Definition and Epidemiology
Blount’s disease (BD) is the progressive bowing of the legs produced by abnormal ossification at the medial aspect of the tibial epiphysis.
- Early or infantile: debuts at 1–3 years of age
- Late; subdivided into:
- Juvenile form: presents at 4–10 years of age
- Adolescent form: occurs in children older than 10 years of age
- Increased incidence in people of African descent
- Early BD:
- Boys > girls
- Bilateral in 50% of cases
- Late BD:
- Less common and less severe than the early form
- More likely to be unilateral
- Prevalent in areas with higher rates of obesity and vitamin D deficiency
Etiology and Pathophysiology
The exact cause of BD is not known and is believed to be multifactorial or due to a combination of hereditary and developmental factors:
- Genetic susceptibility
- Mechanical overload of the joint (associated with obesity)
- Basis of the development of BD:
- Excessive force
- Altered endochondral ossification of the leg
- Mechanical forces (obesity and early ambulation) on the medial physis of the tibia → growth retardation
- Histological changes can be seen on the tibial growth plate:
- Cellular hypertrophy and fibrocartilage islands
- Signifies damaged cartilage
- Damaged cartilage delays bone growth on the medial side → varus deformity
- The procurvatum (backward bending of the tibia) is mainly due to the greatest inhibitory effect being on the posteromedial aspect of the tibial metaphysis.
- Patients are usually overweight or obese.
- 3-dimensional deformity:
- Varus (bowing)
- Procurvatum (backward bending of the tibia)
- Internal tibial rotation (resulting in in-toeing)
- Length discrepancy in lower limbs
- Non-tender bony protuberance at the medial aspect of the proximal tibia
|Early BD||Late BD|
|No pain||Pain in the tibial area|
|Significant deformity||Mild deformity|
|Associated with early ambulation||Associated with obesity|
The diagnosis of BD is achieved through medical history, physical examination, and confirmation via imaging studies.
- Long leg radiograph:
- Varus angulation and sloping at the tibial epiphysis with asymmetrical bowing
- Medial beaking: fragmentation of the medial epiphysis of the tibia
- Irregular ossification and widening at the medial epiphysis
- Magnetic resonance imaging (MRI): mostly used in cases of infantile Blount’s disease to assess extratibial tissues
Severity is based on the metaphyseal-diaphyseal angle (MDA):
- Measures the angle formed by a line going through the metaphyseal beaks and a perpendicular line going through the longitudinal axis of the tibia.
- MDA > 16 degrees is diagnostic of BD.
- MDA between 11 and 15 degrees is a probable diagnosis and warrants follow-up.
Management and Prognosis
The age of the patient and the severity of the deformity determine the type of management.
- Indicated if younger than 3 years of age
- Only used in non-obese patients
- Mainly worn at night
- Surgical management:
- Indicated in severe cases when the disease progresses despite bracing
- Osteotomy is performed prior to the age of 4.
- Hemiepiphysiodesis (gradual angular correction) is performed after the age of 4.
- Fixation methods
- Early infantile BD:
- Must be treated in order to avoid permanent deformity
- With timely treatment → excellent prognosis with rare recurrences
- Mild form may regress spontaneously.
- Late adolescent BD:
- Good prognosis with treatment
- Less likely to have severe deformity
- Physiological genu varum: Children under the age of 2 have physiological, symmetrical, painless bowing with in-toeing that spontaneously regresses with natural growth. Follow-up is usually performed to ensure that the condition does not progress further or has an underlying pathology.
- Rickets: distortion and softening of bones due to vitamin D deficiency or resistance. Patients usually present with bowing of the legs. Can be distinguished from BD via blood work and a lack of beaking on X-ray.
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