Instability

Risk Factors

Surgical Factors

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

Patient Factors

Cognitive impairment
High range of motion (ROM)
Contact sports

Determinants of Hip Stability

Component design
Component position
Soft tissue tension
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:

Greater primary arc range (increased head-neck ratio)
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:

De-tensioning of abductors
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