Orthopedic Basic Science MCQs: AAOS & ABOS OITE Prep

Question 1

A 16-year-old boy has had thigh pain for the past several months. He denies any history of trauma. Examination reveals a large, deeply fixed, soft-tissue mass in the thigh. Laboratory results show an elevated erythrocyte sedimentation rate (ESR) and leukocytosis. A plain radiograph and MRI scan are shown in Figures 1a and 1b. Biopsy specimens are shown in Figures 1c and 1d. What is the most likely diagnosis?

Explanation

Ewing's sarcoma typically can occur in the diaphysis of the long bones (50% to 55%). It is often accompanied by a large soft-tissue mass. Abnormal findings are common, including a low-grade fever, an elevated ESR, and leukocytosis. The histology is consistent with a small round blue cell tumor. The unique pathology and other findings exclude osteosarcoma. Giant cell tumor and chondrosarcoma have a different histologic appearance and typically are more metaphyseal in location. Chondrosarcoma typically is found in older age groups, has a different histologic pattern, and rarely occurs in the midshaft of the femur.

Question 2

A 14-year-old boy who is right handed reports right shoulder pain. Radiographs show a lucent lesion of the proximal humeral epiphysis with a narrow zone of transition. Results of an open biopsy confirm the presence of a chondroblastoma. Based on these findings, the next most appropriate step in management should consist of

Explanation

The patient has a chondroblastoma of the proximal humerus; therefore, the treatment of choice is curettage and bone grafting. Surgical resection of the proximal humerus is not indicated in the initial treatment of an intraosseous chondroblastoma. Mirra JM: Bone Tumors: Clinical, Radiologic, and Pathologic Correlations. Philadelphia, PA, Lea and Febiger, 1989, pp 589-623.

Question 3

A 25-year-old man has had an insidious onset of left hip pain over the past 11 months. A radiograph, coronal MRI scan, and histopathologic specimens are seen in Figures 2a through 2d. What is the most likely diagnosis?

Explanation

Ewing's sarcoma is the second most common primary sarcoma of bone in children and young adults. It is a malignant round cell tumor with uncertain histogenesis. Sheets of uniform small round blue cells with a high nuclear-to-cytoplasm ratio and the absence of osteoid formation differentiate this histologic diagnosis from the other conditions. Immunohistochemical staining and molecular diagnostic studies are useful to verify the diagnosis.

Question 4

What is the most common anatomic location for chondrosarcoma?

Explanation

The most common anatomic location of chondrosarcoma is the pelvis (30%), followed by the proximal femur (20%). Chondrosarcomas appear in the shoulder girdle in 15% of patients but rarely affect the spine or hands. Marcove RC, Mike V, Hutter RV, et al: Chondrosarcoma of the pelvis and upper end of the femur: An analysis of factors influencing survival time in one hundred and thirteen cases. J Bone Joint Surg Am 1972;54:561-572.

Question 5

In the spine, osteoblastomas usually originate in the

Explanation

Osteoblastomas are benign lesions that represent less than 5% of benign bone tumors. Most lesions are located in the spine, followed by the femur, tibia, and skull. Patients with spinal lesions usually have pain that may be associated with scoliosis. The most common location in the spine is within the posterior elements.

Question 6

A 64-year-old man with a history of metastatic lung cancer reports increasing right hip pain over the period of several months. Radiographs are shown in Figures 3a and 3b. Initial management should consist of

Explanation

The patient has lung cancer metastatic to the right proximal femur. The lesion is large, has destroyed a portion of the cortex, and involves the peritrochanteric region. All of these findings put the patient at high risk for pathologic fracture. The lesion is amenable to intramedullary fixation in the form of some type of reconstruction nail. Chemotherapy alone will not restore the bone stock. Given the extent of the lesion, radiation therapy, chemotherapy, or embolization will not prevent fracture. A proximal femoral replacement would be indicated in patients with bone destruction extending into the femoral head and neck. Bisphosphonates may diminish the risk of subsequent lesions but are not sufficient to treat this high-risk lesion. Radiation therapy should be given postoperatively to prevent further bone destruction.

Question 7

A 58-year-old woman has had a painless periscapular mass for the past year. An MRI scan and biopsy specimen are shown in Figures 4a and 4b. What is the most likely diagnosis?

Explanation

Elastofibroma is a rare tumor that most commonly occurs in adults who are older than age 55 years. The lesions usually grow between the chest wall and the scapula, and 10% are bilateral. Histologic analysis shows that they are composed of equal amounts of elastin and collagen with occasional fibroblasts. Briccoli A, Casadei R, Di Renzo M, Favale L, Bacchini P, Bertoni F: Elastofibroma dorsi. Surg Today 2000;30:147-152.

Question 8

A characteristic genetic translocation has been noted in which of the following tumors?

Explanation

There have been no characteristic gene translocations or rearrangements noted in osteosarcoma, chondrosarcoma, neurofibrosarcoma, or epithelioid sarcoma. In contrast, Ewing's sarcoma has been noted to have a consistent genetic translocation t(11:22). Brockstein BE, Peabody TD, Simon MA: Soft tissue sarcomas, in Vokes EE Golomb HM (eds): Oncologic Therapies. New York, NY, Springer-Verlag, 1999, pp 925-952.

Question 9

A 44-year-old man has right hip pain. Radiographs reveal a radiolucent lesion of the femoral head and neck. An MRI scan shows no extraosseous tumor extension. A histopathologic photomicrograph of the biopsy specimen is shown in Figure 5. The chance of metastatic disease developing from this lesion is approximately what percent?

Explanation

The risk of pulmonary metastasis from a benign giant cell tumor is estimated to be 5%.

Question 10

Figures 6a through 6d show the radiographs and biopsy specimens of an 8-year-old girl with leg pain. Management of the lesion should consist of

Explanation

The biopsy specimens show a chondromyxoid fibroma with varying amounts of cartilage, benign fibrous tissue, giant cells, and loose myxoid areas. Chondromyxoid fibroma is a benign active bone lesion that is best treated with aggressive curettage and bone grafting. Although recurrences are common, more aggressive treatment is not warranted initially. Wilson AJ, Kyriakos M, Ackerman LV: Chondromyxoid fibroma: Radiographic appearance in 38 cases and in a review of the literature. Radiology 1991;179:513-518.

Question 11

Which of the following is considered the treatment of choice for a chondroblastoma of the proximal tibial epiphysis without intra-articular extension?

Explanation

Curettage and bone grafting typically is the preferred method of treatment for chondroblastoma, with local recurrence rates of approximately 10%. Some clinicians advocate the addition of adjuvants such as phenol. Left alone, these lesions can destroy bone and invade the joint. Large intra-articular lesions may require major joint reconstruction. Wide local excision rarely is required to eradicate the tumor. Radiation therapy rarely is indicated and only for unresectable or multiply recurrent lesions. Springfield DS, Capanna R, Gherlinzoni F, Picci P, Campanacci M: Chondroblastoma: A review of seventy cases. J Bone Joint Surg Am 1985;67:748-755.

Question 12

An otherwise healthy 78-year-old woman has low back and buttock pain. Rectal examination reveals a large sacral mass. Figures 7a and 7b show a CT scan and a sagittal MRI scan of the lumbosacral spine. A biopsy specimen is shown in Figure 7c. What is the most likely diagnosis?

Explanation

A chordoma is a malignant neoplasm originating from remnants of the notochord. It is usually localized to the midline with 50% at the sacrococcygeal area, 35% at the skull base, and 15% at the mobile portion of the spine. Large vacuolated cells (physaliferous cells) are a characteristic of the tumor. Mindell ER: Chordoma. J Bone Joint Surg Am 1981;63:501-505.

Question 13

Figures 8a through 8c show the lateral radiograph and T1- and T2-weighted MRI scans of a 14-year-old soccer player who reports aching thigh pain. The next most appropriate step in management should consist of

Explanation

Although the MRI findings could be misinterpreted as an aggressive soft-tissue process, the periosteal-based ossification on the radiograph in an athlete most likely suggests myositis ossificans. The radiograph should be repeated to see further maturation of the ossification with a typical "zoning" pattern. The zoning pattern is one of peripheral ossification. This is often best seen on a CT scan. King JB: Post-traumatic ectopic calcification in the muscles of athletes: A review. Br J Sports Med 1998;32:287-290.

Question 14

A 51-year-old male truck driver has had progressive left hip pain for more than 2 years, and he reports that the pain has become severe in the past 9 months. He is now unable to work because of the pain. Examination reveals that range of motion of the hip is limited to 95 degrees of flexion, 0 degrees of internal rotation, and 20 degrees of external rotation. The plain radiograph, MRI scan, and intraoperative gross photographs are shown in Figures 9a through 9d. Management should consist of

Explanation

The diagnosis is synovial chondromatosis. While the plain radiograph fails to show any calcifications, the MRI scan shows an intra-articular mass that involves the capsule. Grossly multiple granular cartilage nodules are seen. Management should consist of removing all loose bodies along with the synovial membrane.

Question 15

Using methylmethacrylate to fill a biopsy hole in the diaphysis of a femur theoretically achieves what purpose?

Explanation

Placing cement over a bone biopsy site prevents tumor contamination by controlling hematoma. Even though the use of cement may impart some strength, the femur is still at significant risk for fracture. The use of bone cement in this manner has not been cleared by the FDA, but many physicians feel that it is appropriate when the patient's health status has been given careful consideration, and the physician has the necessary knowledge and training. The other options are not important reasons to use methylmethacrylate in biopsies. Simon MA, Springfield DS, et al: Biopsy: Surgery for Bone and Soft Tissue Tumors. Philadelphia, PA, Lippincott Raven, 1998, pp 55-65.

Question 16

Gaucher's disease is manifested by reticuloendothelial system macrophage accumulation of

Explanation

Gaucher's disease is characterized by macrophage accumulation of glucocerebroside that is caused by a deficiency of lysosomal enzyme glucocerebrosidase. It is an autosomal-recessive trait and is most commonly found in Ashkenazi Jews. Orthopaedic surgeons see patients with Gaucher's disease usually because of osteonecrosis of the hip. Calcium pyrophosphate is associated with pseudogout. Hydroxyproline is a breakdown product of collagen and is found in high levels in patients with Paget's disease. Homogentisic acid is associated with ochronosis (alkaptonuria). Beatty JH: Orthopaedic Knowledge Update 6. Rosemont, IL, American Academy of Orthopaedic Surgeons, 1999, pp 247-252. Goldblatt J, Sacks S, Beighton P: The orthopedic aspects of Gaucher disease. Clin Orthop 1978;137:208-214.

Question 17

A 14-year-old boy reports pain in the distal thigh. He denies any history of trauma. Examination reveals tenderness and swelling of the distal thigh without effusion. A radiograph and CT scan are shown in Figures 10a and 10b. A biopsy specimen is shown in Figure 10c. Management should consist of

Explanation

Based on these findings, the patient has an aneurysmal bone cyst. Frequently, fluid-fluid levels can be detected on MRI or CT images. The histologic results show a lesion that consists of cavernous spaces filled with blood. The lining of the cavity contains spindle cells, multinucleated giant cells, and reactive bone. Curettage and bone grafting is the preferred treatment method. Without treatment, these lesions can become quite large and destructive. Radiation therapy is not recommended for resectable lesions. Chemotherapy is not required for these benign lesions. Simon MA, Springfield DS, et al: Common Benign Bone Tumors: Surgery for Bone and Soft Tissue Tumors. Philadelphia, PA, Lippincott Raven, 1998, pp 194-200.

Question 18

The dose of the chemotherapeutic agent doxorubicin (Adriamycin) is limited by which of the following factors?

Explanation

Doxorubicin is a chemotherapeutic agent that is best known for dose-limiting cardiotoxicity. This is related not only to the cumulative dose, but also the rate of infusion.

Question 19

A previously asymptomatic 12-year-old girl sustained a direct blow to the right lateral knee from a baseball bat. Examination reveals an area of ecchymosis and tenderness over the lateral thigh. The patient can walk without pain, but range of motion of the knee causes discomfort. Plain radiographs of the knee are shown in Figures 11a and 11b. To address the bone lesion, management should consist of

Explanation

The plain radiographs reveal a pedunculated osteochondroma with a fracture. There is a bony growth in the metaphysis of a long bone, on a stalk that is directed away from the nearby epiphysis. On the AP view, the host cortical and medullary bone are shown as "blending" with lesional bone. There is also a fracture through the lesion. Based on these radiographic findings, the diagnosis is an osteochondroma; therefore, initial management of an acute fracture of an osteochondroma is symptomatic treatment alone. Additional imaging studies are not indicated in this patient. At times it may be difficult to distinguish a sessile osteochondroma from a parosteal osteosarcoma. In the latter case, the host medullary bone and lesion bone are not confluent. A CT scan may be helpful to distinguish if the host medullary and cortical bone are confluent with the lesion.

Question 20

What is the most common pediatric soft-tissue sarcoma?

Explanation

Soft-tissue sarcomas are the sixth most common cancer in children. Rhabdomyosarcoma is the most common type of pediatric soft-tissue sarcoma. Nearly 50% of rhabdomyosarcomas are diagnosed in children who are age 5 years or younger. Unfortunately, there has not been a significant increase in survival in children with metastatic rhabdomyosarcoma despite aggressive therapy including multiple-drug chemotherapy regimens.

Question 21

A 53-year-old man has a 4- x 5-cm high-grade soft-tissue sarcoma in the midthigh. As part of the staging evaluation, regional nodes should be assessed by

Explanation

In general, soft-tissue metastases to regional nodes are a relatively rare occurrence (less than 5% overall). The incidence of lymphatic metastasis is highest for synovial sarcoma, rhabdomyosarcoma, clear cell sarcoma, and epithelioid sarcoma. Regional nodes should be assessed clinically. CT is not used to routinely assess regional nodes. Evaluation of a sentinal node is not indicated because of the low incidence of regional nodal involvement. Fine needle aspiration may be indicated to assess clinically suspicious nodes. Prophylactic inguinal node dissection is contraindicated because it may lead to unnecessary complications such as lymphedema.

Question 22

The risk of local recurrence after surgical resection of a soft-tissue sarcoma is most closely related to

Explanation

A positive margin is most closely related to subsequent local recurrence. The other factors cited, including the size and site of the tumor, may be related to local recurrence; however, they are more commonly prognostic because of the difficulty in obtaining wide surgical margins about large or proximal tumors. Radiation therapy has been noted to decrease the incidence of recurrence but is not felt to be as important as the surgical margin. The grade of the tumor has more influence on the prevalence of metastatic disease than the incidence of local recurrence. Lewis JJ, Leung D, Heslin M, Woodruff JM, Brennan MF: Association of local recurrence with subsequent survival in extremity soft tissue sarcoma. J Clin Oncol 1997;15:646-652.

Question 23

In a patient with a soft-tissue sarcoma treated by wide excision and radiation therapy, the risk of subsequent fracture is probably most influenced by

Explanation

While most pathologic fractures are in the lower extremity in patients treated for soft-tissue sarcomas by wide excision and adjuvant radiation therapy, risk factors for such fractures are bone resection associated with excision of the tumor and soft-tissue sarcomas of the thigh that require periosteal stripping at the time of resection. Such fractures can occur late, often more than 6 months after surgery, are difficult to treat, and often result in nonunion. Bell RS, O'Sullivan B, Nguyen C, et al: Fractures following limb-salvage surgery and adjuvant irradation for soft-tissue sarcoma. Clin Orthop 1991;271:265-271.

Question 24

Figures 12a and 12b show the radiographs of a 50-year-old patient who reports acute knee pain after sustaining a twisting injury while playing tennis. Examination is unremarkable. The next most appropriate step in management should consist of

Explanation

The radiographs show localized diffuse cortical thickening that is characteristic of melorheostosis. The condition may be monostotic or it may involve many bones in one extremity (monomelic) in the distribution of a sclerotome. Bone scans will show increased uptake at the site or sites of skeletal involvement. Long tubular bones are most commonly involved. Melorheostosis is usually asymptomatic and requires no treatment. On rare occasions, there may be associated soft-tissue contractures. Dorfman H, Czerniak B: Bone Tumors. St Louis, MO, Mosby Inc, 1998, pp 1105-1107. Campbell CJ, Papademetriou T, Bonfiglio M: Melorheostosis: A report of the clinical, roentgenographic, and pathological findings in fourteen cases. J Bone Joint Surg Am 1968;50:1281-1304.

Question 25

An adult patient has an 8- x 4- x 10-cm soft-tissue mass located within the adductor compartment of the thigh. Staging studies should consist of

Explanation

The appropriate staging studies should consist of MRI and a radiograph of the primary lesion and CT of the chest. MRI is superior to CT for soft-tissue imaging. CT may be useful for evaluating the cortex of bone for invasion by tumor. Bone scans are not commonly used because soft-tissue sarcomas rarely metastasize to bone. CT of the abdomen and pelvis is not typically ordered except for possible liposarcoma. With liposarcoma, there may be a synchronous or metastatic retroperitoneal liposarcoma. Demetri GD, Pollock R, Baker L, et al: NCCN sarcoma practice guidelines: National Comprehensive Cancer Network. Oncology (Huntingt) 1998;12:183-218.

Question 26

A novel osteoporosis medication targets sclerostin. What is the primary mechanism by which this medication increases bone mass?

Explanation

Sclerostin is produced by osteocytes and inhibits the Wnt/beta-catenin pathway, which normally stimulates osteoblastogenesis. Monoclonal antibodies against sclerostin block this inhibition, thereby directly increasing bone formation.

Question 27

An orthopedic surgeon decides to over-ream the femoral canal to insert a solid intramedullary nail with a 12-mm outer diameter rather than a 10-mm outer diameter. Assuming the material is identical, the bending rigidity of the nail increases by approximately what factor?

Explanation

The bending rigidity of a solid cylinder is proportional to its area moment of inertia, which is calculated based on the radius to the fourth power (r^4). Increasing the diameter from 10 mm to 12 mm (a 1.2-fold increase) increases the bending rigidity by a factor of 1.2^4, which is approximately 2.1.

Question 28

A 28-year-old man sustains a displaced fracture of the talar neck. Which of the following provides the primary blood supply to the body of the talus and is at greatest risk of disruption in this injury?

Explanation

The artery of the tarsal canal, a branch of the posterior tibial artery, provides the dominant blood supply to the talar body. Disruption of this specific blood supply in talar neck fractures is a primary cause of subsequent avascular necrosis.

Question 29

Which of the following best describes the structural composition and biomechanical role of the superficial zone of normal articular cartilage?

Explanation

The superficial zone of articular cartilage contains the highest water content and lowest proteoglycan content. Its collagen fibers are oriented parallel to the joint surface to effectively resist applied shear forces.

Question 30

During the surgical correction of a relapsed clubfoot, a surgeon applies a sequential series of corrective casts. The gradual stretching of the contracted soft tissues over time under a constant applied load is an example of which viscoelastic property?

Explanation

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

Question 31

Rigid internal fixation of a transverse diaphyseal radius fracture with absolute stability and no fracture gap promotes bone healing primarily through which of the following cellular processes?

Explanation

Rigid internal fixation with absolute stability results in primary bone healing. This occurs via direct osteonal remodeling, where cutting cones cross the fracture site without an intermediate cartilaginous stage or macroscopic callus.

Question 32

A patient presents with an inability to actively cross the index and middle fingers following a penetrating injury to the proximal forearm that isolated the median nerve. Testing shows normal sensation over the ulnar digits. Which anatomic variant best explains this intrinsic hand weakness?

Explanation

The Martin-Gruber anastomosis involves motor fibers crossing from the median nerve to the ulnar nerve in the forearm. A proximal median nerve injury could therefore cause weakness of ulnar-innervated intrinsic hand muscles if these crossover fibers are involved.

Question 33

Which of the following enzymes is primarily responsible for the degradation of the organic bone matrix within the Howship lacuna during osteoclastic bone resorption?

Explanation

Osteoclasts dissolve inorganic minerals using hydrogen ions and degrade the organic matrix (primarily Type I collagen) through the secretion of Cathepsin K. Carbonic anhydrase II is essential for generating the protons used to dissolve the mineral phase.

Question 34

In the stress-strain curve of a cortical bone specimen under tension, which of the following mechanical properties is determined by the total area under the curve up to the point of failure?

Explanation

Toughness is the ability of a material to absorb energy and deform before fracturing, represented by the total area under the stress-strain curve. Ductility refers only to the amount of plastic deformation before failure.

Question 35

During a posterior approach to the humerus, the radial nerve is identified as it travels through the spiral groove. Which of the following vascular structures travels directly with the radial nerve in this location?

Explanation

The radial nerve runs intimately with the profunda brachii artery (deep brachial artery) in the spiral groove on the posterior aspect of the humerus. The posterior circumflex humeral artery travels with the axillary nerve in the quadrangular space.

Question 36

Sclerostin plays a critical role in the regulation of bone mass by osteocytes. What is the primary mechanism by which sclerostin exerts its inhibitory effect on bone formation?

Explanation

Sclerostin, secreted by osteocytes, inhibits osteoblastic bone formation by competitively binding to LRP5/6 receptors. This action antagonizes the canonical Wnt/beta-catenin signaling pathway.

Question 37

Which of the following alterations to a cortical bone screw design will most significantly increase its pullout strength?

Explanation

Screw pullout strength is directly proportional to the major (outer) diameter and length of thread engagement. Increasing the major diameter provides the greatest increase in holding power.

Question 38

Which of the following forms of fracture fixation relies primarily on intramembranous ossification and cutting cones for bone healing, without the formation of a visible fracture callus?

Explanation

Compression plating provides absolute stability and abolishes interfragmentary strain, leading to primary bone healing. This process relies on direct Haversian remodeling via cutting cones rather than secondary callus formation.

Question 39

The anterior cruciate ligament (ACL) is composed of two primary bundles. Which of the following best describes the biomechanical function of the posterolateral (PL) bundle?

Explanation

The posterolateral (PL) bundle of the ACL is tightest in extension and primarily controls rotatory stability (internal rotation). The anteromedial (AM) bundle is tightest in flexion and primarily restricts anterior tibial translation.

Question 40

Recombinant human parathyroid hormone (teriparatide) is utilized in the treatment of severe osteoporosis. What is the underlying mechanism by which it exerts an anabolic effect on bone?

Explanation

While continuous PTH exposure upregulates RANKL and causes net bone resorption, intermittent administration of PTH (teriparatide) stimulates osteoblast proliferation and survival, resulting in net bone formation.

Question 41

Which of the following arteries provides the predominant blood supply to the weight-bearing portion of the adult femoral head?

Explanation

The medial femoral circumflex artery (MFCA) is the primary blood supply to the femoral head. Specifically, its lateral epiphyseal branches penetrate the joint capsule to supply the critical superior and weight-bearing regions.

Question 42

When applying a bridge plate to a comminuted diaphyseal fracture, increasing the 'working length' of the construct has which of the following biomechanical effects?

Explanation

Working length is the distance between the innermost screws on either side of a fracture. Increasing the working length decreases the overall stiffness of the construct, which distributes strain and permits controlled micromotion for secondary bone healing.

Question 43

Which of the following characteristics accurately describes the superficial zone of normal articular cartilage compared to the deeper zones?

Explanation

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

Question 44

The sciatic nerve typically exits the greater sciatic foramen inferior to the piriformis muscle. In the most common anatomical variant (Beaton and Anson Type B), how does the sciatic nerve traverse the piriformis?

Explanation

In the most common anatomical variation of the sciatic nerve (approx. 10-15% of the population), the common peroneal division pierces the piriformis muscle, while the tibial division exits normally beneath the muscle.

Question 45

Which of the following transcription factors is essential for the commitment of mesenchymal stem cells to the osteoblast lineage?

Explanation

Runx2 (also known as Cbfa1) is the master transcription factor required for osteoblast differentiation. Sox9 drives chondrogenesis, while PPAR-gamma drives adipogenesis.

Question 46

What is the primary function of Cathepsin K in osteoclastic bone resorption?

Explanation

Cathepsin K is a lysosomal protease secreted by osteoclasts that degrades type I collagen and other bone matrix proteins. It functions optimally in the acidic environment created by the vacuolar H+-ATPase pump.

Question 47

On a stress-strain curve, the total area under the curve up to the point of material failure represents which of the following mechanical properties?

Explanation

Toughness is the ability of a material to absorb energy and plastically deform before fracturing, represented by the total area under the stress-strain curve. Stiffness is represented by the slope of the elastic region (Young's modulus).

Question 48

Which zone of articular cartilage is characterized by the highest water content and collagen fibers oriented parallel to the joint surface?

Explanation

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

Question 49

According to the principles of biomechanics, the bending rigidity of a solid cylindrical intramedullary nail is proportional to its radius raised to which power?

Explanation

The area moment of inertia, which determines resistance to bending, is proportional to the radius to the fourth power (r^4) for a solid cylinder. A small increase in nail diameter significantly increases its bending stiffness.

Question 50

Following a complete peripheral nerve transection, which cells are primarily responsible for clearing myelin debris during Wallerian degeneration in the distal nerve stump?

Explanation

Wallerian degeneration involves the breakdown of the axon and myelin distal to a nerve injury. Schwann cells initially degrade myelin, followed by recruited macrophages which clear the remaining debris to allow for axonal regeneration.

Question 51

In the load-deformation curve of a healthy tendon, the initial non-linear 'toe region' is primarily due to:

Explanation

The toe region corresponds to the stretching out, or 'uncrimping,' of the naturally crimped type I collagen fibers when a load is first applied. Once uncrimped, the tendon enters the linear elastic region.

Question 52

Commercially prepared demineralized bone matrix (DBM) is best described as possessing which of the following biological properties?

Explanation

DBM provides a collagenous scaffold (osteoconductive) and contains viable bone morphogenetic proteins (BMPs) (osteoinductive). Because it lacks living cells, it is not osteogenic.

Question 53

Which type of wear in total joint arthroplasty is characterized by the transfer of material from one surface to another due to localized micro-welding and subsequent tearing?

Explanation

Adhesive wear occurs when two surfaces slide against each other, creating localized micro-welds that break and tear material away. Abrasive wear occurs when a harder surface or particle plows into a softer surface.

Question 54

A slipped capital femoral epiphysis (SCFE) typically represents a mechanical failure through which specific zone of the physis?

Explanation

SCFE and most Salter-Harris type I fractures occur through the hypertrophic zone of the physis. This zone lacks substantial collagen and is mechanically the weakest area.

Question 55

During the remodeling phase of ligament healing, the structural integrity of the extracellular matrix improves primarily due to which of the following processes?

Explanation

In the initial phases of healing, fibroblasts predominantly synthesize disorganized type III collagen. During remodeling, this is gradually replaced by stronger, highly cross-linked type I collagen.

Question 56

Primary (strain-free) bone healing relies on absolute stability and achieves union via which of the following mechanisms?

Explanation

Primary bone healing occurs without a cartilaginous callus when absolute stability (e.g., compression plating) is achieved. It proceeds directly via osteoclastic cutting cones followed by osteoblastic bone deposition (Haversian remodeling).

Question 57

Which lubrication mechanism predominates in articular cartilage under conditions of high contact load and low sliding velocity?

Explanation

Boundary lubrication is mediated by molecules like lubricin (PRG4) on the cartilage surface and operates under high-load, low-velocity conditions. Fluid-film lubrication typically dominates during high-velocity motions.

Question 58

Fretting corrosion in orthopedic implants is most commonly observed at which of the following interfaces?

Explanation

Fretting corrosion occurs due to repetitive micromotion between two closely apposed metal surfaces, mechanically removing the passive oxide layer. It is classically seen at the screw-plate interface.

Question 59

The primary blood supply to the adult femoral head is derived from the:

Explanation

The medial femoral circumflex artery (MFCA) provides the dominant blood supply to the adult femoral head via its lateral epiphyseal branches. The artery of the ligamentum teres contributes a negligible amount in adults.

Question 60

Which of the following characteristics best describes Type I (slow-twitch) skeletal muscle fibers?

Explanation

Type I muscle fibers are 'slow-twitch' fibers that rely on aerobic metabolism. They are rich in mitochondria and myoglobin, granting them high oxidative capacity and significant resistance to fatigue.

Question 61

Parathyroid hormone (PTH) stimulates bone resorption primarily by binding to receptors on the surface of which cell type?

Explanation

PTH binds to the PTH1R receptor on osteoblasts. This stimulates osteoblasts to upregulate RANKL and downregulate OPG, indirectly driving osteoclast differentiation and bone resorption.

Question 62

Which of the following geometric modifications most significantly increases the pull-out strength of a bone suture anchor?

Explanation

Decreasing the thread pitch means having more threads per inch, which increases the surface area of bone-thread engagement and maximizes pull-out strength. Increasing the outer diameter or decreasing the inner core diameter also improves fixation.

Question 63

In the context of tissue biomechanics, the viscoelastic property of load relaxation is defined as:

Explanation

Load (or stress) relaxation is the phenomenon where the stress within a material decreases over time while it is held at a constant strain. Creep, conversely, is increasing deformation under a constant load.

Question 64

What is the most abundant structural protein in the extracellular matrix of bone, constituting roughly 90% of its organic matrix?

Explanation

Type I collagen forms a triple helix and makes up about 90% of the organic matrix of bone, providing significant tensile strength. Osteocalcin and osteopontin are noncollagenous proteins involved in mineralization.

Question 65

Which of the following bone graft options possesses osteogenic, osteoinductive, and osteoconductive properties?

Explanation

Autograft contains live cells (osteogenic), growth factors (osteoinductive), and a scaffold (osteoconductive). Demineralized bone matrix is only osteoinductive and osteoconductive, while allograft is mainly osteoconductive.

Question 66

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

Explanation

The superficial zone contains the highest water content and has collagen fibers aligned parallel to the joint surface to resist shear stress. The deep zone features vertical fibers to resist compressive forces.

Question 67

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

Explanation

Screw pull-out strength is directly proportional to outer diameter and length of engagement, and inversely proportional to thread pitch. Decreasing thread pitch increases the number of threads engaged per unit of length, enhancing pull-out strength.

Question 68

What is the primary mode of polyethylene wear in total hip arthroplasty that generates submicron particles leading to osteolysis?

Explanation

Adhesive wear is the primary mode of wear between ultra-high-molecular-weight polyethylene and the metallic femoral head in total hip arthroplasty. This process breaks off tiny fragments of polyethylene, creating submicron debris that triggers macrophage-mediated osteolysis.

Question 69

On a stress-strain curve for a ligament, the initial 'toe region' represents which of the following physiological events?

Explanation

The toe region of the stress-strain curve represents the initial unfolding or 'uncrimping' of the wavy collagen fibers under low strain. Once uncrimped, the ligament enters the linear elastic region where stress becomes proportional to strain.

Question 70

Which of the following molecules acts as a decoy receptor to bind RANKL, thereby inhibiting osteoclast differentiation and bone resorption?

Explanation

Osteoprotegerin (OPG) is produced by osteoblasts and binds to RANKL as a decoy receptor. This interaction prevents RANKL from binding to RANK on osteoclast precursors, effectively inhibiting osteoclastogenesis and bone resorption.

Question 71

Which structural layer of a peripheral nerve is the primary barrier to the diffusion of foreign substances and maintains the blood-nerve barrier?

Explanation

The perineurium surrounds fascicles of nerve fibers and functions as the primary blood-nerve barrier. It provides significant tensile strength to the nerve and protects axons from foreign substances.

Question 72

Which of the following orthopedic implant materials has a modulus of elasticity most closely resembling that of normal cortical bone?

Explanation

Titanium alloy has a modulus of elasticity (roughly 110 GPa) closer to that of cortical bone (15-20 GPa) compared to stainless steel (200 GPa) and cobalt-chromium (220 GPa). This closer match helps minimize stress shielding.

Question 73

Rigid plate fixation of a transverse radial shaft fracture with absolute stability primarily heals through which of the following processes?

Explanation

Absolute stability achieved through rigid internal fixation prevents micromotion and leads to primary (direct) bone healing. This process bypasses callus formation and relies on Haversian remodeling via cutting cones traversing the fracture site.

Question 74

A patient undergoing gradual clubfoot correction with serial casting relies on a specific viscoelastic property of soft tissues. The progressive elongation of these contracted tissues under a constant load over time is best described as:

Explanation

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

Question 75

Which type of joint lubrication relies primarily on a thin layer of fluid generated by the sliding motion and deformation of articular surfaces under load?

Explanation

Elastohydrodynamic lubrication occurs when a fluid film separates sliding joint surfaces, facilitated by the elastic deformation of articular cartilage under dynamic loads. Boundary lubrication operates primarily during static loading or extreme loads, relying on surface molecules like lubricin.

Question 76

During the normal human gait cycle, which muscle group undergoes an eccentric contraction during the initial contact (heel strike) to loading response phase to prevent foot slap?

Explanation

The ankle dorsiflexors, primarily the tibialis anterior, contract eccentrically after heel strike to lower the foot smoothly to the ground. A failure of this mechanism results in an audible foot slap.

Question 77

Which of the following biochemical changes is most characteristic of the physiological aging process of the intervertebral disc?

Explanation

As the intervertebral disc ages, it undergoes a decrease in total proteoglycan content, a decline in water content, and a reduction in the ratio of chondroitin sulfate to keratin sulfate. This leads to diminished ability of the nucleus pulposus to absorb compressive loads.

Question 78

In bridge plating of a comminuted diaphyseal fracture, increasing the 'working length' of the construct results in which of the following biomechanical outcomes?

Explanation

The working length of a plate is the distance between the two innermost screws spanning the fracture. Increasing the working length makes the construct more flexible, decreasing axial stiffness and allowing more interfragmentary motion to stimulate secondary bone healing.

Question 79

Parathyroid hormone (PTH) helps regulate serum calcium levels. Which of the following mechanisms is NOT a direct action of PTH?

Explanation

PTH does not directly stimulate the intestinal absorption of calcium. Instead, it promotes intestinal calcium absorption indirectly by upregulating 1-alpha-hydroxylase in the kidneys, which converts 25-hydroxyvitamin D to its active form, 1,25-dihydroxyvitamin D.

Question 80

Which of the following markers is essential for the differentiation of macrophages into osteoclasts?

Explanation

RANKL binds to the RANK receptor on osteoclast precursors to stimulate their differentiation into mature osteoclasts. Runx2 is a transcription factor for osteoblasts, while SOX9 is essential for chondrocytes.

Question 81

In the context of viscoelastic materials, what term describes the progressive deformation of a material over time when subjected to a constant load?

Explanation

Creep refers to the time-dependent progressive deformation of a material under a constant load. Stress relaxation describes a decrease in stress over time when the material is held at a constant deformation.

Question 82

Which type of bone healing is primarily characterized by the formation of a fracture callus and occurs under conditions of relative stability?

Explanation

Secondary bone healing via a fracture callus primarily involves endochondral ossification, where a cartilage intermediate forms before being replaced by bone. This process occurs when relative stability (some micromotion) is present.

Question 83

Which type of collagen is the primary structural component of normal articular cartilage?

Explanation

Normal hyaline articular cartilage is primarily composed of Type II collagen, which provides the matrix with its essential tensile strength. Type X collagen is found in the calcified cartilage zone.

Question 84

When comparing titanium alloys to stainless steel for use in orthopedic implants, titanium primarily exhibits:

Explanation

Titanium has a lower modulus of elasticity compared to stainless steel, making it biomechanically closer to cortical bone. This reduces the effect of stress shielding compared to stiffer metals.

Question 85

The primary mode of wear in a well-functioning metal-on-highly cross-linked polyethylene total hip arthroplasty is:

Explanation

Adhesive wear is the predominant mode of wear in standard metal-on-polyethylene bearings, producing submicron particles that can lead to osteolysis. Abrasive wear occurs when a harder surface or third body scratches a softer one.

Question 86

Which mechanism of joint lubrication operates primarily under high loads and low sliding speeds, utilizing a thin layer of surface-active molecules like lubricin on the articular surface?

Explanation

Boundary lubrication relies on molecules such as lubricin and hyaluronic acid to prevent direct cartilage-to-cartilage contact under high loads and low speeds. Fluid-film mechanisms, like hydrodynamic lubrication, operate at higher speeds.

Question 87

During skeletal muscle contraction, calcium binds directly to which of the following proteins to initiate the exposure of myosin-binding sites?

Explanation

Calcium binds to Troponin C, triggering a conformational change that moves tropomyosin away from the myosin-binding sites on the actin filament. This allows cross-bridge cycling to begin.

Question 88

During the normal human gait cycle, maximum dorsiflexion of the ankle occurs during which phase?

Explanation

Maximum ankle dorsiflexion (approximately 10 degrees) occurs during terminal stance, just before heel-off, as the body's center of mass advances over the planted foot.

Question 89

The radial nerve is formed as the terminal continuation of which cord(s) of the brachial plexus?

Explanation

The radial nerve is the terminal branch of the posterior cord of the brachial plexus. It receives nerve root contributions from C5 through T1.

Question 90

Which bone morphogenetic protein (BMP) is an FDA-approved osteoinductive agent commonly utilized in specific spinal fusion surgeries?

Explanation

Recombinant human BMP-2 (rhBMP-2) is an FDA-approved osteoinductive agent used in anterior lumbar interbody fusions and open tibial shaft fractures. BMP-7 (OP-1) was previously used but had a humanitarian device exemption.

Question 91

The "toe region" of the load-elongation curve in normal ligament biomechanics primarily represents:

Explanation

The toe region reflects the initial uncrimping and straightening of the naturally wavy collagen fibers under low loads. Once straightened, the ligament enters the linear elastic region.

Question 92

According to Wolff's Law, bone remodels in response to mechanical stress. The mechanotransduction signal is primarily sensed by which resident cell type?

Explanation

Osteocytes act as the primary mechanosensory cells in bone. They detect mechanical strain and fluid flow through the canalicular network, coordinating osteoblast and osteoclast activity.

Question 93

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

Explanation

Galvanic corrosion is an electrochemical process that occurs when two dissimilar metals, such as stainless steel and titanium, are in physical contact within an electrolytic environment (body fluids).

Question 94

According to the Seddon classification, a nerve injury characterized by the disruption of axons and myelin sheaths but preservation of the endoneurium, perineurium, and epineurium is termed:

Explanation

Axonotmesis refers to axonal damage with intact connective tissue supporting structures (endoneurium, perineurium, epineurium). This allows for spontaneous, directional axonal regeneration.

Question 95

When utilizing a plate for absolute stability (compression plating) in a transverse midshaft radius fracture, the primary goal is to achieve:

Explanation

Absolute stability eliminates micromotion and interfragmentary strain, bypassing callus formation. This results in primary (direct) bone healing via cutting cones and Haversian remodeling.

Question 96

The primary restraint to valgus stress at the knee when tested at 30 degrees of flexion is the:

Explanation

The superficial medial collateral ligament (sMCL) provides the vast majority of restraint to valgus forces at the knee. This is best isolated clinically at 30 degrees of flexion.

Question 97

Rivaroxaban, an oral anticoagulant commonly prescribed for DVT prophylaxis following total joint arthroplasty, exerts its therapeutic effect by directly inhibiting:

Explanation

Rivaroxaban and apixaban are direct oral anticoagulants that reversibly inhibit Factor Xa. This interrupts both the intrinsic and extrinsic pathways of the coagulation cascade.

Question 98

Which of the following cytokines is most strongly associated with the catabolic breakdown of articular cartilage in osteoarthritis?

Explanation

IL-1β, along with TNF-α, are key pro-inflammatory cytokines that drive cartilage degradation in osteoarthritis. They upregulate matrix metalloproteinases (MMPs) and suppress matrix synthesis.

Question 99

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

Explanation

Screw pullout strength is directly proportional to the outer thread diameter, the length of bone engagement, and the shear strength of the bone material.

Question 100

A researcher is studying the molecular mechanisms of bone remodeling. They identify a glycoprotein secreted primarily by osteocytes that negatively regulates bone formation by binding to the LRP5/6 coreceptor on osteoblasts, thereby inhibiting the Wnt/beta-catenin signaling pathway. Which of the following therapeutic agents is specifically designed to target and inhibit this glycoprotein?

Explanation

The glycoprotein described is sclerostin, which is secreted by osteocytes and inhibits the Wnt/beta-catenin pathway, reducing bone formation. Romosozumab is a monoclonal antibody that targets and inhibits sclerostin, thereby promoting osteoblast activity and bone formation.

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

AAOS & ABOS Orthopedic Basic Science MCQs (Set 1): Anatomy, Biomechanics & Bone Biology | OITE Prep

Key Takeaway

AAOS & ABOS Orthopedic Basic Science MCQs (Set 1): Anatomy, Biomechanics & Bone Biology | OITE Prep

Chapter Index