Biomechanics of Hip

Hip Anatomy & Kinematics - Joint's Basic Moves

• Articular Surfaces: Femoral head & acetabulum (lunate surface, notch).
• Acetabular Labrum: Fibrocartilage rim; deepens socket, ↑ stability.
• Joint Capsule: Strong, fibrous; encloses joint.
• Key Ligaments: 📌
  • Iliofemoral (Y-ligament): Strongest; prevents hyperextension.
  • Pubofemoral: Limits abduction, extension.
  • Ischiofemoral: Limits internal rotation, extension.
  • Ligamentum Teres: Artery to femoral head (children).
• Movements (ROM): (Plane / Axis)
  • Flexion: 120°, Ext: 20° (Sagittal / Frontal).
  • Abduction: 45°, Add: 30° (Frontal / Sagittal).
  • Ext. Rot: 45°, Int. Rot: 35° (Transverse / Vertical).
• Blood Supply (Femoral Head):
  • Main: Medial Circumflex Femoral A. (MCFA).
  • Lateral Circumflex Femoral A. (LCFA).
  • Artery of Lig. Teres (children).

⭐ Iliofemoral ligament (Y-ligament): strongest in body, prevents hip hyperextension.

Forces on the Hip - Balancing Act

• Key Forces & JRF:
  • Body Weight (BW): Gravitational force vector.
  • Abductor Muscle Force (AMF): Gluteus medius/minimus; crucial for pelvic stability in single leg stance.
  • Joint Reaction Force (JRF): Net force across the hip. Magnitude: 3-6x BW (walking). Concept: $JRF \approx BW + AMF$ (vector sum).
• Center of Rotation: Femoral head's center; reference for force moments.
• Femoral Angles:
  • Neck-Shaft Angle (Inclination): Normal 125-135°. (<125° coxa vara; >135° coxa valga).
  • Anteversion Angle: Normal 10-15° (adults). (↑ leads to in-toeing).
• Lever Arm Mechanics (Single Leg Stance):
  • BW lever arm (hip center to body CoG) > Abductor lever arm (hip center to greater trochanter).
  • Abductors must generate high force to balance.
• Pauwels' Classification (Femoral Neck Fractures): Angle of fracture line to horizontal.
  • Type I: <30° (Stable; compression).
  • Type II: 30-50° (Less stable; shear).
  • Type III: >50° (Unstable; high shear; ↑non-union risk).

⭐ During normal walking, the hip joint reaction force can reach 3-6 times body weight, peaking during the mid-stance phase due to dynamic loading and muscle activity.

Gait & Clinical Links - Walking & Woes

• Gait Cycle & Hip:
  • Stance: Heel strike (30° flex) → midstance (neutral) → toe off (10° hyperext). JRF 3-5x body weight.
  • Swing: Hip flexes for clearance.
• Pelvic Stability:
  • Abductors (gluteus medius/minimus) crucial in single leg stance.
  • Trendelenburg Sign: Pelvic drop (swing side) if stance abductors weak.
  • Trendelenburg Gait: Trunk lean over stance limb.
• Hip OA Biomechanics:
  • ↑ Medial load, osteophytes, ↓ joint space, subchondral sclerosis.
• THA Principles:
  • Restore: Rotation center, femoral offset, leg length.
  • Acetabular cup: Inclination ~40°±10°, Anteversion ~15°±10° (Lewinnek's zone).
• Cane Use:
  • Contralateral hand preferred.
  • ↓ Abductor force & JRF. 📌 COAS: Cane Opposite Affected Side.

⭐ Using a cane contralateral to the affected hip reduces hip JRF by counteracting body weight moment, decreasing abductor force.

High‑Yield Points - ⚡ Biggest Takeaways

• Hip joint reaction force (JRF) is 2.5-3 times body weight during single-leg stance, increasing with activity.
• Abductor muscles (gluteus medius & minimus) are crucial for pelvic stability during gait.
• Trendelenburg sign/gait indicates weak hip abductors or mechanical disadvantage (e.g., coxa vara, NAI).
• Normal femoral neck anteversion is 10-15 degrees; excessive anteversion causes in-toeing.
• Medializing the hip center of rotation in THA ↓ JRF and abductor muscle force needed.
• Pauwels' angle predicts femoral neck fracture stability based on fracture line orientation to the horizontal.
• Shear forces across a femoral neck fracture ↑ with ↑ Pauwels' angle, impacting healing potential.