THR Biomechanics

Understanding the biomechanics of the hip joint and applying them to Total Hip Replacement (THR) surgery is essential for achieving long-term success. The principles discussed here revolve around minimizing joint reaction forces (JRF) and optimizing functional outcomes.

Free Body Analysis in THR

A free body analysis of the hip assumes the body is in a single-leg stance.

Key points include:

- Leg weight: Each leg is approximately 1/6th of the total body weight.

- Joint Reaction Force (JRF): Determined by the interaction of the abductor muscles and the body weight (5/6th of total body weight to account for the non-weight-bearing leg).

- Abductor Force: The force generated by the abductor muscles counters body weight to elevate the contralateral pelvis during normal gait.

- Osteoarthritis (OA): As cartilage wears away in OA, friction increases, necessitating a higher abductor force to maintain movement, which in turn raises the JRF and causes more pain.

Methods to Alter Forces

  • Increase Abductor Force:
    • Carrying a suitcase on the same (ipsilateral) side.
    • Lateralizing the greater trochanter (GT).
    • Using a high offset stem.
  • Reduce Body Weight Force:
    • Losing weight.
    • Adopting a Trendelenberg gait.
    • Using a walking stick on the opposite (contralateral) side.
  • Other Biomechanical Adjustments:
    • Medializing the acetabular cup.
    • Using a smaller femoral head size.
    • Employing thinner polyethylene.

Charnley’s Principles of THR

The pioneering principles established by Charnley were based on the free body analysis of the hip. His primary goals were to minimize the joint reaction force (JRF) and provide low-friction bearing surfaces. To achieve these, Charnley made several modifications:

- Lateralized the Abductors: By increasing the offset and osteotomizing the greater trochanter (GT) to reposition it more distally and laterally.

- Medialized the Center of Rotation: Achieved by using a smaller femoral head size, medializing the acetabular cup, and employing thin polyethylene.

However, these changes introduced new challenges:

- Cup Failure: Thin polyethylene led to insufficient dissipation of force at the cup-bone interface, resulting in loosening and wear. Additionally, cups implanted into soft, cancellous bone were prone to failure.

Current Recommendations for Acetabular Cups

  • Fixation: Implant the acetabular cup onto the subchondral plate while avoiding excessive medialization.
  • Cemented Cups: For cemented implants, use 3 large keyholes while preserving the subchondral plate. Modern designs feature grooves and flanges to increase surface area and cement contact. However, avoid deep grooves that may compromise polyethylene thickness.

Femoral Component Considerations

  • Offset: Increasing the offset increases the abductor lever arm, which is beneficial. However, increasing neck length (instead of adjusting prosthetic design) transfers more stress to the tip of the implant and the medial cement mantle, leading to potential failure.

  • Fixation Methods:

    1. Uncemented.
    2. Cemented: Ensures both the cement-bone and cement-implant interfaces are firmly fixed (composite beam design).
    3. Taper-Slip Stem: The cement-bone interface is fixed, but the cement-implant interface is not, allowing for some movement.

  • Stresses at Cement-Bone Interface:

    • Uncemented and Composite Beam Designs: High shear, medium tensile, and low compressive stresses.
    • Taper-Slip Stems: Low shear, almost no tensile stress, and high compressive stress due to the viscoelastic properties of cement (e.g., creep and stress relaxation).

Bearing Surfaces in THR

The choice of bearing surfaces is critical, particularly depending on patient age:

Older Patients: Metal-on-polyethylene has a proven track record.

  • Use polyethylene that has been manufactured using direct compression molding.

  • Ensure proper sterilization (gamma irradiation in inert gas) and appropriate storage (vacuum packaging, maximum 2-year shelf life).

  • Consider highly cross-linked polyethylene to improve wear resistance.

Younger Patients: Alternative bearing surfaces may be considered based on longevity and wear characteristics.

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