Causes of Dislocation of Total Hip Arthroplasty : WHAT mnemonic

Mnemonic: WHAT caused the hip dislocation?

1. Wear:

  • Eccentric wear of a polyethylene acetabular cup (common cause of late instability occuring >5 years after procedure) – brings the implant neck closer to the acetabular rim, promoting leverage.
  • Implant loosening

2. Head–neck ratio:

  • Larger head-neck ratios allow greater arc range of motion prior to impingement
  • Large femoral heads are seated deeper within the acetabulum, increasing jump-distance (amount of translation prior to dislocation) and hence increases stability
  • Many revision hip arthroplasty surgeons advocate the use of 36 mm heads in order to maximize stability and accept the trade-off of a higher rate of volumetric wear
femoral head center

3. Alignment: Restoring the patients’ native length, offset and version gives the best chance of implant stability.

  • Vertical offset: Lesser trochanter to Center of femoral head (essential for correction of leg length)
  • Medial offset: Center of femoral head to the line through the axis of distal part of stem
    • If inadequate – shortens moment arm (increase impingement and dislocation)
    • If excessive – increases stress (stress fracture or loosening)
  • Version and position:
    • Acetabular anteversion: 5-25 degrees
    • Acetabular abduction: 30-50 degrees
    • Femoral stem anteversion: 10-15 degrees

4. Tension: refers to adequate abductor function; weakness of abductors may be caused by –

  • Posterior surgical approach
  • CNS or neuromuscular diseases
  • Decreased femoral offset

Reference: Crozier-Shaw G, Magill P. WHAT causes dislocation of a total hip arthroplasty; a simple mnemonic for the orthopaedic resident. Int Orthop. 2015 Mar;39(3):605-6. doi: 10.1007/s00264-014-2661-y. Epub 2015 Jan 21. PMID: 25720360.

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