Are synchondrosis joints found in adults? A look at permanent and temporary joints

By age 25, your skeleton is fully formed, a process that involves the fusion of many childhood bones. But are synchondrosis joints found in adults? It’s a common point of confusion, as these critical growth centers often disappear entirely once skeletal maturity is reached.

Quick Summary

The majority of synchondrosis joints are temporary structures that fuse completely into solid bone during development, but a few permanent ones do remain throughout adulthood, primarily in the chest and skull.

Key Points

• Mostly Temporary: The majority of synchondrosis joints are temporary structures, present in the growing skeletons of children and adolescents.

• Ossification Process: As the skeleton matures, temporary synchondroses are replaced by bone in a process called ossification, forming a bony union known as a synostosis.

• Permanent Examples: A few synchondrosis joints, such as the connection between the first rib and the sternum, are permanent and remain throughout adulthood.

• Healthy Maturation: The fusion of temporary synchondroses is a normal, non-pathological part of skeletal development, not a sign of age-related joint degeneration.

• Distinction from Aging Ailments: Unlike osteoarthritis, which involves cartilage breakdown in mobile joints, the fate of a synchondrosis is either permanent stability or complete fusion.

• Crucial for Growth: In childhood, these joints are vital growth centers, particularly in the long bones and the cranial base.

Understanding Synchondrosis Joints

A synchondrosis is a primary cartilaginous joint, meaning it is composed entirely of hyaline cartilage. These joints allow for little to no movement, and their primary function is to facilitate bone growth and provide a rigid connection between bones. The most well-known examples are the growth plates found in the long bones of children. These plates are crucial for the lengthening of the skeleton during a person's developmental years.

The Journey from Synchondrosis to Synostosis

For most of these joints, their time is limited. During the late teenage years and early twenties, hormonal changes signal the end of a bone's growth. The hyaline cartilage within the synchondrosis begins to ossify, a process where it is replaced by bone tissue. Once the cartilage is completely gone and the two adjacent bones have fused, the joint becomes a synostosis, or a bony union.

Where Temporary Synchondroses Fuse

This natural process of fusion is evident in many parts of the body as we mature:

• Epiphyseal Plates: The growth plates at the ends of long bones are the most classic example. Once growth is complete, the epiphysis (end) and diaphysis (shaft) fuse into a single bone, and the plates disappear.
• Hip Bones: The hip bone is actually three separate bones in childhood: the ilium, ischium, and pubis. These are joined by synchondroses that fuse during adolescence, creating the single, strong adult hip bone.
• Cranial Base: Certain synchondroses at the base of the skull, such as the spheno-occipital synchondrosis, are crucial for cranial base growth and typically ossify by the late teens.

Yes, Some Synchondrosis Joints Remain in Adults

Despite the temporary nature of many, the answer to "are synchondrosis joints found in adults?" is yes. A few permanent synchondroses persist throughout life. These serve as strong, immovable connections and do not undergo the complete ossification process seen in temporary growth joints.

Examples of Permanent Synchondroses

• First Sternocostal Joint: This joint connects the first rib to the manubrium (the top part of the sternum or breastbone). Unlike the other sternocostal joints, which are synovial, this one remains a synchondrosis permanently.
• Certain Cranial Base Joints: Some joints at the base of the skull, such as the petro-occipital fissure, may remain as synchondroses in adulthood, although the timing can be variable.

Synchondrosis vs. Symphysis: A Key Distinction

To further clarify the landscape of cartilaginous joints, it is helpful to distinguish a synchondrosis from a symphysis. Both lack a joint cavity, but differ significantly in their composition and function.

Feature	Synchondrosis (Primary)	Symphysis (Secondary)
Cartilage Type	Hyaline Cartilage	Fibrocartilage and Hyaline Cartilage
Movement	Immovable (Synarthrosis)	Slightly Movable (Amphiarthrosis)
Location	First rib to sternum, childhood growth plates	Pubic symphysis, intervertebral discs
Fate with Age	Most ossify and fuse; some remain permanent	Persist throughout life
Primary Function	Bone growth and rigid connection	Shock absorption and some flexibility

Implications for Healthy Aging

Understanding the natural life cycle of synchondrosis joints is particularly important in the context of healthy aging. The disappearance of growth plates is a normal part of skeletal maturation, not a disease or degenerative condition. By contrast, many age-related joint issues, such as osteoarthritis, affect synovial joints, where protective articular cartilage erodes over time. This erosion can cause pain, inflammation, and reduced mobility, a very different process from the programmed fusion of a synchondrosis. Recognizing this distinction helps prevent misinterpreting normal developmental changes as signs of decline.

For older adults, the permanent synchondroses, such as the first sternocostal joint, function as stable anchor points. They do not contribute to the pain and stiffness associated with conditions like osteoarthritis, which are related to wear and tear in more mobile joints. The bone-to-bone fusion that replaces most synchondroses is a testament to the body's remarkable ability to build a durable, mature skeleton. For more detailed anatomical information on the development and classification of joints, authoritative sources like the NCBI Bookshelf are excellent resources.

Frequently Asked Questions

What is the primary difference between a temporary and permanent synchondrosis?

The primary difference is their fate with age. Temporary synchondroses, like growth plates, ossify and fuse completely by early adulthood. Permanent synchondroses, like the first sternocostal joint, retain their hyaline cartilage and do not fuse.

Are synchondrosis joints ever the source of adult joint pain?

No, typically not. Unlike more mobile joints susceptible to osteoarthritis, synchondroses are immovable and the ones that remain are highly stable. Pain is usually associated with other types of joints or conditions.

How does the fusion of synchondrosis joints affect an adult's height?

The fusion of the epiphyseal plates, a type of synchondrosis, marks the end of bone lengthening and, therefore, the end of growth in height. This is a natural part of becoming an adult.

What is a synostosis and how does it relate to synchondrosis?

A synostosis is a fused, bony joint. It is the end product of a temporary synchondrosis after the hyaline cartilage has fully ossified and the adjacent bones have united.

Can doctors see synchondrosis joints on an x-ray?

Yes. In children, the hyaline cartilage of a synchondrosis (like a growth plate) appears as a darker gap on an x-ray because it's less dense than bone. In an adult, a fused synostosis would not show this gap, but permanent synchondroses might still be visible.

Do all bones in a child's body have synchondrosis joints?

No, not all bones. Synchondroses are specifically located at sites of primary cartilaginous connections, such as the growth plates of long bones, specific areas of the pelvis, and the base of the skull, where bone growth and joining are necessary.

Is the first sternocostal joint the only permanent synchondrosis in adults?

It is the most cited and prominent example. Other less-common permanent synchondroses exist, for example, certain joints at the cranial base, though the timing of fusion can vary among individuals for some of these.