PATHOLOGICAL FRACTURES (HUMERUS)
Metastatic Disease | Mirels Score | Prophylactic Fixation | Workup First
MIRELS STABILITY SCORE
Critical Must-Knows
- Workup First: Never fix a pathological fracture without knowing the primary (unless impending emergency). Biopsy track might spread tumor.
- BLT with a Kosher Pickle: Breast, Lung, Thyroid, Kidney, Prostate (Most common bone mets).
- Renal/Thyroid tumors: Highly vascular & Radio-resistant. Pre-op embolization often needed.
- Survival Estimate: Fixation should outlast the patient's life expectancy (durable construct).
- Fixation Logic: Fixation spans the whole bone (prevent new fractures). Cement augmentation improves stability.
Examiner's Pearls
- "Always ask about constitutional symptoms (weight loss, night sweats)
- "Solitary lesion greater than 40y is Mets/Myeloma until proven otherwise
- "Solitary lesion less than 20y is Primary Bone Tumor until proven otherwise
- "Do not nail a primary bone sarcoma (contaminates whole compartment)
Critical Exam Points
The Solitary Lesion
Biopsy Trap: If the primary is unknown, DO NOT perform internal fixation. You must stage and biopsy first. An inappropriate nail can upstage a sarcoma to Stage IV or necessitate amputation.
Vascular Tumors
Renal & Thyroid: These metastases are significantly vascular. Pre-operative embolization is mandatory to prevent exsanguination on the table.
Mirels Score
Know it cold: Site, Pain, Lesion Type, Size. This is the gold standard answer for "When to operate prophylactically".
Whole Bone Fixation
Span the Bone: Disease can progress elsewhere. Use a long IM nail or long plate to protect the entire humerus, not just the lesion.
At a Glance - Management Decision
| Mirels Score | Fracture Status | Primary Known | Treatment |
|---|---|---|---|
| Score less than 7 | None (Impending) | Yes | Radiotherapy |
| Score greater than 9 | Impending | Yes | Prophylactic Nail/Plate |
| Fractured | Complete | Yes | Acute Fixation + Post-op RT |
| Any | Any | NO (Unknown) | Workup first (Biopsy) |
BLT-KPCommon Bone Metastases
Memory Hook:BLT with a Kosher Pickle for lunch.
SPLSMirels Score Components
Memory Hook:Some People Like Statistics.
CRABMultiple Myeloma Features
Memory Hook:Beware the CRAB in patients over 40 with back pain.
Overview
Pathological fractures of the humerus occur when bone weakened by disease (cystic, metabolic, or neoplastic) fails under physiological loads. In adults over 40, metastatic disease and myeloma are the overwhelming causes. The humerus is the second most common site for long bone metastases (after femur).
Epidemiology
Incidence:
- Bone is the 3rd most common site of metastatic disease.
- Humerus involvement: 20% of bone mets.
- Increasing due to improved cancer survival rates.
Prognosis
Survival (Median):
- Prostate/Breast: Years (24-36 months).
- Renal/Thyroid: Intermediate (12 months).
- Lung: Short (less than 6 months).
- Fixation must be durable enough for expected survival.
Anatomy and Pathophysiology
Pathophysiology of Metastasis
Hematogenous Spread:
- Tumor emboli deposit in vascular marrow (Red marrow).
- Batson's Plexus (low pressure venous system) facilitates spread.
- Cytokines (RANKL/PTHrP) stimulate osteoclasts.
Osteolytic vs Osteoblastic:
- Lytic: Lung, Renal, Thyroid, Breast (mixed), Myeloma. Bone destruction leads to Fracture.
- Blastic: Prostate, Breast (mixed). Bone formation leads to Brittle bone.
Lytic lesions are more prone to fracture and harder to fix.
Classification
Mirels Scoring System
Used to predict fracture risk in impending pathological fractures.
| Variable | Score 1 | Score 2 | Score 3 |
|---|---|---|---|
| Site | Upper Limb | Lower Limb | Peritrochanteric |
| Pain | Mild | Moderate | Functional |
| Lesion | Blastic | Mixed | Lytic |
| Size | Less than 1/3 | 1/3 - 2/3 | Greater than 2/3 |
Score Calculation: Sum of all 4 variables. Minimum 4, Maximum 12.
Mirels Interpretation
| Total Score | Fracture Risk | Recommendation |
|---|---|---|
| Less than or equal to 7 | 0-4% | Radiotherapy / Observation |
| 8 | 15% | Clinical Judgement |
| Greater than or equal to 9 | 33% | Prophylactic Fixation |
Exam Pearl
Mirels score applies to metastatic disease in long bones. It does NOT apply to primary bone tumors or spinal metastases.
History
Key Questions:
- Known history of cancer?
- Constitutional symptoms: Weight loss, night sweat, fatigue.
- Pain history: Mechanical ( fracture) vs Biological (night pain).
- Functional demand.
Night pain typically indicates active tumor.
Examination
Inspection:
- Soft tissue mass? (Sarcoma concern).
- Previous mastectomy scars?
- Thyroid enlargement?
Neurovascular:
- Radial nerve commonly involved in humeral shaft fractures.
- Check Lymph nodes (Axillary/Supraclavicular).
Examination focuses on finding a primary source.
Investigations
Imaging & Staging
X-ray:
- "Moth-eaten" or "Permeative" appearance.
- Cortical destruction.
- Pathological fracture (transverse, minimal trauma).
CT Scan:
- Chest/Abdomen/Pelvis (Staging).
- Defines bone stick remaining.
Bone Scan / PET:
- Identifies other skeletal metastases.
- Solitary vs Multiple (Prognostic).
Complete staging is mandatory.
Management Algorithm
Treatment Decision Making
Indications:
- Low Mirels score (less than 8).
- Terminally ill (Life expectancy less than 6 weeks).
- Non-displaced fracture in severe comorbidities.
- Radiosensitive tumor (Myeloma/Lymphoma/Small Cell Lung).
Treatment:
- Radiotherapy (RT).
- Functional bracing.
- Bisphosphonates.
Pain relief is the primary goal.
Surgical Technique
Fixation Strategy
Standard for Diaphysis:
- Load sharing device.
- Prophylactic or Acute.
- Reaming: Careful! Can spread tumor or cause fat embolism.
- Locking: Static locking (proximal and distal). Or Cement screw augmentation.
Advantages: Minimally invasive, preserves soft tissue, immediate stability.
Complications
Potential Complications
Tumor Progression
Local Recurrence: If RT is not given post-op, tumor continues to grow and destroys fixation. Hardware failure is inevitable if patient survives long enough.
Bleeding
Intra-operative Hemorrhage: Especially Renal Cell/Thyroid. Embolize pre-op!
Non-union
Radiation Effect: RT inhibits bone healing. Pathological fractures often rely on the hardware for life (union is not guaranteed).
Embolism
Fat/Tumor Embolism: Reaming increases intramedullary pressure. Vent the canal or use unreamed nails in high-risk pulmonary patients.
Postoperative Care
Rehabilitation Protocol
- Sling for comfort.
- Immediate use: Construct should allow weight bearing (as tolerated).
- No restrictions usually (construct must be strictly stable).
- Radiotherapy planning begins (once wound dry).
- Bisphosphonate therapy (Zoledronic acid / Denosumab).
- Surveillance for hardware failure.
- Systemic therapy for underlying disease.
Outcomes
Prognosis
- Pain Relief: Excellent (80-90% improvement) after fixation.
- Function: Restoration of ability to feed/groom.
- Ambulatory: Upper limb fixation allows use of walking aids.
- Hardware Failure: 5-10%, usually due to disease progression or long survival.
Evidence Base
Key Studies
Mirels - Scoring System
- Developed scoring system for impending pathological fractures
- Score greater than 9 predicted fracture in 33% (recommend prophylactic fixation)
- Score less than 7 predicted fracture in 4% (radiation safe)
Capanna et al. - Cement Augmentation
- Cement augmentation improves screw purchase in tumor/osteoporosis
- Reduces hardware failure rates
- Allows immediate weight bearing
Katagiri et al. - Prognostic Scoring
- Developed prognostic score for bone mets survival
- Factors: Primary site, visceral mets, performance status
- Helps decide between simple fixation vs durable reconstruction
Patchell et al. - Surgery + Radiation vs Radiation
- Randomized trial for metastatic spinal cord compression (applicable principle)
- Surgery + RT superior to RT alone for ambulatory status
- Direct decompressive surgery remains the gold standard for compression
Harrington - Prophylactic Fixation
- Established criteria for prophylactic fixation
- Greater than 50% cortical destruction
- Lesion greater than 2.5cm
- Pain after radiotherapy
Viva Scenarios
Practice these scenarios to excel in your viva examination
"You are called to ED for a 60-year-old male with a solitary lytic lesion of the humerus and a fracture. He has no history of cancer. What is your plan?"
"Describe the Mirels Score and how you use it."
"How do you manage a pathological humerus fracture from Renal Cell Carcinoma?"
MCQ Practice
Self-Assessment Questions
Q1: Mirels Score
Q: Which factor is weighted most heavily (highest score) in the Mirels scoring system for "Site"?
- A) Humerus
- B) Radius
- C) Femoral Shaft
- D) Peritrochanteric region
- E) Tibia
A: D - The Peritrochanteric region is assigned a score of 3 due to the high biomechanical loads and consequences of failure. Upper limb is 1, Lower limb (shaft) is 2.
Q2: Primary Source
Q: What is the most common source of bone metastasis in women?
- A) Lung
- B) Thyroid
- C) Breast
- D) Kidney
- E) Cervix
A: C - Breast cancer is the most common source of bone metastases in women (about 70%). Prostate is most common in men.
Q3: Vascularity
Q: Which primary tumor gives rise to highly vascular bone metastases requiring pre-operative embolization?
- A) Breast
- B) Prostate
- C) Renal Cell Carcinoma
- D) Lung
- E) Melanoma
A: C - Renal Cell and Thyroid carcinomas are classically highly vascular. Pre-operative embolization is recommended to control bleeding.
Q4: Prophylaxis Threshold
Q: A Mirels score of 10 indicates:
- A) Low risk of fracture (less than 4%)
- B) Moderate risk, Observation indicated
- C) High risk, Prophylactic fixation indicated
- D) Imminent death
- E) Need for amputation
A: C - A score of 9 or greater indicates a high risk of fracture (greater than 33%) and is the threshold for recommending prophylactic fixation.
Q5: Contraindication
Q: Internal fixation of a pathological fracture is potentially contraindicated if:
- A) The patient has multiple metastases
- B) The primary tumor is unknown (solitary lesion)
- C) The Mirels score is 12
- D) The fracture is displaced
- E) The patient is on bisphosphonate therapy
A: B - If the primary is unknown and the lesion is solitary, internal fixation (nailing) is contraindicated until a primary bone sarcoma is ruled out via biopsy/staging, to avoid compartment contamination.
Australian Context
Australian Context
- MDT: All bone tumors (even presumed mets) should be discussed in a Sarcoma/Bone Tumor MDT or with a specialist unit.
- Referral: State-based sarcoma services (e.g., Peter Mac, PA Hospital, Chris O'Brien Lifehouse) exist for complex cases.
- Funding: PBS covers Denosumab/Zoledronic acid for bone mets prevention.
Pathological Fractures - Exam Quick Reference
High-Yield Exam Summary
Key Facts
- •Causes: Breast, Lung, Thyroid, Kidney, Prostate
- •Threshold: Mirels score greater than 9
- •Trap: Solitary lesion (Unknown primary) = STOP
- •Vascular: Renal/Thyroid (Embolize)
- •Fixation: Durable (Nail or Cemented Plate)
- •Survival: Fixation must last longer than the patient
Mirels Score (SPLS)
- •Site (Upper/Lower/Troch)
- •Pain (Mild/Mod/Func)
- •Lesion (Blastic/Mixed/Lytic)
- •Size (less than 1/3, 1/3-2/3, greater than 2/3)
Surgical Steps
- •Positioning (Beach chair/Supine)
- •Approach (Deltopectoral/Split)
- •Biopsy (If needed, frozen section)
- •Reaming (Gentle/Unreamed if pulmonary risk)
- •Nail Insertion (Span whole bone)
- •Cement Augmentation (If proximal/distal voids)
- •Closure (Layered)
Common Pitfalls
- •Nailing a primary sarcoma
- •Missing hypercalcemia
- •Underestimating bleeding in Renal mets
- •Inadequate fixation length (Must span bone)
- •Ignoring radial nerve in percutaneous nailing
Examiner Favorites
- •Calculate Mirels for this X-ray...
- •Workup of solitary lesion
- •Role of radiotherapy vs surgery
- •Mechanism of bisphosphonates
- •What to do if fixation fails?