Residency · Residency · Orthopedic Surgery

Musculoskeletal Imaging for the Orthopedic Resident

Plain Radiographs

Principles

Plain radiographs remain the first-line imaging modality for nearly all musculoskeletal complaints. A minimum of two orthogonal views (typically AP and lateral) should always be obtained, as a fracture or dislocation may be invisible on a single projection. For long bone injuries, imaging should include the joint above and the joint below to identify associated injuries. In pediatric patients, comparison views of the contralateral side can be invaluable for distinguishing normal developmental variants from pathology. A systematic approach to reading radiographs prevents missed findings.

Systematic Approach to Radiograph Interpretation (ABCDEs)

The ABCDEs mnemonic provides a reliable framework for musculoskeletal radiograph interpretation. "A" stands for Alignment, assessing joint congruity, fracture displacement, angulation, and rotation. "B" is for Bone, evaluating cortical integrity, trabecular pattern, bone density, and focal lesions. "C" represents Cartilage, assessed indirectly through joint space width, subchondral sclerosis, and the presence of osteophytes. "D" refers to Distribution of findings, noting whether changes are focal or diffuse, symmetric or asymmetric. "E" encompasses Everything else, particularly soft tissues, including swelling, effusion (fat pad signs), calcification, foreign bodies, and gas.

Key Radiographic Signs

Several radiographic signs carry specific diagnostic significance. The posterior fat pad sign at the elbow indicates an intra-articular effusion and should prompt a search for an occult fracture. A lipohemarthrosis (fat-fluid level on a cross-table lateral view) indicates an intra-articular fracture. The FBI (fat-blood interface) sign serves the same purpose in tibial plateau or distal femur fractures. A Segond fracture, which is a lateral tibial capsular avulsion, is pathognomonic for an ACL tear. The reverse Segond fracture (medial tibial avulsion) is associated with PCL injury. The Terry-Thomas sign describes a widened scapholunate interval (greater than 3 mm) on a PA wrist radiograph, indicating scapholunate ligament injury. The signet ring sign represents scaphoid foreshortening on a PA wrist view, suggesting rotatory subluxation of the scaphoid.

Stress and Weight-Bearing Views

Weight-bearing radiographs are essential for accurate assessment of knees (true joint space narrowing), ankles (syndesmotic stability), and feet (Lisfranc alignment). Gravity stress views help assess deltoid ligament competence at the ankle. Valgus and varus stress views evaluate collateral ligament integrity at the knee. The Zanca view (10-15 degree cephalad tilt) is optimal for visualizing AC joint pathology.

Computed Tomography (CT)

Principles

CT provides superior cortical bone detail and cross-sectional anatomy compared to plain radiographs. Three-dimensional reconstructions are invaluable for fracture classification and preoperative surgical planning. The radiation exposure associated with CT should be considered, particularly in young patients, and should be weighed against the clinical need.

Key Indications in Orthopedics

CT is indicated for intra-articular fractures requiring surgical planning (tibial plateau, pilon, calcaneus, and acetabulum), complex pelvic ring injuries, scaphoid fracture assessment when radiographs are equivocal, and spinal trauma evaluation (cervical spine clearance in obtunded patients and thoracolumbar injury classification). CT angiography evaluates for vascular injury in knee dislocations and high-energy pelvic ring injuries. CT also assesses implant positioning (tunnel placement after ACL reconstruction, component alignment after arthroplasty) and is superior to plain radiographs for evaluating bridging callus in suspected nonunions.

CT Arthrography

CT arthrography combines CT scanning with intra-articular contrast injection. It is useful for detecting labral tears, cartilage lesions, and loose bodies, and serves as an alternative to MRI when MRI is contraindicated (such as in patients with pacemakers or severe claustrophobia).

Magnetic Resonance Imaging (MRI)

Basic Physics and Sequences

MRI exploits the behavior of hydrogen protons aligned within a strong magnetic field. Different pulse sequences produce images with varying tissue contrast. T1-weighted images display fat as bright (high signal) and water as dark, making them ideal for anatomic detail, bone marrow evaluation, and assessing post-gadolinium enhancement. T2-weighted images show water as bright and fat as intermediate signal, making them excellent for detecting edema, effusion, and soft tissue pathology. Proton density (PD) sequences provide exceptional detail for meniscal and ligament evaluation. STIR (Short Tau Inversion Recovery) sequences suppress fat signal while highlighting water, making them highly sensitive for bone marrow edema, stress fractures, and occult fractures. Gradient echo (GRE) sequences are useful for cartilage evaluation and produce characteristic blooming artifact around hemosiderin deposits or metal.

Key Indications

MRI is the definitive imaging modality for ligament and meniscal injuries (knee, ankle), rotator cuff tears and labral pathology, occult fractures (particularly femoral neck stress fractures and scaphoid fractures not visible on radiographs), osteonecrosis (avascular necrosis of the femoral head), bone and soft tissue tumor staging, spinal cord compression and disc pathology, articular cartilage assessment, and infection evaluation (osteomyelitis presents as marrow edema with periosteal and soft tissue enhancement on contrast-enhanced sequences).

Common MRI Findings by Region

Knee

ACL tears appear as discontinuity of the ligament with abnormal signal intensity and altered fiber orientation on sagittal images. Secondary signs include a characteristic bone bruise pattern affecting the lateral femoral condyle and posterolateral tibial plateau. Meniscal tears are diagnosed when abnormal intrameniscal signal reaches the articular surface on at least two consecutive images. PCL tears show increased signal within the normally uniformly dark PCL substance. Bone bruise patterns correlate with specific injury mechanisms and help confirm the clinical diagnosis.

Shoulder

Rotator cuff tears manifest as tendon discontinuity with variable retraction, and chronic tears show fatty infiltration of the muscle belly (graded by the Goutallier classification adapted for MRI). Labral tears appear as detachment or irregularity of the labrum, sometimes with an associated paralabral cyst. Hill-Sachs lesions present as posterolateral humeral head impression fractures from anterior dislocation.

Spine

Disc herniations appear as disc material extending beyond the vertebral body margin. Spinal stenosis is identified as narrowing of the central canal or lateral recesses. Modic changes in the vertebral endplates are classified as Type 1 (edema pattern, suggesting acute/active process), Type 2 (fatty replacement, suggesting chronic stable disease), and Type 3 (sclerotic, least common).

MRI Limitations

Metal artifact from orthopedic implants can significantly degrade image quality, although metal reduction sequences have improved this issue. Motion artifact limits interpretation in uncooperative patients. Claustrophobia prevents some patients from completing studies. Certain implants (some cochlear implants and cardiac devices) are absolute contraindications. Cost, limited availability, and the frequent overdiagnosis of incidental findings (especially in the spine and shoulder of asymptomatic individuals) are additional practical limitations.

Ultrasound

Advantages

Ultrasound offers real-time, dynamic assessment without ionizing radiation. It is inexpensive, portable, and can be performed at the bedside. It provides excellent guidance for procedures such as joint aspiration and therapeutic injection.

Orthopedic Applications

In experienced hands, ultrasound approaches MRI sensitivity for rotator cuff assessment. It is excellent for Achilles tendon evaluation, effusion detection and guided aspiration (particularly useful for pediatric hip effusions), foreign body detection, developmental dysplasia of the hip screening in infants (using the Graf classification), tendon pathology evaluation, and characterization of soft tissue masses.

Nuclear Medicine

Bone Scan (Technetium-99m MDP)

Bone scintigraphy offers high sensitivity but low specificity for detecting areas of increased bone turnover. The three-phase bone scan helps evaluate for infection by assessing blood flow, blood pool, and delayed bone uptake. Bone scans are useful for identifying stress fractures, performing metastatic surveys, and diagnosing reflex sympathetic dystrophy. However, this modality is being largely supplanted by MRI and PET-CT for many indications.

PET-CT (FDG)

PET-CT using fluorodeoxyglucose is primarily employed for oncologic staging but has an increasing role in musculoskeletal infection, particularly chronic osteomyelitis and periprosthetic infection. Areas of high metabolic activity demonstrate increased FDG uptake, though this finding is not specific to malignancy.

WBC-Labeled Scan (Indium-111 or Tc-99m HMPAO)

White blood cell-labeled scans are the most specific nuclear medicine study for infection. When combined with a sulfur colloid scan (to map normal marrow distribution), specificity around prosthetic joints is further increased. The main disadvantage is the lengthy imaging protocol, requiring 24-48 hours for indium-labeled studies.

When to Order Advanced Imaging: A Decision Framework

Fracture Assessment

Plain radiographs should always be obtained first. CT should be added for intra-articular fractures requiring surgical planning, pelvic and acetabular injuries, and spine trauma. MRI is indicated for occult fractures when radiographs are negative but clinical suspicion remains high, and for suspected stress fractures.

Soft Tissue Injuries

MRI is the gold standard for evaluating ligament, tendon, meniscal, and labral pathology. Ultrasound is an excellent alternative for superficial tendons (rotator cuff, Achilles) and situations requiring dynamic assessment.

Infection

Radiographs may be normal in early infection. MRI with gadolinium is the most sensitive and specific modality for osteomyelitis. WBC-labeled scans are reserved for cases where MRI is contraindicated or for periprosthetic infection evaluation.

Tumors

Plain radiographs remain the most important initial study for characterizing bone tumors. MRI provides local staging information including intramedullary extent, soft tissue involvement, and neurovascular relationships. CT chest screens for pulmonary metastases. Bone scan or PET-CT provides systemic staging.

<image>A teaching illustration showing the systematic ABCDEs approach to musculoskeletal radiograph interpretation. Display an AP knee radiograph with labeled annotations pointing out: A — alignment of the joint (mechanical axis), B — bone density and cortical integrity, C — cartilage/joint space (medial narrowing), D — distribution of findings, E — soft tissue swelling and suprapatellar effusion. Include a checklist sidebar.</image>

<image>A comparative MRI sequence illustration showing the same knee in T1-weighted, T2-weighted, and STIR sequences side by side. On T1: show fat as bright in marrow, water/edema as dark. On T2: show fluid in joint as bright. On STIR: show bone marrow edema (occult fracture) as bright with fat suppression. Label the key signal characteristics of each sequence and the clinical utility.</image>

<image>An illustrated flowchart showing the decision algorithm for musculoskeletal imaging. Start with clinical suspicion, branch to: suspected fracture (radiographs first, then CT or MRI), suspected soft tissue injury (MRI or ultrasound), suspected infection (radiographs + MRI with contrast or WBC scan), suspected tumor (radiographs + MRI for staging). Color-code each pathway and include the key indications for each modality.</image>

Clinical Pearls

A negative radiograph does not rule out a fracture; if clinical suspicion is high, advanced imaging with MRI (for occult fractures) or CT (for complex anatomy) should be pursued. Weight-bearing radiographs are essential for knees and feet/ankles because non-weight-bearing views underestimate both joint space narrowing and ligamentous instability. The posterior fat pad sign on a lateral elbow radiograph is always pathologic in adults and should be treated as a fracture until proven otherwise. For musculoskeletal tumors, the plain radiograph remains the single most important diagnostic study and should always be obtained before MRI. A simple rule for MRI interpretation: T1 sequences show anatomy, T2/STIR sequences reveal pathology. CT is superior to MRI for cortical bone detail and characterizing fracture patterns. Ultrasound is significantly underutilized in orthopedics and provides real-time dynamic assessment along with excellent procedural guidance. Finally, imaging findings must always be correlated with the clinical examination, as MRI abnormalities are common in asymptomatic individuals, particularly in the spine and shoulder.

References

• Jacobson JA. Fundamentals of Musculoskeletal Ultrasound. 3rd ed. Elsevier; 2017.
• Helms CA. Fundamentals of Skeletal Radiology. 5th ed. Elsevier; 2019.
• Tuite MJ, et al. ACR Appropriateness Criteria for acute and chronic musculoskeletal conditions. J Am Coll Radiol. Various years.
• Stoller DW. Magnetic Resonance Imaging in Orthopaedics and Sports Medicine. 3rd ed. Lippincott; 2007.

Read this lecture as Markdown