Blount's Disease

Blount's disease is an acquired growth disturbance of the medial aspect of the proximal tibial physis, causing varus angulation of the tibia in the metaphyseal region

Aetiology

• Enchondral ossification is abnormal
• Possibly, a parallel disorder to coxa vara as triangular shaped bony fragment often present
• Exact aetiology is unknown, following factors have been proposed :
  • Obesity
  • Walking too early
  • Genetic predisposition
• Weight bearing is required for development of Blount's disease, as the condition is not seen in non ambulatory patients
• Adolescent variety is caused by physeal growth arrest, as there is often a history of trauma or infection

Classification

Based on the time of presentation

• Infantile
  • Onset at 5 y/o or less
  • Occurs predominantly in black race 
  • Male : Female 2:1
  • 50 - 75% bilateral
  • Patients are usually over weight
• Juvenile
  • Onset between 6 and 9 years
• Adolescent
  • Usually presents between 8 - 13 years
• Varus usually does not exceed 20°
• There are two types of adolescent Blount's disease:
  •  Partial closure of the physis
    • Usually due to trauma or infection
    • Unilateral in 90% of cases
  • "late-onset" tibia vara
    • Idiopathic
    • Most often occurs in obese African-American males
      • less than 50% have bilateral involvement

Clinically

• The usual age at presentation is 2 y/o or more, as opposed to physiologic bowing which should be improving by 2 y/o
• Pain is frequently the presenting symptom
• Pain and tenderness often present over the medial prominence of the proximal tibia
• Deformity is slight but slowly progressive*,* as opposed to physiologic bowing which resolves with time
• Leg length discrepancy averages 2 cm
• Deformity varies from 10^o to 45^o (average 20^o)
• Lateral thrust and internal tibial torsion are suggestive of Blounts

Classification

Langenskiold classified the condition based on the degree of depression of the medial plateau

• Stage I
  • Age 2 - 3 years
  • Irregularity of entire metaphysis
  • Medial metaphyseal beaking
• Stage II
  • Age 2.5 - 4 years
  • Sharp antero-medial depression evident
  • There is a propensity for healing at this stage
• Stage III
  • Age 4 - 6 years
  • Deepening of the depression makes a step
• Stage IV
  • Age 5 - 10 years
  • Developed step is filled in by the epiphysis
  • Growth plate is irreversibly damaged at this stage
• Stage V
  • Age 9 - 11 years
  • A triangular fragment separates
• Stage VI
  • Age 10 - 13 years
  • Medial growth plate ossification
  • Growth continues in the normal lateral part

X-Ray

• Infantile Blounts
  • Varus angualtion
  • Medial metaphysical beaking
  • Medial metaphysical depression
  • Medial metaphyseal triangular bone fragment
  • Bony bridge may be seen
• Adolescent Blounts
  • Thinning of the growth plate
  • The epiphysis is normal in shape 
  • There is no step
• The metaphyseal- diaphyseal (MD) angle (aka Drennan Angle)
  • Angle between a line drawn through the most distal point of the medial and lateral beaks of the tibial metaphysis and the line perpendicular to axis of the tibia 
  • An angle less than 11 degrees is normal and an angle greater than 16 degrees suggests Blounts
  • A similar angle can be drawn for the femur. The ratio of the femoral MD angle to the tibial MD angle is typically less than 1.
    • In physiological bow leg, average metaphyseal-diaphyseal angle was 9° ± 4°
    • In Blounts, average metaphyseal-diaphyseal angle was 19° ± 5.7°
    • The difference was significant to p less than 0.0000001

• Case Example 1:

  Fig 3. Eight year old female with Blount's disease	Fig 4. Standing AP radiograph of 8 year old female with Blount's disease	Fig 5. AP and lateral tiba radiographs of 8 year old female with Blount's disease

   

Pathology

• Postero-medial portion of proximal tibial physis fails to grow normally
• Marked changes on the medial side of the physis
  • The zone of proliferation is thin 
  • Cartilage columns are stunted and irregular
  • Matrix calcification is normal
  • The primary trabeculae are oriented horizontally
• It is not clear whether these changes are primary or secondary to the eccentric compressive forces
• A bony bridge may be evident (especially if secondary to trauma)

Treatment

Based on the age of the patient and the Langenskiold staging

• Stage 1 + 2
  • Long leg bracing (KAFO)
    • Try night bracing first, as it does not disable the child 
    • If still no improvement, proceed to day bracing
    • Resolution to normal axis may take up to 1 year
  • Deformity is often reversible at this stage
• Stage 3 + 4
  • If the deformity persists or increases to these stages, osteotomy is indicated
  • Osteotomise prior to age 4, if possible
  • The deformity is recurrent in stages IV or higher and patient needs to undergo multiple osteotomies until tall enough to do epiphysiodesis
• Stage 5 + 6
  • If tomograms show a bony physeal bridge, can either excise the bridge or perform an osteotomy, esp. if significant growth remaining
  • Premature medial growth plate closure : epiphyseodesis of the lateral plate plus tibial osteotomy
  • Osteotomy indicated if there is :
    • Severe deformity
    • Severe joint depression
  • In osteotomy aim to correct to 10° valgus
  • Use of an opening wedge may obviate the need for operative correction of LLD
  • Need to warn parents of risk of recurrence post surgery

Prognosis

• Infantile Form
  • The first 4 years is the main period for progression, after which the deformity remains the same or progresses slowly
  • Deformity may range from 10° - 60°
  • From 9 years to skeletal maturity, there is usually a gradual increase in deformity
• Adolescent Form
  • Varus deformity increases incidence of early-onset degenerative joint disease

References

• Feldman D.S. Lower Limb Disorders. In Spivak JM, Di Cesare PE, Feldman DS : Orthopedics; A Study Guide; 1999
• Milbrandt T.A., Sucato D.J : Pediatric Orthoprdics. In Mark D. Miller : Review of Orthopedics; 2008

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