MRI in Orthopedics

1. Basic Principles Relevant to Orthopedics

"T1-weighted images are primarily suited for anatomic orientation and tissue characterization, while PDFS sequences play an important role in the diagnosis of joint damage because they are very sensitive to edematous changes and allow for the high signal intensity of hyaline articular cartilage." - THIEME Atlas of Musculoskeletal System

2. MRI of the Knee

Cruciate Ligaments

• Sagittal plane is preferred for viewing both cruciate ligaments in continuity
• The PCL is clearly visible in its entirety in sagittal sections; the ACL requires 15-20° external rotation of the knee for complete visualization
• ACL tear: loss of normal linear fibers, replaced by diffuse high T2 signal or discontinuity
• 1.5T MRI has high accuracy for ACL and meniscal tear diagnosis

T1-weighted sagittal MRI of the knee. (a) Level of the outer meniscus - note gastrocnemius. (b) Level of the cruciate ligaments - ACL, PCL, patella, patellar ligament, and Hoffa's fat pad clearly delineated. (THIEME Atlas of Musculoskeletal System)

Menisci

• Meniscal tears: best seen on sagittal and coronal sequences
• Normal meniscus = low signal on all sequences (fibrocartilage)
• A tear = linear high signal extending to the articular surface
• Bucket-handle tears are a surgical emergency in young patients who cannot fully extend the knee

Articular Cartilage

• PDFS sequences best resolve cartilage thickness and focal defects
• Grading of chondral lesions (OCD, early OA)

Coronal MRI showing articular cartilage, anterior cruciate ligament, external and internal menisci. High signal of cartilage and ligament detail are well appreciated. (THIEME Atlas)

Axial MRI of the knee: lateral femoral condyle, ACL, PCL, articular cartilage, popliteal vessels, and semimembranosus bursa are all delineated. (THIEME Atlas)

3. MRI of the Spine

Spinal Infection (Spondylodiscitis / Spinal Epidural Abscess)

• Entire spine must be imaged because noncontiguous infection occurs in 15-20% of cases
• T1 sequences: decreased signal in infected vertebral bodies and disc spaces; disc-vertebral body margin cannot be differentiated
• T2 sequences: increased signal in the intervertebral disc, markedly increased in the vertebral body; paravertebral abscesses and thecal sac compression shown as areas of increased signal
• Fat suppression + post-gadolinium sequences improve sensitivity of T2 and T1 respectively
• DWI helps characterize fluid collections and differentiate spondylodiscitis from benign reactive marrow changes
• Gadolinium enhances delineation of epidural abscesses and determines their level/position relative to neural elements
• Serial MRI is NOT indicated to follow treatment response unless patient is clinically failing - bony findings often worsen or remain unchanged despite clinical improvement; monitor soft-tissue findings (paraspinal abscesses, epidural collections, T2 disc changes) instead

4. MRI of the Hip - Avascular Necrosis (AVN / Osteonecrosis)

• Negative radiographic findings with clinical suspicion should always prompt MRI
• T1-weighted coronal: geographic lesion of low signal in the weight-bearing surface of the femoral head - "double line sign" is pathognomonic
• STIR (coronal): confirms the geographic lesion with high signal intensity corresponding to marrow edema
• ACR recommends bilateral hip MRI to detect contralateral early osteonecrosis before collapse
• Radionuclide bone scan is the second-line substitute when MRI is contraindicated

Left: Pelvic X-ray showing vague increased density of the left femoral head. Center: Coronal STIR MRI confirming AVN on the left (geographic lesion). Right: Coronal T1-weighted MRI showing bilateral geographic low-signal lesions - note the right femoral head is X-ray normal but MRI-positive for early osteonecrosis. (Rheumatology, Elsevier 2022)

5. MRI of the Ankle and Foot

• Axial MRI with local gradient: best for anterior talofibular ligament (ATFL), posterior talofibular ligament (PTFL), deep layers/tibionavicular component of deltoid ligament
• Coronal MRI: best for calcaneofibular ligament (CFL), posterior talofibular ligament, tibiocalcaneal and posterior tibiotalar ligaments; also differentiates deep vs. superficial layers of the medial collateral ligament
• MRI is more specific than sensitive for CFL injuries
• Bone bruises occur in a significant number of ankle sprains; multiple bone bruises suggest multi-ligament injury; can persist >1 year after injury
• 3T MRI with AI-based reconstruction can produce high-quality ligament/tendon images in <10 minutes - improving accuracy and efficiency

6. MRI in Rheumatoid Arthritis (RA)

• MRI uniquely images bone, articular structures, and soft tissues without ionizing radiation
• Initially used for large joints (hips, knees, shoulders); improved technology now resolves smaller joints
• 3.0T MRI provides quantitative measurement of bone marrow edema; combined with CT gives comprehensive analysis of joint inflammation and bone damage
• Coupling of serologic tests with symmetric synovitis + bone edema or bone erosion on MRI helps discriminate RA from undifferentiated arthritis and spondyloarthritis
• Extremity MRI (E-MRI): smaller units for peripheral joints; lower cost, better positioning; growing role in clinical rheumatology

7. MRI Advantages Over Other Modalities in Orthopedics

8. Contraindications and Practical Points

• Absolute contraindications: ferromagnetic implants (older pacemakers, cochlear implants, certain intracranial aneurysm clips), intraocular metallic foreign bodies
• Passive orthopedic implants (prosthetic joints, plates, screws) are generally MRI-safe but require manufacturer confirmation; newer scanners with MARS (Metal Artifact Reduction Sequences) significantly reduce streak artifact from hardware
• Claustrophobia: managed with open scanners or anxiolytics
• Gadolinium contrast is avoided in severe renal impairment (risk of nephrogenic systemic fibrosis)
• Not indicated routinely for initial evaluation of low back pain (no red flags) - clinical history + exam + plain films are sufficient first steps per AAFP guidelines

Summary of Key Orthopedic Indications for MRI

• Campbell's Operative Orthopaedics 15th Ed 2026 (Spinal MRI sections)
• THIEME Atlas of General Anatomy and Musculoskeletal System (Knee MRI)
• Rheumatology 2-Volume Set, Elsevier 2022 (RA imaging, AVN of hip)
• Bailey & Love's Short Practice of Surgery 28th Ed (ACL injury)