AAOS Basic Science MCQs Set 3: Bone & Biomechanics

Question 1

What is the most common location of osteosarcoma?

Basic Science 2005 Practice Questions: Set 3 (Solved) - Figure 1

Explanation

The most common location of osteosarcoma is the knee area (50% to 55%), followed by the proximal humerus and iliac wing. The most commonly involved long bone is the femur (40% to 45%), followed by the tibia (15% to 25%). Within these bones, tumors are typically adjacent to the epiphyses in most patients. The flat bones of the pelvis and spine are less frequently involved. Malawer MM, Sugarbaker PH, Malawer M: Musculoskeletal Cancer Surgery: Treatment of Sarcomas and Allied Diseases. Kluwer Academic Publishers, 2001.

Question 2

A 40-year-old man with an acetabular chondrosarcoma has a small soft-tissue mass. Treatment should consist of

Basic Science 2005 Practice Questions: Set 3 (Solved) - Figure 2

Explanation

The treatment of choice for pelvic chondrosarcoma is wide resection via an internal hemipelvectomy. Chondrosarcoma requires surgical resection for control and does not respond to traditional chemotherapy or external beam radiation. Hip arthroplasty with acetabular reconstruction and curettage and cementation of the lesion are intralesional procedures that result in a higher incidence of local recurrence of tumor. Pring M, Weber KL, Unni K, Sim FH: Chondrosarcoma of the pelvis: A review of sixty-four cases. J Bone Joint Surg Am 2001;83:1630-1642.

Question 3

Figures 29a and 29b show the AP radiograph and CT scan of a 70-year-old man who has left thigh pain. Serum protein electrophoresis shows a monoclonal gammopathy. Additional radiographs of the femur show other lesions. Management should consist of

Explanation

The underlying diagnosis is multiple myeloma. Because the patient has a large lucent lesion in the peritrochanteric region of the left proximal femur, the risk of pathologic fracture is high. Consideration should be given to prophylactic internal fixation with a locked intramedullary rod. The lesion does not appear to be a sarcoma requiring wide resection and endoprosthetic reconstruction. Neither chemotherapy nor radiation therapy alone is likely to result in long-term stabilization of the proximal femur. Postoperative treatment with bisphosphonates and radiation therapy is indicated to decrease the risk of future pathologic fractures. The patient should also be referred to a medical oncologist for medical management. Menendez LR (ed): Orthopaedic Knowledge Update: Musculoskeletal Tumors. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2002, p 364.

Question 4

What pharmacologic agents are preferred for the treatment of symptomatic active Paget's disease?

Basic Science 2005 Practice Questions: Set 3 (Solved) - Figure 5

Explanation

Recent medical literature supports the use of bisphosphonates as the treatment of choice for active Paget's disease.

Question 5

A 7-year-old girl has pain and a mass in the left scapula. A MRI scan and biopsy specimen are shown in Figures 30a and 30b. After staging studies, initial management should consist of

Explanation

The histology shows small round blue cells that are uniform in appearance; these findings are consistent with Ewing's sarcoma. The MRI scan shows infiltration of the marrow and a large surrounding soft-tissue mass. Based on these findings, the management of choice is systemic chemotherapy. Local control of the primary lesion is addressed by either surgical resection or radiation therapy or a combination of the two after the patient receives systemic chemotherapy. The clinical, radiographic, and histologic presentation of Ewing's sarcoma often can be confused with osteomyelitis. The histology shows an absence of inflammatory cells. Grier HE: The Ewing family of tumors: Ewing's sarcoma and primitive neuroectodermal tumors. Pediatr Clin North Am 1997;44:991-1004.

Question 6

A 73-year-old man reports increasing back and lower extremity pain. A bone scan is shown in Figure 31. What is the most likely diagnosis?

Basic Science 2005 Practice Questions: Set 3 (Solved) - Figure 8

Explanation

The bone scan reveals lesions throughout the skeleton. The patient's age, gender, and pain pattern are consistent with metastatic prostate cancer. Multiple myeloma typically does not have enough osteoblastic activity to produce this bone scan. The patient's age is not consistent with metastatic neuroblastoma (a pediatric disease). Polyostotic fibrous dysplasia may involve multiple active lesions in younger patients but does not have such a widespread distribution of lesions. Hodgkin's lymphoma can involve bone, but the widespread discrete appearance on this bone scan is most consistent with metastatic prostate cancer. In a patient with widespread bone metastases from prostate cancer, bisphosphonates may play a critical role in treatment by decreasing pain and the number of fractures. Roudier MP, Vesselle H, True LD, Higano CS, Ott SM, King SH, Vessella RL: Bone histology at autopsy and matched bone scintigraphy findings in patients with hormone refractory prostate cancer: The effect of bisphosphonate therapy on bone scintigraphy results. Clin Exp Metastasis 2003;20:171-180.

Question 7

A 16-year-old girl has had pain in the left groin for the past 4 months. She notes that the pain is worse at night; however, she denies any history of trauma and has no constitutional symptoms. There is no history of steroid or alcohol use. Examination reveals pain in the left groin with rotation of the hip. There is no associated soft-tissue mass. A radiograph and MRI scan are shown in Figures 32a and 32b, and biopsy specimens are shown in Figures 32c and 32d. What is the most likely diagnosis?

Explanation

Based on the epiphyseal location and sharp, well-defined borders, the radiograph suggests chondroblastoma. Histologically, multinucleated giant cells are scattered among mononuclear cells. The nuclei are homogenous and contain a characteristic longitudinal groove. Although not seen here, "chicken-wire calcification" with a bland giant cell-rich matrix is also typical for chondroblastoma. Clear cell chondrosarcoma occurs in epiphyseal locations but has a more aggressive histologic pattern and occurs in an older age group. Giant cell tumors occur in the epiphysis but have a more uniform giant cell population histologically. Aneurysmal bone cyst often results in bone remodeling and has a different pathologic appearance. Osteonecrosis has a typical histologic pattern of empty lacunae and necrotic bone. Springfield DS, Capanna R, Gherlinzoni F, et al: Chondroblastoma: A review of seventy cases. J Bone Joint Surg Am 1985;67:748-755. Simon M, Springfield D, et al: Chrondroblastoma: Surgery for Bone and Soft Tissue Tumors. Philadelphia, PA, Lippincott Raven, 1998, p 190.

Question 8

Ewing's sarcoma of bone most commonly occurs in which of the following locations?

Explanation

Ewing's sarcoma typically occurs in the major long tubular bones, with the femur the most common location. The flat bones of the pelvis are the second most common location. Ewing's sarcoma occurs in the fibula but with a lower incidence than that seen in the major tubular bones. Ewing's sarcoma infrequently occurs in the metacarpals or the vertebral bodies. Simon M, Springfield D, et al: Ewing's Sarcoma: Surgery for Bone and Soft Tissue Tumors. Philadelphia, PA, Lippincott Raven, 1998, p 287.

Question 9

A previously healthy 14-year-old boy now reports fatigue, and has a bilateral Trendelenburg gait, right hip pain, and bilateral knee and foot pain. Biopsy of a right sacral mass reveals intermediate grade osteosarcoma. There are no metastases. Laboratory studies reveal a serum calcium level of 7.7 mg/dL (normal 8.5 to 10.5), a phosphate level of 2.0 mg/dL (normal 2.7 to 4.5), a 1,25-dihydroxyvitamin D level of less than 10 pg/mL (normal 18 to 62), a parathyroid hormone level of 19 pg/mL (normal 10 to 60), and an alkaline phosphatase level of 428 U/L (normal 15 to 351). What is the most likely cause of the patient's symptoms?

Explanation

The laboratory findings are typical for rickets. Oncogenic rickets is a paraneoplastic syndrome that results from a substance secreted by the tumor that interferes with renal tubule reabsorption of phosphate. This substance previously had been called phosphatonin but recently has been identified as fibroblast growth factor 23. Nutritional rickets is rare in developed countries. Delayed onset familial hypophosphatemic rickets is possible, but the likelihood of having two rare diseases is unlikely. Osteosarcoma does not sequester calcium. Alkaline phosphatase levels can be elevated in osteosarcoma but does not cause muscle weakness. Tumor cachexia would occur only with advanced metastatic disease. A unilateral sacral mass would not cause a bilateral L5 neuropathy or the abnormal laboratory findings. Case records of the Massachusetts General Hospital. Weekly clinicopathological exercises. Case 29-2001. A 14-year-old with abnormal bones and a sacral mass. N Engl J Med 2001;345:903-908.

Question 10

Which of the following staging studies should be obtained for an adult with an 8-cm deep, high-grade malignant fibrous histiocytoma of the extremity?

Explanation

MRI is the preferred imaging study to evaluate the local tumor extension for soft-tissue lesions, but CT can be used if MRI is contraindicated (eg, patients with pacemakers). CT of the chest is always recommended in patients with high-grade sarcomas because 80% of metastases occur in the lungs. CT of the abdomen and pelvis is indicated in patients with lower extremity liposarcoma because some patients also have synchronous retroperitoneal liposarcoma. Lymph node metastasis occurs in up to 5% of patients with soft-tissue sarcoma. If the nodes are clinically enlarged, biopsy is indicated. Routine sentinel node biopsy currently is not recommended. Bone scan is not used in the staging of soft-tissue sarcoma as it has not been shown to be cost-effective. Demetri GD, Pollock R, Baker L, Balcerzak S, Casper E, Conrad C, et al: NCCN sarcoma practice guidelines: National Comprehensive Cancer Network. Oncology (Huntingt) 1998;12:183-218.

Question 11

An 18-year-old boy has had pain in the right knee for the past 6 months. Examination reveals some fullness behind the knee but no significant palpable soft-tissue mass. There is no effusion, and he has full knee range of motion. The remainder of the examination is unremarkable. A radiograph and MRI scans are shown in Figures 33a through 33c, and biopsy specimens are shown in Figures 33d and 33e. What is the most likely diagnosis?

Explanation

The patient has parosteal osteosarcoma. The posterior aspect of the distal femur is the typical location for this variant of osteogenic sarcoma. The imaging studies indicate a surface lesion with no involvement of the adjacent intramedullary canal. The histologic appearance is that of a low-grade fibroblastic osteosarcoma, consisting of relatively mature bone and a bland fibroblastic stroma lacking cytologic atypia and mitotic activity. A cartilaginous component is also frequently seen. Classic osteosarcoma typically has a more aggressive radiologic and histologic appearance. Sessile osteochondromas, while common behind the knee, have a presence of hematopoietic marrow and fat. The cartilage found in the associated cartilaginous cap is oriented. Chondrosarcomas are more typical in an older age group and have a histologic pattern consisting of malignant chondroid. Wold LA, et al: Atlas of Orthopaedic Pathology. Philadelphia, PA, WB Saunders, 1990, pp 20-21.

Question 12

A 30-year-old patient has wrist pain. A radiograph and biopsy specimen are shown in Figures 34a and 34b. What is the most likely diagnosis?

Explanation

Aneurysmal bone cysts typically present as radiolucent lesions with an expansile remodeled cortex. The histologic appearance consists of blood-filled lakes surrounded by a benign lining that contains fibroblasts, giant cells, and hemosiderin. Although the other lesions are in the radiographic differential diagnosis, these histologic findings indicate an aneurysmal bone cyst. Bieselker JL, Marcove RC, Huvos AG, Mike V: Aneurysmal bone cyst: A Clinico-pathologic study of 66 cases. Cancer 1973;26:615.

Question 13

Mutations of what gene are associated with subsequent development of osteosarcoma?

Explanation

The mutation of the retinoblastoma gene has been associated with an increased prevalence of osteosarcoma. The mutation resulting in EWS-FLI1 is associated with Ewing's sarcoma. The other mutations are associated with tumor formation and proliferation but not necessarily with osteosarcoma formation. Menendez LR (ed): Orthopaedic Knowledge Update: Musculoskeletal Tumors. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2002, p 4. Scholz R, Kabisch H, Delling G, Winkler K: Homozygous deletion within the retinoblastoma gene in a native osteosarcoma specimen of a patient cured of a retinoblastoma of both eyes. Pediatr Hematol Oncol 1990;72:65.

Question 14

A 12-year-old girl has painless bowing of the tibia. Radiographs and a biopsy specimen are shown in Figures 35a through 35c. What is the most likely diagnosis?

Explanation

The patient has osteofibrous dysplasia. The radiographic differential diagnosis includes osteofibrous dysplasia, fibrous dysplasia, and adamantinoma. Histology shows a fibro-osseous lesion with prominent osteoblastic rimming but a lack of epithelial nests. Adamantinoma is a low-grade malignancy that typically is located in the anterior tibial cortex and has a soap bubble appearance. Histologically, it is similar to osteofibrous dysplasia but includes epithelial nests of cells. Treatment requires resection. Fibrous dysplasia usually does not require biopsy; however, in this patient the radiographs do not distinguish it from adamantinoma. The radiographic findings are not typical of Ewing's sarcoma or osteosarcoma. Repeat biopsy should be considered if clinical or radiographic features change.

Question 15

A 54-year-old man with metastatic renal cell carcinoma has had increasing pain in the left hip for the past 6 weeks. A radiograph is shown in Figure 36. Prophylactic stabilization will most likely result in

Basic Science 2005 Practice Questions: Set 3 (Solved) - Figure 23

Explanation

Prophylactic stabilization of impending fractures does not directly affect the overall survival rate, but it does improve factors related to intraoperative and postoperative complications and decreased recovery time. Mirels H: Metastatic disease in long bones: A proposed scoring system for diagnosing impending pathologic fractures. Clin Orthop 1989;249:256-264.

Question 16

Which of the following is considered the treatment of choice for a 3-cm chondroblastoma of the distal femoral epiphysis with no intra-articular extension?

Explanation

Curettage and bone grafting typically are the preferred treatment of chondroblastoma, yielding acceptable local recurrence rates of less than 10%. Some surgeons advocate adjuvant therapies such as phenol, liquid nitrogen, or argon beam coagulation. Untreated, these lesions can destroy bone and invade the joint to a significant degree. Large intra-articular lesions may require major joint reconstruction. Wide local excision is rarely required to control the tumor. Radiation therapy is indicated only in unresectable lesions. Springfield DS, Capanna R, Gherlinzoni F, et al: Chondroblastoma: A review of seventy cases. J Bone Joint Surg Am 1985;67:748.

Question 17

A radiograph, MRI scans, and a biopsy specimen of a 9-year-old boy with thigh pain are shown in Figures 37a through 37d. Management should consist of

Explanation

The patient has Ewing's sarcoma. Management options for local tumor control include radiation therapy, resection, or a combination; however, in this patient wide resection is preferred over radiation therapy. Radiation therapy is associated with damage to the growth plate, pathologic fracture, radiation-induced sarcomas, and a local recurrence rate of approximately 10% to 12%. Radiation therapy is used for positive margins, unresectable tumors, or for tumors that have a poor response to chemotherapy. Amputation is not necessary since the tumor is resectable. Chemotherapy has improved overall survival rates to over 60% of patients. Sailer SL: The role of radiation therapy in localized Ewing' sarcoma. Semin Radiat Oncol 1997;7:225-235. Shankar AG, Pinkerton CR, Atra A, Ashley S, Lewis I, Spooner D, et al: Local therapy and other factors influencing site of relapse in patients with localised Ewing's sarcoma. United Kingdom Children's Cancer Study Group (UKCCSG). Eur J Cancer 1999;35:1698-1704. Carrie C, Mascard E, Gomez F, Habrand JL, Alapetite C, Oberlin O, et al: Nonmetastatic pelvic Ewing sarcoma: Report of the French society of pediatric oncology. Med Pediatr Oncol 1999;33:444-449.

Question 18

A 47-year-old woman has had a 1-month history of left hip and medial thigh pain that is exacerbated by sitting. Laboratory studies show a total protein level of 8.2 g/dL (normal 6.0 to 8.0) and an immunoglobulin G (IGG) level of 2,130 mg/dL (normal 562 to 1,835). A radiograph, CT scan, and biopsy specimen are shown in Figures 38a through 38c. What is the most likely diagnosis?

Explanation

The laboratory studies and histology are both consistent with myeloma. Infection should show white blood cells other than plasma cells on histology. Lymphoma would show lymphocytes, not plasma cells. The lack of bone formation on the imaging studies and the lack of osteoid on histology rule out osteosarcoma. The cells have too much cytoplasm and nuclear chromatin to be Ewing's sarcoma cells.

Question 19

A 14-year-old boy has an anteromedial distal thigh mass. A radiograph and MRI scan are shown in Figures 39a and 39b. An open biopsy of the mass should include

Explanation

Biopsy of the soft-tissue component is often diagnostic. Alternatively, in centers with pathologists familiar with bone tumors, needle biopsy is usually successful. The principles of biopsy of bone tumors include avoiding contamination of uninvolved structures and compartments, taking the most direct path to the tumors, making an excisable biopsy tract, and obtaining diagnostic tissue. Transverse biopsy incisions should be avoided because they hinder the definitive surgical procedure. Peabody TD, Simon MA: Making the diagnosis: Keys to a successful biopsy in children with bone and soft-tissue tumors. Orthop Clin North Am 1996;27:453-459. Mankin HJ, Mankin CJ, Simon MA: The hazards of the biopsy, revisited. Members of the Musculoskeletal Tumor Society. J Bone Joint Surg Am 1996;78:656-663.

Question 20

A 60-year-old man has pain at the tip of the index finger. A radiograph and biopsy specimen are shown in Figures 40a and 40b. Management should consist of

Explanation

The radiograph and histology findings are most consistent with squamous cell carcinoma. This tumor is best treated with wide surgical resection margins alone in the absence of metastasis; in this patient, management should consist of amputation through the distal interphalangeal joint. The other treatments are not indicated. Soltani K, Krunic A: Non melanoma skin neoplasms, in Vokes E, Golomb H (eds): Oncologic Therapies, ed 2. Berlin, Germany, Springer, pp 646-647.

Question 21

An infant is born with a mass that involves both the volar and dorsal compartments of the left arm. A clinical photograph and biopsy specimen are shown in Figures 41a and 41b. What is the best initial course of action?

Explanation

The patient has infantile fibrosarcoma. For unresectable lesions, the treatment of choice is chemotherapy with vincristine, actinomycin-D, and cyclophosphamide, followed by excision if there is an adequate decrease in the size of the lesion.

Question 22

Which of the following processes does not account for decreased hematopoiesis in patients with metastatic disease?

Explanation

Paucytopenia is a common problem in patients with metastatic disease. Causes include chemotherapy, external beam radiation, marrow replacement by tumor, and anemia of chronic disease. There is no correlation with decreased calcium and a decrease in hematopoiesis. Supportive care with granulocyte-colony stimulating factor (G-CSF) and neupogen can stimulate hematopoiesis.

Question 23

A 43-year-old woman has an enlarging mass in the left groin. A radiograph, CT scan, and a biopsy specimen are shown in Figures 42a through 42c. Treatment should consist of

Explanation

The patient has a pelvic chondrosarcoma. The radiograph shows a lytic bone lesion emanating from the left inferior pubic ramus and extending into the soft tissues. Punctate calcifications are revealed on the radiograph and CT scan. The histology is consistent with a malignant cartilage lesion. Appropriate treatment for a pelvic chondrosarcoma is wide resection. In this location, wide resection of the ischiopelvic region (type 3 internal hemipelvectomy) is the treatment of choice. A type 2 internal hemipelvectomy involves resection of the periacetabular region. A type 1 internal hemipelvectomy involves resection of the ilium. No reconstruction is required for a type 3 resection. A classic hemipelvectomy is not necessary because the tumor can be removed with an adequate margin while maintaining the neurovascular structures and hip joint. Pring M, Weber KL, Unni K, Sim FH: Chondrosarcoma of the pelvis: A review of sixty-four cases. J Bone Joint Surg Am 2001;83:1630-1642. Sheth DS, Yasko AW, Johnson ME, Ayala AG, Murray JA, Romsdahl MM: Chondrosarcoma of the pelvis: Prognostic factors for 67 patients treated with definitive surgery. Cancer 1996;78:745-750.

Question 24

A 66-year-old man has a high-grade angiosarcoma of the right tibia. A radiograph is shown in Figure 43. Treatment should consist of

Basic Science 2005 Practice Questions: Set 3 (Solved) - Figure 1

Explanation

Angiosarcoma is a locally aggressive sarcoma. The radiograph shows extensive multiple discontinuous lesions throughout the entire tibia. The extent of bone involvement precludes resection; therefore, the treatment of choice is amputation, either above the knee or through the knee. Radiation therapy is not needed after amputation, and chemotherapy remains investigational for soft-tissue sarcoma.

Question 25

Figures 44a and 44b show the radiographs of a 28-year-old woman who has had progressive hip pain for the past 3 months. What is the most likely diagnosis?

Explanation

The patient has multiple hereditary exostoses and a secondary chondrosarcoma arising from a proximal femoral exostosis. The radiograph of the knee shows multiple osteochondromas typical in a patient with multiple hereditary exostoses. Patients with this diagnosis are at an increased risk for malignant degeneration of an osteochondroma. The lateral radiograph of the hip shows a bony lesion emanating from the anterior aspect of the femoral neck that is not well defined in the surrounding soft tissues. There are punctate calcifications and a large soft-tissue mass. The most likely diagnosis is a secondary chondrosarcoma developing from a benign osteochondroma. An enchondroma is an intramedullary benign cartilage lesion. Ollier's disease and Maffucci's syndrome involve multiple enchondromas. Scarborough M, Moreau G: Benign cartilage tumors. Orthop Clin North Am 1996;27:583-589.

Question 26

A viscoelastic material, such as articular cartilage, is subjected to a constant applied load over an extended period, resulting in a progressive increase in deformation. This biomechanical phenomenon is best described as:

Explanation

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

Question 27

In orthopedic biomechanics, for a solid cylindrical intramedullary nail, the bending stiffness is proportional to the radius raised to which power?

Explanation

The bending stiffness of a solid cylinder is directly proportional to its area moment of inertia. For a solid cylinder, the area moment of inertia is proportional to the radius to the fourth power (r^4).

Question 28

Which zone of normal articular cartilage has the highest concentration of water and contains collagen fibers oriented primarily parallel to the joint surface to resist shear forces?

Explanation

The superficial zone of articular cartilage contains the highest water content, the lowest proteoglycan concentration, and type II collagen fibers aligned parallel to the surface to optimally resist shear stress.

Question 29

Denosumab is utilized in the management of osteoporosis and certain skeletal-related events in metastatic bone disease. It alters bone biology by directly binding to and inhibiting which of the following molecules?

Explanation

Denosumab is a monoclonal antibody that targets and neutralizes RANKL, preventing its interaction with the RANK receptor on osteoclast precursors. This inhibits osteoclast formation, function, and survival.

Question 30

Sclerostin, a glycoprotein produced primarily by mature osteocytes, serves as a negative regulator of bone formation. It achieves this by inhibiting which of the following intracellular signaling pathways?

Explanation

Sclerostin binds to LRP5/6 receptors on the surface of osteoblasts, effectively blocking the canonical Wnt/beta-catenin signaling pathway, which is essential for osteoblast differentiation and bone formation.

Question 31

Which type of fracture healing is primarily achieved when absolute stability and rigid internal fixation (e.g., a lag screw and neutralization plate) are applied to a simple fracture pattern?

Explanation

Rigid internal fixation with absolute stability prevents interfragmentary motion, facilitating primary bone healing. This process occurs via cutting cones (osteoclasts followed by osteoblasts) without the formation of an intermediate fracture callus.

Question 32

During the tensile biomechanical testing of a normal ligament, the initial "toe region" of the load-elongation curve represents which physiological event?

Explanation

The toe region of the stress-strain curve for ligaments and tendons corresponds to the uncrimping or straightening of the naturally wavy collagen fibers as tension is initially applied.

Question 33

What is the predominant large aggregating proteoglycan responsible for drawing water into articular cartilage and providing compressive stiffness?

Explanation

Aggrecan is the most abundant large proteoglycan in articular cartilage. Its highly negatively charged chondroitin and keratan sulfate glycosaminoglycan chains create an osmotic gradient that draws water into the matrix, providing compressive resistance.

Question 34

Stress shielding around a femoral stem can lead to proximal bone resorption. To minimize this, engineers seek materials with a modulus of elasticity closer to cortical bone. Which material has a modulus most similar to cortical bone?

Explanation

Titanium alloy has a modulus of elasticity (approx. 110 GPa) that is much closer to that of cortical bone (15-20 GPa) compared to cobalt-chromium (approx. 210 GPa) or stainless steel (approx. 200 GPa), thereby reducing stress shielding.

Question 35

In total hip arthroplasty, accelerated generation of polyethylene wear debris due to a fragment of bone cement becoming trapped between the femoral head and acetabular liner is an example of:

Explanation

Third-body wear occurs when hard particles (e.g., bone fragments, cement, or metal debris) become trapped between two articulating surfaces, leading to accelerated gouging and wear of the softer surface.

Question 36

Bone morphogenetic proteins (BMPs) exert their potent osteoinductive effects by binding to cell-surface receptors and primarily activating which of the following intracellular signaling molecules?

Explanation

BMPs bind to serine/threonine kinase receptors on the cell membrane, which subsequently phosphorylate and activate downstream intracellular Smad proteins (typically Smads 1, 5, and 8) to upregulate osteogenic gene expression.

Question 37

During internal fixation of a diaphyseal fracture, which design modification of a cortical bone screw will most effectively increase its pullout strength?

Explanation

Screw pullout strength is most significantly increased by increasing the outer (major) diameter. This dictates the volume of bone captured between the screw threads and is the single most important factor for pullout resistance.

Question 38

Galvanic corrosion of orthopedic implants is most likely to occur under which of the following specific conditions?

Explanation

Galvanic corrosion occurs when two electrochemically dissimilar metals (e.g., stainless steel and titanium) are in direct contact within an electrolytic environment, leading to accelerated corrosion of the less noble metal.

Question 39

While most of the appendicular skeleton develops via an intermediate cartilage model, which of the following bones develops primarily via intramembranous ossification?

Explanation

The clavicle, along with the flat bones of the skull and portions of the mandible, develops primarily via intramembranous ossification, where mesenchymal stem cells differentiate directly into osteoblasts without a cartilaginous template.

Question 40

During prolonged standing, which exposes a joint to high static loads at zero or very low speeds, which mechanism is primarily responsible for the lubrication of articular cartilage?

Explanation

Boundary lubrication is the primary mechanism protecting articular cartilage under high-load, low-speed conditions. It relies on a monolayer of molecules, such as lubricin and superficial zone protein, bound to the articular surface.

Question 41

Osteoclasts are specialized multinucleated giant cells responsible for the resorption of bone matrix. From which distinct cellular lineage do osteoclasts originate?

Explanation

Osteoclasts are derived from hematopoietic stem cells of the monocyte-macrophage lineage. In contrast, osteoblasts, chondrocytes, and adipocytes share a common origin from mesenchymal stem cells.

Question 42

The biomechanical function of the nucleus pulposus within the intervertebral disc is primarily designed to resist and distribute which type of applied force?

Explanation

The nucleus pulposus is a highly hydrated, gel-like center rich in Type II collagen and aggrecan, perfectly adapted to resist and uniformly distribute compressive axial loads. The surrounding annulus fibrosus handles tensile and torsional forces.

Question 43

Because mature articular cartilage is avascular and hypoxic, chondrocytes must rely predominantly on which metabolic pathway for their cellular energy (ATP) production?

Explanation

Due to the avascular nature of mature articular cartilage, chondrocytes reside in a hypoxic environment and rely almost entirely on anaerobic glycolysis for ATP synthesis, a process regulated by hypoxia-inducible factor 1-alpha (HIF-1 alpha).

Question 44

During the intracellular biosynthesis of collagen, vitamin C (ascorbic acid) is an absolute requirement as a cofactor for which specific enzymatic process?

Explanation

Vitamin C is an essential cofactor for the enzymes prolyl hydroxylase and lysyl hydroxylase. Defective hydroxylation of proline and lysine leads to structurally unstable collagen triple helices, resulting in scurvy.

Question 45

According to the principles of external fixation biomechanics, which of the following modifications will most significantly increase the bending stiffness of a half-pin?

Explanation

While bringing the bar closer to the bone increases construct stiffness, increasing the core diameter of the pin has the most profound effect on the individual pin's bending stiffness, as it is proportional to the radius to the fourth power (r^4).

Question 46

Which zone of articular cartilage is characterized by the highest water content, highest collagen concentration, and lowest proteoglycan concentration?

Explanation

The superficial zone of articular cartilage has the highest water and collagen content, but the lowest proteoglycan concentration. Its collagen fibers are oriented parallel to the joint surface to primarily resist shear forces.

Question 47

What process characterizes creeping substitution during the incorporation of a cortical bone graft?

Explanation

Creeping substitution in cortical bone grafts involves osteoclastic resorption of the dead graft bone followed closely by osteoblastic bone formation. This process relies on cutting cones and proceeds much slower than the incorporation of cancellous bone grafts.

Question 48

What is the primary cellular mechanism of action for nitrogen-containing bisphosphonates?

Explanation

Nitrogen-containing bisphosphonates inhibit farnesyl pyrophosphate synthase within the mevalonate pathway. This prevents the prenylation of small GTPases that are essential for normal osteoclast function and survival.

Question 49

On a load-deformation or stress-strain curve for a structural material, what biomechanical property is represented by the slope of the linear portion?

Explanation

Young's modulus (or modulus of elasticity) is the slope of the linear, elastic portion of the stress-strain curve. It represents the intrinsic stiffness of the material being tested.

Question 50

The time-dependent increase in strain of articular cartilage when subjected to a constant, prolonged compressive load is termed:

Explanation

Creep is defined as the progressive time-dependent deformation (increase in strain) of a viscoelastic material under a constant load. In contrast, stress relaxation is the decrease in stress over time when held at a constant strain.

Question 51

Which of the following combinations of orthopedic implants is most likely to result in significant galvanic corrosion if placed in direct physical contact in vivo?

Explanation

Galvanic corrosion occurs when two electrochemically dissimilar metals are in direct contact within an electrolytic environment. The combination of titanium and stainless steel is highly prone to severe galvanic corrosion and must be avoided.

Question 52

Following a marrow-stimulating procedure (e.g., microfracture) for a full-thickness chondral defect, the resulting repair tissue is predominantly composed of:

Explanation

Microfracture stimulates a marrow-based healing response that typically fills the defect with fibrocartilage rather than native hyaline cartilage. Fibrocartilage is structurally inferior and is predominantly composed of Type I collagen.

Question 53

An activating mutation in the FGFR3 gene primarily affects which zone of the physis, leading to the clinical presentation of achondroplasia?

Explanation

Achondroplasia is caused by a gain-of-function mutation in FGFR3, which pathologically inhibits chondrocyte proliferation. Therefore, the proliferative zone of the physis is primarily affected, leading to short-limbed dwarfism.

Question 54

Aggrecan, the primary proteoglycan found in articular cartilage, is responsible for providing which essential biomechanical property to the tissue?

Explanation

Aggrecan is highly negatively charged due to its glycosaminoglycan chains, which attract water via the Donnan osmotic effect. This swelling pressure, constrained by the type II collagen network, provides cartilage with its compressive stiffness.

Question 55

Which design modification to a cortical bone screw will most effectively increase its pull-out strength?

Explanation

The pull-out strength of a screw is increased by maximizing the volume of bone caught between the threads. This is achieved by increasing the outer diameter, decreasing the inner core diameter, increasing engagement length, and decreasing the thread pitch.

Question 56

The biological principle stating that bone will adapt its mass and architecture to the mechanical loads and stresses placed upon it is known as:

Explanation

Wolff's Law states that healthy bone remodels in response to the mechanical stresses it experiences, becoming thicker in areas of high stress. The Heuter-Volkmann principle applies specifically to physeal growth under compression or tension.

Question 57

Which type of fracture healing is characterized by the sequential formation of a soft cartilaginous callus followed by a hard bony callus?

Explanation

Secondary bone healing occurs via enchondral ossification, proceeding through stages of hematoma formation, soft (cartilaginous) callus, hard (woven bone) callus, and remodeling. Primary bone healing occurs without callus formation under conditions of absolute stability.

Question 58

Osteoclasts are multinucleated giant cells derived from which of the following cellular lineages?

Explanation

Osteoclasts are derived from the fusion of mononuclear precursors of the hematopoietic macrophage-monocyte lineage. In contrast, osteoblasts, chondrocytes, and fibroblasts are derived from mesenchymal stem cells.

Question 59

Compared to a total hip arthroplasty, the primary mode of ultra-high-molecular-weight polyethylene (UHMWPE) wear in a well-functioning total knee arthroplasty is:

Explanation

Polyethylene wear in total knee arthroplasty is primarily due to fatigue and delamination secondary to repetitive subsurface shear stresses from rolling and sliding. Total hip arthroplasty typically experiences adhesive and abrasive wear due to highly conforming surfaces.

Question 60

Which mechanism of joint lubrication operates primarily under conditions of low speeds and high loads, relying on surface-active molecules like lubricin?

Explanation

Boundary lubrication relies on a thin molecular layer of glycoproteins (like lubricin) attached to the articular surface to reduce friction, operating best under low speeds and high loads. Fluid-film mechanisms predominate at higher speeds.

Question 61

On a stress-strain curve of a normal human tendon, what does the initial non-linear "toe region" represent?

Explanation

The initial toe region occurs at low strains and represents the straightening or "uncrimping" of the naturally wavy collagen fibers in the resting tendon. Beyond this region, the curve becomes linear as the fibers themselves begin to stretch.

Question 62

The primary inorganic mineral constituent that provides mature human bone with its compressive strength is:

Explanation

The mineral phase of bone is primarily composed of carbonated, calcium-deficient hydroxyapatite. This inorganic crystalline matrix is embedded within the collagen network and provides bone with its high compressive strength.

Question 63

A patient with severe, prolonged vitamin D deficiency develops osteomalacia. Which of the following serum laboratory profiles is most characteristic of this condition?

Explanation

Severe vitamin D deficiency leads to decreased intestinal calcium and phosphate absorption, resulting in hypocalcemia. This triggers secondary hyperparathyroidism, which further increases renal phosphate excretion, culminating in low calcium, low phosphate, and high PTH.

Question 64

Platelet-Rich Plasma (PRP) exerts its biological and healing effects primarily through the degranulation of which intra-platelet structure?

Explanation

PRP relies on the degranulation of platelet alpha granules, which release a high concentration of anabolic growth factors (e.g., PDGF, TGF-beta, VEGF). These factors are thought to promote angiogenesis, cellular chemotaxis, and tissue healing.

Question 65

In an adult human long bone, the inner two-thirds of the diaphyseal cortex receives its primary blood supply from the:

Explanation

The nutrient artery system operates under high pressure to provide the primary blood supply to the medullary cavity and the inner two-thirds of the diaphyseal cortex. The periosteal vessels supply the outer one-third.

Question 66

During biomechanical testing of a new orthopedic implant material, a load is applied and the material undergoes deformation. The slope of the linear portion of the resulting stress-strain curve represents which of the following mechanical properties?

Explanation

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

Question 67

Articular cartilage is divided into distinct structural zones. 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 content. Its collagen fibers run parallel to the articular surface to resist shear forces.

Question 68

Osteoclasts bind to the bone matrix to initiate resorption by forming a sealing zone. Which specific integrin receptor on the osteoclast membrane is primarily responsible for binding to osteopontin and bone sialoprotein to create this sealing zone?

Explanation

The alpha-v beta-3 integrin is the primary cell surface receptor on osteoclasts that interacts with RGD-containing bone matrix proteins to form the sealing zone required for bone resorption.

Question 69

A 25-year-old athlete undergoes a knee arthroscopy, during which a full-thickness chondral defect is noted. The primary extracellular matrix structural protein in the healthy adjacent hyaline articular cartilage is:

Explanation

Type II collagen constitutes 90% to 95% of the collagen in healthy articular cartilage and provides its tensile strength.

Question 70

Bone remodeling is strictly regulated by intercellular communication between osteoblasts and osteoclasts. Which of the following molecules acts as a decoy receptor for RANKL, thereby inhibiting osteoclastogenesis?

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 activation.

Question 71

Polymer materials used in orthopedics, such as ultra-high molecular weight polyethylene (UHMWPE), exhibit viscoelastic properties. The phenomenon where a material continues to slowly deform over time while subjected to a constant load is known as:

Explanation

Creep is a viscoelastic property defined as progressive, time-dependent deformation under a constant load or stress.

Question 72

A modular total hip arthroplasty fails due to severe tribocorrosion at the head-neck junction. What specific type of wear is characterized by microscopic oscillatory motion between two solid surfaces under load?

Explanation

Fretting wear occurs due to repetitive microscopic motion (micromotion) between two contacting surfaces under load, commonly seen at modular interfaces of joint replacements.

Question 73

Achondroplasia is the most common form of short-limb dwarfism. It is caused by an activating mutation in the FGFR3 gene, which predominantly inhibits chondrocyte activity in which zone of the physis?

Explanation

Achondroplasia results from a mutation in FGFR3 that abnormally suppresses chondrocyte proliferation in the proliferative zone of the growth plate, impairing endochondral ossification.

Question 74

Highly cross-linked ultra-high molecular weight polyethylene (UHMWPE) was introduced to improve the longevity of total hip replacements. Which of the following describes the mechanical tradeoff associated with heavily cross-linking UHMWPE?

Explanation

Cross-linking UHMWPE significantly improves its wear resistance but at the cost of decreasing its mechanical properties, including fatigue strength, ductility, and fracture toughness.

Question 75

Osteocytes secrete a protein called sclerostin to regulate bone mass in response to mechanical loading. Sclerostin inhibits osteoblastic bone formation primarily by antagonizing which intracellular signaling pathway?

Explanation

Sclerostin binds to the LRP5/6 receptors on osteoblasts, effectively inhibiting the canonical Wnt/beta-catenin signaling pathway, which leads to decreased bone formation.

Question 76

A diaphyseal tibia fracture is treated with a cast. According to Perren's strain theory, what biomechanical environment is necessary to promote secondary bone healing with robust callus formation?

Explanation

Secondary bone healing (via callus formation) requires relative stability and an interfragmentary strain of approximately 2% to 10%, which stimulates enchondral ossification.

Question 77

When examining normal human articular cartilage microscopically, chondrocytes and collagen fibers in the deep (basal) zone are oriented in what direction relative to the articular surface?

Explanation

In the deep zone of articular cartilage, chondrocytes are arranged in vertical columns, and collagen fibers are oriented perpendicular to the joint surface to anchor the cartilage to the subchondral bone.

Question 78

Secondary fracture healing proceeds through distinct biological phases. Which phase is characterized by a hypoxic environment, chondrocyte proliferation, and peak expression of Type II collagen?

Explanation

The soft callus phase involves enchondral ossification, where a cartilaginous matrix rich in Type II collagen is produced to bridge the fracture gap before being replaced by bone.

Question 79

In materials science, 'toughness' is a critical property for fracture fixation hardware. Toughness is best defined mechanically as:

Explanation

Toughness is the capacity of a material to absorb energy and deform plastically before fracturing. It is represented by the total area under the stress-strain curve.

Question 80

Aggrecan is the most abundant proteoglycan in articular cartilage. What is its primary biomechanical function within the cartilage matrix?

Explanation

Aggrecan contains highly negatively charged glycosaminoglycan chains that draw water into the matrix, generating an osmotic swelling pressure that imparts compressive stiffness to the cartilage.

Question 81

During the early phases of fracture healing or in high-turnover states like Paget's disease, woven bone is formed. Compared to mature lamellar bone, woven bone is characterized by:

Explanation

Woven bone is immature bone that forms rapidly during early fetal development or fracture repair. It is characterized by randomly oriented collagen fibers and lower mechanical strength.

Question 82

Orthopedic implants are manufactured from various biomaterials. Which of the following metallic alloys has a modulus of elasticity closest to that of human cortical bone, thereby theoretically reducing stress shielding?

Explanation

Titanium alloys have a modulus of elasticity (approx. 110 GPa) that is lower than that of cobalt-chromium or stainless steel, making it closer to cortical bone (approx. 15-20 GPa) and reducing stress shielding.

Question 83

The pull-out strength of a cortical screw used in fracture fixation is heavily dependent on screw design and bone quality. Based on screw geometry, pull-out strength is directly proportional to which of the following dimensions?

Explanation

Screw pull-out strength is directly proportional to the outer (thread) diameter, the length of thread engagement, and the shear strength of the surrounding bone. Inner core diameter primarily determines the screw's tensile and torsional strength.

Question 84

Mineralization of the osteoid matrix relies heavily on the local concentration of calcium and phosphate. Which osteoblast-derived enzyme promotes mineralization by hydrolyzing and removing inorganic pyrophosphate, a potent inhibitor of calcification?

Explanation

Alkaline phosphatase (ALP) promotes bone mineralization both by increasing the local concentration of inorganic phosphate and by cleaving pyrophosphate, which normally inhibits mineralization.

Question 85

Aseptic loosening of cementless total hip stems can occur secondary to localized proximal femoral osteopenia. This phenomenon, known as stress shielding, occurs because the rigid metal implant:

Explanation

Stress shielding occurs when an implant with a higher modulus of elasticity (stiffness) bears the majority of the physiological load. The under-loaded surrounding bone then remodels and resorbs according to Wolff's law.

Question 86

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

Explanation

The superficial zone contains the highest water content, parallel collagen fibers to resist shear stress, and the lowest proteoglycan concentration. This structural arrangement minimizes friction and protects the deeper zones from shear forces.

Question 87

A ligament subjected to a constant, sustained load elongates progressively over time. This viscoelastic property is best described as:

Explanation

Creep is the progressive deformation (elongation) of a viscoelastic material when subjected to a constant load over time. Stress relaxation, conversely, occurs when a material is held at a constant length and the internal force decreases.

Question 88

Osteoclasts resorb bone matrix by creating an acidic microenvironment at the ruffled border. Which intracellular enzyme is primarily responsible for generating the protons required for this acidification process?

Explanation

Carbonic anhydrase II catalyzes the hydration of CO2 to form carbonic acid, which then dissociates into protons and bicarbonate. The protons are actively pumped into the resorption pit via a vacuolar H+-ATPase to dissolve bone mineral.

Question 89

An orthopedic surgeon decides to change an intramedullary nail from a solid titanium rod with a radius of 'r' to a solid titanium rod with a radius of '2r'. Assuming identical material properties, by what factor does the bending rigidity of the nail increase?

Explanation

The bending rigidity is proportional to the area moment of inertia, which for a solid cylinder is proportional to the radius to the fourth power (r^4). Therefore, doubling the radius (2r) increases the bending rigidity by a factor of 16 (2^4).

Question 90

Which of the following components of the extracellular matrix is primarily responsible for the compressive stiffness of articular cartilage?

Explanation

Aggrecan, a large proteoglycan, provides compressive stiffness to articular cartilage. This is due to the high negative charge of its glycosaminoglycan side chains, which attract water and create significant osmotic swelling pressure.

Question 91

Sclerostin (SOST) regulates bone mass by inhibiting a specific cellular signaling pathway. Which of the following pathways is directly antagonized by sclerostin?

Explanation

Sclerostin, produced primarily by osteocytes, acts as a negative regulator of bone formation by binding to LRP5/6 receptors. This binding directly inhibits the canonical Wnt/beta-catenin signaling pathway in osteoblasts.

Question 92

On a stress-strain curve for an orthopedic biomaterial, the area under the curve strictly within the elastic region represents the material's:

Explanation

The area under the stress-strain curve in the elastic region represents resilience, defined as the energy a material can absorb and release without undergoing permanent deformation. Toughness, by contrast, is the total area under the entire curve until failure.

Question 93

In the epiphyseal growth plate, which zone is primarily responsible for longitudinal bone growth through rapid cellular division and column formation?

Explanation

The proliferative zone is characterized by active chondrocyte replication and column formation, directly driving longitudinal bone growth. This zone is highly responsive to hormonal influences, including IGF-1 and growth hormone.

Question 94

According to Perren's strain theory of bone healing, what is the maximum interfragmentary strain that allows for the formation of primary lamellar bone without intermediate soft callus formation?

Explanation

Primary (direct) bone healing requires absolute stability with an interfragmentary strain of less than 2%. Strains between 2% and 10% prevent primary bone formation but allow for secondary bone healing via endochondral ossification (callus formation).

Question 95

Parathyroid hormone (PTH) stimulates bone resorption primarily by binding to receptors located directly on which of the following cell types?

Explanation

PTH binds to receptors on osteoblasts, stimulating them to increase the expression of RANKL and decrease the expression of OPG. This alters the local RANKL/OPG ratio, which subsequently drives osteoclast precursors to mature and resorb bone.

Question 96

When applying a conventional non-locking plate to a diaphyseal fracture, over-tightening the screws compresses the plate directly to the bone. This construct relies on which fundamental biomechanical principle for stability?

Explanation

Conventional non-locking plates rely entirely on friction generated between the underside of the plate and the bone surface to resist functional loads. Locking plates, in contrast, rely on fixed-angle screw-plate interfaces and do not require bone contact for stability.

Question 97

During prolonged standing, resulting in static high loads across the knee joint, which mechanism of articular cartilage lubrication is primarily responsible for minimizing friction between the joint surfaces?

Explanation

Boundary lubrication predominates under high-load, low-velocity, or static conditions where fluid-film lubrication fails. It relies on a monomolecular layer of lubricin (PRG4) and surface-active phospholipids to prevent direct surface-to-surface wear.

Question 98

Which type of collagen is the predominant organic structural component of the soft callus during the early stages of secondary fracture healing?

Explanation

The soft callus in secondary fracture healing is primarily composed of cartilaginous tissue, which is rich in Type II collagen. As endochondral ossification progresses, this cartilage is replaced by a hard bony callus predominantly containing Type I collagen.

Question 99

An orthopedic implant undergoes cyclical loading well below its ultimate yield strength but eventually fails. On an S-N (Stress-Number of cycles) curve, what does the endurance limit represent?

Explanation

The endurance limit (or fatigue limit) is the specific stress threshold on an S-N curve below which a material can endure an infinite number of loading cycles without fatigue failure. Not all materials, such as aluminum, exhibit a true endurance limit.

Question 100

During the remodeling phase of cortical bone healing, cutting cones progress longitudinally through the haversian systems. What is the leading cell type at the forefront of a functioning cutting cone?

Explanation

Cutting cones are responsible for primary cortical bone remodeling and are led by osteoclasts at the cutting edge that actively resorb bone. They are followed by a vascular loop and osteoblasts that deposit new concentric lamellae to form secondary osteons.

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

AAOS Basic Science MCQs (Set 3): Bone & Cartilage Biology, Ortho Biomechanics | ABOS Exam Prep

Key Takeaway

AAOS Basic Science MCQs (Set 3): Bone & Cartilage Biology, Ortho Biomechanics | ABOS Exam Prep

Chapter Index