The Hip: Primary Joint and Associated Pelvic Girdle Joints

How many joints are in the hip?

While commonly referred to in the singular, the "hip" region functionally encompasses a complex interplay of several joints. The primary hip joint is the coxofemoral joint, a single ball-and-socket joint, but its optimal function is critically dependent on the stability and mobility of the surrounding pelvic girdle joints, namely the sacroiliac joints and the pubic symphysis.

The Primary Hip Joint: The Coxofemoral Joint

The joint most people refer to when they say "the hip" is the coxofemoral joint. This is a true synovial joint, designed for significant mobility and stability.

• Anatomy:
  • It is formed by the articulation of the spherical head of the femur (thigh bone) and the cup-shaped acetabulum of the pelvis.
  • The acetabulum is part of the innominate bone, which itself is formed by the fusion of the ilium, ischium, and pubis.
• Type: It is a ball-and-socket joint, allowing for a wide range of motion in all three planes:
  • Flexion/Extension: Moving the leg forward and backward.
  • Abduction/Adduction: Moving the leg away from and towards the midline.
  • Internal/External Rotation: Rotating the leg inward and outward.
• Stability: Despite its mobility, the coxofemoral joint is incredibly stable due to:
  • The deep fit of the femoral head into the acetabulum.
  • A strong joint capsule.
  • Numerous powerful ligaments (iliofemoral, pubofemoral, ischiofemoral) that restrict excessive movement.
  • Surrounding musculature providing dynamic stability.

Associated Joints of the Pelvic Girdle (The Hip Complex)

For a comprehensive understanding of hip function, it's crucial to consider the joints of the pelvic girdle, which work in concert with the coxofemoral joint. These are often referred to as part of the "hip complex" due to their direct influence on hip mechanics and overall lower body stability.

The Sacroiliac (SI) Joints

There are two sacroiliac joints, one on each side of the sacrum.

• Anatomy: Formed by the articulation between the sacrum (the triangular bone at the base of the spine) and the ilium (the largest part of the hip bone).
• Type: They are synovial joints, but with very limited movement. Their primary role is to transmit forces between the upper body and the lower limbs.
• Function: While subtle, movements like nutation (forward tilt of the sacrum relative to the ilium) and counternutation are crucial for shock absorption, gait, and maintaining pelvic stability during activities like walking, running, and lifting. Dysfunction here can significantly impact hip and lower back health.

The Pubic Symphysis

There is one pubic symphysis joint.

• Anatomy: Located at the very front of the pelvis, where the two pubic bones meet in the midline.
• Type: It is a cartilaginous joint (symphysis), specifically a fibrocartilaginous disc, allowing for minimal movement.
• Function: It provides stability to the anterior aspect of the pelvic ring and acts as an anchor point for numerous muscles. Its slight mobility is particularly important during childbirth.

The Lumbo-Pelvic-Hip Complex: An Integrated System

While not in the hip itself, the lumbosacral joint (between the last lumbar vertebra and the sacrum) and the other intervertebral joints of the lumbar spine are inextricably linked to hip function. Movements at the hip often involve compensatory or synergistic movements in the lumbar spine. For example, hip flexion beyond a certain point will typically involve pelvic tilt and lumbar spine flexion. Understanding this lumbo-pelvic-hip complex is vital for analyzing human movement, diagnosing pain, and designing effective exercise programs.

Conclusion

In summary, while there is fundamentally one primary hip joint (the coxofemoral joint) on each side of the body, the functional "hip" region encompasses a network of interconnected joints. These include the two sacroiliac joints and the single pubic symphysis, all of which contribute to the stability, mobility, and overall biomechanics of the lower body. For fitness professionals, kinesiologists, and anyone interested in optimal movement, appreciating this integrated system is key to understanding performance, preventing injury, and promoting long-term health.