Hip

The hip is a ball-and-socket synovial joint that forms the connection between the pelvis and the lower limb. It is one of the most stable and weight-bearing joints in the human body, allowing for a wide range of motion while supporting the body’s weight during movement and standing.^[6] The joint is formed by the articulation of the head of the femur (ball) and the acetabulum of the pelvis (socket), surrounded by strong ligaments and muscles for stability.

Location

The hip joint is located in the pelvic region, on either side of the lower torso, where the femur (thigh bone) meets the acetabulum of the pelvis. It lies deep within the hip region, lateral to the groin and just below the waistline.

Anatomy

The hip joint is a ball-and-socket synovial joint that forms the connection between the pelvis and the femur. It is one of the largest and most stable joints in the human body, designed to provide strength, stability, and mobility. The hip joint plays a vital role in weight-bearing and movement, enabling activities such as standing, walking, and running. Below is a detailed description of its anatomy:

Type of Joint

• The hip joint is a ball-and-socket synovial joint.^[4]
• The ball is the rounded head of the femur, and the socket is the acetabulum of the pelvis.

Articulating Surfaces

The two articulating surfaces of the hip joint are:

Head of the Femur:

• A smooth, spherical structure covered with hyaline cartilage.
• Forms approximately two-thirds of a sphere.
• It articulates with the acetabulum of the pelvis.

Acetabulum:

• A deep, cup-shaped socket formed by the fusion of the ilium, ischium, and pubis bones of the pelvis.
• The rim of the acetabulum is surrounded by the acetabular labrum, a fibrocartilaginous structure that deepens the socket and increases joint stability.
• The acetabulum’s articular surface is horseshoe-shaped and covered with hyaline cartilage, except for the central acetabular fossa.

The ball-and-socket configuration allows for extensive movement while maintaining stability.

Joint Capsule

The hip joint is enclosed by a strong fibrous joint capsule, which is thick and dense to provide stability.^[1]

Attachments

• Proximally: The capsule attaches to the rim of the acetabulum and the labrum.
• Distally: It attaches to the intertrochanteric line anteriorly and the femoral neck posteriorly.

Synovial Membrane

• Lines the internal surface of the joint capsule and produces synovial fluid for lubrication.
• The capsule is reinforced by ligaments for additional strength.

Ligaments

The hip joint is stabilized by intrinsic ligaments that reinforce the joint capsule and ligaments of the head of the femur:

Intrinsic Ligaments (Capsular Ligaments):

• Iliofemoral Ligament (Y-shaped ligament of Bigelow):
  • Location: Lies anterior to the joint.
  • Attachments: From the anterior inferior iliac spine (AIIS) to the intertrochanteric line of the femur.
  • Function: It is the strongest ligament in the body and prevents hyperextension of the hip joint.
• Pubofemoral Ligament:
  • Location: Lies inferior and anterior to the joint.
  • Attachments: From the superior pubic ramus to the intertrochanteric line.
  • Function: Limits abduction and extension of the hip.
• Ischiofemoral Ligament:
  • Location: Lies posterior to the joint.
  • Attachments: From the ischial part of the acetabulum to the greater trochanter of the femur.
  • Function: Stabilizes the hip and limits internal rotation.

 Ligament of the Head of the Femur (Ligamentum Teres):

• Location: Extends from the acetabular notch to the fovea capitis (a depression on the femoral head).
• Contains the artery to the head of the femur (a branch of the obturator artery).
• Although weak, it provides vascular supply to the femoral head.

Acetabular Labrum

• A fibrocartilaginous rim that surrounds the acetabulum.
• Deepens the socket to increase joint stability.
• Provides a suction effect to maintain the head of the femur in the acetabulum.

Blood Supply

The hip joint has a rich blood supply from multiple arteries:

• Medial and Lateral Circumflex Femoral Arteries:
  • Branches of the profunda femoris artery.
  • Supply the majority of the femoral head and neck.
• Obturator Artery: Provides the artery to the head of the femur via the ligamentum teres.
• Superior and Inferior Gluteal Arteries: Supply the surrounding muscles and joint capsule.

The rich blood supply is essential for maintaining joint health and preventing avascular necrosis.

Nerve Supply

The hip joint is innervated by nerves derived from the lumbar plexus and sacral plexus:

• Femoral Nerve: Anterior aspect of the joint.
• Obturator Nerve: Medial aspect of the joint.
• Superior Gluteal Nerve: Lateral aspect of the joint.
• Nerve to Quadratus Femoris: Posterior aspect of the joint.

These nerves provide both motor and sensory innervation, including proprioception.^[8]

Muscles Acting on the Hip Joint

The hip joint is surrounded by powerful muscles that facilitate movement and stability:

Flexors:

• Iliopsoas
• Rectus femoris
• Sartorius

Extensors:

• Gluteus maximus
• Hamstrings (biceps femoris, semitendinosus, semimembranosus)

Abductors:

• Gluteus medius
• Gluteus minimus
• Tensor fasciae latae

Adductors:

• Adductor longus, brevis, and magnus
• Gracilis
• Pectineus

Internal Rotators:

• Gluteus medius (anterior fibers)
• Gluteus minimus
• Tensor fasciae latae

External Rotators:

• Piriformis
• Obturator internus and externus
• Gemelli (superior and inferior)
• Quadratus femoris

Bursae

The hip joint contains bursae (fluid-filled sacs) to reduce friction between muscles, tendons, and bones:

• Trochanteric Bursa: Lies between the gluteus maximus and the greater trochanter.
• Iliopsoas Bursa: Lies between the iliopsoas tendon and the lesser trochanter.
• Ischial Bursa: Located over the ischial tuberosity.

Function

The hip joint is a ball-and-socket synovial joint that serves as the primary connection between the lower limb and the trunk of the body. It is a highly stable and mobile joint, balancing both strength and flexibility to perform vital functions for locomotion, weight-bearing, and body stability. Below is a detailed explanation of its functions:

Primary Movements of the Hip Joint

The hip joint allows movement in three planes due to its ball-and-socket structure:

 Flexion

• Definition: Movement where the thigh moves anteriorly, reducing the angle between the thigh and the trunk.
• Range of Motion: Approximately 120–140 degrees.
• Muscles Involved:
  • Primary: Iliopsoas (psoas major and iliacus), rectus femoris, sartorius.
  • Assistants: Tensor fasciae latae and pectineus.
• Examples: Lifting the leg to walk, climb stairs, or sit.^[7]

 Extension

• Definition: Movement where the thigh moves posteriorly, increasing the angle between the thigh and trunk.
• Range of Motion: Approximately 10–20 degrees.
• Muscles Involved:
  • Primary: Gluteus maximus.
  • Assistants: Hamstrings (biceps femoris, semitendinosus, semimembranosus) and the adductor magnus.
• Examples: Pushing off the ground when walking or running.

 Abduction

• Definition: Movement where the thigh moves away from the midline of the body.
• Range of Motion: Approximately 40–50 degrees.
• Muscles Involved:
  • Primary: Gluteus medius and gluteus minimus.
  • Assistants: Tensor fasciae latae and sartorius.
• Examples: Lifting the leg sideways or balancing on one leg.

 Adduction

• Definition: Movement where the thigh moves toward the midline of the body.
• Range of Motion: Approximately 20–30 degrees.
• Muscles Involved:
  • Primary: Adductor longus, adductor brevis, adductor magnus, gracilis, and pectineus.
• Examples: Crossing the legs or stabilizing the body when standing.

 Internal (Medial) Rotation

• Definition: Rotation of the thigh inward, so the knee and toes face toward the midline.
• Range of Motion: Approximately 30–40 degrees.
• Muscles Involved:
  • Primary: Gluteus medius and gluteus minimus (anterior fibers), tensor fasciae latae.
• Examples: Turning the leg inward while sitting or walking.^[5]

 External (Lateral) Rotation

• Definition: Rotation of the thigh outward, so the knee and toes face away from the midline.
• Range of Motion: Approximately 40–50 degrees.
• Muscles Involved:
  • Primary: Piriformis, obturator internus, obturator externus, gemelli (superior and inferior), and quadratus femoris.
  • Assistants: Gluteus maximus.
• Examples: Turning the leg outward, such as positioning for sports or ballet movements.

Weight-Bearing Function

• The hip joint is a major weight-bearing joint in the human body. It transmits forces between the upper body and the lower limbs during activities such as standing, walking, running, and jumping.
• The head of the femur distributes body weight evenly across the acetabulum, while the joint’s ligaments and cartilage absorb forces and prevent damage.
• During weight-bearing, the joint capsule and ligaments provide stability, preventing dislocation or excessive movement.

Stability of the Trunk and Pelvis

The hip joint maintains stability and alignment of the trunk and pelvis during various activities:

• During standing on one leg, muscles such as the gluteus medius and gluteus minimus stabilize the pelvis to prevent it from tilting (Trendelenburg mechanism).
• The hip ligaments, particularly the iliofemoral ligament, provide passive stability by resisting excessive extension.
• The adductor muscles stabilize the thigh during lateral movements.

This function ensures balance and proper posture during weight-bearing.

Locomotion (Gait and Movement)

The hip joint plays a central role in locomotion by coordinating with the knee and ankle joints. The gait cycle involves:

• Flexion: Initiates forward swing of the leg.
• Extension: Pushes the body forward during the stance phase.
• Abduction and Adduction: Stabilize the pelvis as the opposite leg swings forward.
• Rotation: Adjusts the orientation of the foot and leg for balance and propulsion.

This coordination allows for smooth and energy-efficient walking, running, and climbing.

Shock Absorption

• The hip joint absorbs and distributes impact forces during high-impact activities like running, jumping, or landing.
• The articular cartilage covering the femoral head and acetabulum cushions the joint and reduces stress.
• The labrum and synovial fluid further distribute forces, protecting the joint surfaces.

Range of Motion and Flexibility

The hip joint provides a wide range of motion due to its ball-and-socket structure:

• It allows movement in multiple planes (flexion, extension, abduction, adduction, and rotation).^[3]
• This flexibility enables functional activities like squatting, sitting, dancing, and performing athletic movements.

Support for Upper Body Movement

• The hip joint acts as a foundation for the trunk, head, and arms.
• Movements of the upper body, such as twisting, bending, or lifting, are stabilized by the hip joint.
• The joint’s ligaments and muscles counterbalance these movements, maintaining stability.

Integration with the Knee and Spine

The hip joint functions as part of a kinetic chain, integrating with the knee, ankle, and spine:

• Movements at the hip affect the alignment and function of the knee joint and lumbar spine.
• For example, restricted hip mobility can lead to compensatory movements in the lower back or knees, causing pain or dysfunction.

Proprioception and Balance

The hip joint contains sensory receptors within its capsule, ligaments, and surrounding muscles, providing proprioceptive feedback:

• Proprioception helps the brain understand the position, movement, and stability of the hip joint.
• It is essential for balance, coordination, and adjusting posture during activities like walking on uneven surfaces.

Clinical Significance

The hip joint is a vital weight-bearing joint and plays a key role in mobility and stability.^[2] Its structural complexity and constant use make it prone to various clinical issues. Key aspects of its clinical significance include:

Osteoarthritis

A common degenerative condition where the articular cartilage wears down, causing pain, stiffness, and reduced mobility.

Hip Fractures

Most commonly occur in the femoral neck or intertrochanteric region, particularly in elderly individuals with osteoporosis.

Dislocations

High-impact trauma (e.g., car accidents) can cause hip dislocation, typically posterior dislocations, leading to instability and pain.

Avascular Necrosis (AVN)

Disruption of blood supply to the femoral head (often due to trauma or steroids) can lead to bone death and joint collapse.

Hip Labral Tears

Damage to the acetabular labrum causes pain, clicking, and reduced joint function, common in athletes.

Developmental Dysplasia of the Hip (DDH)

A congenital condition where the acetabulum is shallow, leading to instability or dislocation.