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Musculoskeletal

Musculoskeletal Anatomy (Lower Extremity)

The lower extremity includes the hip, knee, ankle, and foot with their weight-bearing joints, menisci, ligaments, tendons, and neurovascular structures. MRI is central to internal derangement and marrow assessment, complemented by radiographs, CT, and ultrasound. Recognizing meniscal and cruciate anatomy, ankle ligament complexes, and neurovascular courses is key to accurate musculoskeletal diagnosis.

Key Structures

  • Hip: femoral head/neck and acetabulum with the labrum, ligamentum teres, and surrounding abductor/flexor musculature
  • Knee: femorotibial and patellofemoral joints, medial and lateral menisci, anterior and posterior cruciate ligaments, collateral ligaments, and extensor mechanism
  • Ankle: tibiotalar, subtalar, and distal tibiofibular (syndesmosis) joints; lateral (ATFL/CFL/PTFL) and medial (deltoid) ligament complexes
  • Foot: tarsal bones (talus, calcaneus, navicular, cuboid, cuneiforms), metatarsals, phalanges, plantar fascia, and the Lisfranc ligament
  • Tendons: quadriceps/patellar, hamstrings, Achilles, peroneals, tibialis posterior/anterior, and flexor/extensor groups
  • Neurovascular: femoral and sciatic nerves (tibial and common peroneal divisions), and the femoral/popliteal/tibial arterial tree
  • Cartilage and marrow: hyaline articular cartilage and subchondral bone assessed for chondral and stress injury

Imaging Planes & Modalities

  • Knee MRI: sagittal for menisci and cruciates, coronal for collaterals/menisci, axial for the patellofemoral joint and retinacula
  • Hip MRI/MR arthrography with radial images for the labrum and femoroacetabular impingement morphology
  • Ankle/foot MRI in three planes tailored to ligaments and tendons; oblique planes for Lisfranc and syndesmosis
  • Radiographs first-line (including weight-bearing) for alignment, arthritis, and fracture; CT for complex fractures and tarsal coalition
  • Ultrasound for dynamic tendon assessment (Achilles, peroneals), effusions, and guided injections
  • MR/CT angiography for the runoff vessels in trauma and vascular disease

Landmarks & Normal Values

  • Alpha angle (>~55 degrees) indicates cam-type femoroacetabular impingement; center-edge angle assesses dysplasia
  • Normal meniscus is uniformly low signal; grade 3 signal reaching the surface indicates a tear
  • PCL angle and cruciate contours help assess ACL/PCL integrity indirectly
  • Achilles tendon normally has a flat or concave anterior margin; AP thickness typically <7-8 mm
  • Tibiofibular clear space and overlap on radiographs assess syndesmotic integrity
  • Q-angle and tibial tuberosity-trochlear groove (TT-TG) distance evaluate patellar maltracking

Anatomical Variants

  • Discoid lateral meniscus predisposing to tears
  • Bipartite patella (usually superolateral) mimicking fracture
  • Accessory ossicles: os trigonum, accessory navicular, and os peroneum
  • Tarsal coalition (calcaneonavicular, talocalcaneal) causing rigid flatfoot
  • Variant/accessory muscles (accessory soleus, peroneus quartus) presenting as soft-tissue masses
  • Meniscal variants (meniscofemoral ligaments of Humphrey/Wrisberg) mimicking loose bodies

Pathology Correlation

  • Meniscal and cruciate anatomy underlies tear patterns (bucket-handle, root tears) and the O'Donoghue triad
  • Femoroacetabular impingement morphology drives labral tears and early osteoarthritis
  • Ankle ligament complexes explain inversion (lateral) versus eversion/syndesmotic injuries and instability
  • Lisfranc ligament disruption underlies midfoot instability and fracture-dislocation
  • Marrow edema patterns localize stress fractures, bone contusions (pivot-shift bruise pattern in ACL tears), and osteonecrosis
  • Neurovascular courses relate to compartment syndrome, tarsal tunnel syndrome, and popliteal entrapment

Pitfalls & Mimics

  • Magic-angle artifact in curved tendons (peroneals, tibialis posterior) simulating tendinopathy
  • Meniscofemoral ligaments and the popliteus tendon mimicking meniscal tears near the posterior horn
  • Accessory ossicles and bipartite patella mistaken for acute fractures
  • Vacuum phenomenon and normal physes in skeletally immature patients misread as pathology
  • Postoperative meniscus and residual signal confounding re-tear assessment (MR arthrography helps)
  • Pes anserine and other bursae mistaken for cystic masses

References

  • Stoller. Magnetic Resonance Imaging in Orthopaedics and Sports Medicine
  • Helms, Major, Anderson. Musculoskeletal MRI
  • Manaster et al. Diagnostic Imaging: Musculoskeletal
  • Netter. Atlas of Human Anatomy

⚕️ Educational reference — correlate with clinical context and confirm with a qualified radiologist.