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https://upload.orthobullets.com/topic/5007/images/offset_900.jpg
https://upload.orthobullets.com/topic/5007/images/head-neck_ratio.jpg
https://upload.orthobullets.com/topic/5007/images/skirt.jpg
https://upload.orthobullets.com/topic/5007/images/jump..jpg
https://upload.orthobullets.com/topic/5007/images/liners.jpg
Introduction
  • 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
    • femoral offset
      • see "soft tissue tensioning" below
  • 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°
    • 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
  • Restoration of offset
    • definition
      • perpendicular distance from femoral head center of rotation to the axis of the femur  
    • importance
      • increased offset leads to
        • increased soft-tissue tension
        • decreased impingement
        • decreased joint reaction force  
      • 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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