MYOSITIS OSSIFICANS
Heterotopic Bone in Muscle | Zoning Phenomenon | Wait for Maturity | Surgical Excision
MYOSITIS OSSIFICANS TYPES
Critical Must-Knows
- Zoning phenomenon: Mature bone peripherally, immature centrally - pathognomonic on X-ray
- Wait for maturity: Minimum 6 months, ideally 12 months before excision
- Maturity assessment: Bone scan (cold), ALP normal, corticated margins on X-ray
- Never excise immature: High recurrence rate (50%+) if excised before maturity
- Post-excision prophylaxis: Indomethacin or radiation to prevent recurrence
Examiner's Pearls
- "Zoning phenomenon (mature periphery, immature center) is pathognomonic on X-ray
- "Wait minimum 6 months for maturity - bone scan must be cold before excision
- "Never excise immature myositis ossificans - high recurrence rate
- "Post-excision prophylaxis (indomethacin or radiation) reduces recurrence from 50% to 10-20%
Clinical Imaging
Imaging Gallery
Critical Myositis Ossificans Exam Points
Zoning Phenomenon
Mature bone peripherally, immature centrally - this is pathognomonic on X-ray and distinguishes myositis ossificans from osteosarcoma (which has immature periphery, mature center). This is a key diagnostic feature.
Wait for Maturity
Never excise immature myositis ossificans - high recurrence rate (50%+). Wait minimum 6 months, ideally 12 months. Confirm maturity with bone scan (cold), ALP normal, corticated margins on X-ray.
Maturity Assessment
Bone scan must be cold (no uptake) before excision. ALP should be normal. X-ray shows corticated margins. If any of these suggest immaturity, wait longer. Early excision leads to high recurrence.
Post-Excision Prophylaxis
Prophylaxis is mandatory after excision - indomethacin 75mg daily for 6 weeks OR radiation 700cGy within 72 hours. Without prophylaxis, recurrence is 50%+. With prophylaxis, 10-20%.
Myositis Ossificans - Quick Reference
| Timing | Radiographic Finding | Management | Recurrence Risk |
|---|---|---|---|
| 3-4 weeks | Appears on X-ray | Conservative, NSAIDs if early | N/A |
| 3-6 months | Zoning phenomenon | Wait for maturity | High if excised |
| 6-12 months | Mature (cold scan) | Excision if symptomatic | 10-20% with prophylaxis |
ZONEMyositis Ossificans Features
Memory Hook:ZONE: Zoning phenomenon (mature periphery), Ossification in muscle, Never excise immature, Excision when mature!
CABMaturity Assessment
Memory Hook:CAB: Cold bone scan, ALP normal, Bone scan confirms maturity!
PROPHYLAXISPrevention of Recurrence
Memory Hook:PROPHYLAXIS: Post-excision mandatory, Recurrence high without, Only when mature, Prophylaxis reduces, Hemostasis meticulous, Yield to maturity, Long wait, ALP check, X-ray corticated, Indomethacin or radiation, Surgical excision!
Overview and Epidemiology
Myositis ossificans is heterotopic bone formation within muscle, typically occurring after trauma or in association with neurological conditions. The term "myositis" is a misnomer - there is no inflammation, and the bone forms in muscle, not from muscle.
Historical context:
- First described in 1883
- Originally thought to be inflammatory
- Now recognized as heterotopic ossification
- "Zoning phenomenon" described as pathognomonic
Epidemiology:
- Most common after: Quadriceps contusions, elbow trauma, hip trauma
- Incidence: 9-17% of severe quadriceps contusions
- Peak age: Young adults (20-40 years)
- Male:Female ratio: 2:1
- Risk factors: Severe trauma, delayed treatment, early aggressive therapy
The Zoning Phenomenon
Zoning phenomenon is pathognomonic for myositis ossificans: Mature bone peripherally, immature bone centrally. This distinguishes it from osteosarcoma (which has immature periphery, mature center). This is a key diagnostic feature on X-ray.
Anatomy and Pathophysiology
Pathophysiology: Myositis ossificans results from:
- Muscle trauma (contusion, hematoma)
- Hematoma organization into fibrous tissue
- Metaplasia of fibroblasts to osteoblasts
- Heterotopic bone formation within muscle
- Maturation process over 6-12 months
Zoning Phenomenon:
- Peripheral zone: Mature lamellar bone (forms first)
- Central zone: Immature woven bone (forms later)
- Pathognomonic on X-ray
- Distinguishes from osteosarcoma (reverse pattern)
Maturation Process:
Maturation Timeline
| Timeline | Radiographic Finding | Bone Scan | ALP |
|---|---|---|---|
| 3-4 weeks | Appears on X-ray | Hot (increased uptake) | Elevated |
| 3-6 months | Zoning phenomenon | Warm (decreasing) | Normalizing |
| 6-12 months | Corticated margins | Cold (no uptake) | Normal |
Key Pathophysiological Concept:
- Bone forms from outside in (periphery first)
- This is opposite to osteosarcoma (center first)
- Maturation takes 6-12 months
- Early excision disrupts this process and causes recurrence
Never Excise Immature
Never excise immature myositis ossificans - recurrence rate is 50%+ if excised before maturity. Wait minimum 6 months, ideally 12 months. Confirm maturity with bone scan (cold), ALP normal, corticated margins on X-ray. Early excision is the most common error.
Classification Systems
Myositis Ossificans Types
Myositis Ossificans Classification
| Type | Cause | Features | Treatment |
|---|---|---|---|
| Traumatic (MOT) | After trauma/contusion | Most common, single site | Wait for maturity, excise if symptomatic |
| Neurogenic | After head injury/SCI | Multiple sites, bilateral | Longer maturation, higher recurrence |
| Progressive (FOP) | Genetic, rare | Progressive, multiple sites | Not surgical - different entity |
Traumatic (Myositis Ossificans Traumatica - MOT):
- Most common type
- After muscle contusion or trauma
- Single site typically
- Best prognosis
Neurogenic:
- After head injury or spinal cord injury
- Multiple sites, often bilateral
- Longer maturation (12+ months)
- Higher recurrence risk
Progressive (Fibrodysplasia Ossificans Progressiva - FOP):
- Rare genetic condition
- Progressive, multiple sites
- Not surgical - different entity
- Excision contraindicated (worsens condition)
Traumatic myositis ossificans is the most commonly encountered type in clinical practice.
Clinical Assessment
History:
- Recent trauma or contusion (weeks to months)
- Quadriceps contusion (most common)
- Elbow trauma
- Hip trauma
- Head injury or spinal cord injury (neurogenic)
Physical Examination:
Early (3-4 weeks)
- Persistent pain beyond expected
- Palpable firm mass in muscle
- Decreased ROM
- Pain with activity
- Swelling may persist
Established (3-6 months)
- Firm, fixed mass
- ROM limitation
- Pain with stretch
- May cause functional impairment
- X-ray shows zoning phenomenon
Key Clinical Signs:
- Palpable mass: Firm, fixed within muscle
- ROM limitation: Out of proportion to injury
- Pain with stretch: Aggravated by passive stretch
- Functional impairment: May limit activities
Clinical Suspicion
Persistent pain and ROM limitation beyond expected recovery after muscle contusion should raise suspicion for myositis ossificans. X-ray at 3-4 weeks will show the lesion. Zoning phenomenon confirms the diagnosis.
Investigations
Radiographs:
- Appears at 3-4 weeks after injury
- Zoning phenomenon: Mature bone peripherally, immature centrally
- Pathognomonic - distinguishes from osteosarcoma
- Maturation: Corticated margins develop over 6-12 months
Three-Phase Bone Scan:
- Early (immature): Hot (increased uptake)
- Mature: Cold (no uptake)
- Most important for maturity assessment
- Must be cold before excision
CT Scan:
- Shows zoning phenomenon clearly
- Useful for surgical planning
- Assesses relationship to neurovascular structures
- 3D reconstruction helpful
MRI:
- Shows soft tissue involvement
- May help distinguish from tumor
- Not diagnostic but supportive
- Shows muscle edema early
Laboratory:
- ALP (alkaline phosphatase): Elevated early, normalizes with maturity
- Normal ALP confirms maturity
- Monitor serially if uncertain
Bone Scan is Key
Bone scan must be cold (no uptake) before excision. This is the most important test for maturity assessment. If bone scan is still hot, the lesion is immature and excision will have high recurrence rate (50%+). Wait until bone scan is cold.
Management Algorithm
Conservative Management
Indications: Lesion less than 6 months old, bone scan hot, ALP elevated
Management:
- NSAIDs: May limit progression if caught early
- Physiotherapy: Gentle ROM, avoid aggressive stretching
- Monitor: Serial X-rays, bone scan, ALP
- Wait: Do not excise - high recurrence if immature
Key: Wait for maturity. Never excise immature lesion.
Surgical Technique
Note: Surgical excision is only performed when the lesion is mature and symptomatic. This section describes the technique for excising mature myositis ossificans.
Pre-operative Assessment
Confirm Maturity:
- Bone scan: Cold (no uptake) - most important
- ALP: Normal
- X-ray: Corticated margins, stable size
- Timeline: Minimum 6 months, ideally 12 months
Imaging:
- CT scan: 3D reconstruction, assess extent
- Relationship to neurovascular structures
- Plan approach
- Identify margins
Prophylaxis Plan:
- Indomethacin 75mg daily for 6 weeks (start post-op day 1)
- OR Radiation 700cGy within 72 hours
- Both equally effective
- Mandatory to prevent recurrence
Maturity confirmation is essential before proceeding.
Complications
Myositis Ossificans Complications
| Complication | Cause | Prevention/Management |
|---|---|---|
| Recurrence | Excision before maturity | Wait for maturity, confirm with bone scan |
| Recurrence | No post-excision prophylaxis | Mandatory prophylaxis (indomethacin or radiation) |
| Nerve injury | During excision | Careful technique, identify nerves early |
| Infection | Surgical site | Antibiotics, meticulous technique |
| Hematoma | Inadequate hemostasis | Meticulous hemostasis, drain |
| Functional limitation | ROM loss | Physiotherapy, early mobilization |
Recurrence:
- Most common complication
- 50%+ if excised before maturity
- 10-20% if mature with prophylaxis
- 50%+ if mature without prophylaxis
- Prevention: Wait for maturity, use prophylaxis
Nerve Injury:
- Risk during excision
- Identify nerves early
- Careful dissection
- May require neurolysis
Recurrence Prevention
Recurrence is the most common complication. Wait for maturity (bone scan cold) and use post-excision prophylaxis (indomethacin or radiation) to reduce recurrence from 50%+ to 10-20%. These are the two most important prevention strategies.
Postoperative Care
After Excision:
Post-Excision Protocol
- Start prophylaxis (indomethacin day 1 OR radiation within 72 hours)
- Pain management
- Elevation if limb
- Monitor neurovascular status
- Continue prophylaxis
- Begin gentle ROM exercises
- Physiotherapy consultation
- Monitor wound healing
- Continue prophylaxis (total 6 weeks if indomethacin)
- Active ROM exercises
- Strengthening as tolerated
- Functional restoration
- Serial X-rays to assess for recurrence
- Clinical assessment
- ROM measurement
- Functional assessment
Prophylaxis:
- Indomethacin: 75mg daily for 6 weeks (start post-op day 1)
- OR Radiation: 700cGy single dose within 72 hours
- Both equally effective
- Mandatory - reduces recurrence from 50%+ to 10-20%
Outcomes and Prognosis
Recurrence Rates:
Recurrence Rates by Timing and Prophylaxis
| Timing | Prophylaxis | Recurrence Rate |
|---|---|---|
| Immature excision | Any | 50%+ |
| Mature excision | None | 50%+ |
| Mature excision | Prophylaxis | 10-20% |
Prognostic Factors:
- Maturity: Mature lesions have lower recurrence
- Prophylaxis: Dramatically reduces recurrence
- Complete excision: Incomplete excision increases recurrence
- Hemostasis: Hematoma promotes recurrence
- Location: Some locations have higher recurrence
Functional Outcomes:
- Most patients have good functional recovery
- ROM improvement expected
- Pain relief usually achieved
- Recurrence may require repeat surgery
Evidence Base
Myositis Ossificans Traumatica
- Zoning phenomenon pathognomonic
- Mature bone peripherally, immature centrally
- Distinguishes from osteosarcoma
- Wait for maturity before excision
Surgical Excision of Myositis Ossificans
- Recurrence 50%+ if excised before maturity
- Recurrence 10-20% if mature with prophylaxis
- Wait minimum 6 months
- Bone scan must be cold
Post-Excision Prophylaxis
- Indomethacin reduces recurrence
- Radiation equally effective
- Prophylaxis mandatory after excision
- Reduces recurrence from 50%+ to 10-20%
Maturity Assessment
- Bone scan is most important for maturity
- Cold scan indicates maturity
- ALP normalizes with maturity
- Corticated margins on X-ray
Quadriceps Contusion and Myositis Ossificans
- 9-17% of severe quadriceps contusions develop MO
- Risk factors: severe grade, delayed treatment
- Early aggressive therapy increases risk
- Conservative management initially
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
Scenario 1: Immature Lesion
"A 25-year-old athlete presents 3 months after a severe quadriceps contusion. X-ray shows a lesion with zoning phenomenon (mature periphery, immature center). Bone scan is still hot. The patient wants it removed because it's causing pain and limiting ROM. How do you manage this?"
Scenario 2: Mature Lesion Excision
"A 30-year-old man has a mature myositis ossificans in the quadriceps (12 months old, bone scan cold, ALP normal, corticated margins). It's causing significant pain and limiting knee flexion to 90 degrees. Describe your surgical approach and post-operative management."
MCQ Practice Points
Zoning Phenomenon Question
Q: What is the zoning phenomenon in myositis ossificans? A: Mature bone peripherally, immature bone centrally - this is pathognomonic on X-ray and distinguishes myositis ossificans from osteosarcoma (which has immature periphery, mature center). This is a key diagnostic feature.
Maturity Assessment Question
Q: How do you assess maturity of myositis ossificans before excision? A: Bone scan must be cold (no uptake), ALP normal, X-ray shows corticated margins, minimum 6 months old (ideally 12 months). Bone scan is the most important test. Never excise if bone scan is still hot - recurrence is 50%+.
Recurrence Prevention Question
Q: How do you prevent recurrence after excision of myositis ossificans? A: Post-excision prophylaxis is mandatory - indomethacin 75mg daily for 6 weeks OR radiation 700cGy within 72 hours. This reduces recurrence from 50%+ to 10-20%. Both modalities equally effective. Never excise without prophylaxis.
Timing Question
Q: When is it safe to excise myositis ossificans? A: Minimum 6 months after formation, ideally 12 months, when bone scan is cold, ALP is normal, and X-ray shows corticated margins. Never excise immature lesion - recurrence is 50%+.
Recurrence Rate Question
Q: What is the recurrence rate after excision of mature myositis ossificans with prophylaxis? A: 10-20% with prophylaxis, 50%+ without prophylaxis - this is why prophylaxis is mandatory. Recurrence is also 50%+ if excised before maturity, regardless of prophylaxis.
Australian Context and Medicolegal Considerations
Healthcare System:
- Myositis ossificans management available in major hospitals
- Bone scan services available
- Surgical excision available
- Radiation therapy available for prophylaxis
Medicolegal Considerations:
- Documentation: Timing of injury, radiographic findings, maturity assessment, treatment decisions
- Maturity confirmation: Document bone scan results, ALP levels, X-ray findings
- Prophylaxis: Document that prophylaxis was used and type
- Timing: Document that minimum 6 months waited, maturity confirmed
- Communication: Clear communication with patient about need to wait for maturity
Common Issues:
- Excising immature lesion (high recurrence)
- Not using prophylaxis (high recurrence)
- Not confirming maturity before surgery
- Poor documentation of maturity assessment
Medicolegal Risk
Excising immature myositis ossificans leads to high recurrence (50%+) and poor outcomes. Always confirm maturity (bone scan cold, ALP normal, corticated margins) before excision. Document all maturity assessments. Use post-excision prophylaxis.
MYOSITIS OSSIFICANS
High-Yield Exam Summary
Key Facts
- •Heterotopic bone formation in muscle
- •Zoning phenomenon pathognomonic (mature periphery, immature center)
- •Appears on X-ray at 3-4 weeks
- •Matures over 6-12 months
Maturity Assessment (CAB)
- •Cold bone scan (no uptake) - most important
- •ALP normal (alkaline phosphatase)
- •Bone scan confirms maturity
- •Corticated margins on X-ray
- •Minimum 6 months, ideally 12 months
Recurrence Prevention
- •Never excise immature (50%+ recurrence)
- •Wait for maturity (bone scan cold)
- •Post-excision prophylaxis mandatory
- •Indomethacin 75mg daily for 6 weeks OR radiation 700cGy within 72 hours
- •Recurrence: 10-20% with prophylaxis, 50%+ without
Zoning Phenomenon
- •Mature bone peripherally, immature centrally
- •Pathognomonic on X-ray
- •Distinguishes from osteosarcoma (reverse pattern)
- •Forms from outside in (periphery first)