# CIM Case: Unicameral Bone Cyst - Pathological Fracture ## Clinical Scenario **Patient:** 12-year-old male **Presentation:** Right shoulder pain after minor tackle during football, unable to move arm **Relevant history:** No previous injuries, no prior symptoms, healthy otherwise, no family history of bone disorders **Examination findings:** - Holding right arm against body, reluctant to move - Localised tenderness over proximal humerus - Swelling around shoulder region - No deformity visible - Intact sensation, moving fingers normally - Radial pulse palpable - No other injuries --- ## Investigations Provided ### Laboratory Results | Test | Result | Normal Range | Interpretation | |------|--------|--------------|----------------| | N/A | - | - | Bloods not routinely required for UBC | ### Imaging **Image 1: AP and Lateral Proximal Humerus Radiographs** **Radiological features:** - Central lytic lesion in proximal humeral metaphysis - Well-defined margins with thin sclerotic rim - Lesion width less than physeal width - Transverse fracture through the lesion - Cortical fragment visible within the lesion (fallen fragment sign) - No periosteal reaction prior to fracture - Slight expansion of bone (<50% beyond normal diameter) - No soft tissue mass - Physis intact - Lesion extends from physis to mid-diaphysis (~8cm length) **Classification:** - Active UBC: Adjacent to physis (as seen here) - Cyst Index: >3.5 (higher recurrence risk) ## Questions & Model Answers **Diagnosis:** Unicameral Bone Cyst (UBC) / Simple Bone Cyst with Pathological Fracture **Fallen Fragment Sign:** This is PATHOGNOMONIC for UBC. A cortical fragment from the fracture falls to the dependent portion of the cyst, indicating: - The lesion is FLUID-filled (not solid) - Confirms diagnosis without biopsy in most cases **Characteristic Radiographic Features of UBC:** | Feature | Description | |---------|-------------| | **Location** | Central, metaphyseal | | **Margins** | Well-defined, thin sclerotic rim | | **Width** | Less than physeal width (<50% expansion) | | **Contents** | Fluid-filled (hence fallen fragment) | | **Periosteal reaction** | Absent (unless fractured) | | **Septations** | Usually none (simple cyst) | **Differential Diagnosis of Lytic Lesion in Child:** | Diagnosis | Distinguishing Features | |-----------|------------------------| | **Aneurysmal Bone Cyst (ABC)** | Eccentric, more expansile (>50%), blood-filled, fluid-fluid levels on MRI | | **Fibrous Dysplasia** | Ground-glass matrix, "shepherd's crook" | | **Non-ossifying Fibroma** | Eccentric, cortical-based, metaphyseal | | **Ewing Sarcoma** | Permeative, periosteal reaction ("onion-skin"), aggressive | | **Osteosarcoma** | Mixed lytic/sclerotic, periosteal reaction, soft tissue mass | **UBC Epidemiology:** - Peak age: 5-15 years - Male predominance (2:1) - Most common locations: Proximal humerus (50%), proximal femur (25%) - 75% present with pathological fracture > **Consultant Tip:** The fallen fragment sign is diagnostic for UBC. If you see it, no biopsy needed initially. **Active vs Latent Classification:** | Feature | Active Cyst | Latent Cyst | |---------|-------------|-------------| | **Location** | Adjacent to physis | Separated from physis by normal bone | | **Growth** | Expands with skeletal growth | Static or regressing | | **Recurrence rate** | Higher (50-60%) | Lower (10-20%) | | **Age** | Usually younger (<10 years) | Usually older | | **Treatment response** | Less predictable | Better response | **This Patient:** Active cyst (adjacent to physis) - higher recurrence risk **Cyst Index (Predictive of Recurrence):** Cyst Index = Cyst length / Cyst width - Index >3.5 = Higher recurrence risk **Natural History:** | Scenario | Outcome | |----------|---------| | **Fracture through cyst** | 10-15% will heal spontaneously | | **Without fracture** | Most persist, may enlarge | | **At skeletal maturity** | Many become latent and stabilise | | **Risk of refracture** | 50% if untreated cyst persists | **Pathophysiology (Theories):** | Theory | Mechanism | |--------|-----------| | **Venous obstruction** | Blocked venous drainage → fluid accumulation | | **Growth plate disturbance** | Abnormal endochondral ossification | | **Cyst fluid** | Contains prostaglandins, cytokines → bone resorption | **Why Treatment May Fail:** - Active cyst with continued growth - Inadequate drainage/decompression - Persistent cyst lining not addressed - Young age with significant growth remaining > **Consultant Tip:** Active cysts (adjacent to physis) in young children are the most challenging. Multiple treatments are often required. **Initial Management Algorithm:** **Step 1: Immobilisation** - Collar and cuff or sling for proximal humerus fracture - Allow fracture to heal (6-8 weeks typically) - Serial X-rays to monitor healing **Step 2: Observe for Cyst Resolution** - 10-15% of cysts heal with the fracture - "Biological effect" of fracture haematoma may promote healing - Re-image at 3-4 months to assess cyst status **Step 3: If Cyst Persists (85-90%)** - Proceed with definitive treatment once fracture healed - Do not inject through acute fracture **Why Not Treat the Cyst Immediately?** | Reason | Explanation | |--------|-------------| | **Fracture takes priority** | Must heal before cyst treatment | | **Spontaneous resolution possible** | 10-15% heal with fracture | | **Technical difficulty** | Injecting through fracture line unreliable | | **Risk of displacement** | Instrumentation may destabilise fracture | **When to Consider Immediate Intervention:** | Scenario | Intervention | |----------|--------------| | **Impending fracture in femur** | Prophylactic fixation + cyst treatment | | **Complete fracture in weight-bearing bone** | Internal fixation + possible cyst treatment | | **Very large cyst with cortical thinning** | Prophylactic stabilisation | **This Patient's Management:** - Sling immobilisation for 6 weeks - Serial X-rays at 2, 4, 6 weeks - Assess cyst at 3-4 months post-fracture - If cyst persists → proceed with intralesional treatment > **Consultant Tip:** For proximal humerus UBC with fracture, treat the fracture conservatively first. Most heal uneventfully. Address the cyst once the fracture has united. **Treatment Options for Persistent UBC:** **Option 1: Corticosteroid Injection (Most Common Initial Treatment)** | Aspect | Detail | |--------|--------| | **Agent** | Methylprednisolone acetate (40-200mg) | | **Technique** | Fluoroscopic guidance, aspirate cyst fluid first | | **Mechanism** | Anti-inflammatory, reduces prostaglandins, promotes healing | | **Success rate** | 60-70% (may need 2-3 injections) | | **Interval** | Every 2-3 months if not healed | **Technique:** 1. Position under fluoroscopy 2. Insert two trocar needles into cyst 3. Aspirate straw-coloured fluid (confirms diagnosis) 4. Inject contrast to confirm cyst extent 5. Inject corticosteroid **Option 2: Bone Marrow Injection** | Aspect | Detail | |--------|--------| | **Source** | Iliac crest aspiration | | **Technique** | Inject autologous bone marrow into cyst | | **Mechanism** | Osteogenic precursor cells, growth factors | | **Success rate** | Similar or slightly better than steroid (60-80%) | **Option 3: Curettage and Bone Grafting** | Aspect | Detail | |--------|--------| | **Technique** | Open or percutaneous extended curettage + autograft/allograft | | **Indication** | Recurrent cyst, large cyst, failed injections | | **Success rate** | 80-90% | | **Disadvantage** | More invasive, donor site morbidity | **Option 4: Continuous Decompression (Cannulated Screw/Nail)** | Aspect | Detail | |--------|--------| | **Technique** | Cannulated screw or IM nail left in situ | | **Mechanism** | "Venting" - continuous drainage, prevents pressure build-up | | **Success rate** | 80-90% | | **Advantage** | Also provides structural support | **Treatment Algorithm:** ``` First-line: Steroid injection (1-3 attempts) ↓ If failed: Consider bone marrow injection or curettage ↓ If still recurrent: Extended curettage + bone graft ± internal fixation ↓ For large/recurrent cysts: Continuous decompression (cannulated screw/nail) ``` **This Patient's Recommended Treatment:** Start with corticosteroid injection. If not healed after 2-3 injections, proceed to curettage and bone grafting. > **Consultant Tip:** Most UBCs are treated with serial steroid injections first. Reserve curettage for failures. **Predictors of Recurrence:** | Factor | Higher Recurrence Risk | Lower Recurrence Risk | |--------|------------------------|----------------------| | **Age** | <10 years | >10 years | | **Location** | Active (adjacent to physis) | Latent (separated from physis) | | **Size** | Larger cysts | Smaller cysts | | **Cyst index** | >3.5 | <3.5 | | **Initial treatment** | Single injection | Multiple injections or surgery | | **Fracture healing** | Did not heal cyst | Healed with fracture | **This Patient's Risk:** Active cyst, 12 years old (moderate risk), proximal humerus - moderate-high recurrence risk **Management of Recurrent UBC:** **Step 1: Repeat Injection** - Second or third injection often successful - Allow 2-3 months between injections - Consider bone marrow injection if steroid failed **Step 2: Extended Curettage + Bone Grafting** | Technique | Detail | |-----------|--------| | **Exposure** | Open approach to cyst | | **Curettage** | Remove cyst lining completely | | **Burr** | High-speed burr to remove lining remnants | | **Graft** | Autograft (iliac crest), allograft, or synthetic | | **Fixation** | If cortex weakened, may add internal fixation | **Step 3: Adjuvants for Recalcitrant Cysts** | Adjuvant | Mechanism | |----------|-----------| | **Phenol** | Chemical cautery of cyst lining | | **Argon beam** | Thermal destruction of lining | | **Cryotherapy** | Freeze cyst lining | **Step 4: Internal Fixation with Continuous Decompression** **Flexible IM Nailing:** - Titanium elastic nails (TEN/TENS) - Provide stability and continuous venting - Studies show accelerated healing - Left in place until cyst healed (6-12 months) **Cannulated Screws:** - Partially threaded - Allow continuous drainage - Structural support for weakened cortex **Management of Very Large Recurrent Cysts:** - Staged treatment may be needed - Structural allograft for large defects - Consider prophylactic fixation before definitive treatment > **Consultant Tip:** For recurrent UBC in a young child, flexible IM nailing provides both stability and continuous decompression - increasingly favoured approach. **Proximal Femur UBC - Key Differences:** **Increased Concern for Femoral UBC:** | Issue | Rationale | |-------|-----------| | **Weight-bearing bone** | Fracture has significant consequences | | **Risk of AVN** | Blood supply to femoral head at risk | | **Functional impact** | Hip function critical | | **Prophylactic treatment** | Often indicated before fracture | **Risk of Pathological Fracture - Femoral Neck:** - Femoral neck fracture through UBC is a serious injury - AVN risk similar to traumatic fractures - Non-union risk increased - May require internal fixation **Management Algorithm - Proximal Femur UBC:** **Scenario 1: Incidental Finding, No Fracture** | Cyst Size | Management | |-----------|------------| | **Small (<25% cortical involvement)** | Observe, may inject | | **Moderate (25-50%)** | Injection or curettage | | **Large (>50% or impending fracture)** | Prophylactic fixation + curettage | **Scenario 2: Pathological Fracture Through Femoral Neck** | Type | Management | |------|------------| | **Displaced** | Urgent reduction and internal fixation (screws or DHS) + cyst treatment | | **Undisplaced** | Internal fixation (prophylactic) + cyst treatment | | **Intertrochanteric** | May treat fracture conservatively, then cyst | **Surgical Technique for Femoral UBC:** | Step | Detail | |------|--------| | **Approach** | Lateral (for screw fixation) or anterolateral | | **Curettage** | Fenestration into cyst, extended curettage | | **Bone graft** | Autograft or allograft to fill defect | | **Fixation** | Cannulated screws ± DHS for support | **Protected Weight-Bearing:** - Non-weight-bearing 6-12 weeks - Progressive weight-bearing based on healing - Serial X-rays to monitor **This Patient's Proximal Humerus:** - Non-weight-bearing location - Fracture can heal conservatively - Less urgency for internal fixation - Injection is reasonable first-line treatment **Key Differences:** | Feature | Proximal Humerus | Proximal Femur | |---------|------------------|----------------| | Weight-bearing | No | Yes | | Fracture treatment | Usually conservative | Often surgical | | Prophylactic treatment | Rarely needed | Often indicated | | Internal fixation | Rarely needed | Frequently needed | | Functional concern | Moderate | High | > **Consultant Tip:** Proximal femur UBC needs more aggressive treatment. Don't wait for fracture - prophylactically stabilise and treat large cysts. --- ## Key Teaching Points ### Pattern Recognition **This pattern suggests UBC:** - Child/adolescent (5-15 years) - Proximal humerus or proximal femur - Central metaphyseal lytic lesion - Well-defined margins - No periosteal reaction (until fracture) - Lesion width < physeal width - Fallen fragment sign on X-ray (pathognomonic) **Distinguish from ABC:** | Feature | UBC | ABC | |---------|-----|-----| | Location | Central | Eccentric | | Expansion | <50% | >50% (often "blowout") | | Contents | Serous fluid | Blood products | | Fluid levels | No | Yes (on MRI) | | Age | 5-15 years | Any age (peak 10-20) | ### Critical Management Points 1. **Fallen fragment sign is pathognomonic** - confirms diagnosis 2. **Treat the fracture first** - allow 6-8 weeks to heal 3. **10-15% heal with fracture** - watch before treating cyst 4. **Steroid injection is first-line** - 60-70% success, may need repeats 5. **Active cysts recur more** - young age, adjacent to physis 6. **Femoral cysts need more aggressive treatment** - prophylactic fixation often indicated --- ## Common Examiner Follow-ups **Q: "What is in the cyst fluid and why does it matter?"** UBC fluid characteristics: - Straw-coloured, clear or slightly yellow - Similar to serum (protein content) - Contains elevated prostaglandins, interleukins - These cytokines promote bone resorption - Corticosteroids inhibit this inflammatory cascade - hence their efficacy ABC fluid is bloody/dark and may have blood-fluid levels on imaging. --- **Q: "What is the 'cyst index' and how do you use it?"** Cyst Index = Cyst length ÷ Cyst width | Index | Interpretation | |-------|----------------| | <3.5 | Lower recurrence risk | | >3.5 | Higher recurrence risk | Higher index = longer, narrower cyst = higher recurrence. Useful for prognostic counselling and planning frequency of follow-up. --- **Q: "Can you use flexible nails for proximal humerus UBC?"** Yes, flexible intramedullary nails (TENS) can be used: - Provide continuous decompression ("venting") - Give structural support - Studies show accelerated healing - Retrograde insertion from lateral condyle - Left in place 6-12 months until healed Increasingly popular approach, especially for recurrent or large cysts. --- ## Related Topics - Aneurysmal Bone Cyst - Pathological Fractures - Fibrous Dysplasia - Paediatric Bone Tumours - Bone Grafting Techniques - Proximal Femur Fractures in Children