ABOS Part I & OITE Orthopaedic Review: Sarcoma MCQs

Question 1

A 14-year-old male presents with left distal femoral pain and swelling. Imaging reveals a large metaphyseal lesion with cortical destruction and soft tissue extension. Biopsy confirms high-grade osteosarcoma. A whole-body PET-CT scan shows no distant metastases but highlights a separate, small, enhancing lesion within the same femur, proximal to the primary tumor. According to the 8th edition UICC staging criteria for bone sarcomas, what is the correct T-stage for this patient's primary tumor?

Explanation

Correct Answer: C

Academic Rationale:

For bone sarcomas (like osteosarcoma), the 8th edition UICC T-staging system is as follows: T1 is for tumors ≤ 8 cm, T2 for tumors > 8 cm. T3 is specifically designated for discontinuous tumor foci in the primary bone, commonly known as 'skip lesions.' The description of a separate, enhancing lesion within the same femur proximal to the primary tumor perfectly fits the definition of a skip lesion, regardless of the primary tumor's size. Therefore, the T-stage is T3.

Question 2

A 65-year-old male presents with a progressively enlarging, painless mass in his right thigh for 6 months. MRI reveals a 12 cm deep-seated, heterogeneous mass. Biopsy confirms high-grade pleomorphic undifferentiated sarcoma (PUS). Imaging of the chest, abdomen, and pelvis is negative for metastases, and lymph nodes are unremarkable. What is the correct UICC T-stage for this soft tissue sarcoma of the extremity?

Explanation

Correct Answer: C

Academic Rationale:

For soft tissue sarcomas of the trunk and extremities, the 8th edition UICC T-staging is based primarily on tumor size: T1 for tumors ≤ 5 cm, T2 for tumors > 5 cm and ≤ 10 cm, and T3 for tumors > 10 cm. The described tumor is 12 cm, which places it in the T3 category. T4 is reserved for tumors of any size with invasion of major neurovascular structures, bone, joint, or visceral organs. Since invasion of these critical structures is not mentioned, T3 is the correct stage based on size alone.

Question 3

In the context of the UICC staging system for musculoskeletal sarcomas, which factor is not a primary determinant of the Grade (G) component?

Explanation

Correct Answer: E

Academic Rationale:

The Grade (G) component in the UICC staging system for musculoskeletal sarcomas, particularly for soft tissue sarcomas (often using the FNCLCC system), is determined by histopathological features such as tumor differentiation, mitotic activity, and the extent of tumor necrosis. Pleomorphism and cellularity are aspects of differentiation. Tumor size, however, is a primary determinant of the T-stage (primary tumor extent), not the G-stage (histological grade). Therefore, tumor size is not a primary determinant of the Grade component.

Question 4

A 28-year-old female is diagnosed with a high-grade synovial sarcoma in her left forearm. Staging workup reveals a single, isolated pulmonary nodule consistent with metastatic disease. There is no regional lymph node involvement. According to the 8th edition UICC staging for soft tissue sarcomas, what is the overall stage group?

Explanation

Correct Answer: D

Academic Rationale:

The presence of distant metastases (M1) automatically upstages any sarcoma to Stage IV. For soft tissue sarcomas in the 8th edition UICC, M1 disease is classified as Stage IVA if there are no regional lymph node metastases (N0). If regional lymph node metastases (N1) are present alongside distant metastases, it would be Stage IVB. In this case, with M1 and N0, the overall stage is IVA.

Question 5

Which of the following describes the M1a designation for bone sarcomas in the 8th edition UICC staging system?

Explanation

Correct Answer: A

Academic Rationale:

In the 8th edition UICC staging system for bone sarcomas (e.g., osteosarcoma, Ewing sarcoma), M1a specifically refers to distant metastasis to the lung only. M1b refers to distant metastasis to bone or other distant sites (e.g., liver, brain). This distinction can be important prognostically, as isolated lung metastases sometimes have a better prognosis and are amenable to surgical resection in select cases.

Question 6

In the Enneking surgical staging system for musculoskeletal sarcomas, which of the following is characteristic of Stage IIB?

Explanation

Correct Answer: D

Academic Rationale:

The Enneking surgical staging system classifies musculoskeletal sarcomas based on Grade (G), Location (T), and Metastasis (M). Grade is either Low (G0) or High (G1). Location is either Intra-compartmental (T0) or Extra-compartmental (T1). Metastasis is either None (M0) or Present (M1). Stage II corresponds to high-grade lesions (G1). Stage IIA is high-grade, intra-compartmental (G1T0M0). Stage IIB is high-grade, extra-compartmental (G1T1M0). Stage III involves any grade with metastasis (M1). Therefore, IIB is High-grade, extra-compartmental, with no regional or distant metastases.

Question 7

Which of the following imaging modalities is considered the gold standard for assessing the local extent and relationship of a primary soft tissue sarcoma to neurovascular structures, critical for surgical planning?

Explanation

Correct Answer: D

Academic Rationale:

MRI with contrast is considered the gold standard for evaluating the local extent of a soft tissue sarcoma. It provides superior soft tissue resolution compared to CT and is excellent for delineating the tumor's margins, its relationship to critical neurovascular bundles, muscle compartments, and bone, which are all crucial for planning wide surgical excision and determining the feasibility of limb salvage. While CT is useful for bone involvement and chest staging, and PET-CT for metabolic activity and distant metastases, MRI offers the best local anatomical detail for surgical planning.

Question 8

In the management of osteosarcoma, what is the primary purpose of obtaining a 'surgical margin' and how is it defined?

Explanation

Correct Answer: C

Academic Rationale:

The primary purpose of obtaining a 'surgical margin' in osteosarcoma (and other sarcomas) is to achieve local tumor control. It is defined by the microscopic distance (or lack thereof) between the tumor cells and the cut edge of the resected surgical specimen. A 'wide' or 'negative' margin (R0 resection) means no tumor cells are seen at the cut edge, which significantly reduces the risk of local recurrence. This is distinct from tumor grade, chemotherapy response, metastatic potential, or reconstruction choice, although all are interrelated aspects of oncology.

Question 9

Which of the following is considered the primary 'oncological' function of neoadjuvant (preoperative) chemotherapy in the treatment of high-grade osteosarcoma?

Explanation

Correct Answer: C

Academic Rationale:

While neoadjuvant chemotherapy can reduce tumor size (facilitating limb salvage) and alleviate pain, its primary oncological function in high-grade osteosarcoma is to treat micrometastatic disease that is presumed to be present at diagnosis, even if not detectable on imaging. Osteosarcoma has a high propensity for early systemic spread. Preoperative chemotherapy aims to eradicate these micrometastases, thereby improving overall survival. Evaluating tumor sensitivity (pathological response) is a crucial secondary benefit, as it guides adjuvant therapy. Sterilizing tumor margins is an indirect benefit if the tumor shrinks, but achieving clear surgical margins remains paramount, and chemotherapy doesn't guarantee margin sterility.

Question 10

For patients with primary bone sarcoma, which of the following is the most significant factor influencing the prognosis of those with localized disease?

Explanation

Correct Answer: D

Academic Rationale:

While patient's age, tumor size, location, and histological subtype all have prognostic significance for localized bone sarcomas, the response to neoadjuvant chemotherapy, as measured by the percentage of tumor necrosis in the resected specimen, is consistently one of the most powerful and independent prognostic factors. A 'good response' (typically >90-95% necrosis) is strongly associated with improved event-free and overall survival. This is a key 'Nevin Insight' into evaluating the efficacy of systemic therapy. The other factors are important, but chemotherapy response integrates the biological aggressiveness of the tumor with the effectiveness of treatment.

Question 11

A 45-year-old male presents with acute onset back pain and progressive neurological deficit after a fall from a height. Imaging reveals a burst fracture of L1 with retropulsion into the spinal canal causing cauda equina compression. Neurological exam shows bilateral lower extremity weakness (3/5), saddle anesthesia, and urinary retention. Which of the following is the most appropriate initial management strategy?

Explanation

Correct Answer: A

Immediate posterior decompression and fusion with instrumentation is the most appropriate initial management. The presence of acute neurological deficit (cauda equina syndrome) due to spinal canal compromise warrants urgent surgical intervention. Posterior decompression addresses the canal compromise, and instrumentation provides stability to prevent further neurological deterioration and allow early mobilization. Conservative management is contraindicated due to neurological deficits. Anterior decompression might be considered in some cases but is generally not the initial approach for acute neurological deficits in a burst fracture with posterior element involvement, especially with posterior element injury. Laminectomy alone can destabilize the spine further and is associated with poor outcomes. Steroids are not proven to improve outcomes in cauda equina syndrome.

Question 12

A 68-year-old female undergoes a primary total hip arthroplasty for osteoarthritis. On postoperative day 2, she develops acute onset of severe hip pain and inability to bear weight. X-rays show no dislocation or periprosthetic fracture. Lab work reveals a CRP of 150 mg/L (normal <5 mg/L) and a WBC count of 18,000 cells/µL. Her temperature is 38.8°C. What is the most likely diagnosis and subsequent management?

Explanation

Correct Answer: C

The constellation of acute severe pain, inability to bear weight, systemic inflammatory response (high CRP, WBC, fever), and recent surgery is highly suggestive of acute periprosthetic joint infection (PJI). Urgent irrigation and debridement (I&D) with component retention (DAIR - Debridement, Antibiotics, and Implant Retention) is the preferred initial management for acute PJI, especially if the infection is diagnosed early (within 3-4 weeks of onset) and the components are well-fixed. If DAIR fails or components are unstable, a one-stage or two-stage exchange might be necessary. Aseptic loosening usually presents later and without acute systemic inflammatory signs. Hematoma can cause pain but usually not such a strong systemic response without infection. DVT typically presents with calf swelling and tenderness, not severe hip pain and systemic signs of infection. Nerve palsy would not explain the pain, fever, and inflammatory markers.

Question 13

A 28-year-old professional basketball player presents with chronic anterior knee pain and instability, worsening with activity. He previously sustained a twisting injury to his knee. Examination reveals a positive Lachman test, pivot shift test, and a firm endpoint on anterior drawer. His X-rays are normal. MRI shows a complete tear of the anterior cruciate ligament (ACL) and a horizontal tear of the posterior horn of the medial meniscus. What is the most appropriate management?

Explanation

Correct Answer: D

For a young, active individual with a complete ACL tear and symptomatic instability, ACL reconstruction is indicated. Given the concomitant horizontal tear of the posterior horn of the medial meniscus, and assuming it's a reparable tear (e.g., in the red-red or red-white zone, stable, and of sufficient size), primary repair of the meniscus in conjunction with ACL reconstruction is often the preferred approach. ACL reconstruction provides a stable environment that can promote meniscus healing and reduces the risk of future degenerative changes. Partial meniscectomy, while common, removes meniscal tissue and increases the risk of osteoarthritis, and is generally avoided if repair is feasible, especially in younger patients with concomitant ACL injury. Conservative management is not suitable for a professional athlete with symptomatic instability.

Question 14

A 72-year-old female presents with acute, severe, intractable pain in her right shoulder following a fall. She has significant medical comorbidities including atrial fibrillation on anticoagulation, poorly controlled diabetes, and severe osteoporosis. X-rays reveal a displaced 3-part fracture of the proximal humerus. She has limited active and passive range of motion. What is the most appropriate surgical management option?

Explanation

Correct Answer: B

In an elderly patient with severe osteoporosis, a 3-part displaced proximal humerus fracture, significant comorbidities, and likely poor bone quality, reverse total shoulder arthroplasty (rTSA) has emerged as a favorable option. ORIF often has high complication rates (screw cut-out, avascular necrosis) in this population. Hemiarthroplasty can lead to glenoid erosion and pain. Non-operative management is typically reserved for non-displaced or minimally displaced fractures, or very low-demand patients, and is unlikely to provide adequate pain relief or function for a severe displaced fracture. Total shoulder arthroplasty is not indicated for fracture care in this setting due to rotator cuff compromise.

Question 15

A 6-year-old boy presents with a limp and pain in his right hip. On examination, he has limited abduction and internal rotation of the hip. X-rays show flattening and increased density of the right femoral epiphysis. What is the most likely diagnosis?

Explanation

Correct Answer: D

The clinical presentation of a 6-year-old boy with a limp, limited abduction and internal rotation, and radiographic findings of flattening and increased density (sclerosis) of the femoral epiphysis are classic for Legg-Calvé-Perthes disease (LCPD). SCFE typically occurs in older, often obese, adolescents. DDH presents earlier in infancy/toddlerhood. Transient synovitis is a self-limiting inflammatory condition without radiographic changes of epiphyseal collapse. Septic arthritis would present with acute, severe pain, fever, and systemic signs of infection, and often rapid joint destruction, not chronic epiphyseal changes.

Question 16

A 34-year-old construction worker sustains an open right tibial shaft fracture (Gustilo-Anderson Type IIIA) after being crushed by a steel beam. He is hemodynamically stable. What is the most critical initial management step after addressing life-threatening injuries?

Explanation

Correct Answer: C

For an open fracture, especially Gustilo-Anderson Type IIIA, urgent irrigation and debridement (I&D) within 6-8 hours is paramount to minimize the risk of infection. This is a surgical emergency. Following thorough debridement, stabilization of the fracture (often with external fixation initially for severe open fractures, or intramedullary nailing in less contaminated Type I/II/IIIA fractures, depending on surgeon preference and wound status) is performed. Definitive intramedullary nailing is typically performed after initial debridement and often after several days if the wound requires observation or further debridement. Primary wound closure is generally contraindicated in contaminated open fractures until the wound is clean and infection risk is minimized. Systemic antibiotics are crucial but are adjunctive to surgical debridement.

Question 17

Which of the following statements regarding osteosarcoma is FALSE?

Explanation

Correct Answer: E

Radiation therapy is generally NOT the primary treatment modality for localized osteosarcoma. Surgical resection with wide margins is the cornerstone of local control. Osteosarcoma is relatively radioresistant. Chemotherapy, both neoadjuvant (pre-operative) and adjuvant (post-operative), is critical for controlling micrometastatic disease and improving survival. The other statements are true: osteosarcoma is the most common primary malignant bone tumor in children/adolescents, commonly affects the metaphysis of long bones, and frequently metastasizes to the lungs.

Question 18

A 55-year-old male presents with severe, bilateral knee pain, worse with activity and stair climbing. Radiographs reveal tricompartmental osteoarthritis with significant joint space narrowing, subchondral sclerosis, and osteophytes. He has failed extensive conservative management including NSAIDs, physical therapy, and intra-articular injections. His BMI is 32. He is otherwise healthy. What is the most appropriate next step in management?

Explanation

Correct Answer: C

Given the severe tricompartmental osteoarthritis, failure of conservative management, and the patient's age and activity level, bilateral total knee arthroplasty (TKA) is the most appropriate next step. UKA is only suitable for isolated unicompartmental disease. HTO is typically for younger, more active patients with unicompartmental varus malalignment and good bone stock, not tricompartmental disease. Arthroscopic debridement and lavage have shown limited long-term benefits for advanced osteoarthritis. While weight loss is beneficial, it's unlikely to fully resolve symptoms in severe, end-stage osteoarthritis and should be pursued as an adjunct to TKA, not as a replacement for surgical intervention when conservative measures have failed.

Question 19

A 30-year-old male sustains a high-energy impaction injury to his elbow. Radiographs show a comminuted fracture of the radial head with significant displacement and involvement of the coronoid process (Mason Type III, Regan-Morrey Type II coronoid). He has associated elbow instability. Which of the following is the most appropriate management strategy?

Explanation

Correct Answer: C

This patient has a 'terrible triad' injury of the elbow: comminuted radial head fracture, coronoid fracture, and associated elbow dislocation/instability (implied by high-energy injury and significant displacement). For Mason Type III radial head fractures with instability and coronoid involvement, radial head arthroplasty is often preferred over ORIF, especially if the radial head is irreparable, to restore stability and maintain length. Concomitant repair of the coronoid process (if a significant fragment) and the lateral collateral ligament complex is crucial for restoring elbow stability. Excision of the radial head alone in the presence of elbow instability and coronoid fracture can lead to persistent instability and proximal migration of the radius. ORIF of a comminuted radial head often fails. Closed reduction and casting are inadequate for such a complex, unstable injury.

Question 20

Which factor is most strongly associated with an increased risk of chronic regional pain syndrome (CRPS) Type 1 following an orthopedic injury or surgery?

Explanation

Correct Answer: D

Pre-existing psychological distress, anxiety, depression, and somatization disorders are strongly associated with an increased risk of developing CRPS Type 1. While CRPS can occur at any age, younger to middle-aged adults are more commonly affected than those over 60. Peripheral neuropathy is a risk factor for CRPS Type 2 (causalgia), not typically Type 1. Delayed mobilization can contribute to stiffness but is not as strong a predictor for CRPS as psychological factors. CRPS primarily affects the extremities, not the axial skeleton.

Question 21

A 24-year-old male presents with a deep-seated mass near his knee joint. Biopsy reveals a biphasic spindle cell tumor. What is the characteristic chromosomal translocation associated with this pathology?

Explanation

Synovial sarcoma is characterized by the t(X;18)(p11;q11) translocation, resulting in the SYT-SSX fusion gene. It commonly presents in young adults as a deep-seated soft tissue mass near a joint.

Question 22

A 68-year-old female presents with a painful total knee arthroplasty 2 years postoperatively. Serum CRP is 15 mg/L and ESR is 35 mm/hr. Synovial fluid aspiration yields a WBC count of 3,500 cells/uL with 75% PMNs. Which of the following synovial fluid biomarkers is most specific for confirming a periprosthetic joint infection (PJI)?

Explanation

Alpha-defensin is a highly specific and sensitive antimicrobial peptide released by neutrophils in response to infection. It is considered a major criterion or strong adjunct in modern PJI diagnostic algorithms.

Question 23

A 35-year-old male is brought to the ED after a high-speed motorcycle collision. He is hypotensive with a systolic BP of 70 mmHg. Pelvic radiograph shows a completely disrupted pubic symphysis with severe widening and disruption of the posterior sacroiliac ligaments bilaterally. After initiating massive transfusion, what is the most appropriate next step in orthopedic management?

Explanation

In a hemodynamically unstable patient with an anteroposterior compression (APC) pelvic ring injury, the initial step is applying a pelvic binder over the greater trochanters. This reduces pelvic volume and helps tamponade venous bleeding before considering angiography or surgery.

Question 24

A 16-year-old female with classic osteosarcoma of the distal femur completes neoadjuvant chemotherapy followed by wide surgical resection. Histological analysis of the resected specimen shows 95% tumor necrosis. What is the most significant prognostic implication of this finding?

Explanation

Tumor necrosis of 90% or greater following neoadjuvant chemotherapy in osteosarcoma indicates a good histologic response. This is the single most important prognostic factor for improved overall survival.

Question 25

A 72-year-old male falls and sustains a periprosthetic femur fracture around a cemented polished taper-slip stem placed 8 years ago. Radiographs show a spiral fracture around the tip of the stem. The stem is radiographically loose with subsidence. What is the Vancouver classification and appropriate treatment?

Explanation

A fracture around the stem (Type B) with a loose prosthesis but adequate bone stock is a Vancouver B2 fracture. The standard of care is revision arthroplasty using a long-stem prosthesis that bypasses the fracture.

Question 26

A 45-year-old male sustains a high-energy pilon fracture (OTA/AO 43-C3) with severe soft tissue swelling and fracture blisters over the medial ankle. What is the most appropriate initial management?

Explanation

High-energy pilon fractures with severe soft tissue compromise should be managed with staged treatment. Initial spanning external fixation allows soft tissues to heal prior to definitive ORIF, significantly reducing the risk of wound complications and infection.

Question 27

A 45-year-old male is diagnosed with a massive deep myxoid liposarcoma of the thigh. Staging chest CT is negative. Which additional imaging study is uniquely critical for complete staging of this specific histologic subtype?

Explanation

Myxoid liposarcoma has a unique propensity for extrapulmonary metastases, particularly to the spine and other bone marrow sites. An MRI of the total spine (or whole-body MRI) is recommended for accurate staging.

Question 28

A 60-year-old male with a metal-on-polyethylene total hip arthroplasty presents 5 years postoperatively with groin pain. Radiographs show no loosening. Aspiration is negative for infection. Serum cobalt levels are highly elevated, while chromium levels are only mildly elevated. What is the most likely etiology?

Explanation

Markedly elevated cobalt levels relative to chromium in a metal-on-polyethylene THA are classic for mechanically assisted crevice corrosion (trunnionosis) at the head-neck junction. This can lead to an adverse local tissue reaction (ALTR).

Question 29

A 28-year-old male sustains a closed midshaft tibia fracture. Four hours post-admission, he develops severe pain out of proportion to the injury, pain with passive stretch of the hallux, and paresthesias in the first web space. Compartment pressures measure 45 mmHg in the anterior compartment with a diastolic BP of 65 mmHg. What is the indicated treatment?

Explanation

The patient has clinical signs of acute compartment syndrome and a delta-P (diastolic BP minus compartment pressure) of 20 mmHg, confirming the diagnosis. Immediate four-compartment fasciotomy of the leg is the standard of care to prevent irreversible muscle and nerve necrosis.

Question 30

According to the Enneking surgical staging system for malignant bone tumors, how is a high-grade osteosarcoma that has broken through the cortex into the surrounding soft tissue but lacks regional or distant metastases classified?

Explanation

In the Enneking staging system, Stage II denotes a high-grade tumor. The suffix "B" indicates an extracompartmental lesion, making a tumor extending into surrounding soft tissue a Stage IIB.

Question 31

A 65-year-old female undergoes a primary total hip arthroplasty via a posterior approach. Six weeks later, she experiences a posterior dislocation while sitting in a low chair. Which of the following component malpositions most likely contributed to this specific type of instability?

Explanation

Posterior dislocation of a THA is often associated with inadequate anteversion (i.e., retroversion) of the acetabular component or femoral stem. This causes premature impingement during hip flexion and internal rotation, levering the head out posteriorly.

Question 32

A 78-year-old osteoporotic female sustains a 4-part proximal humerus fracture with varus impaction and head ischemia. She has significant shoulder osteoarthritis. What is the most reliable surgical option to predictably restore elevation and provide pain relief?

Explanation

Reverse total shoulder arthroplasty (RTSA) is the treatment of choice for elderly patients with complex 4-part proximal humerus fractures and poor bone quality or pre-existing osteoarthritis. RTSA relies on the deltoid for elevation and is less dependent on tuberosity healing compared to hemiarthroplasty.

Question 33

A 10-year-old boy presents with fever, elevated ESR, and thigh pain. Radiographs reveal a permeative diaphyseal lesion in the femur with a laminated periosteal reaction. Biopsy shows small round blue cells strongly positive for CD99. Which of the following characterizes the optimal definitive management of this condition?

Explanation

Ewing sarcoma is a highly aggressive, systemic disease from the outset. Standard management involves neoadjuvant systemic chemotherapy, followed by local control via wide surgical resection or radiation, and then adjuvant chemotherapy.

Question 34

During a primary total knee arthroplasty, the surgeon notes poor patellar tracking with a tendency for lateral subluxation. Which of the following technical errors most commonly causes this complication?

Explanation

Internal rotation of either the femoral or tibial components increases the Q-angle, exacerbating lateral patellar maltracking. To optimize patellar tracking, the femoral component should be placed in slight external rotation and the patellar component placed medially.

Question 35

A 30-year-old male sustains a displaced Pauwels type III (vertical) femoral neck fracture. Which fixation construct provides the greatest biomechanical stability to resist the high shear forces inherent to this fracture pattern?

Explanation

Pauwels type III fractures have a highly vertical orientation, resulting in significant shear forces. Biomechanical studies demonstrate that a sliding hip screw provides superior stability against shear stress compared to multiple parallel cancellous screws.

Question 36

A 55-year-old male presents with deep shoulder pain. Imaging shows a lytic lesion in the proximal humerus with popcorn calcifications, endosteal scalloping, and soft tissue extension. Biopsy confirms a grade 3 conventional chondrosarcoma. What is the recommended treatment?

Explanation

High-grade (Grade 2 and 3) conventional chondrosarcomas are generally resistant to chemotherapy and radiotherapy. The standard treatment is wide surgical resection to achieve negative margins.

Question 37

A 70-year-old male is undergoing evaluation for a total hip arthroplasty. Standing and sitting lateral spinopelvic radiographs demonstrate a stiff lumbar spine with less than 10 degrees of pelvic tilt change between positions. What adjustment to acetabular cup positioning should be considered to minimize dislocation risk?

Explanation

In patients with a stiff spine, the pelvis fails to retrovert during sitting, increasing the risk of anterior impingement and posterior dislocation. Surgeons often compensate by placing the acetabular cup in slightly increased anteversion and inclination.

Question 38

A 40-year-old female sustains a Schatzker Type VI tibial plateau fracture. Postoperatively, she develops foot drop and numbness in the first web space. During the surgical approach, which structure was most likely injured, and where is its most vulnerable anatomical location in this context?

Explanation

Foot drop and numbness in the first web space indicate a peroneal nerve injury. The common peroneal nerve wraps around the fibular neck, making it highly vulnerable during lateral or posterolateral approaches to complex tibial plateau fractures.

Question 39

A 12-year-old boy presents with a diaphyseal femur lesion accompanied by an "onion skin" periosteal reaction. Biopsy reveals small round blue cells that are strongly positive for CD99. Which chromosomal translocation is most commonly associated with this pathology?

Explanation

Ewing sarcoma is characterized by small round blue cells, CD99 positivity, and an "onion skin" periosteal reaction. It is defined by the t(11;22) translocation, which creates the EWS-FLI1 fusion protein.

Question 40

A 72-year-old female sustains a fall and presents with a periprosthetic fracture around her cemented total hip arthroplasty. Radiographs demonstrate a fracture around the stem, which is visibly loose, but the proximal femoral bone stock remains adequate. According to the Vancouver classification, what is the most appropriate surgical management?

Explanation

This is a Vancouver B2 fracture (fracture around a loose stem with adequate bone stock). The standard of care is revision arthroplasty using a long cementless stem that bypasses the fracture by at least two cortical diameters.

Question 41

A 25-year-old male sustains a high-energy Pauwels type III femoral neck fracture. To maximize biomechanical stability and minimize the risk of varus collapse, which fixation construct provides the highest resistance to the vertical shear forces inherent to this fracture pattern?

Explanation

Pauwels type III fractures have a highly vertical orientation, resulting in significant shear forces that promote varus collapse. A sliding hip screw (fixed-angle device) provides superior biomechanical stability against vertical shear compared to multiple parallel cancellous screws.

Question 42

A 30-year-old male presents with a slowly enlarging, painful mass near his ankle joint. MRI shows a deep mass with heterogeneous signal intensity. Core needle biopsy reveals a biphasic tumor with both spindle and epithelial cell components. Which genetic abnormality is diagnostic for this tumor?

Explanation

The clinical presentation and biphasic histology are classic for synovial sarcoma. This aggressive soft tissue sarcoma is genetically defined by the t(X;18) translocation, resulting in the SYT-SSX fusion gene.

Question 43

A 68-year-old male presents with a painful total knee arthroplasty 3 years postoperatively. Serum ESR is 45 mm/hr and CRP is 25 mg/L. Joint aspiration yields a synovial WBC count of 4,500 cells/uL with 85% polymorphonuclear neutrophils, and the alpha-defensin test is positive. According to the current consensus criteria, what is the most appropriate next step?

Explanation

The patient meets definitive criteria for a chronic periprosthetic joint infection (positive alpha-defensin, elevated synovial WBC > 3000, and PMN > 80%). The gold standard treatment for chronic PJI in the United States is a two-stage revision arthroplasty.

Question 44

A 32-year-old male sustains a closed comminuted tibial shaft fracture. Two hours post-injury, he complains of severe pain exacerbated by passive stretch of the hallux. His blood pressure is 90/60 mmHg. Intracompartmental pressure testing reveals an anterior compartment pressure of 35 mmHg. What is the most definitive indication for a four-compartment fasciotomy in this patient?

Explanation

A delta pressure (diastolic blood pressure minus intracompartmental pressure) of less than 30 mmHg is the most reliable threshold for diagnosing acute compartment syndrome. Relying on absolute pressure can lead to overtreatment, especially in hypotensive patients.

Question 45

A 28-year-old female presents with a lytic, expansile lesion in the distal femur epiphysis extending to the subchondral bone. Biopsy confirms a Giant Cell Tumor of bone. She is started on denosumab therapy preoperatively. What is the precise mechanism of action of this medication?

Explanation

Denosumab is a monoclonal antibody that binds to and inhibits RANKL. This prevents the differentiation and function of the reactive osteoclast-like giant cells, thereby reducing osteolysis and allowing for peripheral tumor ossification.

Question 46

During a primary total knee arthroplasty, after making the standard initial bone cuts, trial components are placed. The knee is found to be tight in flexion but well-balanced in extension. Which of the following surgical adjustments is most appropriate to balance the gaps?

Explanation

A knee that is tight in flexion but well-balanced in extension requires an adjustment that selectively increases the flexion gap. Increasing the posterior tibial slope will open the flexion gap without significantly altering the extension gap.

Question 47

A 45-year-old female sustains an intra-articular distal humerus fracture involving the capitellum and the entire trochlea, lacking posterior comminution (Dubberley Type 3A). Which surgical approach provides the most optimal visualization for direct reduction of this specific articular shear fracture?

Explanation

Capitellar and trochlear shear fractures (Dubberley classification) are best visualized and fixed via an extended lateral approach. Olecranon osteotomy provides excellent posterior exposure but suboptimal visualization of anterior shear fragments.

Question 48

A 15-year-old male with conventional osteosarcoma of the proximal tibia undergoes 10 weeks of neoadjuvant MAP (methotrexate, doxorubicin, cisplatin) chemotherapy, followed by wide surgical resection. Which histological finding in the resected specimen is most strongly associated with a favorable overall survival?

Explanation

The degree of tumor necrosis following neoadjuvant chemotherapy is the single most important prognostic factor in osteosarcoma. Greater than 90% necrosis (Huvos Grade III or IV) indicates an excellent histologic response and correlates with significantly improved survival.

Question 49

A 60-year-old female suffers recurrent posterior dislocations following a primary total hip arthroplasty performed via a posterior approach. Radiographs demonstrate the acetabular cup is positioned in 45 degrees of inclination and 0 degrees of anteversion. What is the primary mechanical cause of her instability?

Explanation

The standard safe zone for acetabular component positioning is approximately 40 +/- 10 degrees of inclination and 15 +/- 10 degrees of anteversion. A cup with 0 degrees of anteversion is retroverted, which heavily predisposes the hip to posterior dislocation.

Question 50

A 28-year-old female presents with a slow-growing mass in her popliteal fossa. Biopsy reveals a biphasic tumor with both epithelial and spindle cell components. Which chromosomal translocation is most characteristic of this specific sarcoma?

Explanation

The patient has a synovial sarcoma, which is classically associated with the t(X;18)(p11;q11) translocation resulting in the SYT-SSX fusion gene. The t(11;22) translocation is characteristic of Ewing sarcoma, while t(12;16) is seen in myxoid liposarcoma.

Question 51

A 65-year-old man presents with knee pain 2 years after a total knee arthroplasty. Synovial fluid aspiration yields a white blood cell count of 3,500 cells/µL with 75% neutrophils. Which of the following is considered a major criterion for definitive periprosthetic joint infection (PJI) according to the 2018 International Consensus Meeting (ICM) criteria?

Explanation

According to the 2018 ICM criteria, a major criterion for PJI is either two positive periprosthetic cultures with the same organism or the presence of a sinus tract communicating with the joint. The other options are considered minor criteria.

Question 52

A 35-year-old male is involved in a high-speed motor vehicle collision. Pelvic radiographs show a widened symphysis pubis of 3.5 cm. CT scan reveals disruption of the anterior sacroiliac ligaments but intact posterior sacroiliac ligaments. What is the correct Young-Burgess classification of this injury?

Explanation

An APC II injury is characterized by a symphyseal diastasis greater than 2.5 cm and disruption of the anterior sacroiliac ligaments. The posterior sacroiliac ligaments remain intact, providing vertical stability but rotational instability.

Question 53

A 50-year-old male presents with dull right shoulder pain. Radiographs reveal a lytic lesion in the proximal humerus with 'popcorn' calcifications. MRI demonstrates endosteal scalloping greater than two-thirds of the cortical thickness. Biopsy confirms a Grade II chondrosarcoma. What is the mainstay of treatment?

Explanation

The standard of care for Grade II (intermediate) and Grade III (high-grade) conventional chondrosarcomas is wide surgical resection. Conventional chondrosarcomas are generally resistant to both chemotherapy and radiation therapy.

Question 54

A 55-year-old female undergoes a total hip arthroplasty utilizing a ceramic-on-ceramic bearing. One year postoperatively, she complains of a high-pitched squeaking noise during gait. Which component factor is most strongly associated with the development of this phenomenon?

Explanation

Squeaking in ceramic-on-ceramic total hip arthroplasty is most heavily linked to edge loading of the bearing surfaces. This is typically caused by component malposition, specifically excessive acetabular cup inclination or malversion.

Question 55

A 28-year-old male sustains a highly vertical, displaced femoral neck fracture (Pauwels Type III) after a fall from a height. Open reduction and internal fixation is planned. Which fixation construct provides the highest biomechanical stability against shear forces for this specific fracture pattern?

Explanation

Pauwels Type III fractures possess a high degree of verticality, subjecting them to intense shear forces. Biomechanical studies indicate that a fixed-angle device, such as a sliding hip screw with a derotation screw, offers superior resistance to shear compared to multiple cancellous screws.

Question 56

A 15-year-old boy completes neoadjuvant chemotherapy and undergoes wide resection for conventional high-grade osteosarcoma of the distal femur. Pathological examination of the resected specimen reveals 95% tumor necrosis. What does this histopathologic finding indicate regarding his prognosis?

Explanation

Tumor necrosis of 90% or greater (Huvos Grade III or IV) following neoadjuvant chemotherapy is the most significant positive prognostic indicator for overall survival in osteosarcoma. However, patients must still complete their standardized postoperative adjuvant chemotherapy course.

Question 57

Which wear mechanism is primarily responsible for the generation of submicron polyethylene wear debris, which ultimately triggers macrophage-mediated osteolysis in total hip arthroplasty?

Explanation

Adhesive wear occurs when asperities on the femoral head momentarily 'micro-weld' to the polyethylene surface, shearing off submicron particles during joint motion. These tiny submicron particles are the primary instigators of the macrophage cascade leading to periprosthetic osteolysis.

Question 58

A 42-year-old female presents with a posteromedial shear fracture of the tibial plateau (Schatzker IV variant). Which surgical approach provides the most direct visualization and allows for optimal anti-glide buttress plating of this specific fragment?

Explanation

The posteromedial approach, exploiting the interval between the pes anserinus anteriorly and the medial head of the gastrocnemius posteriorly, grants direct access to the posterior aspect of the medial tibial plateau. This is critical for applying a posterior buttress plate to counteract vertical shear forces.

Question 59

A 9-year-old boy presents with a 1-month history of progressive thigh pain and low-grade fevers. Radiographs demonstrate a permeative diaphyseal lesion with a classic 'onion-skin' periosteal reaction. A core needle biopsy confirms Ewing sarcoma. Which fusion gene is most likely present in this patient's tumor cells?

Explanation

Ewing sarcoma is driven by a balanced translocation, most commonly t(11;22)(q24;q12), which fuses the EWS gene on chromosome 22 to the FLI1 gene on chromosome 11. This creates the EWS-FLI1 fusion oncoprotein, present in approximately 85-90% of cases.

Question 60

A 68-year-old female sustains a posterior dislocation of her total hip arthroplasty 4 weeks postoperatively. The index procedure was performed via a posterior approach. She is successfully closed reduced in the emergency department. What is the most common etiology for early posterior instability in this specific clinical setting?

Explanation

Early dislocation (within the first few months) following a total hip arthroplasty is most frequently associated with component malposition. For posterior dislocations, this typically involves inadequate acetabular anteversion (retroversion) or inadequate offset.

Question 61

A 30-year-old male is admitted with a closed, severely comminuted tibial shaft fracture. Two hours later, he complains of excruciating leg pain that is unresponsive to IV opioids. Examination reveals exquisite pain with passive stretch of the hallux. What absolute threshold of delta pressure (Diastolic Blood Pressure minus Intracompartmental Pressure) definitively mandates a four-compartment fasciotomy?

Explanation

A delta pressure (Diastolic Blood Pressure minus Intracompartmental Pressure) of less than 30 mmHg is a highly sensitive and specific objective threshold indicating acute compartment syndrome. When clinical suspicion is high and delta pressure falls below 30 mmHg, emergent fasciotomy is mandated.

Question 62

A 35-year-old female presents with a massive, unresectable recurrent Giant Cell Tumor (GCT) of the sacrum. The multidisciplinary tumor board recommends initiation of denosumab therapy. What is the specific mechanism of action of this medication in treating GCT of bone?

Explanation

Denosumab is a fully human monoclonal antibody that specifically binds to the Receptor Activator of Nuclear factor Kappa-B Ligand (RANKL). In GCT, neoplastic stromal cells express RANKL, which recruits and activates the osteoclast-like giant cells; denosumab breaks this cycle, halting osteolysis.

Question 63

A 62-year-old man presents with progressive groin pain 5 years after receiving a modern metal-on-polyethylene total hip arthroplasty. Serum cobalt and chromium levels are significantly elevated, and an MRI with MARS sequencing reveals a large fluid collection around the hip joint. What is the most likely source of the elevated metal ions?

Explanation

In a metal-on-polyethylene THA presenting with Adverse Local Tissue Reaction (ALTR) and elevated cobalt/chromium levels, the source of the metal ions is the modular junction. This phenomenon, known as trunnionosis, involves fretting and crevice corrosion at the head-neck taper.

Question 64

A 24-year-old male sustains a displaced Hawkins Type III fracture of the talar neck following a motorcycle accident. Which specific arterial blood supply is considered the dominant contributor to the talar body and is most predictably disrupted in this severe injury pattern?

Explanation

The artery of the tarsal canal is a branch of the posterior tibial artery and provides the dominant, most critical blood supply to the talar body. Disruption of this artery, along with others in displaced talar neck fractures, dramatically increases the risk of avascular necrosis.

Question 65

A 12-year-old boy presents with a diaphyseal femur lesion. Biopsy shows a proliferation of small blue round cells. Which chromosomal translocation and resulting fusion gene are most characteristic of this diagnosis?

Explanation

Ewing sarcoma is classically associated with the t(11;22)(q24;q12) translocation, resulting in the EWS-FLI1 fusion protein. This acts as an aberrant transcription factor that drives tumorigenesis.

Question 66

Following a total hip arthroplasty via a posterior approach, a patient experiences recurrent posterior dislocations. Radiographs reveal the acetabular component is placed in 30 degrees of abduction and 5 degrees of retroversion. What is the most appropriate definitive management?

Explanation

The acetabular component is retroverted, predisposing the patient to posterior dislocation. The target for cup placement is typically 15-20 degrees of anteversion and 40-45 degrees of abduction, making cup revision the definitive treatment.

Question 67

A 28-year-old male sustains a Hawkins Type III talar neck fracture following a motor vehicle collision. Based on the classification, what is the approximate risk of developing avascular necrosis (AVN) of the talar body?

Explanation

A Hawkins Type III talar neck fracture involves subluxation or dislocation of both the subtalar and tibiotalar joints. This severe injury disrupts the major blood supplies to the talus, leading to an AVN risk nearing 75-100%.

Question 68

A 55-year-old female presents with shoulder pain. Radiographs show a lytic lesion in the proximal humerus with intralesional popcorn calcification and endosteal scalloping greater than two-thirds of the cortical thickness. Biopsy confirms Grade 2 chondrosarcoma. What is the most appropriate surgical treatment?

Explanation

Grade 2 (intermediate-grade) chondrosarcomas have a higher risk of local recurrence and metastasis compared to Grade 1 lesions. They require wide en bloc resection and appropriate reconstruction, as they are notoriously unresponsive to chemotherapy and radiation.

Question 69

A 35-year-old farm worker sustains a severely contaminated open tibial shaft fracture with a 12 cm laceration and significant periosteal stripping, requiring a rotational flap for coverage. According to the Gustilo-Anderson classification, what is the most appropriate initial antibiotic regimen?

Explanation

This is a Gustilo-Anderson Type IIIB open fracture occurring in a farm environment, significantly increasing the risk of Clostridium infection. Standard protocol requires a first-generation cephalosporin, an aminoglycoside, and high-dose penicillin for anaerobic coverage.

Question 70

A 68-year-old male presents with a painful total knee arthroplasty 3 years post-operatively. Synovial fluid analysis reveals a WBC count of 4,500 cells/mcL with 85% PMNs. Alpha-defensin is positive. No sinus tract is present. According to the MSIS criteria, what is the diagnostic conclusion?

Explanation

According to major PJI diagnostic criteria, a positive alpha-defensin test combined with an elevated synovial WBC count (>3000 cells/mcL) and PMN percentage (>80%) in the chronic setting definitively confirms periprosthetic joint infection.

Question 71

A 22-year-old male complains of severe, aching pain in the posterior elements of his L4 vertebra, which is not completely relieved by NSAIDs. Imaging reveals a 2.5 cm expansile, radiolucent lesion with a sclerotic rim in the L4 lamina. What is the most likely diagnosis?

Explanation

Osteoblastomas are benign bone-forming tumors often found in the posterior elements of the spine and are typically larger than 2 cm. Unlike osteoid osteomas, the pain is less predictably relieved by NSAIDs and they often lack a dense reactive sclerotic zone of the same proportion.

Question 72

A patient presents after a severe crush injury with an anteroposterior compression (APC) Type III pelvic ring injury. What primary anatomical structure's complete disruption differentiates an APC III from an APC II injury?

Explanation

In the Young-Burgess classification, APC II involves disruption of the anterior sacroiliac, sacrotuberous, and sacrospinous ligaments but leaves the posterior sacroiliac ligaments intact. APC III involves complete disruption of the posterior sacroiliac ligaments, resulting in complete global pelvic instability.

Question 73

A 45-year-old female undergoes a metal-on-metal hip resurfacing arthroplasty. Two years later, she develops groin pain. Workup reveals an elevated serum cobalt-to-chromium ratio and a solid, cystic mass communicating with the joint. What is the most critical biomechanical risk factor for developing this adverse local tissue reaction (ALTR)?

Explanation

Adverse local tissue reactions (ALTR) in metal-on-metal articulations are heavily driven by edge loading. Edge loading is most commonly caused by malposition of the acetabular component, specifically excessive inclination (steep cup) and excessive anteversion.

Question 74

A 32-year-old male presents with a slow-growing mass in the popliteal fossa. Histology reveals a biphasic pattern of spindle cells and epithelial cells. Cytogenetic testing demonstrates a t(X;18)(p11;q11) translocation. What is the resulting fusion gene critical for this tumor's pathogenesis?

Explanation

Synovial sarcoma is characterized by the t(X;18)(p11;q11) chromosomal translocation. This results in the fusion of the SYT gene on chromosome 18 with one of the SSX genes on the X chromosome, creating the oncogenic SYT-SSX fusion protein.

Question 75

When fixing a displaced 4-part proximal humerus fracture, preservation of the humeral head blood supply is critical. Based on recent quantitative perfusion studies, which artery provides the primary blood supply to the articular segment of the humeral head?

Explanation

Historically, the anterior humeral circumflex was thought to be the primary vascular supply. However, recent anatomic studies demonstrate that the posterior humeral circumflex artery provides the vast majority (up to 64%) of the blood supply to the humeral head.

Question 76

Which of the following is considered an absolute contraindication to medial unicompartmental knee arthroplasty (UKA) for isolated medial compartment osteoarthritis?

Explanation

Inflammatory arthritis, such as rheumatoid arthritis, is an absolute contraindication for UKA due to the systemic nature of the disease and high likelihood of progression to other compartments. Modern criteria have relaxed historical contraindications like obesity and older age.

Question 77

A 34-year-old female presents with a recurrent giant cell tumor of the distal radius that is now deemed unresectable without significant morbidity. Which targeted medical therapy is most appropriate to inhibit the recruitment and activity of the osteoclast-like giant cells?

Explanation

Denosumab is a monoclonal antibody against RANKL. It prevents RANKL from binding to RANK on the osteoclast-like giant cells, thereby inhibiting their recruitment and bone-resorbing activity, making it highly effective for unresectable GCTs.

Question 78

In the pathophysiology of acute compartment syndrome following a tibia fracture, which physiological sequence correctly describes the initial cascade leading to muscle ischemia?

Explanation

Compartment syndrome begins with elevated interstitial pressure that exceeds venous outflow pressure, leading to venous hypertension. This reduces the arteriovenous pressure gradient, which collapses capillaries and impairs local tissue perfusion long before actual arterial occlusion occurs.

Question 79

A 62-year-old female complains of a painful catch and an audible pop when extending her knee from a flexed position, one year after a posterior-stabilized total knee arthroplasty. What is the most likely pathophysiologic cause of this specific complication?

Explanation

Patellar clunk syndrome is a complication classically associated with posterior-stabilized TKA designs. It occurs when a fibrous nodule forms at the superior pole of the patella and catches in the intercondylar box of the femoral component during knee extension.

Question 80

In the AJCC staging system for soft tissue sarcomas of the extremity, what finding automatically upgrades the tumor to at least Stage IV, regardless of tumor size or histologic grade?

Explanation

According to the AJCC staging system for soft tissue sarcomas, the presence of regional lymph node metastasis (N1) is a profound poor prognostic factor. It automatically categorizes the disease as Stage IV, identical to the presence of distant metastasis.

Question 81

A 25-year-old male sustains a vertically oriented (Pauwels Type III) basicervical femoral neck fracture. Which biomechanical factor makes this fracture pattern particularly prone to failure of internal fixation?

Explanation

Pauwels Type III fractures possess a highly vertical fracture line (angle > 50 degrees). This vertical orientation converts normal weight-bearing forces into high shear forces across the fracture site, significantly increasing the risk of varus collapse and nonunion.

Question 82

An 80-year-old female with a cemented total hip arthroplasty sustains a fall. Radiographs show a periprosthetic femur fracture around the tip of the stem. The stem remains well-fixed within the cement mantle. What is the Vancouver classification and appropriate treatment?

Explanation

A fracture around or just below the stem tip with a well-fixed stem is classified as Vancouver Type B1. Because the stem is stable, the gold standard treatment is open reduction and internal fixation utilizing locking plates and cables, sparing the well-fixed implant.

Orthopaedic Surgery Board Exam Review: ABOS Part I & AAOS OITE Prep Questions | Part 22210

ABOS Part I & AAOS OITE Orthopaedic Surgery Review: Sarcoma, Trauma, Arthroplasty | Part 22211

Key Takeaway

ABOS Part I & AAOS OITE Orthopaedic Surgery Review: Sarcoma, Trauma, Arthroplasty | Part 22211

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Chapter Index