Uncemented Stem

Principle

  • Primary Objective: Attain immediate stem stability.
  • Secondary Objective: Achieve subsequent biologic fixation.

Characteristics

  • Larger Stems: Allow more cortical contact.
  • Fixation is Biologic:
    • Micro-fractures at the implant-bone interface can remodel.
    • Fixation depends on achieving biologic integration.

Reasons for Failure to Achieve Biologic Fixation

  1. Poor Bone Quality: Inadequate in-growth or on-growth.
  2. Suboptimal Stem Design: Insufficient in-growth or on-growth capability.
  3. Faulty Implantation Technique:
    • Inadequate initial stability.
    • Leads to fibrous in-growth instead of bony in-growth.

Types of Uncemented Stems

1. Porous Coated

  • Mechanism: In-growth of bone.
  • Features:
    • Metallic stem with pores allowing bone growth.
    • Material types:
      • Titanium with fibers and beads.
      • CoCr with sintered beads.

Optimal Parameters

Parameter Value
Pore Size 50–150 µm
Bone-Implant Gap Distance 50 µm
Porosity 50%
Maximum Micromotion 50–150 µm
Ideal HA Coating Thickness 50 µm
  • Deeper Pores: Provide shear stability but risk coating shear.

2. Grit Blasted or HA Coated

  • Mechanism: On-growth of bone.
  • Biologic Fixation: Bone grows onto the roughened surface.

Grit Blasting

  • Process: Surface roughened by particle spray.
  • Features:
    • Bone adheres to pits and valleys.
    • Surface roughness determines shear resistance.

HA Coating

  • Material: Osteoconductive ceramic ([Ca₁₀(PO₄)₆(OH)₂]).
  • Features:
    • Bidirectional biologic fixation (bone-prosthesis and vice versa).
    • Reduces gaps faster than other methods.
    • Ideal for revision surgeries.
  • Optimal Composition:
    • High crystallinity for biological activity.
    • Thickness of 50 µm (thicker coatings risk shearing).

Techniques of Uncemented Fixation

Principles

  1. Early Stability: Minimize micromotion to <50 µm for bony bonding.
  2. Avoid Fibrous Bonding: Prevent subsidence.

Two Techniques

  1. Press Fit
    • Bone preparation is smaller than the implant.
    • Implant impaction generates hoop stresses, stabilizing the implant.
    • Risks: Higher risk of fracture.
  2. Line-to-Line Fit
    • Bone preparation matches the implant size.
    • Extensively porous-coated implants distribute shear stress.
    • Provides a strong grip on the bone.

Supplementary Fixation

  • Screws in Cup: Often used to provide additional fixation.
  • Cortical Contact: Preferred for stability.
    • Cancellous bone allows ingrowth but leads to more micromotion.
    • In the cup, aim for cortical rim fit.
    • In the stem, the largest possible size ensures some cortical contact.

Stress Shielding

  • Definition: Bone density reduction proximal to or around a well-fixed stem due to decreased physiological loading, caused by the stiffer implant sharing the load.
  • More Common with Uncemented Stems:
    • Cemented stems have a damping effect due to the flexibility of cement, which provides a transitional zone between stem and bone.
    • Stress Shielding is Less Common in Cemented Stems: Cement’s flexibility reduces modulus mismatch.

Consequences of Stress Shielding

  1. Does not affect longevity: As long as the stem is well fixed.
  2. Predisposes to fractures: Around the stem, especially with uncemented stems.
  3. Revision Challenges: Loss of bone stock makes revision surgery difficult.

Main Factors Contributing to Stress Shielding

  1. Stem Stiffness
  2. Extent and Distribution of Porous Coating

Worst combination for stress shielding: A long, fat, cylindrical, fully porous-coated CoCr stem.

Stem Stiffness

Influencing Factors:
1. Size: Stiffness increases by the 4th power of stem radius.
2. Material: CoCr is stiffer than titanium.
3. Shape:
- Cylindrical stems are stiffer than tapered ones.
- Solid stems are stiffer than fluted/hollow ones.
- Long stems stress shield more.

Measures to Reduce Stem Stiffness

  • Tapering the stem
  • Adding flutes
  • Using a Cemented Stem: Cement provides a dampening effect on stress transfer, although cemented stems must be stiffer due to their smaller size.
    • Using Titanium: Although titanium is less stiff, it can generate more wear particles, which are undesirable.

Extent of Porous Coating

  • Full Coating: Results in diaphyseal (shaft) ingrowth and causes proximal stress shielding.
    • Spot Welding: Increased density around the distal tip; load dissipates distally.
    • Proximal Coating: Distributes the load more evenly.

Collared Implants

In Cemented Stems

  • Collar Function: Promotes loading through the proximal medial bone (where stress shielding occurs).
    • Risk: Collar may not seat directly against bone, leading to lysis under the collar.

In Uncemented Stems

  • Controversial Use:
    • Collar may prevent full seating of the prosthesis, leading to a loose stem.
    • If seated properly, provides the most physiologically similar strain to native bone.
    • Protects against subsidence, but cementing may be a better option in such cases.

Morse Taper

  • Definition: A cone-in-cone taper used to join rotating parts like drill bits and chucks.
  • Working Principle:
    • The trunion (male) and bore (female) are nearly the same taper, with the bore being slightly smaller.
    • Impaction of the bore onto the trunion generates opposing stresses.
    • Result: A reliable interlock.
  • In THR (Total Hip Replacement):
    • The taper may be subject to fretting and corrosion.
    • Precautions: Components must be clean and dry before assembly.
    • Taper sizes vary between manufacturers; mixing is not recommended unless the exact taper is matched.
    • Common Taper Size: 12/14 taper.
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