Pelvic Ligaments: Sacrotuberous, Sacrospinous, and Pelvic Stability

What is the ligament between the tailbone and the hip?

While no single major ligament directly connects the coccyx (tailbone) solely to the hip bone, the sacrotuberous ligament and sacrospinous ligament are the principal structures that connect the sacrum (the bone directly above the coccyx) to parts of the hip bone (ischium), playing a crucial role in pelvic stability and indirectly influencing the tailbone's position.

Understanding the Anatomy of the Pelvis and Tailbone

To accurately identify the ligaments in question, it's essential to understand the key bony structures of the lower spine and pelvis.

• Coccyx (Tailbone): The most inferior (lowest) part of the vertebral column, typically composed of 3-5 fused or semi-fused vertebrae. It articulates superiorly with the sacrum.
• Sacrum: A large, triangular bone formed by the fusion of five sacral vertebrae, located at the base of the spine. It articulates with the fifth lumbar vertebra superiorly and the coccyx inferiorly. Laterally, it articulates with the ilium (part of the hip bone) to form the sacroiliac joints.
• Hip Bones (Pelvic Bones): Each hip bone (os coxae) is a large, irregular bone, formed by the fusion of three bones:
  • Ilium: The largest and most superior part, forming the prominence of the hip.
  • Ischium: The posteroinferior part, which bears the weight when sitting. It features the ischial spine and the ischial tuberosity.
  • Pubis: The anteroinferior part, forming the front of the pelvis.

While the question specifically asks about a ligament between the tailbone and the hip, the primary robust ligaments in this region span from the sacrum to the hip bone, with the coccyx often receiving some reinforcing fibers from the sacrotuberous ligament.

The Primary Ligaments: Sacrotuberous and Sacrospinous

The two most significant ligaments connecting the sacrum (and by extension, the coccyx) to the ischium are:

The Sacrotuberous Ligament

• Origin: This broad, strong ligament originates from the posterior superior iliac spine (PSIS), the posterior surface of the sacrum, and the lateral margins of the coccyx.
• Insertion: It extends downwards and laterally to insert onto the medial margin of the ischial tuberosity (the bony prominence you sit on).
• Course: The fibers run obliquely downwards, laterally, and anteriorly. Some fibers of the gluteus maximus muscle blend with its superficial surface.
• Relevance to Coccyx: While its primary connection is to the sacrum, some of its fibers directly attach to the lateral border of the coccyx, providing indirect support and stability to the tailbone.

The Sacrospinous Ligament

• Origin: This thinner, triangular ligament originates from the lateral margin of the sacrum and the coccyx.
• Insertion: It extends laterally to insert onto the ischial spine (a sharp, pointed projection on the posterior border of the ischium).
• Course: It runs almost horizontally, deep to the sacrotuberous ligament.
• Relevance to Coccyx: It has direct attachments to the coccyx, reinforcing the connection between the coccyx and the pelvic girdle.

Functions and Biomechanical Significance

These ligaments are crucial for maintaining the structural integrity and stability of the pelvis, particularly the sacroiliac (SI) joint, and for supporting the pelvic floor.

• Pelvic Stability: Both ligaments work together to stabilize the sacrum and prevent its anterior rotation (nutation) relative to the ilium during weight-bearing activities. This is vital for efficient force transmission between the trunk and lower limbs.
• Counteracting Shearing Forces: They resist the tendency of the sacrum to shear downwards and forwards, especially during movements like standing up, walking, and running.
• Formation of Foramina: The sacrospinous and sacrotuberous ligaments, along with the ischium, convert the greater and lesser sciatic notches of the hip bone into the greater sciatic foramen and lesser sciatic foramen, respectively. These foramina serve as passageways for nerves (like the sciatic nerve), blood vessels, and muscles (like the piriformis) to exit or enter the pelvis.
• Support for the Pelvic Floor: They provide an attachment point for certain muscles of the pelvic floor (e.g., coccygeus muscle), contributing to the support of pelvic organs.

Clinical Relevance and Common Issues

Dysfunction or injury to these ligaments can lead to pain and impaired movement.

• Coccygodynia (Tailbone Pain): While often related to direct trauma to the coccyx or inflammation of the sacrococcygeal joint, tension or injury to the sacrotuberous or sacrospinous ligaments can contribute to or mimic coccygeal pain.
• Pelvic Girdle Pain (PGP): Dysfunction of the sacroiliac joints, often involving laxity or tightness of these stabilizing ligaments, can result in pain in the lower back, buttocks, and even radiating down the leg. This is common during pregnancy due to hormonal changes affecting ligamentous laxity.
• Trauma: Falls directly onto the buttocks, childbirth, or repetitive strain can injure these ligaments, leading to sprains, inflammation (ligamentitis), or chronic pain.
• Nerve Entrapment: Due to their proximity to major nerves exiting the pelvis (e.g., pudendal nerve, sciatic nerve), tightness or inflammation of these ligaments can sometimes contribute to nerve irritation or entrapment, causing referred pain or sensory changes.

Implications for Movement and Exercise

As an Expert Fitness Educator, understanding these ligaments is paramount for effective programming and injury prevention.

• Core Stability: These ligaments are integral components of the deep posterior core system. Their proper function supports the pelvis and spine, contributing to overall core stability during movement.
• Force Transmission: They play a critical role in transferring forces between the upper body and lower extremities. During activities like squatting, deadlifting, or jumping, these ligaments help to stabilize the pelvis, allowing for efficient power generation.
• Postural Influence: Chronic poor posture or prolonged sitting can lead to tightness or weakness in the muscles and connective tissues surrounding these ligaments, potentially contributing to pain or dysfunction.
• Rehabilitation and Training Considerations:
  • Pelvic Floor Integration: Exercises that engage the pelvic floor muscles often indirectly influence the tension and health of these ligaments.
  • Gluteal and Hamstring Strength: The sacrotuberous ligament has close fascial connections with the gluteus maximus and hamstrings. Strengthening these muscles can provide synergistic support to the ligament.
  • Mobility vs. Stability: While maintaining adequate mobility in the hip and lumbar spine is important, exercises should also emphasize controlled stability of the pelvis, especially in multi-planar movements.
  • Mindful Movement: Awareness of pelvic position and controlled movement patterns during exercises like hip hinges, squats, and lunges can help protect these structures.

Conclusion

While the question "What is the ligament between the tailbone and the hip?" might imply a single, direct connection, a more accurate anatomical understanding points to the sacrotuberous and sacrospinous ligaments. These robust structures connect the sacrum (just above the coccyx) to the ischium (part of the hip bone), with the coccyx receiving direct attachments from both. Their primary role is to provide critical stability to the pelvic girdle, facilitate efficient force transfer, and protect neural structures. For fitness professionals and enthusiasts, recognizing the importance of these ligaments is key to optimizing movement, preventing injury, and addressing lower back and pelvic pain. Any persistent pain in this region warrants consultation with a qualified healthcare professional.