Pelvic Anatomy: Pubic Symphysis, Acetabulum, and Hip Joint

What is the joint between pubic bones of acetabulum?

The query regarding the "joint between pubic bones of acetabulum" touches upon two distinct, yet interconnected, anatomical structures: the pubic symphysis, which is the joint directly connecting the two pubic bones, and the acetabulum, the socket of the hip joint, to which the pubic bone contributes significantly.

The Pubic Symphysis: The Primary Joint Between Pubic Bones

The most direct answer to "the joint between pubic bones" is the pubic symphysis. This is a midline cartilaginous joint (specifically, a secondary cartilaginous joint or symphysis) located at the front of the pelvis.

• Structure: It consists of a fibrocartilaginous disc sandwiched between the two pubic bones. The articular surfaces of the pubic bones are covered with a thin layer of hyaline cartilage, which adheres to the fibrocartilaginous disc.
• Ligaments: This joint is reinforced by several strong ligaments, including the superior pubic ligament, inferior (arcuate) pubic ligament, anterior pubic ligament, and posterior pubic ligament. These ligaments provide stability and limit excessive movement.
• Function: While often considered a relatively immobile joint, the pubic symphysis allows for slight movements, particularly important during childbirth when hormonal changes (like relaxin) increase its flexibility. Its primary role is to provide stability to the pelvic girdle and absorb shock during locomotion.

The Acetabulum: A Socket Formed by Multiple Bones

The "of acetabulum" part of the query refers to the hip socket. The acetabulum is not a separate bone but a deep, cup-shaped depression on the lateral aspect of the hip bone (os coxae). It serves as the articulation point for the head of the femur, forming the hip joint.

• Composition: The acetabulum is formed by the fusion of three primary pelvic bones:
  • Ilium: Contributes the superior and posterior part.
  • Ischium: Forms the inferior and posterior part.
  • Pubis: Forms the inferior and anterior part.
• Fusion: In children, these three bones are separate and united by a triradiate cartilage within the acetabulum. This cartilage ossifies and fuses by late adolescence or early adulthood, creating a single, strong hip bone. Therefore, the pubic bone contributes directly to the formation of the acetabular socket, but it doesn't form a "joint with the acetabulum" as a separate entity; rather, it is part of the acetabulum.

The Hip Joint: The Articulation Within the Acetabulum

While the pubic symphysis is the joint between the pubic bones, the hip joint is the crucial articulation within the acetabulum, involving the pubic bone as a structural component of the socket.

• Type of Joint: The hip joint is a classic ball-and-socket synovial joint, allowing for a wide range of motion in multiple planes (flexion, extension, abduction, adduction, internal rotation, external rotation, and circumduction).
• Articulating Surfaces: The head of the femur (the "ball") articulates with the acetabulum (the "socket"). Both surfaces are covered with smooth articular cartilage to facilitate frictionless movement.
• Labrum: The acetabulum is deepened by a fibrocartilaginous rim called the acetabular labrum, which enhances joint stability and congruency.
• Ligaments: The hip joint is heavily reinforced by strong ligaments (iliofemoral, pubofemoral, ischiofemoral) that limit excessive motion and contribute to its remarkable stability, making it one of the most stable joints in the body. The pubofemoral ligament, for example, extends from the pubic part of the acetabular rim and superior pubic ramus to the intertrochanteric line of the femur, limiting excessive abduction and extension.

Clinical Significance and Common Conditions

Understanding these joints is crucial for diagnosing and treating various musculoskeletal conditions:

• Pubic Symphysis Dysfunction (PSD): Common during pregnancy, where increased laxity of the pubic symphysis can lead to pain and instability.
• Osteitis Pubis: Inflammation of the pubic symphysis and surrounding muscle attachments, often seen in athletes due to repetitive stress.
• Hip Osteoarthritis: Degeneration of the articular cartilage within the hip joint, leading to pain and reduced mobility.
• Acetabular Fractures: Traumatic injuries to the acetabulum can severely impact hip joint function and stability.
• Hip Impingement (FAI): Abnormal contact between the femoral head/neck and the acetabular rim, often due to subtle anatomical variations, including those affecting the pubic contribution to the acetabulum.

Anatomy in Motion: Why This Matters for Movement

From a kinesiology perspective, the integrity and function of both the pubic symphysis and the hip joints are paramount for efficient human movement:

• Pelvic Stability: The pubic symphysis, along with the sacroiliac joints, forms the pelvic ring, providing a stable base for the spine and lower limbs. This stability is essential for transmitting forces during walking, running, and lifting.
• Lower Limb Mobility: The hip joint, with its extensive range of motion, allows for the powerful and nuanced movements of the lower limb, critical for locomotion, balance, and athletic performance. The pubic bone's precise contribution to the acetabulum directly influences the mechanics of this ball-and-socket joint.
• Force Transmission: The pubic bones, as part of the pelvic girdle, are integral in transmitting forces from the lower limbs to the axial skeleton and vice versa. Any dysfunction in these joints can disrupt kinetic chains and lead to compensatory movements and injuries.

In summary, while there isn't a "joint between pubic bones of acetabulum" as a single, distinct entity, the pubic bones form the pubic symphysis with each other and are a critical component of the acetabulum, which in turn forms the highly mobile and stable hip joint with the femur.