Orthopedics Hyperguide Review | Dr Hutaif General Ortho -...

Question 1

Which of the following material properties best explains why a titanium alloy implant exhibits a lower modulus of elasticity compared to a stainless steel implant, thereby reducing stress shielding in orthopaedic fixation?

Explanation

The modulus of elasticity (Young's modulus) is an intrinsic material property that dictates the stiffness of an implant. It is fundamentally determined by the atomic bonding and crystalline lattice structure of the metal. Titanium alloys have a modulus of elasticity closer to that of cortical bone compared to stainless steel or cobalt-chrome, reducing the stress-shielding effect. Yield strength, density, and fatigue strength are separate properties that do not dictate the intrinsic stiffness of the material.

Question 2

During the healing of a diaphyseal fracture treated with rigid internal fixation providing absolute stability, bone healing occurs primarily via which of the following mechanisms?

Explanation

Rigid internal fixation that provides absolute stability (e.g., compression plating) bypasses callus formation and results in primary bone healing. This occurs via direct Haversian remodeling, where cutting cones of osteoclasts cross the fracture site, followed immediately by osteoblasts laying down new bone. Endochondral ossification (callus formation) is seen in secondary bone healing (relative stability). Creeping substitution refers to the incorporation of bone grafts.

Question 3

A 45-year-old male sustains a distal third femoral shaft fracture.

If the surgeon opts for retrograde intramedullary nailing, what is the ideal entry point?

Explanation

The ideal starting point for a retrograde femoral nail is intra-articular: in the center of the intercondylar notch (medial-lateral), directly in line with the anatomic axis of the femur, and just anterior to the origin of the posterior cruciate ligament (PCL). An entry point that is too anterior increases the risk of damaging the patellofemoral joint cartilage, and too posterior risks damaging the PCL or missing the diaphyseal axis.

Question 4

A 4-week-old female infant is placed in a Pavlik harness for a dislocated left hip (Graf Type IV). After 3 weeks of strict full-time wear, dynamic ultrasound demonstrates that the hip remains completely dislocated. What is the most appropriate next step in management?

Explanation

If a dislocated hip fails to reduce after 3 to 4 weeks of proper Pavlik harness treatment, the harness must be discontinued. Continuing the harness in a persistently dislocated hip places the child at high risk for 'Pavlik harness disease' (erosion of the posterior rim of the acetabulum). The standard next step after failed Pavlik harness treatment for a dislocated hip is typically a closed reduction with spica casting under general anesthesia, utilizing an arthrogram.

Question 5

When performing an anatomic anterior cruciate ligament (ACL) reconstruction, understanding the native footprint is critical. In 90 degrees of knee flexion, the femoral origin of the anteromedial (AM) bundle is best described as being located:

Explanation

The native ACL consists of the anteromedial (AM) and posterolateral (PL) bundles, named for their tibial insertion. On the femoral side, when the knee is flexed to 90 degrees (the standard position for viewing during arthroscopy), the AM bundle footprint is located high (proximal) and deep (posterior) in the notch. The PL bundle is located lower (distal) and shallower (anterior) in the notch.

Question 6

A 25-year-old male arrives in the trauma bay following a motor vehicle collision. He has no motor or sensory function below the T4 dermatome. A digital rectal examination reveals absent rectal tone and an absent bulbocavernosus reflex. Which of the following statements regarding his neurologic status is most accurate?

Explanation

An absent bulbocavernosus reflex (BCR) signifies the presence of spinal shock. Spinal shock is a state of transient physiologic reflex depression of spinal cord function below the level of the injury. Until spinal shock resolves (marked by the return of the BCR), a clinician cannot definitively classify the spinal cord injury as complete or incomplete, because neurologic function may return once the shock phase dissipates.

Question 7

A 14-year-old boy presents with a destructive diaphyseal lesion of the femur with an associated soft tissue mass. Biopsy reveals small round blue cells. Which of the following molecular abnormalities is most classically associated with this patient's likely diagnosis?

Explanation

The patient's clinical and histologic picture (diaphyseal lesion, small round blue cells) is classic for Ewing sarcoma. Ewing sarcoma is heavily characterized by the t(11;22)(q24;q12) chromosomal translocation, which results in the EWSR1-FLI1 fusion protein (found in roughly 85% of cases). t(X;18) is associated with Synovial sarcoma. t(12;16) is associated with Myxoid liposarcoma. MDM2 amplification is characteristic of well-differentiated liposarcoma/parosteal osteosarcoma.

Question 8

The primary stabilizing structure of the second tarsometatarsal joint complex is the Lisfranc ligament. Which of the following best describes the precise anatomic attachments of the Lisfranc ligament?

Explanation

The Lisfranc ligament is an oblique, stout intra-articular ligament that provides the primary dorsal-plantar and transverse stability to the midfoot. It originates from the lateral aspect of the medial cuneiform and inserts onto the medial aspect of the base of the second metatarsal. Notably, there is no direct ligamentous connection between the first and second metatarsal bases.

Question 9

A 30-year-old carpenter sustains a sharp knife laceration to the volar aspect of his index finger at the level of the middle phalanx.

On physical examination, he is unable to flex the distal interphalangeal (DIP) joint, but proximal interphalangeal (PIP) joint flexion is fully intact. Based on the Verdan zones of flexor tendon injury, this injury is classified as:

Explanation

The inability to flex the DIP joint with preserved PIP flexion localizes the injury to the flexor digitorum profundus (FDP) distal to the insertion of the flexor digitorum superficialis (FDS). The FDS inserts at the middle phalanx. Injuries to the flexor tendon system distal to the FDS insertion are classified as Zone I injuries, and by definition, involve only the FDP tendon.

Question 10

Articular cartilage relies on multiple mechanisms to maintain extremely low friction coefficients. At the exact initiation of joint movement from a stationary position (under high load and low speed), which type of lubrication mechanism is predominantly responsible for reducing friction?

Explanation

Boundary lubrication is the primary mechanism operating at the initiation of movement, under high loads, and at low speeds. It is mediated by superficial molecules, primarily lubricin (PRG4), which attach to the articular surface and prevent direct surface-to-surface contact. Fluid-film lubrication mechanisms (like elastohydrodynamic and squeeze-film) dominate during sustained dynamic joint motion at higher speeds.

Question 11

The introduction of highly cross-linked polyethylene (HXLPE) in total hip arthroplasty has significantly reduced volumetric wear rates compared to conventional ultra-high-molecular-weight polyethylene (UHMWPE). However, the cross-linking and subsequent thermal stabilization processes alter the material's bulk mechanical properties. Which of the following represents a known mechanical tradeoff of HXLPE?

Explanation

Irradiation to create cross-links improves wear resistance but decreases the mechanical robustness of the polyethylene. Specifically, highly cross-linked polyethylene (HXLPE) exhibits decreased yield strength, ultimate tensile strength, ductility, and resistance to fatigue crack propagation compared to conventional UHMWPE. Thermal processing (e.g., remelting) removes free radicals to prevent oxidation but further reduces these mechanical properties.

Question 12

A 24-year-old male is admitted with a severe closed tibia fracture. The surgical team is concerned about acute compartment syndrome. The patient is hypotensive with a blood pressure of 80/45 mmHg (Mean Arterial Pressure = 57 mmHg). Intracompartmental pressure testing reveals a pressure of 28 mmHg in the anterior compartment. What is the most appropriate next step in management?

Explanation

The decision to perform a fasciotomy should be based on the 'delta pressure' (Delta P), which is calculated as the Diastolic Blood Pressure minus the Compartment Pressure. A Delta P of less than or equal to 30 mmHg is highly suggestive of compartment syndrome and is an absolute indication for fasciotomy. In this patient, Delta P = 45 - 28 = 17 mmHg. The absolute compartment pressure of 28 is less significant than the critical narrowing of the perfusion gradient.

Question 13

A 12-year-old boy presents with left knee pain and an obligate externally rotated gait.

Radiographs reveal a left slipped capital femoral epiphysis (SCFE). Which of the following is a recognized indication for prophylactic in situ pinning of the asymptomatic contralateral right hip?

Explanation

Prophylactic pinning of the contralateral asymptomatic hip in SCFE is controversial but is strictly indicated in certain high-risk groups. These include patients with underlying endocrinopathies (such as hypothyroidism or growth hormone deficiency), renal osteodystrophy, prior pelvic radiation, or patients presenting at a very young age (e.g., <10 years for girls or <11 for boys) with an open triradiate cartilage. Obesity alone is a risk factor for SCFE but not an absolute indication for prophylactic pinning.

Question 14

Biomechanical models of the rotator cuff emphasize load transmission properties. Burkhart et al. described a thick, band-like structure composed of bundled collagen fibers running perpendicular to the supraspinatus and infraspinatus tendons. This structure transmits load to the humerus and stress-shields the thinner, more avascular crescent region. What is this structure called?

Explanation

The 'rotator cable' is a thick, macroscopic bundle of fibers that courses perpendicular to the supraspinatus and infraspinatus tendon fibers. It acts biomechanically like a suspension bridge, transmitting forces from the muscle belly to the humerus while shielding the thinner, often avascular tissue lateral to it, known as the 'rotator crescent'. This explains why some patients with crescent tears maintain excellent function.

Question 15

Articular cartilage is highly resilient and capable of withstanding massive compressive loads during normal joint kinematics. This remarkable compressive stiffness and resilience is primarily mediated by which of the following extracellular matrix components?

Explanation

Aggrecan, the major proteoglycan in articular cartilage, possesses a high concentration of negatively charged glycosaminoglycans (chondroitin sulfate and keratan sulfate). These negative charges repel each other and draw water into the matrix via Donnan osmotic pressure, creating a swelling pressure that resists compressive loads. While Type II collagen provides the tensile framework, it is the aggrecan-water interaction that provides compressive stiffness.

Question 16

According to the Young-Burgess classification of pelvic ring injuries, an anteroposterior compression type II (APC II) injury is defined by pubic symphysis diastasis and the specific disruption of which of the following posterior pelvic ligamentous structures?

Explanation

An APC II injury involves an external rotation force that opens the anterior pelvic ring (symphysis diastasis). In the posterior ring, it specifically tears the anterior sacroiliac ligament, the sacrotuberous ligament, and the sacrospinous ligament. The robust posterior sacroiliac ligaments remain completely intact, acting as a hinge. This creates rotational instability but preserves vertical stability. Disruption of both anterior and posterior SI ligaments would constitute an APC III injury.

Question 17

A 70-year-old female with severe osteoporosis and a recent vertebral compression fracture is prescribed a bone-modifying agent.

She is initiated on teriparatide. What is the primary mechanism of action of this medication when administered as a daily subcutaneous injection?

Explanation

Teriparatide is a recombinant human parathyroid hormone analog (PTH 1-34). While continuous exposure to high levels of PTH (as in hyperparathyroidism) leads to bone resorption, intermittent administration (such as daily subcutaneous injection) preferentially stimulates osteoblast activity over osteoclast activity. This results in a net anabolic (bone-building) effect, differentiating it from antiresorptive agents like bisphosphonates (farnesyl pyrophosphate synthase inhibitors) or denosumab (RANKL inhibitor).

Question 18

A 4-year-old child presents with a 2-day history of refusal to bear weight on the right leg. Clinical evaluation reveals a temperature of 38.6°C (101.5°F). Laboratory workup shows a WBC count of 14,000/mm³, an ESR of 50 mm/hr, and a CRP of 3.5 mg/dL. According to the classic Kocher criteria, what is the approximate probability that this child has septic arthritis of the hip?

Explanation

The Kocher criteria are used to differentiate septic arthritis from transient synovitis in children. The four classic criteria are: history of fever (>38.5°C), non-weight bearing, ESR >40 mm/hr, and serum WBC >12,000/mm³. The probability of septic arthritis based on the number of criteria present is approximately: 1 predictor = 3%, 2 predictors = 40%, 3 predictors = 93%, and 4 predictors = 99%. This patient meets all 4 criteria.

Question 19

A competitive cyclist presents with progressive numbness in the volar aspect of his small finger and the ulnar half of his ring finger. He also notes weakness in spreading his fingers apart. Sensation over the dorsoulnar aspect of the hand is fully intact. This clinical picture is most consistent with compression of the ulnar nerve at which specific anatomic location?

Explanation

The patient has both motor (interossei) and sensory (volar pinky/ring) deficits of the ulnar nerve, but spared dorsal sensation. The dorsal ulnar cutaneous nerve branches off the ulnar nerve approximately 5-8 cm proximal to the wrist. Because dorsal sensation is intact, the lesion must be at the wrist (Guyon's canal) rather than the elbow. In Guyon's canal, Zone 1 is proximal to the bifurcation and contains both mixed motor and sensory fibers. Zone 2 is pure motor, and Zone 3 is pure sensory.

Question 20

A patient with a previously implanted stainless steel plate for a femur fracture undergoes revision surgery. The surgeon inadvertently uses a titanium screw to secure an adjunctive piece of hardware in direct physical contact with the stainless steel plate. Over time, rapid corrosion and eventual failure of the less noble metal occurs.

This process is best described as:

Explanation

Galvanic corrosion occurs when two dissimilar metals are placed in direct contact with each other within an electrolytic solution (like human bodily fluids). An electrochemical cell is created, and the less noble metal (the anode) undergoes accelerated corrosive degradation. Fretting corrosion is mechanically induced by micromotion between components, whereas crevice corrosion occurs in restricted spaces with oxygen depletion. Using titanium and stainless steel in direct contact is a classic setup for galvanic corrosion.

Question 21

What is the primary cellular abnormality that drives the initial phase of Paget disease of bone (osteitis deformans)?

Explanation

Paget disease occurs in three phases: an initial lytic (osteoclastic) phase, a mixed osteoclastic-osteoblastic phase, and a late sclerotic (osteoblastic) phase. The primary defect lies in the osteoclast. Pagetic osteoclasts are larger and contain an excessive number of nuclei (up to 100 per cell, compared to the normal 3-5). This leads to massive localized bone resorption.

Question 22

In total hip arthroplasty, aseptic loosening is most frequently driven by the biological response to ultra-high-molecular-weight polyethylene (UHMWPE) wear debris. Which specific particle size range is most readily phagocytosed by macrophages, initiating the osteolytic cascade?

Explanation

Macrophages preferentially phagocytose UHMWPE wear particles in the 0.1 to 10 micrometer range. Particles smaller than this often do not stimulate an aggressive immune response, while particles larger than 10-15 micrometers are too large for individual macrophage phagocytosis and typically elicit a foreign body giant cell response. Macrophage activation by 0.1-10 μm particles leads to the release of osteolytic cytokines such as TNF-alpha, IL-1, IL-6, and PGE2.

Question 23

What is the predominant mechanism of corrosion seen at the modular head-neck taper junction (trunnionosis) in modern total hip arthroplasty?

Explanation

The primary mechanism of corrosion at modular junctions (like the head-neck trunnion) is Mechanically Assisted Crevice Corrosion (MACC). Micromotion (fretting) at the taper junction mechanically disrupts the protective passivation layer (titanium or chromium oxide). The fluid-filled gap (crevice) between the components becomes depleted of oxygen, leading to an acidic microenvironment that prevents repassivation and accelerates active metal dissolution.

Question 24

A 6-year-old child presents with a refusal to walk, bleeding gums, and petechiae. Radiographs reveal a 'Frankel line' and 'Wimberger ring sign'. The underlying pathophysiology of this condition is a defect in which of the following processes?

Explanation

This clinical and radiographic presentation is classic for scurvy (Vitamin C deficiency). Vitamin C is an essential cofactor for the enzymes prolyl hydroxylase and lysyl hydroxylase. Without hydroxylation of proline and lysine, collagen chains cannot form stable triple helices or undergo proper extracellular cross-linking, resulting in fragile blood vessels, poor wound healing, and defective bone formation.

Question 25

Achondroplasia is the most common form of short-limb dwarfism, caused by an activating mutation in FGFR3. This genetic anomaly primarily exerts its inhibitory effect on chondrocyte function within which specific zone of the physis?

Explanation

Achondroplasia is caused by a gain-of-function mutation in the Fibroblast Growth Factor Receptor 3 (FGFR3) gene. FGFR3 normally acts as a negative regulator of bone growth. When constitutively active, it severely inhibits chondrocyte proliferation and differentiation, predominantly affecting the proliferative zone of the physis, leading to diminished enchondral ossification.

Question 26

A 15-year-old boy presents with a destructive metaphyseal lesion in the distal femur with an associated soft tissue mass and a 'sunburst' periosteal reaction. Which of the following pairs of tumor suppressor genes are most commonly mutated or inactivated in the pathogenesis of this specific tumor?

Explanation

The clinical and radiographic presentation describes conventional osteosarcoma. The pathogenesis of osteosarcoma is strongly associated with mutations in the tumor suppressor genes RB1 (Retinoblastoma gene) and TP53 (p53). Patients with hereditary retinoblastoma (RB1 mutation) and Li-Fraumeni syndrome (TP53 mutation) have a significantly increased risk of developing osteosarcoma.

Question 27

A 65-year-old man presents with an acutely swollen and painful knee. Synovial fluid aspiration yields a yellow fluid with a WBC count of 45,000 cells/uL (predominantly PMNs). Polarized light microscopy reveals weakly positively birefringent rhomboid-shaped crystals. What is the composition of these crystals?

Explanation

The classic presentation of pseudogout (calcium pyrophosphate deposition disease, CPPD) includes an acutely inflamed joint with synovial fluid showing rhomboid-shaped crystals that exhibit weak positive birefringence under polarized light. Monosodium urate crystals (gout) are needle-shaped and strongly negatively birefringent.

Question 28

A 10-year-old boy suffers a subtrochanteric femur fracture. Radiographs show a diffusely dense, 'bone-in-bone' appearance and loss of the medullary canal. The underlying cause of this condition is most commonly due to a defect in:

Explanation

The clinical picture describes osteopetrosis (marble bone disease). It results from defective osteoclast function, preventing normal bone resorption and remodeling. The most well-known functional defect is a mutation in Carbonic Anhydrase II (or the TCIRG1 gene affecting the vacuolar proton pump), which prevents the osteoclast from creating the acidic environment necessary for bone resorption, often resulting in osteoclasts that lack a functional ruffled border.

Question 29

A 5-year-old child is evaluated for severe kyphoscoliosis, knock-knees, and corneal clouding, but displays normal intelligence. Radiographs show universal platyspondyly with central anterior beaking. Urine analysis shows elevated levels of keratan sulfate. What is the deficient enzyme?

Explanation

This is Morquio syndrome (Mucopolysaccharidosis Type IV). It is characterized by severe skeletal dysplasia (platyspondyly with central anterior beaking, atlantoaxial instability), normal intelligence, and corneal clouding. Type IVA is caused by a deficiency in N-acetylgalactosamine-6-sulfatase (galactose-6-sulfatase), leading to the accumulation of keratan sulfate. Alpha-L-iduronidase deficiency causes Hurler syndrome (MPS I).

Question 30

Polymethylmethacrylate (PMMA) bone cement is frequently used in arthroplasty for implant fixation. Which of the following additives is responsible for making PMMA radiopaque on standard plain radiographs?

Explanation

In standard PMMA formulations, barium sulfate (BaSO4) or zirconium dioxide (ZrO2) is added as a radiopacifier so the cement mantle can be evaluated radiographically. Benzoyl peroxide is the initiator in the powder, N,N-dimethyl-p-toluidine is the accelerator in the liquid, and hydroquinone is added to the liquid as a stabilizer to prevent premature polymerization.

Question 31

A 68-year-old woman is 7 days postoperative from a total knee arthroplasty. She has been receiving unfractionated heparin for DVT prophylaxis. Her platelet count precipitously drops from 250,000 to 90,000. What is the underlying pathophysiology of this complication?

Explanation

Heparin-induced thrombocytopenia (HIT) Type II is a severe, immune-mediated complication of heparin therapy. It typically occurs 5-10 days after starting heparin and is caused by the formation of IgG antibodies against complexes of heparin and Platelet Factor 4 (PF4). These immune complexes activate platelets, leading to paradoxical widespread thrombosis and a consumptive drop in platelet count.

Question 32

According to Sunderland's classification of peripheral nerve injuries, a third-degree nerve injury is characterized by the disruption of the axon and which of the following connective tissue structures?

Explanation

Sunderland classified nerve injuries into five degrees: 1st degree (Neurapraxia) involves focal demyelination; 2nd degree (Axonotmesis) involves axonal disruption with an intact endoneurium; 3rd degree involves disruption of the axon and endoneurium (but perineurium and epineurium remain intact); 4th degree involves disruption of the axon, endoneurium, and perineurium (only epineurium is intact); 5th degree (Neurotmesis) is complete transection of the nerve.

Question 33

During the rehabilitation phase following an Achilles tendon repair, a patient performs 'heel drops', slowly lowering their heel below the level of a stair step. Which type of muscle contraction is predominantly utilized by the gastrocnemius-soleus complex during the lowering phase?

Explanation

An isotonic eccentric contraction occurs when the muscle lengthens while under tension. During the lowering phase of a heel drop, the gastrocnemius-soleus complex is actively firing to control the descent, but the muscle-tendon unit is lengthening. Concentric contraction occurs when the muscle shortens (e.g., raising the heel). Isometric occurs when length does not change.

Question 34

Among the commonly used solid orthopedic implant metals, which of the following has a modulus of elasticity (Young's modulus) closest to that of cortical bone, thereby theoretically reducing the risk of stress shielding?

Explanation

Cortical bone has a modulus of elasticity of approximately 15-20 GPa. While all standard solid metals are significantly stiffer than bone, Titanium alloy (Ti-6Al-4V) has a modulus of ~110 GPa, which is roughly half that of 316L Stainless Steel (~200 GPa) and Cobalt-chromium (~210-240 GPa). Therefore, titanium implants cause less stress shielding compared to stainless steel or CoCr implants.

Question 35

A 12-year-old boy with multiple palpable bony bumps around his knees and shoulders is diagnosed with Multiple Hereditary Exostoses (MHE). The genetic mutation associated with this condition typically affects the EXT1 or EXT2 genes. What is the normal physiological function of the proteins encoded by these genes?

Explanation

Multiple Hereditary Exostoses (MHE), or Multiple Osteochondromas, is an autosomal dominant condition caused by mutations in the EXT1 or EXT2 genes. These genes encode glycosyltransferases that are responsible for the synthesis of heparan sulfate proteoglycans. A deficiency disrupts Indian Hedgehog (Ihh) signaling and normal chondrocyte proliferation, leading to the formation of osteochondromas.

Question 36

A 4-year-old boy presents with severe bowing of the lower extremities. Laboratory tests reveal normal serum calcium, significantly decreased serum phosphate, normal PTH, and normal 25-hydroxyvitamin D. He is diagnosed with X-linked hypophosphatemic rickets. The defective gene (PHEX) in this condition leads to an overproduction or decreased degradation of which circulating factor?

Explanation

X-linked hypophosphatemic rickets is the most common heritable form of rickets. It is caused by a mutation in the PHEX gene. PHEX normally cleaves and inactivates Fibroblast Growth Factor 23 (FGF23). When PHEX is defective, excess FGF23 accumulates. FGF23 decreases renal phosphate reabsorption (by downregulating NaPi-IIa/c cotransporters) and inhibits 1-alpha-hydroxylase, leading to profound phosphaturia and hypophosphatemia.

Question 37

During secondary bone healing of a closed tibial shaft fracture treated with a cast, what is the primary mode of ossification occurring within the relatively hypoxic central region of the fracture callus?

Explanation

Secondary bone healing relies on callus formation. In the central, relatively hypoxic and mechanically unstable core of the fracture gap, healing proceeds via endochondral ossification (cartilage forms first and is subsequently replaced by bone). Intramembranous ossification (direct bone formation without a cartilage intermediate) occurs at the more stable, oxygen-rich periphery of the hard callus.

Question 38

A neonate is noted to have multiple fractures, blue sclerae, and profound hearing loss. A defect in the synthesis of type I collagen is suspected. At the molecular level, this defect most commonly involves the substitution of which essential amino acid in the collagen triple helix?

Explanation

Osteogenesis Imperfecta is typically caused by mutations in the COL1A1 or COL1A2 genes. The classic molecular defect is a single base pair mutation that results in the substitution of glycine by a bulkier amino acid. Glycine, being the smallest amino acid, is required at every third position (the Gly-X-Y repeating sequence) to allow the three alpha chains to pack tightly into a stable triple helix.

Question 39

In the pathogenesis of Rheumatoid Arthritis, the chronically inflamed synovium forms a pannus that aggressively destroys cartilage and bone. Which cytokine is the primary mediator responsible for stimulating osteoclastogenesis and subsequent periarticular bone erosions?

Explanation

Tumor Necrosis Factor-alpha (TNF-alpha) is a central cytokine in the pathogenesis of Rheumatoid Arthritis. It is secreted by macrophages and T-cells in the pannus. TNF-alpha aggressively upregulates RANKL expression on synovial fibroblasts and T-cells, which directly stimulates osteoclast differentiation and activity, resulting in the characteristic marginal bone erosions seen in RA.

Question 40

A 3-year-old child presents with a disproportionately short trunk, coxa vara, and severe myopia. Radiographs reveal delayed ossification of the pubic bones and flattened vertebral bodies (platyspondyly). The underlying pathophysiology involves a genetic defect in which of the following structural proteins?

Explanation

The clinical presentation is consistent with Spondyloepiphyseal Dysplasia Congenita (SEDC). SEDC is caused by a mutation in the COL2A1 gene, which results in defective Type II collagen. Because Type II collagen is the primary collagen in articular cartilage, the nucleus pulposus, and the vitreous humor of the eye, these patients typically present with severe spinal deformity (platyspondyly), epiphyseal dysplasia, and ocular complications like myopia and retinal detachment.

Question 41

Galvanic corrosion is an electrochemical process that can lead to early failure of orthopedic implants when two dissimilar metals are in contact in an electrolyte solution (such as body fluid). Which of the following metal combinations presents the highest theoretical risk for galvanic corrosion?

Explanation

Stainless steel and Titanium have the greatest difference in their anodic index (galvanic potential difference). When placed in contact within an electrolytic environment, stainless steel acts as the anode and corrodes rapidly, while titanium acts as the cathode. Thus, mixing stainless steel and titanium implants (e.g., a stainless steel screw in a titanium plate) is contraindicated.

Question 42

Articular cartilage is highly specialized to withstand compressive forces. Which zone of articular cartilage has the highest concentration of water and collagen fibers oriented parallel to the joint surface?

Explanation

The superficial (tangential) zone has the highest water content (up to 80%) and lowest proteoglycan content. The Type II collagen fibers in this zone are oriented parallel to the joint surface to resist shear forces.

Question 43

A 35-year-old mechanic presents with a swollen, painful index finger 3 days after a puncture wound. Examination raises suspicion for pyogenic flexor tenosynovitis. Kanavel's cardinal signs for this condition include all of the following EXCEPT:

Explanation

Kanavel's four cardinal signs of flexor tenosynovitis are: 1) flexed resting posture, 2) fusiform (sausage-like) swelling, 3) tenderness along the flexor sheath, and 4) excruciating pain on passive EXTENSION (not flexion), which stretches the inflamed tendon sheath.

Question 44

A 24-year-old polytrauma patient with a bilateral femur fractures arrives in the emergency department. The team debates Early Total Care (ETC) versus Damage Control Orthopedics (DCO) with external fixation. Which of the following physiological parameters strongly indicates that DCO is the safest approach?

Explanation

Indications for Damage Control Orthopedics (DCO) include parameters that indicate a patient is 'in extremis' or borderline. These include: Base excess worse than -8 mmol/L, pH < 7.24, Temperature < 35°C, Lactate > 2.5 mmol/L, or significant coagulopathy. A base excess < -8 reflects severe metabolic acidosis and hypoperfusion.

Question 45

A 14-year-old boy presents with a painful, destructive diaphyseal lesion of the left femur and an associated large soft tissue mass. Biopsy reveals uniform, small, round, blue cells expressing CD99. What is the most common cytogenetic abnormality associated with this tumor?

Explanation

The clinical presentation and histology are classic for Ewing sarcoma. Over 85% of Ewing sarcomas are characterized by the t(11;22)(q24;q12) translocation, resulting in the EWS-FLI1 fusion protein. t(X;18) is seen in synovial sarcoma; t(12;16) in myxoid liposarcoma; t(2;13) in alveolar rhabdomyosarcoma.

Question 46

A 12-year-old obese boy presents with acute on chronic left hip pain and inability to bear weight. Examination shows an obligatory external rotation of the hip with passive flexion. Slipped capital femoral epiphysis (SCFE) is diagnosed. What is the primary arterial supply to the femoral head that is at risk of disruption in this condition?

Explanation

In older children and adults, the predominant blood supply to the femoral head is the lateral epiphyseal branches of the Medial Femoral Circumflex Artery (MFCA). This precarious blood supply is at risk for disruption in a displaced or unstable SCFE, potentially leading to avascular necrosis (AVN).

Question 47

Following anterior cruciate ligament (ACL) reconstruction with a bone-patellar tendon-bone autograft, the graft undergoes a biological process termed 'ligamentization'. During which postoperative timeframe is the graft structurally at its weakest?

Explanation

The ACL graft undergoes four stages: necrosis, revascularization/proliferation, cellular repopulation, and remodeling. The graft is weakest during the revascularization and proliferation phase, typically between 6 to 8 weeks post-surgery, when necrotic tissue is being resorbed and new vascular networks are forming, drastically reducing the structural mechanical strength.

Question 48

A 22-year-old man dives into shallow water and sustains a severe cervical spine injury resulting in complete tetraplegia. In the emergency department, his vital signs show a heart rate of 50 bpm and blood pressure of 85/50 mmHg. His extremities are warm and well-perfused. Which of the following best explains this clinical picture?

Explanation

Neurogenic shock occurs after a spinal cord injury (usually above T6) due to the loss of sympathetic tone. This results in unopposed vagal parasympathetic tone leading to bradycardia and systemic vasodilation (hypotension with warm extremities). Spinal shock refers to the temporary loss of spinal reflexes below the level of injury and is a neurological, rather than hemodynamic, phenomenon.

Question 49

Primary bone healing occurs without callus formation and requires absolute stability. Which of the following fracture fixation constructs is designed to promote primary bone healing?

Explanation

Primary bone healing requires absolute stability (strain <2%) and direct contact between fracture ends, allowing for osteonal cutting cones to cross the fracture site. Compression plating of a transverse fracture provides this environment. Intramedullary nails, bridge plates, external fixators, and casts typically allow micro-motion, resulting in secondary bone healing via endochondral ossification (callus formation).

Question 50

In total hip arthroplasty, the use of highly cross-linked polyethylene (HXLPE) has significantly reduced the incidence of wear and subsequent osteolysis. However, increasing the radiation dose to increase cross-linking has a known deleterious effect on the material. What is the primary disadvantage of increased cross-linking?

Explanation

While high doses of irradiation (cross-linking) significantly decrease adhesive and abrasive wear, it alters the mechanical properties of the polyethylene, specifically decreasing its yield strength, ductility, and fracture toughness. This makes the material more brittle and susceptible to fatigue failure (e.g., rim cracking or catastrophic failure in thin poly liners).

Question 51

Tranexamic acid (TXA) is widely used in orthopedic surgery to reduce perioperative blood loss and transfusion requirements. What is the exact mechanism of action of TXA?

Explanation

Tranexamic acid is an antifibrinolytic agent. It is a synthetic analog of the amino acid lysine. It reversibly binds to the lysine-binding sites on plasminogen, preventing its activation to plasmin, thereby inhibiting the degradation of fibrin clots (fibrinolysis).

Question 52

Osteopetrosis is a rare genetic disorder characterized by dense, heavy, and brittle bones. The malignant infantile form is most commonly caused by a genetic defect that impairs the osteoclast's ability to acidify the resorption pit. A mutation affecting which of the following enzymes is responsible?

Explanation

Osteopetrosis is caused by defective osteoclast function. The malignant infantile form (autosomal recessive) is frequently due to mutations in the TCIRG1 gene (V-ATPase) or the Carbonic Anhydrase II (CAII) gene. CAII is essential for generating protons (H+) from water and carbon dioxide; these protons are then pumped into the ruffled border to create the acidic environment necessary for bone resorption.

Question 53

A 30-year-old male sustains a hyperplantarflexion injury to his foot resulting in midfoot swelling and pain. Radiographs demonstrate a 'fleck sign' at the base of the second metatarsal. The primary ligament injured in this condition connects which two bony structures?

Explanation

The Lisfranc ligament is an intra-articular ligament that connects the lateral aspect of the medial cuneiform to the medial aspect of the base of the 2nd metatarsal. It is critical for the stability of the tarsometatarsal (Lisfranc) joint. The 'fleck sign' is an avulsion fracture at the attachment of this ligament.

Question 54

Bacterial biofilms on orthopedic implants render infections highly resistant to antibiotics and host immune clearance. The life cycle of a biofilm occurs in distinct stages. What is the final stage that facilitates the spread of infection to distant sites?

Explanation

The stages of biofilm formation are: 1) Reversible attachment of planktonic bacteria to the surface, 2) Irreversible attachment, 3) Maturation (growth, exopolysaccharide matrix production, quorum sensing), and 4) Dispersion/Detachment, where single cells or clumps of bacteria shed from the biofilm to disseminate and colonize new surfaces.

Question 55

During a physical examination of the hand, a patient is asked to tightly hold a piece of paper between their thumb and lateral aspect of the index finger (key pinch). The examiner pulls the paper away, and the patient's thumb interphalangeal (IP) joint noticeably flexes. This is a positive Froment's sign. Which muscle is compensating, and what is its innervation?

Explanation

Froment's sign tests for ulnar nerve palsy. The adductor pollicis (innervated by the ulnar nerve) is normally responsible for the key pinch. When it is weak or paralyzed, the patient compensates by using the Flexor Pollicis Longus (FPL) to flex the thumb IP joint to hold the paper. The FPL is innervated by the Anterior Interosseous Nerve (AIN), a branch of the median nerve.

Question 56

The Ponseti method is the gold standard for the treatment of idiopathic congenital talipes equinovarus (clubfoot). What is the correct sequence of deformity correction using serial casting?

Explanation

The sequence of correction in the Ponseti method follows the mnemonic C-A-V-E: Cavus (corrected first by supinating the forefoot to align with the hindfoot), Adductus, Varus, and finally Equinus (which often requires a percutaneous Achilles tenotomy).

Question 57

Humoral hypercalcemia of malignancy is a paraneoplastic syndrome commonly caused by tumor secretion of Parathyroid Hormone-related Protein (PTHrP). Which of the following primary carcinomas is most classically associated with PTHrP secretion?

Explanation

Squamous cell carcinomas (especially of the lung, head, and neck), renal cell carcinoma, and breast carcinoma are classic sources of PTHrP. PTHrP mimics PTH by binding to the PTH-1 receptor, leading to increased bone resorption and hypercalcemia. Small cell lung cancer is more commonly associated with ACTH or ADH secretion. Prostate cancer usually causes osteoblastic lesions, not typically hypercalcemia via PTHrP.

Question 58

In biomechanics, the mechanical behavior of a material is plotted on a stress-strain curve. The total area under the stress-strain curve from the origin up to the point of ultimate failure represents which mechanical property?

Explanation

Toughness is the amount of energy a material can absorb before it fails, represented by the total area under the stress-strain curve. Stiffness is the slope of the elastic region (Young's modulus). Yield strength is the point where plastic deformation begins. Ultimate tensile strength is the peak stress on the curve. Ductility is the amount of plastic strain before failure.

Question 59

The medial meniscus is more frequently injured than the lateral meniscus, in part because it is less mobile. Which of the following structures is responsible for firmly attaching the medial meniscus to the joint capsule, limiting its excursion?

Explanation

The deep portion of the medial collateral ligament (the meniscofemoral and meniscotibial/coronary ligaments) firmly attaches the peripheral border of the medial meniscus to the joint capsule and tibia, restricting its mobility. The lateral meniscus has no attachment to the lateral collateral ligament, making it more mobile and less prone to being trapped and torn during rotary stresses. Wrisberg and Humphrey are meniscofemoral ligaments of the lateral meniscus.

Question 60

According to Seddon's classification of nerve injuries, a lesion that involves complete disruption of the axon and myelin sheath, but preservation of the endoneurium, perineurium, and epineurium, is classified as:

Explanation

Seddon classified nerve injuries into Neuropraxia (conduction block, intact axon), Axonotmesis (disrupted axon/myelin, but intact connective tissue framework, allowing for Wallerian degeneration and spontaneous regeneration), and Neurotmesis (complete transection). The described lesion is Axonotmesis, which correlates with Sunderland Grade II.

Question 61

What is the primary purpose of heating (melting or annealing) highly cross-linked polyethylene (HXLPE) above its melting point after gamma irradiation during manufacturing?

Explanation

Irradiation of polyethylene creates cross-links that improve wear resistance, but it also leaves behind free radicals. If left untreated, these free radicals react with oxygen in vivo, leading to oxidative chain scission and degradation of the material's mechanical properties. Post-irradiation thermal treatment (either melting above the melting point or annealing just below it) quenches these free radicals, preventing oxidation.

Question 62

A 35-year-old male presents after a motorcycle crash with a hemodynamically unstable anteroposterior compression type III (APC-III) pelvic ring injury. A pelvic binder is urgently indicated. What is the most appropriate anatomic landmark for the optimal placement of the pelvic binder to effectively reduce pelvic volume?

Explanation

The optimal placement for a pelvic binder is centered directly over the greater trochanters. Placing it higher (over the iliac crests or ASIS) is a common error that can fail to reduce the pelvic volume effectively and may even paradoxical open the pelvic ring in some fracture patterns.

Question 63

In a patient with an irreversible high radial nerve palsy, which of the following is the most standard and biomechanically reliable tendon transfer to restore wrist extension?

Explanation

The classic transfer for restoring wrist extension in a radial nerve palsy is the Pronator Teres (PT) to the Extensor Carpi Radialis Brevis (ECRB). The ECRB is chosen over the ECRL because its central insertion at the base of the third metacarpal provides balanced wrist extension, whereas ECRL insertion results in radial deviation.

Question 64

A patient undergoes a Dial test during a knee examination. There is 15 degrees of increased external rotation at 30 degrees of knee flexion compared to the contralateral side, but symmetric external rotation at 90 degrees of flexion. This finding is most indicative of an isolated injury to which of the following structures?

Explanation

The Dial test evaluates external rotation of the tibia. Increased external rotation (>10 degrees compared to the normal side) at 30 degrees of flexion, but not at 90 degrees, indicates an isolated posterolateral corner (PLC) injury. If increased external rotation is present at both 30 and 90 degrees, it indicates a combined PLC and PCL injury.

Question 65

A 55-year-old female presents with medial foot pain and a progressive flatfoot deformity. Examination reveals a flexible hindfoot valgus, a 'too-many-toes' sign, and inability to perform a single-limb heel rise. Weight-bearing radiographs show 40% talonavicular uncovering. According to the Johnson and Strom classification modified by Myerson, which of the following surgical interventions is most appropriate for this stage IIb deformity?

Explanation

The patient has a Stage IIb posterior tibial tendon dysfunction (PTTD), characterized by a flexible deformity with significant forefoot abduction (>30% talonavicular uncovering). Stage IIa is treated with FDL transfer and MDCO. Stage IIb requires the addition of a lateral column lengthening (such as an Evans osteotomy) to correct the severe forefoot abduction.

Question 66

A 45-year-old male complains of neck pain radiating down his right arm. Examination reveals weakness in wrist flexion and finger extension, with a diminished triceps reflex. Sensation is decreased over the dorsal aspect of the long finger. Which cervical nerve root is most likely compressed?

Explanation

A C7 radiculopathy is characterized by weakness in the triceps (elbow extension), wrist flexors, and finger extensors. The triceps reflex is typically diminished, and numbness or tingling is usually felt in the middle (long) finger.

Question 67

A 13-year-old obese male presents to the emergency department with acute-onset right hip pain. He is entirely unable to bear weight on the right leg, even with crutches. Radiographs demonstrate a Slipped Capital Femoral Epiphysis (SCFE). What is the most significant prognostic factor for the development of avascular necrosis (AVN) in this patient?

Explanation

The most important prognostic factor for the development of AVN in SCFE is the stability of the slip. An unstable SCFE (defined by Loder as the inability to bear weight, even with crutches) has a high rate of AVN (up to 47%), whereas stable slips have an AVN rate approaching zero.

Question 68

A 25-year-old male is evaluated 6 hours after intramedullary nailing of a closed tibia fracture. He complains of severe pain out of proportion, unrelieved by opioids. His blood pressure is 110/70 mmHg. Compartment pressures of the anterior, lateral, superficial posterior, and deep posterior compartments are 35 mmHg, 30 mmHg, 25 mmHg, and 45 mmHg, respectively. What is the most appropriate next step in management?

Explanation

The diagnosis of compartment syndrome can be confirmed using the Delta P (Diastolic BP - highest compartment pressure). A Delta P of less than 30 mmHg is an absolute indication for fasciotomy. In this patient, Delta P = 70 (diastolic) - 45 (deep posterior compartment) = 25 mmHg. Immediate four-compartment fasciotomy is required.

Question 69

Under which of the following mechanical and biological conditions does primary (contact) bone healing predominantly occur?

Explanation

Primary (contact) bone healing occurs via direct cutting cone remodeling (Haversian remodeling) across the fracture site without callus formation. This requires absolute stability (strain <2%), viable bone ends, and no fracture gap. Any gap or movement leads to secondary healing with callus formation.

Question 70

A 65-year-old female with metastatic breast cancer presents with a lytic lesion in the peritrochanteric region of the femur. The lesion measures 50% of the cortical diameter. She reports moderate pain with weight-bearing. Using Mirels' criteria, what is her calculated score and the most appropriate management?

Explanation

Mirels' criteria score is calculated based on 4 variables (Site, Pain, Lesion, Size), assigning 1, 2, or 3 points for each. Site: Peritrochanteric = 3. Pain: Moderate = 2. Lesion: Lytic = 3. Size: 1/3 to 2/3 (50%) = 2. Total score = 3 + 2 + 3 + 2 = 10. A score of >/= 9 indicates impending pathologic fracture and prophylactic internal fixation is highly recommended.

Question 71

A 68-year-old male presents with instability and swelling in his knee 2 years after a primary total knee arthroplasty (TKA). On examination, the knee is stable in full extension but exhibits significant anteroposterior laxity at 90 degrees of flexion. Which of the following technical errors during the index procedure most likely caused this complication?

Explanation

Flexion instability (stable in extension, loose in flexion) is caused by a mismatched flexion gap relative to the extension gap. Excessive resection of the posterior femoral condyles directly enlarges the flexion space without altering the extension space, leading to this presentation.

Question 72

An 18-year-old contact athlete undergoes surgical stabilization for recurrent anterior shoulder instability. Preoperative imaging reveals 15% glenoid bone loss and a large, engaging Hill-Sachs lesion. An arthroscopic Bankart repair with a remplissage procedure is performed. Which structure is tenodesed into the humeral head defect during the remplissage?

Explanation

The remplissage procedure is an adjunctive technique used for engaging Hill-Sachs lesions to prevent the defect from engaging the anterior glenoid rim. It involves tenodesis of the infraspinatus tendon and the posterior capsule into the bony defect on the posterolateral humeral head.

Question 73

A 70-year-old male presents with bilateral leg pain and heaviness that worsens with walking. Which of the following historical or physical examination findings is most specific for neurogenic claudication (lumbar spinal stenosis) rather than vascular claudication?

Explanation

Neurogenic claudication is posture-dependent. Spinal extension narrows the canal, exacerbating symptoms, while spinal flexion (e.g., leaning forward on a shopping cart or riding a bicycle) increases canal volume and relieves symptoms. Vascular claudication is strictly distance/exertion-dependent and is quickly relieved by simply stopping and standing still.

Question 74

A 24-year-old rugby player presents after grabbing an opponent's jersey. He is unable to actively flex the distal interphalangeal (DIP) joint of his ring finger. Radiographs show a small bony avulsion fragment volar to the proximal interphalangeal (PIP) joint. According to the Leddy and Packer classification, what type of injury is this, and what is its vascular implication?

Explanation

This is a 'Jersey finger' (FDP avulsion). In a Leddy and Packer Type II injury, the tendon retracts to the level of the PIP joint. It is caught by the intact vincula longus, which preserves some blood supply, allowing for a slightly delayed repair compared to a Type I injury (where the tendon retracts to the palm, rupturing vincula, making it ischemic and requiring repair within 7-10 days).

Question 75

In the evaluation of a suspected Lisfranc injury, which of the following ligamentous connections defines the primary component of the Lisfranc ligament complex?

Explanation

The Lisfranc ligament complex connects the medial cuneiform to the base of the second metatarsal. There is no direct transverse ligamentous connection between the bases of the first and second metatarsals, making this interval mechanically vulnerable.

Question 76

In normal articular cartilage, which zone is characterized by the highest concentration of proteoglycans, the lowest concentration of water, and chondrocytes arranged in vertical columns?

Explanation

The deep (radial) zone of articular cartilage is defined by chondrocytes arranged in vertical columns, collagen fibers oriented perpendicular to the joint surface, the lowest water content, and the highest proteoglycan concentration, providing immense resistance to compressive forces.

Question 77

A 60-year-old female presents 6 weeks after non-operative management of a minimally displaced distal radius fracture. She reports sudden inability to actively extend her thumb interphalangeal joint. The extensor pollicis longus (EPL) tendon is suspected to have ruptured. What is the most widely accepted mechanism for this complication in non-displaced or minimally displaced fractures?

Explanation

EPL ruptures are most commonly associated with non-displaced or minimally displaced distal radius fractures. Because the extensor retinaculum remains intact, bleeding and edema increase pressure within the tightly constrained third dorsal compartment, leading to microvascular ischemia and subsequent delayed rupture of the EPL tendon.

Question 78

A 4-month-old infant is being treated with a Pavlik harness for developmental dysplasia of the hip (DDH). At the two-week follow-up, the parents report the infant is not kicking her left leg as much. On examination, there is decreased active extension of the left knee, but normal ankle movements. What is the most likely cause of this complication?

Explanation

The most common nerve injury associated with Pavlik harness use is a transient femoral nerve palsy, typically caused by hyperflexion of the hip. This presents with decreased active extension of the knee (quadriceps weakness). Treatment involves adjusting the harness to reduce flexion; the palsy typically resolves spontaneously.

Question 79

When performing a direct anterior approach to the hip for a total hip arthroplasty, the internervous plane utilized superficially is between muscles supplied by which two nerves?

Explanation

The direct anterior approach (Smith-Petersen) utilizes an internervous plane. Superficially, it runs between the Sartorius (femoral nerve) and the Tensor Fasciae Latae (superior gluteal nerve). Deeply, it separates the Rectus Femoris (femoral nerve) from the Gluteus Medius (superior gluteal nerve).

Question 80

A 15-year-old male presents with a destructive, sunburst-pattern bone tumor in the distal femur. Biopsy confirms high-grade intramedullary osteosarcoma. A thorough family history reveals that his mother had breast cancer at age 32, and his younger sister was recently diagnosed with an adrenocortical carcinoma. Which underlying genetic mutation is most likely present in this patient?

Explanation

The strong family history of early-onset breast cancer, adrenocortical carcinoma, and osteosarcoma is the hallmark of Li-Fraumeni syndrome. This autosomal dominant cancer predisposition syndrome is caused by a germline mutation in the TP53 tumor suppressor gene.

Question 81

Denosumab is utilized in the management of severe osteoporosis and certain bone metastases. What is its exact mechanism of action at the cellular level?

Explanation

Denosumab is a monoclonal antibody that specifically binds to RANKL, preventing it from activating the RANK receptor on osteoclasts. This arrests osteoclast maturation, function, and survival, unlike Osteoprotegerin (OPG) which acts as the body's natural decoy receptor.

Question 82

When a constant mechanical load is applied to a viscoelastic material such as a ligament, the material will undergo progressive deformation over time. This biomechanical phenomenon is referred to as:

Explanation

Creep describes the progressive deformation of a viscoelastic material when subjected to a constant load. In contrast, stress relaxation is the decrease in internal stress over time when the material is held at a constant deformation.

Question 83

Tranexamic acid (TXA) is frequently administered in orthopedic trauma and arthroplasty to minimize perioperative blood loss. Which of the following best describes its primary pharmacological action?

Explanation

TXA is a synthetic analog of the amino acid lysine that competitively inhibits the activation of plasminogen to plasmin. This prevents plasmin from degrading fibrin clots, effectively stabilizing formed thrombi.

Question 84

Pediatric physeal fractures (Salter-Harris type I-III) typically propagate through the structurally weakest layer of the growth plate. Which histologic zone does this represent?

Explanation

The zone of hypertrophy is structurally the weakest area of the physis due to the large volume of swollen chondrocytes and comparatively sparse extracellular matrix. Most physical fractures propagate through this specific layer.

Question 85

In damage control orthopedics (DCO) for a polytraumatized patient, which metabolic marker is considered the most reliable indicator of adequate end-organ resuscitation to clear the patient for definitive fracture fixation?

Explanation

Serum lactate less than 2.0-2.5 mmol/L and a resolving base deficit (approaching zero) are highly reliable markers of restored tissue perfusion. Normalizing these values suggests the patient has moved past the initial systemic inflammatory response and can safely undergo definitive surgery.

Question 86

Demineralized bone matrix (DBM) is widely used to supplement bone grafting in nonunions and fusions. Which of the following accurately describes its innate biological properties?

Explanation

DBM is both osteoconductive (providing a structural scaffold) and osteoinductive (due to exposed bone morphogenetic proteins [BMPs] from the demineralization process). It lacks living cells, so it is not osteogenic.

Question 87

Which of the following proteoglycans is primarily responsible for providing the osmotic swelling pressure and compressive resilience in healthy articular cartilage?

Explanation

Aggrecan is the most abundant proteoglycan in articular cartilage matrix. Its heavily sulfated, negatively charged glycosaminoglycan chains draw water into the tissue, establishing a swelling pressure that resists compressive loads.

Question 88

During a Smith-Petersen (anterior) approach to the hip, the superficial internervous plane is developed. Which two nerves supply the specific muscles that define this superficial plane?

Explanation

The superficial plane of the Smith-Petersen approach lies between the sartorius (innervated by the femoral nerve) and the tensor fasciae latae (innervated by the superior gluteal nerve). This establishes a true internervous corridor.

Question 89

If an orthopedic surgeon inadvertently uses a stainless steel screw through a titanium alloy locking plate, what mode of hardware failure or degradation is most likely to be accelerated?

Explanation

Galvanic corrosion occurs when two dissimilar metals are placed in physical contact within an electrolytic environment (such as bodily fluids). This sets up an electrochemical cell that accelerates the degradation of the more anodic metal.

Question 90

Four weeks after sustaining a closed humeral shaft fracture, a patient exhibits complete radial nerve palsy. An electromyogram (EMG) reveals fibrillation potentials in the brachioradialis but no voluntary motor unit action potentials. According to Seddon's classification, this nerve injury is best termed:

Explanation

Fibrillation potentials confirm denervation, ruling out neuropraxia (which has preserved distal conduction and no fibrillations). Given the intact nerve sheath typically seen in closed fractures (as opposed to full transection in neurotmesis), this represents axonotmesis undergoing Wallerian degeneration.

Question 91

A 68-year-old male presents with severe knee pain, erythema, and a draining sinus 3 weeks following a total knee arthroplasty. Radiographs show well-fixed implants. What is the standard of care surgical management?

Explanation

For early acute periprosthetic joint infections (typically defined as <4 weeks postoperatively) with stable implants, DAIR with modular component exchange is the preferred treatment. This minimizes morbidity while addressing the immature biofilm.

Question 92

In the evaluation of a patient with suspected acute compartment syndrome of the leg, which pressure measurement parameter is the most universally accepted absolute threshold for performing an emergency fasciotomy?

Explanation

A delta pressure (defined as the diastolic blood pressure minus the intracompartmental pressure) of less than 30 mmHg is the standard threshold indicating critical tissue ischemia. Absolute pressures are less reliable due to individual variations in baseline perfusion pressure.

Question 93

The primary cellular defect in the initial active phase of Paget's disease of bone (osteitis deformans), which eventually leads to chaotic woven bone architecture, is localized to:

Explanation

Paget's disease begins with an aggressive osteolytic phase driven by abnormally large, multinucleated osteoclasts that rapidly resorb bone. This is followed by a disorganized and frantic osteoblastic response, resulting in mechanically weak, structurally chaotic woven bone.

Question 94

Hypertrophic nonunions are radiographically characterized by the classic "elephant foot" appearance with abundant callus formation that fails to bridge. The primary etiology of this specific type of nonunion is:

Explanation

Hypertrophic nonunions occur when the fracture has an excellent biological environment and blood supply (evident by exuberant callus) but lacks sufficient mechanical stability to allow bridging. Management consists of improving fixation rigidity (e.g., reamed exchange nailing).

Question 95

A 15-year-old male presents with worsening nocturnal distal femur pain. Imaging reveals an aggressive metaphyseal lesion with a "sunburst" periosteal reaction and Codman's triangle. Which genetic mutation profile is most strongly associated with this primary bone tumor?

Explanation

The clinical and radiographic picture represents classic osteosarcoma. Osteosarcomas are closely linked to mutations in tumor suppressor genes, particularly p53 (as seen in Li-Fraumeni syndrome) and RB1 (hereditary retinoblastoma).

Question 96

A 65-year-old patient undergoes a workup for pathological vertebral compression fractures, anemia, and hypercalcemia. Radiographs show multiple punched-out lytic lesions in the calvarium. Which laboratory test is the most diagnostic for this suspected malignancy?

Explanation

Multiple myeloma is the most common primary bone malignancy in adults. It is definitively diagnosed by the presence of a monoclonal spike (M-spike) on SPEP or UPEP, which represents the overproduction of abnormal immunoglobulins by clonal plasma cells.

Question 97

Under the Gustilo-Anderson classification system, how would you classify an open tibial shaft fracture featuring a 10 cm laceration, severe periosteal stripping, and exposed desiccated bone that will necessitate a free vascularized tissue transfer for closure?

Explanation

A Gustilo-Anderson Type IIIB fracture involves extensive soft-tissue injury with periosteal stripping and inadequate soft-tissue coverage, mandating a local or free flap for closure. Type IIIC would additionally require major arterial repair.

Question 98

In the pathogenesis of Slipped Capital Femoral Epiphysis (SCFE), the classic displacement of the capital femoral epiphysis relative to the femoral neck is in which direction?

Explanation

In SCFE, the capital femoral epiphysis is held within the acetabulum while the femoral neck displaces anteriorly and superiorly. Thus, relative to the neck, the epiphysis "slips" posteriorly and inferiorly.

Question 99

The organic extracellular matrix of normal mature lamellar bone is essential for tensile strength and ductility. Which collagen type accounts for approximately 90% of this organic matrix?

Explanation

Type I collagen is the overwhelmingly predominant structural protein in bone matrix, tendons, and ligaments. Type II collagen is primarily found in hyaline and articular cartilage.

Question 100

The progressive loss of articular cartilage characteristic of osteoarthritis is primarily mediated by the enzymatic cleavage of aggrecan and Type II collagen. Which class of enzymes is the primary driver of this catabolic degradation?

Explanation

Matrix metalloproteinases (MMPs), specifically MMP-13 (collagenase 3) and ADAMTS-5 (an aggrecanase), are the principal enzymes that degrade the cartilage extracellular matrix in osteoarthritis.

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