development of a physeal bar.

Ultrasonography

Findings

Ultrasonography has no known role in the evaluation of Blount disease.

Nuclear Imaging

Findings

Multiphase bone scintigraphy is sensitive in assessing normal and abnormal growth plate functions in the growing skeleton.¹³ Mechanical loading and stress factors influence scintigraphic uptake at the growth plate. When immobilization is prolonged and when closure begins, growth-plate activity decreases. In patients with angular deformities of the legs, the half of the growth plate with greater mechanical loading becomes more active than the other half. In patients with Blount disease, increased uptake occurs medially in the tibial plate, and scintigraphic changes may also be seen in the distal femur. Scintigraphy is not used for diagnosis, but it can be useful in making treatment decisions (see Image 4).

Angiography

Findings

Angiography has no role in the evaluation of Blount disease.

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