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Heterotopic Ossification

- See: Ossification of Soft Tissues: and Myositis Ossificans

- Discussion:
    - occurence and formation of mature bone in non-osseous tissue; 
    - may present w/ signs of localized inflammation or pain, elevated skin temp, ect.
    - tends to occur after thr, spinal injury, head injury (11%), burns, bruises, elbow trauma, total hip replacement;
           - ref: Osteoblasts Have a Neural Origin in Heterotopic Ossification..
    - Events leading to HO;
           - mesenchymal cell (XRT prevents induced differentiation)
           - osteoblast
           - matrix (EHDP)
           - osteocyte
    - HIP HO:
HO following THR:
           - IM Nailing:
                   - Heterotopic ossification about the hip after intramedullary nailing for fractures of the femur.
                   - Heterotopic ossification around the hip with intramedullary nailing of the femur.
           - HO following acetabular fracture  (acetabular frx)
                   - Heterotopic ossification as a complication of acetabular fracture. Prophylaxis with low-dose irradiation.
                   - Heterotopic ossification following operative treatment of acetabular fracture. An analysis of risk factors.
                   - Heterotopic ossification prophylaxis following operative treatment of acetabular fracture.
                   - Extended iliofemoral versus triradiate approaches in management of associated acetabular fractures.
                   - Prophylaxis with indomethacin for heterotopic bone. After open reduction of fractures of the acetabulum.
                   - Heterotopic Ossification following acetabular fixation: Incidence and risk factors: 10-year experience of a tertiary centre.

HO of the elbow:
           - Early excision of heterotopic ossification about the elbow followed by radiation therapy.

    - HO following spinal cord trauma:
           - radiographically develops in 3-5% of patients, 1-4 mo (or upto 18 mo) after injury;
           - it occurs below the level of the injury, usually at major joints;
           - incidence is 40% (1/2 of these are clinically significant);
           - after transection of spinal cord, ossification often takes place in soft tissues adjacent to large joints;
           - lower extremities are particularly suseptible & most dramatic changes are seen in Knee > Pelvis > Hip;
           - characteristic finding is a bilateral accretion of well defined bone which encircle the joints;
                    - Resection of heterotopic ossification in patients with spinal cord injuries.
                    - Resection of heterotopic ossification in the adult with head trauma.
                    - Clinical observations on fractures and heterotopic ossification in the spinal cord and traumatic brain injured populations.
                    - Surgical approaches for resection of heterotopic ossification in traumatic brain-injured adults.
                    - Periarticular heterotopic ossification in head-injured adults. Incidence and location.
                    - Heterotopic ossification around the hip in spinal cord-injured patients. A long-term follow-up study.

- Radiographic Studies:
     - x-rays: soft tissue ossification usually does not appear in 1st mo
     - bone scans may reveal incr isotopic intake by the second weak;

- Labs:
     - alkaline phosphatase activity correlates w/ bone scans;

- Non Operative Treatment:
    - NSAIA
          - Indomethacin 25 mg PO tid for 3-6 months;
          - Prophylaxis with indomethacin for heterotopic ossification after Chiari osteotomy of the pelvis.
    - Etidronate
          - theoretically works by delaying mineralization of osteoid
          - diphosphonates do not prevent heterotopic bone formation in lab animals but they do delay of mineralization of osteoid.
          - delay in mineralization caused by diphosphanates are reversed when the disphosphonates are discontinued;
    - Radiation Therapy:
          - single does of 600 to 800 centgray of radiation given within 24 hours of surgery preoperatively or 72 hours postoperatively;
          - a relative contra-indication of XRT may be posterior hip dislocation w/ femoral head frx, since there is a theoretical risk of
                    contributing to AVN or non-union;
          - cost: $ 2,000 to 2,500;
          - references:
                 - Preoperative irradiation versus the use of nonsteroidal anti-inflammatory drugs for prevention of heterotopic ossification following total hip replacement: the results of a randomized trial.
                 - Radiotherapy vs. nonsteroidal anti-inflammatory drugs for the prevention of heterotopic ossification after major hip procedures: a meta-analysis of randomized trials.
                 - Prevention of heterotopic ossification in high-risk patients by radiation therapy.
                 - The use of radiation to discourage ectopic bone. A nine-year study in surgery about the hip.

- Operative Resection:
    - main disadvantage is risk of recurrence heterotopic ossification;
    - operative timing:
            - if HO has caused the loss of motion, some recommend allowing process to mature (sharp cortical and trabecular markings)
                    before operative resection;
           - some recommend waiting 12 months before operative resection;
           - once serial radiographs have shown that the ossification is mature w/ sharp peripheral edges and no indication of expansion,
                    resection is considered;
           - bone scans and alkaline phosphatase may not be helpful in predicting maturity of the ossification;
    - references:
           - What Risk Factors Predict Recurrence of Heterotopic Ossification After Excision in Combat-related Amputations?
    - in the study by Genet, et al., a consecutive series of 143 patients w/ traumatic brain injury and spinal cord injury;
           - some patients had complete ankylosis, and some had severe limitation of motion;
           - patients with ankylosis had improvement from an average of 0° to 90° of hip flexion but had only 63° of motion;
           - patients without ankylosis had improvement from an average of 38° to 95° of motion, with 83° of motion;
           - authors noted the importance of estimating bone mineral density prior to surgery to assess the risk of intraoperative fracture;
           - delayed surgery can increase in intra-articular pathology and osteoporosis;
           - Impact of late surgical intervention on heterotopic ossification of the hip after traumatic neurological injury.

Botulinum Toxin-induced Muscle Paralysis Inhibits Heterotopic Bone Formation.
A clinical perspective on common forms of acquired heterotopic ossification.
Keeping Bugs Bunny on the Move 

Heterotopic ossification in complex orthopaedic combat wounds: quantification and characterization of osteogenic precursor cell activity in traumatized muscle.

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