Mnemonic: WHAT caused the hip dislocation?
- 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
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.