Causes
Instability is multifactorial.
Patient factors
- neuromuscular disease
- cognitive impairment
- previous hip surgery
- abnormal spinopelvic motion
- poor abductor function
Implant factors
- head size and head-neck ratio
- offset
- liner design
- impingement
Surgical factors
- cup and stem orientation
- leg length and soft-tissue tension
- surgical approach and repair
- trochanteric/abductor problems
THA instability is multifactorial.
Patient factors:
- neuromuscular disease
- cognitive impairment
- poor compliance
- abductor deficiency
- spinal stiffness or deformity
Surgical/mechanical factors:
- cup orientation
- stem version
- combined anteversion
- inadequate offset
- impingement
- soft-tissue deficiency
- small head or low jump distance
- component loosening
Acute dislocation
Assess:
- direction of dislocation
- neurovascular status
- fracture
- component position
Closed reduction is performed with appropriate analgesia/anaesthesia unless there is a reason for immediate open treatment.
Post-reduction radiographs confirm concentric reduction and exclude fracture.
Recurrent instability
Determine the mechanism rather than repeatedly reducing the hip without a plan.
Ask:
- is a component malpositioned?
- is there impingement?
- is offset/soft-tissue tension inadequate?
- is the abductor mechanism deficient?
- is there infection or loosening?
- is spinopelvic mechanics contributing?
Recurrent dislocation requires systematic analysis.
Ask:
- Is the cup malpositioned?
- Is stem version inappropriate?
- Is offset/length insufficient?
- Is there bony or prosthetic impingement?
- Are abductors deficient?
- Is there spinopelvic abnormality?
- Are components loose?
CT can help define component version. Functional standing/sitting imaging can be useful where spinopelvic mechanics are suspected.
Operative options
Depending on cause:
- remove impingement
- revise malpositioned components
- restore offset and length
- repair/reconstruct abductors
- dual-mobility articulation
- constrained liner in selected salvage situations
A constrained liner treats the consequence of instability but transfers higher forces to the implant interfaces and should not be used to avoid correcting a fixable mechanical cause.
First dislocation
Assess:
- direction
- mechanism
- neurovascular status
- radiographs before and after reduction
- component position
- fracture
Closed reduction is usually appropriate when there is no contraindication.
After reduction, determine whether the event was an isolated positional episode or evidence of a mechanical problem.
Spinopelvic relationship
Pelvic orientation changes between standing and sitting. A stiff lumbar spine or fused spine can reduce adaptive pelvic motion and alter functional cup position, increasing instability risk.
This is increasingly important in revision planning.
Surgical options
Depending on cause:
- reposition malaligned component
- restore offset/length
- remove impingement
- increase head size where safe
- use dual-mobility articulation
- use constrained liner in selected severe deficiency
- repair/reconstruct abductors where possible
A constrained liner should not be used to conceal correctable malposition.
FRCS synthesis
The principle is: identify the reason for instability and correct it. Device choice comes after mechanical analysis.