DNB Theory Questions Explained: Arthroplasty & AVN of Femoral Head By Dr. Vishal Huggi

Estimated reading time: 4 minutes

DNB theory exams often look intimidating, but most questions are actually straightforward scoring opportunities—if you know how much to write and how to structure your answer. In this session, Dr. Vishal Huggi breaks down two commonly asked 3–5 mark questions from Orthopedics in a very practical, exam-focused way.

Let’s understand both questions exactly the way an examiner expects.

Question 1: Advantages and Disadvantages of Robotic Knee Replacement

(Asked in DNB Paper 3 – 3 Marks)

This is a direct question, and most students lose marks by either writing too much theory or missing key points. For a 3-mark question, aim for 4–5 crisp advantages and 4–5 disadvantages.

Advantages of Robotic-Assisted Knee Arthroplasty

1. Accurate Pre-operative Planning
   Robotic systems allow detailed planning before surgery, improving precision.
   
2. Optimal Implant Selection
   The surgeon can accurately choose the implant size and positioning.
   
3. Minimal Soft Tissue Release
   Less soft tissue damage makes it a more minimally invasive procedure.
   
4. Accurate Alignment
   Planned alignment can be precisely achieved—mechanical, functional, kinematic, or reverse kinematic alignment.
   
5. Precise Bone Cuts & Implant Placement
   High accuracy in bone cuts ensures better implant positioning.

These are the keywords examiners want to see: accuracy, alignment, precision, minimal soft tissue damage.

Disadvantages of Robotic Knee Replacement

1. Limited Long-Term Data
   Robotic knee replacement has only about 10 years of data, unlike conventional TKR with 40–50 years of outcomes.
   
2. Closed Implant Platform
   Most robots are company-specific—limited implant choice across brands.
   
3. Increased Surgical Time
   Longer operative time increases the risk of infection and complications.
   
4. Higher Radiation Exposure
   Some robotic systems require CT scans, increasing radiation dose.
   
5. High Learning Curve & Cost
   Expensive equipment, training requirements, and significant financial investment.
   
6. Pin Site Fracture (Important Point)
   Fixation pins used during robotic surgery can cause pin site fractures—a commonly overlooked but important disadvantage.

Exam Tip

• Do not write long paragraphs.
  
• Stick to objective points.
  
• For 3 marks → 3–4 advantages + 3–4 disadvantages are enough.

Question 2: Avascular Necrosis (AVN) of Femoral Head

Causes, Pathogenesis & Classification

(3–5 Marks Question)

This is a high-yield, must-not-miss question.

Causes of AVN of the Femoral Head

The easiest way to remember and present this answer is to divide it into Traumatic and Non-Traumatic causes.

Traumatic Causes

Anything affecting the head and neck of femur:

• Femoral head & neck fractures
  
• Hip dislocations
  
• SCFE (Slipped Capital Femoral Epiphysis) in pediatrics

Non-Traumatic Causes

• Alcohol abuse
  
• Steroid use
  
• Idiopathic
  
• Sickle cell anemia

Other Systemic Causes:

• SLE
  
• Post-organ transplantation
  
• Radiation therapy
  
• Smoking
  
• Pregnancy

Writing under headings automatically fetches more marks.

Etiopathogenesis of AVN (Write as a Flow)

You do not need pages of explanation. A simple flowchart is enough.

Traumatic Pathway

Trauma → Vascular disruption → Ischemia → AVN

Non-Traumatic Pathway

Risk factors → Altered coagulation / altered lipid metabolism
→ Endothelial dysfunction
→ Thrombus formation
→ Critical ischemia
→ AVN

Exam Tip

For a 3–5 mark question, flowcharts > paragraphs.
Long explanations waste time without adding marks.

Classification of AVN: Modified Ficat–Arlet Classification

Always write classification in a table format.

Key Stages to Remember

• Stage 1 – Normal X-ray
  
• Stage 4 – Complete collapse with osteoarthritic changes

The confusion usually lies in Stage 2 & 3—here’s a simple trick.

How to Remember the Crescent Sign

Think alphabetically:

• A → B → C
  
• C = Crescent
  
• Crescent appears in Stage 2B

What does Crescent Sign indicate?

• Subchondral fracture
  
• Subchondral collapse
  
• Bone below becomes sequestrum
  
• Bone above shows sclerosis

Simple Memory Trick

• Stage 1 → Normal
  
• Stage 2A → Early changes
  
• Stage 2B → Crescent sign
  
• Stage 3 → Collapse
  
• Stage 4 → OA changes

Once crescent appears, head salvage is no longer possible.

Treatment Based on Stage (Very Important)

• Stage 1 & Stage 2A
  → Head salvage procedures
  
  • Core decompression
    
  • Vascularized grafts
    
• Stage 2B onwards
  → Total Hip Replacement (THR)

Crescent sign = no head salvage.

Final Takeaway for DNB Exams

• Answer exactly what is asked
  
• Stick to headings, tables, and flowcharts
  
• Do not overwrite
  
• Smart presentation saves time and improves scores

If you revise these questions in this structured manner, they become easy scoring areas in DNB theory.

arthroplasty

To watch more insightful sessions:  Conceptual Orthopedics Youtube Channel