Definition
Extraskeletal bone formation in periarticular tissues
- HA crystals within osteoid matrix
Different to calcification
- osteoid matrix laid down
Types
1. Myositis Ossificans
- post traumatic
2. Heterotopic Ossification / associated with TJR
3. Neoplastic Ossification
Epidemiology
Occurs 50-70% THR's
- significant in 20%
- clinically significant in 1%
More common in men x 2
Risk factors
High
- previous hip / other hip HO
- incidence is 80%
Moderate
- hypertrophic OA
- active Ankylosing Spondylitis
- hyperostosis
- DISH
- active Paget's
- fracture (acetabular trauma, pre or intra operative)
Low
- THR
- posterior approach < Hardinge < transtrochanteric
Pathology
Similar cascade to fracture healing
- unknown trigger
- undifferentiated mesenchymal cells differentiate in osteoblasts
- occurs within first few days
- produce osteoid
- mineralised to bone (mature lamellar bone)
Two Precursor cells about the Hip / Friedenstein
1. Determined Osteogenic Progenitor Cells
- from bone marrow
- develop into osteoblasts with inflammation
2. Inducible Osteogenic Progenitor Cells
- need BMP to develop into osteoblasts
Brooker Classification
Only Type IV interferes with function
Type I: Isolated islands of bone
Type II: Bony spurs from pelves and proximal femur, gap > 1 cm
Type III: Gap < 1 cm
Type IV: Apparent ankylosis
Clinical Features
Usually none
- pain usually as it matures
- decreased ROM
- dislocation 2° impingement (rare due to loss of ROM)
- nerve irritation
- trochanteric bursitis
- hip can appear red, swollen and tender
X-ray
New bone in peri prosthetic soft tissues
- visible by 3-6/52
- extent determined by 3/12
Maturation continues for 12-18/12
Bone Scan
Increased uptake = continued activity
- remain increased for 12/12
Bloods
Rise in serum alkaline phosphatase post surgery
- associated with HO
Management
1. Prevention
Identify at patients risk preoperatively
- Ankylosing Spondylitis / Pagets / Previous HO / DISH
A. Surgical Technique
Gentle handling of tissues
- avoid muscle stripping
- lavage tissues
- drain wound
B. NSAID
Indomethacin 50 mg bd for 1 week
- significant reduction in risk of HO
- 7 days as effective as 14 days
- as effective as postoperative radiation
Risk of GIT side-effects ~ 20%
- interaction with anticoagulants
- double risk of significant bleeding with DVT prophylaxis
Results
Knelles JBJS 1997
- 685 Primary THR
- 50mg bd Indocid for 1 week
- as effective as 1 x 7 Gy Post-op
C. DXRT
For very high risk patients
- previous HO / indocid contraindicated because of PUD
- post surgical excision of HO
Dosing
- 700 Rad / 7 Gy < day 5
- 800 Rad 6h pre-op prevents HO
Side effects
1. Delay incorporation of bone graft / union of trochanter
2. May delay porous ingrowth with uncemented components
3. Can make patient nauseous
4. Nil evidence wound problems (shielded, low dose)
5. Risk of malignancy - nil evidence at this low dose
D. Biphosphonates
Delay calcification and delay Xray appearance of bone
- doesn't prevent osteoid formation
- calcification occurs once drug stopped
- no longer used
2. Surgical Excision
Indications
Significant symptoms / reduced ROM & > Brooker III
- revision of prosthesis
Timing
Usually 12-18 / 12 post-operatively
- mature appearance on XR
- cold Bone Scan
- serum ALP normal
Prophylaxis
Radiotherapy post oeratively as high risk
Results
Usually increased ROM
- unreliable effect on pain
- bone often reforms
Case