Protrusio Acetabuli

Definition

• Medialisation of the medial wall of the acetabulum with a centre-edge angle > 40° on an AP XR of the pelvis
• Result of remodelling of weak, medial acetabular bone after multiple, recurring stress fractures
• Most often resulting secondary to inflammatory cause (inflammatory destruction) or metabolic cause (qualitative deficiency in the bone)
• Migration occurs along the resultant joint-reaction force vector (McCullum et al 1980)

Aetiology

Primary

• Otto Pelvis (Arthrokatadysis)
  • Bilateral 1/3rd
  • Middle aged females
  • Pain & Decreased ROM early
  • Coxa Vara & Osteoarthritis common
  • ? Causally related to osteomalacia
  • Diagnosis of exclusion

Secondary

• Inflammatory arthritis
  • 15% of Rh arthritis of the hip
  • 33% of ankylosing spondylitis of the hip
• Metabolic
  • Osteomalacia – up to 50%
  • Renal osteodystrophy
  • Hyperparathyroidism
  • Paget’s disease
  • Osteoporosis
• Connective Tissue disorder
  • Marfan’s
  • Erler-Danlos
  • Osteogenesis imperfecta
• Traumatic
  • Central fracture-dislocation
• Septic arthritis
• Neoplastic
• Existing Prosthesis
  • Hemiarthroplasty
• Seen in 5% of those with Osteoarthritis

Clinical findings

• Progressive pain & stiffness in groin
• Limp
• Trendelenberg +
• Antalgic gait

Investigations

Xray

• Position of femoral head in relation to medial wall acetabulum & pelvic brim = iliopectineal line
  • Grade I Mild 1-5mm
  • Grade II Moderate 6-15mm
  • Grade III Severe > 15mm
• Medial wall (acetabular line) & it’s relation to ilioischial line (Kohler’s line)
  • Men Normal = acetabular line 2mm lateral to I-I line
  • Women Normal = 1mm medial to I-I line
    • Grade I 3-8mm 6-11mm
    • Grade II 8-13mm 12-17mm
    • Grade III > 13mm > 17mm
• CE > 40°

Blood

• FBC
• ESR
• CRP
• RhF
• ANA
• Se electrolytes

Synovial Bx

Treatment

Skeletally immature

• Triradiate epiphyseodesis
• If coxa varus present then consider adding valgus intertrochanteric osteotomy

Skeletally mature adolescent/ young adult

• Valgus intertrochanteric osteotomy
  • Indications
  • significant symptoms with minimal degenerative changes in patient < 40 years
  • Principle
    • redirect joint-reaction force superiorly
  • Pointers
    • Amount of correction determined by amount of preoperative adduction
    • Generally 20-30° correction desirable
    • Lateralisation of the femur to correct mechanical alignment
    • Soft-tissue release (esp psoas) to effect lateralisation & improve hip ROM
• Arthroplasty
  • Replacement
  • Resection
  • Interposition
• Arthrodesis

Older adult

• THR
  • THR in protrusio acetabuli
    • Principles
      • 1. Place hip centre in anatomic location
        • To restore biomechanics
      • 2. Restore bone stock
        • Reconstruct segmental/ cavitatory defects of medial wall with bone graft
      • Use intact peripheral rim for component fixation
    • Preoperative
      • Define acetabular defect with appropriate imaging ~ 3D CT
      • Template preoperative LLD
    • Intraoperative
      • Sciatic nerve should be identified as closer to joint
      • Trochanteric Osteotomy may be required for exposure
      • Dislocation may be very difficult
      • Removal of rim bone – first try removing portion of posterior acetabulum
      • Head delivery piecemeal
      • May have to do neck osteotomy in situ
      • Significant soft tissue release from femur needed
      • Don’t ream medially or use centering hole
      • Remove cartilage & soft tissue with curette
      • Enlarge rim only with reamer
      • Take care not to create peripheral defect
      • Use one or more techniques to prevent medial migration
      • Use wire mesh in floor
      • Graft floor with morsellized bone
      • Cup may be
      • Uncemented rim fit cup
      • Cemented cup into impaction graft
      • Cemented cup into protrusio ring
      • Large cup with flange
    • Outcome
      • adequacy of correction of the deformity correlates with long-term prosthetic survival
      • Medial cup placement lead to high medial stresses cf. anatomic placement
      • Metal-backed component effective due to superior stress dissipation of the metal cup
      • Metal protrusio ring more reliably transfers stress from the medial wall to the rim than does protrusio cup
      • Reinforcement of medial wall with cement & wire mesh not effective
      • 50% reconstructed hips with cup centre > 10mm from anatomic hip centre failed
      • Increased loosening cup with protrusio grade 1
      • Multiple studies have shown excellent results with bone graft used to (1) lateralise the cup & (2) restore bone stock (usually femoral head morcellized or fragmented with or without cement)
      • Most widely used approach today is fill defect with morcellized graft then use porous coated cup
    • Ranawat & Zahn

Degree of Protrusi	Management
Protrusio < 5mm	THR without BG
Protrusio > 5mm with intact medial wall	THR with BG
Protrusio > 5mm with deficient medial wall	THR with BG & additional fixation (uncemented cup with screw fixation or antiprotrusio ring)