---

Bone Scan - Complete Guide for Orthopaedic Postgraduate Exam

What is a Bone Scan?

Key exam point: Bone scan detects the body's reaction to a problem (increased blood flow, metabolism, osteoblastic activity), NOT the problem itself directly.

---

The Radiotracer Used

• How it works: ⁹⁹ᵐTc-MDP is absorbed onto hydroxyapatite crystals in bone
• It concentrates wherever there is increased blood flow and increased bone metabolism (osteoblastic activity)
• A gamma camera detects the radiation emitted and creates the image

---

Three-Phase Bone Scan - The Most Important Concept for Exams

Three-Phase Scan Interpretation Table (High-yield!)

• In osteomyelitis: all three phases are positive (hot in all phases)
• In cellulitis: only first two phases are positive (hot soft tissue, normal bone phase)
• This is the KEY way to differentiate osteomyelitis from cellulitis on a bone scan

---

Orthopaedic Uses of Bone Scan (MUST KNOW LIST)

1. Osteomyelitis

• Can detect osteomyelitis within 48 hours of clinical onset (before X-ray becomes positive)
• X-ray takes 10-21 days to show changes
• Use in patients with metallic implants where MRI is limited
• Sensitivity ~90% for detecting vertebral osteomyelitis

2. Stress Fractures

• Bone scan shows abnormality 1-2 weeks before it appears on plain X-ray
• Nearly 100% sensitive for stress injuries of bone
• Negative bone scan effectively rules out a stress fracture
• MRI is more specific, but bone scan is often used first due to availability
• Used for: pars interarticularis (spondylolysis), metatarsal stress fractures, tibial stress fractures

3. Bone Metastases

• Highly sensitive for osteoblastic (bone-forming) metastases (e.g., prostate, breast)
• Helps determine extent of metastatic disease throughout the entire skeleton at one shot (whole-body scan)
• Important limitation: Purely lytic lesions (renal cell, lung cancer) can be falsely negative because there is no osteoblastic activity to detect
• Multiple myeloma: Bone scan is unreliable - use skeletal survey with skull films instead

4. Avascular Necrosis (AVN)

• Early AVN (hypoperfusion stage): Bone scan shows a "cold" spot (decreased uptake) due to loss of blood supply
• Reparative phase: Shows increased uptake as revascularization occurs
• Bone scan can detect AVN earlier than X-ray

5. THA / TKA Component Loosening

• Used to evaluate painful joint replacements
• Best for femoral components
• Used in conjunction with gallium scan to distinguish aseptic loosening from infection
• Important: ⁹⁹ᵐTc + ¹¹¹In scans for detecting loosening have low specificity (78%) and sensitivity (64%) - not recommended for routine use

6. Reflex Sympathetic Dystrophy (Complex Regional Pain Syndrome)

• Three-phase bone scan is helpful
• Shows diffuse increased uptake in the affected limb in all phases

7. Spondylolysis

• SPECT (Single-Photon Emission CT) bone scan is useful
• Identifies active stress fracture at pars interarticularis even when X-ray is negative

8. Tumours

• Can detect primary bone tumours and assess polyostotic involvement
• Helps in staging and identifying skip lesions

---

The Three Radiotracer Agents - Summary (Exam High-Yield)

Gallium scan special rules:

• Requires 24 hours delay for appendicular skeleton
• Requires 72 hours delay for axial skeleton
• Combined Tc + Gallium scan: sensitivity 70%, specificity up to 93% for osteomyelitis
• In inflammation: Gallium uptake exceeds Technetium uptake (or shows different distribution) - this distinguishes infection from reactive bone

Indium scan:

• Patient's blood drawn, WBCs separated and labeled with ¹¹¹In in the lab, then reinjected
• Images at 24-48 hours
• Reliable for acute osteomyelitis
• Unreliable for chronic osteomyelitis (which is lymphocyte-dominated, not neutrophil) - may give "cold" scan
• Also unreliable for aseptic vs. septic arthroplasty loosening

---

Cold Scan vs. Hot Scan

Exam trap: Early osteomyelitis with vascular compromise (pus under pressure, necrosis, joint effusion, soft tissue swelling) can give a false-negative "cold" scan with ⁹⁹ᵐTc.

---

Advantages of Bone Scan

1. Detects disease before X-ray becomes positive
2. Surveys the entire skeleton in one study (whole-body)
3. Useful when MRI is contraindicated (metallic implants)
4. Relatively low cost vs. MRI
5. High sensitivity (~90-100%) for many conditions

---

Limitations / Disadvantages

1. Low specificity - many conditions look the same (infection, tumour, fracture, degenerative change all show increased uptake)
2. Poor spatial resolution - cannot pinpoint exactly which bone in a region (e.g., cannot separate hindfoot from midfoot well)
3. Radiation exposure
4. False negatives with purely lytic lesions (myeloma, RCC)
5. False positives in Charcot arthropathy (WBC scans)
6. Delayed imaging needed for gallium (24-72 hours)
7. Poor for differentiating infection from Charcot joint

---

SPECT and PET - Modern Additions

• SPECT (Single-Photon Emission CT): Combines scintigraphy with CT - better localization; used for spondylolysis, femoral head osteonecrosis, patellofemoral syndrome
• PET scan: Uses F-18 fluorodeoxyglucose (FDG); taken up by areas of high metabolic activity; best for small lesions especially in lung cancer; also useful for assessing treatment response
• PET/CT has gained favour for staging metastatic bone disease but indications are still evolving

---

Quick Exam Summary Table

---