Definition
Migration of the femoral head past the medial wall of the acetabulum / ilioischial line
Centre edge angle > 40o
Aetiology
Primary
Otto's Disease
- bilateral in one third
- middle aged females
- pain & decreased ROM early
- coxa vara & OA common
- ? causally related to osteomalacia
Secondary
PROFSHAMN
- Paget's
- RA
- osteomalacia / OI
- fracture / central dislocation
- septic arthritis especially TB
- hemiarthroplasty
- Ankylosing Spondylitis
- Marfan's syndrome, malignancy
- Neurofibromatosis
Charnley classification 1978
Defined medial wall of acetabulum as ilioischial line
Grade I 1-5mm Grade II 6-15 mm Grade III >15 mm
Eldstein & Murphy 1983
Medial wall is acetabular line & ilio-ischial line
- men acetabular line 2mm lateral to ilioischial line
- women 1mm medial to ilio-ischial line is normal
| Grade | Men | Women | |
| I | 3 - 8 mm | 6 - 11 mm | |
| II | 8 - 13 mm | 12 - 17 mm | |
| III | > 13 mm | > 17 mm with fragmentation |
NHx
Inexorable progression of deformity
- axis of migration is same direction as joint reaction force in stance phase
Management
Medical Workup
Identify and treat any underlying cause
- FBC, ESR, RF, ANA, ELFT, Ca
Options
A. Skeletally immature
Triradiate fusion
- can combine with valgising osteotomy
Steel et al JPO 1996
- 22 patients with Marfan's syndrome
- 12 of 19 restored to normal
- 4 improved
- 3 unchanged
B. Young adult
Valgising intertrochanteric femoral osteotomy (VITO)
- patient < 40, minimal OA
- may delay THR for 10 years
Aim for 20-30° valgus correction
- if neck shaft angle is 130° aim for 155°
- trapezoid shortening to minimise LLD
Lateralization of femur to restore mechanical alignment
Require soft tissue release especially psoas
C. Middle aged / elderly
THR
THR Protrusio
Principle
Place hip center anatomically
Restore joint biomechanics
- outcome depends on cup position
- adequacy of correction of the deformity & biomechanics correlates with long-term prosthetic survival
- medial joint positioning leads to high medial stresses
Results
Ranawat JBJS Am 1980
- 35 hips with protrusio secondary to RA
- 16 of 17 THR >10 mm from hip centre loosened
- 13 THR with <5 mm out good survival
Determine Hip Centre
1. Teardrop
- average 2 cm vertical & 4 cm horizontal from teardrop
- average coordinates reported in normal adults 14 mm vertical & 37 mm horizontal
2. Ranawat Method
Draw parallel horizontal lines at the levels of the iliac crests and ischial tuberosity and mark 3 points
- Point 1: 5mm lateral to intersection of Shenton's and Kohler's lines
- Point 2: located superior to point 1 by a distance 1/5 of the pelvic height
- Point 3: similar distance horizontally from vertical line
Isosceles triangle between 1/2/3 locates the acetabulum
- line 2/3 through subchondral bone
Management Bone Defects
1. Assess medial wall integrity with CT
2. Algorithm / Ranawat J Arthroplasty 1986
A. < 5mm - no graft required
B. > 5mm but medial wall intact - morcellised bone graft
C. No medial wall - mesh / cage + morcellised bone graft
Technique
Preoperative
- template LLD (max 4cm)
- define acetabular defect with CT
- ensure intact medially
Approach
1. Sciatic nerve is nearer the joint than normal
- identify and protect early
2. Dislocation of the hip can be difficult
- femoral osteotomy in situ + femoral head removal piecemeal may be required
- trochanteric osteotomy may be required for exposure
Reaming
- enlarge rim only
- avoid creating peripheral defect
Contained acetabular defect
Morcellised bone graft
- rim fit uncemented cup
- cemented cup
Uncontained acetabular defect
A. Wire mesh / bone gaft / cemented cup
B. Wafer bone graft / morcellised bone graft / cage / cemented cup