Aseptic Loosening of THR

Classification

Gruen Zones

1-7 starting at GT ending at Calcar

Charnley Zones

1-3 starting laterally ending medially

Key Feature

Progression of lucent lines is the key feature

Reference: Canale T (2002) Campbell’s Operative Orthopaedics, 10th edition, Mosby

Femoral Stem

Cemented Stem Loosening

9 Possible Radiologic Findings to Note:

  1. Radiolucency between stem and cement mantle
  2. Radiolucency between cement and bone
  3. Subsidence of stem within cement mantle
  4. Subsidence of cement mantle within bone
  5. Stem position changed to varus
  6. Bending of the stem
  7. Fracture of the stem
  8. Rarefaction of the cement or fragmentation of cement
  9. Cement mantle fracture (usually at stem tip – zone 4)

Technical Errors Contributing to Poor Cement Mantle

  1. Removal of all cancellous bone
  2. Stem movement while cement hardening
  3. Cement voids (pigtail catheter, bone, blood, etc.)
  4. Inadequate pressurization
  5. Poor stem position – point contact with bone
  6. Inadequate cement mantle size (<2mm or <1/3 canal)

Modes of Cemented Stem Failure

Modes of Failure:

Dependent on the quality of cement fixation in various zones around the stem.

  1. Pistoning (stem inside cement mantle)
  2. Pistoning (cement mantle within bone)
  3. Pivoting of mid-stem
  4. Pivoting at calcar
  5. Bending - cantilever

Mode 1a: Pistoning (Stem Inside Cement Mantle)

  • Cause: Inadequate bonding of cement with the whole stem
  • Associated Finding: Zone 4 cement fracture due to axial pistoning

XR Findings:
- Stem subsidence
- Lucency in zones 1 and 2
- Distal cement fracture

Mode 1b: Pistoning (Cement Mantle Within Bone)

  • Cause: Poor bonding of cement with bone
    • Inadequate pressurization
    • Removal of all cancellous bone

XR Findings:
- Lucency in all 7 zones
- Whole cement mantle subsidence with stem still incorporated

Mode 2: Pivoting of Mid-Stem

  • Cause: Poor cement fixation proximal medially and distal laterally
  • Mechanism: Stem pivots around mid-point into varus

XR Findings:
- Medial migration of proximal stem
- Lateral migration of distal stem
- Fracture around mid-stem pivot point (zones 2 and 6)

Mode 3: Pivoting at Calcar

  • Cause: Poor distal cement fixation
  • Mechanism: Distal stem acts like a windscreen wiper while pivoting around the well-fixed calcar

XR Findings:
- Lucency in zones 4 and 5
- Sclerosis and thickening around the stem tip

Mode 4: Bending - Cantilever

  • Cause: Poor proximal fixation with solid distal fixation
  • Outcome: Stem bends as it is anchored distally and eventually fractures

XR Findings:
- Lucency in zones 1, 7, 2, and 6
- Stem fracture

Uncemented Stem Loosening

Classification of Uncemented Stem Fixation:

  1. Well-Fixed Stem with Bony Ingrowth
  2. Stable Stem with Fibrous Ingrowth
  3. Loose Stem

XR Features:

Well-Fixed Stems:

  • No radiolucent lines
  • May have cortical hypertrophy where porous coating ends
  • Proximal stress shielding indicates good, stable distal fixation

Stable Fibrous Stems:

  • Uniform radiolucent line <1mm
  • Sclerotic line outlining the radiolucency
  • No subsidence

Unstable Stems:

  • Divergent, irregular radiolucent lines
  • Spot welds or areas of localized bone hypertrophy (e.g., at calcar or stem tip)
  • Indicate stem movement and lack of uniform loading

Summary of Key Findings:

  • Well-Fixed Stem: No lucent lines, uniform hypertrophy
  • Localized Hypertrophy: At stem tip indicating pistoning or abnormal loading
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