Orthopedic Board Review MCQs: Lipoma & Bone Tumors

Question 1

A 55-year-old male presents with a large, deep-seated painless mass in his thigh. MRI shows a predominantly lipomatous tumor with thickened septa. Histopathology shows mature adipocytes with focal nuclear atypia.

Which genetic alteration is most specific to confirm the diagnosis of an atypical lipomatous tumor?

Explanation

Atypical lipomatous tumors (well-differentiated liposarcomas) are characterized by ring or giant rod chromosomes leading to MDM2 and CDK4 gene amplification on chromosome 12q13-15. Testing for MDM2 amplification distinguishes it from a benign lipoma.

Question 2

A 28-year-old male with Multiple Hereditary Exostoses presents with recent growth and pain in a distal femur lesion. MRI shows a cartilage cap thickness of 25 mm.

What is the most likely pathological finding in this growing lesion?

Explanation

Malignant transformation of an osteochondroma is almost exclusively to a secondary chondrosarcoma. A cartilage cap thickness greater than 2 cm (20 mm) in an adult is highly suspicious for malignant transformation.

Question 3

A 65-year-old male with long-standing ankylosing spondylitis presents to the ED after a minor fall. He complains of severe neck pain but has no neurological deficits. Radiographs demonstrate a fracture through the C6-C7 intervertebral disc space extending into the posterior elements.

What is the most appropriate management?

Explanation

Cervical spine fractures in AS are highly unstable, acting as long lever arms, and carry a high risk of neurologic deterioration. Rigid internal fixation, typically posterior or combined anteroposterior, is required due to high failure rates with non-operative management.

Question 4

A 45-year-old male presents with chronic knee pain, catching, and restricted range of motion. Radiographs reveal multiple uniform, calcified intra-articular loose bodies.

Primary synovial chondromatosis is currently best described as:

Explanation

Primary synovial chondromatosis was historically considered a reactive metaplasia. However, cytogenetic studies identifying recurrent FN1-ACVR2A fusions have reclassified it as a benign neoplastic process.

Question 5

Deep lipomas of the extremities are classified as intramuscular or intermuscular. Which of the following statements regarding intramuscular lipomas is true compared to their subcutaneous counterparts?

Explanation

Intramuscular lipomas frequently infiltrate between muscle fibers, making complete enucleation more difficult. This leads to a higher local recurrence rate (up to 15%) compared to well-encapsulated subcutaneous lipomas.

Question 6

A 12-year-old boy with multiple bony bumps around his knees and wrists is diagnosed with Multiple Hereditary Exostoses (MHE).

The pathogenesis of this condition involves a mutation in EXT1 or EXT2 genes, which leads to a defect in the synthesis of what molecular component?

Explanation

EXT1 and EXT2 genes encode glycosyltransferases essential for the synthesis of heparan sulfate. Deficiency in heparan sulfate disrupts Indian Hedgehog (Ihh) signaling, causing abnormal chondrocyte proliferation.

Question 7

A 50-year-old patient with known ankylosing spondylitis presents with localized, worsening back pain. Radiographs reveal a destructive disco-vertebral lesion with reactive sclerosis.

What is the primary etiology of this specific lesion?

Explanation

This describes an Andersson lesion, a non-infectious, destructive disco-vertebral lesion seen in AS. It represents a pseudarthrosis resulting from continued motion at a single unfused segment or an occult fracture in a rigid spine.

Question 8

A 38-year-old female undergoes arthroscopic removal of numerous loose bodies from her shoulder joint. Pathology confirms primary synovial chondromatosis. Which of the following features most reliably differentiates primary from secondary synovial chondromatosis?

Explanation

Primary synovial chondromatosis features hundreds of loose bodies that are relatively uniform in size due to synchronized neoplastic proliferation. Secondary synovial chondromatosis typically yields fewer, irregularly sized loose bodies formed from mechanical wear.

Question 9

An atypical lipomatous tumor (well-differentiated liposarcoma) located in which of the following anatomic areas carries the highest risk for disease-specific mortality and dedifferentiation?

Explanation

Lesions in the retroperitoneum carry a much worse prognosis because complete surgical margins are difficult to achieve. The high rate of local recurrence in the retroperitoneum increases the time-dependent risk of dedifferentiation.

Question 10

An 18-year-old presents with a painless mass at the proximal medial tibia.

Which of the following radiographic or MRI findings is the absolute sine qua non for diagnosing a true osteochondroma?

Explanation

The pathognomonic imaging feature of an osteochondroma is direct cortico-medullary continuity with the native host bone. This helps differentiate it from surface lesions like parosteal osteosarcoma.

Question 11

A 55-year-old male presents with a deep, painless, slow-growing mass in his thigh.

Biopsy reveals mature adipocytes with focal nuclear atypia. To differentiate an atypical lipomatous tumor from a benign lipoma, which molecular finding is most diagnostic?

Explanation

Atypical lipomatous tumors (well-differentiated liposarcomas) are characterized by ring or giant marker chromosomes that result in the amplification of MDM2 and CDK4 genes. Benign lipomas lack this genetic amplification.

Question 12

A 25-year-old male with Multiple Hereditary Exostoses (MHE) reports rapid growth of a previously stable mass on his proximal tibia.

MRI is obtained. Which of the following MRI findings is the most concerning indicator of malignant transformation to secondary chondrosarcoma?

Explanation

In a skeletally mature patient, a cartilage cap thickness exceeding 1.5 to 2.0 cm on MRI is highly suspicious for malignant transformation to secondary chondrosarcoma. Medullary continuity is a diagnostic feature of a benign osteochondroma, not a sign of malignancy.

Question 13

A 60-year-old male with a 20-year history of ankylosing spondylitis presents after a ground-level fall. He has severe neck pain. Radiographs reveal a displaced C6-C7 discovertebral fracture.

What is the most appropriate definitive management?

Explanation

Fractures in the ankylosed spine act like long bone fractures and are highly unstable, often involving all three columns. They require rigid, long-segment posterior instrumentation and fusion to prevent catastrophic neurologic decline or nonunion.

Question 14

A 40-year-old male presents with chronic knee catching and swelling. Imaging shows multiple intra-articular calcific loose bodies.

Which histopathologic feature most reliably differentiates primary synovial chondromatosis from secondary osteochondromatosis?

Explanation

Primary synovial chondromatosis represents benign neoplastic metaplasia of the synovium, characterized by clustering of chondrocytes and cellular atypia (which can mimic chondrosarcoma). Secondary chondromatosis bodies simply represent detached articular fragments and lack this metaplastic atypia.

Question 15

Which of the following MRI sequences is most useful for confirming that a well-circumscribed soft tissue mass is a benign lipoma rather than a fluid-filled cyst?

Explanation

Both fat and proteinaceous fluid/hemorrhage can appear bright on standard T1-weighted images. A T1-weighted fat-suppressed sequence will show a drop in signal for a lipoma, definitively identifying it as fat.

Question 16

Multiple hereditary exostoses (MHE) is an autosomal dominant condition linked to mutations in the EXT1 and EXT2 genes. What is the primary function of the proteins encoded by these genes?

Explanation

The EXT1 and EXT2 genes encode glycosyltransferases essential for the polymerization of heparan sulfate. Mutations lead to defective heparan sulfate proteoglycans, disrupting normal chondrocyte proliferation and leading to osteochondroma formation.

Question 17

A 45-year-old male with advanced Ankylosing Spondylitis develops progressive lower back pain. Radiographs reveal a destructive, radiolucent lesion at the T12-L1 discovertebral junction with sclerotic margins.

What is the most likely diagnosis?

Explanation

An Anderson lesion is an inflammatory or traumatic pseudarthrosis that occurs in the ankylosed spine. It often presents as a destructive discovertebral lesion with reactive sclerosis and is managed with surgical stabilization if painful and unstable.

Question 18

A 55-year-old female with a long history of primary synovial chondromatosis of the hip presents with new, rapidly worsening rest pain and progressive joint destruction on radiographs. What is the most likely complication?

Explanation

Primary synovial chondromatosis has a known, albeit rare, risk of malignant transformation into secondary synovial chondrosarcoma (approximately 5% incidence). Rapid clinical deterioration and bone destruction are hallmark signs.

Question 19

An atypical lipomatous tumor (ALT) and a well-differentiated liposarcoma (WDLS) are histologically identical. The terminology differs based purely on anatomical location. In which location is the term "well-differentiated liposarcoma" utilized due to the high risk of unresectability and mortality from local recurrence?

Explanation

The term well-differentiated liposarcoma (WDLS) is reserved for lesions in the retroperitoneum or mediastinum, where wide excision is frequently impossible, leading to high local recurrence rates and death. In the appendicular skeleton, they are termed atypical lipomatous tumors (ALT).

Question 20

Which of the following is a strict diagnostic requirement for the radiographic identification of a benign osteochondroma?

Explanation

The pathognomonic radiographic hallmark of an osteochondroma is direct continuity of its cortex and medullary cavity with the cortex and medullary cavity of the host bone.

Question 21

A 35-year-old male with Ankylosing Spondylitis presents for a total hip arthroplasty (THA) due to severe bilateral hip autofusion.

Compared to patients undergoing THA for primary osteoarthritis, this patient is at significantly higher risk for which of the following postoperative complications?

Explanation

Patients with Ankylosing Spondylitis are at a notably higher risk for developing massive heterotopic ossification (HO) following THA. Prophylaxis with NSAIDs (e.g., Indomethacin) or single-dose radiation is strongly recommended.

Question 22

Primary synovial chondromatosis typically involves which of the following pathophysiological processes?

Explanation

Primary synovial chondromatosis is a benign neoplastic process involving spontaneous chondroid metaplasia of the synovial membrane, producing cartilaginous nodules that may detach and ossify.

Question 23

A 40-year-old male notes a slowly enlarging mass in his trapezius muscle. MRI reveals a well-defined mass with signal intensity identical to subcutaneous fat, interspersed with distinct, thick striations of skeletal muscle fibers. What is the most likely diagnosis?

Explanation

An intramuscular lipoma characteristically infiltrates between skeletal muscle bundles. On MRI, this appears as a predominately fatty mass with traversing linear striations representing entrapped muscle fibers, distinct from the thick septations seen in liposarcomas.

Question 24

A 14-year-old female with Multiple Hereditary Exostoses (MHE) presents with a visible forearm deformity. Which of the following descriptions represents the most common sequence of forearm deformity in this condition?

Explanation

Osteochondromas in MHE commonly affect the distal ulna due to its small cross-sectional area, causing growth retardation. This relative ulnar shortening leads to a tethering effect, causing secondary bowing of the radius, ulnar deviation of the hand, and potentially radial head dislocation.

Question 25

A 24-year-old male with chronic back pain and stiffness is suspected of having Ankylosing Spondylitis. During the physical examination, the physician makes a mark at the level of the posterior superior iliac spines (dimples of Venus) and another mark 10 cm above it, then asks the patient to flex forward. This test is known as the:

Explanation

The Schober test assesses the restriction of lumbar spine flexion, a hallmark clinical sign of Ankylosing Spondylitis. Normal flexion should increase the distance between the two marks by at least 5 cm.

Question 26

Which major articulation is overwhelmingly the most common site of involvement for primary synovial chondromatosis?

Explanation

The knee is the most commonly affected joint in primary synovial chondromatosis, accounting for approximately 50-70% of cases, followed by the hip, elbow, and shoulder.

Question 27

A 55-year-old male presents with a large, deep intramuscular mass in his right thigh. Biopsy reveals mature adipocytes with focal areas of hyperchromatic, atypical stromal cells.

Which of the following molecular or genetic findings is diagnostic for distinguishing this lesion from a benign lipoma?

Explanation

Atypical lipomatous tumors (well-differentiated liposarcomas) are characterized by the amplification of MDM2 and CDK4 genes on chromosome 12q13-15. This genetic finding reliably distinguishes them from benign lipomas, which lack these amplifications.

Question 28

A 45-year-old male with long-standing ankylosing spondylitis presents to the emergency department after a minor fall. He complains of severe neck pain. Radiographs show a fracture extending through the C5-C6 disc space and involving the posterior elements.

What is the most appropriate definitive management?

Explanation

Spinal fractures in ankylosing spondylitis are highly unstable, often involving all three columns, and carry a high risk of neurologic compromise. Due to altered biomechanics, standard treatment requires long-segment posterior instrumentation and fusion to ensure adequate stabilization.

Question 29

A 25-year-old male with a known distal femur osteochondroma reports newly onset pain and a palpable increase in the size of the mass. MRI reveals a cartilage cap thickness of 2.8 cm with soft tissue edema. What is the next best step in management?

Explanation

In a skeletally mature patient, a cartilage cap thicker than 2 cm (or 1.5 cm in some texts) combined with new pain and growth is highly suspicious for malignant transformation to secondary chondrosarcoma. Wide surgical resection is the definitive treatment.

Question 30

A 40-year-old male presents with chronic knee catching and pain. Radiographs reveal numerous uniformly sized, calcified loose bodies within the joint space.

What is the primary pathophysiologic mechanism for this condition?

Explanation

Primary synovial chondromatosis is a benign neoplastic process caused by cartilaginous metaplasia of the synovial membrane. This results in the formation of multiple uniformly sized cartilaginous nodules that may detach and ossify.

Question 31

A 14-year-old boy presents with multiple bony prominences around his knees and ankles. Family history is positive for a similar condition. Which of the following statements regarding his diagnosis is most accurate?

Explanation

Multiple hereditary exostoses (MHE) is an autosomal dominant condition linked to mutations in the EXT1 and EXT2 genes, which are involved in heparan sulfate synthesis. The risk of malignant transformation to chondrosarcoma is approximately 1-5%, much lower than 50%.

Question 32

A 35-year-old male with ankylosing spondylitis has a severe chin-on-chest deformity, rendering him unable to look straight ahead.

If surgical correction is planned, at which spinal level is an extension osteotomy most safely and commonly performed to correct the cervicothoracic kyphosis?

Explanation

The C7-T1 junction is the preferred site for a cervicothoracic extension osteotomy in ankylosing spondylitis. This level is chosen because the spinal canal is relatively wide here, and the vertebral artery typically enters the transverse foramen at C6, leaving C7-T1 safer for bony resection.

Question 33

A 50-year-old female presents with a soft, painless, mobile mass on her upper back. On MRI, the mass is well-circumscribed, hyperintense on T1-weighted images, and demonstrates complete signal loss on Short Tau Inversion Recovery (STIR) sequences. What is the most likely diagnosis?

Explanation

A benign lipoma is composed of mature adipose tissue, which appears hyperintense on T1-weighted MRI. Complete suppression of the signal on fat-saturated (STIR) sequences is pathognomonic for fat.

Question 34

An 18-year-old male presents with a painless, hard bony mass extending from the proximal lateral fibula. He recently developed a foot drop and decreased sensation over the dorsum of his foot. Compression of which nerve is most likely responsible for his symptoms?

Explanation

An osteochondroma located at the proximal lateral fibula can cause extrinsic compression of the common peroneal nerve as it wraps around the fibular neck. This results in foot drop and sensory loss over the lateral leg and dorsum of the foot.

Question 35

A 38-year-old male with a history of primary synovial chondromatosis of the hip, treated with arthroscopic synovectomy 4 years ago, presents with rapidly worsening pain. Radiographs show a destructive, lytic lesion invading the acetabulum with an associated soft tissue mass. What is the most likely diagnosis?

Explanation

Primary synovial chondromatosis carries a rare (up to 5%) risk of malignant transformation into secondary chondrosarcoma. Rapidly worsening symptoms, bone destruction, and soft tissue invasion are hallmarks of this malignant change.

Question 36

A 50-year-old male with long-standing ankylosing spondylitis presents with localized, non-radiating thoracolumbar back pain. Radiographs reveal a destructive, irregular discovertebral lesion at T12-L1 with reactive sclerosis, mimicking discitis. His CRP is only mildly elevated. What is this lesion called?

Explanation

An Andersson lesion is a non-infectious inflammatory discovertebral lesion or pseudoarthrosis seen in ankylosing spondylitis. It occurs due to localized stress fractures through an ankylosed spine segment and can closely mimic infection on imaging.

Question 37

A 12-year-old boy presents with a hard, painless bump on his distal femur. Radiographs show a pedunculated bony exostosis pointing away from the joint.

What is the pathognomonic radiographic feature required to confirm an osteochondroma?

Explanation

The hallmark of an osteochondroma is the direct continuity of both the cortex and the medullary cavity of the lesion with those of the underlying host bone.

Question 38

A 60-year-old male is diagnosed with an atypical lipomatous tumor (well-differentiated liposarcoma) located deep in the retroperitoneum. Why is the prognosis for this retroperitoneal lesion poorer than for a histologically identical lesion in the deep soft tissues of the thigh?

Explanation

Atypical lipomatous tumors/well-differentiated liposarcomas do not typically metastasize but can dedifferentiate over time. In the retroperitoneum, achieving wide, negative surgical margins is difficult, leading to multiple local recurrences and a higher risk of dedifferentiation into a higher-grade sarcoma.

Question 39

Primary synovial chondromatosis is a benign neoplasm characterized by synovial metaplasia. Which specific genetic fusion is most frequently implicated in the pathogenesis of this disease?

Explanation

Recent molecular studies have identified recurrent fusions involving the Fibronectin 1 (FN1) gene, most commonly FN1-ACVR2A or FN1-NCOA2, as the driving genetic abnormalities in primary synovial chondromatosis.

Question 40

A 28-year-old male with HLA-B27 positive ankylosing spondylitis presents for a routine follow-up. He complains of chronic, inflammatory lower back pain. Which of the following is the most common extra-articular manifestation associated with his underlying condition?

Explanation

Acute anterior uveitis (iritis) is the most common extra-articular manifestation of ankylosing spondylitis, occurring in approximately 25-30% of patients. It typically presents as an acutely painful, red eye with photophobia.

Question 41

A 10-year-old boy presents with an incidentally discovered sessile osteochondroma of the proximal humerus. Over the next several years of skeletal growth, how will the relative position of this lesion change?

Explanation

Osteochondromas arise from aberrant physeal cartilage. As the host bone undergoes longitudinal growth at the physis, the osteochondroma is progressively 'left behind,' making it appear to migrate towards the diaphysis over time.

Question 42

A 32-year-old male with active ankylosing spondylitis has persistent axial disease (sacroiliitis and spondylitis) despite maximally tolerated doses of two different NSAIDs. He has no peripheral joint involvement. What is the most appropriate next step in his medical management?

Explanation

For patients with active axial ankylosing spondylitis who fail NSAID therapy, TNF inhibitors (e.g., infliximab, adalimumab) or IL-17 inhibitors are the recommended next step. Conventional synthetic DMARDs like methotrexate and sulfasalazine are ineffective for pure axial disease.

Question 43

A 45-year-old female presents with a deep intramuscular lipoma in the proximal anterior forearm near the supinator muscle. She reports insidious, progressive weakness in extending her thumb and fingers, without any sensory deficits. Which nerve is most likely compressed by this mass?

Explanation

A mass near the supinator muscle (arcade of Frohse) can compress the posterior interosseous nerve (PIN). PIN compression results in motor weakness of thumb and finger extensors, without sensory loss (as the sensory component travels via the superficial radial nerve).

Question 44

A 65-year-old female with severe, end-stage osteoarthritis of the knee undergoes arthroplasty. Intraoperatively, multiple calcified loose bodies are found in the joint. Pathologic evaluation confirms secondary synovial chondromatosis. How does this condition characteristically differ from primary synovial chondromatosis?

Explanation

Secondary synovial chondromatosis results from the shedding of cartilage due to underlying joint pathology, most commonly osteoarthritis. Unlike the primary form, the loose bodies in secondary chondromatosis are typically fewer in number, of varying sizes, and have concentric rings of ossification.

Question 45

A 55-year-old man presents with a deep 12 cm thigh mass. Biopsy shows mature adipose tissue with focal nuclear atypia. Molecular testing reveals MDM2 gene amplification. What is the most appropriate management?

Explanation

MDM2 amplification confirms an Atypical Lipomatous Tumor (ALT) / Well-Differentiated Liposarcoma. In the extremity, ALTs have no metastatic potential, so marginal excision that preserves critical structures is adequate management, though local recurrence remains a risk.

Question 46

A 22-year-old male with multiple hereditary exostoses (MHE) reports new-onset, enlarging mass and pain around his right proximal femur. Which of the following is the strongest indicator of malignant transformation on imaging?

Explanation

Growth of an osteochondroma after skeletal maturity or a cartilage cap >1.5-2.0 cm on MRI strongly suggests malignant transformation to a secondary chondrosarcoma. Observation is no longer appropriate for growing, painful lesions in adults.

Question 47

A 65-year-old man with advanced Ankylosing Spondylitis presents after a minor fall with severe neck pain. Radiographs are inconclusive. What is the most appropriate next step in management?

Explanation

Patients with Ankylosing Spondylitis have a rigid, brittle spine highly susceptible to unstable fractures even from minor trauma. If standard radiographs are negative but the patient has pain, cross-sectional imaging (CT or MRI) is mandatory to rule out occult fractures.

Question 48

A 45-year-old woman complains of chronic right knee pain and locking. Imaging reveals multiple intra-articular calcified loose bodies of uniform size. What is the classic pathophysiologic mechanism of this condition?

Explanation

Primary synovial chondromatosis involves benign cartilaginous metaplasia of the synovial membrane, producing uniform loose bodies. This contrasts with secondary synovial chondromatosis, which results from mechanical fragmentation in degenerative joint disease and produces loose bodies of varying sizes.

Question 49

A 12-year-old boy is diagnosed with Multiple Hereditary Exostoses (MHE). Genetic testing is likely to show a mutation in genes responsible for which of the following cellular processes?

Explanation

MHE is an autosomal dominant condition caused by mutations in the EXT1 or EXT2 genes, which are tumor suppressors involved in heparan sulfate biosynthesis. Defective heparan sulfate leads to abnormalities in the Indian hedgehog (Ihh) signaling pathway, resulting in disordered chondrocyte proliferation.

Question 50

A 50-year-old man with severe bilateral hip pain and a known history of Ankylosing Spondylitis is scheduled for a total hip arthroplasty (THA). He is at significantly increased risk for which of the following postoperative complications?

Explanation

Patients with ankylosing spondylitis have a high risk of developing heterotopic ossification (HO) following total hip arthroplasty. Prophylaxis with nonsteroidal anti-inflammatory drugs (e.g., indomethacin) or localized radiation therapy is highly recommended.

Question 51

A 45-year-old woman presents with weakness in wrist extension and finger extension. MRI demonstrates a large mass composed uniformly of fat sitting intimately on the periosteum of the proximal radius. What is the most likely diagnosis?

Explanation

Parosteal lipomas are rare, benign adipose tumors intimately associated with the periosteum, often presenting in the proximal radius. They frequently cause compressive neuropathy of the posterior interosseous nerve (PIN), leading to extensor weakness.

Question 52

A 30-year-old male with a history of primary synovial chondromatosis of the hip undergoes arthroscopic synovectomy. Ten years later, he presents with sudden, rapidly progressive hip pain and an enlarging mass. Radiographs show aggressive bone destruction. What is the most likely diagnosis?

Explanation

Primary synovial chondromatosis has a known, albeit rare (1-5%), risk of malignant transformation into secondary chondrosarcoma. Rapid clinical deterioration, new mass formation, and aggressive bony destruction or marrow invasion on MRI are hallmarks of this transformation.

Question 53

An osteochondroma is generally considered to stop growing at skeletal maturity. If an osteochondroma-like lesion originates within the epiphysis rather than the metaphysis, leading to joint deformity and restricted motion, what is this condition termed?

Explanation

Trevor disease, or dysplasia epiphysealis hemimelica, is characterized by an osteochondroma-like growth arising from the epiphysis, typically affecting one side (hemimelic) of a joint in the lower extremity. It leads to joint asymmetry, restricted motion, and premature osteoarthritis.

Question 54

A 68-year-old man with long-standing Ankylosing Spondylitis presents with a severe chin-on-chest deformity. Which cervical level is most commonly chosen for a closing wedge extension osteotomy to correct this deformity?

Explanation

The cervicothoracic junction (C7-T1) is the preferred level for extension osteotomies in ankylosing spondylitis. This level has a wider spinal canal minimizing spinal cord injury risk, and the mobility of the spinal cord is relatively greater here than in the mid-cervical spine.

Question 55

A painless soft tissue mass in the posterior neck of a 55-year-old man is biopsied. Histology reveals mature adipocytes, uniform spindle cells, and thick, rope-like collagen bundles without lipoblasts. Immunohistochemistry is strongly positive for CD34. What is the diagnosis?

Explanation

Spindle cell lipomas classically occur in the posterior neck, shoulder, or upper back of older men. They are characterized by mature fat, CD34-positive spindle cells, and ropey collagen, and they are entirely benign.

Question 56

A 55-year-old male with long-standing Ankylosing Spondylitis (AS) presents to the emergency department with severe neck pain following a minor fall from standing height. A CT scan of the cervical spine demonstrates a through-and-through chalk-stick fracture at the C5-C6 level. What is the most appropriate surgical management for this patient?

Explanation

Spinal fractures in ankylosing spondylitis are highly unstable (often three-column injuries) and are managed similarly to long-bone fractures. Long-segment posterior instrumentation (often 3 levels above and below) is required to provide adequate biomechanical stability and prevent catastrophic neurologic injury.

Question 57

A 35-year-old male presents with a slowly enlarging, painless mass over the distal femur. Radiographs reveal a sessile bony exostosis pointing away from the joint. An MRI is obtained to evaluate the cartilage cap. Which of the following cartilage cap thicknesses is most indicative of malignant transformation to secondary chondrosarcoma in an adult?

Explanation

In adults, a cartilage cap thickness greater than 1.5 to 2 cm (>15-20 mm) on MRI is highly suspicious for malignant transformation of an osteochondroma into a secondary chondrosarcoma. In skeletally immature patients, thicker caps are normal and expected during active growth.

Question 58

A 40-year-old female presents with progressive knee pain and mechanical catching. Imaging reveals multiple intra-articular calcified loose bodies with relative preservation of the joint space. A diagnosis of primary synovial chondromatosis is suspected. What is the underlying pathogenesis of this condition?

Explanation

Historically considered a reactive metaplastic process, primary synovial chondromatosis is now recognized as a benign neoplastic proliferation of the synovium. It is often associated with specific clonal chromosomal aberrations, notably FN1-ACVR2A fusions.

Question 59

A 60-year-old male presents with a large, deep-seated fatty mass in the thigh. Biopsy shows mature adipose tissue with focal atypia and hyperchromatic stromal cells. Cytogenetic analysis is ordered to differentiate an Atypical Lipomatous Tumor (ALT) from a benign lipoma. Which of the following genetic markers is characteristic of an ALT?

Explanation

Atypical Lipomatous Tumors (ALT), also known as well-differentiated liposarcomas, are characterized by supernumerary ring chromosomes leading to the amplification of the MDM2 and CDK4 genes on chromosome 12q. This molecular signature reliably differentiates ALT from benign lipomas.

Question 60

A 45-year-old male with advanced Ankylosing Spondylitis presents with fixed global positive sagittal imbalance. Surgical correction using a pedicle subtraction osteotomy (PSO) is planned. To maximize lordosis restoration and minimize the risk of spinal cord injury, what is the ideal spinal level for the PSO?

Explanation

The L3 level is considered the ideal site for a pedicle subtraction osteotomy (PSO) in patients with AS. It allows for significant restoration of lumbar lordosis while remaining well below the level of the conus medullaris, reducing the risk of permanent neurologic injury.

Question 61

A 10-year-old boy presents with multiple bony bumps around his knees, ankles, and shoulders. He is short in stature and exhibits a mild forearm deformity. A diagnosis of Multiple Hereditary Exostoses (MHE) is made. What is the inheritance pattern and the most common genetic mutation associated with this condition?

Explanation

Multiple Hereditary Exostoses (MHE) is an autosomal dominant condition characterized by mutations in the EXT1 or EXT2 genes. These genes are tumor suppressors involved in heparan sulfate synthesis, and their mutation leads to the formation of multiple osteochondromas.

Question 62

A 60-year-old male presents with a deep intramuscular mass in the thigh. MRI reveals a large lipomatous lesion with thick septations.

Core needle biopsy shows adipocytes with variation in cell size and hyperchromatic stromal cells. Which of the following cytogenetic abnormalities is diagnostic for this tumor?

Explanation

Atypical lipomatous tumors (well-differentiated liposarcomas) are characterized by ring or giant rod chromosomes containing amplified sequences of the 12q13-15 region. This results in the specific amplification of the MDM2 and CDK4 genes.

Question 63

A 45-year-old male complains of chronic knee pain, swelling, and catching. Radiographs and MRI are shown.

What is the primary pathophysiological mechanism underlying this condition?

Explanation

Primary synovial chondromatosis is a benign neoplastic process involving cartilaginous metaplasia of the synovial membrane. This leads to the formation of multiple, typically uniform, intra-articular loose bodies.

Question 64

A 72-year-old male with a known history of ankylosing spondylitis presents with severe back pain after minor trauma. Imaging reveals a transvertebral fracture.

Which of the following is the most appropriate definitive management for this patient?

Explanation

Fractures in ankylosing spondylitis are highly unstable due to the altered biomechanics of the fused "bamboo" spine. They typically require long-segment posterior spinal instrumentation spanning at least three levels above and below the fracture to prevent secondary neurological injury.

Question 65

A 14-year-old boy is diagnosed with multiple hereditary exostoses (MHE). What is the normal physiological function of the proteins encoded by the genes most commonly mutated in this condition?

Explanation

MHE is primarily caused by autosomal dominant mutations in the EXT1 or EXT2 genes. These genes encode glycosyltransferases that are essential for the synthesis of heparan sulfate, which regulates normal chondrocyte proliferation and maturation.

Question 66

A 50-year-old male with long-standing ankylosing spondylitis presents with localized, worsening back pain without a history of trauma. Radiographs show a destructive discovertebral lesion with reactive sclerosis.

What is the most likely diagnosis?

Explanation

Andersson lesions represent non-infectious, destructive discovertebral lesions seen in advanced ankylosing spondylitis. They often result from pseudoarthrosis at the site of unhealed stress fractures in the rigid, fused spine.

Question 67

A 40-year-old female presents with a painless, slow-growing mass on her proximal forearm. Radiographs demonstrate focal cortical hyperostosis of the proximal radius adjacent to a radiolucent soft-tissue mass. What is the most likely diagnosis?

Explanation

Parosteal lipomas often cause a classic reactive cortical hyperostosis or bowing of the adjacent underlying bone. The purely radiolucent mass adjacent to the hyperostosis differentiates it from bone-forming tumors.

Question 68

A 25-year-old male with a known distal femur osteochondroma presents with a rapidly enlarging, pulsatile popliteal mass and calf pain.

What is the most likely etiology of the new mass?

Explanation

Vascular complications, such as pseudoaneurysm formation, can occur due to repetitive friction of an artery over the hard cartilage cap of an osteochondroma. This is most classically described with the popliteal artery adjacent to a distal femoral osteochondroma.

Question 69

Which of the following features best distinguishes primary synovial chondromatosis from secondary synovial chondromatosis?

Explanation

Primary synovial chondromatosis typically presents with multiple loose bodies of uniform size due to synchronous synovial metaplasia. Secondary synovial chondromatosis (from trauma or osteoarthritis) features a few loose bodies of varying sizes alongside underlying degenerative joint disease.

Question 70

A 45-year-old male incidentally undergoes a radiograph of his foot after a mild sprain. The lateral view shows a well-defined lytic lesion in the calcaneus with central calcification creating a target sign. What is the appropriate management?

Explanation

Intraosseous lipomas are benign tumors commonly found in the calcaneus, often displaying a classic target sign due to central calcified fat necrosis. They are typically asymptomatic and require only observation.

Question 71

A 32-year-old male with human leukocyte antigen (HLA)-B27 positive ankylosing spondylitis presents with acute unilateral eye pain, photophobia, and blurred vision. What is the most likely extra-articular manifestation he is experiencing?

Explanation

Acute anterior uveitis (iritis) is the most common extra-articular manifestation of ankylosing spondylitis, occurring in 25-30% of patients. It requires prompt ophthalmological evaluation to prevent synechiae and vision loss.

Question 72

A 15-year-old boy presents with a hard mass on his proximal tibia. Radiographs show a sessile bony exostosis pointing away from the joint space.

Which radiographic finding is required to definitively classify this lesion as an osteochondroma?

Explanation

The pathognomonic radiographic feature of an osteochondroma is direct continuity of its cortex and medullary cavity with that of the native host bone. The lesion classically grows in a direction pointing away from the adjacent joint.

Question 73

A 45-year-old male with severe ankylosing spondylitis presents with a fixed chin-on-chest deformity causing difficulty swallowing and impaired horizontal gaze. He is scheduled for an extension osteotomy. Which level is most appropriate for the osteotomy to correct the cervical deformity?

Explanation

Cervicothoracic kyphosis in AS is typically corrected with an extension osteotomy at the C7-T1 level. This level has a wide spinal canal and is below the vertebral artery's usual entry point at C6, minimizing neurological and vascular risks.

Question 74

A 16-year-old boy with a known history of multiple hereditary exostoses (MHE) presents with worsening deformity of his left forearm and decreased range of motion. Radiographs reveal a characteristic forearm deformity associated with this condition. Which of the following best describes the typical pathoanatomy of the forearm in MHE?

Explanation

Forearm deformity in MHE typically begins with relative shortening of the ulna due to osteochondroma formation. This tethering effect leads to secondary bowing of the radius and eventual dislocation of the radial head.

Question 75

A 35-year-old male is evaluated for a large, deep intramuscular thigh mass. Core needle biopsy confirms the presence of well-differentiated adipocytes, but the pathologist requests fluorescence in situ hybridization (FISH) to differentiate between a benign lipoma and an atypical lipomatous tumor (ALT). Amplification of which of the following gene loci is pathognomonic for ALT?

Explanation

Atypical lipomatous tumors (well-differentiated liposarcomas) are characterized by supernumerary ring chromosomes leading to the amplification of the MDM2 and CDK4 genes. Benign lipomas typically lack this amplification.

Question 76

A 60-year-old male with long-standing ankylosing spondylitis presents with increasing difficulty looking forward due to a severe rigid cervicothoracic kyphosis. A cervical extension osteotomy is planned to correct his chin-on-chest deformity. At which anatomic level is this osteotomy most safely and classically performed?

Explanation

Cervical extension osteotomies in ankylosing spondylitis are classically performed at the C7-T1 junction. The spinal canal is relatively wide at this level, and the flexibility of the adjacent shoulders and rib cage allows for safer correction with lower risk to the vertebral arteries.

Question 77

A 45-year-old male presents with recurrent catching and locking in his left knee. Imaging reveals numerous uniformly sized, small calcified bodies confined within the joint capsule.

What is the most definitive histological hallmark distinguishing this condition from secondary osteochondromatosis?

Explanation

Primary synovial chondromatosis is characterized by metaplastic cartilage formation within the synovial tissue itself, leading to uniformly sized nodules. Secondary chondromatosis results from fragmented intra-articular cartilage/bone (e.g., severe osteoarthritis) and features variably sized, larger bodies.

Question 78

A 14-year-old girl is evaluated for a painless, hard mass about the medial aspect of the proximal tibia.

Radiographs and MRI confirm the diagnosis of an osteochondroma. Which of the following imaging features is an absolute requirement to confidently make this diagnosis?

Explanation

The pathognomonic radiographic and MRI finding for an osteochondroma is the continuity of both the cortex and the medullary canal of the lesion with that of the underlying host bone. A cartilage cap >2 cm in an adult is a warning sign for malignant transformation.

Question 79

A 68-year-old male with a 30-year history of ankylosing spondylitis presents to the emergency department after a minor ground-level fall. He complains of moderate neck pain. Neurological examination is completely normal. Lateral cervical radiographs are largely obscured by his shoulders but show a fused spine. What is the most appropriate next step in management?

Explanation

Patients with ankylosing spondylitis are at high risk for highly unstable "chalk stick" fractures even after minor trauma. A CT scan of the entire spine is mandatory as standard radiographs frequently miss these fractures due to altered anatomy and osteopenia.

Question 80

A 35-year-old male with long-standing primary synovial chondromatosis of the hip presents with rapid worsening of pain and joint swelling over the last three months. MRI reveals permeative destruction of the femoral neck with marrow replacement and a large soft tissue mass breaking through the joint capsule. What complication has most likely occurred?

Explanation

While primary synovial chondromatosis is benign, it carries a rare risk (approx. 1-5%) of malignant transformation to secondary chondrosarcoma. Rapid clinical deterioration, bone marrow invasion, and cortical destruction on imaging are key indicators of this complication.

Question 81

Which of the following genetic mechanisms is fundamentally responsible for the pathogenesis of multiple hereditary exostoses (MHE)?

Explanation

MHE is an autosomal dominant disorder caused by loss-of-function mutations in EXT1 or EXT2. These genes encode glycosyltransferases essential for heparan sulfate chain elongation, disrupting normal chondrocyte proliferation and differentiation.

Question 82

A 52-year-old male is referred for a painless, slow-growing soft tissue mass in his proximal forearm. MRI reveals a well-defined lipomatous mass located entirely within the belly of the brachioradialis muscle. Biopsy demonstrates mature adipocytes interdigitating with normal skeletal muscle fibers, without cytological atypia or lipoblasts. Which of the following statements regarding this lesion is true?

Explanation

Intramuscular lipomas are benign but uniquely exhibit an infiltrative growth pattern where mature adipocytes interdigitate with normal skeletal muscle. Despite this appearance, they lack atypia, though they have a higher recurrence rate than superficial lipomas if incompletely excised.

Question 83

A 45-year-old male with a 20-year history of ankylosing spondylitis presents with severe bilateral hip pain and stiffness. Bilateral total hip arthroplasty (THA) is planned. Which of the following prophylactic measures is highly recommended postoperatively for this specific patient population?

Explanation

Patients with ankylosing spondylitis undergoing THA are at a significantly increased risk of developing heterotopic ossification (HO). Prophylaxis with NSAIDs (e.g., indomethacin) or a single dose of localized radiation is strongly recommended.

Question 84

An 18-year-old male presents with a pulsatile mass and a palpable thrill in the popliteal fossa of his right knee. He has a known history of a distal femur osteochondroma in the same leg. What is the most likely etiology of this vascular presentation?

Explanation

A classic vascular complication of an osteochondroma, particularly in the distal femur or proximal tibia, is mechanical impingement and friction on the adjacent popliteal artery, leading to the formation of a pseudoaneurysm.

Question 85

A 65-year-old male presents with a slowly enlarging, painless soft tissue mass in the posterior subcutaneous neck. Excisional biopsy is performed. Histology demonstrates a mixture of mature adipocytes, uniform spindle cells, and thick, ropey collagen bundles in a myxoid background. Immunohistochemistry is positive for CD34. What is the most likely diagnosis?

Explanation

Spindle cell lipoma is a benign lipomatous tumor that classically occurs in the posterior neck/back of older males. Histologically, it is defined by a mixture of mature fat, uniform CD34-positive spindle cells, and distinct ropy collagen bundles.

Question 86

A patient with long-standing ankylosing spondylitis sustains a cervical spine fracture extending through the C5-C6 intervertebral disc space and posterior elements.

Several hours after admission, he develops progressive bilateral lower extremity weakness and urinary retention. What is the most critical immediate diagnostic or therapeutic step?

Explanation

Progressive neurological deficit in an AS patient with a spinal fracture is highly suspicious for an expanding epidural hematoma, which occurs more frequently in AS patients due to altered dural adherence and bleeding from fractured bone/vessels. Urgent MRI is required.

Question 87

During the surgical excision of a symptomatic solitary osteochondroma of the distal femur in a 20-year-old patient, which of the following oncologic principles is most critical to minimize the risk of local recurrence?

Explanation

Symptomatic osteochondromas are treated with marginal excision. It is critical to completely excise the entire cartilaginous cap along with its overlying perichondrium; failure to do so leaves active chondrocytes behind, resulting in local recurrence.

Question 88

A 55-year-old woman is evaluated for a large mass deep in her thigh.

MRI reveals a predominantly high T1-signal mass with thick, nodular, enhancing septations (>2 mm). Biopsy confirms an Atypical Lipomatous Tumor (ALT). Which of the following statements best describes the biological behavior and appropriate treatment for this specific tumor?

Explanation

Atypical Lipomatous Tumors (ALT) are locally aggressive but do not possess metastatic potential unless they undergo dedifferentiation. The standard of care is marginal surgical excision to minimize morbidity, recognizing that local recurrence may occur.

Question 89

Which of the following is the primary histological and pathophysiological initial event in the development of spinal ankylosis in ankylosing spondylitis?

Explanation

The hallmark primary lesion in ankylosing spondylitis is enthesitis (inflammation at the insertion sites of ligaments, tendons, and capsules into bone). This is eventually followed by reactive bone formation (ossification) leading to syndesmophytes and ankylosis.

Question 90

A 30-year-old male with an established diagnosis of multiple hereditary exostoses presents with a newly enlarging, painful osteochondroma on his right iliac wing. He reached skeletal maturity 12 years ago. Imaging shows a cartilage cap measuring 3.5 cm in thickness. What is the most appropriate management?

Explanation

A growing, painful osteochondroma with a cartilage cap >2 cm in a skeletally mature adult is highly suspicious for malignant transformation to secondary chondrosarcoma. The appropriate treatment for low-grade chondrosarcoma is wide surgical resection.

Question 91

Which of the following extraskeletal manifestations is most frequently associated with the HLA-B27 positive condition ankylosing spondylitis?

Explanation

Acute anterior uveitis (iritis) is the most common extra-articular manifestation of ankylosing spondylitis, occurring in 25-30% of patients. While cardiovascular and pulmonary complications occur, they are much less frequent.

Question 92

A patient is undergoing arthroscopy for primary synovial chondromatosis of the shoulder. During the procedure, the surgeon removes dozens of loose bodies. To minimize the rate of local recurrence of the condition, what additional intraoperative step is required?

Explanation

In primary synovial chondromatosis, the underlying pathology resides in the metaplastic synovium. Removing loose bodies alone is insufficient; a thorough synovectomy must be performed to decrease the risk of local recurrence.

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

Orthopedic Board Review: Lipoma, Osteochondroma, AS, Synovial Chondromatosis MCQs | Part 15

Key Takeaway

Orthopedic Board Review: Lipoma, Osteochondroma, AS, Synovial Chondromatosis MCQs | Part 15

Chapter Index