Sacrococcygeal Joint: Anatomy, Function, and Clinical Considerations

What is the name of the joint between the sacrum and the coccyx?

The joint between the sacrum and the coccyx is primarily known as the sacrococcygeal joint or sacrococcygeal symphysis, a cartilaginous joint that allows for limited movement.

The Sacrococcygeal Joint: An Overview

The sacrococcygeal joint represents the terminal segment of the vertebral column, connecting the inferior aspect of the sacrum to the superior aspect of the coccyx. While often overlooked, this small joint plays a subtle yet significant role in the biomechanics of the pelvis and trunk, particularly in relation to weight-bearing, shock absorption, and pelvic floor function. It is a type of cartilaginous joint, specifically a symphysis, characterized by the presence of fibrocartilage between the articulating bones.

Anatomy of the Sacrococcygeal Joint

Understanding the structure of the sacrococcygeal joint is crucial for appreciating its function and potential dysfunctions.

• Joint Type: The sacrococcygeal joint is classified as a symphysis, a secondary cartilaginous joint. This means the articulating bone surfaces are covered by hyaline cartilage and united by a strong disc of fibrocartilage. In some individuals, particularly with age, the joint may fuse completely, forming a bony union known as a synostosis.
• Articulating Surfaces:
  • The apex (inferior tip) of the sacrum forms the superior articulating surface.
  • The base (superior aspect) of the coccyx forms the inferior articulating surface.
• Interposed Disc: A fibrocartilaginous disc, similar in composition to intervertebral discs but much smaller, lies between the sacrum and coccyx. This disc contributes to the joint's limited flexibility and shock-absorbing capacity.
• Ligaments: Several strong ligaments reinforce the sacrococcygeal joint, providing stability and limiting excessive movement:
  • Anterior Sacrococcygeal Ligament: A continuation of the anterior longitudinal ligament, reinforcing the anterior aspect of the joint.
  • Posterior Sacrococcygeal Ligament: Composed of superficial and deep layers, analogous to the posterior longitudinal ligament and ligamentum flavum, respectively. It strengthens the posterior aspect.
  • Lateral Sacrococcygeal Ligaments: Connect the inferolateral angles of the sacrum to the transverse processes of the first coccygeal vertebra.
  • Intercornual Ligaments: Connect the sacral cornua (small bony projections) to the coccygeal cornua.

Function and Biomechanics

The sacrococcygeal joint allows for a very limited range of motion, primarily consisting of flexion and extension (forward and backward tilting) of the coccyx relative to the sacrum. This movement is typically only a few degrees.

• Shock Absorption: The fibrocartilaginous disc and the slight mobility of the joint contribute to absorbing forces transmitted through the vertebral column, especially during sitting or impact.
• Muscle Attachment: The coccyx serves as an important attachment point for several muscles, including:
  • Parts of the gluteus maximus muscle.
  • Muscles of the pelvic floor, such as the levator ani (specifically the iliococcygeus and pubococcygeus parts) and coccygeus muscles. These muscles play vital roles in supporting pelvic organs, maintaining continence, and contributing to core stability.
• Adaptive Movement: The slight mobility of the coccyx is particularly important during activities like defecation and childbirth, where it can flex posteriorly to increase the outlet of the pelvis.

Common Conditions and Clinical Considerations

While small, the sacrococcygeal joint can be a source of significant discomfort and pain.

• Coccydynia: This is the most common condition affecting the joint, characterized by localized pain in the coccyx region. It often results from:
  • Trauma: Direct falls onto the buttocks, leading to contusions, sprains, or even fractures or dislocations of the coccyx.
  • Childbirth: Pressure on the coccyx during delivery can cause injury.
  • Repetitive Strain: Prolonged sitting on hard surfaces or poor posture.
  • Idiopathic Causes: In some cases, the cause of coccydynia remains unknown.
• Fractures and Dislocations: Though less common than soft tissue injuries, the coccyx can be fractured or dislocated, leading to acute pain and functional limitations.
• Anatomical Variations: The number of coccygeal vertebrae can vary (typically 3-5), and the degree of angulation of the coccyx can differ between individuals, potentially influencing susceptibility to pain. As mentioned, fusion of the joint (synostosis) is common with age and is generally asymptomatic unless it occurs following an injury that prevents proper healing.

Significance for Fitness and Health Professionals

For fitness enthusiasts, personal trainers, and student kinesiologists, understanding the sacrococcygeal joint is critical for a holistic approach to movement and pain management.

• Pelvic Floor and Core Stability: The coccyx's role as an attachment point for pelvic floor muscles highlights its indirect importance in core stability and continence. Dysfunction or pain in this region can impact the ability to effectively engage these muscles.
• Posture and Seating: Awareness of the sacrococcygeal joint helps in advising clients on optimal seated posture to minimize pressure and prevent coccydynia. Ergonomic considerations, such as using a coccyx cushion, can be beneficial for those experiencing pain.
• Rehabilitation: For clients presenting with lower back, hip, or pelvic pain, the sacrococcygeal joint should be considered in a comprehensive assessment. Addressing coccydynia may involve manual therapy, stretching of tight muscles (e.g., piriformis, levator ani), strengthening of supporting musculature, and activity modification.
• Exercise Modification: Understanding the joint's limitations helps in modifying exercises that may put excessive pressure or strain on the coccyx, such as certain types of abdominal exercises or supine positions on hard surfaces.

Conclusion

The joint between the sacrum and the coccyx, known as the sacrococcygeal joint or sacrococcygeal symphysis, is a small but functionally important cartilaginous articulation at the very end of the vertebral column. While its range of motion is limited, its role in shock absorption, providing muscle attachments for the vital pelvic floor, and adapting during specific physiological processes underscores its understated significance. For fitness and health professionals, recognizing its anatomy, biomechanics, and common pathologies is key to providing comprehensive care and education to clients, ensuring optimal function and alleviating pain in this often-overlooked region.