AAOS & ABOS Basic Science MCQs: Anatomy & Bone Healing

Question 1

An otherwise healthy 75-year-old man has a painful mass in the popliteal fossa of his right knee. A lateral radiograph of the knee, a CT scan of the distal femur, and a histopathologic specimen are shown in Figures 13a through 13c. Management should consist of

Explanation

The patient has a parosteal osteosarcoma of the distal femur. The findings of mild knee pain, radiographic evidence of a radiodense mass involving the parosseous space or surface of the distal femur, and histologic findings of a spindle cell lesion forming immature osteoid with little to no necrosis most likely suggest a parosteal osteosarcoma. The treatment of choice is surgical resection. Okada K, Frassica FJ, Sim FH, Beabout JW, Bond JR, Unni KK: Parosteal osteosarcoma: A clinicopathological study. J Bone Joint Surg Am 1994;76:366-378.

Question 2

What type of metastatic tumor most often has a lytic radiographic appearance?

Explanation

Lung carcinoma most often has a lytic radiographic appearance. Bladder and prostate carcinoma are usually blastic. Breast carcinoma can be both blastic and lytic. Thyroid carcinoma may be difficult to visualize radiographically and may be seen only on MRI scans. Metastatic osteosarcomas typically produce bone. Frassica FJ, Frassica DA, McCarthy EF, Riley LH III: Metastatic bone disease: Evaluation, clinicopathologic features, biopsy, fracture risk, nonsurgical treatment, and supportive management. Instr Course Lect 2000;49:453-459.

Question 3

A 6-year-old boy has leg pain. A radiograph, MRI, CT, and bone scans, and a biopsy specimen are shown in Figures 14a through 14e. What is the most likely diagnosis?

Explanation

From an imaging point of view, all of the diagnoses are possible. Biopsy results and cultures are necessary to make the diagnosis. The biopsy specimen shows inflammatory cells and necrotic bone, consistent with osteomyelitis. Fletcher BD, Hanna SL: Pediatric musculoskeletal lesions simulating neoplasms. Magn Reson Imaging Clin N Am 1996;4:721-747.

Question 4

A 13-year-old girl has had right groin pain for the past 3 weeks. She denies any history of trauma. Examination of the hip reveals no palpable soft-tissue mass or lymphadenopathy, and there is full range of motion. A plain radiograph and MRI scan are shown in Figures 15a and 15b. Biopsy specimens are shown in Figures 15c and 15d. What is the most likely diagnosis?

Explanation

Fibrous dysplasia frequently occurs in the proximal femur. Microfractures and remodeling can lead to the classic "Shepherd's crook deformity." The lesion has a ground-glass appearance on plain radiographs. The histologic appearance shows proliferating fibroblasts in a loose spindle cell background. Dysplastic metaplastic trabeculae are arranged in an irregular or "Chinese letter" appearance. Enneking WF, Gearen PF: Fibrous dysplasia of the femoral neck: Treatment by cortical bone grafting. J Bone Joint Surg Am 1986;68:1415-1422.

Question 5

What process is often found associated with other neoplasms?

Explanation

Aneurysmal bone cyst may be either a "pattern" or a "diagnosis." Therefore, aneurysmal bone cyst should be viewed as a diagnosis of exclusion. Hemorrhage into a variety of primary bone lesions (eg, giant cell tumor of bone, chondroblastoma, osteoblastoma, fibrous dysplasia, osteosarcoma, or vascular neoplasms) may result in intralesional, membrane-bone blood-filled cysts. Such secondary changes may be confused with aneurysmal bone cyst, resulting in inappropriate therapy because assessment should be focused on identifying the underlying primary process. The entire specimen should be examined histologically in an effort to locate an underlying primary bone tumor. Bonakdarpour A, Levy WM, Aegerter E: Primary and secondary aneurysmal bone cyst: A radiological study of 75 cases. Radiology 1978;126:75-83. Levy WM, Miller AS, Bonakdarpour A, Aegerter E: Aneurysmal bone cyst secondary to other osseous lesions: Report of 57 cases. Am J Clin Pathol 1975;63:1-8.

Question 6

A 14-year-old girl reports hip pain that is exacerbated by weight bearing. A radiograph and biopsy specimen are shown in Figures 16a and 16b. The best course of management should be

Explanation

The patient has a unicameral bone cyst. Because the subtrochanteric part of the femur is a high-stress region, the treatment of choice is bone curettage and grafting. Azouz EM, Karamitsos C, Reed MH, Baker L, Kozlowski K, Hoeffel JC: Types and complications of femoral neck fractures in children. Pediatr Radiol 1993;23:415-420.

Question 7

A 37-year-old man has left shoulder pain and weakness. Coronal T1- and axial T2-weighted MRI scans are shown in Figures 17a and 17b. The biopsy specimen is shown in Figure 17c. What is the most likely diagnosis?

Explanation

A ganglion cyst arising from the posterior labrum of the shoulder is a known cause of suprascapular nerve impingement. The MRI characteristics of low-signal intensity on T1 and high-signal intensity on T2 are seen in lesions with a high fluid content. The histology shows a cavity with a thin fibrous lining. Ferrick MR, Marzo JM: Suprascapular entrapment neuropathy and ganglion cysts about the shoulder. Orthopedics 1999;22:430-434.

Question 8

Of the following factors, which is considered the most important prognostic indicator in soft-tissue sarcomas?

Explanation

Histologic grade, the presence or absence of metastatic disease, and tumor size are important prognostic factors. Of the available choices, however, the size of the sarcoma is the most important prognostic indicator. A tumor size of greater than 5 cm is a more important prognostic factor than tumor location. Patients with sarcomas that measure 5 cm or less have nearly identical 3-year survival rates regardless of whether the tumor is subcutaneous or deep. Histologic grade (high versus low) is an important factor. However, histologic subtype frequently is not as important a factor as tumor size.

Question 9

Radiographs of a 15-year-old girl with knee pain reveal a radiopaque lesion of the distal femoral metaphysis and epiphysis with a small associated soft-tissue mass. A biopsy specimen shows osteoid and pleomorphic cells with multiple mitotic figures. Staging studies show no other sites of disease. Treatment should consist of

Explanation

Based on the location and extent of the tumor, most patients are candidates for limb salvage surgery. The patient has an osteosarcoma; therefore, the treatment of choice is preoperative chemotherapy, wide resection, and reconstruction. There is no difference in survival rates between patients who undergo limb salvage surgery and those who undergo amputation. Curettage and placement of methylmethacrylate is used to treat benign lesions of bone and would not be appropriate in this patient.

Question 10

What form of fixation is associated with the highest incidence of osseous union when using segmental allograft reconstruction following tumor resection?

Explanation

Plate and screw fixation of allograft is associated with the highest incidence of union but also the highest prevalence of allograft fracture. Conversely, intramedullary fixation is associated with a higher incidence of nonunion but fewer fractures. Step-cut osteotomies are not associated with a higher incidence of union.

Question 11

A 51-year-old woman has had progressively increasing right knee pain for the past 6 months. She has a history of metastatic renal cell carcinoma to the lung and the skeletal system. Radiographs are seen in Figures 18a and 18b. The next step in management of the right distal femur lesion should consist of

Explanation

In a patient with known metastatic disease, the surgeon must rule out additional lesions throughout the femur prior to surgical management. Lesions located in the diaphysis or in the peritrochanteric region may influence the surgical procedure. Frassica FJ, Gitelis S, Sim FH: Metastatic bone disease: General principles, pathophysiology, evaluation, and biopsy. Instr Course Lect 1992;41:293-300.

Question 12

Following resection of malignant tumors, complications related to endoprosthetic reconstruction are most common in what anatomic location?

Explanation

It is generally accepted that reconstructions of the proximal tibia are associated with the highest incidence of failure, probably because of poor soft-tissue coverage, the need for extensor mechanism reconstruction, and other anatomic issues. It also may be related to the fact that patients with tumors of the proximal tibia, in general, have a better prognosis and better survival rates than patients with tumors located elsewhere in the body. Reconstructions of the proximal humerus may be more durable because they are not involved in weight-bearing activities.

Question 13

Figures 19a and 19b show the AP and lateral radiographs of an 18-year-old man who has had knee pain for 3 months. Figure 19c shows a histopathologic photomicrograph of the biopsy specimen. Which of the following factors is most likely to affect his survival?

Explanation

The presence of metastatic disease at diagnosis is the most significant prognostic factor in a patient with osteosarcoma. Limb-sparing surgery, when compared with amputation, is not noted to be associated with any difference in the rate of survival. The presence of a mutated p53 gene has been noted in a minority of patients with osteosarcoma; however, the prognostic importance on an individual basis is unclear. Tumor location and a relative poor response to chemotherapy would be secondary to the presence of metastatic disease in estimating a patient's prognosis. Weis L: Common malignant bone tumors: Osteosarcoma, in Simon MA, Springfield DS (eds): Surgery for Bone and Soft Tissue Tumors. Philadelphia, PA, Lippincott-Raven, 1998, pp 265-274.

Question 14

What anatomic site is considered at highest risk for pathologic fracture?

Explanation

The subtrochanteric femur has been identified as an anatomic site that is particularly prone to pathologic fracture. An avulsion fracture of the lesser trochanter is a sign of impending femoral fracture. While the other anatomic locations are also frequently involved in metastatic bone disease, pathologic fractures occur less commonly. Simon MA, Springfield DS, et al: Surgery for Bone and Soft Tissue Tumors. Philadelphia, PA, Lippincott Raven, 1998, p 683.

Question 15

What clinical finding is associated with the least favorable prognosis in an adolescent patient who has been diagnosed with a high-grade osteosarcoma of the distal femur?

Explanation

The presence of synchronous bone disease in young patients carries a dismal prognosis, one that is even worse than the presence of resectable pulmonary metastasis. Many osteosarcomas cross the physis; therefore, this has not been shown to be of prognostic importance. Similarly, the presence of the soft-tissue mass has less prognostic significance.

Question 16

A 65-year-old man has a painful mass of the middle finger. A clinical photograph, lateral radiograph, coronal MRI scan, and biopsy specimen are seen in Figures 20a through 20d. What is the most likely diagnosis?

Explanation

Although the degeneration of an isolated benign cartilaginous lesion into a chondrosarcoma is rare, it occurs in roughly 10% of patients with Ollier's disease. Pain is the most common symptom of chondrosarcoma. The treatment of low-grade chondrosarcoma ranges from intralesional excision to wide amputation. The intent of the surgery is to remove all the disease to decrease the chance of local recurrence. Lee FY, Mankin HJ, Fondren G, et al: Chondrosarcoma of bone: An assessment of outcome. J Bone Joint Surg Am 1999;81:326-338.

Question 17

Figures 21a and 21b show the radiograph and CT scan of a 14-year-old patient with thigh pain. The next most appropriate step in management should consist of

Explanation

The radiographs show increased density and reactive bone formation. A faint nidus can be seen on the radiograph but is obvious on the CT scan. The initial therapy for an osteoid osteoma should be oral anti-inflammatory drugs. A biopsy or SPECT bone scan is not indicated because the osteoid osteoma is clearly seen on the CT scan. If the patient fails to respond to nonsurgical therapy, CT-guided radiofrequency ablation or surgical excision is indicated depending on the anatomic location. Frassica FJ, Waltrip RL, Sponseller PD, Ma LD, McCarthy EF Jr: Clinicopathologic features and treatment of osteoid osteoma and osteoblastoma in children and adolescents. Orthop Clin North Am 1996;27:559-574. Kneisl JS, Simon MA: Medical management compared with operative treatment for osteoid osteoma. J Bone Joint Surg Am 1992;74:179-185.

Question 18

A 14-year-old boy has a midshaft fibular lesion. Biopsy results are consistent with Ewing's sarcoma. Following induction chemotherapy, local control typically consists of

Explanation

Current treatment regimens for Ewing's sarcoma typically involve induction chemotherapy followed by local control and further chemotherapy. Local control consists of surgery alone, radiation therapy alone, or a combination of the two. In bones that are easily resectable (or expendable) with wide margins, surgery alone is usually recommended. For areas that are unresectable (ie, large, bulky pelvic tumors), radiation therapy alone is sometimes the preferred method of local control. If surgery is chosen and margins are close, radiation therapy can be used as an adjuvant. Amputation rarely is required for an isolated fibular lesion. Observation without adequate local therapy results in local recurrence. Nesbit ME Jr, Gehan EA, Burgert EO Jr, et al: Multimodal therapy for the management of primary, nonmetastatic Ewing's sarcoma of bone: A long-term follow-up of the First Intergroup study. J Clin Oncol 1990;8:1664-1674.

Question 19

An otherwise healthy 65-year-old man reports thigh pain of insidious onset. He states that the pain is increased with weight bearing and also occurs at night. He denies any history of cancer. Radiographs are shown in Figures 22a and 22b. A bone scan shows an isolated lesion. CT scans of the chest and abdominal are negative for any other lesions. Initial management should consist of

Explanation

The patient has a solitary lesion that is at high risk for fracture. While metastatic lesions are most common in this age group, a stabilization procedure is contraindicated until results of a biopsy confirm the presence of a metastasis. Because of the lytic lesion and the associated calcification shown on the radiograph, the most likely diagnosis is a chondrosarcoma. Therefore, any type of stabilization procedure with a rod or plate would compromise a wide surgical excision to remove the entire proximal femur. Radiation therapy and chemotherapy are also contraindicated until biopsy results are obtained. Because of these factors, obtaining a frozen section biopsy specimen is considered the next most appropriate step in management. If a metastatic lesion is confirmed on the frozen section, a stabilization procedure could then be performed under the same anesthetic. Therefore, it is important to have a pathologist available at the time of a biopsy. Frassica FJ, Frassica DA, McCarthy EF, Riley LH III: Metastatic bone disease: Evaluation, clinicopathologic features, biopsy, fracture risk, nonsurgical treatment, and supportive management. Instr Course Lect 2000;49:453-459.

Question 20

What is the most appropriate next step in the work-up of a patient with the asymptomatic lesion shown in Figure 23?

Explanation

The eccentric metaphyseal location, skeletal maturity, narrow zone of transition, and lack of symptoms suggest a benign process and are consistent with a healed nonossifying fibroma. These lesions typically fill in (ossify) with skeletal maturity, eventually remodeling and disappearing. Radiographic monitoring is indicated. Biopsy is not recommended unless the lesion changes radiographically. Marks KE, Bauer TW: Fibrous tumors of bone. Orthop Clin North Am 1989;20:377-393. Bullough PG, Walley J: Fibrous cortical defect and non-ossifying fibroma. Postgrad Med J 1965;41:672-676.

Question 21

A 63-year-woman has an elbow flexion contracture. History reveals that she underwent three previous surgeries to remove a malignant fibrous histiocytoma of the forearm. An MRI scan reveals a locally recurrent tumor at the site of the previous surgery. Which of the following is considered the most predictive factor for local recurrence?

Explanation

The greatest risk factor for local recurrence is an inadequate surgical margin. The tumor grade, histologic subtype, and size are predictive of systemic relapse. Sarcomas that arise in some anatomic sites, such as the forearm or retroperitoneum, may be more difficult to completely resect compared with other sites. The optimum margin is generally considered to be a cuff of normal tissue beyond the tumor. Bell RS, O'Sullivan B, Liu FF, et al: The surgical margin in soft-tissue sarcoma. J Bone Joint Surg Am 1989;71:370-375. Sadoski C, Suit HD, Rosenberg A, Mankin H, Efird J: Preoperative radiation, surgical margins, and local control of extremity sarcomas of soft tissues. J Surg Oncol 1993;52:223-230.

Question 22

A 19-year-old woman has a painful right knee. A radiograph, MRI scan, CT scan, and histopathologic specimen are shown in Figures 24a through 24d. What is the most likely oncologic stage of the lesion?

Explanation

The patient has a high-grade osteosarcoma of the distal femur with a skip lesion, and pulmonary metastasis is seen on the CT scan. This corresponds to a stage III lesion according to the Musculoskeletal Tumor Society System as adopted from Enneking.

Question 23

Calcitonin acts as an antiresorptive agent by

Explanation

Calcitonin is a hormone that binds to osteoclasts and acts to decrease both osteoclast activity and number. Calcitonin is most effective in reducing vertebral compression fractures in high-turnover osteoporosis. It is also effective in treating regional osteoporosis. Because of its analgesic effect, it is helpful in treating painful acute compression fractures associated with osteoporosis.

Question 24

An 8-year-old girl has had a painless enlarging mass of insidious onset in the left thigh for the past 3 weeks. Her mother denies any history of trauma, fever, or disease. Examination reveals a nontender, mobile mass in the left medial thigh. Her gait is normal. Figures 25a through 25d show the frog-lateral radiograph, the axial and coronal T1-weighted MRI scans, and the axial T2-weighted MRI scan. Biopsy results reveal a nonrhabdomyosarcoma soft-tissue sarcoma. The most appropriate treatment should consist of

Explanation

In childhood, the more common soft-tissue sarcomas are rhabdomyosarcoma, synovial sarcoma, and fibrosarcoma. Rhabdomyosarcoma, treated with radiation therapy and chemotherapy, is a round cell tumor and is inconsistent with this patient's histologic findings. Synovial sarcoma can be monophasic or biphasic with both spindle and epithelial-like cells and is associated with the characteristic reciprocal chromosomal translocation of t(x:18)(p11;q11) which is not found in fibrosarcoma. Synovial sarcoma also can be associated with cystic loculated areas best seen in a T2-weighted MRI scan. Nonrhabdomyosarcoma childhood soft-tissue sarcomas are treated with surgical excision in conjunction with chemotherapy and/or radiation therapy. The histology reveals no inflammatory cells to suggest an abscess; therefore, antibiotics and drainage are unnecessary. The MRI scans clearly show a mass of soft tissue and no bone involvement; therefore, proximal femoral resection is not appropriate. Serial observation is not appropriate because of the history of enlargement and insidious onset. Enzinger FM, Weiss SW: Soft Tissue Tumors, ed 3. St Louis, MO, Mosby Year Book, 1995, p 757.

Question 25

Histologically, synovial chondromatosis is characterized by

Explanation

Histologically, there is metaplastic cartilage arising from the synovium. These lobules of zonates hyaline cartilage are of variable size, are embedded within edematous synovium, and protrude into the joint. The lobules calcify and ossify, leading to the characteristic radiographic appearance. Inflammatory synovitis is not characteristic of synovial chondromatosis. The fluid is clear and serosanguin, not blood tinged. Milgram JM: Synovial osteochondromatosis: A histopathological study of thirty cases. J Bone Joint Surg Am 1977;l59:792-801.

Question 26

Increasing the inner (core) diameter of a cortical bone screw, while maintaining the same outer diameter and thread pitch, will have which of the following biomechanical effects?

Explanation

Decreasing thread depth by increasing the core diameter reduces the volume of bone engaged, lowering pullout strength. However, the core diameter dictates the area moment of inertia, meaning a thicker core significantly increases bending strength.

Question 27

What mechanical property of a material is defined by the slope of the linear portion of its stress-strain curve?

Explanation

The modulus of elasticity (Young's modulus) is the measure of a material's stiffness, represented by the slope of the linear (elastic) portion of the stress-strain curve. Toughness is represented by the total area under the entire stress-strain curve.

Question 28

Which of the following fracture fixation constructs relies entirely on primary bone healing for fracture union?

Explanation

Primary bone healing occurs under conditions of absolute stability, achieved with techniques like a lag screw and neutralization plate. This involves direct Haversian remodeling without the formation of a fracture callus.

Question 29

During an anterior (Smith-Petersen) approach to the hip, the internervous plane between the sartorius and tensor fasciae latae is utilized. Which of the following nerves is at greatest risk of iatrogenic injury during the superficial dissection?

Explanation

The lateral femoral cutaneous nerve crosses the sartorius approximately 2 cm distal to the anterior superior iliac spine (ASIS). It is highly susceptible to traction or transection injury during the superficial dissection of the anterior approach to the hip.

Question 30

The pullout strength of a cortical screw is directly proportional to which of the following variables?

Explanation

Screw pullout strength is directly proportional to the outer (thread) diameter, the length of thread engagement, and the shear strength of the surrounding bone. It is inversely proportional to the thread pitch.

Question 31

Which of the following represents the correct sequence of events during the incorporation of a structural cortical bone allograft?

Explanation

Cortical allograft incorporation occurs via a process known as creeping substitution. This process begins with osteoclastic resorption of the graft material, followed subsequently by osteoblastic bone formation.

Question 32

A patient with severe right hip osteoarthritis uses a cane to reduce joint pain. To most effectively minimize the joint reaction force across the right hip, how should the cane be utilized?

Explanation

Using a cane in the contralateral (left) hand reduces the joint reaction force on the right hip by counteracting the body weight moment. This significantly decreases the force required by the right hip abductor muscles, which are the primary contributors to hip joint reaction forces.

Question 33

In a posterior approach to the humerus, the radial nerve is identified crossing the posterior humerus. At what average distance proximal to the olecranon fossa does the radial nerve cross the posterior aspect of the humerus?

Explanation

The radial nerve crosses the posterior humerus in the spiral groove approximately 14 to 15 cm proximal to the radiocapitellar joint (or olecranon fossa) and roughly 20 cm distal to the acromion.

Question 34

In a metal-on-polyethylene total hip arthroplasty, which of the following wear mechanisms is primarily responsible for the generation of submicron polyethylene particles that lead to macrophage-mediated osteolysis?

Explanation

Adhesive wear is the primary mechanism generating submicron ultra-high-molecular-weight polyethylene (UHMWPE) particles in total joint arthroplasty. These submicron particles are phagocytosed by macrophages, initiating an inflammatory cascade that leads to osteolysis.

Question 35

Which intracellular signaling pathway is directly activated by the binding of Bone Morphogenetic Proteins (BMPs) to their cell surface receptors during osteoinduction?

Explanation

BMPs bind to serine/threonine kinase receptors, leading to the phosphorylation and activation of Smad 1, 5, and 8. These activated Smads form a complex with Smad 4, which translocates to the nucleus to regulate the transcription of osteogenic genes.

Question 36

A solid titanium intramedullary nail's resistance to bending (bending rigidity) is directly proportional to its radius raised to what power?

Explanation

The bending rigidity of a solid cylindrical implant is proportional to its area moment of inertia, which scales with the radius to the fourth power (r^4). Therefore, small increases in nail diameter drastically increase bending stiffness.

Question 37

During surgical reconstruction of the posterolateral corner of the knee, the fibular collateral ligament (FCL) and popliteus tendon (PT) are identified. What is the anatomic relationship of their femoral attachments?

Explanation

On the lateral femoral condyle, the attachment of the fibular collateral ligament (FCL) is consistently located proximal and posterior to the attachment of the popliteus tendon.

Question 38

According to Perren's strain theory, what type of tissue is expected to form in a fracture gap when the interfragmentary strain is between 2% and 10%?

Explanation

Perren's strain theory dictates that tissue tolerance limits healing. Fibrocartilage can tolerate strains up to 10-15%, making it the predominant tissue in a 2-10% strain environment. Primary bone healing requires less than 2% strain.

Question 39

A surgeon considers using a stainless steel screw through a titanium plate. What biomechanical principle strongly discourages this specific practice?

Explanation

Galvanic corrosion occurs when two dissimilar metals are placed in physical contact within a conductive fluid medium, such as body fluids. The less noble metal (in this case, stainless steel) acts as an anode and undergoes accelerated corrosion.

Question 40

In the plantar aspect of the foot, the "Master Knot of Henry" is a critical anatomic landmark where which two structures cross?

Explanation

The Master Knot of Henry is located in the medial plantar midfoot. At this site, the flexor hallucis longus (FHL) tendon crosses dorsal to the flexor digitorum longus (FDL) tendon.

Question 41

Following a displaced, severely comminuted midshaft tibia fracture, the endosteal blood supply is entirely disrupted. Which vascular system provides the initial compensatory blood supply to the healing fracture callus?

Explanation

When the primary endosteal (nutrient artery) blood supply is disrupted by a fracture or intramedullary reaming, the periosteal blood supply undergoes transient hypertrophy. This reversed flow provides the major blood supply to the early healing callus.

Question 42

How does decreasing the "working length" of a bridge plate construct affect its mechanical properties?

Explanation

The working length of a plate is the distance between the two closest screws on either side of the fracture. Decreasing the working length increases the overall construct stiffness, which concentrates stress over a shorter segment of the plate and may increase the risk of implant failure.

Question 43

During a posterior cervical foraminotomy, the surgeon must be acutely aware of the vertebral artery's course. The vertebral artery typically enters the transverse foramen at which cervical level?

Explanation

The vertebral artery typically arises from the first part of the subclavian artery and ascends to enter the transverse foramen of the cervical spine at the C6 level in approximately 90% of individuals.

Question 44

The piezoelectric effect in bone is characterized by which of the following phenomena during mechanical loading?

Explanation

According to the piezoelectric effect, mechanical compression of bone generates an electronegative potential that stimulates osteoblastic bone formation. Conversely, tension generates an electropositive potential favoring osteoclastic resorption.

Question 45

Ligaments exhibit viscoelastic properties, meaning their mechanical behavior is highly dependent on the rate of loading. How does a ligament respond to a rapid, high-velocity load compared to a slow load?

Explanation

Viscoelastic materials like ligaments and bone become stiffer (increased modulus of elasticity) and can absorb more energy before failure when subjected to high loading rates compared to slow loading rates.

Question 46

Which of the following structures is NOT considered a border or content of the rotator interval in the shoulder?

Explanation

The rotator interval is bordered by the supraspinatus superiorly, the subscapularis inferiorly, and the coracoid medially. It contains the coracohumeral ligament, superior glenohumeral ligament, and long head of the biceps; the teres minor is located posteriorly.

Question 47

In the stress-strain curve of a given orthopaedic material, what does the area under the curve in the elastic region represent?

Explanation

The area under the stress-strain curve in the elastic region represents the resilience of the material. The total area under the entire curve (elastic and plastic regions combined) represents the material's toughness.

Question 48

When increasing the outer diameter of a solid intramedullary nail by a factor of 2, the torsional rigidity increases by a factor of:

Explanation

For a solid cylinder, torsional rigidity is proportional to the polar moment of inertia, which scales with the radius to the fourth power (r^4). Therefore, doubling the diameter increases the torsional rigidity by a factor of 16.

Question 49

According to Perren's strain theory, what is the maximum interfragmentary strain compatible with the formation of lamellar bone?

Explanation

Perren's strain theory states that lamellar bone can only form under conditions of low strain, typically less than 2%. Granulation tissue tolerates up to 100% strain, while fibrocartilage tolerates up to 10% strain before tearing.

Question 50

During the reparative phase of secondary fracture healing, which growth factor is primarily responsible for inducing the differentiation of mesenchymal stem cells into chondrocytes?

Explanation

Bone morphogenetic proteins (such as BMP-2) are osteoinductive and strongly induce mesenchymal stem cell differentiation into chondrogenic and osteogenic lineages. TGF-beta stimulates matrix synthesis, and VEGF is essential for angiogenesis.

Question 51

A viscoelastic material subjected to a constant load demonstrates a progressive increase in deformation over time. This phenomenon is known as:

Explanation

Creep is the time-dependent increase in deformation (strain) under a constant load (stress). In contrast, stress relaxation is the decrease in stress over time under a constant state of deformation.

Question 52

Following a displaced diaphyseal fracture of the adult tibia, what is the primary source of blood supply to the healing callus during the early stages of secondary bone healing?

Explanation

Following a fracture, the medullary nutrient artery is often disrupted. The extraosseous periosteal blood supply temporarily becomes the dominant source of blood flow to the healing fracture callus early in the reparative process.

Question 53

Which of the following combinations of orthopaedic implant materials is most likely to result in severe galvanic corrosion when placed in direct contact in vivo?

Explanation

Galvanic corrosion occurs when two dissimilar metals are placed in physical contact in an electrolytic environment. The combination of stainless steel and titanium creates a significant difference in electrochemical potential, leading to severe galvanic corrosion of the less noble metal (stainless steel).

Question 54

The Wnt/beta-catenin signaling pathway plays a crucial role in bone homeostasis. Sclerostin (SOST) regulates bone mass by:

Explanation

Sclerostin is produced by osteocytes and binds to LRP5/6 receptors on osteoblasts, inhibiting the Wnt/beta-catenin signaling pathway. This inhibition leads to decreased osteoblastogenesis and a net reduction in bone formation.

Question 55

In plate osteosynthesis of a diaphyseal fracture, decreasing the "working length" of the plate has what effect on the construct?

Explanation

The working length of a plate is the distance between the two closest screws on either side of the fracture. Decreasing the working length increases the overall stiffness of the construct (both axial and torsional) and decreases interfragmentary strain.

Question 56

The pullout strength of a cortical screw is directly proportional to which of the following geometric parameters?

Explanation

The pullout strength of a screw is directly proportional to the outer (major) diameter of the thread, the length of thread engagement, and the shear strength of the bone. The core (inner) diameter determines the screw's tensile strength and resistance to fatigue failure.

Question 57

During normal gait, the peak joint reaction forces acting on the hip joint occur during which phase of the gait cycle?

Explanation

Peak joint reaction forces across the hip often reach 2.5 to 3 times body weight and occur primarily during the mid-stance phase of gait. This is driven largely by the strong contraction of the hip abductor muscles necessary to stabilize the pelvis.

Question 58

As a fracture callus matures from woven to lamellar bone, what biomechanical change occurs?

Explanation

As woven bone remodels into lamellar bone, the tissue becomes more organized, anisotropic, and highly mineralized. This remodeling phase results in a significant increase in stiffness, ultimate strength, and elastic modulus, while the material becomes relatively more brittle.

Question 59

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

Explanation

The deep (radial) zone of articular cartilage contains collagen fibers oriented perpendicularly to the subchondral bone. It also possesses the highest concentration of proteoglycans, providing maximal resistance to compressive forces.

Question 60

Osteoclasts resorb bone by creating an acidic environment in the sealed zone. Which enzyme is responsible for the production of hydrogen ions required for this process?

Explanation

Carbonic anhydrase II catalyzes the conversion of CO2 and water into carbonic acid, which then dissociates into hydrogen ions and bicarbonate. The hydrogen ions are pumped into the resorption pit to dissolve the inorganic hydroxyapatite, while Cathepsin K degrades the organic matrix.

Question 61

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

Explanation

Anisotropy refers to a material exhibiting varying mechanical properties when loaded in different directions. Cortical bone is highly anisotropic, being strongest when resisting longitudinal compression and weakest under transverse tension.

Question 62

The normal tendon-to-bone insertion (enthesis) consists of four zones. Following surgical repair, the regenerated insertion site typically heals by:

Explanation

The native enthesis consists of tendon, uncalcified fibrocartilage, calcified fibrocartilage, and bone. After surgical repair, this highly specialized transitional zone does not typically regenerate; instead, it heals via a biomechanically weaker fibrovascular scar tissue interface.

Question 63

Which of the following modifications will most significantly increase the overall stiffness of a unilateral external fixator construct?

Explanation

Bringing the rod closer to the bone (decreasing the offset) significantly increases the stiffness of an external fixator construct. Increasing pin diameter also has a profound effect on stiffness, as it is proportional to the radius to the fourth power.

Question 64

In total joint arthroplasty, cross-linking of ultra-high-molecular-weight polyethylene (UHMWPE) alters its mechanical properties by:

Explanation

Highly cross-linked UHMWPE demonstrates a significantly decreased wear rate, which effectively reduces particle-induced osteolysis. However, the cross-linking process negatively alters mechanical properties, leading to decreased toughness, reduced fatigue strength, and lower ductility.

Question 65

According to the Sunderland classification, a third-degree nerve injury involves disruption of the axon and which of the following structures?

Explanation

A Sunderland third-degree injury involves disruption of the axon, myelin sheath, and the endoneurium, while the perineurium and epineurium remain intact. Wallerian degeneration occurs, and clinical recovery can be unpredictable due to intrafascicular scarring.

Question 66

Demineralized bone matrix (DBM) products primarily rely on which of the following components for their osteoinductive properties?

Explanation

Demineralized bone matrix (DBM) is processed by removing the mineral component of bone, which exposes the underlying osteoinductive proteins, primarily Bone Morphogenetic Proteins (BMPs). DBM serves as an osteoconductive and osteoinductive graft but inherently lacks live osteogenic cells.

Question 67

Which of the following alterations to a cortical screw design most significantly increases its pull-out strength in diaphyseal bone?

Explanation

Pullout strength of a screw is most dependent on the outer (major) diameter of the thread. Other factors increasing pullout strength include decreased pitch (more threads per unit length) and increased length of engagement.

Question 68

According to Perren’s strain theory, what is the maximum interfragmentary strain that can be tolerated for the formation of lamellar bone without an intervening cartilage stage?

Explanation

Primary bone healing occurs without a cartilaginous callus when interfragmentary strain is strictly below 2%. Strain between 2% and 10% allows for secondary healing via endochondral ossification.

Question 69

A patient undergoes internal fixation with a titanium alloy plate. Compared to stainless steel, titanium alloy exhibits which of the following mechanical properties?

Explanation

Titanium alloy has a lower modulus of elasticity compared to stainless steel, bringing it closer to that of cortical bone. This leads to less stress shielding of the underlying bone, preserving bone density.

Question 70

During the remodeling phase of tendon healing, which of the following biochemical changes predominantly occurs?

Explanation

During the remodeling phase of tendon healing, the initial, structurally weaker Type III collagen is gradually replaced by stronger, highly cross-linked Type I collagen. This process increases the ultimate tensile strength of the tendon.

Question 71

During the volar (Henry) approach to the radius, the surgeon dissects between the brachioradialis and the flexor carpi radialis. What is the internervous plane at the proximal portion of this exposure?

Explanation

The proximal internervous plane for the volar approach to the radius is between the brachioradialis (innervated by the radial nerve) and the pronator teres (innervated by the median nerve).

Question 72

In total hip arthroplasty, the use of a ceramic-on-ceramic bearing surface is primarily intended to minimize which type of wear?

Explanation

Adhesive wear is the primary mode of wear in metal-on-polyethylene bearings, leading to osteolysis. Ceramic-on-ceramic bearings have a highly wettable surface that improves fluid film lubrication, drastically reducing adhesive wear.

Question 73

Which of the following terms best describes the process by which a structural cortical allograft is slowly resorbed by osteoclasts and replaced by host osteoblasts?

Explanation

Creeping substitution is the simultaneous process of bone resorption by host osteoclasts and deposition by host osteoblasts into a graft. Cortical allografts undergo this process slowly, initially losing structural strength before full incorporation.

Question 74

Which bone morphogenetic protein (BMP) is an FDA-approved osteoinductive agent for use in open tibial shaft fractures treated with intramedullary nailing?

Explanation

Recombinant human BMP-2 (rhBMP-2) is FDA-approved for use in acute open tibial shaft fractures treated with an IM nail, as well as anterior lumbar interbody fusion. BMP-7 (OP-1) was previously utilized for recalcitrant tibial nonunions under an HDE.

Question 75

When a constant load is applied to a viscoelastic material such as a ligament over a prolonged period, the material will undergo a time-dependent increase in length. This phenomenon is known 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 length.

Question 76

A surgeon uses a stainless steel screw to secure a titanium plate during fracture fixation. The construct is at high risk for which of the following complications?

Explanation

Galvanic corrosion occurs when two dissimilar metals (e.g., stainless steel and titanium) are placed in contact within a conductive fluid environment like the human body. The less noble metal acts as an anode and undergoes accelerated corrosion.

Question 77

A patient presents with a positive Trendelenburg gait following a total hip arthroplasty via the direct lateral approach. Which nerve was most likely injured during the splitting of the gluteus medius?

Explanation

The superior gluteal nerve innervates the gluteus medius, gluteus minimus, and tensor fasciae latae. Injury to this nerve during the direct lateral (Hardinge) approach, especially if splitting more than 5 cm proximal to the greater trochanter, leads to abductor weakness.

Question 78

The bending stiffness of a fracture fixation plate is proportional to the plate’s thickness raised to what power?

Explanation

The bending stiffness (area moment of inertia) of a rectangular plate is proportional to its width times the thickness cubed (base × height³ / 12). Therefore, a small increase in plate thickness exponentially increases its bending stiffness.

Question 79

In which zone of articular cartilage are the collagen fibers oriented perpendicular to the articular surface to provide the greatest resistance to compressive forces?

Explanation

In the deep (radial) zone of articular cartilage, collagen fibers are oriented perpendicularly to the articular surface and cross into the tidemark. This structural orientation provides the primary resistance to compressive loads.

Question 80

Activation of the Wnt/beta-catenin signaling pathway in osteoblasts primarily results in which of the following effects?

Explanation

The Wnt/beta-catenin signaling pathway promotes osteoblast differentiation, proliferation, and survival, leading to increased bone formation. Sclerostin, targeted by drugs like romosozumab, is a natural inhibitor of this pathway.

Question 81

Which of the following biomechanical principles is the primary advantage of utilizing a locked plating construct over a conventional compression plate in osteoporotic bone?

Explanation

Locked plating constructs function as single-beam, fixed-angle devices where the screw heads thread directly into the plate. This resists toggling and secondary loss of reduction, making them mechanically superior in osteoporotic bone where friction-based conventional plates often fail.

Question 82

Which of the following best describes the phenomenon of 'creep' in the viscoelastic behavior of ligaments?

Explanation

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

Question 83

Primary (strain-free) bone healing is characterized by which of the following processes?

Explanation

Primary bone healing occurs under conditions of absolute stability (strain <2%) and involves direct intramembranous healing via osteoclastic cutting cones. Subsequent osteoblast bone deposition occurs without intermediate callus formation.

Question 84

Increasing the working length of a bridging plate construct leads to which of the following biomechanical changes?

Explanation

Increasing the working length (the distance between the innermost screws on either side of the fracture) decreases the axial stiffness of the construct. This makes the fixation more flexible, promoting secondary bone healing through callus formation.

Question 85

Bone morphogenetic proteins (BMPs) primarily induce osteoblast differentiation through which of the following intracellular signaling molecules?

Explanation

BMPs bind to serine/threonine kinase receptors on the cell surface, which subsequently phosphorylate intracellular Smad proteins (typically Smad 1, 5, and 8). These proteins then translocate to the nucleus to regulate the transcription of osteogenic genes.

Question 86

In articular cartilage, which zone is characterized by the highest concentration of water, collagen fibers oriented parallel to the articular surface, and flattened chondrocytes?

Explanation

The superficial zone of articular cartilage features collagen fibers aligned parallel to the surface to resist shear stress. It contains flattened chondrocytes and possesses the highest water content compared to deeper zones.

Question 87

Which enzyme is essential for osteoclasts to create the acidic environment required for the dissolution of bone mineral in the resorption pit (Howship's lacuna)?

Explanation

Carbonic anhydrase II generates protons (H+) from carbon dioxide and water. These protons are then actively pumped across the ruffled border into the resorption pit to lower the pH and dissolve hydroxyapatite.

Question 88

According to the muscle length-tension curve, maximum active muscle force is generated at which of the following resting lengths?

Explanation

The length-tension relationship dictates that maximum active force is produced at the optimal resting length of the sarcomere. At this length, there is maximal overlap of actin and myosin filaments for optimal cross-bridge formation.

Question 89

Highly cross-linked polyethylene used in total joint arthroplasty aims to reduce wear but is associated with which of the following trade-offs?

Explanation

Cross-linking polyethylene with gamma irradiation generates free radicals that can cause oxidative degradation over time. Thermal treatments like remelting or annealing are required to quench these free radicals, though remelting can reduce fatigue strength.

Question 90

During the repair phase of secondary fracture healing, the soft callus is gradually replaced by hard callus primarily through which of the following processes?

Explanation

In secondary fracture healing, the initial cartilaginous soft callus undergoes endochondral ossification. Chondrocytes hypertrophy, the matrix is calcified, and it is subsequently replaced by woven bone to form the hard callus.

Question 91

Sclerostin, a glycoprotein produced by osteocytes, regulates bone mass by inhibiting which of the following pathways?

Explanation

Sclerostin inhibits the Wnt/beta-catenin pathway by binding to LRP5/6 receptors on osteoblasts, thereby decreasing osteoblast differentiation and bone formation. Romosozumab is a monoclonal antibody that targets sclerostin to treat osteoporosis.

Question 92

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

Explanation

Galvanic corrosion occurs when two dissimilar metals (such as stainless steel and titanium) are in contact within a conductive fluid. This creates an electrochemical gradient leading to the corrosion of the less noble metal.

Question 93

The pullout strength of a cortical bone screw is most significantly increased by increasing which of the following parameters?

Explanation

Pullout strength is directly proportional to the outer (thread) diameter, the length of thread engagement in the bone, and the shear strength of the bone. Increasing the outer diameter has the greatest mathematical impact on maximizing pullout strength.

Question 94

Which of the following molecules is primarily responsible for boundary lubrication at the articular cartilage surface during high-load, low-velocity conditions?

Explanation

Lubricin (PRG4) is a glycoprotein synthesized by superficial zone chondrocytes and synovial fibroblasts. It binds to the articular surface to provide boundary lubrication, drastically reducing friction under high-load and low-velocity conditions.

Question 95

Which phase of tendon healing is characterized by the highest synthesis of disorganized Type III collagen?

Explanation

During the proliferative phase (days 3 to 6 weeks), fibroblasts migrate into the wound and synthesize a disorganized matrix primarily composed of Type III collagen. This is later replaced by stronger, organized Type I collagen during the remodeling phase.

Question 96

Which of the following orthopedic implant materials has a modulus of elasticity most similar to that of human cortical bone?

Explanation

Titanium alloys have a modulus of elasticity (approx. 100-110 GPa) that is much closer to cortical bone (approx. 15-20 GPa) than stainless steel (approx. 200 GPa) or cobalt-chromium (approx. 240 GPa). This closer match helps minimize stress shielding.

Question 97

According to Wolff's Law, bone undergoes remodeling in response to mechanical stress. Which cellular mechanism directly senses fluid shear stress within the bone matrix?

Explanation

Osteocytes are the primary mechanosensory cells in bone. Fluid flow through the lacunocanalicular network resulting from mechanical loading creates shear stress, which is sensed by osteocyte dendritic processes to regulate bone remodeling.

Question 98

The primary blood supply to the proximal pole of the scaphoid is delivered via vessels entering the bone at which anatomical location?

Explanation

The major blood supply to the scaphoid is retrograde, originating from branches of the radial artery that enter the bone dorsally at or distal to the scaphoid waist. This retrograde flow leaves the proximal pole highly susceptible to avascular necrosis following fractures.

Question 99

In the normal human brachial plexus, the posterior cord is formed by the union of which of the following structures?

Explanation

The posterior cord of the brachial plexus is formed by the convergence of the posterior divisions of all three trunks (upper, middle, and lower). It receives contributions from nerve roots C5 through T1.

Question 100

The normal human intervertebral disc exhibits distinct collagen distributions to resist different biomechanical loads. What is the predominant collagen type found in the nucleus pulposus?

Explanation

The nucleus pulposus is rich in proteoglycans and Type II collagen, which gives it a gel-like consistency to resist compressive forces. In contrast, the annulus fibrosus is composed primarily of Type I collagen to resist tensile and torsional forces.

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

AAOS & ABOS Basic Science MCQs (Set 2): Biomechanics, Anatomy & Bone Healing

Key Takeaway

AAOS & ABOS Basic Science MCQs (Set 2): Biomechanics, Anatomy & Bone Healing

Chapter Index