Blount's Disease

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Original Editors - Rachael Lowe

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Description

Blount's disease, also known as tibia vara, is a developmental growth disorder of the tibia that causes the lower leg to angle outwards, causing bowing of the leg. It is characterised by progressive multiplanar deformities of the leg caused by disordered endochondral ossification of the proximal medial tibial physis.

The cause of Blount's disease is assumed to be multifactoral, mostly mechanically due to childhood obesity. This can be described by the effects of increased weight on the growth plates. The medial proximal tibia fails to develop normally, resulting in genu varum.  Unlike bowlegs, which tend to straighten as the child develops, Blount's disease is progressively worsening. It can cause severe bowing and can affect one or both legs.

Pathogenesis

Looking at the Heuter-Volkmann principle, the pathogenesis of Blount's disease can be described as growth inhibition caused by excessive compressive forces. This pressure on the medial proximal tibia causes structure and function alterations on the cartilage of the epiphysis, further causing chondrocytes and delaying in epiphysis ossification. The increase in medial load with obesity usually lead to genu varum.

Dietz et al.16 examined the relationship between body weight and angular deformities in fifteen young children with Blount disease. They observed a significant correlation between body weight and the tibiofemoral shaft angle (r = 0.75) and noted an even stronger relationship between body weight and varus deformity when nine obese children were considered independently (r = 0.92). Using gait analysis, Gushue et al.17 studied the effect of childhood obesity on three-dimensional knee joint biomechanics. Compared with children of normal weight, overweight children showed a substantially higher peak internal knee abduction moment during early stance, with increased loading of the medial compartment of the knee joint. My colleagues and I18 recently reported a linear relationship between the magnitude of obesity and biplanar radiographic deformities in children with the early-onset form of Blount disease and in patients with a body-mass index of >40 kg/m2 irrespective of the age at the onset of the Blount disease. Despite having a lower body-mass index, children with early-onset Blount disease had more severe varus and procurvatum deformities of the proximal part of the tibia than did adolescents with Fig. 1-A Fig. 1-B Clinical photograph (Fig. 1-A) and standing anteroposterior radiograph (Fig. 1-B) of a five-year-old girl with bilateral early-onset Blount disease. 1759 TH E JOURNAL OF BONE & JOINT SURGERY d JBJS . ORG VOLUME 91-A d NUMBER 7 d JULY 2009 BLOUNT DISEASE Blount disease18. Wenger et al.11 suggested that the proximal tibial growth plate responds differently at various stages of skeletal maturity, with the increased pliability of the unossified epiphyses of younger patients causing more growth inhibition than occurs in adolescents. Davids et al.15 examined gait deviations related to increased thigh girth associated with adolescent obesity. An obese child with large thighs has difficulty adducting the hips adequately and this may result in ‘‘fat-thigh gait’’ by producing a varus moment on the knees, thus increasing pressure at the medial part of the proximal tibial physis. This concept supports the observation that preexisting varus alignment of the knee is not necessary to initiate the pathologic changes seen in some patients with late-onset Blount disease21. Recent studies suggest that childhood obesity reduces bone mineral content to levels below what would be predicted on the basis of body weight22. Such factors may further predispose obese children with Blount disease to the development of progressive deformities with increasing weight. The mechanical etiology does not fully explain the unilateral or asymmetrical involvement of the limbs that is often seen in these children or the observation that some children with classic radiographic findings of Blount disease, especially the early-onset type, are not overweight18,23. One report showed no difference in terms of obesity or early walking age between black children with early-onset Blount disease and their peers in a local population23. However, no details about the children’s weight, body-mass index, or radiographic findings were provided. Currently, the etiology of Blount disease remains unknown and is probably multifactorial. It is likely that various genetic, humoral, biomechanical, and environmental factors control physeal growth and influence the development of normal lower-limb alignment. The clinical manifestations of both forms of Blount disease may represent an alteration in the normal growth and development of the lower limb in genetically predisposed children through distinct but related pathways.

Etiology / Epidemiology

The following are known predisposing factors of Blount's disease:

• This condition is more common among children of African and Scandanavian ancestry.
• It is associated with obesity, short stature, and early walking.
• There does not appear to be an obvious genetic factor.

Classification

Blount's disease is mostly catagorised into early-onset if it develops in children under 4 years old, and late-onset, when it develops after the age of 4. The late-onset type can further be classified into juvenile (age 4 - 10) and adolescent (after the age of 10) Blount's disease.

t. Also, there are comparable histologic findings at the proximal tibial growth plate9-11

Clinical Presentation

• Unilateral or bilateral (mostly with early onset) presentation
• Multiplanar deformities of the lower leg includes:
  • Tibial varus
  • Procurvatum
  • Internal torsion
  • Limb shortening
  • Distal femoral varus (in late-onset)

• Early onset knee osteoarthritis as result of deformities
  

This entity can lead to a progressive deformity with gait deviations, limb-length discrepancy, and premature arthritis of the knee6-8.

Diagnostic Procedures

X-rays (AP and lateral views) are done as the main tool for diagnosis.

Langenskiöld classification

This classification system are used to describe the radiographic changes and are divided into 6 progressive type in early onset Blount's disease, describing changes with aging.

• Stage 1: Age 2-3
  • Metaphyseal irregularities in ossification zone
  • Slow development of epiphysis in medial aspect of tibia
  • Medial and/or distal beaking of medial epiphysis
• Stage 2: Age 2.5-4
  • Medial physeal line depresses sharply
  • Beaking medially
  • Wegde-shaping and underdevelopment of medial epiphysis
• Stage 3: Age 4-6
  • Metaphyseal beak depression deepens
  • Metaphysis "step"
  • Medial part of epiphysis more wedge-shaped and underdeveloped
• Stage 4: Age 5-10
  • Narrowing of physis
  • Enlargement of epiphysis
  • Metaphysis "step" deepens
  • Depression in medial metaphysis ocupied by epiphysis
• Stage 5: Age 9-11
  • Clearly separated epiphysis in two parts
  • Epiphysial plate partially double
  • Articular surface medially sloping
• Stage 6: Age 10-13
  • Ossification of medial physis with cessation of growth
  • Normal lateral tibial growth

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Outcome Measures

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Medical management

Conservative management

Children who develop severe bowing before the age of 3 may be treated with bracing. However, bracing may fail, or bowing may not be detected until the child is older.

Surgical management

• Realignment tibial osteotomy: To be done before the age of 4 to decrease the risk of recurrent lower extremity deformity and to restore leg length where needed.
• Distraction osteogenesis: For late-onset disease:
  • Aim to achieve multiplanar correction
• The growth of just the outer half of the tibia can be surgically restricted to allow the child’s natural growth to reverse the bowing process. This much smaller surgery is most effective in children with less severe bowing and significant growth remaining.

Differential Diagnosis

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Resources

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Case Studies

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References

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