The Hip Complex: Structure & Function

💡 Core Concept: The Hip Joint (Acetabulofemoral joint) is a ball-and-socket joint designed for Stability and Weight Bearing. Unlike the shoulder, which sacrifices stability for mobility, the hip has deep congruency and strong ligaments to support the body's mass.

1. Structure of the Hip Joint

A. The Proximal Surface: Acetabulum

• Lunate Surface: The horseshoe-shaped, cartilage-covered area that actually bears weight.
• Acetabular Fossa: The deep, central part. It contains fat and blood vessels but does not bear weight.
• Acetabular Labrum: A wedge-shaped fibrocartilage rim that deepens the socket and creates a suction seal (Negative pressure) to maintain stability.

[Image of Acetabulum anatomy with labrum and lunate surface]

B. The Distal Surface: Femur

The Femoral Head is slightly larger than a hemisphere. Two critical angulations of the femur determine hip biomechanics:

Angle	Normal Value	Abnormalities	Biomechanical Effect
Angle of Inclination	~125°	Coxa Valga: >125° Coxa Vara: <125°	Vara: Increases moment arm (better force), but increases shear on neck (fracture risk). Valga: Decreases moment arm (muscle works harder), but more stable loading.
Angle of Torsion	15-20° (Anteversion)	Excessive Anteversion: >20° Retroversion: <15°	Anteversion: Leads to "Toe-in" gait (pigeon-toed) to align the head in the socket.

C. Capsule and Ligaments

The capsule is thickened by three massive ligaments. All three tighten in Extension (Closed Packed Position).

• Iliofemoral (Y Ligament): Strongest. Checks Extension. Allows "hanging on ligaments" in stance (Paraplegic stance).
• Pubofemoral: Anterior/Inferior. Checks Abduction and Extension.
• Ischiofemoral: Posterior. Checks Internal Rotation and Extension.

2. Functional Biomechanics

A. Osteokinematics & Arthrokinematics

• Femur on Pelvis (Open Chain): Convex Femoral Head moves on Concave Acetabulum.
  • Flexion: Anterior Roll, Posterior Slide.
  • Abduction: Lateral Roll, Medial (Inferior) Slide.
• Pelvis on Femur (Closed Chain): Concave Acetabulum moves on Convex Femoral Head.
  • Anterior Pelvic Tilt (Flexion): Acetabulum rolls and slides Anteriorly.

B. Lumbopelvic Rhythm

• Ipsidirectional: Pelvis and Spine move in same direction (bending over to touch toes). Max mobility.
• Contradirectional: Pelvis and Spine move in opposite directions (keeping eyes level while walking).

3. Joint Forces and Muscle Function

A. The Hip Abductor Mechanism

In single-leg stance, the Hip Abductors (Gluteus Medius) must generate massive force to counteract the torque of gravity acting on the body weight.

⚠️ The Trendelenburg Sign: If the Right Gluteus Medius is weak, when the patient stands on the Right leg, the pelvis will drop on the Left side. This indicates the abductors cannot counterbalance the body weight.

B. Joint Reaction Forces (JRF)

During walking, the hip sustains forces of 2.5x to 3x body weight. This force is the sum of Body Weight + Muscle Contraction Force.

🏆 Key Points: Cane Biomechanics

To reduce pain/force in an arthritic Right hip:

• Hold the cane in the Left (Contralateral) hand.
• Why? The cane creates a long moment arm that assists the Glute Medius.
• Pushing down on the cane generates a torque that counteracts gravity, allowing the Glute Medius to relax. Less muscle force = Less Joint Compression.

4. Hip Pathology

• Arthrosis (OA): Degeneration of articular cartilage. Load is concentrated on the superior-anterior aspect of the acetabulum.
• Fracture: Common in elderly with Osteoporosis. Coxa Vara increases shear force on the femoral neck, increasing fracture risk.
• SCFE (Slipped Capital Femoral Epiphysis): Common in adolescents. The femoral head slips posteriorly/inferiorly on the growth plate.

📝 20 High-Yield MCQs

Test your knowledge for Exams.

Q1. The "Closed Packed Position" of the hip joint is:

A. Full Extension, Slight Abduction, and Internal Rotation B. 90 degrees Flexion, Slight Adduction C. Full Flexion and External Rotation D. 30 degrees Flexion, 30 degrees Abduction (Loose packed)

Rationale: In full extension, the spiral ligaments (Iliofemoral, etc.) twist and tighten, maximizing stability.

Q2. A patient with excessive "Anteversion" of the hip (angle > 20°) will typically compensate by walking with:

A. Toe-out gait B. Toe-in gait (Pigeon-toed) C. Increased stride length D. Vaulting

Rationale: To center the femoral head in the acetabulum (improve congruence), the patient internally rotates the femur, turning the toes in.

Q3. Which ligament is considered the strongest in the hip and primarily limits Hyperextension?

A. Ischiofemoral B. Iliofemoral (Y Ligament) C. Pubofemoral D. Ligamentum Teres

Rationale: The Iliofemoral ligament (Ligament of Bigelow) is incredibly strong and allows paraplegics to stand by resting on it in extension.

Q4. Coxa Vara (Angle of Inclination < 125°) results in:

A. Increased shear force on the femoral neck B. Decreased moment arm for the Gluteus Medius C. Longer leg length D. Decreased risk of fracture

Rationale: Coxa Vara bends the neck more horizontally. This increases the lever arm for muscles (good) but drastically increases shear stress (bad/fracture risk).

Q5. During Open Chain hip flexion, the femoral head:

A. Rolls Anteriorly and Slides Anteriorly B. Rolls Anteriorly and Slides Posteriorly C. Rolls Posteriorly and Slides Anteriorly D. Spins only

Rationale: Convex femur on Concave acetabulum = Roll and Slide in OPPOSITE directions.

Q6. To effectively reduce joint reaction forces in the Right hip, a cane should be held in:

A. The Right hand B. The Left hand C. Both hands D. No cane is recommended

Rationale: Holding the cane in the opposite (Left) hand creates a long lever arm that helps the abductors counteract gravity, reducing the muscle force required.

Q7. A positive Trendelenburg sign involving a drop of the Left pelvis indicates weakness of:

A. The Right Hip Abductors B. The Left Hip Abductors C. The Right Hip Extensors D. The Left Quadratus Lumborum

Rationale: The stance leg (Right) muscles are responsible for holding the pelvis level. If they fail, the opposite (swing) side drops.

Q8. The primary function of the Acetabular Labrum is to:

A. Connect the femur to the spine B. Deepen the socket and create a negative pressure seal C. Bear weight directly D. Provide blood supply

Rationale: The labrum deepens the acetabulum and maintains a suction seal, which is vital for joint stability and fluid distribution.

Q9. Anterior Pelvic Tilt is produced by a force couple of which muscles?

A. Hip Flexors and Erector Spinae B. Hip Extensors and Abdominals C. Hip Abductors and Adductors D. Hamstrings and Glutes

Rationale: Hip flexors pull the front of the pelvis down, while back extensors pull the back of the pelvis up, increasing lordosis.

Q10. The Center of Edge (CE) angle measures:

A. Acetabular coverage of the femoral head (Stability) B. Femoral torsion C. Angle of inclination D. Knee valgus

Rationale: A smaller CE angle indicates dysplasia (less coverage, instability). A larger CE angle can lead to impingement.

Q11. The Piriformis muscle acts as an External Rotator at neutral, but becomes an Internal Rotator at:

A. 0 degrees flexion B. > 90 degrees hip flexion C. Full extension D. It never changes function

Rationale: As the hip flexes past 60-90 degrees, the line of pull of the Piriformis shifts anterior to the axis of rotation, reversing its action.

Q12. Which part of the acetabulum does NOT bear weight?

A. Lunate Surface B. Acetabular Fossa C. Superior Rim D. Posterior Wall

Rationale: The Acetabular Fossa is deep and non-articular; it contains a fat pad and the Ligamentum Teres.

Q13. Passive Insufficiency of the Rectus Femoris limits:

A. Knee Flexion when the Hip is Extended B. Knee Extension when Hip is Flexed C. Hip Flexion when Knee is Flexed D. Hip Abduction

Rationale: The Rectus Femoris crosses both joints. Lengthening it at both ends (Hip Ext + Knee Flex) stretches it to its limit (Passive Insufficiency).

Q14. Ward's Triangle in the femoral neck is significant because:

A. It is an area of weakness with low trabecular density (fracture risk) B. It is the strongest part of the bone C. It is where muscles attach D. It stores marrow

Rationale: It is the zone between the primary compressive and tensile trabecular groups. It has minimal bone density and is prone to osteoporotic fractures.

Q15. What is the primary function of the Ligamentum Teres in an adult?

A. Primary stabilizer of the hip B. Proprioception and secondary stabilization (sling effect) C. Main blood supply source D. Prevents extension

Rationale: While it carries a small artery (obturation), its mechanical role is minimal compared to the capsule. It may provide proprioceptive feedback.

Q16. Ipsidirectional Lumbopelvic Rhythm is used to:

A. Increase the reach of the upper extremities (e.g., touching toes) B. Keep the head upright while walking C. Maintain balance D. Sit down

Rationale: The spine and pelvis move in the same direction (flexion + anterior tilt) to maximize range of motion.

Q17. During single-leg stance, the Hip Joint Reaction Force is approximately:

A. 1x Body Weight B. 2.5 - 3x Body Weight C. 0.5x Body Weight D. 10x Body Weight

Rationale: The joint must support body weight PLUS the massive contraction force of the Abductors pulling down to level the pelvis.

Q18. Coxa Valga (Angle > 125°) results in:

A. Increased moment arm for abductors B. Decreased moment arm for abductors (Muscle must work harder) C. Increased shear force D. Increased fracture risk

Rationale: A straighter neck (Valga) brings the trochanter closer to the center of rotation, shortening the lever arm. This requires more muscle force but provides vertical stability.

Q19. Slipped Capital Femoral Epiphysis (SCFE) involves slippage of the femoral head in which direction?

A. Posteriorly and Inferiorly B. Anteriorly and Superiorly C. Medially D. Laterally

Rationale: "Ice cream falling off the cone." The epiphysis slips down and back relative to the neck.

Q20. Which nerve innervates the primary Hip Abductors (Glute Med/Min)?

A. Inferior Gluteal Nerve B. Superior Gluteal Nerve C. Femoral Nerve D. Obturator Nerve

Rationale: The Superior Gluteal Nerve supplies the Gluteus Medius, Minimus, and TFL. (Inferior supplies Glute Max).