Heterotopic Ossification vs Myositis Ossificans: Diagnostic and Treatment Approach
Heterotopic ossification (HO) and myositis ossificans (MO) are essentially the same pathologic process—abnormal formation of mature lamellar bone within soft tissues where bone does not normally exist—with MO being a specific subtype of HO that occurs following trauma. 1, 2
Understanding the Terminology
Heterotopic ossification is the broader term encompassing all non-neoplastic bone formation in soft tissues, classified into posttraumatic, neurogenic (non-traumatic), and the rare genetic form fibrodysplasia ossificans progressiva (FOP). 1
Myositis ossificans specifically refers to the posttraumatic form of HO, also called myositis ossificans traumatica, where osseous and cartilaginous elements develop within injured soft tissue. 3, 4
The new bone forms outside the joint capsule without disrupting it and can be contiguous with the skeleton but generally does not involve the periosteum. 1
Diagnostic Imaging Algorithm
Start with plain radiographs immediately—they are mandatory as the first-line study to identify the presence of ossified tissue and distinguish it from other pathology. 5, 6, 7
Initial Radiographic Assessment
Radiographs may show characteristic features such as peripherally more mature ossification in myositis ossificans, which helps differentiate it from malignant processes. 5
Compare with the contralateral asymptomatic side when findings are subtle. 7
Radiographs can identify calcification (present in 27% of soft tissue masses), bone involvement (22%), and intrinsic fat (11%). 5
Advanced Imaging When Radiographs Are Insufficient
CT is superior to all other modalities for visualizing and quantifying established heterotopic bone formation and should be used to define the extent and maturity of ossification. 6, 7
MRI is less sensitive than radiographs for detecting heterotopic ossification/calcification but may be useful for assessing soft tissue involvement and excluding differential diagnoses. 6, 5
Three-phase technetium-99m bone scan is the most sensitive imaging modality for early detection and assessing the maturity of HO, which is critical for timing surgical intervention. 1, 2
Timing of Imaging
Initial imaging can detect HO as early as 7 days to 36 months after surgery or trauma (mean 6.7 months). 4
Serial quantitative bone scans are used to determine lesion maturity before considering surgical resection. 2
Treatment Algorithm
Conservative Management (First-Line)
During the acute inflammatory stage, rest the involved joint in a functional position; once acute inflammatory signs subside, initiate passive range of motion exercises and continued mobilization. 1
NSAIDs (particularly indomethacin) and gentle range-of-motion exercises are recommended for established HO. 7, 1
Indomethacin and radiation therapy are appropriate for prophylaxis or early treatment of HO. 1
Bisphosphonates (such as ethane-1-hydroxy-1-diphosphate or disodium etidronate) are effective prophylaxis if initiated shortly after trauma, though mineralization resumes after drug discontinuation. 1, 3
Surgical Intervention
Delay surgery for 12 to 18 months until radiographic evidence of HO maturation and maximal recovery after neurological injury. 1
Surgical Indications
Improvement of function, standing posture, sitting or ambulation, independent dressing, feeding and hygiene, or repeated pressure sores from underlying bone mass. 1
Mechanical symptoms such as locking or catching from loose bodies. 8, 7
Criteria for Earlier Surgery (Before 18 Months)
The ideal candidate must have ALL of the following: 1
- No joint pain or swelling
- Normal alkaline phosphatase level
- Three-phase bone scan indicating mature HO
Post-Surgical Considerations
HO is likely to recur and possibly progress if resection is undertaken before the lesion has matured. 2
Prophylactic measures (NSAIDs, radiation, or bisphosphonates) should be considered to prevent recurrence. 1, 2
Critical Differential Diagnosis
Fibrodysplasia Ossificans Progressiva (FOP)
Surgical excision is absolutely contraindicated in FOP as it triggers more extensive ossification. 5, 6
FOP is characterized by congenital malformations of the great toes and progressive heterotopic endochondral ossification in characteristic anatomic patterns (axial > appendicular; dorsal > ventral; proximal > distal). 5
Consider FOP when evaluating suspected "masses" in toes. 5
Other Conditions to Exclude
Malignant bone tumors: Look for unexplained weight loss, solitary lesion with quick growth, cortical destruction, or perpendicular periosteal new bone formation. 5
Infectious osteomyelitis: Consider with systemic symptoms (fever, chills), presumable port of entry, solitary lesion, significantly elevated CRP/ESR, or bacteremia. 5
Calcific tendonitis, loose bodies, or septic arthritis in the acute setting with swelling. 7
Common Pitfalls to Avoid
Do not order MRI before plain radiographs—MRI is less sensitive for calcifications and ossification. 6, 7
Do not perform surgery before confirming lesion maturity with serial bone scans and normalization of alkaline phosphatase. 1, 2
Do not biopsy suspected FOP lesions—this will trigger catastrophic progression of ossification. 5, 6
Failing to exclude septic arthritis when swelling is acute with systemic signs can lead to delayed treatment of infection. 7
Complete immobilization leads to muscle atrophy and joint stiffness—gentle mobilization should begin once acute inflammation subsides. 7, 1