Biomechanics of the Hip Joint

1. Introduction

2. Geometry and Osseous Architecture

Femoral Neck-Shaft (Neck-Shaft) Angle

• Normal: 125-130° (mean ~130° in men, ~128° in women)
• Coxa vara: angle < 120° - reduces abductor lever arm, Trendelenburg gait
• Coxa valga: angle > 135° - reduces abductor leverage, increases joint contact force

Femoral Neck Anteversion

• The femoral neck subtends an angle of 15-25° anterior to the transcondylar axis (anteversion)
• Increased anteversion: toe-in gait, reduces anterior hip stability
• Decreased anteversion (retroversion): toe-out gait, predisposes to cam-type impingement

Acetabular Geometry

• The acetabulum faces laterally, anteriorly, and inferiorly (~45° inclination, ~15-20° anteversion)
• The lunate articular surface is horseshoe-shaped; the acetabular fossa is filled with fibroadipose tissue
• The labrum deepens the socket, increasing coverage and stability

Trabecular Architecture (Ward's Systems)

1. Medial (compressive) system: arises from the medial cortex/calcar femorale, fans into the femoral head - resists compressive loads
2. Lateral (tensile) system: from the lateral cortex to the superior femoral head - resists tensile forces

• The Ward's triangle (between the two systems) is an area of relative weakness - fractures occur here in osteoporosis
• The calcar femorale: a dense vertical plate of cortical bone at the posteromedial femoral neck that provides the principal structural support

3. Range of Motion

• Flexors: iliopsoas (primary), rectus femoris, sartorius
• Extensors: gluteus maximus, hamstrings
• Abductors: gluteus medius and minimus (primary stabilizers in stance)
• External rotators: piriformis, obturators, gemelli, quadratus femoris ("POGO-Q")
• Internal rotators: anterior fibers of gluteus medius/minimus, tensor fascia lata

4. Forces Acting on the Hip Joint - The Lever Arm Concept

The Two-Lever System

• Load lever arm (B-X): from the center of the femoral head (B) to the body's center of gravity (X) - approximately 2.5 times longer than the abductor lever arm
• Force (abductor) lever arm (A-B): from the greater trochanter (insertion of abductors, A) to the center of the femoral head (B)

Mathematical Derivation of Joint Reaction Force (JRF)

Abductor moment = Body weight moment

Abductor Force (AF) × Abductor Lever Arm = Body Weight (BW) × Body Weight Lever Arm

AF × d_A = BW × d_BW

Since d_BW / d_A ≈ 2.5:1

AF ≈ 2.5 × BW

JRF = AF + BW ≈ 2.5 BW + BW ≈ 3.5 × BW (in single-limb stance)

5. Joint Reaction Forces - Summary Table

• Bailey & Love's Surgery, 28th Ed. + Campbell's Operative Orthopaedics, 15th Ed., 2026

6. Trendelenburg Sign and Gait

• Weakness or paralysis of ipsilateral abductors (gluteus medius)
• Shortened abductor lever arm (e.g., coxa valga, unreduced hip dislocation, THA malposition)
• Superior gluteal nerve palsy
• Painful hip (antalgic pseudo-Trendelenburg)

7. Clinical Application - Surgical Manipulation of Biomechanics

Charnley's Low-Friction Arthroplasty Concept

1. Medialization of the acetabulum: deepens the cup medially, shortening the body weight lever arm (B-X → B1-X), reducing required abductor force
2. High-offset femoral stem: lateralizes the greater trochanter, lengthening abductor lever arm (A-B → A1-B1)
3. Lateral + distal reattachment of greater trochanter: further lengthens abductor lever arm (A2-B2), tightens abductors

Hip Offset (Femoral Offset)

• Defined as the perpendicular distance from the femoral shaft axis to the center of the femoral head
• Restoring normal offset is critical in THA:
  • Reduced offset: weakens abductor complex, increases JRF, causes Trendelenburg gait, increases polyethylene wear risk
  • Increased offset: improves abductor tension and lever arm, but may increase torsional stresses on stem

Forces in Different Planes

• Coronal plane: medial deflection of the stem
• Sagittal plane: posterior deflection (especially with hip flexion - rising from a chair, ascending stairs)
• Combined: produces torsion of the femoral stem - critical for implant design

8. Stability Mechanisms

1. Bony congruence: deep socket, femoral head coverage by acetabulum (~270°)
2. Labrum: creates a suction effect, deepens socket, increases stability by ~10%
3. Capsular ligaments (static stabilizers):
   • Iliofemoral (Y-ligament of Bigelow): strongest ligament in the body; limits extension and external rotation
   • Pubofemoral ligament: limits abduction and extension
   • Ischiofemoral ligament: limits internal rotation in extension
4. Negative intra-articular pressure: suction mechanism resists distraction
5. Ligamentum teres: minor role in adults; transmits the medial circumflex artery branch in children
6. Periarticular muscles: primary dynamic stabilizers

9. Biomechanics in Gait

• Absorbs ground reaction forces (3-5 × BW peak)
• Requires adequate abductor function to maintain pelvic stability
• Normal hip extension (10-15°) in late stance propels the body forward

• Hip flexes to ~30-40° (minimum needed for foot clearance)
• Iliopsoas is the primary swing initiator
• Losing hip extension in stance increases pelvic anterior tilt and compensatory lumbar lordosis

10. Patho-Biomechanics - Clinical Correlates

Summary Points (for Quick Recall)

• JRF = Body weight component + Abductor force component
• Abductor lever arm ≈ 1/2.5 of body weight lever arm in normal hip
• 3× BW during walking; 10× BW during running/jumping
• Positive Trendelenburg = inadequate ipsilateral abductors
• Hip stability: bony congruence + Y-ligament of Bigelow + capsule + negative intra-articular pressure
• Key NSA: 125-130° normal; anteversion 15-25° normal
• Surgical strategies to reduce JRF: medialization of cup + restoration of offset

Hip Biomechanics - All Diagrams

Diagram 1 - Three Axes of Motion at the Hip Joint

• Transverse axis → Flexion / Extension
• Sagittal (AP) axis → Abduction / Adduction
• Longitudinal axis → Internal / External Rotation

Diagram 2 - Range of Motion: Flexion / Extension

Diagram 3 - Range of Motion: Abduction / Adduction

Diagram 4 - Range of Motion: Internal / External Rotation (hip flexed 90°)

Diagram 5 - Lever Arms Acting on the Hip (THA Biomechanics)

• B: Medialization of acetabulum shortens the body weight lever arm (B1-X); high-offset neck lengthens abductor lever arm (A1-B1)
• C: Lateral + distal reattachment of greater trochanter lengthens abductor lever arm further (A2-B2)

Diagram 6 - Hip Abductor Force: Maintaining Level Pelvis in Single-Limb Stance

Diagram 7 - Hip Joint Load vs Neck-Shaft Angle: Normal, Coxa Valga, Coxa Vara

• Normal CCD angle (~126°): Body weight lever arm (a) ≈ 3× muscle lever arm (b) → R = 4× body weight
• Coxa valga (increased CCD): Muscle lever arm (b) decreases → muscle must work harder → R = 7× body weight
• Coxa vara (decreased CCD): Muscle lever arm (b) increases → less muscle force needed → R = 3× body weight

Diagram 8 - Reducing Hip Load: Carrying Weight on Affected Side (Duchenne Limp Mechanism)

Diagram 9 - Reducing Hip Load: Walking Stick on Unaffected Side

Diagram 10 - Trendelenburg Gait: Normal vs Positive Trendelenburg vs Trendelenburg Lurch

• A - Normal: shoulders and pelvis level in single-limb stance
• B - Positive Trendelenburg lurch + negative Trendelenburg sign: trunk leans over the affected (stance) hip; pelvis remains level because the lean brings center of gravity over the stance leg
• C - Positive Trendelenburg lurch + positive Trendelenburg sign: pelvic obliquity present (contralateral pelvis drops); trunk leaning over involved hip

Diagram 11 - Trabecular Pattern of the Proximal Femur (Ward's Triangle)

1. Principal compressive group: medial cortex → femoral head (resists compressive loads)
2. Principal tensile group: lateral cortex → superior femoral head (resists tensile forces)
3. Secondary compressive group: intertrochanteric region, perpendicular to neck
4. Secondary tensile group + Greater trochanter group
5. Ward's triangle: area of paucity between groups in inferomedial femoral neck → weakest zone, first to lose trabeculae in osteoporosis

Diagram 12 - Forces Producing Torsion of Femoral Stem (THA)

• A (coronal plane): forces tend to deflect the femoral stem medially
• B (sagittal plane): forces (especially with hip flexed - rising from chair, climbing stairs) tend to deflect the stem posteriorly
• Combined effect: produces torsion of the femoral stem - critical design consideration for implants