Cemented Stem

Best Long-Term Results:
Partly because there are many uncemented designs without equivalent longevity.

Evolution of Cementing Technique

Generation Technique Features
1st Finger Packing Sharp-edged stem.
2nd (1975) Cement Gun (retrograde) Pulse lavage, pressurization.
3rd (1982) Vacuum Mixing (porosity reduction) Canal brushed/dried, stem centralizer, cement restrictor, rough stem finish.

Cement Mantle Size

  • Minimum: 2 mm.
  • Ideal: 4 mm proximally, 2 mm distally.
  • Ratio: Stem fills 2/3 of the canal; 1/3 is cement.
  • Challenges: Narrow canals make achieving this difficult.

Note: Proximal mantle is where stems typically loosen first.

Cemented Stems

  • Stiffness:
    • High Young’s modulus directs load away from the proximal mantle, reducing early loosening.
    • Can lead to stress shielding, though typically associated with uncemented stems.
  • Design:
    • Should avoid sharp edges to prevent high contact pressure points in the cement mantle.

Two Broad Design Types

Composite Beam

  • Example: Charnley.
  • Reliant on: Immediate stability, no micromotion within the cement mantle.
  • Features:
    • Matt finish or slightly roughened.
    • Collar or flange to prevent migration.
  • Load Dissipation: Primarily at the tip of the stem.

Taper Slip

  • Example: Exeter.
  • Mechanism:
    • Initial migration into the cement mantle achieves a stable mechanical state.
    • Generates radial (hoop) stresses, placing the prosthesis, cement, and bone interface under compression.
  • Load Dissipation: More evenly around the stem.
  • Features:
    • Highly polished, tapered design.

Cemented Stem Failure

Failure Mechanism

  • Crack formation at the cement–stem interface (microfractures).
  • Inadequate stem stiffness → bending → crack propagation.
  • Cement: Non-biologic fixation; cracks spread to the cement–bone interface, leading to loosening.

Composite Beam Failure

  • Micromotion between cement and stem.
  • Debris generation.
  • Cement fracture and crack propagation.

Taper Slip Stem Failure

  • Stable inherently, but rotational motion poses a risk.
  • Axial Loading: Sudden hoop stresses from events like falls can cause fractures.
  • Bone Vulnerability: Anisotropic properties of bone exacerbate the risk.
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