Correct Answer: Increased joint reaction force

Femoral offset is defined as the horizontal 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 abductor moment arm. Because the body weight moment arm remains constant, the abductor muscles must generate a significantly greater force to maintain a level pelvis during single-leg stance. The joint reaction force across the hip is the vector sum of the body weight and the abductor muscle force. Therefore, an increase in required abductor force directly leads to an increased overall joint reaction force. This can lead to increased polyethylene wear and a persistent Trendelenburg gait due to abductor fatigue or weakness. Increasing offset would increase the bending moment on the stem, not decreasing it.

Correct Answer: Tensor fasciae latae

The direct anterior approach to the hip utilizes the internervous plane between the sartorius (femoral nerve) and the tensor fasciae latae (superior gluteal nerve) superficially, and the rectus femoris (femoral nerve) and gluteus medius (superior gluteal nerve) deep. During the exposure, the ascending branches of the lateral femoral circumflex artery are consistently encountered crossing the field transversely. These vessels must be identified and ligated to prevent significant postoperative hematoma. These ascending branches primarily supply the tensor fasciae latae (TFL) muscle.

Correct Answer: Component malpositioning leading to edge loading

Squeaking is a known complication specific to ceramic-on-ceramic (CoC) total hip arthroplasties, occurring in up to 1-10% of patients. The most significant risk factor for squeaking is component malpositioning, specifically acetabular cup anteversion and inclination outside the safe zone, which leads to edge loading. Edge loading disrupts the fluid film lubrication between the ceramic surfaces, causing stripe wear, increased friction, and micro-separation, which manifests clinically as an audible squeak. Other associated factors include younger age, higher activity level, and larger femoral head sizes (not smaller), but edge loading due to malposition is the primary mechanical driver.

Correct Answer: Corticosteroid injection into the iliopsoas bursa

This patient's clinical presentation is classic for iliopsoas impingement following total hip arthroplasty. Symptoms include activity-related groin pain, particularly with active hip flexion (e.g., getting into a car). It is often caused by a prominent anterior edge of the acetabular component irritating the iliopsoas tendon. The initial management for iliopsoas impingement should always be nonoperative, consisting of physical therapy, NSAIDs, and an image-guided corticosteroid injection into the iliopsoas bursa. This provides both diagnostic confirmation and therapeutic relief. If conservative measures fail after a prolonged trial, surgical intervention (such as arthroscopic or open iliopsoas tenotomy, or acetabular revision if the component is severely malpositioned or loose) may be considered.

Correct Answer: Two-stage exchange arthroplasty

This patient meets the criteria for a chronic periprosthetic joint infection (PJI). According to the MSIS criteria, a synovial WBC > 3,000 cells/µL or > 80% PMNs in a hip > 90 days post-op is highly indicative of infection. Elevated ESR/CRP and a positive alpha-defensin further confirm the diagnosis. Because the infection is chronic (occurring 3 years postoperatively with new-onset symptoms, not an acute hematogenous spread within 3 weeks), irrigation and debridement with modular exchange (DAIR) is contraindicated due to the presence of mature biofilm. The gold standard treatment for chronic PJI in North America is a two-stage exchange arthroplasty, which involves removal of all components, placement of an antibiotic spacer, a course of IV antibiotics, and subsequent reimplantation.

Correct Answer: Vancouver B2; Revision to a long fully porous-coated or fluted tapered stem

The 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.

Correct Answer: Insufficient acetabular anteversion

Component malposition is a leading cause of instability following total hip arthroplasty. The "safe zone" described by Lewinnek for the acetabular component is 40 ± 10 degrees of inclination (abduction) and 15 ± 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.

Correct Answer: Adaptive bone remodeling due to load bypass

The 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).

Correct Answer: Mechanically assisted crevice corrosion at the head-neck taper

This 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.

Correct Answer: Macrophage

The patient is presenting with periprosthetic osteolysis secondary to particulate wear debris (most commonly polyethylene). The biological cascade of osteolysis is initiated when macrophages phagocytose wear particles that are between 0.1 and 1.0 micrometers in size. Upon phagocytosis, the macrophages become activated and release a cascade of pro-inflammatory cytokines, including TNF-alpha, IL-1, IL-6, and PGE2. These cytokines stimulate the expression of RANKL, which ultimately leads to the recruitment, differentiation, and activation of osteoclasts. While the osteoclast is the effector cell that directly resorbs the bone, the macrophage is the primary cell responsible for phagocytosing the debris and initiating the entire inflammatory cascade.

Correct Answer: Decreased trunnion length and diameter

This 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.

Correct Answer: Increased anteversion and increased inclination

This patient has a "stiff spine" due to her prior L2-pelvis fusion, indicated by a change in pelvic tilt of less than 10 degrees between standing and sitting. In a normal patient, transitioning from standing to sitting causes the lumbar spine to flex and the pelvis to tilt posteriorly, which functionally increases acetabular anteversion and accommodates hip flexion, preventing anterior impingement and posterior dislocation. In a patient with a stiff spine, the pelvis fails to tilt posteriorly during sitting. To compensate for this lack of dynamic functional anteversion and prevent posterior dislocation, the surgeon must place the acetabular component in greater operative anteversion and inclination than the standard "safe zone."

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

This 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.

Correct Answer: Remelting; decreases ultimate tensile strength and fatigue crack propagation resistance

Irradiation of polyethylene creates cross-links that drastically improve wear resistance, but it also cleaves polymer chains, leaving free radicals. If left untreated, these free radicals react with oxygen in vivo, leading to oxidation, embrittlement, and catastrophic failure. To manage free radicals, the polyethylene is thermally treated. Remelting involves heating the polyethylene above its melting point (approx. 135-150°C). This completely eliminates all free radicals, providing excellent oxidation resistance. However, it decreases the crystallinity of the polymer, which reduces its mechanical properties, including ultimate tensile strength, yield strength, and fatigue crack propagation resistance. Annealing (heating below the melting point) preserves mechanical properties but fails to eliminate all free radicals, leaving the plastic susceptible to long-term oxidation. Vitamin E (alpha-tocopherol) can be added to quench free radicals without the need for remelting, thereby preserving mechanical strength while preventing oxidation.

Correct Answer: Superior gluteal nerve and femoral nerve

The direct anterior approach (Smith-Petersen) utilizes both superficial and deep internervous planes. The superficial plane is between the tensor fasciae latae (TFL), innervated by the superior gluteal nerve, and the sartorius, innervated by the femoral nerve. The deep plane is between the gluteus medius (and minimus), innervated by the superior gluteal nerve, and the rectus femoris, innervated by the femoral nerve. Therefore, the nerves defining the boundaries of this approach are the superior gluteal nerve laterally and the femoral nerve medially.

Correct Answer: To prevent sciatic nerve palsy

Crowe IV DDH is characterized by a high hip dislocation with greater than 100% subluxation. The native acetabulum is hypoplastic, and a false acetabulum forms superiorly. When performing a THA in these patients, placing the cup in the true anatomic hip center provides the best biomechanical advantage and longevity. However, bringing the femur down to the true hip center requires significant lengthening of the limb. Lengthening the limb by more than 4 cm poses a high risk of stretch-induced sciatic nerve palsy. To safely place the cup at the true center without overstretching the sciatic nerve, a subtrochanteric shortening osteotomy of the femur is routinely performed.

Correct Answer: Edge loading and stripe wear

Squeaking 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.

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

Protrusio 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.

Correct Answer: Increased joint reactive forces and increased abductor weakness

Femoral 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.

Correct Answer: One-stage or two-stage revision arthroplasty for chronic PJI

This patient has a chronic periprosthetic joint infection (PJI). According to the 2018 ICM criteria, a synovial WBC count > 3,000 cells/µL 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.

Correct Answer: Increased abductor muscle force required during the stance phase of gait

Femoral 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.

Correct Answer: Component malposition leading to edge loading

Squeaking 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.

Correct Answer: Revision to a long fully porous-coated or fluted tapered cementless stem

This 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.

Correct Answer: Lateral femoral cutaneous nerve; anterolateral thigh

The direct anterior approach to the hip utilizes the superficial internervous plane between the tensor fasciae latae (superior gluteal nerve) and the sartorius (femoral nerve). The deep plane is between the rectus femoris (femoral nerve) and the gluteus medius (superior gluteal nerve). The lateral femoral cutaneous nerve (LFCN) exits the pelvis medial to the anterior superior iliac spine (ASIS) and courses distally over the sartorius muscle. It is at high risk of stretch or transection during the superficial dissection and retraction of the direct anterior approach. Injury to the LFCN results in numbness, paresthesias, or meralgia paresthetica in the anterolateral thigh. The femoral nerve is located more medially in the femoral triangle and is rarely injured unless retractors are placed carelessly medial to the psoas.

Correct Answer: Perivascular lymphocytic infiltration

The 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.

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

According 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.

Correct Answer: Revision of the acetabular component to increase anteversion

The 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.

Correct Answer: Pore size 50-300 micrometers; Micromotion < 150 micrometers

Successful 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.

Correct Answer: Medial femoral circumflex artery

During 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.

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.

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.

Correct Answer: It passes deep to the inguinal ligament, approximately 1 to 2 cm medial to the anterior superior iliac spine.

The direct anterior approach utilizes the internervous plane between the sartorius (femoral nerve) and the tensor fasciae latae (superior gluteal nerve). The nerve most at risk during the superficial dissection is the lateral femoral cutaneous nerve (LFCN). The LFCN typically passes deep to the inguinal ligament, about 1 to 2 cm medial to the anterior superior iliac spine (ASIS), and courses distally over the sartorius muscle. Injury to this nerve results in meralgia paresthetica (numbness/dysesthesia over the anterolateral thigh). The superior gluteal nerve exits superior to the piriformis. The femoral nerve travels between the psoas and iliacus. The obturator nerve exits through the obturator canal. The sciatic nerve passes posterior to the hip joint.

Correct Answer: Dense perivascular infiltrate of T-lymphocytes and macrophages with tissue necrosis.

This patient is presenting with an adverse local tissue reaction (ALTR) or aseptic lymphocyte-dominated vasculitis-associated lesion (ALVAL), commonly referred to as a pseudotumor, which is a known complication of metal-on-metal hip arthroplasty. The histological hallmark of ALVAL is a dense, perivascular infiltrate of T-lymphocytes, macrophages, and plasma cells, often accompanied by extensive tissue necrosis and fibrin exudation. This represents a delayed-type (Type IV) hypersensitivity reaction to metal ions (cobalt and chromium). Polymorphonuclear leukocytes indicate acute infection. Multinucleated giant cells with birefringent particles are characteristic of polyethylene wear-induced osteolysis. Monoclonal plasma cells suggest multiple myeloma. Non-caseating granulomas are seen in sarcoidosis.

Correct Answer: Two-stage revision arthroplasty.

This patient meets the Musculoskeletal Infection Society (MSIS) criteria for a chronic periprosthetic joint infection (PJI) of the hip. She has elevated inflammatory markers (ESR >30, CRP >1.0) and an elevated synovial WBC count (>3,000 cells/µL) with a high neutrophil percentage (>80%). Because the infection is chronic (occurring 3 years postoperatively with an insidious onset), a DAIR procedure is contraindicated, as the mature biofilm cannot be eradicated without removing the implants. The gold standard treatment for chronic PJI in North America is a two-stage revision arthroplasty, which involves complete removal of all components and cement, placement of an antibiotic-loaded spacer, a course of IV antibiotics, and subsequent reimplantation once the infection is cleared.

Correct Answer: Increased wear resistance and decreased fracture toughness.

Highly cross-linked polyethylene (HXLPE) is created by exposing UHMWPE to gamma or electron beam irradiation, which breaks polymer chains and allows them to recombine (cross-link). This process dramatically increases wear resistance. However, irradiation also creates free radicals that can lead to oxidation and degradation over time. To eliminate these free radicals, the material is thermally treated (remelted or annealed). Remelting eliminates all free radicals (decreasing oxidation potential) but alters the crystalline structure, which decreases the material's mechanical properties, specifically reducing its fracture toughness, yield strength, and ultimate tensile strength compared to conventional UHMWPE.

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

This patient has a Vancouver B2 periprosthetic femur fracture. The Vancouver classification is based on fracture location, implant stability, and bone stock. Type B fractures occur around or just distal to the stem. B1 indicates a well-fixed stem, B2 indicates a loose stem with adequate bone stock, and B3 indicates a loose stem with poor bone stock. The radiographic findings of subsidence and varus shift confirm the stem is loose (B2). The standard of care for a Vancouver B2 fracture is revision of the femoral component using a long, uncemented, extensively porous-coated or fluted tapered stem that bypasses the most distal aspect of the fracture by at least two cortical diameters. ORIF alone (Options 1 and 2) is reserved for B1 fractures. A standard-length cemented stem would not provide adequate distal fixation.

Correct Answer: The true acetabulum is located inferior and medial to the false acetabulum, and the femur exhibits excessive anteversion.

In severe developmental dysplasia of the hip (Crowe Type IV), the femoral head is completely dislocated from the true acetabulum and often articulates with a false acetabulum (neoacetabulum) on the ilium. The true acetabulum is located inferior and medial to this false acetabulum and is typically shallow, deficient anteriorly and superiorly, and filled with fibrofatty tissue (pulvinar). On the femoral side, the anatomy is characterized by a narrow (stovepipe) medullary canal, excessive femoral neck anteversion, a short femoral neck, and a posteriorly displaced greater trochanter. The sciatic nerve is often shortened due to the proximal migration of the femur and is at high risk for stretch injury when the hip is brought down to the true acetabulum, often necessitating a femoral shortening osteotomy.

Correct Answer: Impingement of the femoral neck against the anterior acetabular rim during hip flexion and internal rotation.

Posterior dislocation of a total hip arthroplasty typically occurs with the hip in flexion, adduction, and internal rotation. The "safe zone" for acetabular component positioning is generally considered to be 40 ± 10 degrees of inclination and 15 ± 10 degrees of anteversion. In this patient, the cup has 0 degrees of anteversion (it is relatively retroverted). When the cup lacks adequate anteversion, the anterior rim of the acetabular component is prominent. During hip flexion and internal rotation, the anterior aspect of the femoral neck impinges against this prominent anterior rim, which acts as a fulcrum to lever the femoral head out of the socket posteriorly. Therefore, anterior impingement leads to posterior dislocation.

Correct Answer: Fretting and crevice corrosion at the modular head-neck junction.

This patient has a metal-on-polyethylene (MoP) bearing surface, yet presents with elevated cobalt levels and normal chromium levels. This specific ion profile (high cobalt, normal/low chromium) in a non-metal-on-metal hip is the hallmark of mechanically assisted crevice corrosion (MACC), also known as trunnionosis. This occurs at the modular junction between the titanium femoral stem neck (trunnion) and the cobalt-chromium femoral head. Micro-motion at this interface leads to fretting wear, which disrupts the passivation layer, allowing crevice corrosion to occur and releasing cobalt ions into the surrounding tissue, potentially causing an adverse local tissue reaction (ALTR). Bearing surface wear in this construct would produce polyethylene debris, not cobalt.

Correct Answer: Stage III; Total hip arthroplasty.

The patient has osteonecrosis (avascular necrosis) of the femoral head. The Ficat and Arlet classification is based on radiographic findings: Stage I has normal x-rays (MRI positive); Stage II shows cystic/sclerotic changes but a spherical head; Stage III is characterized by subchondral collapse (the "crescent sign") and flattening of the femoral head, with a preserved joint space; Stage IV involves secondary osteoarthritis with joint space narrowing and acetabular changes. This patient has a crescent sign and mild flattening with preserved joint space, making it Stage III. Once subchondral collapse has occurred (Stage III or IV), joint-preserving procedures like core decompression are highly prone to failure. Total hip arthroplasty is the most reliable and successful treatment for Stage III and IV osteonecrosis to relieve pain and restore function.

Correct Answer: Decreased abductor muscle tension required for pelvic stability.

Increasing the femoral offset increases the lever arm of the abductor musculature. According to the biomechanics of the hip, the joint reaction force is a balance between the body weight (and its lever arm) and the abductor force (and its lever arm). By increasing the abductor lever arm (offset), less abductor force is required to counteract the body weight and maintain a level pelvis. Consequently, this decreases the overall joint reaction force across the hip. Increasing offset also increases the impingement-free range of motion and soft tissue tension, reducing the risk of dislocation. However, a negative consequence of increasing femoral offset is that it increases the bending moment (stress) on the femoral stem, potentially increasing the risk of stem fatigue failure or loosening if extreme.

Correct Answer: Squeaking

Ceramic-on-ceramic (CoC) bearings offer the lowest wear rates of all bearing couples, making them an attractive option for young, active patients. However, a unique complication associated with CoC bearings is "squeaking," which has been reported in up to 10% of patients. Squeaking is often associated with component malposition (specifically edge loading), loss of fluid film lubrication, or third-body wear. Trunnionosis (corrosion at the head-neck junction) is typically seen with large metal heads on titanium stems. Osteolysis is most classically associated with polyethylene wear debris. Elevated metal ions and ALVAL are characteristic of metal-on-metal bearing surfaces.

Correct Answer: Superior gluteal nerve and femoral nerve

The direct anterior approach to the hip utilizes a true internervous and intermuscular plane. The superficial dissection is between the tensor fasciae latae (TFL), which is innervated by the superior gluteal nerve, and the sartorius, which is innervated by the femoral nerve. The deep dissection utilizes the plane between the rectus femoris (femoral nerve) and the gluteus medius (superior gluteal nerve). This approach avoids detaching any muscles from the pelvis or femur, which is theorized to allow for a faster early recovery.

Correct Answer: Revision to a long fully porous-coated cementless stem.

This patient has a Vancouver B2 periprosthetic femur fracture. The Vancouver classification is based on fracture location, implant stability, and bone stock. Type A fractures are in the trochanteric region. Type B fractures are around or just distal to the stem. Type C fractures are well below the stem. Type B is subdivided into B1 (well-fixed stem), B2 (loose stem, adequate bone stock), and B3 (loose stem, poor bone stock). Because this stem is loose (subsidence) but bone stock is adequate, it is a B2 fracture. The standard of care for a Vancouver B2 fracture is revision to a long cementless stem that bypasses the most distal aspect of the fracture by at least two cortical diameters. ORIF alone (Option A) is reserved for B1 or C fractures. Proximal femoral replacement (Option D) is indicated for B3 fractures.

Correct Answer: Perivascular lymphocytic infiltrate.

The patient is presenting with an adverse local tissue reaction (ALTR), specifically an aseptic lymphocyte-dominated vasculitis-associated lesion (ALVAL), which is a known complication of metal-on-metal hip arthroplasty. ALVAL is a Type IV delayed hypersensitivity reaction to metal ions (cobalt and chromium). Histologically, it is characterized by a dense perivascular lymphocytic infiltrate. Polymorphonuclear leukocytes (Option A) would indicate an acute bacterial infection. Sheets of macrophages with birefringent particles (Option B) are characteristic of polyethylene wear-induced osteolysis. Caseating granulomas (Option D) are seen in tuberculosis.

Correct Answer: Lateral epiphyseal branch of the medial circumflex femoral artery

The predominant blood supply to the adult femoral head is the medial circumflex femoral artery (MCFA). Specifically, the lateral epiphyseal branches of the MCFA supply the critical weight-bearing superior and lateral portions of the femoral head. The lateral circumflex femoral artery supplies the anterior and inferior portions of the head and neck but is less critical. The artery of the ligamentum teres (a branch of the obturator artery) provides a negligible amount of blood supply in the adult, though it is more significant in the pediatric population.

Correct Answer: Stress shielding

The radiographic findings describe stress shielding, which is a manifestation of Wolff's Law. Wolff's Law states that bone remodels in response to the mechanical stresses placed upon it. When a stiff, extensively porous-coated diaphyseal-fitting stem is used, the implant achieves rigid distal fixation and assumes the majority of the mechanical load, bypassing the proximal femur. Because the proximal femur is "shielded" from normal physiological stress, it undergoes disuse osteopenia and calcar resorption. This is typically asymptomatic and rarely leads to clinical failure, but it is a distinct radiographic phenomenon compared to osteolysis (which is driven by a macrophage response to particulate wear debris).

Correct Answer: Insufficient acetabular anteversion

The "safe zone" for acetabular component positioning, as classically described by Lewinnek, is 40 +/- 10 degrees of abduction (inclination) and 15 +/- 10 degrees of anteversion. This patient's cup is in 0 degrees of anteversion, meaning it is retroverted relative to the safe zone. Insufficient anteversion (or retroversion) of the acetabular component strongly predisposes the hip to posterior dislocation, especially during activities involving hip flexion, adduction, and internal rotation (like bending over to tie a shoe). Her abduction angle (45 degrees) is within the acceptable range.

Correct Answer: Ascending branch of the medial circumflex femoral artery

During the posterior approach to the hip, the short external rotators are detached from the femur. The ascending branch of the medial circumflex femoral artery (MCFA) consistently runs near the superior border of the quadratus femoris. If the quadratus femoris is released too far medially or without prior identification and coagulation of this vessel, brisk bleeding can occur. To avoid this, many surgeons leave the quadratus femoris intact or only partially release its superior edge while carefully cauterizing the ascending branch of the MCFA.

Correct Answer: Decreases the required force of the hip abductors.

A Trendelenburg gait (abductor lurch) is a compensatory mechanism used by patients with hip pain or abductor weakness. By shifting the torso laterally over the affected hip during the stance phase, the patient moves their center of gravity closer to the center of rotation of the hip joint. This significantly decreases the lever arm of the body weight. Because the torque created by body weight is reduced, the opposing force required by the hip abductors to maintain a level pelvis is also proportionally decreased. Consequently, this compensatory lurch decreases the overall joint reaction force across the painful hip, thereby reducing pain during ambulation.