MCQs – Hip Recon 3

Modern Study Review (AI-Generated)

High-Yield Summary

Prosthetic joint infection (PJI) microbiology and total hip arthroplasty (THA) biomechanics are core topics in the Royal College exam, emphasizing infection management and implant mechanics. The critical clinical decision often hinges on distinguishing indolent infections (Staphylococcus epidermidis) from fulminant infections (Staphylococcus aureus) and tailoring antibiotic therapy accordingly. In dysplastic hips and post-Salter osteotomy THA, acetabular and femoral anatomical variations dictate implant positioning and surgical technique to avoid impingement and ensure stability. Biomechanically, stem design parameters (modulus, length, cross-section) influence stress distribution and risk of stress shielding, a key factor in implant longevity. While classic teaching favors rigid stems, modern trends increasingly consider modularity and bone preservation strategies.

High-Yield Decision Matrix

Active Recall Q&A

Q: What is the best antibiotic treatment for Staphylococcus epidermidis infections in THA?
A: Vancomycin.
Related Pearl: Vancomycin covers methicillin-resistant Staph epidermidis strains, which are common in prosthetic joint infections and require prolonged therapy.

Q: Which bacteria are hardest to eradicate in total hip arthroplasty infections?
A: Gram-negative bacteria, especially Pseudomonas, are hardest to eradicate; Staphylococcus epidermidis is also difficult.
Related Pearl: Gram-negative infections often necessitate staged revision surgery and extended antibiotic courses due to biofilm formation.

Q: What is the most common organism in an indolent infection 6 months after THA?
A: Staphylococcus epidermidis.
Related Pearl: Indolent infections present with subtle symptoms and require high clinical suspicion and advanced diagnostics like synovial fluid analysis.

Q: What is the most common organism in a fulminant infection after THA?
A: Staphylococcus aureus.
Related Pearl: Fulminant infections present acutely with systemic signs and require urgent surgical debridement and intravenous antibiotics.

Q: What must be considered when performing THA after a Salter osteotomy?
A: The acetabulum is retroverted with lack of posterior wall; the cup should be placed with adequate anteversion, and anterior lip overhang must be resected to avoid impingement.
Related Pearl: Failure to resect the anterior lip leads to anterior impingement, increasing risk of early implant failure.

Q: What femoral factors complicate THA in dysplastic hips?
A: Short neck, small and straight canal, posteriorly positioned trochanter, significant anteversion, and possible need for shortening osteotomy.
Related Pearl: Preoperative CT imaging is critical to assess femoral anatomy and plan osteotomies or modular implants.

Q: What acetabular factors complicate THA in dysplastic hips?
A: Anterosuperior bone defect, high-riding acetabulum, and need to identify the native acetabular floor.
Related Pearl: Reconstruction may require bone grafting or specialized implants to restore hip center and achieve stable fixation.

Q: How should a wound hematoma with complete femoral nerve palsy in an anticoagulated patient be managed?
A: Reverse anticoagulation and consider surgical decompression or radiologic drainage.
Related Pearl: Early decompression reduces risk of permanent nerve injury; observation alone risks irreversible deficits.

Q: How does increasing the cross-sectional area of a prosthetic stem affect stress?
A: It decreases stress by increasing the moment of inertia (I), which reduces bending stress.
Related Pearl: Moment of inertia increases with the radius to the fourth power, so small increases in diameter greatly reduce stress.

Q: What is the formula for stress in a prosthetic stem under bending?
A: Stress = My/I, where M = bending moment, y = distance from neutral axis, I = moment of inertia.
Related Pearl: Increasing I by increasing radius exponentially reduces stress, improving implant durability.

Q: How do increasing modulus of elasticity, stem length, and cross-sectional area affect stress distribution in a prosthetic stem?
A: They increase stress in the stem but decrease stress on the cement and proximal femur, potentiating stress shielding.
Related Pearl: Stress shielding leads to proximal bone resorption and may cause implant loosening over time.

Q: What is the difference between modulus of elasticity and stiffness?
A: Modulus of elasticity is a material property independent of geometry; stiffness depends on geometry (length, cross-section).
Related Pearl: Viscoelastic materials like polyethylene exhibit time-dependent modulus changes affecting implant performance.

Q: What does lucency in zones 1 and 7 indicate in a PCA acetabular component?
A: Loosening where the beads are located superiorly.
Related Pearl: Radiolucent lines in these zones correlate with micromotion and early implant failure risk.

Q: How should a patient be managed after a THA dislocation reduced in the post-anesthesia recovery room?
A: Bed rest and abduction pillow if components are stable.
Related Pearl: Early mobilization with precautions reduces risk of recurrent dislocation and improves functional outcomes.

Q: What vascular injury is suggested by massive bleeding after placing a retractor around the anterior column?
A: Femoral artery injury.
Related Pearl: Immediate vascular control and repair are critical to prevent limb-threatening hemorrhage.

Q: Which artery injury is most commonly associated with AVN of the adult femoral head?
A: Lateral epiphyseal artery injury.
Related Pearl: This artery is the main blood supply to the femoral head; its injury leads to ischemic necrosis.

Q: What is the typical location of AVN in the femoral head?
A: Anterosuperior aspect.
Related Pearl: This region is most vulnerable due to terminal blood supply and mechanical loading.

Q: Is alcaptonuria a cause of AVN of the femoral head?
A: No, alcaptonuria (ochronosis) is not a cause of AVN.
Related Pearl: Alcaptonuria causes ochronotic arthropathy, a degenerative joint disease distinct from AVN.

Q: Which conditions are known causes of AVN of the femoral head?
A: Gaucher’s disease, sickle cell disease, Caisson’s disease, and dislocation.
Related Pearl: These conditions disrupt blood flow or increase intraosseous pressure, precipitating necrosis.

Q: What is the characteristic MRI appearance of AVN?
A: Double line sign on T2; the dead area is dark on T1 and bright on T2 sequences.
Related Pearl: The double line sign represents granulation tissue and sclerosis at the necrotic margin, a pathognomonic feature.

Classic Clinical Notes

• The best treatment for Staph epidermidis is vancomycin.

• The hardest bug to eradicate in THA is Pseudomonas; it may in fact be Staph epidermidis, but in general, the gram negatives are hardest to get rid of.

• The most common bug in an indolent infection 6 months postop THA is going to be Staph epidermidis; in a fulminant infection, it is most likely Staph aureus.

• In doing a THA after a patient has a Salter osteotomy – beware the lack of posterior wall. Their acetabulum has now been retroverted, so you don’t want to put your component in the same alignment as the native acetabulum. Note that when you put your cup in with adequate anteversion, because the acetabulum is so retroverted, there may be quite a bit of anterior lip overhanging – you must cut this out or you’ll impinge.

• There are many factors to consider in doing THA in dysplastics:
• Femur: short neck, small canal, straight canal, trochanter way around the back, significant anteversion, need for shortening osteotomy
• Acetabulum: antero/superior bone defect, high riding, need to find native floor

• The anticoagulated patient who develops a wound hematoma and complete femoral nerve palsy – reverse the anticoagulation and consider what to do next. Many questions suggest that you can just observe. Some authors suggest that you should decompress this. Hard to know what to do. I think I’m gonna go in and decompress this, or get radiology to put in a drain.

• To decrease the stress on a prosthetic stem, you can increase the cross-sectional area of the stem.

• Stress = My/I where M = bending moment, y = linear distance from neutral axis, and I = moment of inertia. The moment of inertia is significantly increased (r to the 4th power) by increasing the radius, and hence the stress would decrease.

• From Campbell’s: “Increasing the modulus of elasticity, the stem length, and the cross-sectional area of the stem increases the stress in the stem, but decreases the stress on the cement and proximal third of the femur.” I think he means that you increase the stiffness of the stem and thus potentiate stress shielding.

• Remember: modulus of elasticity is a material property, not a structural property. Modulus does not change with thickness, length, cross-sectional area, etc. The overall stiffness of an implant may change though, but this is not the same as the modulus. Note that viscoelastic materials do change with time, and so their modulus would change a little over time (e.g., polyethylene).

• Lucency in zones 1 and 7 represent loosening in a PCA component where the beads are all up at the top (in zones 1 and 7).

• If you see a patient who dislocates her THA during the transfer but you get it reduced in the PAR – treat the patient with bed rest and an abduction pillow if you are happy with the components otherwise.

• If you get huge bleeding after putting a retractor around the anterior column – you’ve hit the femoral artery.

• AVN in adults is most likely to be associated with injury to the lateral epiphyseal artery.

• AVN typically involves the anterosuperior aspect of the head.

• Alcaptonuria is not a cause of AVN of the femoral head. Alcaptonuria is the excretion of black urine and is seen in patients with ochronosis, a deficiency of the enzyme to break down homogentisic acid. AVN is seen with Gaucher’s, sickle cell, Caisson’s disease, and dislocation.

• The appearance of AVN on MRI – would see a double line sign on T2; on T1 the dead area is DARK, on T2 the dead area is BRIGHT.

Last Updated on January 25, 2026 by Christian Veillette