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- Ideal positioning places the implant's primary arc range within the patient's functional range
- Lewinnek proposed Safe zones in 1978
- Cup inclination 40° ± 10°; anteversion 15° ± 10°
- Stem anteversion ~15°
- However, studies show that dislocations still occur even when implants are placed within these safe zones
- Recent publications suggest alternative positioning methods, such as the Combined anteversion concept proposed by Dorr et al. in 2009. Target is 35° ± 10°
- Without navigation, this can be assessed intraoperatively using the Ranawat Sign, though less accurately. Patient must be in lateral position with hip stabilized
- This assessment is performed by placing the operated leg in extension and internally rotating from neutral foot position until the base of the femoral head is parallel to the acetabular component
- This angle represents the combined anteversion angle. Target is 35° ± 10°
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- Patient – Femoral neck fracture, prior spine fusion, neuromuscular conditions, alcoholism, spinopelvic abnormalities, High ASA (more comorbidities)
- Implant – Small head diameter, use of skirt, lipped liner
- Surgeon – Approach (NJR shows no differences, but posterior approach requires soft tissue repair), malposition, osteophyte impingement, failure to restore offset, non-union of greater trochanter fracture causing poor soft tissue tension
- Pathology for THR - Femoral neck fracture carries higher risk of dislocation
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- Malkani 2019 in Arthroplasty –
- Spine fusion before THR (even up to 5 years) leads to higher risk of dislocation (50% more)
- Buckland et al. JBJS 2017
- THA after spinal fusion: Risk increases with each level of fusion
- Control group: 1.55% dislocation rate
- 1-2 fusion levels: 2.96%
- 3-7 fusion levels: 4.12%
- THR before fusion - lower risk
- Reasons may include decreasing pelvic mobility in a stable, well-healed THA or early postoperative spine precautions after lumbar spinal fusion restricting positions that could cause dislocation.
- Consider using Dual mobility cups in THR patients with prior fusion.
Malkani 2019.pdf634.8 KiB
Buckland 2017.pdf1.7 MiB
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- When transitioning from standing to sitting, lumbar lordosis decreases and the pelvis should tilt posteriorly (reducing sacral slope and increasing pelvic tilt) to accommodate femoral flexion
- If the spinopelvic junction is fused (surgically or biologically), the pelvis has minimal movement and the acetabular cup becomes "fixed in space"
- As a result, the femur may impinge anteriorly and subsequently dislocate posteriorly.
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7340719/
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- Head diameter corresponds to:
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- Def: Range between impingement
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- It is proportional to the radius of the head (but not the radius itself)
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- The head–neck ratio is affected by femoral head size, femoral neck geometry, and the use of a skirt on the femoral head.
- Skirt is used to extend the neck length when the plus head is still insufficient
- Trapezoidal shaped neck gives a better head-neck ratio
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- Chamfer geometry of the PE cup rim
- Presence of extended-rim hooded liner (if placed incorrectly)
- Too horizontal/closed/too low anteversion
- Failing to remove acetabular osteophytes that impinge against the neck
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- ATLS, ensure isolated injury, reduce with Allis maneuver under sedation
- If unsuccessful, consider bringing patient to OT for reduction under GA and paralysis
- Temporary immobilization with abduction pillow/knee immobilizer
- Assess risk factors TRO infection
- Need to rule out infection in history, bloods (FBC, CRP, ESR)
- Assess surgical notes – approach and repair of capsule, elevated rim liner?
- Interval XR to assess for osteolysis, PE wear (superior migration of head)
- First time or recurrent? Interval symptom-free period?
- MOI – significant trauma?
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- Do not rush into open reduction
- Investigate with advanced imaging, rule out infection
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- Conservative management
- Consider abduction bracing
- Address any obvious, treatable cause (e.g., greater trochanter non-union)
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- Up to 2/3 of patients do not need surgical revision
- In this paper by Fraser in 1981, incidence was 0.63% with 92 patients out of 14,000 THR procedures. Only 16 dislocated more than once and required revision
fraser1981.pdf537.6KB
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- CT scan to measure implant positioning - with specific instruction to include cuts at distal femur to reference the distal femoral intercondylar axis
- Acetabular component - inclination, anteversion
- Femoral component - anteversion, offset
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- Correct alignment of femur and acetabulum
- Correct soft tissue tension
- Fix GT fracture if present
- Consider GT advancement
- Remove impinging osteophytes
- Correct offset
- Change implants as required:
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- Femoral head that moves within a PE component, which also moves within a shell
- Consider in patients with prior spine fusion or dislocating THR
- Advantages: Greater ROM and stability, smaller head causes less wear
- ➖ Higher constraint = increased wear
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- Head locked in cup
- Higher constraint = increased wear
- Miller's Course options:
- 1. Most cases addressed with 36mm head
- 2. Dual mobility
- 2017 Hip Society Otto Aufranc Award – Compared to 40mm head, dual mobility cup showed lower risk of dislocation, revision, and reoperation
- 3. Constrained head rarely used
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- Irradiated cancer cases → hip abductors do not function
- Previous hip fusion take down → Hip abductors often do not function well
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- This is a complex revision case
- Pre-operative planning
- Thorough templating
- Ensure all necessary equipment is on standby
- Intraoperative approach
- Use extensile posterior approach
- Correct component alignment
- Replace implants as needed
- Post-operative care follows standard THR protocol
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- Bipolar conversion - maximizes head/neck ratio
- Drawbacks: groin pain and medial migration
- Girdlestone excision arthroplasty
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- Non-operative treatment is much less successful for dislocated bipolar hemiarthroplasties compared to THRs
- In paper by Gill et al., Single closed reduction of a dislocated hemiarthroplasty was only successful in 16% of the patients
- Timing characteristics:
- 70% occur in first month, 98% within first 3 months
- Only 16% successfully treated with first closed reduction
- 10% required successful open reduction
Gill 2018.pdf765 KiB
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- Smaller center edge angle with CEA < 45.4° or acetabular depth of 19.12mm more likely to suffer dislocation
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6883609/
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- Treat as PJI — approach depends on patient comorbidities
- Single-stage option - conversion to THR with dual mobility consideration (SOA Pang Hee Nee)
- Step 1: Remove components and thoroughly debride
- Step 2: Change surgical equipment and insert new components
- Two-stage revision option
- Salvage options:
- Girdlestone excision arthroplasty
- Reaming of acetabulum with bipolar reinsertion
- Conversion to THR
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- Replace liner if implant positioning is reasonable
- Perform revision THR if alignment is poor
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Whoever diligently seeks good seeks favor, but evil comes to him who searches for it. Proverbs 11:27