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Introduction
  • Formation of bone in atypical, extraskeletal tissues
    • usually occurs
      • spontaneously or following trauma
      • within 2 months of neurologic injury (brain or spinal cord)
      • following THA and TKA
    • most common location is between muscle and joint capsule
  • Epidemiology
    • incidence
      • (see table below)
    • demographics
      • male:female = 2:1
      • especially men with hypertrophic osteoarthritis, and women >65y
    • location
      • traumatic brain injury or stroke
        • hip > elbow > shoulder > knee
        • elbow HO more common following brain trauma  
        • occurs on affected (spastic) side
        • rarely in the knee (TBI)
      • spinal cord injury 
        • hip > knee > elbow > shoulder
        • hip flexors and abductors > extensors or adductors
        • medial aspect of the knee
    • risk factors 
      • (see table below)
  • Pathophysiology
    • exact cause of HO is not known but there appears to be a genetic disposition
    • experimental HO associated with
      • tissue expression of BMP
  • Associated conditions
    • orthopaedic manifestations
      • pathologic fractures
        • from decreased joint ROM and osteoporotic bone
      • nerve impingement
      • soft tissue contractures, contributing to the formation of decubitus ulcers
      • CRPS (more common in patients with HO)
      • joint ankylosis
      • HO after THA adversely affects outcome of THA
    • nonorthopaedic conditions
      • skin maceration and hygiene problems
Risk Factors for Heterotopic Ossification
Injury severity score (ISS) q   High ISS is a risk 11%
Traumatic brain injury (TBI) Higher incidence in the spastic limbs of the patient      11%
Spinal cord injury Complete SCI produces more HO than incomplete SCI. Cervical and thoracic SCI produces more HO than lumbar SCI. Younger age produces more HO (20-30yo). Higher incidence in the spastic limbs of the patient.   20%
Neurologic compromise Prolonged coma in young patient (20-30yo), and prolonged ventilator use  
Other diseases
DISH, ankylosing spondylitis, hypertrophic osteoarthritis (prominent osteophytes)
 
Decubitus ulcers Worse with concomitant decubitus ulcers and SCI or TBI 70% (with concomitant SCI)
Antegrade femoral nail entry site Worse with piriformis fossa entry point. 25%
Distal femur traction pins  HO in distal quadriceps. Higher incidence in patients with other concomitant injuries, use of large diameter Steinmann pins (5mm) because of hematoma, soft tissue injury from percutaneous insertion. rare
Amputation through zone of injury   Worse with blast mechanism  63%
Surgical approaches Extended iliofemoral > Kocher-Langenbeck > ilioinguinal approach (acetabular fracture). Anterior approach > posterior approach for femoral head fracture fixation.  25% (acetabular fracture fixation)
Total hip arthroplasty THA Complications  Increased risk with psoas tenotomy and cementless components (more particulate debris and marrow spillage, muscle trauma from difficult broaching). Smith-Petersen and Hardinge > transtrochanteric > posterior (posterior has lowest risk of HO). 53% (significant in only 5%)
Total knee arthroplasty 

TKA complications  Increased risk with notching anterior femur, surgical trauma to quadriceps, distal femur exposure and periosteal stripping, and postop manipulation under anesthesia, and high lumbar BMD

 
 
Classification
  • Subtypes
    • neurogenic HO (discussed here)
    • traumatic myositis ossificans 
    • fibrodysplasia ossificans progressiva (Munchmeyer's Disease)
Presentation
  • Symptoms
    • painless loss of ROM
    • interferes with ADL
    • CRPS symptoms
    • fever
  • Physical exam
    • inspection
      • warm, painful, swollen joint
      • may have effusion
      • skin problems
        • decubitus ulcers
          • from contractures around skin, muscles, ligaments 
        • skin maceration and hygiene problems
    • motion
      • decreased joint ROM
      • joint ankylosis
      • with HO after TKA, might develop quad muscle snapping or patella instability
    • neurovascular
      • peripheral neuropathy
        • HO often impinges on adjacent NV structures
Imaging
  • Radiographs 
    • recommended views
      • Judet view valuable for evaluation of hip HO
    • findings
      • ossification usually easy to visualize  
      • maturity of HO 
        • the appearance of a bony cortex suggests mature HO
        • sharp demarcation from surrounding tissue
        • trabecular pattern
    • sensitivity and specificity
      • not useful for early diagnosis
      • only useful at 1 week after onset of symptoms
        • calcium is deposited 7-10 days later than symptom onset
  • Ultrasound 
    • indications
      • for early diagnosis of hip HO  
    • findings
      • echogenic surfaces with posterior acoustic shadowing
  • CT 
    • indications
      • useful for preoperative planning 
  • Triphasic bone scan 
    • indications
      • best for early diagnosis  
      • most commonly used diagnostic study
Studies
  • Labs
    • elevated serum alkaline phosphatase (>250IU/L)
      • ALP removes inhibitors of mineralization
      • nonspecific, may be elevated with skeletal trauma
      • cannot determine maturity of HO
      • elevated 12wks after surgery is predictor
    • elevated CRP
      • correlates with inflammatory activity of HO better than ESR
      • normalization of CRP may correlate with maturity of HO
    • elevated ESR (>35mm/h)
      • 12wks after THA is predictor
    • elevated CK
      • correlates with involvement of muscle, extent of muscle involvement
  • Histology
    • mature fatty bone marrow
    • mature trabecular bone
Treatment
  • Prophylaxis
    • bisphosphonates & NSAIDS
      • indications
        • although no literature supports, are commonly used
      • technique
        • indomethacin is most commonly used
          • dose is 75mg/day for 10days to 6 weeks
    • perioperative radiation
      • indications
        • although no literature supports, commonly used
        • is thought to be effective by blocking osteoblast differentiation
      • technique
        • a single perioperative dose of 700cGy can be given either 4 hours preop or within 72 hours postoperatively 
        • <550cGy not effective
  • Posttraumatic
    • wide exposure and surgical resection 
      • indications
        • severe loss of motion and decreased function
      • technique
        • wide exposure required to identify all neurovascular structures that may be involved
      • timing of resection (controversial)
        • marked decrease in bone scan activity AND normalization of ALP
        • 6 months following general trauma
        • 1 year following SCI
        • 1.5 years following TBI
          • some data suggests equivalent results when comparing early versus late resection
      • postop
        • follow with 5 day course of indomethacin
        • early gentle joint mobilization
  • Arthroplasty
    • treatment for HO following THA 
    • treatment for HO following TKA 
Complications
  • Hematoma and intraoperative bleeding
  • Infection
    • higher rate of infection following joint arthroplasty if HO is present
  • Fractures of osteoporotic bone
    • osteopenic from disuse
    • during surgery or physiotherapy
  • Recurrence
    • recurrence rate correlates with neurological injury
      • greater recurrence if severe neurological compromise
  • AVN 
    • if extensive dissection or stripping is required
IBank
  Location
Xray
Xray
CT
B. Scan
MRI
MRI
Histo(1)

Case A

tibia  
 
 
 
   
 
   
Case B hip              
(1) - histology does not always correspond to case


 

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