Hip Anatomy

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Description

The hip articulation is true diarthroidal balland-socket style joint, formed from the head of the femur as it articulates with the acetabulum of the pelvis.  This joint serves as the main connection between the lower extremity and the trunk, and typically works in a closed kinematic chain.

Motions Available

• Flexion: forward and upward movement of the femur at the hip occurs in the sagittal plane about an medial-lateral axis.
• Extension: upward movement toward the rear of the body of the femur at the hip occuring in the sagittal plane.
• Abduction: movement of the femur on the hip in a direction away from the midline of the body in the frontal plane.
• Adduction: movement of the femur on the hip in a direction toward the midline of the body in the frontal plane.
• Internal Rotation: rotation of the femur toward the midline of the body in the transverse plane.
• External Rotation: rotation of the femur away from the midline of the body in the transverse plane.

Anterior view of the right hip joint.

Ligaments & Joint Capsule

Notable Ligaments:

• Ligamentum Teres: This ligament is located entirely within the hip joint.  It spans the hip running from the acetabular notch to the fovis capitis of the femur, attaching the femoral head to the inferior acetabular rim.  This ligament is joined with nerves and vessels that pass to the femoral head.  The vascular component of this structure is important during development, but is less significant in children.  This ligament becomes taut during adduction, flexion, and external rotation, but only minimally contributes to joint stability.
  
• Ischiofemoral Ligament: Is the only ligament located on the posterior aspect of the hip.  It attaches to the posterior surface of the acetabular rim and labrum.  This ligament is said to "wind" around the joint and insert on the anterior aspect of the femur.  The location and orientation of this ligament reinforces the joint capsule posteriorly and checks against excessive extension and internal rotation of the hip joint.
  
• Iliofemoral Ligament (Y Ligament of Bigelow): Attaches to the AIIS and then fans out to attach along the intertrochanteric line of the femur.  The iliofemoral ligament is the strongest ligament in the body, and checks extension, adduction (superior fibers), and abduction (inferior fibers). In addition, because this ligament limits hip extension, it allows maintenance of the upright posture by reducing the need for muscle contractions.
  
• Pubofemoral Ligament: Is located on the anterior portion of the joint, arising from the anterior aspect of the pubic ramus and passing to the anterior surface of the intertrochanteric fossa.  This ligament's main purpose is to check hip abduction and extension.  This ligament may blend with the inferior fibers of the iliofemoral ligament.
  

Joint Capsule:

• The hip joint is extremely strong, due to its reinforcement by strong ligaments and musculatrue, providing a relatively stable joint.  Unlike the weak articular capsule of the shoulder, the hip joint capsule is a substantial contributor to joint stability.  The capsule is thicker anterosuperiorly where the predominant stresses of weight bearing occur, and is thinner posteroinferiorly.
  

Muscles

Flexors:

• Psoas
• Iliacus
• Pectineus
• Rectus Femoris

Extensors:

• Gluteus Maximus
• Semitendinosus
• Semimembranosus
• Biceps Femoris (long head)

Adductors:

• Adductor Magnus
• Adductor Longus
• Adductor Brevis
• Gracilis
• Pectineus

Abductors:

• Gluteus Medius
• Tensor Fascia Latae

Internal Rotators:

• Tensor Fascia Latae
• Gluteus Minimus

External Rotators:

• Gluteus Maximus
• Gemellus Superior
• Gemellus Inferior
• Obturator Externus
• Obturator Internus
• Quadratus Femoris
• Piriformis

Closed Packed Position

Full extension of the hip joint is the closed packed postion because this position draws the strong ligaments of the joint tight, resulting in stability.

Open Packed Position

The hip joint is one of the only joints where the position of optimal articular contact (combined flexion, abduction, and external rotation) is the open-packed, rather than closed packed position, since flexion and external rotation tend to uncoil the ligaments and make them slack.

Other Important Information

• Labrum: The labrum forms a fibrocartilagenous extension of the bony acetabulum which increases the containment of the femoral head. In addition to this function, the labrum also obstructs fluid flow in and out of the joint through a sealing action which is often referred to as a “suction effect” in view of the resistance generated to distraction of the head from the acetabular socket. This sealing function not only enhances joint stability, but is thought to more uniformly distribute compressive loads applied to the articular surfaces, thereby reducing peak cartilage stresses during weight-bearing.
  
• Femoral Triangle
  
• Angle of Inclination
  
• Angle of Torsion