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Protrusio Acetabulae

Medial wall of acetabulum is on average 2 mm lateral of ilio-ischial line in a male and 1 mm medial to that line in the female. Diagnosis of protrusio is established, if the medial wall of the acetabulum is 3 mm or more medial to the ilio-ischial line in men or 6 mm medial to it in women.

Classification

Hirst etal JBJSB 69: 229, 1987

  • Grade I : 3 - 8 mm medial in a male and 6 - 11 mm in women; i.e. 5-10 mm protrusion
  • Grade II : 8 - 13 mm medial in males and 12 - 17 mm in women; i.e. 10-15 mm protrusion
  • Grade III : 13 mm medial in a male and greater than 17 mm in women (with fragmentation); i.e. greater than 15 mm protrusion

Overgaard; according to the shap of tear drop

  • Closed
  • Crossed
  • Reversed

Aetiology

  • Probably results from remodeling of a weak medial wall after repeated stress fractures
  • In primary cases, medial displacement continue till the greater trochanter impinges on the acetabular rim, then ossification of the medial wall begins
  • Healing is rare in secondary prutrosion

Primary
The idiopathic form is termed arthrokatadysis; aka Otto's disease or "Otto pelvis"

  • Precursor to OA
  • More common in women
  • Develops soon after puberty
  • OA in mid adult life

Secondary

  • Congenital : Marfan syndrome
  • Tauma
  • Infection
  • Inflammatory : RA
  • Metabolic bone disease
  • Osteomalacia
  • Rickets
  • Osteoporosis
  • Paget’s disease

Associated with

  • Decreased bone density
    • Osteoporosis or osteomalacia
    • Osteogenesis imperfecta
    • Rickets
    • RA
  • Normal bone density
    • OA
    • Idiopathic
  • Increased density
    • Hypophosphataemia
    • X-linked inherited

Clinical

  • Usually asymptomatic
  • Decreased ROM, especially abduction
  • Pain after development of secondary OA
  • A lateral globular mass may be palpable in DRE over the rectal wall

Treatment

  • Conservative in asymptomatic cases
  • Surgery for severe OA or limitation of ROM
    • Valgus trochanteric osteotomy reduces pain, but does not improve ROM
    • Cup arthroplasty : temporary and needs thick medial wall
    • Arthrodesis : in young unilateral cases
    • THR : graft the floor and lateralize the cup at the time of THR or use a protrusio prosthesis
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