ABOS Basic Science MCQs (Set 4): Bone Physiology Questions

Question 1

A 23-year-old man has had right posterolateral knee pain and occasional lateral calf dysesthesias for the past 8 months. A radiograph, CT scan, MRI scans, and a biopsy specimen are shown in Figures 62a through 62e. What is the most likely diagnosis?

Explanation

The radiograph shows an eccentric, cortically based lytic lesion in the proximal fibula. The CT and MRI scans confirm that it is well circumscribed and cortically based with significant surrounding edema. The radiographic differential diagnosis would be a Brodie's abscess or osteoid osteoma. An osteoblastoma would have to be greater than 2 cm in size. A chondroblastoma may also have significant edema around it, but it is an epiphyseal-based lesion, not cortically based. The well-circumscribed nature of the lesion is not consistent with osteosarcoma. The pathology shows a very cellular and vascular stroma with plump, but not atypical osteoblast cells making a matrix of immature woven bone. There are no abundant inflammatory cells or dead bone suggestive of osteomyelitis or a Brodie's abscess. Therefore, the clinical and histologic picture is most consistent with an osteoid osteoma. Percutaneous radiofrequency ablation, usually with CT guidance, has become the preferred method for treating most cases of osteoid osteoma. Rosenthal DI: Radiofrequency treatment. Orthop Clin North Am 2006;37:475-484.

Question 2

Figures 63a and 63b show the radiographs of an 11-year-old girl who sustained a twisting injury of the knee playing soccer. She is now asymptomatic. What is the appropriate treatment of the lesion?

Explanation

This is a nonossifying fibroma of the proximal tibia. The lesion is eccentric, cortically based, with sclerotic margins and no evidence of a soft-tissue mass. Nonossifying fibromas are benign lesions that need no biopsy or surgical treatment when classic findings appear on radiographs. A follow-up radiograph should be performed 2 to 3 months after the initial presentation to ensure that the lesion is not progressive. Surgery is reserved for large lesions with risk of pathologic fracture or for cases where a displaced pathologic fracture has occurred and internal fixation is needed for fracture treatment. Nondisplaced pathologic fractures through nonossifying fibromas are best treated by allowing the fracture to heal and observation of the lesion. Vaccaro AR (ed): Orthopaedic Knowledge Update 8. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2005, pp 197-215.

Question 3

A 21-year-old man is referred for evaluation of a lesion in the left proximal femur that was discovered when he was undergoing an upper gastrointestinal series. He reports no hip or thigh pain. Radiographs are shown in Figures 64a and 64b. What is the best course of action?

Explanation

Melorheostosis is a rare disorder characterized by the classic radiographic appearance of flowing hyperostosis in a long bone. The hyperostosis may be on the periosteal or endosteal surface of the bone and frequently gives the appearance of wax falling down the side of a candle. The radiographs are diagnostic; therefore, no further work-up is indicated. Menendez LR (ed): Orthopaedic Knowledge Update: Musculoskeletal Tumors. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2002, pp 87-102.

Question 4

A 17-year-old boy has had elbow pain for the past 6 weeks. A radiograph, MRI scans, and biopsy specimens are shown in Figures 65a through 65e. What is the most likely diagnosis?

Explanation

The findings are consistent with an osteoblastoma. The radiographs show a bone-forming lesion of the distal humerus. The lesion has an osseous component extending out of the native cortex with a thin sclerotic border. The T2-weighted MRI scan shows the lesion extending anteriorly beyond the native cortex. No fluid-fluid levels are seen. Histology shows large osteoblasts producing osteoid and woven bone. The tissue between the spicules of bone and osteoid contains thin fibrous tissue and capillaries. Osteoid osteoma is a smaller lesion usually with sclerotic reactive bone around a small nidus. The histology differentiates osteoblastoma from osteosarcoma because no malignant cells are seen. There is no cartilage production or chondroblasts in the histologic specimen, eliminating chondroblastoma. Giant cell tumors of bone typically occur in a epiphyseal metaphyseal location, most commonly after skeletal maturity, and contain numerus giant cells. Menendez LR (ed): Orthopaedic Knowledge Update: Musculoskeletal Tumors. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2002, pp 87-102.

Question 5

What is the most common site of metastases from a soft-tissue sarcoma?

Basic Science Board Review 2008: High-Yield MCQs (Set 4) - Figure 15

Explanation

The most common site of metastases from a soft-tissue sarcoma is the lungs and occurs in 40% to 60% of patients. The second most common site of metastases in soft-tissue sarcomas is the lymph nodes. Nodal metastases are seen with regularity in synovial sarcoma, epithelioid sarcoma, and rhabdosarcoma. The liver, brain, bone, and muscle are occasional sites of spread, but the occurrence is very rare. Simon SR (ed): Orthopaedic Basic Science. Rosemont, IL, American Academy of Orthopaedic Surgeons, 1994, pp 219-276.

Question 6

A 45-year-old woman has had right wrist pain for the past 2 months. A radiograph, bone scan, and MRI scan are shown in Figures 66a through 66c, and a photomicrograph is shown in Figure 66d. What is the most likely diagnosis?

Explanation

The radiograph shows a purely radiolucent lesion without matrix mineralization in the epiphysis of the distal radius. The lesion is "hot" on bone scan, and the MRI scan reveals cortical destruction with a soft-tissue mass. These findings are most consistent with giant cell tumor. The distal radius is a common location for giant cell tumors. The other options would be very uncommon in this location in a 45-year-old patient. The photomicrograph demonstrates multinucleated giant cells in a sea of mononuclear cells. The nuclei of the giant cells and the nuclei of the mononuclear stromal cells are identical. This feature helps distinguish giant cell tumor from other lesions that might contain giant cells. Cheng CY, Shih HN, Hsu KY, et al: Treatment of giant cell tumor of the distal radius. Clin Orthop Relat Res 2001;383:221-228.

Question 7

An otherwise healthy 16-year-old dancer reports a 1-month history of leg pain. AP and lateral radiographs of the distal femur are shown in Figures 67a and 67b. What is the next most appropriate step in management?

Explanation

The radiographs show a very ill-defined, aggressive, moth-eaten bony destruction involving the distal left femoral diaphysis just above the metaphyseal junction. The differential diagnosis includes Ewing's sarcoma, osteosarcoma, lymphoma of bone, eosinophilic granuloma, osteomyelitis, and others. MRI would further define the lesion, and soft-tissue and intramedullary extension. There is aggressive periosteal reaction in the posteromedial aspect of the adjacent lesion with some multilayered components in the distal interface of the periosteum. The lateral radiograph shows cortical penetration and irregular periosteal reaction of the posterior margin, suggesting some posterior soft-tissue extension at this site as well. Gebhardt MC, Ready JE, Mankin HJ: Tumors about the knee in children. Clin Orthop Relat Res 1990;255:86-110.

Question 8

Acral metastases are most commonly seen in what type of carcinoma?

Explanation

Metastatic lesions to bone are usually located in the axial and proximal appendicular skeleton. Metastases below the elbow and knee are rare, but when they do occur they are most commonly from lung carcinoma. Hayden RJ, Sullivan LG, Jebson PJ: The hand in metastatic disease and acral manifestations of paraneoplastic syndromes. Hand Clin 2004;20:335-343.

Question 9

A 47-year-old woman has had left medial clavicle pain for the past 6 months. History is remarkable for mediastinal non-Hodgkin's lymphoma, treated with mantel radiation 22 years ago. A radiograph, CT scan, MRI scan, and a biopsy specimen are shown in Figures 68a through 68d. What is the most likely diagnosis?

Explanation

Radiation-associated sarcomas typically occur at least 5 years following radiation therapy, in the radiation therapy field, and with different histology than the original disease. The radiograph shows a lytic destructive lesion of the medial clavicle. The radiographic differential could include any of the above etiologies. The CT and MRI scans show this same reaction with extension into the adjacent soft tissue and periosteal reaction. These findings eliminate a degenerative process or radiation-induced osteonecrosis but do not distinguish between a neoplastic and infectious process. A PET scan showed marked uptake in the distal clavicle, which is more consistent with a malignant neoplastic process than a reactive process, like that of an infection. These findings, combined with the cellular atypia and bone formation on the biopsy specimen, confirm the diagnosis of radiation-associated sarcoma. In this older patient, radiation-associated sarcoma appears in an unusual location secondary to her previous radiation treatment in that region, which can occur 3 to 50 years after previous radiation therapy. Shaheen M, Deheshi BM, Riad S, et al: Prognosis of radiation-induced bone sarcoma is similar to primary osteosarcoma. Clin Orthop Relat Res 2006;450:76-81.

Question 10

In 1980, a 32-year-old woman was found to have right breast mass, and a biopsy revealed adenocarcinoma. She underwent a mastectomy at that time, with no other treatment. Five years later, she noticed a lump in the left breast and underwent a left mastectomy. Seven lymph nodes were positive. In 2006, she now reports hip and thigh pain for the past 3 months. Figures 69a and 69b show AP and lateral radiographs of the femur. A bone scan shows a solitary lesion. Following radiographic staging, what is the next most appropriate step in management?

Explanation

Solitary bone lesions require biopsy, for there is the possibility that the lesion may represent a primary bone sarcoma, which will necessitate a different treatment plan. This is especially true in patients with remote histories of cancer. The most likely cause of a lytic bone lesion in a patient older than age 40 years is a metastatic lesion. Rougraff BT, Kneisl JS, Simon MA: Skeletal metastases of unknown origin: A prospective study of a diagnostic strategy. J Bone Joint Surg Am 1993;75:1276-1281.

Question 11

What is the mechanism of action of bisphosphonates?

Explanation

Bisphosphonates are stable analogues of pyrophosphate that have a strong affinity for bone hydroxyapatite; these agents inhibit bone resorption by reducing the recruitment and activity of osteoclasts and increasing apoptosis. Bone formed while patients are receiving bisphosphonate treatment is histologically normal. Bisphosphonates have been shown to be effective in decreasing pathologic fractures, bone pain, and the need for radiation therapy in patients with multiple myeloma and metastatic carcinoma to bone. The most effective method of administration is via monthly intravenous infusion. Osteonecrosis of the mandible is sometimes a complication of this treatment. Gass M, Dawson-Hughes B: Preventing osteoporosis-related fractures: An overview. Am J Med 2006;119:S3-S11.

Question 12

A 16-year-old girl has had hip pain for 1 year. Approximately 2 months ago she noted the development of a hard mass in the right buttock that has steadily increased in size. She now reports severe pain in the right buttock, with radiation down the leg and numbness involving the right foot and toes. A radiograph is shown in Figure 70a and an axial postcontrast T1-weighted MRI scan is shown in Figure 70b. A biopsy specimen is shown in Figure 70c. The chest CT shows multiple lung metastases. Treatment of this lesion should consist of

Explanation

Ewing's sarcoma is the second most common primary tumor of bone in children. Depending on the site and extent of disease, chemotherapy, radiation therapy, and surgery are all treatment options. In this patient with extensive pelvic and metastatic disease, chemotherapy and radiation therapy offer the best oncologic control while preserving functional outcome. Gibbs CP Jr, Weber K, Scarborough MT: Malignant bone tumors. Instr Course Lect 2002;51:413-428. Thacker MM, Temple HT, Scully SP: Current treatment for Ewing's sarcoma. Expert Rev Anticancer Ther 2005;5:319-331.

Question 13

A 55-year-old woman has slowly increasing pain at the distal end of her little finger that is exacerbated by cold temperatures. She denies any history of trauma to her hands and is employed as a school teacher. The histology of the resected specimen is shown in Figure 71. What is the most likely diagnosis?

Basic Science Board Review 2008: High-Yield MCQs (Set 4) - Figure 31

Explanation

Glomus tumors are rare vascular lesions typically occurring about the nail of the distal phalanx of the hand. The diagnostic "triad" of glomus tumors consists of local pain, sensitivity to cold, and paroxysmal pain. They tend to present with pain as the most typical symptom and this can be exacerbated by changes in temperature that is felt to cause a vascular response within the lesion. The biopsy specimen confirms a glomus tumor showing the typical vascular spaces surrounded by glomus epithelioid glomus cells. Zook EG, Brown RE: The perionychium, in Green DP, Hotchkiss RN, Pederson WC (eds): Green's Operative Hand Surgery, ed 4. Philadelphia, PA, Churchill Livingstone, 1999, vol 2, pp 1353-1380.

Question 14

A 69-year-old woman reports a painful clicking in her right shoulder. A soft-tissue mass is palpated at the lower portion of the scapula. Based on the MRI scan and biopsy specimens shown in Figures 72a through 72c, what is the most likely diagnosis?

Explanation

Elastofibroma is a rare, benign soft-tissue pseudotumor characteristically located in the subscapular region. Most patients are between 50 and 70 years of age and have pain, decreased shoulder range of motion, or a mass. The lesion usually is best visualized with the arm elevated forward and adducted to displace the scapula laterally and allow the mass to protrude from the chest wall. They are frequently bilateral. Grossly, the mass is ill-defined, oblong or spherical, firm, and ranges in size from 5 cm to 10 cm. These masses appear infiltrative and frequently are mistaken for a sarcomatous lesion. Histologically, the mass is composed of a mixture of intertwining eosinophilic collagen and elastic fibers, and scattered fibroblasts, mucoid material, and fat. Vastamaki M: Elastofibroma scapulae. Clin Orthop Relat Res 2001;392:404-408. Nielsen T, Sneppen O, Mykre-Jensen O, et al: Subscapular elastofribroma: A reactive pseudotumor. J Shoulder Elbow Surg 1996;5:209-213.

Question 15

A 40-year-old man has a painless mass around his left ankle. He notes minimal growth over the past year. An MRI scan is shown in Figure 73a, and biopsy specimens are shown in Figures 73b and 73c. What is the most likely diagnosis?

Explanation

The biopsy specimen is a low-power view of a soft-tissue sarcoma with a biphasic pattern of epithelial cells and fibrous spindle cells that are typical of a synovial sarcoma. A deep, painless soft-tissue mass greater than 5 cm in size is suspicious for a sarcoma. The imaging in this patient is indeterminate and the patient requires a biopsy for an accurate diagnosis. The biopsy reveals a tumor with a biphasic appearance consistent with a synovial sarcoma. These tumors are slow growing, occur primarily in the lower extremities, and are found in a younger demographic population compared to malignant fibrous histiocytoma and liposarcoma. They can occur in a biphasic pattern with clumps of epithelial cells and fibrous spindle cells or in a monophasic pattern. Synovial sarcomas stain positively for keratin. Keratin is positive in nearly all biphasic types and in many tumors of the monophasic fibrous type. Sixty percent of these tumors are found in the lower extremity. The area around the knee is the most common location, followed by the ankle and foot. Epithelioid and clear cell sarcomas are found associated with tendon sheaths of the hand and feet and are generally smaller in size. Epithelioid sarcoma may resemble granulomatous inflammation histologically; clear cell sarcoma is composed of nests of clear cells with occasional multinucleated giant cells. Pigmented villonodular synovitis is found within the synovium and is characterized by a villous histologic appearance with hemosiderin-laden macrophages. Malignant fibrous histiocytoma has a storiform histologic pattern with an abundance of pleomorphic cells. Enzinger FM, Weiss SW: Soft Tissue Tumors, ed 3. St Louis, MO, Mosby, 1995, p 757.

Question 16

A 28-year-old woman has left shoulder pain and a tender soft-tissue mass. Based on the MRI scan and biopsy specimens shown in Figures 74a through 74c, what is the most likely diagnosis?

Explanation

Schwannomas (neurilemomas) occur at all ages but are most frequently seen in persons between the ages of 20 and 50 years. MRI features of schwannomas are fairly nonspecific, but when they are associated with a large named nerve, the identification of a mass in continuity with that nerve is highly suggestive of a schwannoma. Most have a fairly homogeneous appearance with a high water content and often fusiform shape. Classically, the histology shows alternating Antoni A (dense spindle cell region) areas and Antoni B (loose myxoid tissue) areas. They also demonstrate uniform intense immunostaining with S-100 protein. Damron TA, Sim FH: Soft-tissue tumors about the knee. J Am Acad Orthop Surg 1997;5:141-152.

Question 17

Which of the following prognostic indicators is associated with the least favorable outcome for patients newly diagnosed with osteosarcoma?

Explanation

Distant bone metastasis is associated with an extremely poor prognosis for patients with osteosarcoma (5-year survival rate of less than 10%). Most osteosarcomas are high grade and extracompartmental, and approximately half are greater than 8 cm at presentation. The 5-year survival rate for these patients is approximately 70%. Patients with a solitary pulmonary metastasis have a prognosis worse than patients without detectable metastases but not as bad as those with bone metastases. Bielack SS, Kempf-Bielack B, Delling G, et al: Prognostic factors in high-grade osteosarcoma of the extremities or trunk: An analysis of 1,702 patients treated on neoadjuvant cooperative osteosarcoma study group protocols. J Clin Oncol 2002;20:776-790. Heck RK, Stacy GS, Flaherty MJ, et al: A comparison study of staging systems for bone sarcomas. Clin Orthop Relat Res 2003;415:64-71.

Question 18

A 12-year-old girl has had increasing left knee pain for the past 3 months. A radiograph is shown in Figure 75a, and low- and high-power photomicrographs are shown in Figures 75b and 75c. What is the most appropriate treatment?

Explanation

The radiograph reveals an aggressive purely lytic lesion of the distal femoral metaphysis. There is no apparent matrix mineralization or periosteal reaction. The photomicrographs show a malignant spindle cell neoplasm in a storiform pattern. Based on these findings, the diagnosis is malignant fibrous histiocytoma of bone. At most institutions, patients with this tumor are treated similar to patients with osteosarcoma with multi-agent chemotherapy and surgery with wide margins (resection or amputation). Menendez LR (ed): Orthopaedic Knowledge Update: Musculoskeletal Tumors. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2002, pp 203-209.

Question 19

A 58-year-old woman has had a slowly progressing mass over the distal interphalangeal (DIP) joint of her dominant hand with a worsening deformity of her nail. She has no significant medical history but underwent bilateral knee arthroplasties 1 year ago. Radiographs reveal a small osteophyte at the DIP joint dorsally. A clinical photograph and a biopsy specimen are shown in Figures 76a and 76b. What is the most likely diagnosis?

Explanation

A mucous cyst is thought to be a ganglion arising from the DIP joint in patients with osteoarthritis. They are frequently associated with nail deformities. Treatment involves removal of the cyst with debridement of DIP joint osteophytes. Fritz GR, Stern PJ, Dickey M: Complications following mucous cyst excision. J Hand Surg Br 1997;22:222-225.

Question 20

A 13-year-old boy has had pain and swelling in his ankle for the past several months. Based on the radiograph, MRI scan, and biopsy specimen shown in Figures 77a through 77c, what is the best course of action?

Explanation

Chondroblastomas are benign cartilage lesions frequently seen in adolescents or young adults. They are found in the epiphyseal or apophyseal regions of bones. The radiograph shows a radiolucent lesion with mineralization and a well-marginated rim of reactive bone. The lesion is composed of sheets of immature chondroblasts (polygonal cells with a clear, bluish cytoplasm and a small round central nucleus). In some regions, classic "chicken-wire" matrix calcifications and a "cobblestone" pattern of cell arrangement may be seen. Treatment consists of curettage and bone grafting. Campanacci M: Bone and Soft Tissue Tumors, ed 2. New York, NY, Springer-Verlag, 1999, pp 247-264.

Question 21

A 39-year-old man has had a foot mass for the past several months. MRI scans are shown in Figures 78a through 78c. A core biopsy specimen reveals synovial sarcoma, and a staging chest CT scan is normal. Which of the following treatments offers the best local tumor control and expedites the patient's return to normal function?

Explanation

Certain histologic subtypes of soft-tissue sarcoma have been noted to arise preferentially in the hand and the foot, such as epithelioid sarcoma, clear cell sarcoma, and synovial sarcoma. Synovial sarcoma is the most common foot sarcoma. Frequently there is a delay in diagnosis because the lesions are rare. The lesions tend to occur in younger adults, typically between the ages of 15 and 40 years. Patients with hand and foot sarcomas have been described as having improved overall survival, but this is likely a result of the smaller size of tumors arising in these locations. In this patient, the tumor has grown to a substantial size and involves many of the bones of the midfoot. Limb salvage may be a possibility when incorporated into a multidisciplinary treatment program, but this will entail months of adjuvant treatment and significant morbidity. Amputation and early prosthetic fitting still have a role in management of some soft-tissue sarcomas, most frequently in the foot. Ferguson PC: Surgical considerations for management of distal extremity soft tissue sarcomas. Curr Opin Oncol 2005;17:366-369.

Question 22

A 10-month-old boy has multiple skeletal lesions and a skin rash that he has had since he was a newborn. Based on the radiographs and biopsy specimens shown in Figures 79a through 79d, what is the most likely diagnosis?

Explanation

Langerhans cell histiocytosis or eosinophilic granuloma is a nonneoplastic lesion that is part of a spectrum of clinical diseases featuring histiocytes. Most occur during the first two decades of life within any bone. Radiographs show a radiolucent lesion, frequently diaphyseal in location. A periosteal response is occasionally seen and can resemble more aggressive lesions such as osteomyelitis or Ewing's sarcoma. Histology demonstrates CD1a positive histiocytes with large oval-shaped nuclei with indentation, and a variable presence of eosinophils. Plasschaert F, Craig C, Bell R, et al: Eosinophilic granuloma: A different behaviour in children than in adults. J Bone Joint Surg Br 2002;84:870-872.

Question 23

Which of the following is not a characteristic of synovial sarcomas?

Explanation

Synovial sarcomas have a number of features that differentiate them from other soft-tissue sarcomas. They often have small areas of calcifications within the lesion. They occur in a younger patient population than most soft-tissue sarcomas. A subset of patients with synovial sarcoma tend to be chemosensitive. They often contain the SYT-SSX translocation. Although they can occur intra-articular, this is rare, despite their name. Campanacci M: Bone and Soft Tissue Tumors, ed 2. New York, NY, Springer-Verlag, 1999, pp 1109-1126.

Question 24

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

Explanation

Patients with a mutation of the retinoblastoma gene (RB) have an increased likelihood for the development of osteosarcoma. The EWS-FLI1 gene is the fusion product of a chromosomal translocation of 11:22. EGF-R and IGF-R are growth factor receptors, but mutations in these genes have not been correlated with osteosarcoma. VEGF is a proangiogenic molecule that is involved in tumor formation in multiple sites but not the development of osteosarcoma. Scholz RB, Kabisch H, Delling G, et al: 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;7:265-273.

Question 25

A 27-year-old man has had pain in the right index finger for the past 9 months. The pain is completely relieved with ibuprofen. An AP radiograph and CT scan are shown in Figures 80a and 80b. What is the most likely diagnosis?

Explanation

Osteoid osteoma is a round or oval, well-circumscribed lesion with a radiolucent nidus. A small area of calcification may be present within the center of the nidus. The radiolucent nidus is surrounded by a thick rim of sclerotic bone. These diagnostic features are frequently better seen on CT. An increase in cyclooxygenase activity has been demonstrated within osteoid osteomas, which may explain why aspirin and other nonsteroidal anti-inflammatory drugs classically relieve the pain associated with these lesions. Kneisl JS, Simon MA: Medical management compared with operative treatment for osteoid-osteoma. J Bone Joint Surg Am 1992;74:179-185.

Question 26

Highly cross-linked ultra-high molecular weight polyethylene (UHMWPE) is widely used in total joint arthroplasty to reduce wear rates. However, the process of extensive cross-linking significantly decreases which of the following mechanical properties?

Explanation

While high cross-linking of UHMWPE significantly improves wear resistance, it inversely reduces fatigue crack propagation resistance, yield strength, and fracture toughness. This increases the risk of component fracture under high stress.

Question 27

Romosozumab has recently been approved for the treatment of severe osteoporosis. What is the precise molecular target of this monoclonal antibody?

Explanation

Romosozumab is a monoclonal antibody that binds to and inhibits sclerostin. By blocking sclerostin, it disinhibits the Wnt/beta-catenin signaling pathway, leading to a dual effect of increasing bone formation and decreasing bone resorption.

Question 28

When analyzing the stress-strain curve of an orthopedic biomaterial, the total area under the curve prior to the point of material failure represents which biomechanical property?

Explanation

Toughness is defined as the total amount of energy a material can absorb before catastrophic failure, represented by the entire area under the stress-strain curve. Resilience is the energy absorbed only up to the elastic limit.

Question 29

An intramedullary nail is being selected for the fixation of a diaphyseal femur fracture. Biomechanically, the torsional stiffness of a solid cylindrical intramedullary nail is directly proportional to the radius raised to what power?

Explanation

The torsional stiffness of a solid cylinder is determined by its polar moment of inertia, which is proportional to the radius to the fourth power (r^4). Therefore, a small increase in the nail's radius drastically increases its resistance to torsion.

Question 30

A patient with a modular uncemented total hip arthroplasty presents with unexplained hip pain and rising serum cobalt levels. Radiographs show no loosening. Fretting and mechanically assisted crevice corrosion are most likely occurring at which of the following interfaces?

Explanation

Mechanically assisted crevice corrosion (MACC), often referred to as trunnionosis, occurs at the modular head-neck taper junction. Micromotion disrupts the passive oxide layer, leading to localized corrosion and release of metal ions.

Question 31

During bone resorption, osteoclasts create an acidic microenvironment in Howship's lacunae by secreting protons via a vacuolar H+-ATPase. Which enzyme is then primarily responsible for the degradation of the organic type I collagen matrix?

Explanation

Cathepsin K is a lysosomal cysteine protease secreted by osteoclasts that functions optimally in an acidic environment to cleave type I collagen. While TRAP is a marker of osteoclasts, Cathepsin K is the primary matrix-degrading enzyme.

Question 32

In an experiment, a cadaveric anterior cruciate ligament is rapidly stretched to a constant, fixed length. Over the next several minutes, the force required to maintain this specific length gradually decreases. This viscoelastic phenomenon is known as:

Explanation

Stress relaxation occurs when a viscoelastic material is held at a constant strain (length), and the stress (internal force) decreases over time. Creep, conversely, is the gradual increase in strain when held at a constant stress.

Question 33

Titanium alloy (Ti-6Al-4V) is frequently chosen over Cobalt-Chromium for the manufacturing of diaphyseal-engaging femoral stems in uncemented total hip arthroplasty. What is the primary biomechanical advantage of Titanium in this application?

Explanation

Titanium alloy has a modulus of elasticity approximately half that of Cobalt-Chromium, making it significantly closer to the modulus of cortical bone. This increased flexibility reduces stress shielding and subsequent proximal femoral osteolysis.

Question 34

Ceramic-on-ceramic bearings in total hip arthroplasty provide the lowest volumetric wear rates of all modern bearing surfaces. However, their use is specifically associated with which of the following unique complications?

Explanation

Ceramics are extremely hard and scratch-resistant, but they are brittle materials. They carry a unique risk of catastrophic brittle fracture and can produce an audible squeaking sound during the gait cycle.

Question 35

An adult sustains a transverse radius fracture and undergoes open reduction and internal fixation with a dynamic compression plate. If absolute stability and rigid compression are achieved, the fracture will heal via which mechanism?

Explanation

Under conditions of absolute stability with interfragmentary strain less than 2%, primary bone healing occurs. This process is driven by osteoclastic cutting cones crossing the fracture site, directly replacing necrotic bone without callus formation.

Question 36

Polymethylmethacrylate (PMMA) bone cement is supplied as a powder and a liquid monomer. What is the specific function of Barium sulfate or Zirconium dioxide, which are added to the powder component?

Explanation

Barium sulfate and Zirconium dioxide are added to PMMA purely as radiopacifiers to allow visualization of the cement mantle on postoperative radiographs. They do not strengthen the cement and can actually act as stress risers.

Question 37

Parathyroid hormone (PTH) plays a critical role in calcium homeostasis. At the cellular level within bone tissue, what is the primary direct action of PTH?

Explanation

Osteoclasts lack PTH receptors. PTH binds to receptors on osteoblasts, stimulating them to express RANKL, which subsequently binds to RANK on osteoclast precursors to induce differentiation and bone resorption.

Question 38

In orthopedic biomechanics, materials are often compared by their stiffness. Which of the following sequences correctly ranks the materials in order of DECREASING modulus of elasticity (stiffest to most flexible)?

Explanation

Cobalt-Chromium is the stiffest (approx. 200 GPa), followed by Titanium alloy (approx. 100 GPa). Cortical bone is much less stiff (15-20 GPa), and PMMA bone cement is highly flexible (2-3 GPa).

Question 39

A nonunion of the tibia requires bone grafting. The surgeon desires a graft material that possesses osteoconductive, osteoinductive, and osteogenic properties simultaneously. Which of the following materials is the only one to fulfill all three criteria?

Explanation

Autologous bone graft (like iliac crest) provides a scaffold (osteoconductive), growth factors like BMPs (osteoinductive), and live mesenchymal stem cells/osteoblasts (osteogenic). Allografts and synthetics lack live cells and are not osteogenic.

Question 40

Articular cartilage is a highly specialized connective tissue adapted to withstand compressive loads. Which structural component of the extracellular matrix is primarily responsible for generating the tissue's swelling pressure and compressive stiffness?

Explanation

Aggrecan, the primary proteoglycan in cartilage, contains highly negatively charged glycosaminoglycans (GAGs). These negative charges repel each other and attract water via the Donnan osmotic effect, creating the swelling pressure that resists compression.

Question 41

Orthopedic stainless steel implants are commonly manufactured from 316L alloy. In this designation, the "L" indicates an important compositional alteration designed to prevent intergranular corrosion. What does the "L" stand for?

Explanation

The 'L' in 316L stands for Low carbon content (less than 0.03%). Minimizing carbon prevents the formation of chromium carbides at the grain boundaries, which would otherwise deplete chromium and lead to intergranular corrosion.

Question 42

When two dissimilar metals are placed in physical contact within the electrolytic environment of the human body, galvanic corrosion may occur. Which of the following implant combinations represents the greatest risk for severe galvanic corrosion?

Explanation

Stainless steel and titanium are far apart on the anodic index. Mixing them (e.g., a stainless steel screw in a titanium plate) creates a strong electrochemical cell, leading to accelerated pitting and corrosion of the less noble stainless steel.

Question 43

A ligament subjected to a constant, prolonged tensile load demonstrates a gradual increase in length over time. Which of the following terms best describes this biomechanical property?

Explanation

Creep is defined as the progressive deformation of a viscoelastic material over time under a constant load. Stress relaxation, conversely, is the decrease in stress over time when the material is held at a constant length.

Question 44

A 45-year-old patient receives a cementless total hip arthroplasty. To minimize stress shielding of the proximal femur, the femoral stem should be manufactured from a material with a modulus of elasticity closest to that of cortical bone. Which of the following materials has the lowest modulus of elasticity?

Explanation

Titanium alloy has a lower modulus of elasticity (approx 110 GPa) compared to stainless steel (approx 200 GPa) and cobalt-chromium (approx 210 GPa). This makes it biomechanically closer to cortical bone (approx 15-20 GPa), effectively reducing proximal femoral stress shielding.

Question 45

In total joint arthroplasty, highly cross-linked ultra-high molecular weight polyethylene (UHMWPE) is commonly used. What is the primary intended effect of increasing the cross-linking of UHMWPE, and what is a known negative biomechanical consequence?

Explanation

Highly cross-linked polyethylene significantly reduces adhesive and abrasive wear rates in joint replacements. However, this cross-linking process reduces the material's fracture toughness and ductility, making it more susceptible to fatigue crack propagation.

Question 46

A 68-year-old woman is prescribed alendronate for the treatment of severe osteoporosis. Nitrogen-containing bisphosphonates inhibit bone resorption primarily by blocking which of the following enzymes in the osteoclast?

Explanation

Nitrogen-containing bisphosphonates inhibit farnesyl pyrophosphate synthase in the mevalonate pathway. This prevents the prenylation of small GTPases essential for osteoclast function and survival, ultimately leading to osteoclast apoptosis.

Question 47

During the incorporation of a massive structural cortical bone allograft, the process of creeping substitution is initiated. Which of the following biological events must occur first to allow creeping substitution to proceed?

Explanation

The incorporation of a cortical bone allograft begins with host vascular invasion into the existing haversian systems of the graft. This revascularization is requisite for the subsequent influx of osteoclasts to resorb dead bone, followed by osteoblastic bone formation.

Question 48

When inserting a cortical screw for fracture fixation, the ultimate pull-out strength of the screw is directly proportional to which of the following design parameters?

Explanation

The pull-out strength of a screw is directly proportional to its outer (thread) diameter and the total length of thread engagement in the bone. It is inversely proportional to the pitch (the distance between adjacent threads).

Question 49

Sclerostin is a key regulatory protein in bone metabolism and is the target of the monoclonal antibody romosozumab. Sclerostin exerts its primary osteo-regulatory effect by inhibiting which of the following intracellular signaling pathways?

Explanation

Sclerostin is produced by osteocytes and binds to LRP5/6 receptors on osteoblasts to inhibit the canonical Wnt/beta-catenin signaling pathway. Inhibition of this pathway decreases osteoblastic differentiation and bone formation.

Question 50

Recombinant human bone morphogenetic protein-2 (rhBMP-2) is utilized to enhance bone healing in specific fusion environments. BMPs transmit their osteoinductive intracellular signals primarily through which of the following secondary messengers?

Explanation

BMPs bind to cell surface serine/threonine kinase receptors, which subsequently phosphorylate intracellular Smad proteins (typically Smad 1, 5, and 8). The resulting Smad complex translocates to the nucleus to initiate the transcription of osteogenic genes.

Question 51

On a stress-strain curve representing a typical human tendon, a non-linear "toe region" is observed at very low strains. This specific region primarily represents which of the following microscopic events?

Explanation

The "toe region" of the tendon stress-strain curve represents the initial straightening out, or "uncrimping," of the wavy collagen fibers as tension is applied. Once the fibers are fully straightened, the curve enters the steeper, linear elastic region.

Question 52

According to Perren's strain theory of fracture healing, the mechanical environment dictates the type of tissue that forms in a fracture gap. Which type of tissue will predominantly form if the interfragmentary strain is between 2% and 10%?

Explanation

According to Perren's strain theory, lamellar bone forms only under very low strain (<2%). When the interfragmentary strain is intermediate (between 2% and 10%), fibrocartilage (soft callus) formation is tolerated and promotes secondary fracture healing.

Question 53

Polymethylmethacrylate (PMMA) bone cement is widely used in arthroplasty for implant fixation. The primary mechanism by which PMMA provides stability to the implant is best described as:

Explanation

PMMA bone cement does not possess adhesive properties nor does it chemically bond to host bone or metal implants. It functions essentially as a grout, providing rigid fixation via an intricate mechanical interlock by penetrating the trabecular interstices of cancellous bone.

Question 54

A 55-year-old patient with end-stage chronic kidney disease presents with hypocalcemia, osteomalacia, and secondary hyperparathyroidism. This metabolic bone disease is primarily driven by a deficiency in which of the following steps of vitamin D metabolism?

Explanation

In severe chronic kidney disease, the kidneys have a drastically reduced capacity to produce the enzyme 1-alpha-hydroxylase. This leads to an inability to convert 25-hydroxyvitamin D into its active form, 1,25-dihydroxyvitamin D, resulting in poor calcium absorption and secondary hyperparathyroidism.

Question 55

During a complex revision trauma surgery, a surgeon considers using a stainless steel screw to secure a cobalt-chromium plate. This practice is contraindicated due to the risk of accelerated degradation of the less noble metal when bathed in host body fluids. This phenomenon is termed:

Explanation

Galvanic corrosion occurs when two dissimilar metals are in direct physical contact within an electrolytic environment, such as human tissue fluid. The less noble metal acts as an anode and undergoes accelerated oxidative degradation.

Question 56

Cortical bone exhibits distinct mechanical properties depending on the direction of the applied load. For instance, it is significantly stronger in longitudinal compression than in transverse tension. This structural biomechanical property is known as:

Explanation

Anisotropy describes a material that exhibits different mechanical properties depending on the direction of the applied load. Cortical bone is highly anisotropic because its osteonal structure aligns to optimally resist physiological longitudinal compressive loads.

Question 57

During the reparative phase of secondary fracture healing, the soft fibrocartilaginous callus is gradually replaced by a hard, bony callus. This specific tissue transition occurs primarily via which of the following biological mechanisms?

Explanation

Secondary fracture healing proceeds through the formation of a cartilage intermediate known as a soft callus. This cartilaginous soft callus is subsequently converted into a bony hard callus primarily through the process of endochondral ossification.

Question 58

Osteoprotegerin (OPG) plays a critical role in maintaining bone mass and preventing excessive resorption. What is the primary molecular function of OPG within the bone remodeling microenvironment?

Explanation

Osteoprotegerin (OPG) is secreted by osteoblasts and functions as a soluble decoy receptor that binds to RANKL. By sequestering RANKL, OPG prevents it from interacting with the RANK receptor on osteoclast precursors, thereby halting osteoclast differentiation and activity.

Question 59

Which of the following noncollagenous proteins is the most abundant in the mature extracellular bone matrix, is highly dependent on vitamin K for its synthesis, and regulates mineralization by binding directly to calcium?

Explanation

Osteocalcin is the most abundant noncollagenous protein in bone and is synthesized exclusively by mature osteoblasts. Its production requires vitamin K-dependent gamma-carboxylation, which allows it to strongly bind calcium and regulate hydroxyapatite crystal growth.

Question 60

A 72-year-old woman with severe osteoporosis and a recent vertebral compression fracture is being evaluated for teriparatide therapy. Which of the following elements of her medical history represents an absolute contraindication to the use of this medication?

Explanation

Teriparatide is an anabolic recombinant parathyroid hormone (PTH) analog. It carries a black box warning and is strictly contraindicated in patients with an increased baseline risk of osteosarcoma, which includes a history of prior skeletal radiation therapy or Paget's disease.

Question 61

The remarkable compressive stiffness and load-bearing capacity of articular cartilage are primarily generated by which of the following molecular interactions within its extracellular matrix?

Explanation

Articular cartilage achieves its profound compressive stiffness largely due to the high density of negatively charged sulfate and carboxyl groups on glycosaminoglycans (GAGs). These fixed negative charges create strong electrostatic repulsion and a high Donnan osmotic swelling pressure that fiercely resists compression.

Question 62

Under high-load, low-velocity conditions (such as sustained standing), boundary lubrication is the predominant mode of tribological protection in a healthy synovial joint. Which of the following molecules is primarily responsible for mediating boundary lubrication at the articular surface?

Explanation

Lubricin, also known as proteoglycan 4 (PRG4), is a mucinous glycoprotein synthesized by superficial zone chondrocytes and synovial cells. It binds directly to the articular cartilage surface, providing essential boundary lubrication that dramatically reduces friction under high-load, low-speed conditions.

Question 63

An orthopaedic surgeon decides to use a thicker plate to stabilize a comminuted fracture. According to the biomechanical principles of rectangular plates, if the thickness of the plate is doubled, how does its bending stiffness change?

Explanation

The bending stiffness (area moment of inertia) of a rectangular plate is proportional to its width and to the cube of its thickness (h^3). Therefore, doubling the thickness increases the bending stiffness by a factor of 8 (2^3).

Question 64

Highly cross-linked ultra-high molecular weight polyethylene (UHMWPE) is frequently used in total joint arthroplasty to reduce wear debris. What mechanical property is most significantly decreased as a direct result of increasing the cross-linking in UHMWPE?

Explanation

While highly cross-linking UHMWPE significantly reduces adhesive and abrasive wear, it simultaneously decreases its ductility, toughness, and resistance to fatigue crack propagation. This makes it more susceptible to fracture in high-stress applications or thin liners.

Question 65

A 65-year-old woman is prescribed denosumab for osteoporosis. This medication works by inhibiting bone resorption through a mechanism that most closely mimics which of the following endogenous molecules?

Explanation

Denosumab is a monoclonal antibody that binds to RANKL, preventing it from interacting with the RANK receptor on osteoclasts. This mechanism effectively mimics the action of endogenous osteoprotegerin (OPG), which acts as a decoy receptor for RANKL.

Question 66

A massive structural cortical allograft is used for reconstruction after tumor resection. During the process of creeping substitution, at what postoperative time frame does the graft typically experience its maximum mechanical weakness?

Explanation

Structural cortical allografts undergo creeping substitution, where osteoclastic resorption precedes osteoblastic bone formation. This resorptive phase causes an increase in porosity, resulting in maximum mechanical weakness between 6 to 12 months post-implantation, increasing the risk of fracture.

Question 67

Stress shielding in cementless femoral stems leads to proximal bone loss. Which of the following implant materials has a Young's modulus closest to that of cortical bone, thereby theoretically minimizing stress shielding?

Explanation

Titanium alloy has a Young's modulus (approx. 110 GPa) that is lower than Cobalt-Chromium (approx. 210 GPa) and Stainless Steel (approx. 200 GPa), making it closer to that of cortical bone (approx. 15-20 GPa). This reduced stiffness mismatch helps decrease stress shielding.

Question 68

When evaluating the pullout strength of a cortical screw, which of the following design modifications will yield the greatest increase in pullout strength?

Explanation

Screw pullout strength is most heavily influenced by the outer diameter, thread depth, thread pitch, and the length of engagement in the bone. Increasing the outer diameter has the most profound effect on maximizing pullout resistance.

Question 69

A 55-year-old patient receives a recombinant human Bone Morphogenetic Protein (rhBMP-2) graft during a spinal fusion. Which intracellular signaling molecules are directly phosphorylated after BMP-2 binds to its cell surface receptors?

Explanation

BMP-2 binds to serine/threonine kinase receptors, leading to the intracellular phosphorylation of Smad 1, 5, and 8. These receptor-regulated Smads then form a complex with Smad 4 and translocate to the nucleus to induce osteogenic gene transcription.

Question 70

A 70-year-old woman is treated with intermittent low-dose teriparatide. The anabolic effect of this drug on bone is primarily mediated by the stimulation of which of the following?

Explanation

Teriparatide (recombinant PTH 1-34) given intermittently has an anabolic effect by stimulating osteoblast differentiation, activity, and extending their lifespan by inhibiting osteoblast apoptosis. Continuous PTH, conversely, leads to net bone resorption.

Question 71

Which of the following forms of corrosion is most likely to occur at the modular junction between a titanium femoral stem and a cobalt-chromium femoral head due to variations in local oxygen tension?

Explanation

Crevice corrosion occurs in restricted spaces (like modular tapers) where oxygen depletion prevents the reformation of the protective oxide layer. While galvanic corrosion can also occur with dissimilar metals, taper corrosion is primarily a mechanically assisted crevice corrosion (fretting-crevice mechanism).

Question 72

When testing the viscoelastic properties of cortical bone, how does an increased rate of loading (strain rate) affect the biomechanical behavior of the bone?

Explanation

Because bone is a viscoelastic material, its mechanical properties change with the rate of loading. At higher strain rates (e.g., trauma), bone becomes both stiffer and stronger, absorbing more energy before failure.

Question 73

Polymethylmethacrylate (PMMA) bone cement is primarily utilized in arthroplasty for implant fixation. Which of the following best describes its mechanical characteristics?

Explanation

PMMA does not act as an adhesive; it provides fixation via mechanical interlock. It is highly resistant to compressive forces but is notably weak in tension and shear, and it undergoes an exothermic reaction, shrinking slightly during polymerization.

Question 74

During the soft callus phase of secondary fracture healing, the initial cartilaginous matrix is predominantly composed of which type of collagen?

Explanation

The soft callus is a cartilaginous intermediate in endochondral fracture healing and is predominantly composed of Type II collagen. As the soft callus is replaced by hard callus (woven bone), Type I collagen becomes predominant.

Question 75

A surgeon decides to treat a nonunion with distraction osteogenesis. The process of bone formation during distraction osteogenesis occurs primarily through which mechanism?

Explanation

Distraction osteogenesis heals primarily through intramembranous ossification, where bone forms directly from mesenchymal cells without a cartilaginous intermediate. A stable mechanical environment and adequate vascularity are essential for this to occur.

Question 76

In the stress-strain curve of a normal human ligament, the initial 'toe region' is characterized by low stiffness. What microscopic structural change corresponds to this region?

Explanation

The non-linear 'toe region' of a ligament or tendon stress-strain curve occurs at low strains. It represents the straightening (uncrimping) of the resting, wavy collagen fibers before they begin to stretch elastically.

Question 77

Romosozumab is a bone-building medication recently approved for severe osteoporosis. It exerts its dual effect of increasing bone formation and decreasing bone resorption by binding to and inhibiting:

Explanation

Romosozumab is a monoclonal antibody that binds to and inhibits sclerostin. Sclerostin is an inhibitor of the Wnt/beta-catenin signaling pathway, so its inhibition leads to increased osteoblastic bone formation and secondarily decreased bone resorption.

Question 78

Ceramic-on-ceramic bearings in total hip arthroplasty are chosen for their excellent wear properties. However, a specific complication unique to ceramic bearings is:

Explanation

Ceramic bearings are highly wettable and hard, leading to the lowest wear rates of all bearing surfaces. However, they are brittle (lacking toughness) and have a risk of catastrophic fracture, as well as the unique phenomenon of 'squeaking'.

Question 79

In designing an intramedullary nail, if the solid cross-sectional radius of the nail is increased by 10%, how is its torsional rigidity affected?

Explanation

The torsional rigidity of a solid cylinder is proportional to the polar moment of inertia, which varies with the radius to the fourth power (r^4). An increase of 10% (1.1) results in a rigidity of 1.1^4 = 1.4641, which is approximately a 46% increase.

Question 80

Passivation is a manufacturing process used to increase the corrosion resistance of orthopedic metal implants. For stainless steel implants, this process creates a protective surface layer primarily composed of:

Explanation

Passivation of stainless steel (and cobalt-chromium alloys) involves treating the metal with nitric acid to produce a highly adherent, protective, and self-healing layer of chromium oxide on the surface. This layer significantly improves corrosion resistance.

Question 81

Vitamin D plays a critical role in calcium homeostasis and bone health. The final step in the formation of the most active form of Vitamin D (1,25-dihydroxyvitamin D3) occurs in the kidney and is catalyzed by which enzyme?

Explanation

Cholecalciferol is hydroxylated in the liver to 25-hydroxyvitamin D3 by 25-hydroxylase. The final conversion to the active 1,25-dihydroxyvitamin D3 occurs in the kidney, catalyzed by 1-alpha-hydroxylase, an enzyme stimulated by PTH.

Question 82

A fracture is rigidly fixed with a compression plate, achieving absolute stability and a gap of less than 0.01 mm. Under these conditions, the fracture will heal via primary bone healing, which is microscopically characterized by:

Explanation

Primary (direct) bone healing occurs under conditions of absolute stability and minimal gap strain. It bypasses callus formation and relies on remodeling units (cutting cones), where osteoclasts drill across the fracture line followed closely by bone-depositing osteoblasts.

Question 83

A 65-year-old woman is treated with a monoclonal antibody for osteoporosis that mimics the physiological role of osteoprotegerin (OPG). What is the primary mechanism of action of this medication?

Explanation

Denosumab is a monoclonal antibody that acts like OPG by binding to RANKL, preventing it from activating RANK on osteoclasts. This inhibits osteoclast survival and function, thereby decreasing bone resorption.

Question 84

When selecting an intramedullary nail for a tibial shaft fracture, the surgeon considers the material properties. Which of the following orthopedic biomaterials has a Young's modulus closest to that of cortical bone?

Explanation

Titanium has a Young's modulus of approximately 110 GPa, which is closer to cortical bone (15-30 GPa) compared to stainless steel (200 GPa) or cobalt-chromium (220 GPa). This closer match helps reduce stress shielding.

Question 85

Highly cross-linked ultra-high-molecular-weight polyethylene (UHMWPE) is widely used in total hip arthroplasty to reduce wear. What is the primary trade-off regarding the mechanical properties of UHMWPE when the degree of cross-linking is increased?

Explanation

Increasing cross-linking in UHMWPE significantly improves its resistance to adhesive and abrasive wear. However, this comes at the expense of decreased fatigue resistance, ductility, and fracture toughness.

Question 86

A 35-year-old man undergoes closed intramedullary nailing for a transverse midshaft femur fracture. Callus formation is noted at 6 weeks. Which of the following mechanical environments optimally promotes this type of secondary bone healing?

Explanation

Secondary bone healing (callus formation) is stimulated by moderate interfragmentary strain (2% to 10%). Strain below 2% leads to primary bone healing, while strain above 10% promotes fibrous tissue or nonunion.

Question 87

A ligament graft used for anterior cruciate ligament (ACL) reconstruction elongates over time when subjected to a constant physiological load. This time-dependent biomechanical property is best described as:

Explanation

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

Question 88

A 50-year-old active man receives a ceramic-on-ceramic total hip arthroplasty. Which of the following is a recognized complication specific to this bearing surface compared to metal-on-polyethylene?

Explanation

Ceramic-on-ceramic bearings offer excellent wear properties but carry unique risks, including audible squeaking and catastrophic bearing fracture. They do not typically generate large volumes of osteolytic submicron particles.

Question 89

A surgeon decides to use a solid intramedullary nail instead of a hollow nail of the same outer diameter. According to the polar moment of inertia, how does increasing the inner diameter (hollowing the nail) affect its torsional rigidity?

Explanation

The polar moment of inertia for a hollow cylinder is proportional to the difference between the outer radius to the fourth power and the inner radius to the fourth power. Therefore, hollowing a solid nail decreases its torsional rigidity proportionally to the inner radius to the fourth power.

Question 90

Demineralized bone matrix (DBM) is used during a spinal fusion. Which of the following components is primarily responsible for the osteoinductive properties of this graft material?

Explanation

Demineralized bone matrix (DBM) is primarily an osteoinductive graft. Its osteoinductive properties are derived from retained bone morphogenetic proteins (BMPs) exposed during the acid demineralization process.

Question 91

A patient requires revision of a femoral shaft fracture nonunion. The original stainless steel plate is left in place, and a titanium screw is used for supplementary fixation. Which of the following processes is most likely to occur at the interface of these two metals?

Explanation

Galvanic corrosion occurs when two dissimilar metals (e.g., stainless steel and titanium) are placed in contact within an electrolytic medium like body fluid. This leads to accelerated corrosion of the less noble metal.

Question 92

During the inflammatory phase of fracture healing, which of the following cell types is primarily responsible for the initial clearing of necrotic tissue and secretion of cytokines to recruit mesenchymal stem cells?

Explanation

Macrophages play a crucial role during the early inflammatory phase of fracture healing. They clear necrotic debris and secrete key cytokines that recruit mesenchymal stem cells to the fracture site.

Question 93

Calcium phosphate cements are often used to fill metaphyseal bone defects. Which of the following statements best describes their primary biomechanical advantage and clinical limitation?

Explanation

Calcium phosphate cements are osteoconductive and provide excellent compressive strength, making them ideal for filling metaphyseal voids. However, they are brittle and have poor shear and tensile strength.

Question 94

A novel therapeutic agent for osteoporosis works by binding to sclerostin. What is the downstream effect of this medication on bone metabolism?

Explanation

Medications like romosozumab bind and inhibit sclerostin. Since sclerostin naturally inhibits the Wnt/beta-catenin pathway, its blockade leads to increased osteoblast proliferation and bone formation.

Question 95

A patient presents with hypophosphatemic rickets. Laboratory evaluation reveals elevated levels of Fibroblast Growth Factor 23 (FGF23). What is the primary physiological effect of FGF23 on the kidneys?

Explanation

FGF23 is secreted by osteocytes and acts on the kidneys to induce phosphaturia. It also decreases active Vitamin D synthesis by directly inhibiting the enzyme 1-alpha-hydroxylase.

Question 96

Polymethylmethacrylate (PMMA) is utilized for implant fixation in a cemented total knee arthroplasty. Which of the following statements regarding the properties of PMMA is accurate?

Explanation

PMMA bone cement provides initial implant fixation by acting as a grout via mechanical interlock with cancellous bone, rather than a true chemical adhesive. Its setting reaction is highly exothermic, and additives like barium sulfate decrease its mechanical strength.

Question 97

A patient with a cementless total hip arthroplasty shows significant proximal femoral bone loss at 5-year follow-up. The femoral stem is fully porous-coated and well-fixed distally. This phenomenon is a clinical manifestation of:

Explanation

Wolff's law states that bone remodels in response to the mechanical stresses placed upon it. A distally fixed, stiff femoral stem unloads the proximal femur, leading to adaptive bone resorption known as stress shielding.

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

AAOS Basic Science MCQs (Set 4): Bone Physiology, Biomechanics, & Biomaterials

Key Takeaway

AAOS Basic Science MCQs (Set 4): Bone Physiology, Biomechanics, & Biomaterials

Chapter Index