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3. Adult Reconstruction (Hip & Knee) MCQs

Practice Set 209 of 326

This practice set contains high-yield board review questions covering key concepts in 3. Adult Reconstruction (Hip & Knee). Each clinical scenario is designed to test your diagnostic and management skills relevant to this subspecialty.

Question 4161

Topic: 3. Adult Reconstruction (Hip & Knee)

A 78-year-old female sustains a fall 5 years after a primary total hip arthroplasty. Radiographs reveal a periprosthetic fracture of the femur. The fracture line extends from just distal to the lesser trochanter to the tip of the femoral stem. The stem has subsided 2 cm and is in varus alignment. The proximal femoral bone stock is of good quality. According to the Vancouver classification, what is the fracture type and the most appropriate surgical treatment?

. Vancouver B1; Open reduction and internal fixation with cables and a locking plate
. Vancouver B2; Revision to a long fully porous-coated or fluted tapered stem
. Vancouver B3; Proximal femoral replacement
. Vancouver C; Open reduction and internal fixation with a locking plate
. Vancouver A; Nonoperative management with protected weight-bearing

Correct Answer & Explanation

. Vancouver B2; Revision to a long fully porous-coated or fluted tapered stem

Explanation

Correct Answer: Vancouver B2; Revision to a long fully porous-coated or fluted tapered stemThe Vancouver classification is used for postoperative periprosthetic femoral fractures. Type A fractures involve the trochanters. Type B fractures occur around or just distal to the stem. Type C fractures occur well below the stem. Type B is subdivided based on stem stability and bone stock: B1 (well-fixed stem), B2 (loose stem, adequate bone stock), and B3 (loose stem, poor bone stock). In this vignette, the fracture is around the stem (Type B), and the stem has subsided and shifted into varus, indicating it is loose. The bone stock is described as good. Therefore, this is a Vancouver B2 fracture. The standard of care for a B2 fracture is revision of the femoral component to a long stem (typically a fluted, tapered, modular stem) that bypasses the most distal fracture line by at least two cortical diameters, along with cerclage wiring of the fracture fragments.

Question 4162

Topic: 3. Adult Reconstruction (Hip & Knee)

A 68-year-old male sustains a posterior dislocation of his total hip arthroplasty 6 weeks postoperatively while bending over to tie his shoes. Closed reduction is successful in the emergency department. Post-reduction radiographs and a CT scan reveal that the acetabular component has 35 degrees of inclination and 5 degrees of anteversion. The femoral stem has 15 degrees of anteversion. What is the primary mechanical cause of this patient's instability?

. Excessive acetabular anteversion
. Insufficient acetabular anteversion
. Excessive acetabular inclination
. Insufficient femoral offset
. Abductor muscle deficiency

Correct Answer & Explanation

. Insufficient acetabular anteversion

Explanation

Correct Answer: Insufficient acetabular anteversionComponent malposition is a leading cause of instability following total hip arthroplasty. The "safe zone" described by Lewinnek for the acetabular component is 40 ยฑ 10 degrees of inclination (abduction) and 15 ยฑ 10 degrees of anteversion. In this patient, the acetabular component has only 5 degrees of anteversion, which is significantly under-anteverted (retroverted). Insufficient anteversion of the cup predisposes the hip to posterior dislocation, particularly during maneuvers involving hip flexion, adduction, and internal rotation (such as bending over to tie shoes). The inclination of 35 degrees is slightly low but acceptable, and the femoral anteversion of 15 degrees is normal.

Question 4163

Topic: 3. Adult Reconstruction (Hip & Knee)

A 60-year-old male returns for a routine 5-year follow-up after a primary total hip arthroplasty utilizing an extensively porous-coated, diaphyseal-fitting cylindrical femoral stem. He is asymptomatic. Radiographs demonstrate significant osteopenia of the proximal femur with rounding and resorption of the calcar. The distal portion of the stem shows spot welds and no radiolucent lines. What is the primary mechanism responsible for these radiographic findings?

. Particulate wear debris leading to macrophage activation
. Low-grade indolent periprosthetic joint infection
. Adaptive bone remodeling due to load bypass
. Micromotion at the bone-implant interface
. Galvanic corrosion at the head-neck junction

Correct Answer & Explanation

. Adaptive bone remodeling due to load bypass

Explanation

Correct Answer: Adaptive bone remodeling due to load bypassThe radiographic findings describe stress shielding, which is a classic phenomenon seen with extensively porous-coated, diaphyseal-fitting femoral stems. According to Wolff's Law, bone remodels in response to the mechanical stresses placed upon it. Because the stiff, diaphyseal-fitting stem achieves rigid distal fixation, the mechanical load is transferred directly to the distal femur, bypassing the proximal femur. Deprived of normal mechanical stress, the proximal femoral bone undergoes adaptive resorption (osteopenia and calcar rounding). This is a mechanical phenomenon, distinct from osteolysis caused by particulate wear debris (which typically presents as focal, scalloped radiolucencies rather than diffuse proximal osteopenia).

Question 4164

Topic: 3. Adult Reconstruction (Hip & Knee)

A 55-year-old male presents with progressive groin pain and a palpable anterior thigh mass 6 years after a metal-on-polyethylene total hip arthroplasty. The implant utilizes a large-diameter modular cobalt-chromium head on a titanium alloy stem. Radiographs show well-fixed components with no osteolysis. Aspiration yields cloudy, sterile fluid with a WBC count of 1,500 cells/ยตL. Serum metal ion testing reveals a significantly elevated cobalt level with a normal chromium level. What is the most likely diagnosis?

. Polyethylene wear-induced osteolysis
. Chronic periprosthetic joint infection
. Mechanically assisted crevice corrosion at the head-neck taper
. Iliopsoas bursitis
. Metallosis secondary to bearing surface wear

Correct Answer & Explanation

. Mechanically assisted crevice corrosion at the head-neck taper

Explanation

Correct Answer: Mechanically assisted crevice corrosion at the head-neck taperThis patient is presenting with an adverse local tissue reaction (ALTR) secondary to mechanically assisted crevice corrosion (MACC), commonly referred to as trunnionosis. This occurs at the modular head-neck junction (the trunnion), particularly when a cobalt-chromium head is paired with a titanium stem. The classic laboratory finding for trunnionosis in a metal-on-polyethylene articulation is an elevated serum cobalt level with a normal or disproportionately low chromium level. This distinguishes it from wear of a metal-on-metal bearing surface, where both cobalt and chromium are typically elevated equally. The presentation of a sterile, cystic mass (pseudotumor) with pain is characteristic of ALTR.

Question 4165

Topic: 3. Adult Reconstruction (Hip & Knee)

A 65-year-old male presents with right groin pain 6 years after a primary total hip arthroplasty. He has a metal-on-highly cross-linked polyethylene bearing with a titanium stem and a cobalt-chromium modular head. Serum cobalt levels are significantly elevated compared to chromium. MRI demonstrates a cystic mass in the iliopsoas bursa. Which of the following implant design factors most directly increases the risk of this specific complication?

. Increased trunnion surface area
. Decreased femoral head size
. Decreased trunnion length and diameter
. Use of a ceramic femoral head
. Increased neck-shaft angle

Correct Answer & Explanation

. Decreased trunnion length and diameter

Explanation

Correct Answer: Decreased trunnion length and diameterThis patient is presenting with mechanically assisted crevice corrosion (MACC), also known as trunnionosis, which occurs at the modular head-neck junction. It is characterized by elevated serum cobalt levels (often disproportionately higher than chromium) and adverse local tissue reactions (ALTR) such as pseudotumors. Risk factors for MACC include the use of a cobalt-chromium head on a titanium stem, larger femoral head sizes (which increase the frictional torque at the trunnion), lower neck-shaft angles (varus positioning increases the bending moment), and decreased trunnion size. Modern stems often feature shorter and thinner trunnions (e.g., 12/14 or 11/13 tapers) to increase range of motion and decrease impingement; however, this decreases the contact surface area between the head and neck, thereby increasing micromotion, fretting, and subsequent corrosion.

Question 4166

Topic: 3. Adult Reconstruction (Hip & Knee)

A 78-year-old female sustains a fall 8 years after a cemented total hip arthroplasty. Radiographs reveal a spiral fracture around the femoral stem extending just distal to the tip. The stem has subsided 1.5 cm and there is a radiolucent line at the cement-bone interface. The patient is medically optimized. What is the most appropriate surgical management?

. Open reduction and internal fixation with a lateral locking plate and cerclage cables
. Revision of the femoral component to a standard-length cemented stem
. Revision of the femoral component to a fully porous-coated or fluted tapered stem bypassing the fracture
. Cortical strut allografting alone
. Nonoperative management with a hip spica cast

Correct Answer & Explanation

. Revision of the femoral component to a fully porous-coated or fluted tapered stem bypassing the fracture

Explanation

Correct Answer: Revision of the femoral component to a fully porous-coated or fluted tapered stem bypassing the fractureThis patient has a Vancouver B2 periprosthetic femur fracture. The Vancouver classification is based on fracture location, implant stability, and bone stock. A Type B fracture occurs around the stem or just distal to it. Type B1 indicates a well-fixed stem, Type B2 indicates a loose stem with adequate bone stock, and Type B3 indicates a loose stem with poor bone stock. The subsidence and radiolucent lines in this vignette confirm the stem is loose (Type B2). The gold standard treatment for a Vancouver B2 fracture is revision arthroplasty using a long, diaphyseal-engaging stem (such as a fully porous-coated or fluted tapered modular stem) that bypasses the most distal aspect of the fracture by at least 2 cortical diameters. Open reduction and internal fixation alone (Option A) is reserved for Vancouver B1 fractures and would lead to catastrophic failure in the setting of a loose implant.

Question 4167

Topic: 3. Adult Reconstruction (Hip & Knee)

A 55-year-old male undergoes a total hip arthroplasty using a ceramic-on-ceramic bearing surface. Two years postoperatively, he complains of a new-onset "squeaking" sound emanating from his hip during normal gait, though he denies pain. Radiographs show the acetabular component is placed in 65 degrees of inclination and 35 degrees of anteversion. Which of the following phenomena is most likely responsible for this complication?

. Trunnionosis
. Edge loading and stripe wear
. Aseptic lymphocytic vasculitis-associated lesion (ALVAL)
. Third-body wear from retained cement
. Impingement of the femoral neck on the anterior capsule

Correct Answer & Explanation

. Edge loading and stripe wear

Explanation

Correct Answer: Edge loading and stripe wearSqueaking is a known complication specific to ceramic-on-ceramic (CoC) total hip arthroplasties, occurring in up to 10% of patients. The most common cause of squeaking is edge loading, which occurs when the femoral head contacts the edge of the ceramic liner rather than the smooth inner articulating surface. This is highly associated with cup malposition, specifically excessive inclination (vertical cup, as seen in this patient with 65 degrees of inclination) or excessive anteversion. Edge loading leads to loss of fluid film lubrication, increased friction, and "stripe wear" on the ceramic head, ultimately producing the audible squeak. ALVAL is associated with metal-on-metal bearings. Trunnionosis is associated with metal heads on titanium stems.

Question 4168

Topic: 3. Adult Reconstruction (Hip & Knee)

A 62-year-old female with long-standing rheumatoid arthritis presents with severe bilateral hip pain. Radiographs demonstrate bilateral protrusio acetabuli with the femoral heads migrating medial to the ilioischial line. During total hip arthroplasty, which of the following is the most appropriate technique for managing the acetabular defect?

. Reaming to bleeding subchondral bone medially to ensure biological fixation
. Placement of a jumbo cup to achieve medial fixation
. Use of impaction particulate cancellous bone graft medially and placement of the cup at the anatomic hip center
. Medialization of the acetabular component to increase the lever arm of the abductors
. Routine use of a constrained liner to prevent dislocation

Correct Answer & Explanation

. Use of impaction particulate cancellous bone graft medially and placement of the cup at the anatomic hip center

Explanation

Correct Answer: Use of impaction particulate cancellous bone graft medially and placement of the cup at the anatomic hip centerProtrusio acetabuli is defined by the medial migration of the femoral head past the ilioischial (Kohler's) line. It is commonly seen in rheumatoid arthritis, Paget's disease, and Marfan syndrome. The surgical goal during THA is to restore the anatomic hip center by lateralizing the acetabular component. Medial reaming is strictly contraindicated as it will worsen the defect and risk intrapelvic migration. The standard technique involves peripheral reaming to achieve rim fit, combined with the placement of impacted particulate cancellous bone graft in the medial defect. This lateralizes the cup to the anatomic center, restores normal biomechanics, and allows the medial bone graft to incorporate.

Question 4169

Topic: 3. Adult Reconstruction (Hip & Knee)

During a total hip arthroplasty, the surgeon inadvertently decreases the femoral offset by 10 mm compared to the contralateral normal hip, while maintaining equal leg lengths. Which of the following clinical or biomechanical consequences is most likely to occur?

. Increased joint reactive forces and increased abductor weakness
. Decreased joint reactive forces and increased abductor weakness
. Increased joint reactive forces and decreased risk of dislocation
. Decreased joint reactive forces and increased risk of dislocation
. No change in joint reactive forces, but increased abductor tension

Correct Answer & Explanation

. Increased joint reactive forces and increased abductor weakness

Explanation

Correct Answer: Increased joint reactive forces and increased abductor weaknessFemoral offset is the perpendicular distance from the center of rotation of the femoral head to a line bisecting the long axis of the femur. Decreasing the femoral offset shortens the lever arm of the abductor musculature. To maintain a level pelvis during single-leg stance, the abductors must generate significantly more force to counteract the body weight. This increased muscle force translates directly into increased joint reactive forces across the hip. Clinically, the shortened abductor lever arm leads to abductor weakness, a Trendelenburg gait (limp), and soft tissue laxity, which increases the risk of dislocation.

Question 4170

Topic: 3. Adult Reconstruction (Hip & Knee)

A 70-year-old male presents with a painful total hip arthroplasty 3 years after his index surgery. Aspiration of the hip joint yields synovial fluid with a white blood cell (WBC) count of 4,500 cells/ยตL and 85% polymorphonuclear neutrophils (PMNs). Alpha-defensin testing is positive. According to the 2018 International Consensus Meeting (ICM) criteria for periprosthetic joint infection (PJI), what is the next most appropriate step in management?

. One-stage or two-stage revision arthroplasty for chronic PJI
. Irrigation and debridement with modular component exchange (DAIR)
. Six weeks of intravenous antibiotics followed by repeat aspiration
. Revision of the femoral head and polyethylene liner only
. Observation and oral NSAIDs

Correct Answer & Explanation

. One-stage or two-stage revision arthroplasty for chronic PJI

Explanation

Correct Answer: One-stage or two-stage revision arthroplasty for chronic PJIThis patient has a chronic periprosthetic joint infection (PJI). According to the 2018 ICM criteria, a synovial WBC count > 3,000 cells/ยตL or PMN% > 80% is highly indicative of a chronic PJI. A positive alpha-defensin test provides further definitive evidence. Because the infection is chronic (presenting 3 years postoperatively, well beyond the 4-week window for acute infections), the standard of care is component removal. This is typically achieved via a two-stage exchange arthroplasty (removal of implants, placement of an antibiotic spacer, IV antibiotics, followed by reimplantation) or a carefully selected one-stage revision. Debridement, antibiotics, and implant retention (DAIR) is only indicated for acute postoperative infections (within 4 weeks of surgery) or acute hematogenous infections (within 3 weeks of symptom onset) with well-fixed implants.

Question 4171

Topic: Total Hip Arthroplasty (THA)

A 65-year-old man undergoes a primary total hip arthroplasty. Intraoperatively, the surgeon utilizes a femoral stem that decreases the patient's native femoral offset by 10 mm while perfectly restoring the native leg length. Which of the following biomechanical consequences is most likely to occur as a direct result of this change?

. Decreased joint reactive force across the hip
. Increased abductor muscle force required during the stance phase of gait
. Increased impingement-free range of motion
. Increased tension on the iliotibial band
. Lateralization of the hip center of rotation

Correct Answer & Explanation

. Increased abductor muscle force required during the stance phase of gait

Explanation

Correct Answer: Increased abductor muscle force required during the stance phase of gaitFemoral offset is the perpendicular distance from the center of rotation of the femoral head to the anatomical axis of the femur. Decreasing the femoral offset shortens the abductor moment arm. Because the body weight moment arm remains constant, the abductor muscles must generate significantly more force to maintain a level pelvis during the single-leg stance phase of gait. This increased abductor force consequently increases the overall joint reactive force across the hip. Decreasing offset also increases the risk of bony or implant impingement, thereby decreasing impingement-free range of motion, and decreases tension on the iliotibial band and abductors, potentially leading to instability.

Question 4172

Topic: 3. Adult Reconstruction (Hip & Knee)

A 45-year-old highly active woman undergoes a ceramic-on-ceramic total hip arthroplasty. Two years postoperatively, she presents complaining of a loud squeaking noise emanating from her hip with every step. She denies any pain, and radiographs demonstrate well-fixed components with no evidence of osteolysis. Which of the following factors is most strongly associated with the development of this phenomenon?

. Use of a titanium alloy femoral stem
. Small femoral head diameter (e.g., 28 mm)
. Component malposition leading to edge loading
. High body mass index (BMI > 40)
. Use of a highly cross-linked polyethylene liner

Correct Answer & Explanation

. Component malposition leading to edge loading

Explanation

Correct Answer: Component malposition leading to edge loadingSqueaking is a known complication specific to hard-on-hard bearings, particularly ceramic-on-ceramic (CoC) total hip arthroplasty, occurring in up to 10% of patients. The primary mechanical etiology is edge loading, which occurs when the femoral head contacts the rim of the acetabular liner rather than the congruent spherical surface. This is most commonly caused by component malposition, specifically excessive acetabular cup inclination (steep cup) or excessive anteversion. Edge loading disrupts the fluid film lubrication, leading to stripe wear on the ceramic head, increased friction, and the generation of high-frequency acoustic vibrations (squeaking). While squeaking can be a nuisance, it is not always associated with catastrophic failure, though it indicates suboptimal biomechanics.

Question 4173

Topic: 3. Adult Reconstruction (Hip & Knee)

A 78-year-old woman sustains a fall 5 years after undergoing a cementless total hip arthroplasty. Radiographs reveal a spiral fracture of the proximal femur that extends just distal to the tip of the femoral stem. Comparison with previous radiographs demonstrates that the femoral stem has subsided 2 cm and is in varus. The proximal bone stock is of good quality. According to the Vancouver classification, what is the most appropriate surgical management?

. Open reduction and internal fixation with cerclage wires alone
. Open reduction and internal fixation with a lateral locking plate and strut allografts
. Revision to a long fully porous-coated or fluted tapered cementless stem
. Revision to a standard-length cemented stem
. Nonoperative management with a hip spica cast

Correct Answer & Explanation

. Revision to a long fully porous-coated or fluted tapered cementless stem

Explanation

Correct Answer: Revision to a long fully porous-coated or fluted tapered cementless stemThis patient has a Vancouver B2 periprosthetic femur fracture. The Vancouver classification is based on fracture location, implant stability, and bone stock. Type A fractures involve the trochanters. Type B fractures occur around or just distal to the stem. Type C fractures occur well below the stem. Type B is subdivided into B1 (well-fixed stem), B2 (loose stem, good bone stock), and B3 (loose stem, poor bone stock). Because the stem has subsided and shifted into varus, it is loose (B2). The standard of care for a Vancouver B2 fracture is revision arthroplasty using a long, diaphyseal-fitting stem (such as a fully porous-coated or fluted tapered stem) to bypass the fracture and achieve stable distal fixation, often supplemented with cerclage wires for the fracture itself. ORIF alone (options A and B) is reserved for B1 or C fractures and would fail in the presence of a loose implant.

Question 4174

Topic: 3. Adult Reconstruction (Hip & Knee)

A 55-year-old man with a metal-on-metal total hip arthroplasty placed 8 years ago presents with new-onset groin pain and a palpable anterior groin mass. Laboratory evaluation reveals significantly elevated serum cobalt and chromium levels. Joint aspiration yields sterile, cloudy fluid. MRI with metal artifact reduction sequence (MARS) demonstrates a large, thick-walled cystic mass communicating with the joint space. If a biopsy of the periprosthetic tissue were performed, what would be the predominant histologic finding?

. Abundant neutrophils and gram-positive cocci
. Sheets of lipid-laden macrophages
. Perivascular lymphocytic infiltration
. Birefringent polymeric wear debris
. Extensive woven bone formation

Correct Answer & Explanation

. Perivascular lymphocytic infiltration

Explanation

Correct Answer: Perivascular lymphocytic infiltrationThe patient is presenting with an adverse local tissue reaction (ALTR) or aseptic lymphocyte-dominated vasculitis-associated lesion (ALVAL), which is a known complication of metal-on-metal (MoM) bearings. This is a Type IV delayed hypersensitivity reaction to metal ions (cobalt and chromium). The classic histologic hallmark of ALVAL is a dense perivascular lymphocytic infiltrate, often accompanied by tissue necrosis and macrophage infiltration. Birefringent polymeric wear debris (Option D) is characteristic of polyethylene wear osteolysis, which typically presents with macrophages containing particulate debris, not a massive cystic pseudotumor driven by lymphocytes. Neutrophils (Option A) would indicate an acute infection.

Question 4175

Topic: 3. Adult Reconstruction (Hip & Knee)

A 68-year-old man presents with a painful total hip arthroplasty 3 years postoperatively. His ESR is 45 mm/hr and CRP is 25 mg/L. A diagnostic joint aspiration is performed, yielding synovial fluid with a white blood cell (WBC) count of 4,500 cells/uL and 75% polymorphonuclear leukocytes (PMNs). A synovial alpha-defensin test is positive. According to the 2018 International Consensus Meeting (ICM) criteria for periprosthetic joint infection (PJI), what is the most appropriate next step in management?

. Repeat the joint aspiration in 6 weeks
. Prescribe 6 weeks of oral suppressive antibiotics
. Proceed with surgical intervention (e.g., 1-stage or 2-stage revision) for PJI
. Perform a core biopsy of the proximal femur
. Administer an intra-articular corticosteroid injection for presumed inflammatory synovitis

Correct Answer & Explanation

. Proceed with surgical intervention (e.g., 1-stage or 2-stage revision) for PJI

Explanation

Correct Answer: Proceed with surgical intervention (e.g., 1-stage or 2-stage revision) for PJIAccording to the 2018 ICM criteria for PJI, a score of 6 or greater indicates an infection. The scoring system assigns points as follows: elevated CRP (>10 mg/L) or D-dimer = 2 points; elevated ESR (>30 mm/hr) = 1 point; elevated synovial WBC (>3,000 cells/uL) or leukocyte esterase (++ or +++) = 3 points; positive alpha-defensin = 3 points; elevated synovial PMN (>80%) = 2 points; elevated synovial CRP (>6.9 mg/L) = 1 point. In this patient: ESR is elevated (1 pt), CRP is elevated (2 pts), synovial WBC is elevated (3 pts), and alpha-defensin is positive (3 pts). The synovial PMN is 75% (0 pts). The total score is 1 + 2 + 3 + 3 = 9 points. Because the score is >= 6, the diagnosis of PJI is confirmed, and the appropriate management is surgical intervention (typically a 2-stage exchange arthroplasty in North America, though 1-stage is an option in select cases). Delaying treatment or giving a steroid injection is contraindicated.

Question 4176

Topic: Total Hip Arthroplasty (THA)

A 70-year-old woman undergoes a primary total hip arthroplasty via a posterior approach. Postoperatively, she suffers three recurrent posterior dislocations, all occurring when she attempts to stand up from a low chair. Radiographic evaluation reveals that the acetabular component is placed in 10 degrees of anteversion and 45 degrees of inclination. The femoral stem is in 15 degrees of anteversion. What is the most appropriate surgical intervention to address her instability?

. Revision of the acetabular component to increase anteversion
. Revision of the acetabular component to decrease anteversion
. Revision of the femoral component to decrease anteversion
. Application of a hip spica cast for 6 weeks
. Trochanteric advancement to increase abductor tension

Correct Answer & Explanation

. Revision of the acetabular component to increase anteversion

Explanation

Correct Answer: Revision of the acetabular component to increase anteversionThe patient is experiencing recurrent posterior dislocations due to component malposition. The Lewinnek safe zone for acetabular component placement is 15 +/- 10 degrees of anteversion (i.e., 5 to 25 degrees, though modern targets often aim for 20-25 degrees) and 40 +/- 10 degrees of inclination. However, combined anteversion (acetabular + femoral) is critical and should ideally be between 25 and 45 degrees. In this patient, the acetabular cup is in only 10 degrees of anteversion, which is relatively retroverted/under-anteverted, predisposing her to posterior instability during hip flexion (such as rising from a low chair). The femoral stem is in a normal range (15 degrees). The most appropriate treatment is revision of the acetabular component to increase its anteversion, thereby restoring proper combined anteversion and preventing posterior impingement and dislocation.

Question 4177

Topic: 3. Adult Reconstruction (Hip & Knee)

A surgeon places a cementless hemispherical acetabular cup using a 1 mm under-ream technique to achieve an initial press-fit. For optimal biologic osteointegration (bone ingrowth) into the porous coating of the implant, what are the maximum acceptable thresholds for the gap between the host bone and the implant, and the micromotion at the bone-implant interface?

. Pore size 1-10 micrometers; Micromotion < 150 micrometers
. Pore size 50-300 micrometers; Micromotion < 150 micrometers
. Pore size 500-1000 micrometers; Micromotion < 500 micrometers
. Pore size 50-300 micrometers; Micromotion < 500 micrometers
. Pore size 1-10 micrometers; Micromotion < 500 micrometers

Correct Answer & Explanation

. Pore size 50-300 micrometers; Micromotion < 150 micrometers

Explanation

Correct Answer: Pore size 50-300 micrometers; Micromotion < 150 micrometersSuccessful biologic fixation (bone ingrowth) of cementless implants requires specific mechanical and structural conditions. The ideal pore size for the porous coating is between 50 and 300 micrometers, which allows for vascularization and osteon formation. Additionally, initial mechanical stability is paramount. Micromotion at the bone-implant interface must be minimized. Studies have shown that micromotion less than 40 micrometers is ideal for bone ingrowth. Micromotion between 40 and 150 micrometers results in a combination of bone and fibrous tissue ingrowth. Micromotion greater than 150 micrometers leads exclusively to fibrous tissue formation and subsequent aseptic loosening. Furthermore, the gap between the implant and host bone should ideally be less than 50 micrometers to facilitate direct bone formation.

Question 4178

Topic: Total Hip Arthroplasty (THA)

During a posterior approach to the hip for a total hip arthroplasty, the surgeon identifies the piriformis tendon and the short external rotators. These structures are tagged and released near their femoral insertions. To prevent significant postoperative hematoma, a specific arterial branch located near the inferior border of the obturator externus and superior border of the quadratus femoris must be identified and ligated. From which major artery does this branch originate?

. Lateral femoral circumflex artery
. Medial femoral circumflex artery
. Inferior gluteal artery
. Superior gluteal artery
. Obturator artery

Correct Answer & Explanation

. Medial femoral circumflex artery

Explanation

Correct Answer: Medial femoral circumflex arteryDuring the posterior approach to the hip (Moore or Southern approach), the short external rotators (piriformis, superior gemellus, obturator internus, inferior gemellus) are released from the greater trochanter. The ascending branch of the medial femoral circumflex artery (MFCA) is consistently found coursing vertically near the inferior border of the obturator externus and the superior border of the quadratus femoris. It is critical to identify, coagulate, or ligate this vessel during the deep dissection to prevent excessive intraoperative bleeding and postoperative hematoma formation. The MFCA is the primary blood supply to the adult femoral head, but in the setting of THA, the head is resected, so ligating this branch is standard practice.

Question 4179

Topic: 3. Adult Reconstruction (Hip & Knee)

A 62-year-old man with a metal-on-polyethylene total hip arthroplasty (titanium alloy stem, cobalt-chromium head) presents with insidious onset of groin pain 7 years postoperatively. Radiographs demonstrate well-fixed components with no evidence of osteolysis. Aspiration of the hip yields dark, sterile fluid. Laboratory analysis reveals a significantly elevated serum cobalt level, while the serum chromium level is within normal limits. What is the most likely diagnosis?

. Polyethylene wear-induced osteolysis
. Mechanically assisted crevice corrosion (Trunnionosis)
. Unrecognized low-virulence periprosthetic joint infection
. Aseptic loosening of the femoral stem
. Adverse local tissue reaction from a metal-on-metal bearing

Correct Answer & Explanation

. Mechanically assisted crevice corrosion (Trunnionosis)

Explanation

Correct Answer: Mechanically assisted crevice corrosion (Trunnionosis)The patient has a metal-on-polyethylene bearing, ruling out a true metal-on-metal (MoM) bearing complication at the articular surface. However, he presents with symptoms and lab findings typical of metal toxicity (elevated cobalt). In a patient with a titanium stem and a cobalt-chromium head, the modular head-neck junction (the trunnion) is susceptible to mechanically assisted crevice corrosion (MACC), commonly referred to as trunnionosis. This process selectively releases cobalt ions into the surrounding tissue and bloodstream, leading to a high cobalt-to-chromium ratio (unlike MoM articular wear, which typically elevates both). This can cause an adverse local tissue reaction (ALTR) similar to that seen in MoM hips, presenting with pain, dark fluid, and pseudotumors despite well-fixed components and a non-metal articular bearing.

Question 4180

Topic: 3. Adult Reconstruction (Hip & Knee)

A 68-year-old male undergoes a primary total hip arthroplasty for severe osteoarthritis. Intraoperatively, the surgeon decides to utilize a high-offset femoral stem rather than a standard offset stem of the same size. Which of the following best describes the biomechanical consequence of this decision?

. Increased joint reaction force at the hip.
. Decreased tension on the abductor musculature.
. Increased abductor moment arm and decreased joint reaction force.
. Increased leg length with no change in the abductor moment arm.
. Medialization of the femoral shaft relative to the pelvis.

Correct Answer & Explanation

. Increased abductor moment arm and decreased joint reaction force.

Explanation

Correct Answer: Increased abductor moment arm and decreased joint reaction force.Increasing the femoral offset in a total hip arthroplasty moves the femur laterally relative to the center of rotation of the femoral head. This increases the abductor moment arm. Because the abductor moment arm is increased, the abductor muscles do not need to generate as much force to maintain a level pelvis during the single-leg stance phase of gait. Consequently, the overall joint reaction force across the hip is decreased. This also increases the tension on the abductor musculature (restoring it to normal if it was lax), which helps prevent dislocation. Increasing offset does not inherently increase leg length, as offset is a horizontal vector while leg length is a vertical vector.

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