Cartilaginous joints: what are they, types and characteristics?

Cartilaginous joints show us a lot about how the human body works.

The locomotor system refers to the set of tissues and organs that allow living beings to move and respond to environmental stimuli. In the case of humans, this intricate work of biomechanics has 206 bones, more than 650 muscles and 360 joints, 86 of which are located in the skull.

When we talk about the formation of movement in vertebrates, we automatically think of muscles and bones, since they are the "bulk" that produce force when responding to a stimulus. However, we must not forget the joints, since they are the ones that join two or more bones to maintain their structure or, failing that, make movement possible.

Based on this premise, we find it interesting to explore the world of joints and all that their anatomy and variability entails. In the following lines, we will tell you everything about the cartilaginous joints and their particularities..

What is a cartilaginous joint?

As it indicates the medical dictionary of the Clinical University of Navarra (CUN), an articulation is a specialized structure in the zone of union between two or more bones or, in its defect, between bony and cartilaginous structures.. Normally, when we think of joints, the knee and the elbow come to mind, but these obvious formations are a minority.

Without going any further, 86 joints are found in the head, of which the atlantooccipital head and the temporomandibular joint are the only mobile ones. In the skull, some of the joints are nothing more than the "glue" between the flat bones that protect our brain, but since they meet the classic definition (union of two or more bones), they fall into this large anatomical category.

Within the grouping of the joints, we find three major categories. These are the following:

• Synovial joints: the bones involved are separated by a very narrow joint cavity, which allows a wide range of movements. The elbow and the knee are clear examples.
• Fibrous joints: connections between two bones joined directly to each other, often by fibrocartilage. They are rigid. An example of this are the points of union between the bones of the head, known as sutures.
• Cartilaginous joints: the ones that concern us here.

Cartilaginous joints are at a midpoint at the physiological level, being more mobile than fibrous joints but less mobile than synovial joints, which represent the maximum range of mobility. In addition, it should be noted that cartilaginous joints also form the growth regions of the long bones and intervertebral discs of the human spine.

Types of cartilaginous joints

Cartilaginous joints comprise the symphysis and synchondrosis. We will tell you about their particularities in the following lines.

1. Synchondrosis

In synchondrosis, the connecting element between the bones involved is the hyaline cartilage, as opposed to the fibrocartilage of the symphyses (although some symphyses also have hyaline cartilage). Moreover, on this occasion the formation is transient.

An example of this is the articulation present between the basilar process of the occipital and the body of the sphenoid, when both structures are still cartilaginous because they have not completed their development. Once the relevant tissue maturation occurs, both articular surfaces fuse and the synchondrosis disappears. They usually appear between growing bone structures allowing a certain degree of movement, but they ossify completely with time.

On the other hand, it should also be noted that there are a couple of permanent synchondroses. One of these is the first sternocostal joint, where the first rib and the manubrium of the sternum meet. This one stands out from the rest, as the other rib-sternum joints are of the flat synovial type. The other permanent synchondrosis is the petro-occipital, between the occipital and petrosal bones of the skull.

The synchondroses between the long bones of growth

In the long bones of humans (such as the femur), two very specific structures can be distinguished: the epiphysis and the diaphysis.. The epiphysis is each of the two ends of the long bone, the area where the joints are located, which is wider than the diaphysis. On the other hand, the diaphysis is the area between the two epiphyses, which is covered with a hard periosteum and in its internal area contains the bone marrow, where the circulating cellular elements (red Blood cells and others) are synthesized.

Synchondroses are usually located in growing long bones between both epiphyses and the central diaphysis. These relatively "soft" joints allow the body of the bone to elongate and separate the bony conglomerate into three distinguishable sections, as if they were three bones (epiphysis-cartilage-diaphysis-cartilage-epiphysis). Finally, these cartilages ossify and form an anatomical whole.

2. Symphysis

In this type of cartilaginous joint, the bones in contact are first joined by a sheet of fibrocartilaginous nature (fibrocartilage), which fuses the components into an anatomical structure. Unlike the synchondroses that we will see later, symphyses are permanent throughout the life of the individual.

A clear example of a symphysis is the pubic symphysis, although one is also present in the mandible, in the sacrococcygeal region, in the sternum and, without going any further, between the vertebrae of the spine. Simply put, in the symphysis, two separate bones are connected by cartilage.

The symphysis pubis

The pubic symphysis (the most conspicuous of all) is defined as a cartilaginous joint located at the confluence of the two pubic bones, formed by a small fibrocartilaginous disc interposed between two narrow laminae of hyaline cartilage.. In addition, this disc is reinforced by a pair of particularly interesting ligaments: the inferior and superior pubic ligaments. These provide enormous stability to the osteoarticular conglomerate of the pubis.

Interestingly, the female pubic symphysis is covered by adipose tissue, which forms the well-known "mons pubis". The characteristic female pubic hair grows on this structure, but it also has glands that secrete hormones involved in sexual attraction. Even the most seemingly anecdotal anatomical variation has a clear evolutionary significance.

Cartilaginous joints and spine

As we have said, the intervertebral discs are fibrocartilaginous symphyses, which are located between each of the 26 vertebral bones that provide axial support to the trunk and give protection to the spinal cord, which allows the transmission of information to our nerve endings.

You've probably wondered why the elderly seem to shrink in size as they get older, haven't you? Interestingly, much of this shrinkage is due to the degradation of the intervertebral joints, as well as osteoporosis and other processes of bone damage and remodeling. The force of gravity acts on the spine and, over the years, the vertebrae compress these discs and squeeze them together.

After the age of 40, people usually lose one centimeter of height for every 10 years, partly due to this compression caused by the wear and tear of the intervertebral discs.We remind you once again that these are cartilaginous joints of the symphysis type. As we age, an average human being can lose 2 to 7.5 centimeters in height during his or her lifetime, a value that may seem tiny but is more than remarkable.

Osteoporosis is also essential to understanding this reduction in height, because in osteoporosis, the bones are reabsorbed and the bone is lost, bones are resorbed and destroyed at a higher rate than that of their synthesis.. As a result, some long and short bones become even thinner and shorter over time, becoming much more fragile and prone to fractures. It is no coincidence that almost no one presents a vertebral fracture before the age of 50.

Summary

As you may have noticed, the cartilaginous joints go far beyond the merely anecdotal, since it is thanks to them, for example, that the spine is supported and can explain a large part of the reduction in height in adult humans. On the other hand, thanks to the cartilaginous articulation of the pubis, the mons pubis can manifest itself in women, which seems to play a not inconsiderable role in sexual attraction.

With these lines, it is more than clear that, in the human body, almost everything has a reason. Except for a few vestigial structures (such as wisdom teeth), every tissue, cell and junction point has a specific function, more or less important for maintaining internal homeostasis or carrying out movements in the environment.

Bibliographic references:

• Why do people shrink? Radyschildren.org. Retrieved March 28 from https://www.rchsd.org/health-articles/por-qu-se-encoje-la-gente/.
• Anatomy notes. Types of joints: synovial and solid, Elsevier. Retrieved March 28 from https://www.elsevier.com/es-es/connect/medicina/anatomia-tipos-articulaciones-sinoviales-y-solidas.
• Articulation, Clínica Universidad Navarra (CUN). Retrieved March 28 from https://www.cun.es/diccionario-medico/terminos/articulacion.
• Cartilagous joints, lumenlearning.com. Retrieved March 28 from https://courses.lumenlearning.com/boundless-ap/chapter/cartilaginous-joints/.
• Cartilagous joints, radiopaedia. Retrieved March 28 from https://radiopaedia.org/articles/cartilaginous-joints.
• Montero, S. A. R. (2007). Pubic symphysitis. Bibliographic review. Seminarios de la Fundación Española de Reumatología, 8(3), 145-153.