Amphiarthroses: Definition, Types, Examples, and Clinical Significance

What is also called Amphiarthroses?

Amphiarthroses are also known as cartilaginous joints or slightly movable joints within the human skeletal system. They represent a category of joints that permit limited movement, providing both stability and flexibility.

Understanding Joint Classification

The human body's skeletal system is a complex framework, and its ability to move, bear weight, and maintain posture relies heavily on the various types of joints connecting its bones. Joints, or articulations, are typically classified based on two primary criteria: their structure (the type of tissue binding the bones together) and their function (the degree of movement they permit).

Functionally, joints are categorized into three main types:

• Synarthroses: Immovable joints (e.g., sutures of the skull).
• Amphiarthroses: Slightly movable joints.
• Diarthroses: Freely movable joints (also known as synovial joints).

Structurally, joints are classified as fibrous, cartilaginous, or synovial. Amphiarthroses primarily fall under the cartilaginous joint classification.

Defining Amphiarthroses

Amphiarthroses are joints characterized by the presence of cartilage, either hyaline cartilage or fibrocartilage, connecting the articulating bones. This cartilaginous connection allows for a limited degree of movement, more than an immovable synarthrosis but significantly less than a freely movable diarthrosis.

Characteristics of Amphiarthroses:

• Limited Movement: They permit slight movement, which is crucial for shock absorption and distributing forces across the skeletal structure.
• Cartilaginous Union: The bones are united by cartilage, which can be either hyaline cartilage or fibrocartilage.
• Absence of Synovial Cavity: Unlike diarthroses, amphiarthroses do not possess a synovial cavity, articular capsule, or synovial fluid.
• Strength and Stability: While allowing some movement, they provide considerable strength and stability to the skeletal region they connect.

Types of Amphiarthroses

Based on the specific type of cartilage and the structure of their connection, amphiarthroses are further divided into two main subtypes:

• Symphyses:

  • In symphyses, the articulating bones are covered with a layer of hyaline cartilage, but they are connected by a broad, flat disc of fibrocartilage.
  • Fibrocartilage is exceptionally strong and compressible, allowing for shock absorption and slight movement under significant load.
  • Examples include the intervertebral discs between vertebrae and the pubic symphysis.

• Syndesmoses:

  • In syndesmoses, the bones are united by a band of fibrous tissue, typically a ligament or an interosseous membrane, rather than cartilage directly.
  • While technically fibrous joints, they are often functionally classified as amphiarthroses because the length of the connecting fibers allows for a small amount of movement.
  • The amount of movement depends on the length of the connecting fibers; longer fibers allow for more movement.
  • Examples include the distal tibiofibular joint (connecting the tibia and fibula at the ankle) and the radioulnar syndesmosis (connecting the radius and ulna along their shafts).

Common Examples of Amphiarthroses in the Body

Understanding where these joints are located helps to appreciate their functional significance:

• Vertebral Column (Intervertebral Joints): The intervertebral discs, composed of fibrocartilage, connect adjacent vertebrae. These symphyses allow for slight bending, twisting, and compression of the spine, collectively contributing to the spine's overall flexibility and shock-absorbing capacity.
• Pubic Symphysis: This joint connects the two halves of the pelvis (pubic bones) anteriorly. It is a symphysis composed of fibrocartilage. It allows for minimal movement, which is particularly important during childbirth to widen the pelvic outlet.
• Distal Tibiofibular Joint: Located just above the ankle, this syndesmosis connects the distal ends of the tibia and fibula via a strong interosseous ligament. It provides stability to the ankle joint while allowing for slight movements that accommodate ankle dorsiflexion and plantarflexion.
• Sacroiliac Joint: While often described as a diarthrosis due to a small synovial component, its primary stability and limited movement are provided by strong ligaments, making it functionally behave more like an amphiarthrosis. It connects the sacrum to the ilium, transferring weight from the upper body to the lower limbs.

Functional Significance and Biomechanical Role

The unique design of amphiarthroses provides a crucial balance between stability and flexibility, enabling essential biomechanical functions:

• Shock Absorption: The cartilaginous or fibrous connections act as natural shock absorbers, protecting bones and internal organs from impact forces during activities like walking, running, and jumping.
• Load Distribution: They help distribute weight and forces evenly across adjacent bones, preventing localized stress concentrations that could lead to injury.
• Limited but Essential Movement: While not as mobile as synovial joints, the slight movements permitted by amphiarthroses are vital for overall body mechanics. For instance, the cumulative slight movements of the intervertebral discs allow for significant spinal flexibility.
• Structural Integrity: They contribute significantly to the structural integrity and stability of regions like the pelvis and lower leg, which are subject to considerable mechanical stress.

Clinical Relevance

Understanding amphiarthroses is important in clinical practice, particularly in fields like orthopedics, physical therapy, and chiropractic:

• Degenerative Conditions: Conditions like degenerative disc disease in the spine affect the intervertebral symphyses, leading to pain, reduced mobility, and neurological symptoms.
• Injuries: Sprains of syndesmoses, such as a high ankle sprain (affecting the distal tibiofibular syndesmosis), can be debilitating and require specific rehabilitation protocols due to their role in ankle stability.
• Pregnancy and Childbirth: Hormonal changes during pregnancy (e.g., relaxin) increase the laxity of the pubic symphysis, allowing for greater pelvic expansion during childbirth. However, excessive laxity can lead to pubic symphysis dysfunction (PSD).

Conclusion

Amphiarthroses, also known as cartilaginous joints or slightly movable joints, are indispensable components of the human skeletal system. Whether in the form of robust symphyses or fibrous syndesmoses, they embody a critical compromise between rigidity and mobility. Their unique structure allows them to absorb shock, distribute loads, and provide the subtle yet essential movements necessary for efficient human locomotion and stability, underpinning our everyday movements and resilience against physical stress.