Explain Why Synovial Joints Have Greater Flexibility Than Fixed and Cartilaginous Joint

Explain Why Synovial Joints Have Greater Flexibility Than Fixed and Cartilaginous Joint?

The human body possesses various joints that serve different functions. There are primarily three types according to their structure and the type of movement permitted, namely: synovial joints, cartilaginous joints, and fibrous or fixed joints. Each joint shows variations in flexibility, with synovial joints being the most flexible. Exploring these architectural features enabling movement should give an insight into why synovial joints are more flexible than fixed and cartilaginous joints.

1. Types of Joints in the Human Body

Before examining why synovial joints possess their unique flexibility, it is worthwhile examining the basic types and structures of joints:

1. Synovial Joints

Synovial joints are looser joints that include the synovial fluid-filled cavity and cartilaginous structures. These joints are capable of highly flexible and wide-ranging movements. They are the most abundant joint type in the human body, being found in areas such as the knee, elbow, shoulder and hips. Synovial joints include several subs-types according to the nature of movements allowed:

-Hinge joints (e.g., elbow and knee) permit flexion and extension.

-Ball-and-socket joints (e.g., shoulder and hip) facilitate rotational movement and are able to accommodate movement through almost all angles.

-Pivot joints (e.g., joint between atlas and axis of the neck) permit rotation about a single axis.

-Saddle joints (e.g., joint of the thumb) permit back-and-forth and side-to-side movements.

-Plane joints (e.g., joints between bones of the wrist) allow sliding or gliding motion.

-Condyloid joints (e.g., wrist joint) facilitate movements in two planes: flexion, extension and side-to-side movements.

The flexibility exhibited by synovial joints is made possible due to the existence of certain structures that allow free mobility in the joints: the presence of synovial fluid, articular cartilage, ligaments, and joints of sift. 

2. Cartilaginous Joints

Cartilaginous joints are defined as joints that possess cartilage between bones and allow limited movement. These joints can be subdivided into two types:

Symphyses: These joints are united by fibrocartilage, such as the pubic symphysis (joining the two halves of the pelvis) and the intervertebral discs (between the vertebrae).

Synchondroses: These joints are united by hyaline cartilage, such as epiphyseal plates (growth plates) in growing bones.

Cartilaginous joints allow for more movement compared to fixed joints, but are still very limited compared to synovial joints.

3. Fibrous (Fixed) Joints

Fibrous joints are classified as being immovable joints or joints that allow slight movement. The bones of fibrous joints are held together by dense connective tissue, with no synovial cavity present. The main types of fibrous joints are as follows:

Sutures: These are present in the joints between flat bones in the skull and are the joints which do not allow any movement at all.

Syndesmoses: In these joints, less degree of movement is envisaged due to the amount of connective tissue in the ligament joining the two bones, with an example being the one connecting the tibia and fibula of the leg.

Gomphoses: Found between the teeth and their respective bony sockets in the jaw, where ligament holds the teeth in place.

Next to flexibility, fibrous joints are highly resistant for stability and protection.

image credit: FREEPIK

2. Structure and Function of Synovial Joints

Synovial joints are characterized by their unique structure, which permits a much larger freedom of movement in comparison to cartilaginous and fibrous joints. Some structural features of synovial joints that provide flexibility are listed as follows.

1. Synovial Fluid

Synovial fluid is a slick, viscous fluid that fills the synovial cavity of synovial joints. It has the following functions:

Lubrication: Synovial fluid decreases friction between the articular surfaces (the surfaces of the bones covered by cartilage) to allow for smooth and unhindered movement.

Nutrient Delivery: Synovial fluid helps nourish the articular cartilage by transporting nutrients to the cartilage cells (chondrocytes) since cartilage itself lacks a blood supply. 

Shock Absorption: The fluid also cushions the joint by absorbing forces that are generated during locomotion, thus preserving the joint from injuries. 

Synovial fluid gives high efficiency in joint flexibility and diminishes the wear and tear of joints and cartilage.

2. Articular Cartilage

A layer of articular cartilage that covers the ends of bones in synovial joints is made of hyaline cartilage. This cartilage is smooth and flexible, supporting low friction during movement. The highly elastic properties provide cushioning between those bones to prevent direct bone-on-bone contact. The cushioning effect allows the bones to glide past each other and adds to the joint's flexibility.

In cartilaginous joints, the cartilage is thicker and less elastic, thereby hindering movement, while fibrous joints contain no cartilage at all, contributing to further limitation in flexibility.

3. Joint Capsule and Ligaments

The joint capsule envelops the synovial cavity. Fibrous in nature, it is a strong structure. The inner wall of the joint capsule is lined by a synovial membrane, which generates synovial fluid. The joint capsule serves to stabilize the joint by restricting excessive movement that might result in injury.

Ligaments are fibrous tissue bands that connect one bone to another across a joint as strong yet flexible tissue. Ligaments serve the function of supporting the synovial joint by preventing excessive motions while allowing some controlled movements. They provide further stability to the joint by preventing dislocations or strains.

The balance of the joint capsule, ligaments and synovial fluid generates high flexibility about synovial joints. The structures allow for movements while working to prevent instability or injury.

4. Muscle Tendons

The tendons are thick fibrous connective tissues that link muscles to bones. Across a synovial joint, tendons will cross the joint to tend to facilitate movement, working by pulling on the bones involved when muscles contract. Tendons also act to stabilize the joint and additionally support the joint to keep movements coordinated and efficient.

On the contrary, movement in cartilaginous and fibrous joints does not rely as much on tendons. In other words, fibrous joints do not have any tendons, while cartilaginous joints are much stiffer and have very less tendon support across the joint. 

3. Factors That Influence Synovial Joints to Be More Flexible

Greater flexibility in synovial joints contributes to a greater movement potential relative to fixed or cartilaginous joints. These include the synovial fluid, the articular cartilage, the ligaments and the type of movement that the joint accommodates.

1. The Presence Of Synovial Fluid

The lubricating property of synovial fluid provides a variety of synovial joints with the most flexibility. In the case of fixed and cartilaginous joints, there is no or very limited lubrication, while in synovial joints the continuous presence of this fluid provides friction reduction, allowing smooth and unobstructed motion.

2. Flexible Articular Cartilage

The smooth and elastic articular cartilage of synovial joints enhances the ease of movement between the two bones. Articular cartilage withstands mechanical stresses during movement and provides a flexible surface over which the bones can glide. In contrast, cartilaginous joints have relatively less flexible cartilage; fixed joints do not possess cartilage, thereby limiting their movements even more.

3. Wide Range of Movements

Synovial joints can shear, flex, extend, rotate, abduct, adduction, circumduction, and glide. These movements depend on the type of synovial joint, which gives synovial joints flexibility because of the nature of these joints compared to other types, such as cartilaginous joints or fixed joints. Cartilaginous joints only offer limited small controlled types of movements, whereas fixed joints are mostly immovable or allow very little movement.

4. Stabilization and Flexibility

The joint is highly stabilized by ligaments and tendons but is also simply meant to provide flexibility. The flexibility still allows for great ranges of motion without hyper-extension or dislocation due to stability, giving an entirely different dimension compared to fixed and cartilaginous joints, which can go either way.

image credit: FREEPIK

4. Comparison of Synovial Joints with Cartilaginous and Fixed Joints

1. Synovial vs. Cartilaginous Joints

Cartilaginous joints allow for some limited movement between articulating bones and are more flexible than fixed joints. Although cartilage does connect the bones in these joints, the flexibility is somewhat restricted by the limits on movement imposed by certain other structures. While cartilage in cartilaginous joints is more rigid than articular cartilage in synovial joints, thus curtailing flexibility, 

For example, in intervertebral joints, a form of cartilaginous joint, vertebrae unite through fibrocartilage discs separated by these discs to allow very slight flexion and extension, not at all the complete room allowed by the synovial joints.

2. Synovial vs. Fixed Joints

Fixed joints, also called fibrous joints, do not allow for any movement. These joints support stability and protection, rather than flexibility. Because of this, the sutures between the bones of the skull are fixed joints, which will tightly hold the bones together and will allow no movements. Synovial joints, however, have been set to be allowing movements and they are by far more flexible than fixed joints.

Conclusion: Explain Why Synovial Joints Have Greater Flexibility Than Fixed and Cartilaginous Joint

To sum up, synovial joints are the most flexible type of joint in the human body due to their unique architecture, namely, the synovial cavity, synovial fluid and articular cartilage, plus associated support structures such as ligaments and tendons. These structures work in facilitating the free and smooth movement of synovial joints, a remarkable range of motion. For this reason, cartilaginous joints and fibrous (fixed) joints have become more rigid and limited when it comes to movement, with cartilaginous joints allowing limited movement and fibrous joints being for the most part immovable. 

With the lubrication of synovial fluid, the flexibility of articular cartilage, stabilization by ligaments, with support by muscles, synovial joints are far more flexible than fixed and cartilaginous joints, enabling them to support dynamic movement in daily living.