Orthopedic MCQs: AAOS Basic Science Board Review Set 3

Question 1

Figure 26a shows the radiograph of a 55-year-old woman who has pain in her right leg after falling. Laboratory studies reveal an elevated alkaline phosphatase level. A biopsy specimen from the proximal tibia is shown in Figure 26b. What is the most likely diagnosis?

Explanation

Paget's disease of bone is a metabolic disorder of bone remodeling. The normally coupled process of bone resorption and deposition is lost, resulting in excessive localized bone resorption and compensatory increased bone formation. Pagetic bone tends to be more brittle; therefore, it is susceptible to pathologic fractures and subsequent deformities. Lander PH, Hadjipavlou AG: A dynamic classification of Paget's disease. J Bone Joint Surg Br 1986;68:431-438.

Question 2

Radiographs of a pediatric patient reveal a suspected osteosarcoma of the distal femur. Additional staging studies should consist of

Explanation

CT of the abdomen and pelvis is not part of the staging of osteosarcoma. Staging studies should consist of CT of the chest, radiographs of the chest and primary tumor, MRI of the primary tumor, and a bone scan. The MRI should be obtained prior to the biopsy.

Question 3

Which of the following factors are considered prognostic of survival in patients with soft-tissue sarcomas?

Explanation

The factors that are independently prognostic of patient survival are tumor size, tumor grade, and tumor depth (ie, subfascial versus superficial). These factors are the basis for the American Joint Committee on Cancer staging criteria. Patient age and neurovascular invasion are not prognostic. Surgical margin is prognostic for local recurrence but not conclusively for patient survival or metastasis. Metastatic disease is also predictive of survival. Cheng EY, Thompson RC Jr: New developments in the staging and imaging of soft-tissue sarcomas. Instr Course Lect 2000;49:443-451.

Question 4

Figures 27a through 27c show the AP radiograph, MRI scan, and biopsy specimen of an otherwise healthy man who has a painful wrist. Serum chemistry studies are normal. What is the most likely diagnosis?

Explanation

The osseous sites most frequently involved by giant cell tumor of bone are the distal femur, proximal tibia, and distal radius with approximately 10% of giant cell tumors involving the distal radius. The goals of treatment are to remove the tumor completely and to preserve maximum function of the extremity.

Question 5

What is the most common primary malignant tumor of bone in childhood?

Explanation

Osteosarcoma is the most common primary malignant tumor of bone in childhood, followed by Ewing's sarcoma. Rhabdomyosarcoma is a soft-tissue sarcoma of childhood. Chondrosarcoma rarely occurs in childhood. Osteochondroma is a benign tumor of bone. Simon MA, Springfield DS, et al: Osteogenic Sarcoma: Surgery for Bone and Soft Tissue Tumors. Philadelphia, PA, Lippincott Raven, 1998, p 266.

Question 6

A 44-year-old woman has bilateral knee pain, and history reveals bilateral hip replacements. Radiographs are seen in Figure 28a, and histopathologic specimens from the total hip replacement are shown in Figures 28b and 28c. Laboratory studies reveal anemia. What is the most likely diagnosis?

Explanation

Rheumatoid arthritis is an inflammatory arthritis that usually involves multiple joints. Radiologic findings of periarticular erosion, osteopenia, and minimal osteophyte formation favor rheumatoid arthritis over osteoarthritis. Pigmented villonodular synovitis and Charcot arthropathy are more often considered monoarticular diseases. There are no radiographic findings of Paget's disease.

Question 7

In addition to pain, which of the following factors are considered most predictive of the risk of pathologic fracture?

Explanation

While guidelines for predicting fracture risk are at best imprecise, the scoring system by Mirels (pain, anatomic location, and pattern of bony destruction) has been shown to be most predictive of fracture risk. Functional pain, peritrochanteric location, and lytic bone destruction are the greatest risk factors for pathologic fracture. The factors of patient weight, age, soft-tissue mass, and location within bone are all of lesser importance. Frassica FJ, Frassica DA, McCarthy EF, Riley LH III: Metastatic bone disease: Evaluation, clinicopathologic features, biopsy, fracture risk, nonsurgical treatment, and supportive management. Instr Course Lect 2000;49:453-459.

Question 8

Patients with which of the following primary carcinomas have the shortest overall survival rate after a solitary metastasis to bone?

Explanation

The median survival of patients after discovery of bone metastasis from primary lung carcinoma is shorter compared with other primary sites.

Question 9

A 63-year-old woman has a femoral neck fracture. A biopsy specimen obtained from the fracture site at the time of her hemiarthroplasty reveals metastatic carcinoma. Seven days after surgery, she becomes confused and lethargic. Which of the following laboratory values is most likely implicated in the patient's symptoms at this time?

Explanation

Although many hematologic and electrolyte abnormalities may be present in a patient with advanced metastatic cancer, an elevated serum calcium level is most commonly associated with confusion. Treatment with hydration, diuretics, and bisphosphonates is recommended. Clohishy D: Management of skeletal metastasis in clinical orthopaedics, in Craig E (ed): Operative Orthopaedics. Philadelphia, PA, Lippincott Williams & Wilkins, 1999, pp 994-997.

Question 10

Figure 29a shows the clinical photograph of a 26-year-old woman who has had the leg deformity since birth. She reports difficulty with walking and weight bearing and notes increased discomfort and swelling when the leg is dependent. She denies any history of trauma or family history of a similar disorder. Examination reveals a fixed equinovarus deformity of the foot but no evidence of a limb-length discrepancy. No other cutaneous findings or soft-tissue masses are noted. Sagittal and axial T1- and T2-weighted MRI scans are shown in Figures 29b and 29c. What is the most likely diagnosis?

Explanation

Because the MRI scans show marked dilation and proliferation of lymphatic channels that completely involve all the leg muscles and the clinical photograph shows the severe swelling associated with this disease, the most likely diagnosis is lymphangiomatosis. Poliomyelitis affects the anterior horn cells and manifests as muscle atrophy. Neurofibromatosis can have a similar clinical appearance but usually is associated with other systemic and cutaneous findings. Congenital band syndrome results in amputated or shortened extremities. Chronic venous stasis disease usually is not associated with joint contractures, and typically it affects older individuals. Surgical excision is the only known treatment; this patient underwent an above-knee amputation. Berquist TH (ed): MRI of the Musculoskeletal System, ed 3. Philadelphia, PA, Lippincott Raven, 1997, p 771.

Question 11

Figures 30a and 30b show the MRI scans of a 51-year-old woman who has had an enlarging soft-tissue mass in her right thigh for the past 18 months. Examination reveals no inguinal adenopathy. Results of a biopsy show a high-grade sarcoma. A bone scan is unremarkable, and a CT scan of the chest shows no evidence of pulmonary metastasis. According to the Musculoskeletal Tumor Society (MSTS) staging system, the tumor should be classified as what stage?

Explanation

According to the MSTS staging system, soft-tissue sarcomas are staged according to the following factors: grade, site, and metastasis. Roman numerals are used to designate malignant tumors, and Arabic numerals are used for benign tumors. Low-grade malignant tumors are staged as I while intermediate- and high-grade tumors are staged as II. Site is defined as intracompartmental (A) or extracompartmental (B). Any metastasis is staged as III. This patient has a high-grade tumor (II), and the MRI scans show that it is confined to a single compartment; therefore, it is staged as IIA. There is no evidence of metastatic disease. Enneking WF, Spanier SS, Goodman MA: A system for the surgical staging of musculoskeletal sarcoma. Clin Orthop 1980;153:106-120. Peabody TD, Monson D, Montag A, Schell MJ, Finn H, Simon MA: A comparison of the prognoses for deep and subcutaneous sarcomas of the extremities. J Bone Joint Surg Am 1994;76:1167-1173. Pisters PW, Leung DH, Woodruff J, Shi W, Brennan MF: Analysis of prognostic factors in 1,041 patients with localized soft tissue sarcomas of the extremities. J Clin Oncol 1996;14:1679-1689.

Question 12

An otherwise healthy 45-year-old man has a 3-cm subcutaneous mass on his anterior thigh that is presumed to be a lipoma. Removal of the mass is performed in the office of his primary care physician. Pathologic evaluation shows a high-grade malignant fibrous histiocytoma. Staging reveals no evidence of metastatic disease. Management at this time should consist of

Explanation

Resection of a previously excised soft-tissue sarcoma is recommended. Reoperation is recommended after removal of malignant tumors previously believed to be benign, as approximately one half of the patients will have residual tumor in the re-excised specimen. Observation is not indicated in most patients because local recurrence is likely. Radiation therapy alone may result in long-term local control but is felt to be less effective than reoperation. Chemotherapy and radiation therapy alone are not recommended. Wide excision is the most important factor for local disease control. Radiation therapy after surgical re-excision may also decrease the risk of local recurrence. Peabody TD, Monson D, Montag A, Schell MJ, Finn H, Simon MA: A comparison of the prognoses for deep and subcutaneous sarcomas of the extremities. J Bone Joint Surg Am 1994;76:1167-1173. Giuliano AE, Eilber FR: The rationale for planned reoperation after unplanned total excision of soft-tissue sarcomas. J Clin Oncol 1985;3:1344-1348. Lewis JJ, Leung D, Espat J, Woodruff JM, Brennan MF: Effect of resection in extremity soft tissue sarcoma. Ann Surg 2000;231:655-663.

Question 13

A 13-year-old girl has had a firm mass and pain in her right shoulder for the past several weeks. She denies any history of trauma. A radiograph and MRI scan are shown in Figures 31a and 31b. Biopsy specimens are shown in Figures 31c and 31d. What is the most likely diagnosis?

Explanation

The patient has osteosarcoma. The radiograph suggests an aggressive primary tumor of bone, and the histology shows malignant cells surrounded by osteoid, classic for osteosarcoma. Ewing's sarcoma histologically consists of small round blue cells. Osteochondroma and periosteal chondroma occur in the shoulder but have a different histologic pattern and a less aggressive radiographic appearance. Chondrosarcomas rarely occur in children. Simon MA, Springfield DS, et al: Osteogenic Sarcoma: Surgery for Bone and Soft Tissue Tumors. Philadelphia, PA, Lippincott Raven, 1998, p 266.

Question 14

The use of radiation therapy is most effective in metastatic bone disease from which of the following tumors?

Explanation

Both myeloma and lymphoma are more responsive to radiation therapy. The other types of tumors are relatively more resistant to radiation. Doses of 25 to 50 Gy (2,500 to 5,000 cGy) are usually sufficient for myeloma and lymphoma, while carcinomas frequently require a higher dosage. Large cell lymphoma of bone is usually of B-cell origin and is treated with chemotherapy and radiation therapy. Simon MA, Springfield DS, et al: Surgery for Bone and Soft Tissue Tumors. Philadelphia, PA, Lippincott Raven, 1998, p 683.

Question 15

Figures 32a and 32b show the radiographs of an active 13-year-old boy who has persistent left thigh pain and a limp despite a trial of protected weight bearing. Management should consist of

Explanation

The plain radiographs show an eccentric metaphyseal lesion involving a long bone in a skeletally immature patient. The lesion is longer than it is wide, with distinctly lobular outer edges that are sclerotic. These findings are characteristic of a nonossifying fibroma. Small asymptomatic lesions may be followed clinically. Larger lesions that occupy greater than two thirds of the width of the shaft and are located in areas of high mechanical stress such as the femur are more prone to fracture than smaller lesions. Pain is often a sign of impending fracture or the presence of a small fracture that may not be apparent on radiographs. The natural history of the lesion is to resolve over a period of years. The procedure that would allow the patient to return to contact sports is curettage and bone grafting. Intralesional steroid injection has been advocated in the treatment of unicameral bone cysts and eosinophilic granuloma but not nonossifying fibromas. En block resection is not indicated for a benign lesion. Low-dose radiation therapy has been used for eosinophilic granuloma but not for nonossifying fibromas. Walker RN, Green NE, Spindler KP: Stress fractures in skeletally immature patients. J Pediatr Orthop 1996;16:578-584.

Question 16

The spread of malignant cells to the vertebrae is often through

Explanation

In 1940, Batson described a valveless plexus of veins that extend from the dural venous sinuses of the skull to the sacrum. This system permits retrograde blood flow and enables tumor cells to enter vertebral bodies at multiple levels. Increased intra-abdominal pressure will enhance this retrograde blood flow.

Question 17

A 10-year-boy has had thigh pain for the past several months. He denies any history of trauma. Examination reveals no soft-tissue mass, and mild tenderness. Figures 33a and 33b show the plain radiograph and MRI scan, and the biopsy specimens are shown in Figures 33c and 33d. What is the most likely diagnosis?

Explanation

The diagnosis is eosinophilic granuloma. The plain radiograph and MRI scan show a lesion in the midshaft of the femur. There is no soft-tissue mass. There is reactive bone about the lesion that suggests a less aggressive tumor. The histology reveals eosinophils in an otherwise bland cellular background with no evidence of mitotic figures or malignant cells to suggest sarcoma. The diagnostic elements are the amphophilic (ie, pale purple) histiocytes with cigar-shaped nuclei, some of which have linear longitudinal grooves. There is no histologic evidence of infection. Lymphoma of bone would be an unusual occurrence in this age group, and the histology is not consistent with that diagnosis.

Question 18

A 35-year-old patient has right hip pain. Figures 34a and 34b show the coronal MRI scan and the biopsy specimen. What is the most likely diagnosis?

Explanation

Alcohol abuse, steroids, Caisson disease, sickle cell disease, and radiation therapy are the leading causes of nontraumatic osteonecrosis of the femoral head. Arlet J: Nontraumatic avascular necrosis of the femoral head: Past, present, and future. Clin Orthop 1992;277:12-21.

Question 19

A 16-year-old girl has a painful foot mass. A radiograph, MRI scan, and biopsy specimens are shown in Figures 35a through 35d. What is the most likely diagnosis?

Explanation

Synovial sarcoma should always be considered in the differential diagnosis of a foot mass; however, the histopathology shows a typical example of PVNS, with hemosiderin, giant cells, and synovium. Synovial chondromatosis would have metaplastic cartilage in the synovium. The radiograph shows subtle erosion of the lateral cortex of the cuboid, and the MRI scan shows a soft-tissue mass. Ghert MA, Scully SP, Harrelson JM: Pigmented villonodular synovitis of the foot and ankle: A review of six cases. Foot Ankle Int 1999;20:326-330. Jones BC, Sundaram M, Kransdorf MJ: Synovial sarcoma: MR imaging findings in 34 patients. Am J Roentgenol 1993;161:827-830. Sartoris DJ, Resnick D: Magnetic resonance imaging of pediatric foot and ankle disorders. J Foot Surg 1990;29:489-494.

Question 20

What is the most common diagnosis in a patient older than age 40 years with a destructive bony lesion?

Explanation

The most common destructive lesions in a patient older than age 40 years are, in decreasing incidence, metastatic carcinoma, multiple myeloma, lymphoma, and chondrosarcoma. Osteosarcoma is found primarily in younger patients.

Question 21

A 78-year-old man being seen for routine follow-up studies reports mild to moderate pain in his left arm. The patient has a history of lung cancer and severe heart disease. A radiograph and a bone scan are shown in Figures 36a and 36b. Treatment should consist of

Explanation

The patient has lung cancer metastatic to the left humerus that is mildly to moderately symptomatic. Radiographically, the lesion is small and is not associated with significant cortical destruction. Metastatic lesions in the upper extremity have a lower incidence of pathologic fracture than lesions in the lower extremity.

Question 22

Which of the following benign bone lesions can develop lung metastases?

Explanation

Although considered benign bone lesions, lung metastases can develop in giant cell tumors and chondroblastomas. These often can be treated with multiple thoracotomies, resulting in long-term survival. Roberts PF, Taylor JG: Multifocal benign chondroblastomas: Report of a case. Hum Pathol 1980;11:296-298.

Question 23

The radiographic feature seen in Figure 37 that best indicates a slow-growing process is the

Basic Science 2002 Practice Questions: Set 3 (Solved) - Figure 32

Explanation

For the fibula (or any bone) to bow, a long-standing process needs to be present. Pressure from a rapid process would cause erosion, not allowing the bone to remodel. The other features are helpful confirmatory findings but also may be associated with aggressive processes. In this patient, the fibular deformity is caused by a sessile osteochondroma of the tibia. Lodwick GS, Wilson AJ, Farrell C, Virtama P, Dittrich F: Determining growth rates of focal lesions of bone from radiographs. Radiology 1980;134:577-583.

Question 24

A 52-year-old man has a stage IIB malignant fibrous histiocytoma of the distal femur. Local treatment should consist of

Explanation

Local control of malignant fibrous histiocytoma of bone typically consists of wide excision. Curettage and bone grafting is a procedure with an intralesional surgical margin, with an unacceptable rate of local recurrence. Prophylactic fixation is considered for patients with metastatic disease. Radiation therapy alone is not adequate for local control of this tumor. Neoadjuvant chemotherapy is often used primarily for systemic tumor control.

Question 25

A patient undergoes excision of a 3- x 3-cm soft-tissue sarcoma. The final histology results show tumor at the surgical margin. The surgical procedure performed is classified as

Explanation

Surgical margins are classified according to the plane of dissection in relation to the tumor. An intralesional procedure is where the plane of dissection passes through the tumor. A marginal resection involves a resection where the plane of dissection is through the reactive zone of the tumor. A wide resection is through completely normal tissue outside of the reactive zone. A radical resection removes all of the muscles and/or bone of the compartment that are involved by the tumor.

Question 26

When a constant deformation is applied to a viscoelastic material such as a ligament, the force required to maintain that deformation decreases over time. This phenomenon is known as:

Explanation

Stress relaxation is the decrease in stress over time when a viscoelastic material is held at a constant strain. Creep is the progressive deformation over time under a constant load.

Question 27

A 68-year-old woman is treated with a monoclonal antibody for severe osteoporosis. The drug binds to a specific receptor on osteoblast lineage cells to inhibit osteoclastogenesis. What is the molecular target of this medication?

Explanation

Denosumab is a monoclonal antibody that binds to RANKL, preventing it from interacting with RANK on osteoclasts. This mimics the action of OPG, thereby inhibiting osteoclast maturation and survival.

Question 28

In normal articular cartilage, which of the following best describes the structural arrangement of collagen fibers in the superficial zone?

Explanation

In the superficial zone of articular cartilage, collagen fibers are oriented parallel to the joint surface to resist shear forces. In the deep zone, they are oriented perpendicularly to resist compressive loads.

Question 29

A 4-year-old child presents with bowing of the lower extremities and widening of the physes. Laboratory studies reveal hypocalcemia, hypophosphatemia, and markedly elevated alkaline phosphatase. A defect in which of the following processes is most likely responsible?

Explanation

The clinical and laboratory findings are consistent with rickets, which is characterized by defective mineralization of osteoid and cartilage matrix at the growth plate. This is most commonly due to Vitamin D deficiency.

Question 30

Which of the following points on a stress-strain curve represents the maximum stress a material can withstand before failing?

Explanation

Ultimate tensile strength is the maximum stress plotted on the stress-strain curve before the material fails. The yield point marks the transition from elastic to plastic deformation.

Question 31

Which of the following is the primary glycosaminoglycan responsible for the compressive stiffness of articular cartilage by creating a high osmotic swelling pressure?

Explanation

Chondroitin sulfate is the most abundant glycosaminoglycan in articular cartilage and provides compressive stiffness through its strong negative charge, which draws water into the tissue. Aggrecan contains both chondroitin and keratan sulfate, but chondroitin sulfate predominates.

Question 32

Romosozumab is an anabolic agent used in the treatment of osteoporosis. It exerts its osteogenic effect by binding to and inhibiting which of the following?

Explanation

Romosozumab is a monoclonal antibody that binds to and inhibits sclerostin. Sclerostin normally inhibits the Wnt/beta-catenin pathway, so its inhibition leads to increased osteoblast activity and net bone formation.

Question 33

A patient undergoes revision total hip arthroplasty. The retrieved femoral component shows a localized form of corrosion at the modular head-neck junction. This phenomenon is primarily driven by small relative motions between the components and is known as:

Explanation

Fretting corrosion occurs at contact sites between materials under load, such as modular junctions in arthroplasty, due to micromotion that disrupts the protective passive oxide layer. Galvanic corrosion requires dissimilar metals in an electrolyte.

Question 34

Adult articular chondrocytes exist in an avascular environment. Which of the following pathways is their primary source of energy production?

Explanation

Due to the avascular and hypoxic nature of articular cartilage, chondrocytes rely primarily on anaerobic glycolysis for ATP production. They express high levels of hypoxia-inducible factor 1-alpha (HIF-1a) to survive.

Question 35

A young patient presents with multiple fractures and generalized increased bone density on radiographs. Genetic testing reveals a mutation affecting the ruffled border of osteoclasts. A defect in which of the following enzymes is most likely involved?

Explanation

Osteopetrosis is caused by defective osteoclast resorption. A common mutation involves Carbonic anhydrase II, which is necessary to produce protons for acidifying the resorption pit via the ruffled border.

Question 36

During the normal gait cycle, articular cartilage relies on different lubrication mechanisms. At the initiation of movement under high loads and low speeds, which lubrication mechanism predominates?

Explanation

Boundary lubrication predominates at high loads and low speeds, such as the initiation of motion. It is mediated primarily by lubricin (PRG4) attached to the articular surface.

Question 37

Continuous, high-dose administration of Parathyroid Hormone (PTH) leads to net bone resorption. Which of the following best describes the cellular mechanism responsible for this effect?

Explanation

PTH binds to receptors on osteoblasts and osteocytes, stimulating them to upregulate RANKL expression and downregulate OPG. This indirectly promotes osteoclast differentiation and bone resorption, as osteoclasts do not possess PTH receptors.

Question 38

While Type II collagen constitutes the vast majority of collagen in articular cartilage, a specific minor collagen type is uniquely responsible for anchoring the basal layer of cartilage to the underlying subchondral bone. Which type is it?

Explanation

Type X collagen is produced specifically by hypertrophic chondrocytes and is localized primarily to the calcified zone of articular cartilage. Its primary function is to help anchor the cartilage to the subchondral bone.

Question 39

A severely malnourished patient presents with bleeding gums, petechiae, and joint pain. The underlying defect involves impaired hydroxylation of proline and lysine residues during collagen synthesis. This process requires which of the following as a cofactor?

Explanation

Vitamin C (ascorbic acid) is a required cofactor for prolyl and lysyl hydroxylase enzymes. Deficiency causes scurvy, resulting in defective collagen triple helix formation and fragile connective tissues.

Question 40

In total joint arthroplasty, the phenomenon where microscopic asperities of a harder material cut through a softer material is known as:

Explanation

Abrasive wear occurs when a harder surface roughens or cuts into a softer surface (e.g., a scratched metal femoral head articulating against UHMWPE). Adhesive wear involves material transfer between two articulating surfaces.

Question 41

When a long bone is subjected to bending forces, its resistance to bending is proportional to its area moment of inertia. For a hollow cylinder, area moment of inertia is proportional to the radius raised to which power?

Explanation

The area moment of inertia for a cylinder is proportional to the radius to the fourth power (r^4). Thus, placing intramedullary nails with a larger radius or increasing the outer diameter of a bone exponentially increases bending rigidity.

Question 42

Bone Morphogenetic Proteins (BMPs) are members of the TGF-beta superfamily that induce osteoblastic differentiation. Which intracellular signaling molecules are directly phosphorylated by BMP receptors upon ligand binding?

Explanation

Upon binding to their receptors, BMPs initiate intracellular signaling by phosphorylating Smad proteins, specifically Smads 1, 5, and 8. These then complex with Smad 4 to enter the nucleus and regulate osteogenic gene transcription.

Question 43

During cyclical loading and unloading of a viscoelastic structure such as a tendon, the energy lost as heat is represented by the area between the loading and unloading curves. This phenomenon is termed:

Explanation

Hysteresis is the loss of energy (usually as heat) during the loading and unloading cycle of a viscoelastic material. It explains why the unloading curve does not exactly follow the loading curve on a stress-strain diagram.

Question 44

Which of the following changes accurately describes the typical biochemical alteration in articular cartilage associated with normal aging?

Explanation

In aging cartilage, total proteoglycan and water content decrease, and the size of aggrecan molecules decreases. There is also a relative increase in the ratio of keratin sulfate to chondroitin sulfate.

Question 45

Primary (strain-free) bone healing occurs without callus formation. Which of the following is an absolute requirement for primary bone healing?

Explanation

Primary bone healing requires absolute stability (zero or negligible strain) and direct contact between fracture ends, typically achieved via rigid internal fixation. It bypasses callus formation and relies on direct osteoclastic tunneling.

Question 46

Which of the following molecules acts as a decoy receptor to inhibit osteoclastogenesis by binding to RANKL?

Explanation

Osteoprotegerin (OPG) is secreted by osteoblasts and binds to RANKL, preventing it from interacting with RANK on osteoclast precursors. This inhibits osteoclast differentiation and bone resorption.

Question 47

In the biomechanical behavior of ligaments, the phenomenon where a constant displacement results in a progressive decrease in the load over time is known as:

Explanation

Stress relaxation is a viscoelastic property where the stress (load) decreases over time when a tissue is held at a constant strain (displacement). Creep is increasing deformation under a constant load.

Question 48

In normal articular cartilage, which zone has the highest water content and collagen fibers oriented parallel to the joint surface?

Explanation

The superficial zone has the highest water content and collagen fibers oriented parallel to the joint surface to resist shear forces. The deep zone has the lowest water content and collagen fibers oriented perpendicular to the joint surface.

Question 49

Intermittent administration of parathyroid hormone (PTH) primarily results in which of the following skeletal effects?

Explanation

Intermittent (pulsatile) administration of PTH, such as with teriparatide, has an anabolic effect on bone by stimulating osteoblast activity and increasing their lifespan. Continuous PTH exposure leads to net bone resorption.

Question 50

How does the torsional strength of a solid cylindrical bone change if its radius is doubled?

Explanation

The polar moment of inertia, which determines resistance to torsion for a cylinder, is proportional to the radius to the fourth power (r^4). Therefore, doubling the radius increases the torsional strength by a factor of 16 (2^4).

Question 51

Which glycosaminoglycan is the most abundant in normal mature articular cartilage and is responsible for drawing water into the extracellular matrix?

Explanation

Chondroitin sulfate is the most abundant glycosaminoglycan in articular cartilage aggrecan molecules. It provides a high fixed negative charge density that creates an osmotic gradient, drawing water into the tissue to resist compressive loads.

Question 52

A pediatric patient presents with bowing of the legs and widening of the physes. Laboratory studies show normal calcium, markedly decreased phosphate, and normal vitamin D levels. A mutation in which of the following is most likely responsible?

Explanation

The scenario describes X-linked hypophosphatemic rickets, typically caused by a PHEX gene mutation leading to elevated FGF-23. This results in renal phosphate wasting and impaired mineralization of bone.

Question 53

On a stress-strain curve, the point at which a material begins to undergo permanent deformation and no longer returns to its original shape when unloaded is defined as the:

Explanation

The yield point marks the transition from the elastic region to the plastic region on a stress-strain curve. Loading beyond this point causes permanent (plastic) deformation.

Question 54

Which of the following matrix metalloproteinases (MMPs) is considered the primary enzyme responsible for the cleavage of type II collagen in the pathogenesis of osteoarthritis?

Explanation

MMP-13 (collagenase-3) is the principal enzyme responsible for degrading type II collagen in articular cartilage during the progression of osteoarthritis. Its expression is upregulated by pro-inflammatory cytokines like IL-1 and TNF-alpha.

Question 55

During the endochondral ossification phase of secondary fracture healing, hypertrophic chondrocytes secrete which of the following factors to stimulate angiogenesis?

Explanation

Hypertrophic chondrocytes in the fracture callus secrete VEGF, which is essential for initiating angiogenesis. The invasion of blood vessels brings osteoprogenitor cells and osteoclasts to replace the cartilage model with woven bone.

Question 56

A stainless steel screw is inadvertently used with a titanium plate for fracture fixation. Which type of corrosion is most likely to occur?

Explanation

Galvanic corrosion occurs when two dissimilar metals are in contact within a conductive fluid (like body fluids). The metal with the lower electrochemical potential (anodic) will corrode more rapidly.

Question 57

Which molecule is primarily responsible for boundary lubrication of the articular cartilage surface, reducing friction at low speeds and high loads?

Explanation

Lubricin (proteoglycan 4) is a highly glycosylated protein secreted by superficial zone chondrocytes and synoviocytes. It binds to the articular surface and provides boundary lubrication, preventing wear under low speed and high load conditions.

Question 58

Sclerostin, an inhibitor of bone formation, exerts its effect primarily by blocking which of the following signaling pathways in osteoblasts?

Explanation

Sclerostin is produced by osteocytes and binds to LRP5/6 receptors on osteoblasts, inhibiting the canonical Wnt/beta-catenin signaling pathway. This leads to decreased osteoblast proliferation and bone formation.

Question 59

In total joint arthroplasty, wear particles generated by the physical scratching of the polyethylene bearing surface by a harder metal or ceramic surface is best described as:

Explanation

Abrasive wear occurs when a harder surface roughens or scratches a softer surface. Adhesive wear occurs when micro-welds form between surfaces and shear off, while third-body wear involves loose particles caught between articulating surfaces.

Question 60

Microfracture of a full-thickness articular cartilage defect relies on marrow stimulation to form a repair tissue. Which of the following best characterizes the predominant collagen type in this repair tissue?

Explanation

Microfracture stimulates the formation of fibrocartilage, which is predominantly composed of Type I collagen. Unlike native hyaline cartilage (Type II collagen), fibrocartilage has inferior biomechanical properties and is less durable over time.

Question 61

Denosumab, used in the treatment of osteoporosis, acts by mimicking the physiologic action of which endogenous molecule?

Explanation

Denosumab is a monoclonal antibody that targets and binds to RANKL, preventing it from activating RANK on osteoclasts. This mechanism of action directly mimics the role of endogenous osteoprotegerin (OPG).

Question 62

The endurance limit of an orthopedic implant material is defined as the:

Explanation

The endurance limit (or fatigue limit) is the highest alternating stress that a material can withstand for an infinite number of cycles without failing. It is a critical parameter for materials subjected to repetitive loading, such as joint replacements.

Question 63

Articular chondrocytes primarily rely on which of the following metabolic pathways for energy production?

Explanation

Articular cartilage is avascular and operates in a hypoxic environment. Consequently, chondrocytes rely predominantly on anaerobic glycolysis for ATP production, rather than oxygen-dependent pathways like oxidative phosphorylation.

Question 64

Nitrogen-containing bisphosphonates (e.g., alendronate) inhibit osteoclast function primarily by interfering with which intracellular pathway?

Explanation

Nitrogen-containing bisphosphonates inhibit farnesyl pyrophosphate (FPP) synthase within the mevalonate pathway. This prevents the prenylation of small GTPases (like Ras and Rho), leading to osteoclast apoptosis and decreased bone resorption.

Question 65

Cortical bone exhibits different mechanical properties depending on the direction of the applied load. This material property is termed:

Explanation

Anisotropy refers to a material having directionally dependent mechanical properties. Cortical bone is highly anisotropic, meaning it is stronger and stiffer when loaded longitudinally (along the osteons) compared to transverse loading.

Question 66

In which zone of articular cartilage is the water content the highest and the collagen fibers oriented parallel to the joint surface?

Explanation

The superficial zone contains the highest concentration of water and collagen (Type II). Its collagen fibers are oriented parallel to the joint surface to resist shear forces.

Question 67

A viscoelastic material subjected to a constant strain will demonstrate a gradual decrease in stress over time. This phenomenon is known as:

Explanation

Stress relaxation is the property of viscoelastic materials where the internal stress decreases over time under a constant strain. In contrast, creep is the progressive deformation over time under a constant stress.

Question 68

Osteoprotegerin (OPG) regulates bone resorption by:

Explanation

OPG is a decoy receptor produced by osteoblasts that binds to RANKL. This prevents RANKL from binding to RANK on osteoclast precursors, thereby inhibiting osteoclast differentiation and bone resorption.

Question 69

Which of the following gene mutations is most commonly associated with the pathogenesis of malignant infantile osteopetrosis?

Explanation

Mutations in TCIRG1 (encoding a vacuolar proton pump) are responsible for more than 50% of cases of malignant infantile osteopetrosis. This mutation leads to defective osteoclast acidification and failure of bone resorption.

Question 70

The primary function of aggrecan in articular cartilage is to:

Explanation

Aggrecan is a large proteoglycan containing negatively charged glycosaminoglycans that attract water. This creates a swelling pressure within the collagen network, providing compressive stiffness to articular cartilage.

Question 71

The slope of the linear portion of the stress-strain curve represents a material's:

Explanation

The modulus of elasticity (Young's modulus) is defined by the slope of the linear (elastic) portion of the stress-strain curve. It represents the intrinsic stiffness of the material.

Question 72

Vitamin C deficiency impairs bone formation by disrupting which of the following cellular processes?

Explanation

Vitamin C is an essential cofactor for prolyl and lysyl hydroxylases. Its deficiency impairs the hydroxylation of proline and lysine residues, weakening the collagen triple helix and leading to scurvy.

Question 73

The maximum stress a material can withstand for an infinite number of loading cycles without failure is known as the:

Explanation

The endurance limit (or fatigue limit) is the maximum stress amplitude below which a material can undergo an infinite number of loading cycles without experiencing fatigue failure.

Question 74

Intermittent administration of parathyroid hormone (PTH) analogues (e.g., teriparatide) primarily leads to:

Explanation

Intermittent PTH administration stimulates osteoblastic bone formation by downregulating sclerostin, which enhances Wnt/beta-catenin signaling. In contrast, continuous PTH elevation leads to net bone resorption.

Question 75

Which of the following molecules is primarily responsible for boundary lubrication in articular joints?

Explanation

Lubricin (PRG4) is a highly glycosylated protein secreted by superficial zone chondrocytes and synoviocytes. It provides boundary lubrication, significantly reducing friction at the articular cartilage surface.

Question 76

The primary cellular defect in Paget disease of bone resides in the:

Explanation

Paget disease is primarily characterized by hyperactive, multinucleated osteoclasts that cause excessive and disorganized bone resorption. This is followed by a compensatory but structurally abnormal increase in osteoblastic bone formation.

Question 77

Mixing stainless steel and titanium implants in a highly saline environment (e.g., the human body) increases the risk of which type of corrosion?

Explanation

Galvanic corrosion occurs when two dissimilar metals with different electrochemical potentials are placed in contact within a conductive fluid environment. This leads to accelerated corrosion of the less noble metal.

Question 78

The earliest biochemical change observed in articular cartilage during the development of osteoarthritis is:

Explanation

The earliest biochemical change in osteoarthritic cartilage is an increase in water content. This occurs due to the breakdown of the superficial collagen network, allowing hydrophilic proteoglycans to swell excessively.

Question 79

Nutritional rickets is characterized by an initial defect in which of the following processes?

Explanation

Rickets (and osteomalacia in adults) is typically caused by a deficiency in Vitamin D or calcium. This results in a failure to mineralize the osteoid matrix and, in growing children, the cartilaginous growth plate (physis).

Question 80

Which of the following design modifications will most effectively increase the pullout strength of a bone screw?

Explanation

Screw pullout strength is directly proportional to the outer diameter and the length of engagement, and inversely proportional to thread pitch. Therefore, decreasing the pitch (more threads per unit length) increases pullout strength.

Question 81

A patient presents with generalized bone pain and proximal muscle weakness. Laboratory tests reveal low serum calcium, low phosphorus, elevated alkaline phosphatase, and elevated parathyroid hormone. What is the most likely diagnosis?

Explanation

These laboratory findings represent the classic profile of osteomalacia, often due to severe Vitamin D deficiency. It triggers secondary hyperparathyroidism, leading to low/normal calcium, low phosphorus, and elevated alkaline phosphatase.

Question 82

Articular cartilage has a remarkably limited intrinsic capacity for healing following injury primarily because it:

Explanation

Articular cartilage is unique in that it is avascular, aneural, and alymphatic. The lack of a local blood supply severely restricts its ability to mount a typical inflammatory response and recruit progenitor cells for repair.

Question 83

According to Perren's strain theory, secondary fracture healing with abundant callus formation is optimized when the mechanical strain at the fracture gap is maintained between:

Explanation

Perren's strain theory posits that secondary bone healing (callus formation) requires a moderate interfragmentary strain environment, typically between 2% and 10%. Strain below 2% promotes primary healing, while strain above 10% disrupts tissue formation, potentially leading to nonunion.

Question 84

Denosumab is an effective pharmacologic treatment for severe osteoporosis because it functions as a monoclonal antibody that directly targets and inhibits:

Explanation

Denosumab is a human monoclonal antibody that specifically binds to RANKL, preventing it from activating RANK on the surface of osteoclasts. This mechanism mimics the endogenous action of osteoprotegerin (OPG), robustly inhibiting bone resorption.

Question 85

The total area under the load-deformation (or stress-strain) curve of a bone specimen up to the point of structural failure represents its:

Explanation

The area under the entire load-deformation curve represents the material's toughness. It quantifies the total amount of energy the bone can absorb before fracturing.

Question 86

Parathyroid hormone (PTH) regulates bone remodeling. Continuous administration of PTH leads to net bone resorption. Through which direct cellular mechanism does PTH stimulate osteoclastogenesis?

Explanation

PTH binds to receptors on osteoblasts, stimulating them to express RANKL and M-CSF while inhibiting OPG. RANKL then binds to RANK on osteoclast precursors, promoting their differentiation and activation into mature osteoclasts. Osteoclasts themselves do not possess PTH receptors.

Question 87

In adult articular cartilage, which zone is characterized by the highest concentration of water, the lowest concentration of proteoglycans, and collagen fibers oriented parallel to the joint surface?

Explanation

The superficial zone of articular cartilage contains the highest water content and lowest proteoglycan concentration. Its collagen fibers are densely packed and aligned parallel to the articular surface to resist shear forces.

Question 88

When utilizing an intramedullary nail for fracture fixation, the torsional rigidity of a solid cylindrical nail is proportional to its radius raised to which power?

Explanation

The torsional rigidity of a solid cylinder is proportional to its polar moment of inertia, which scales with the radius to the fourth power (r^4). Therefore, even small increases in nail diameter significantly increase its resistance to torsional loads.

Question 89

Which of the following represents the earliest biochemical and structural change observed in articular cartilage during the pathogenesis of osteoarthritis?

Explanation

The earliest change in osteoarthritis is the damage and cleavage of the collagen network. This breakdown allows the hydrophilic proteoglycans to swell, resulting in a paradoxical increased water content before overall proteoglycan depletion occurs.

Question 90

A 4-year-old child presents with bleeding gums, petechiae, and subperiosteal hemorrhages. Radiographs show a "white line of Frankel." This condition is caused by a nutritional deficiency that impairs which specific step of collagen synthesis?

Explanation

Scurvy is caused by Vitamin C deficiency, an essential cofactor for prolyl and lysyl hydroxylase enzymes. Impaired hydroxylation of proline and lysine residues leads to defective collagen triple-helix formation and consequently weak connective tissue.

Question 91

A surgeon considers using a 316L stainless steel screw to secure a titanium alloy plate. This practice is strongly contraindicated because mixing these distinct metals in an electrolytic biological fluid leads to which type of corrosion?

Explanation

Galvanic corrosion occurs when two dissimilar metals with different anodic indices are placed in contact within a conductive fluid (like physiological body fluids). The less noble metal acts as an anode and undergoes accelerated dissolution.

Question 92

In the extracellular matrix of articular cartilage, aggrecan monomers assemble into massive proteoglycan aggregates by non-covalently binding to which of the following molecules?

Explanation

Aggrecan monomers form large aggregates by binding non-covalently to a central backbone of hyaluronic acid. This critical interaction is highly stabilized by the presence of link proteins.

Question 93

A patient with frequent fractures and a dense "rugger jersey" spine on radiographs is diagnosed with osteopetrosis. Genetic testing reveals a defect in carbonic anhydrase II. This primary defect impairs the function of which cell by what mechanism?

Explanation

Osteopetrosis is characterized by defective osteoclast-mediated bone resorption. Carbonic anhydrase II is required to generate the protons that osteoclasts secrete via vacuolar H+-ATPases to acidify the resorption pit (Howship's lacuna) and dissolve bone mineral.

Question 94

A ligament that is subjected to a constant, sustained load over a prolonged period will progressively elongate. This specific viscoelastic behavior is defined as:

Explanation

Creep is the progressive deformation (elongation) of a viscoelastic material when subjected to a constant load over time. In contrast, stress relaxation refers to the decrease in stress over time when the material is held at a constant length.

Question 95

During normal joint function under high-loading and high-velocity conditions, articular cartilage lubrication is primarily achieved by fluid being dragged into the joint space and deforming the cartilage surfaces. This mechanism is known as:

Explanation

Elastohydrodynamic lubrication is the dominant fluid-film mechanism under high loads in diarthrodial joints. The fluid pressure deforms the resilient articular cartilage, increasing the surface area and maintaining a thick layer of pressurized fluid between the opposing asperities.

Question 96

A 65-year-old male presents with increasing hat size, bowing of tibiae, and hearing loss.

Laboratory values demonstrate normal calcium, normal phosphate, and markedly elevated alkaline phosphatase. Which of the following is the primary cellular abnormality in this disease?

Explanation

The clinical picture is classic for Paget's disease of bone. The primary pathologic event is excessive and haphazard bone resorption driven by hyperactive, highly multinucleated osteoclasts, which is followed by a disorganized osteoblastic response (woven bone).

Question 97

On a standard stress-strain curve for human cortical bone tested in tension, the transition point where the material stops deforming elastically and begins to deform plastically is termed the:

Explanation

The yield point marks the end of the elastic region (where deformation is reversible) and the beginning of the plastic region (where permanent structural damage and deformation occur). The ultimate tensile strength is the maximum stress the material can withstand.

Question 98

Mature articular chondrocytes reside in an avascular environment and must generate cellular energy under characteristically hypoxic conditions. They derive the vast majority of their ATP through which of the following metabolic pathways?

Explanation

Because articular cartilage is totally avascular, mature chondrocytes live in a profoundly hypoxic environment. They rely almost entirely on anaerobic glycolysis for ATP production, which results in a high baseline production of lactic acid.

Question 99

A 45-year-old female with a history of a Roux-en-Y gastric bypass presents with diffuse bone pain and proximal muscle weakness. Laboratory evaluation reveals low serum calcium, low phosphorus, elevated alkaline phosphatase, and elevated parathyroid hormone. What is the most likely diagnosis?

Explanation

The patient's malabsorptive history and laboratory profile indicate severe Vitamin D deficiency leading to osteomalacia. Impaired mineralization of osteoid presents with low calcium and phosphorus, which triggers compensatory secondary hyperparathyroidism.

Question 100

When selecting a biomaterial for a total hip arthroplasty femoral stem to minimize stress shielding, the surgeon should consider the elastic modulus. Which of the following orthopedic biomaterials has an elastic modulus most closely approximating that of human cortical bone?

Explanation

Titanium alloy has an elastic modulus (approximately 110 GPa) that is significantly lower than stainless steel (approx. 200 GPa) or cobalt-chromium (approx. 210-240 GPa). This closer match to human cortical bone (15-20 GPa) helps reduce stress shielding.

AAOS & ABOS Basic Science MCQs (Set 3): Bone Biology, Biomechanics & Anatomy Review

AAOS Basic Science Orthopedic MCQs (Set 3): Bone Metabolism, Biomechanics & Cartilage Biology | Board Review

Key Takeaway

AAOS Basic Science Orthopedic MCQs (Set 3): Bone Metabolism, Biomechanics & Cartilage Biology | Board Review

Chapter Index