Hip Joint: Ligaments Connecting the Femur and Acetabulum

What is the ligament between the femur and the acetabulum?

The primary ligaments connecting the femur to the acetabulum are the strong capsular ligaments—the iliofemoral, pubofemoral, and ischiofemoral ligaments—which collectively form the robust fibrous capsule of the hip joint, along with the intra-articular ligamentum teres.

Introduction to the Hip Joint

The hip joint is a critical ball-and-socket synovial joint, connecting the lower limb to the axial skeleton. It is formed by the articulation of the spherical head of the femur (thigh bone) and the cup-shaped acetabulum of the pelvic bone. This design allows for a wide range of motion—flexion, extension, abduction, adduction, and internal/external rotation—while simultaneously bearing significant body weight and forces during activities like walking, running, and jumping. The remarkable stability of the hip joint, despite its mobility, is largely attributed to the deep fit of the femoral head within the acetabulum, the strong surrounding musculature, and, most critically, a complex network of powerful ligaments.

The Primary Ligaments of the Hip Joint Capsule

The hip joint is encased by a thick, fibrous joint capsule, which is significantly reinforced by three main extrinsic ligaments that spiral around the joint, tightening during extension to enhance stability. These ligaments are named for the specific parts of the pelvis they originate from before attaching to the femur.

• Iliofemoral Ligament (Ligament of Bigelow or Y-ligament):

  • Location and Anatomy: This is the strongest and most robust ligament in the body, situated on the anterior and superior aspects of the hip joint. It originates from the anterior inferior iliac spine (AIIS) and the rim of the acetabulum, then broadens and divides into two distinct bands (giving it its "Y" shape) that attach to the intertrochanteric line of the femur.
  • Function: The iliofemoral ligament is crucial for preventing excessive hip extension, which is vital for maintaining an upright posture without significant muscular effort (passive standing). It also limits external rotation and, to a lesser extent, adduction. Its exceptional strength is a primary contributor to the hip's anterior stability.

• Pubofemoral Ligament:

  • Location and Anatomy: Located on the anterior and inferior aspect of the hip joint, this ligament originates from the superior pubic ramus and the obturator crest. It then blends with the inferior part of the joint capsule and inserts into the lower part of the intertrochanteric line and the neck of the femur.
  • Function: The pubofemoral ligament primarily limits excessive hip abduction and extension. It also provides some restraint against external rotation, working in concert with the iliofemoral ligament to reinforce the anterior and inferior aspects of the capsule.

• Ischiofemoral Ligament:

  • Location and Anatomy: Situated on the posterior aspect of the hip joint, this ligament originates from the ischial part of the acetabular rim. Its fibers spiral superiorly and laterally, attaching to the greater trochanter of the femur and blending with the fibers of the joint capsule.
  • Function: The ischiofemoral ligament is the primary limiter of excessive hip internal rotation, especially when the hip is extended. It also contributes to the limitation of hip extension and provides posterior stability to the joint.

Together, these three ligaments form a cohesive and remarkably strong capsular system that effectively "screws home" the femoral head into the acetabulum during hip extension, thereby minimizing muscular energy required to maintain upright posture and providing significant passive stability.

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

While the capsular ligaments provide the primary structural stability, another important ligament connects the femur to the acetabulum, though its role is distinct.

• Location and Anatomy: The ligamentum teres is an intra-articular (within the joint capsule) ligament. It originates from the acetabular notch (a non-articular part of the acetabulum) and the transverse acetabular ligament, and attaches to the fovea capitis, a small depression on the head of the femur.
• Function: Historically, it was believed to contribute significantly to hip stability. However, current understanding indicates that its mechanical role in stability is relatively minor, especially in healthy adults. Its primary importance lies in carrying a small artery (a branch of the obturator artery) to the head of the femur, particularly crucial for its blood supply during childhood. In adults, this artery often atrophies, with the main blood supply coming from the retinacular arteries.

Functional Significance and Biomechanics

The collective arrangement of these ligaments is a marvel of biomechanical engineering. Their specific orientations and fiber directions mean that as the hip moves into various ranges of motion, different parts of the ligaments become taut, effectively limiting excessive movement and protecting the joint. For instance, during hip extension (as in standing upright), all three capsular ligaments become taut, locking the femoral head securely into the acetabulum and minimizing the need for constant muscle contraction to maintain posture. This "ligamentous locking" mechanism is fundamental to human bipedalism.

Clinical Relevance

Understanding these ligaments is crucial in clinical practice:

• Hip Sprains: While less common than ankle or knee sprains due to their immense strength, severe forces can lead to sprains (stretching or tearing) of the hip ligaments, particularly the iliofemoral ligament.
• Dislocations: The strength of these ligaments makes hip dislocations rare and typically requires significant trauma (e.g., car accidents) to overcome their integrity. Posterior dislocations are the most common, often involving tearing of the posterior capsule and ischiofemoral ligament.
• Rehabilitation: Following injury or surgery, rehabilitation protocols often focus on strengthening the muscles surrounding the hip to compensate for any ligamentous laxity and restore dynamic stability.
• Pathology of Ligamentum Teres: While not a primary stabilizer, injuries to the ligamentum teres (e.g., tears) can occur, often associated with hip impingement or instability, leading to pain and dysfunction.

Conclusion

The ligaments connecting the femur and the acetabulum are an intricate and immensely strong network vital for the stability, function, and integrity of the hip joint. The iliofemoral, pubofemoral, and ischiofemoral ligaments form the robust joint capsule, providing passive stability and limiting extreme ranges of motion, particularly extension. The intra-articular ligamentum teres, while having a minor mechanical role, is important for vascular supply. Together, these structures underpin the hip's ability to support body weight and facilitate efficient movement, making them critical components of human locomotion and posture.