What are the axes of the lower limbs?

Axes of the Lower Limbs: Understanding Alignment and Clinical Significance

The axes of the lower limbs refer to the mechanical and anatomical alignment patterns that determine proper weight distribution, joint function, and overall biomechanics of the legs. These axes are critical for normal gait, joint health, and prevention of musculoskeletal disorders.

Key Axes of the Lower Limbs

Mechanical Axis

• The mechanical axis (also called the weight-bearing line) extends from the center of the femoral head to the center of the ankle joint 1
• This line normally passes slightly medial to or through the center of the knee joint in adults
• Deviation of this axis is measured as:
  • Varus alignment (bowlegs): mechanical axis passes lateral to the knee center
  • Valgus alignment (knock-knees): mechanical axis passes medial to the knee center

Anatomical Axes

• Femoral anatomical axis: Line through the center of the femoral shaft
• Tibial anatomical axis: Line through the center of the tibial shaft
• The angle between these two axes normally forms a slight valgus angle of 5-9° in adults 2

Developmental Changes in Axes

• At birth: Lower extremities typically have a varus alignment (bowlegs) with increased femoral antetorsion 2
• Ages 2-3: Varus alignment gradually shifts toward neutral
• Ages 3-7: Children develop physiologic valgus alignment (knock-knees), peaking around age 4
• Ages 7-10: Valgus gradually decreases to the normal adult alignment of 5-9° 2
• By skeletal maturity: Normal femoral antetorsion decreases to about 15° and the lower leg shows external rotation of approximately 15° 2

Clinical Assessment of Lower Limb Axes

Radiographic Assessment

• Standing hip-to-ankle radiographs (long-leg views) provide the most accurate assessment of the mechanical axis 1
• Key measurements include:
  • Hip-knee-ankle (HKA) angle
  • Tibiofemoral angle
  • Joint orientation angles (distal femoral and proximal tibial angles)
• Standing AP knee radiographs can provide valid information about coronal plane alignment at the knee, but hip-to-ankle radiographs are necessary for accurate assessment of the overall mechanical axis 1

Clinical Examination

• Intermalleolar distance (for valgus) or intercondylar distance (for varus)
• Assessment of gait pattern
• Evaluation of joint range of motion
• Note: Clinical measurements correlate poorly with radiographic measurements, even when corrected for leg length 3

Clinical Significance of Axial Alignment

Impact on Joint Health

• Proper alignment distributes forces evenly across joint surfaces
• Malalignment increases stress on specific compartments of the knee:
  • Varus alignment increases medial compartment loading
  • Valgus alignment increases lateral compartment loading
• Persistent malalignment can lead to premature osteoarthritis 3

Relevance in Total Knee Arthroplasty

• Restoration of neutral mechanical alignment has traditionally been considered crucial for TKA success 1, 4
• Recent evidence suggests mild deviations from neutral alignment (±3°) may not significantly impact short-term functional outcomes 4
• Assessment of alignment after TKA typically involves standing AP and lateral radiographs, with some practitioners using long-leg views for optimal assessment 1

Management of Axial Deformities

• Physiologic variations in children typically resolve spontaneously and require observation only 3, 2
• Pathologic deformities may require intervention:
  • Mild deformities (0-2 cm leg length discrepancy): observation or shoe modification 3
  • Moderate deformities: guided growth techniques or epiphysiodesis in growing children 1, 3
  • Severe deformities: corrective osteotomies 1, 3

Common Pitfalls in Assessment and Management

• Misinterpreting physiologic variations as pathologic, particularly in children under 8 years 3, 2
• Failing to obtain weight-bearing radiographs, which are essential for accurate assessment 1
• Relying solely on clinical measurements without radiographic confirmation 3
• Overlooking the impact of rotational positioning on radiographic appearance of alignment 1
• Unnecessary treatment of physiologic variations that would resolve spontaneously 3, 2

Understanding the axes of the lower limbs is fundamental for proper assessment and management of various orthopedic conditions, from developmental variations to degenerative joint disease and post-surgical evaluation.