A 68-year-old man from Delhi presents with a 3-month history of progressive left hip pain and stiffness. He reports difficulty walking and cannot squat or cross his legs. On examination, the left hip is held in flexion, adduction, and internal rotation. Passive abduction and external rotation are severely restricted and painful. Plain radiograph of the pelvis shows joint space narrowing and osteophytes around the left hip. Which anatomical structure is primarily responsible for the restricted external rotation observed in this patient?

Explanation

## Anatomical Basis of Restricted External Rotation in Hip Osteoarthritis ### Normal Hip Ligament Anatomy The hip joint is stabilized by three major extracapsular ligaments: | Ligament | Origin | Insertion | Primary Function | |----------|--------|-----------|-----------------| | **Iliofemoral (Y-ligament of Bigelow)** | Anterior inferior iliac spine | Intertrochanteric line (two bands) | Limits extension; strongest hip ligament | | **Pubofemoral** | Superior pubic ramus / obturator crest | Iliofemoral ligament / intertrochanteric line | Limits **abduction AND external rotation** | | **Ischiofemoral** | Ischial body (posterior acetabular rim) | Greater trochanter (medial surface) | Limits **internal rotation** in flexion | ### Why the Pubofemoral Ligament Is the Primary Restraint to External Rotation **Key Point:** The **pubofemoral ligament** runs from the pubis obliquely downward and laterally to blend with the inferior band of the iliofemoral ligament. Its fiber orientation specifically resists **abduction and external rotation** of the hip — the two movements most restricted in this patient. - The **iliofemoral ligament** is the strongest hip ligament and primarily limits **hyperextension** and, to a lesser degree, external rotation in the extended position. However, its primary role is anti-extension, not anti-external-rotation. - The **ischiofemoral ligament** (posterior) limits **internal rotation** — it would be taut when the hip is externally rotated, not when external rotation is being attempted from a neutral or internally rotated position. - The **pubofemoral ligament** (anteromedial/inferior) is specifically oriented to resist external rotation and abduction. In osteoarthritis, capsular fibrosis and contracture of this ligament are the primary anatomical reasons external rotation and abduction are the earliest and most severely restricted movements. **Clinical Pearl:** In hip OA, the capsule contracts symmetrically but the pubofemoral ligament — which resists ER and abduction — becomes fibrotic and shortened, explaining why passive abduction and external rotation are the movements most severely restricted and painful on examination (as described in this vignette). This is consistent with the classic "capsular pattern" of the hip described by Cyriax: most restriction of internal rotation, then abduction, then external rotation — all movements resisted by the pubofemoral and inferior capsule. **High-Yield:** Per Moore's Clinically Oriented Anatomy and Gray's Anatomy, the pubofemoral ligament is the primary anatomical restraint to **external rotation and abduction** of the hip. The ischiofemoral ligament limits internal rotation. The iliofemoral ligament limits extension. ### Summary of Movement Restrictions - **External rotation restricted** → Pubofemoral ligament (anteromedial) is taut - **Abduction restricted** → Pubofemoral ligament + inferior capsule - **Extension restricted** → Iliofemoral ligament (Y-ligament) - **Internal rotation restricted** → Ischiofemoral ligament (posterior) [cite: Moore's Clinically Oriented Anatomy 8e Ch 5; Gray's Anatomy 41e, Hip Joint chapter]