Instability

Risk Factors

Surgical Factors

  1. Component malposition
  2. Inadequate offset restoration (abductors de-tensioned)
  3. Impingement – bony or component
  4. Shortening
  5. Poor soft tissue repair (especially posterior approach)
  6. Conversion of failed ORIF to THR

Patient Factors

  1. Cognitive impairment
  2. High range of motion (ROM)
  3. Contact sports

Determinants of Hip Stability

  1. Component design
  2. Component position
  3. Soft tissue tension
  4. Soft tissue function

Component Design

Primary Arc Range

  • Defines the ROM arc before impingement occurs
  • Main determinant: Head-neck ratio
    • Higher head-neck ratio = greater primary arc range

Excursion Distance

  • Distance traveled by the head before dislocation once impingement occurs
  • Main determinant: Head size
    • Excursion distance is half the head size
      • Example: 11mm for a 22mm head, 16mm for a 32mm head

Larger heads are inherently more stable because:
1. Greater primary arc range (increased head-neck ratio)
2. Larger excursion distance

Taper Size

  • Narrower tapers increase the head-neck ratio and enhance stability
  • Collared stems tend to have larger necks, reducing the head-neck ratio

Acetabular Augmentation

  • Hooded cups are designed to aid stability but can lower the primary arc range
  • Constrained liners contain the head but drastically reduce the primary arc range
    • Leads to excessive stress transfer to the cup-bone interface, causing early loosening

Component Position

  • A patient’s native hip has a larger head-neck ratio and larger head, making it inherently more stable

Goal:
- Place THR in the middle of the patient’s functional range, providing leeway if the primary arc range is exceeded

Ideal Component Positioning:
- Cup anteversion: 15–30 degrees
- Combined anteversion: 30–40 degrees
- Cup inclination: 35–45 degrees

Note: This may vary slightly based on the approach and implants used

Soft Tissue Tension

  • Key muscles: Abductors
    • Provide joint reaction force (JRF) to stabilize the hip

Impact of reduced offset or neck length:
1. De-tensioning of abductors
2. Predisposing to trochanteric impingement in abduction

Soft Tissue Dysfunction

  • Causes may be central or peripheral neurologic issues
  • Often multifactorial in elderly patients

Management of the Dislocated THR

  • Hips that dislocate more than twice usually require revision surgery
  • Important to document the position of the hip and activity during dislocation
  • During closed reduction, document the position of instability

Use of Braces

  • Not always necessary
  • Knee splints prevent flexion (posterior instability)
  • Abduction braces allow tissues to contract in a better position

Best predictor of successful revision surgery:
- Understanding the cause of instability

Possible Causes & Solutions

Eccentric Poly Wear

  • Common cause of dislocation
  • Rectified with poly exchange

Component Malposition

  • Diagnosed via X-ray or CT scans
  • Must be revised even if well-fixed

Inadequate Offset or Neck Length

  • Revision needed to restore or increase abductor tension

Increasing Head Size

  • Increases primary arc and excursion distance

Trochanteric Advancement

  • Useful for increasing abductor tension if all else is satisfactory
  • Not effective for trochanteric escape or pull-off

Constrained Liner

  • Last resort after addressing all other factors

Conversion to Hemiarthroplasty

  • Not possible with acetabular defects (risk of intra-pelvic migration)
  • Suitable for low-demand patients

Resection Arthroplasty

  • Absolute last resort for low-demand patients where all else has failed
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