summary Heterotopic Ossification is the formation of bone in atypical, extraskeletal tissues that may occur following localized trauma, following a neurological injury, or as a post-surgical complication. Patients typically present with painless loss of motion of the affected joint. Diagnosis is made radiographically with soft tissue ossification with sharp demarcation from surrounding soft tissues. Treatment is focused on prevention with oral indomethacin and perioperative radiation. Surgical excision is indicated in the presence of mature lesions associated with severe loss of motion and function. Epidemiology Incidence (see table below) Demographics male:female = 2:1 especially men with hypertrophic osteoarthritis, and women >65y Anatomic location most common location is between muscle and joint capsule 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) Etiology Forms usually occurs spontaneously or following trauma within 2 months of neurologic injury (brain or spinal cord) following THA and TKA 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) 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 Burns Both locally under burn and remotely. More common with >20% body surface area 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 Higher incidence with 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 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 the lowest risk of HO). For direct anterior approach, less HO risk with "bikini incision" 53% (significant in only 5%) Total knee arthroplasty 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 Other diseases DISH Ankylosing spondylitis Hypertrophic osteoarthritis (prominent osteophytes) 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 Indomethacin indications although no literature supports, are commonly used technique indomethacin is most commonly used dose is 75mg/day for 10 days 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 irradiation 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