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Updated: Jan 1 2024

THA Stability Techniques

  • summary
    • Four important variables that help determine the stability of THA
      • component design
      • component position
      • soft-tissue tensioning
      • soft tissue function
  • Component Design
    • Femoral component design
      • large femoral heads
        • decreased dislocation rates due to
          • head-neck ratio increased
            • definition
              • diameter of femoral head/diameter of femoral neck
            • importance
              • larger head-neck ratios allow greater arc range of motion prior to impingement
          • skirts can be avoided
            • definition
              • skirts are attachments used to extend the length of the femoral neck
            • importance
              • skirts decrease the head-neck ratio
          • jump-distance is increased
            • definition
              • amount of translation prior to dislocation
            • importance
              • large femoral heads are seated deeper within the acetabulum, increasing jump-distance
              • increase in jump-distance increases joint stability
          • a larger femoral head will not compensate for abductor deficiency or a vertically positioned cup
      • femoral offset
        • see "soft tissue tensioning" below
      • femoral neck-shaft angle
        • increasing neck shaft angle (more valgus) compared to native anatomy can increase leg length and decrease offset
        • decreasing neck-shaft angle (more varus) compared to native anatomy can decrease leg length and increase offset
    • Acetabular component design
      • elevated rim liner
        • a posteriorly placed elevated rim liner may increase joint stability
      • lateralized liner
        • increases soft-tissue tension by increasing offset
        • has been shown to increase the risk of acetabular component loosening
  • Component Position
    • Acetabular position
      • recommendations
        • anteversion
          • 5° - 25°
        • abduction
          • 30° - 50°
        • medialization of the cup increases moment arm of the abductors (gluteus medius & gluteus minimus)
          • increased moment arm leads to decreased joint reactive forces
      • caveats
        • surgical approach may affect optimal position of implants
          • posterior approach should err towards more anteversion
          • anterior approach should err towards less anteversion
        • hypertrophy of the anterior inferior iliac spine may cause component impingement and instability
      • complications
        • excessive retroversion
          • posterior dislocation
        • excessive anteversion
          • anterior dislocation
        • excessive abduction (high theta angle, vertical cup)
          • posterior superior dislocation
          • eccentric polyethylene wear and late instability
        • excessive adduction (low theta angle, horizontal cup)
          • impingement in flexion
          • inferior dislocation
    • Femoral stem position
      • recommendations
        • 10°- 15° of anteversion
      • caveats
        • more difficult to adjust femoral component version in uncemented femoral components
    • Combined version
      • definition
        • femoral component anteversion plus acetabular component anteversion
      • recommendations
        • 37 degrees
  • Soft Tissue Tensioning
    • Abductor Integrity 
      • gluteus medius serves as the major hip abductor muscle; insufficiency/tear leads to Trendelenburg gait
        • gluteus maximus and/or fascia lata transfers can be utilized in chronic abductor insufficiency
    • Restoration of offset
      • definition
        • perpendicular distance from femoral head center of rotation to the axis of the femur
      • importance
        • increased offset leads to
          • decreased impingement
          • increases soft tissue tension without increasing leg length
        • decreased offset may lead to
          • instability
          • abductor weakness
          • gluteus medius lurch
        • increasing offset improves hip stability
      • techniques to increase offset
        • increasing length of femoral neck
        • decreasing neck-shaft angle
        • medializing the femoral neck while increasing femoral neck length
        • trochanteric advancement
        • alteration of the acetabular liner (see "component design" above)
  • Soft Tissue Function
    • Three main factors controlling proper soft tissue function
      • central nervous system
        • pathology that affects the central nervous system
          • stroke
          • cerebellar dysfunction
          • dementia
          • MS
          • Parkinson's
          • myelopathy
          • delirium
          • alcoholism
      • peripheral nervous system
        • pathology that affects the peripheral nervous system
          • spinal stenosis (gluteus medius is L5)
          • peripheral neuropathy
          • radiculopathy
          • paralysis/paresis
      • local soft tissue integrity
        • trauma
        • myoligamentous disruption
        • deconditioning
        • aging process
        • poor health
        • irradiation
        • osteolysis
        • collagen abnormalities
        • myopathy
        • malignancy
        • infection
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