Acetabulum: Differences Between the Notch and Fossa, and Their Functions

What is the difference between acetabular notch and fossa?

The acetabular notch and acetabular fossa are distinct anatomical features within the acetabulum of the hip bone, with the fossa being a central, non-articular depression and the notch representing an inferior deficiency in the acetabular rim, both crucial for hip joint structure and function.

Understanding the Acetabulum

The acetabulum is the cup-shaped depression on the lateral aspect of the hip bone (os coxa), formed by the fusion of the ilium, ischium, and pubis. It serves as the socket for the head of the femur, forming the highly stable and mobile hip joint. While often considered a single structure, the acetabulum comprises several key features that contribute to its intricate biomechanics, including the acetabular fossa and the acetabular notch.

The Acetabular Fossa Explained

The acetabular fossa is a roughened, non-articular depression located centrally within the acetabulum.

• Definition: It is the deepest, non-weight-bearing part of the acetabulum.
• Location: Situated in the floor of the acetabulum, medial to the lunate surface.
• Composition: Unlike the smooth, cartilage-covered lunate surface (which articulates with the femoral head), the fossa is typically devoid of articular cartilage. It is characterized by small nutrient foramina and a rough texture.
• Function:
  • Ligament Attachment: The primary function of the acetabular fossa is to provide an attachment site for the ligament of the head of the femur (also known as the ligamentum teres or round ligament). This ligament contains a small artery (the foveal artery or artery to the head of the femur), which supplies blood to the femoral head, particularly in childhood.
  • Fat Pad: The fossa is also filled with a fat pad (Haversian fat pad), which acts as a shock absorber and allows for lubrication and displacement of synovial fluid during hip movements.
  • Space for Movement: It provides necessary space for the femoral head to move without impinging on the central acetabular bone during extreme ranges of motion.

The Acetabular Notch Explained

The acetabular notch is a distinct gap or deficiency located at the inferior aspect of the acetabular rim.

• Definition: It is an interruption in the otherwise continuous circular rim of the acetabulum.
• Location: Found at the inferior margin of the acetabulum, bridging the ischial and pubic contributions to the acetabulum.
• Formation: It is formed where the lunate surface of the acetabulum does not complete a full circle, leaving an opening.
• Function/Significance:
  • Transverse Acetabular Ligament: The acetabular notch is bridged by the transverse acetabular ligament. This ligament effectively completes the inferior margin of the lunate surface, transforming the notch into a foramen (opening) and deepening the socket for the femoral head. It plays a role in stabilizing the hip joint by reinforcing the inferior capsule.
  • Passage for Neurovascular Structures: The space created by the transverse acetabular ligament and the underlying notch allows for the passage of blood vessels and nerves into and out of the hip joint capsule and the acetabular fossa itself, including branches of the obturator artery and nerve.
  • Labral Attachment: The acetabular labrum, a fibrocartilaginous ring that deepens the acetabular socket, attaches to the rim of the acetabulum, including the margins of the notch, before transitioning into the transverse acetabular ligament.

Key Differences Summarized

Clinical Relevance and Biomechanical Implications

Understanding the distinction between the acetabular fossa and notch is crucial for several clinical and biomechanical considerations:

• Hip Stability: The integrity of the acetabular rim, including the proper functioning of the transverse acetabular ligament spanning the notch, is vital for hip joint stability.
• Impingement Syndromes: Abnormalities in the shape or orientation of the acetabulum, including the depth of the fossa or the configuration of the notch, can contribute to femoroacetabular impingement (FAI), where the femoral head or neck abnormally contacts the acetabular rim.
• Surgical Considerations: Surgeons must be acutely aware of these landmarks during hip arthroscopy or open hip surgery to avoid damaging neurovascular structures passing through the notch or to properly reconstruct the acetabular labrum and ligamentous structures.
• Vascular Supply: The artery within the ligament of the head of the femur, originating in the acetabular fossa, is a critical, albeit variable, blood supply to the femoral head, especially important in pediatric fractures.

Conclusion

While both the acetabular fossa and the acetabular notch are integral components of the acetabulum, they serve distinct anatomical and functional roles. The fossa is a central, non-articular depression primarily housing the ligamentum teres and a fat pad, facilitating smooth joint movement. The notch, conversely, is an inferior opening in the acetabular rim, functionally completed by the transverse acetabular ligament, which enhances socket depth and provides a conduit for essential neurovascular structures. Together, these features contribute significantly to the complex stability, mobility, and overall biomechanics of the human hip joint.