Orthopedic Basic MCQs 2026 | Board Review & Answers

Question 1

A 35-year-old male sustains a diaphyseal femur fracture. Which of the following factors is most critical for the initiation of endochondral ossification during secondary fracture healing?

Explanation

Endochondral ossification, characteristic of secondary fracture healing, is promoted by a degree of interfragmentary motion (relative stability) and robust vascularity. Absolute rigidity (as seen in primary healing) primarily favors intramembranous ossification. While hematoma is essential, motion and vascularity are more direct stimuli for the chondrogenic phase. High oxygen tension inhibits cartilage formation and favors bone. Inflammatory cells are critical for the initial stages of healing.

Question 2

Regarding the biomechanics of cortical bone, which statement is most accurate?

Explanation

Cortical bone is an anisotropic material, meaning its mechanical properties (like Young's modulus) vary with the direction of applied load. It is significantly stronger in compression than in tension. Creep is time-dependent deformation under constant load, not primarily due to rapid loading. Fatigue fractures result from repetitive loading below the ultimate strength. Bone exhibits viscoelastic properties, not perfect elasticity, even below the yield point.

Question 3

Which cytokine is primarily responsible for osteoclast differentiation and activation?

Explanation

RANKL (Receptor Activator of Nuclear factor Kappa-B Ligand) is the primary cytokine responsible for osteoclast differentiation, activation, and survival. It is secreted by osteoblasts and stromal cells and binds to RANK receptors on pre-osteoclasts and mature osteoclasts. TGF-β and IGF-1 are generally anabolic for bone. BMP-2 is critical for osteoblast differentiation. IL-6 has multiple roles, including stimulating osteoclast activity, but RANKL is the most direct and crucial regulator.

Question 4

A 60-year-old patient undergoes total knee arthroplasty. Which type of wear mechanism is typically most concerning for long-term polyethylene liner failure due to microscopic particle release leading to osteolysis?

Explanation

Fatigue wear is generally the most concerning type of wear in total joint arthroplasty polyethylene liners. It involves the propagation of subsurface cracks under cyclic loading, leading to the release of microscopic polyethylene particles. These particles initiate an inflammatory response, leading to osteolysis and aseptic loosening. Adhesive and abrasive wear are also relevant but less often the primary mode of catastrophic failure. Third-body wear involves particles from other sources. Corrosion wear is typically metallic.

Question 5

Regarding articular cartilage, which statement is FALSE?

Explanation

Articular cartilage is avascular, aneural, and alymphatic. It relies on synovial fluid for nutrient exchange and waste removal through diffusion, not direct vascularization. It is indeed composed primarily of type II collagen, and chondrocytes synthesize the matrix. Proteoglycans like aggrecan attract water, providing turgor and compressive strength. The tidemark is a critical histological landmark separating the two layers of cartilage.

Question 6

What is the primary role of osteoprotegerin (OPG) in bone metabolism?

Explanation

Osteoprotegerin (OPG) is a soluble decoy receptor for RANKL. By binding to RANKL, OPG prevents RANKL from binding to its receptor (RANK) on pre-osteoclasts and osteoclasts, thereby inhibiting osteoclast differentiation, function, and survival. This effectively reduces bone resorption. It does not directly stimulate osteoblast differentiation or mineralization, nor does it affect renal calcium reabsorption or PTH secretion directly.

Question 7

Which growth factor is most potent in inducing ectopic bone formation and is commonly used in spinal fusion procedures?

Explanation

Bone Morphogenetic Proteins (BMPs), particularly BMP-2 and BMP-7, are known for their potent osteoinductive properties and ability to induce ectopic bone formation. BMP-2 is widely used clinically in spinal fusion and fracture nonunion treatment. PDGF, FGF, and VEGF are important for angiogenesis and soft tissue healing but are not as osteoinductive. TGF-α is less relevant in this context.

Question 8

A patient presents with signs of an impending pathological fracture in the femoral diaphysis due to metastatic disease. What is the most significant biomechanical factor predicting the risk of fracture in this scenario?

Explanation

Cortical involvement exceeding 50% of the circumference (or 2.5 cm length) is a widely recognized critical threshold for impending pathological fracture risk, often prompting prophylactic fixation. While other factors contribute, the degree of cortical destruction is the most direct biomechanical indicator of structural compromise and increased risk of fracture. The Mirels' score incorporates this along with pain, lesion type, and size, but cortical involvement is key.

Question 9

Regarding the vascular supply of long bones, which statement is most accurate?

Explanation

The nutrient artery enters the diaphysis and branches into ascending and descending medullary arteries, supplying the inner two-thirds to three-quarters of the cortical bone via Haversian and Volkmann's canals. The periosteal arteries supply the outer one-third to one-quarter of the cortex. Metaphyseal arteries are crucial for the growth plate and also contribute to the vascularity of the adjacent diaphysis, especially after skeletal maturity. Venous drainage occurs through nutrient veins, metaphyseal veins, and periosteal veins.

Question 10

What is the primary function of the Volkmann's canals in cortical bone?

Explanation

Volkmann's canals are horizontal channels that connect adjacent Haversian canals (which run longitudinally) and also connect Haversian canals to the periosteal and endosteal surfaces. They contain blood vessels and nerves, facilitating the intricate vascular and neural network within cortical bone. Lacunae house osteocytes, and canaliculi connect lacunae. Cancellous bone structure is different.

Question 11

Which of the following describes the 'stress-shielding' phenomenon observed in orthopedic implants?

Explanation

Stress-shielding refers to the phenomenon where a stiff orthopedic implant (e.g., a total hip stem) bears a significant portion of the mechanical load, thereby 'shielding' the adjacent bone from its normal physiological stress. In accordance with Wolff's Law, this reduction in stress leads to bone resorption and decreased bone density in the shielded areas, which can potentially lead to implant loosening or periprosthetic fracture risk.

Question 12

Regarding muscle physiology, which statement about Type IIx (fast glycolytic) muscle fibers is correct?

Explanation

Type IIx (fast glycolytic) muscle fibers are characterized by a high rate of force generation, rapid contraction speed, and high anaerobic capacity, but they fatigue very quickly due to low mitochondrial density and low oxidative capacity. They are white in color due to low myoglobin content. Type I (slow oxidative) fibers are fatigue-resistant, have high mitochondrial density, and are red. Type IIa (fast oxidative-glycolytic) fibers are intermediate.

Question 13

What is the primary function of the Golgi tendon organ?

Explanation

The Golgi tendon organ (GTO) is a proprioceptor located in the musculotendinous junction. Its primary function is to sense muscle tension. When tension becomes excessive, the GTO sends inhibitory signals via Ib afferents to the motor neurons of the same muscle, causing it to relax, thereby protecting the muscle and tendon from injury. Muscle spindles detect muscle length and stretch.

Question 14

Which of the following statements regarding the role of calcium in muscle contraction is FALSE?

Explanation

Calcium does not directly bind to myosin heads to facilitate cross-bridge formation. Instead, calcium binds to troponin C, which then causes a conformational change in the troponin-tropomyosin complex, exposing the myosin-binding sites on the actin filaments. Myosin heads then bind to actin, initiating the power stroke. All other statements are correct regarding the role of calcium in excitation-contraction coupling.

Question 15

Which type of collagen is predominantly found in fibrocartilage, such as that of the meniscus or intervertebral disc?

Explanation

Fibrocartilage, found in structures like the menisci, intervertebral discs, and labrum, is primarily composed of Type I collagen, providing significant tensile strength. Type II collagen is characteristic of hyaline cartilage. Type III is found in skin and blood vessels. Type IV is found in basement membranes. Type IX is a minor component of hyaline cartilage.

Question 16

What is the primary biomechanical function of the subchondral bone in a healthy joint?

Explanation

The subchondral bone acts as a shock absorber, dissipating compressive loads applied across the joint and protecting the overlying articular cartilage from excessive stress. While it does provide structural support, its primary biomechanical role is energy absorption. Nutrient diffusion primarily occurs from synovial fluid. It is not a direct metabolic reservoir for chondrocytes in the same way as synovial fluid.

Question 17

A fracture demonstrating a 'butterfly fragment' is typically indicative of which type of loading mechanism?

Explanation

A butterfly fragment, or wedge fragment, is characteristic of a fracture caused by a bending moment combined with axial compression. The fragment results from a combination of compressive failure on the concave side and tensile failure on the convex side of the bending bone. Pure tension results in transverse fractures, pure compression in oblique or comminuted fractures, and pure torsion in spiral fractures.

Question 18

Regarding the healing of a meniscal tear, which zone has the highest potential for intrinsic repair?

Explanation

The red-red zone, which is the peripheral 10-30% of the meniscus, has a direct blood supply from the capsular arteries. Tears in this zone have the highest potential for healing through repair due to the presence of blood-borne healing factors. The red-white zone has limited potential, and the white-white zone is avascular with very poor intrinsic healing capacity.

Question 19

Which of the following statements about the role of synovial fluid is FALSE?

Explanation

Synovial fluid is acellular, containing very few cells, primarily synoviocytes, macrophages, and lymphocytes, but not abundant fibroblasts for repair. Its primary roles are lubrication (due to hyaluronic acid and lubricin), nutrition, and waste removal for the avascular articular cartilage. It is a non-Newtonian fluid, meaning its viscosity changes with shear rate, becoming less viscous with movement, which is beneficial for joint function.

Question 20

Which of the following nerve injuries carries the best prognosis for recovery?

Explanation

First-degree neuropraxia (Sunderland classification, equivalent to Seddon's neuropraxia) involves a temporary conduction block with intact axons and connective tissue sheaths. Recovery is complete, usually within days to weeks. Axonotmesis involves axonal damage but intact endoneurium, with Wallerian degeneration and potential for recovery. Neurotmesis involves complete severance of the nerve, requiring surgical repair with the poorest prognosis. Sunderland's classification has 5 degrees, with neuropraxia generally encompassing the mildest forms (1st degree).

Question 21

A common mechanism for bone loss in metabolic conditions involves an imbalance in the RANKL/OPG ratio. An increase in which of these would lead to increased bone resorption?

Explanation

An increase in RANKL expression by osteoblasts and stromal cells leads to more RANKL available to bind to RANK receptors on osteoclast precursors, thereby increasing osteoclast differentiation, activation, and survival, resulting in increased bone resorption. Increased OPG production or increased binding of OPG to RANK would inhibit osteoclast activity. Decreased RANKL production or decreased activation of RANK receptors would also lead to reduced bone resorption.

Question 22

Which of the following describes the 'creep' phenomenon in viscoelastic materials like bone or ligaments?

Explanation

Creep is the time-dependent increase in deformation (strain) of a material when subjected to a constant load (stress) over time. Stress relaxation, conversely, is the time-dependent decrease in stress when a material is held at a constant deformation. Viscoelastic materials exhibit both creep and stress relaxation. Immediate deformation is elastic or plastic response. Fatigue relates to repeated loading. Yield point marks the onset of plastic deformation.

Question 23

What is the key histological feature distinguishing osteoid from mature bone matrix?

Explanation

Osteoid is the unmineralized organic matrix produced by osteoblasts. Mature bone matrix is osteoid that has subsequently undergone mineralization with hydroxyapatite crystals. Both contain collagen fibers (primarily type I) and osteocytes. Haversian systems are features of cortical bone structure, not a distinction of osteoid itself.

Question 24

A patient with a distal radius fracture is treated with closed reduction and casting. Which of the following cells are the first to arrive and initiate the fracture healing cascade?

Explanation

In the initial inflammatory phase of fracture healing, the first cells to arrive at the fracture site are polymorphonuclear leukocytes (neutrophils), followed shortly by macrophages and lymphocytes. These cells are crucial for clearing debris and initiating the inflammatory cascade. Osteoblasts and chondrocytes appear later as the repair phase begins.

Question 25

Which major cell type is primarily responsible for the degradation of the cartilaginous callus during endochondral ossification in fracture healing?

Explanation

During endochondral ossification, osteoclasts are the primary cells responsible for resorbing the calcified cartilage matrix (the cartilaginous callus) to create space for invading blood vessels and subsequent osteoblast-mediated bone formation. Chondrocytes produce the cartilage, and osteoblasts lay down new bone matrix. Macrophages clear debris.

Question 26

What is the primary role of vitamin D in bone metabolism?

Explanation

The primary role of activated vitamin D (calcitriol) is to increase intestinal absorption of calcium and phosphate, thereby ensuring adequate mineral availability for bone mineralization. It also has direct effects on bone remodeling and kidney function, but intestinal absorption is its most prominent and critical function for bone health. It does not directly stimulate osteoclast activity, but adequate calcium levels are essential for bone turnover.

Question 27

A patient with severe osteoarthritis of the knee is prescribed an NSAID. What is the primary mechanism of action of NSAIDs?

Explanation

Non-steroidal anti-inflammatory drugs (NSAIDs) primarily exert their analgesic and anti-inflammatory effects by inhibiting cyclooxygenase (COX) enzymes (COX-1 and COX-2). This inhibition reduces the synthesis of prostaglandins, which are key mediators of pain and inflammation. They do not selectively block TNF-alpha (biologics do), stimulate opioid receptors, or directly enhance cartilage repair. Some NSAIDs also inhibit leukotriene synthesis, but COX inhibition is the main mechanism.

Question 28

Which antibiotic class is commonly used for prophylactic coverage in orthopedic surgery, particularly against Gram-positive bacteria like Staphylococcus aureus?

Explanation

First or second-generation cephalosporins, such as Cefazolin, are the antibiotics of choice for surgical prophylaxis in most orthopedic procedures. They provide excellent coverage against common surgical site infection pathogens, particularly Gram-positive bacteria like Staphylococcus aureus, while having a relatively narrow spectrum compared to broader agents. Fluoroquinolones and aminoglycosides have broader spectrums and are typically reserved for established infections or specific indications. Macrolides and tetracyclines are generally not first-line for surgical prophylaxis.

Question 29

A patient is undergoing revision hip arthroplasty due to a suspected infection. Which laboratory marker is most sensitive for detecting chronic osteomyelitis?

Explanation

While both ESR and CRP are elevated in infection, ESR tends to remain elevated for a longer duration in chronic infections like osteomyelitis compared to CRP, which normalizes more rapidly. CRP is generally more sensitive for acute infections. WBC count is often normal in chronic osteomyelitis. Procalcitonin is typically used for severe systemic bacterial infections (sepsis). Serum creatinine assesses renal function.

Question 30

Which of the following imaging modalities is considered the 'gold standard' for diagnosing early avascular necrosis (AVN) of the femoral head?

Explanation

Magnetic Resonance Imaging (MRI) is the 'gold standard' for diagnosing early avascular necrosis (AVN) of the femoral head. It can detect changes in bone marrow edema and fat necrosis before any radiographic changes are visible. Plain radiographs are typically normal in early AVN. CT scans are good for bony detail but less sensitive than MRI for early marrow changes. Bone scintigraphy can be sensitive but less specific than MRI.

Question 31

Regarding the Mendelian inheritance patterns in orthopedics, which condition is typically inherited in an autosomal dominant manner?

Explanation

Osteogenesis Imperfecta (OI), particularly types I, II, III, and IV, is predominantly inherited in an autosomal dominant fashion, often due to mutations in the COL1A1 or COL1A2 genes, affecting Type I collagen synthesis. Spinal Muscular Atrophy and Cystic Fibrosis are autosomal recessive. Duchenne Muscular Dystrophy is X-linked recessive. Spina Bifida is a multifactorial condition.

Question 32

Which type of biopsy is generally preferred for diagnosing a suspected soft tissue sarcoma due to its ability to provide sufficient tissue for grading and subtyping?

Explanation

An incisional biopsy (where a portion of the tumor is removed) is generally preferred for suspected soft tissue sarcomas. It provides an adequate amount of tissue for full histological analysis, including tumor grading, subtyping, and special stains, which are critical for treatment planning. FNA often provides insufficient tissue. Excisional biopsy might compromise subsequent definitive surgical margins if the diagnosis is malignant. Punch biopsy is usually too small for deep soft tissue tumors. Frozen section is for rapid intraoperative assessment, not definitive diagnosis.

Question 33

What is the primary role of polymethylmethacrylate (PMMA) bone cement in joint arthroplasty?

Explanation

PMMA bone cement functions as a grout, filling the irregularities between the implant and the bone, and then polymerizes to create a mechanical interlock. It is not a biological adhesive and does not promote osteointegration. It transfers stress between the implant and bone but is not typically considered a primary load-bearing structural component in the same way the implant itself is. While antibiotics can be added to cement, its primary role is mechanical fixation.

Question 34

Which of the following is considered a relative contraindication to the use of a tourniquet in orthopedic surgery?

Explanation

Peripheral vascular disease (PVD) is a relative contraindication to tourniquet use due to the risk of exacerbating ischemia, thrombosis, or arterial damage in compromised vessels. While diabetes and sickle cell trait require careful consideration and monitoring, PVD carries a more direct risk of limb ischemia. Local skin infection is an absolute contraindication, not relative. Hypertension is generally managed medically and not a direct contraindication.

Question 35

What is the most common organism responsible for acute hematogenous osteomyelitis in healthy children?

Explanation

Staphylococcus aureus is by far the most common causative organism for acute hematogenous osteomyelitis across all age groups, including healthy children. While Kingella kingae is increasingly recognized in young children, S. aureus remains the dominant pathogen. E. coli and Pseudomonas are more common in neonates or immunocompromised patients. Streptococcus pyogenes is less common in bone infections.

Question 36

Which nerve is most commonly injured in fractures of the midshaft humerus?

Explanation

The radial nerve courses in the spiral groove (radial groove) of the humerus, making it particularly susceptible to injury in fractures of the midshaft humerus. Median and ulnar nerves are more commonly injured around the elbow. The axillary nerve is vulnerable with proximal humerus fractures. The musculocutaneous nerve is less commonly involved in humeral shaft fractures.

Question 37

What is the characteristic histological feature seen in Paget's disease of bone?

Explanation

Paget's disease (osteitis deformans) is characterized by highly disorganized bone remodeling, leading to a 'mosaic' or 'jigsaw' pattern of woven and lamellar bone. There is increased, chaotic osteoclastic resorption followed by rapid, disorganized osteoblastic bone formation. It does not involve decreased osteoclast activity, fibrous tissue replacement, or unmineralized osteoid accumulation primarily.

Question 38

A 70-year-old female presents with acute onset back pain and a vertebral compression fracture. Which of the following is the most effective pharmacologic agent for preventing future osteoporotic fractures by inhibiting osteoclast function?

Explanation

Denosumab is a monoclonal antibody that targets RANKL, preventing osteoclast formation and function, leading to a potent antiresorptive effect and significant reduction in fracture risk. Bisphosphonates also inhibit osteoclast function and are first-line. Teriparatide is an anabolic agent. Calcium/Vitamin D are essential supplements but not direct anti-osteoporosis agents alone. Strontium ranelate has dual action but is less commonly used. Calcitonin has modest efficacy.

Question 39

Which of the following statements regarding osteosarcoma is FALSE?

Explanation

The primary treatment for osteosarcoma involves neoadjuvant chemotherapy, surgical resection (limb-sparing surgery when possible), and adjuvant chemotherapy. Osteosarcoma is generally radioresistant, and radiation therapy is typically reserved for unresectable tumors, positive margins, or palliation. All other statements are true: it is common in children/adolescents, metaphyseal, produces osteoid, and metastasizes to the lungs.

Question 40

What is the typical appearance of Ewing's sarcoma on plain radiographs?

Explanation

Ewing's sarcoma is classically associated with an 'onion-skinning' or lamellated periosteal reaction, especially in the diaphysis of long bones. This pattern results from successive layers of new bone being laid down by the periosteum in response to tumor invasion. 'Sunburst' is more typical of osteosarcoma. 'Soap-bubble' is seen in giant cell tumor. 'Popcorn' calcifications are seen in enchondroma. Geographic lysis with a sclerotic rim suggests a benign lesion.

Question 41

The most important factor in preventing surgical site infections (SSIs) in orthopedic surgery is:

Explanation

While all options except for prolonged broad-spectrum antibiotics play a role in SSI prevention, the appropriate timing and selection of prophylactic antibiotics is widely considered the single most important intervention. Antibiotics should be administered within 60 minutes prior to incision and cover anticipated pathogens. Prolonged post-operative antibiotics are generally not recommended due to resistance concerns. Glycemic control, normothermia, and DVT prophylaxis are important adjuncts.

Question 42

Which of the following is an absolute contraindication to regional anesthesia in orthopedic surgery?

Explanation

Patient refusal is an absolute contraindication to any medical procedure, including regional anesthesia. While anticoagulation, pre-existing neuropathy, and history of epidural abscess are significant concerns and often contraindications, they are typically considered relative depending on the specific block, patient factors, and risk/benefit analysis. An INR of 1.5 is often acceptable for certain peripheral nerve blocks, but not an absolute contraindication across the board for all regional blocks (e.g., neuraxial blocks). Surgical site infection at the injection site is an absolute contraindication due to risk of spreading infection.

Question 43

Regarding local anesthetics, which property determines the duration of action?

Explanation

The duration of action of a local anesthetic is primarily determined by its protein binding. Higher protein binding allows the drug to remain at the nerve membrane for longer, extending its effect. Lipid solubility influences potency and speed of onset. pKa influences the speed of onset. Concentration affects the intensity and onset of the block. Vasoconstrictor additives prolong duration by reducing systemic absorption.

Question 44

Which bone is considered an irregular bone?

Explanation

Vertebrae are classified as irregular bones due to their complex and unique shapes that do not fit into other categories (long, short, flat, sesamoid). The femur is a long bone, patella is a sesamoid bone, scapula and rib are flat bones.

Question 45

What is the primary function of the periosteum in adult long bones?

Explanation

The periosteum, particularly its inner cambium layer, contains osteoprogenitor cells vital for bone repair and appositional growth (increase in bone width). It also contains blood vessels that supply the outer one-third to one-quarter of the cortical bone. Hematopoiesis occurs in the bone marrow. Bone growth in length occurs at the epiphyseal plates.

Question 46

In the context of fracture nonunion, which type of nonunion is generally characterized by sufficient vascularity but insufficient stability?

Explanation

Hypertrophic nonunion is characterized by abundant callus formation (indicating good biological activity and vascularity) but insufficient stability at the fracture site. The bone attempts to heal but cannot bridge the gap due to excessive motion, often resulting in an 'elephant foot' appearance on radiographs. Atrophic nonunion has poor vascularity and little callus formation. Oligotrophic is a variant of atrophic with minimal callus. Pseudarthrosis is a specific type of established nonunion with a false joint. Infected nonunion is due to infection.

Question 47

Which zone of the epiphyseal plate is responsible for the proliferation of chondrocytes, leading to longitudinal bone growth?

Explanation

The Zone of Proliferation contains rapidly dividing chondrocytes arranged in columns. These cells increase in number, pushing the epiphysis away from the diaphysis and thus contributing to longitudinal bone growth. The zone of resting cartilage anchors the plate. Hypertrophy involves cell enlargement. Calcification is where the matrix calcifies, and ossification is where bone replaces cartilage.

Question 48

A patient with a history of intravenous drug use presents with acute pain and swelling in the knee. Arthrocentesis reveals purulent fluid. Which organism is a common cause of septic arthritis in this population, besides Staphylococcus aureus?

Explanation

In intravenous drug users (IVDUs), Pseudomonas aeruginosa is a common cause of septic arthritis, particularly affecting large joints like the knee or sternoclavicular joint, in addition to Staphylococcus aureus. Neisseria gonorrhoeae is common in sexually active young adults. Streptococcus pneumoniae is less common. Borrelia burgdorferi causes Lyme arthritis. Salmonella spp. can cause osteomyelitis in sickle cell patients but is less common for septic arthritis in IVDUs.

Question 49

Which of the following is a recognized complication of using indomethacin for prophylactic ossification after total hip arthroplasty?

Explanation

Indomethacin, a non-selective NSAID, can inhibit prostaglandin synthesis, which is crucial for early stages of fracture healing and bone formation. Therefore, a recognized complication of its use (e.g., for heterotopic ossification prophylaxis) is delayed fracture healing, especially in fresh fractures. It is not typically associated with increased DVT risk (aspirin is used for DVT prophylaxis), hypertension crisis, or auditory disturbances as its main adverse effects.

Question 50

The concept of 'tension band plating' relies on converting which type of force into a compressive force at the fracture site?

Explanation

Tension band plating (or wiring) converts tensile forces (e.g., on the convex side of a bone under bending) into compressive forces at the fracture site. This helps stabilize the fracture and promotes healing by providing interfragmentary compression. It is commonly used for avulsion fractures or fractures on the tension side of eccentrically loaded bones (e.g., olecranon, patella, medial malleolus).

Question 51

Which imaging feature on plain radiographs is most indicative of chronic osteomyelitis?

Explanation

The presence of a sequestrum (a fragment of necrotic bone that has separated from the healthy bone) and an involucrum (a sheath of new bone laid down by the periosteum around the infected, dead bone) are classic radiographic signs of chronic osteomyelitis. Soft tissue swelling and periarticular effusion are nonspecific. Periosteal elevation and lucent lesions can be seen in acute osteomyelitis but are less specific for chronicity. Early stages can be normal.

Question 52

What is the primary advantage of uncemented (press-fit) components in total joint arthroplasty?

Explanation

The primary advantage of uncemented components is the potential for long-term biological fixation through osteointegration, where the patient's own bone grows directly onto or into the implant's porous surface. This can provide durable fixation. While it eliminates PMMA heat, it does not provide immediate biological fixation (mechanical press-fit is immediate), and revision surgery can be more complex due to bone ingrowth. Infection risk is not inherently lower.

Question 53

A patient presents with a painful mass in the distal femur. Biopsy reveals chondrosarcoma. Which statement accurately describes this tumor?

Explanation

Chondrosarcoma is a malignant tumor characterized by the production of cartilage matrix by malignant chondrocytes. It is generally resistant to chemotherapy and radiation, with surgical resection being the primary treatment. While some variants can occur in younger individuals, it primarily affects older adults (over 40-50 years). It typically metastasizes hematogenously, primarily to the lungs, rather than to regional lymph nodes.

Question 54

Which type of fracture pattern is typically associated with a high-energy rotational injury?

Explanation

Spiral fractures are characteristically caused by torsional (rotational) forces. The fracture line spirals along the shaft of the bone. Transverse fractures result from direct bending or tension. Oblique fractures can result from compression or bending. Comminuted fractures involve multiple fragments and are typically from high-energy direct trauma. Avulsion fractures occur when a tendon or ligament pulls off a piece of bone.

Question 55

The 'SAFE' interval in anterior cervical discectomy and fusion (ACDF) refers to the safe surgical corridor between which two anatomical structures?

Explanation

The 'SAFE' (Superior, Anterior, Fascial, Esophageal) interval or 'danger zone' in ACDF refers to the space between the carotid sheath (containing the carotid artery, jugular vein, vagus nerve) laterally and the esophagus/trachea medially, anterior to the prevertebral fascia. This corridor allows access to the cervical spine while minimizing injury to vital structures. The recurrent laryngeal nerve is within the tracheoesophageal groove, and careful retraction is needed to protect it.

Question 56

Which of the following describes the effect of chronic denervation on skeletal muscle?

Explanation

Chronic denervation of skeletal muscle leads to progressive atrophy, significant decrease in force production, and ultimately the replacement of muscle fibers with fibrous and adipose tissue. It also causes reduced metabolic activity and vascularity, and the neuromuscular junction degenerates. Muscle fibers generally convert towards a more 'slow-twitch' phenotype initially, but ultimately severe atrophy dominates, not hypertrophy.

Question 57

What is the primary characteristic that differentiates primary bone tumors from metastatic bone tumors on a population level?

Explanation

Metastatic bone tumors are significantly more common than primary bone tumors, especially in adults. While primary bone tumors can be lytic or blastic (e.g., osteosarcoma is often blastic), and metastatic tumors can be both lytic (e.g., renal cell) or blastic (e.g., prostate), the prevalence difference is the most defining characteristic on a population level. Presentation and margin clarity vary for both.

Question 58

Which type of fracture healing involves direct apposition of fracture ends with minimal callus formation, typically seen with absolute stability provided by rigid internal fixation?

Explanation

Primary (or direct) bone healing occurs when there is absolute stability and direct contact between the fracture ends, typically achieved with rigid internal fixation. This process involves direct remodeling by osteoclasts and osteoblasts, with minimal or no external callus formation. Secondary healing involves a callus, and endochondral ossification is part of secondary healing. Stress fractures heal like secondary fractures. Nonunion is a failure of healing.

Question 59

The concept of 'load sharing' in fracture fixation refers to:

Explanation

Load sharing describes a fixation construct where the bone fragments themselves (e.g., compressed fracture fragments) contribute to bearing the mechanical load, allowing the implant to share that load rather than bearing it entirely. This promotes more physiological stress on the bone, which can encourage healing and reduce stress shielding. Examples include intramedullary nails or plates applied with lag screws and compression.

Question 60

A 25-year-old professional athlete undergoes an osteochondral autograft transfer for a focal chondral defect of the medial femoral condyle. When reviewing the histologic zones of normal articular cartilage, the deepest layer (adjacent to the subchondral bone) is primarily characterized by which of the following structural orientations and functions?

Explanation

Articular cartilage consists of four distinct zones. The deep (radial) zone contains the thickest Type II collagen fibers, which are oriented perpendicular to the joint surface. These fibers cross the tidemark into the calcified zone to firmly anchor the uncalcified cartilage to the underlying subchondral bone. The superficial zone has fibers parallel to the joint surface to resist shear forces.

Question 61

A surgeon utilizes a structural cortical allograft to manage a massive diaphyseal bone defect during a revision total femur arthroplasty. Which of the following best describes the classical biological sequence of graft incorporation for this specific type of structural graft?

Explanation

Cortical bone grafts incorporate via creeping substitution, but unlike cancellous grafts, the sequence is uniquely osteoclastic first. Osteoclasts must bore out the existing Haversian canals to allow vascular ingrowth before osteoblasts can lay down new bone. This initial resorptive phase leads to a temporary, significant decrease in the mechanical strength of cortical allografts. In contrast, cancellous grafts undergo initial osteoblastic apposition on dead trabeculae, followed by remodeling.

Question 62

In optimizing the mechanical fixation of a non-locking plate for a diaphyseal osteoporotic humerus fracture, which of the following geometrical modifications to the cortical screw will most significantly increase its pullout strength?

Explanation

Pullout strength is the force required to pull a screw out of the bone and is proportional to the shear area of the bone engaged by the threads. The formula for pullout strength shows that it is most highly correlated with the major (outer) diameter of the screw. Increasing the core diameter increases the bending strength of the screw itself but does not increase pullout strength. Decreasing pitch increases the number of threads engaged, which helps, but is less impactful than increasing the major diameter.

Question 63

A 45-year-old male is treated with a minimally invasive bridge plate for a severely comminuted midshaft tibia fracture. According to Perren's strain theory, direct (primary) bone healing with lamellar bone formation can only occur when the interfragmentary strain is kept below which of the following thresholds?

Explanation

Perren's strain theory dictates that a specific tissue type can only form if the local mechanical strain does not exceed the strain tolerance of that tissue. Lamellar bone is highly rigid and can tolerate strains up to only 2%; thus, primary bone healing requires absolute stability (<2% strain). Woven bone tolerates up to 10% strain, cartilage tolerates up to 10%, and granulation tissue can tolerate up to 100% strain.

Question 64

A 65-year-old patient presents with aseptic loosening 15 years following a total hip arthroplasty. Radiographs reveal extensive eccentric osteolysis. The primary biological mediator of this osteolysis is activated by which of the following mechanisms?

Explanation

The primary cause of long-term aseptic loosening in joint arthroplasty is wear debris-induced osteolysis. UHMWPE particles (typically between 0.1 and 1.0 micrometers in size) are phagocytosed by macrophages. The macrophages become activated and release pro-inflammatory cytokines (such as TNF-alpha, IL-1, and IL-6), which subsequently stimulate osteoclastogenesis via the RANKL pathway, leading to localized bone resorption.

Question 65

A competitive long-distance marathon runner sustains an acute hamstring strain. Biopsies of the uninjured hamstrings in this athlete would reveal a predominance of muscle fibers characterized by which of the following physiologic profiles?

Explanation

Endurance athletes like marathon runners have a predominance of Type I (slow-twitch) muscle fibers in their continuously acting skeletal muscles. Type I fibers are uniquely adapted for prolonged, sustained contractions. They are characterized by a high density of mitochondria, high myoglobin content (giving them a 'red' appearance), abundant capillary supply, preferential aerobic (oxidative) metabolism, and slow contraction speeds that resist fatigue.

Question 66

A 28-year-old male sustains a closed humerus shaft fracture with an immediate, complete radial nerve palsy. An EMG performed at 4 weeks demonstrates fibrillation potentials in the brachioradialis. Advanced imaging confirms preserved continuity of the endoneurium despite complete axonal disruption. According to the Sunderland classification, what grade of nerve injury does this represent?

Explanation

This is a Sunderland second-degree nerve injury (which corresponds to Seddon's axonotmesis). The axon is completely disrupted, leading to Wallerian degeneration distal to the injury (indicated by fibrillation potentials on EMG at 4 weeks), but the endoneurial tube and all supporting connective tissue (perineurium, epineurium) remain intact. This intact endoneurial tube allows the regenerating axon to reach its specific target, typically resulting in complete clinical recovery.

Question 67

During open reduction and internal fixation of a periprosthetic femur fracture, an inexperienced surgeon opts to use a standard stainless steel cortical screw to secure a commercially pure titanium locking plate. Which of the following best explains why this combination of dissimilar metals increases the risk of premature implant failure?

Explanation

Mixing dissimilar metals (e.g., titanium and stainless steel) in the electrolytic environment of human tissue fluid creates a galvanic cell. Because the metals have different electrochemical potentials, electrons flow from the less noble metal (anode) to the more noble metal (cathode), resulting in accelerated galvanic corrosion of the anodic metal. This compromises structural integrity and releases potentially toxic metallic ions into the surrounding tissues.

Question 68

Following a primary flexor tendon repair in Zone II of the hand, a structured early active mobilization protocol is initiated. At the cellular and tissue level, what is the primary biological advantage of this protocol during the early stages of tendon healing?

Explanation

Flexor tendons heal through both intrinsic (proliferation of epitenon and endotenon cells) and extrinsic (ingrowth of fibroblasts from the surrounding tendon sheath) mechanisms. Extrinsic healing is largely responsible for the formation of motion-limiting peritendinous adhesions. Early controlled mobilization heavily suppresses extrinsic healing and favors intrinsic healing. It also applies mechanical stress that aligns the newly synthesized collagen fibrils longitudinally, greatly improving the tendon's tensile strength and functional gliding capacity.

Question 69

A 72-year-old male presents with deep, aching thigh pain, progressive hearing loss, and noticeable anterior bowing of his femur. Laboratory studies reveal an isolated, significantly elevated serum alkaline phosphatase level with normal calcium and phosphorus. A bone biopsy of the affected femur is most likely to demonstrate which of the following histological hallmarks?

Explanation

The clinical scenario (bone pain, bowing, hearing loss, isolated elevated alk phos in an elderly male) is classic for Paget's disease of bone (osteitis deformans). The disease involves phases of excessive osteoclastic resorption followed by frantic, disorganized osteoblastic bone formation. The resulting histologic hallmark, particularly during the mixed or sclerotic phase, is a 'mosaic' or 'jigsaw puzzle' pattern of woven and lamellar bone with prominent, haphazard cement lines reflecting the chaotic remodeling process.

Question 70

A 60-year-old patient undergoes a total knee arthroplasty. The articular cartilage removed during surgery naturally functions to minimize friction, primarily through boundary lubrication at high loads. Which molecule is most responsible for this boundary lubrication in a healthy joint?

Explanation

Lubricin (proteoglycan 4 or PRG4) is a glycoprotein synthesized by superficial zone chondrocytes and synoviocytes. It is the primary molecule responsible for boundary lubrication in articular cartilage, significantly reducing friction during high-load, low-velocity joint movement. Hyaluronic acid contributes primarily to elastohydrodynamic lubrication in synovial fluid.

Question 71

A surgeon is deciding between a solid and a hollow intramedullary nail of the exact same outer diameter and material for a diaphyseal tibia fracture. Compared to the solid nail, the hollow nail possesses which of the following mechanical characteristics?

Explanation

Torsional rigidity is proportional to the polar moment of inertia. For a cylinder, a hollow core reduces the polar moment of inertia compared to a solid cylinder of the identical outer diameter, thus lowering its torsional rigidity. Bending rigidity (determined by the area moment of inertia) is also lower for a hollow nail compared to a solid nail of the exact same outer diameter.

Question 72

A 4-year-old child presents with multiple fractures after minor trauma, blue sclerae, and hearing loss. Genetic testing reveals a mutation resulting in a defect in the folding of the collagen triple helix. Which of the following best describes the underlying genetic anomaly in the most severe, non-lethal form of this disease?

Explanation

The patient has Osteogenesis Imperfecta (OI). Type I OI (mildest) is a quantitative defect resulting in decreased production of normal type I collagen. Types II, III, and IV are qualitative defects, typically caused by a point mutation replacing glycine with a bulkier amino acid. This substitution disrupts the stable packing of the collagen triple helix. Type III is the most severe non-lethal form.

Question 73

Intravenous tranexamic acid (TXA) is administered to a patient undergoing a complex spinal fusion to reduce perioperative blood loss. Which of the following best describes the precise molecular mechanism of action of this agent?

Explanation

Tranexamic acid (TXA) is a synthetic analog of the amino acid lysine. It reversibly and competitively binds to the lysine receptor sites on plasminogen. This prevents plasminogen from binding to fibrin and being activated into plasmin, thereby effectively inhibiting fibrinolysis and stabilizing clots.

Question 74

In modern total hip arthroplasty, highly cross-linked polyethylene (HXLPE) is utilized to decrease wear rates. Which of the following is an adverse biomechanical consequence of increasing the radiation dose to maximize cross-linking in the polyethylene manufacturing process?

Explanation

While irradiation creates chemical cross-links that significantly improve the wear resistance of polyethylene, it inversely affects certain mechanical properties. Increasing the radiation dose leads to decreased ductility, yield strength, fracture toughness, and ultimate tensile strength, which can increase the risk of mechanical failure or catastrophic cracking of the liner.

Question 75

During the reparative phase of secondary fracture healing, a cartilaginous soft callus forms and is subsequently replaced by bone. Which specific type of collagen is most predominantly synthesized by the cells during the peak of this soft callus formation?

Explanation

Secondary fracture healing involves endochondral ossification. During the soft callus phase, chondrocytes rapidly proliferate and synthesize a cartilage matrix that is predominantly rich in Type II collagen. Later in the process, hypertrophic chondrocytes express Type X collagen to facilitate calcification, before osteoblasts finally lay down Type I collagen to form woven bone.

Question 76

During the normal human gait cycle, the peak force across the hip joint can reach up to 2.5 to 3 times body weight. During which specific phase of the gait cycle does the maximum joint reaction force across the hip occur?

Explanation

The maximum joint reaction force at the hip occurs during the late stance phase, specifically terminal stance (heel off). During this phase, the hip abductor muscles contract forcefully to stabilize the pelvis as the body's center of mass accelerates forward over the supporting limb, creating the highest compressive forces across the joint articulation.

Question 77

A patient presents with localized pain and swelling two years after internal fixation of a distal radius fracture with a stainless steel plate and screws. Radiographs show extensive peri-implant radiolucency. Revision surgery reveals dark grey tissue staining and loosening of one screw, which was found to be inadvertently made of titanium. What type of corrosion primarily caused this phenomenon?

Explanation

Galvanic corrosion occurs when two electrochemically dissimilar metals (e.g., stainless steel and titanium) are placed in direct physical contact within an electrolytic medium (such as human body fluids). An electrochemical cell is created, and the less noble (more anodic) metal corrodes preferentially and rapidly. This is why mixing different implant metals in a single construct is contraindicated.

Question 78

Following a severe stretch injury to the brachial plexus, a patient exhibits a delayed protective muscle reflex to sudden elongation. Which specialized mechanoreceptor is primarily responsible for detecting both the rate and magnitude of muscle stretch to initiate the myotatic (stretch) reflex?

Explanation

The muscle spindle is an encapsulated sensory receptor located parallel to extrafusal muscle fibers. It detects changes in muscle length and the rate of that change, initiating the myotatic (stretch) reflex via Ia afferent fibers. Conversely, the Golgi tendon organ is located in series at the musculotendinous junction, detecting muscle tension to initiate the inverse myotatic reflex (autogenic inhibition).

Question 79

Demineralized bone matrix (DBM) is frequently used in orthopedic surgery to augment spinal fusions. The primary biologic property of DBM that makes it beneficial for bone healing is directly attributed to the presence of which of the following?

Explanation

Demineralized bone matrix (DBM) is an allograft processed by acid extraction to remove the mineral component (hydroxyapatite). This process exposes the underlying osteoinductive proteins, primarily Bone Morphogenetic Proteins (BMPs). DBM is predominantly osteoinductive and osteoconductive, but lacks viable cells (it is non-osteogenic) and provides minimal structural support.

Question 80

A 65-year-old patient receives a total hip arthroplasty with a highly cross-linked polyethylene liner. What is the primary purpose of remelting the polyethylene after gamma irradiation during the manufacturing process?

Explanation

Gamma irradiation induces cross-linking in ultra-high-molecular-weight polyethylene (UHMWPE) to improve wear characteristics, but it also generates free radicals. If left untreated, these free radicals react with oxygen in vivo, leading to oxidative degradation and subsequent implant failure. Remelting allows trapped free radicals to recombine and be eliminated, though it slightly decreases the mechanical properties (such as yield strength and fatigue resistance) compared to annealing.

Question 81

In the pathogenesis of early osteoarthritis, which of the following represents the earliest biochemical change in the articular cartilage matrix?

Explanation

The earliest recognizable biochemical change in osteoarthritis is an increase in the water content of the articular cartilage. This occurs due to the breakdown of the superficial collagen meshwork, which normally constrains the hydrophilic proteoglycans. Once the collagen network is damaged, the proteoglycans swell and absorb more water. As the disease progresses to later stages, the total proteoglycan content eventually decreases.

Question 82

According to Perren's strain theory, what is the maximum interfragmentary strain that allows for the formation of lamellar bone during fracture healing?

Explanation

Perren's strain theory dictates that a specific tissue will not form if the interfragmentary strain exceeds the maximum strain that the tissue can tolerate before failure. Lamellar bone is rigid and can only tolerate up to 2% strain. Woven bone can tolerate up to 10% strain, cartilage can tolerate 10-30%, and granulation tissue can tolerate up to 100%. Therefore, absolute stability (strain <2%) is required for primary (lamellar) bone healing.

Question 83

A 72-year-old female with severe osteoporosis and a recent vertebral compression fracture is started on teriparatide. Which of the following best describes the cellular mechanism of action of this medication?

Explanation

Teriparatide is a recombinant human parathyroid hormone (PTH 1-34) analog. When administered intermittently (e.g., via daily subcutaneous injection), it exerts an anabolic effect by directly stimulating osteoblasts via the PTH/PTHrP receptor, leading to increased bone formation. Continuous exposure to endogenous PTH (as in hyperparathyroidism) leads to net bone resorption. Option A describes bisphosphonates, Option B describes denosumab, and Option D describes romosozumab.

Question 84

During tensile testing of a healthy flexor tendon, the stress-strain curve initially exhibits a non-linear 'toe region'. Which microscopic structural characteristic is primarily responsible for this specific region of the curve?

Explanation

The stress-strain curve of tendons and ligaments typically begins with a non-linear 'toe region' at low strains (up to approximately 2%). This is anatomically due to the straightening, or 'uncrimping', of the naturally crimped type I collagen fibrils. Once straightened, the tendon enters the linear (elastic) region, where stiffness increases significantly until microscopic failure (yield point) and eventual macroscopic rupture occur.

Question 85

A surgeon uses a stainless steel plate with titanium alloy screws for the internal fixation of a diaphyseal fracture. Which of the following types of corrosion is most likely to occur at the screw-plate interface?

Explanation

Galvanic corrosion occurs when two dissimilar metals with different electrochemical potentials are placed in physical contact within an electrolytic medium (such as blood or interstitial fluid). In this scenario, mixing stainless steel and titanium creates a galvanic cell, resulting in accelerated corrosion of the more anodic metal (typically the stainless steel). To avoid this, orthopedic implants of different metal alloys should generally not be mixed.

Question 86

In the physial growth plate, a gain-of-function mutation in the FGFR3 gene predominantly affects which of the following zones, resulting in achondroplasia?

Explanation

Achondroplasia is caused by an autosomal dominant gain-of-function mutation in the Fibroblast Growth Factor Receptor 3 (FGFR3) gene. FGFR3 normally acts as a negative regulator of endochondral bone growth. When overactive, it prematurely inhibits chondrocyte proliferation. This primary defect localizes to the proliferative zone of the physis, leading to the characteristic short-limbed (rhizomelic) dwarfism.

Question 87

A 28-year-old male sustains a closed midshaft humerus fracture with an associated radial nerve palsy. If the nerve has undergone Wallerian degeneration but the endoneurial tubes remain completely intact, what is the approximate expected rate of axonal regeneration?

Explanation

The scenario describes axonotmesis (Sunderland second degree), where the axon is disrupted leading to distal Wallerian degeneration, but the endoneurium, perineurium, and epineurium remain intact. Axonal regeneration in humans proceeds at a rate of approximately 1 mm/day (or about 1 inch per month). This physiologic constant is critical for clinically predicting the timeline of expected motor and sensory recovery.

Question 88

Which zone of articular cartilage is characterized by the highest concentration of water, the lowest concentration of proteoglycans, and collagen fibrils oriented parallel to the joint surface?

Explanation

The superficial (tangential) zone comprises the top 10-20% of articular cartilage. It is characterized by the highest water content (up to 80%), the lowest proteoglycan content, and densely packed type II collagen fibers arranged parallel to the articular surface. This structural arrangement is uniquely suited to withstand the high shear forces experienced at the joint surface.

Question 89

During a prolonged, low-intensity orthopedic rehabilitation exercise (such as endurance cycling), which muscle fiber type is predominantly recruited, and what is its primary metabolic pathway for energy production?

Explanation

Type I muscle fibers are 'slow-twitch' fibers characterized by high endurance, abundant mitochondria, and high myoglobin content. They are fatigue-resistant and recruited primarily during prolonged, low-intensity activities. Their primary source of ATP is aerobic metabolism via oxidative phosphorylation. Conversely, Type IIb fibers are 'fast-twitch', fatigue quickly, and rely mainly on anaerobic glycolysis for rapid bursts of power.

Question 90

A 25-year-old athlete undergoes an osteochondral autograft transfer. Regarding the ultrastructure of normal articular cartilage, which zone is characterized by the highest water content, lowest proteoglycan concentration, and collagen fibers oriented parallel to the joint surface?

Explanation

The superficial (tangential) zone makes up 10-20% of articular cartilage thickness. It has the highest water content, highest concentration of collagen, and lowest concentration of proteoglycans. The collagen fibers are oriented parallel to the articular surface to resist shear forces.

Question 91

During internal fixation of a diaphyseal fracture, a surgeon considers the pullout strength of the cortical screws being used. Which of the following modifications to a screw design will most significantly increase its pullout strength?

Explanation

Pullout strength of a screw is directly proportional to the outer diameter, thread depth, and length of engagement in the bone, and inversely proportional to the pitch. Increasing the outer diameter has the most profound effect on increasing pullout strength.

Question 92

A modular revision total hip arthroplasty fails due to significant wear and metal debris at the head-neck taper junction. Which type of corrosion is primarily responsible for this specific mode of failure at the modular interface?

Explanation

Fretting corrosion occurs at the contact sites between materials under load, such as the modular head-neck taper junction (trunnionosis) in a total hip arthroplasty. Micromotion disrupts the passive protective oxide layer, leading to mechanical wear and subsequent corrosive attack.

Question 93

A 14-year-old boy presents with a painful, destructive mass in his distal femur. Biopsy reveals a highly cellular tumor composed of small round blue cells. Cytogenetic analysis demonstrates a t(11;22) chromosomal translocation. Which fusion gene is most likely responsible for this tumor's pathogenesis?

Explanation

The t(11;22) translocation is characteristic of Ewing sarcoma, resulting in the EWS-FLI1 fusion protein. SYT-SSX1 is seen in synovial sarcoma, PAX3-FKHR in alveolar rhabdomyosarcoma, TLS-CHOP in myxoid liposarcoma, and COL1A1-PDGFB in dermatofibrosarcoma protuberans.

Question 94

Tranexamic acid (TXA) is frequently utilized in major orthopedic surgery to reduce perioperative blood loss. Which of the following best describes its primary mechanism of action?

Explanation

Tranexamic acid is a synthetic derivative of the amino acid lysine. It competitively and reversibly inhibits the activation of plasminogen to plasmin by binding to lysine receptor sites on plasminogen. This prevents plasmin from breaking down fibrin clots, thereby inhibiting fibrinolysis.

Question 95

During mechanical tensile testing of a healthy human anterior cruciate ligament, the initial non-linear 'toe region' observed on the stress-strain curve is primarily due to which of the following phenomena?

Explanation

The initial 'toe region' of the stress-strain curve for ligaments and tendons represents the uncrimping or straightening of the naturally wavy collagen fibers. Once the fibers are straight, the curve enters the linear (elastic) region where further stress causes proportional strain.

Question 96

A 45-year-old female presents with hypophosphatemic rickets, diffuse bone pain, and muscle weakness. Laboratory studies demonstrate significantly elevated levels of Fibroblast Growth Factor 23 (FGF23). What is the primary physiologic effect of FGF23 on the kidneys?

Explanation

FGF23 is a hormone secreted primarily by osteocytes that regulates phosphate homeostasis. In the kidney, it decreases phosphate reabsorption (causing phosphaturia) and inhibits the enzyme 1-alpha-hydroxylase, leading to decreased synthesis of active 1,25-dihydroxyvitamin D. This ultimately reduces intestinal absorption of calcium and phosphate.

Question 97

A patient develops a prosthetic joint infection 6 months after a total knee arthroplasty. Which characteristic of the bacterial biofilm is the primary reason why systemic, culture-directed antibiotics alone frequently fail to eradicate the infection without surgical debridement?

Explanation

Biofilms protect bacteria via a polymeric glycocalyx matrix. Deep within the biofilm, a lack of oxygen and nutrients induces bacteria to become dormant 'persister' cells. Because most antibiotics target actively dividing cells, these metabolically inactive bacteria are highly tolerant to antimicrobial therapy, necessitating surgical removal of the biofilm.

Question 98

Bone morphogenetic proteins (BMPs) play a crucial role in osteoinduction during fracture healing and spine fusion. Which of the following BMPs is most strongly associated with the FDA-approved use for acute, open tibial shaft fractures?

Explanation

Recombinant human BMP-2 (rhBMP-2) is FDA-approved for acute, open tibial shaft fractures treated with an intramedullary nail, as well as for single-level anterior lumbar interbody fusions. rhBMP-7 (OP-1) was previously utilized for recalcitrant long bone nonunions under a Humanitarian Device Exemption. BMP-3 actually acts as an antagonist to osteogenesis.

Question 99

A 32-year-old sustains a closed midshaft humerus fracture. Neurological examination reveals a complete radial nerve palsy. According to the Sunderland classification of nerve injury, which degree corresponds to the disruption of the axon and endoneurium, with preservation of the perineurium and epineurium?

Explanation

According to the Sunderland classification: 1st degree = myelin injury with intact axon (neuropraxia); 2nd degree = axon injury with intact endoneurium (axonotmesis); 3rd degree = axon and endoneurium disrupted, perineurium intact; 4th degree = axon, endoneurium, and perineurium disrupted, epineurium intact; 5th degree = complete transection of the nerve (neurotmesis).

Question 100

A 65-year-old man requires revision of a total hip arthroplasty that was originally performed using a cobalt-chromium modular head and a titanium alloy femoral stem. Intraoperatively, extensive black, soot-like debris is noted at the modular head-neck taper junction, accompanied by localized soft tissue necrosis. Which of the following is the primary pathophysiologic mechanism responsible for the debris generation and implant degradation at this specific interface?

Explanation

The scenario describes 'trunnionosis' (corrosion at the modular head-neck junction), which typically presents with black debris and adverse local tissue reactions (ALTR). The primary mechanism driving this is mechanically assisted crevice corrosion (MACC), also known as fretting-crevice corrosion. Micromotion at the taper junction cyclically disrupts the protective passivation (oxide) layer of the metals (fretting). When this occurs in a confined, oxygen-depleted space like a modular taper (a crevice), the local environment becomes acidic and chloride-rich, preventing the repassivation of the metal and leading to rapid, localized continuous corrosion. Although Cobalt-Chromium and Titanium are dissimilar metals, true macroscopic galvanic corrosion is not the primary driver in modern modular junctions because both rely on stable, similar oxide layers with close resting potentials; rather, it is the mechanical disruption (fretting) combined with the confined chemical environment (crevice) that causes catastrophic failure.

General Orthopedics 2026 Practice Questions: Set 13 (Solved)

Orthopedic Basic 2026 MCQs: Board Review Questions & Answers (Part 5)

Key Takeaway

Orthopedic Basic 2026 MCQs: Board Review Questions & Answers (Part 5)

Chapter Index