Symphysis Joints: Definition, Characteristics, and Anatomical Examples

What type of joint is the symphysis?

A symphysis is a type of cartilaginous joint, specifically classified as a secondary cartilaginous joint (synchondrosis secundaria), characterized by a disc of fibrocartilage that unites two bones, allowing for limited movement.

Understanding Joint Classification

To properly understand what a symphysis is, it's essential to first grasp the fundamental ways joints are classified in anatomy and kinesiology. Joints, or articulations, are sites where two or more bones meet. They are typically classified based on two primary criteria:

• Structural Classification: This method categorizes joints based on the type of connective tissue that binds the bones together and whether a joint cavity is present. The main structural types are fibrous, cartilaginous, and synovial joints.
• Functional Classification: This method categorizes joints based on the degree of movement they permit. The functional types are synarthroses (immovable), amphiarthroses (slightly movable), and diarthroses (freely movable).

The symphysis falls under both classifications: structurally as a cartilaginous joint and functionally as an amphiarthrotic (slightly movable) joint.

The Symphysis: A Cartilaginous Joint

Cartilaginous joints are characterized by bones united by cartilage, without a joint cavity. They are further subdivided into two types:

• Primary Cartilaginous Joints (Synchondroses): These joints are formed by hyaline cartilage and are typically temporary, ossifying with age (e.g., epiphyseal plates in long bones, sternocostal joint of the first rib). They are generally synarthrotic (immovable).
• Secondary Cartilaginous Joints (Symphyses): These joints are formed by fibrocartilage, often with a thin layer of hyaline cartilage covering the bony surfaces. Unlike primary cartilaginous joints, symphyses are persistent throughout life and are designed to provide strong, shock-absorbing connections that allow for limited movement. This makes them functionally amphiarthrotic.

The symphysis, therefore, is distinct due to its composition of fibrocartilage, which provides both strength and flexibility, enabling a degree of movement that is greater than fibrous joints but less than synovial joints.

Key Characteristics of a Symphysis

Several defining features set symphysis joints apart:

• Fibrocartilage Disc: The hallmark of a symphysis is the presence of a strong, resilient disc of fibrocartilage that directly connects the two bones. This material is ideal for resisting compression, absorbing shock, and providing tensile strength.
• Amphiarthrotic Movement: Symphysis joints permit only slight movements. This limited mobility is crucial for maintaining stability in areas that also require some degree of flexibility, such as the spine or pelvis.
• Absence of Synovial Cavity: Unlike synovial joints, symphyses lack a fluid-filled joint capsule, synovial membrane, and articular cartilage. The bones are directly connected by the fibrocartilage.
• Hyaline Cartilage Facets (Often Present): While the primary uniting tissue is fibrocartilage, the bony surfaces articulating with the fibrocartilage disc are often covered by a thin layer of hyaline cartilage.

Examples of Symphysis Joints in the Human Body

Several critical anatomical structures in the human body are symphysis joints, each serving vital functions:

• Pubic Symphysis: Located anteriorly between the two pubic bones of the pelvis. This joint is reinforced by strong ligaments and allows for slight movement, which is particularly important during childbirth to widen the pelvic outlet. It also contributes to pelvic stability during locomotion.
• Intervertebral Discs: These structures are located between the bodies of adjacent vertebrae from the second cervical vertebra (C2) down to the sacrum. Each intervertebral disc consists of an outer annulus fibrosus (fibrocartilage) and an inner nucleus pulposus (gelatinous core). Collectively, these symphyses provide tremendous flexibility to the vertebral column, absorb shock during movement, and distribute compressive forces.
• Manubriosternal Joint: This joint connects the manubrium (the superior part of the sternum) to the body of the sternum. While often considered a synchondrosis in early life, it develops into a symphysis as the hyaline cartilage is replaced by fibrocartilage in adults, allowing for slight movements during respiration.
• Sacrococcygeal Joint: The articulation between the sacrum and the coccyx. This joint allows for limited movement, which can be significant during defecation and childbirth.

Functional Significance and Clinical Relevance

The design of symphysis joints highlights an elegant balance between stability and mobility. Their primary functions include:

• Shock Absorption: The fibrocartilage acts as an effective shock absorber, protecting adjacent bones from compressive forces. This is particularly evident in the intervertebral discs, which cushion the spine during activities like walking, running, and jumping.
• Strength and Stability: While allowing for limited movement, symphyses provide robust connections, essential for maintaining the integrity of structures like the pelvic girdle and vertebral column.
• Limited Flexibility: The slight movement permitted by these joints is crucial for accommodating physiological changes and movements. For example, the pubic symphysis adapts during pregnancy and childbirth, and the intervertebral discs allow for bending and twisting of the torso.

Clinically, issues with symphysis joints can lead to significant discomfort and dysfunction. Conditions such as pubic symphysis dysfunction (PSD) during pregnancy or intervertebral disc herniation underscore the vital role these joints play in daily function and overall musculoskeletal health. Understanding their unique structure and limited mobility is key to diagnosing and managing such conditions.

Conclusion

In summary, a symphysis is a secondary cartilaginous joint characterized by a uniting disc of fibrocartilage, which permits slight movement (amphiarthrotic). These joints are indispensable for providing both stability and limited flexibility in critical areas of the skeleton, such as the vertebral column and pelvic girdle. Their robust yet adaptable nature underscores their importance in human biomechanics and overall physiological function.