Module 6 : Anatomy of the Joints In this module you will learn: About the classification of joints What synovial joints are and how they work Where the hinge joints are located and their functions Examples of gliding joints and how they work About the saddle joint and its function 6.1 Introduction The body has a need for strength and movement, which is why we are rigid. If our bodies were not made this way, then movement would be impossible. We are designed to grow with bones, tendons, ligaments, and joints that all play a part in natural movements known as articulations – these strong connections join up bones, teeth, and cartilage. Each joint in our body makes these links possible and each joint performs a specific job – many of them differ in shape and structure, but all control a range of motion between the body parts that they connect. 6.2 Classifying Joints Joints that do not allow movement are known as synarthrosis joints. Examples of synarthroses are sutures of the skull, and the gomphoses which connect our teeth to the skull. Amphiarthrosis joints allow a small range of movement, an example of this is your intervertebral discs attached to the spine. Another example is the pubic symphysis in your hip region. The freely moving joints are classified as diarthrosis joints. These have a higher range of motion than any other type of joint, they include knees, elbows, shoulders, and wrists. Joints can also be classified depending on the kind of material each one is structurally made up of. A fibrous joint is made up of tough collagen fiber, examples of this are previously mentioned sutures of the skull or the syndesmosis joint, which holds the ulna and radius of your forearm in place. Cartilaginous types are made up of a band of cartilage that binds the bone together. A few examples of cartilaginous joints include those between the ribs and the intervertebral disks in your spine. The most abundant joint found in the human body is a synovial joint, they feature a fluid filled section that runs between cartilage pads, found at the end of articulating bones. Synovial joints are surrounded with dense connective tissue that forms a protective capsule lined with a synovial membrane. Sometimes part of the outer capsule extends into thick, resistant bands known as ligaments. Ligaments support joints and help prevent injury or dangerous movements. Synovial fluid acts as a lubricant to protect the area from general wear and tear. 6.3 Different Types of Synovial Joints Ball and socket joints have the highest range of motion due to their unique structural components. The only ball and socket joints are the shoulder and hips. These two areas have a high need for a range of motion as they partially regulate all upper and lower body movements. They are also responsible for allowing the limbs to move. The two main components of a ball and socket joint are two bones, one has a spherical head and the other has a cup-shaped socket. When referring to the shoulder joint, the spherical part of the upper arm bone is connected to a small cavity known as the glenoid (part of the scapula). The glenoid cavity allows the shoulder region to have the greatest range of motion in the human body. The glenoid cavity is surrounded by a cartilage ring known as the labrum, which allows flexibility and reinforcement to the shoulder joint. The rotator cuff muscles keep the humerus secured within the cavity. Hip joints These are less mobile than shoulder joints, but are much stronger and harder to damage. Hip joints need to be stable because they must uphold the body weight that rests above the legs. The hip region is the link between upper and lower body functions like walking, running and standing. The spherical head of the thigh bone (femur) fits into a deeper socket in the hip bone called the acetabulum. The hip bones are supported by strong muscles and ligaments to aid in holding the head of the femur in place. The deliberate depth of the acetabulum aids in preventing any damage by limiting the movement of the area within the socket. Ball and socket joints These are commonly referred to as multi-axial due to their ability to move bones along several axes. The muscles surrounding these joints allow the upper arms and hip joints to abduct away from your body, move along the body, allow flexion and extension movements. Both the hip and shoulder joint can move in a full circular motion, they can both also rotate in lateral and medial movements. 6.4 Hinge Joints These fall under the class of synovial joints, but have other distinctive features. These include the ankle, knee joint and elbows. Hinge joints are formed in between two bones or more, allowing the bones to move alongside one axis, enabling flexion or extension. The simplest form of hinge joints is the interphalangeal joints that are located between your fingers and toes. These hinge joints provide flexion and the ability to move digits at an angle, i.e. clenching your fist or curling your toes. They also allow the digits to increase angular movement by 180 degrees when feet or hands are flat. Little body weight or mechanical force is needed to exert this type of joint and they are made up of standard synovial material, including small ligaments that aid in reinforcing the area. The elbow This is another type of hinge joint, but one that is much more complex. This forms where the humerus connects to the radius and ulna of the forearm. Elbows have to endure stronger force than your knuckles so they are supported by accessory ligaments and complex bone structures. The radius and ulna ligaments provide support for the humerus in every movement in the area. At the end of the ulna is the olecranon, this forms the tip of your elbow and attaches to a part of the humerus called the olecranon fossa. This acts as a regulator for elbow extension and limits the movement to 180 degrees before it locks in to the olecranon fossa. The ankle joint This is another hinge joint that forms between your fibula and tibia in the lower leg region, along with the talus bone in your foot. Parts of the fibula and tibia bones form a socket around the talus which limits movement of your feet to one single axis. There are 4 accessory ligaments that include deltoid ligament (not to be confused with shoulder) these hold the bones together and reinforce joints, so they can withstand any stress from body weight when standing, jumping, walking or running. The knee joint This is found between the femur (upper leg bone) and tibia/ fibula (lower leg bones). This is the largest, most complex hinge joint in your body. Over time they have evolved to become strong and sturdy while still providing a large range of motion that is necessary for humans to move. The area is supported by internal and external ligaments to make up for the lack of support from surrounding bones in the area. The lack of bone support allows for the knee joint to flex to a larger degree than if it was supported by surrounding bones. A cartilage, known as the meniscus, located between tibia and femur bones, protects the area more acting as a shock absorber during high-impact exercises and movements. 6.5 Gliding Joint A gliding joint is also referred to as a plane joint or planar joint, this is another common type of synovial joint that forms between bones meeting on flat articulate surfaces. Most gliding joints allow bones to glide next to each other in directions alongside the plane of the joint. This can be up/down, left/right or diagonal. They also allow for slight rotation, but this is limited by the shape of surrounding bones, elasticity and how the joint capsule is structured. The typical structure of synovial joints helps provide flexibility in gliding joints and also limits movement in order to minimize risk of injury. The synovial membrane protects the area with oil-like fluid, lubricating the area and reduces friction on joints and bones. The joint capsule and attached ligaments hold the bones together so that they can glide without dislocating. Between the bones lies articular cartilage – a smooth surface that also aids in safe gliding motion and a shock absorber to protect bones from impact. Most gliding joints are formed within the appendicular skeleton located between your carpal wrist bones, carpal, and metacarpal bones of your palms and tarsal bones of your ankle. Also between the metatarsal and tarsals of your feet. Such bones form flattened surfaces between each other to provide flexibility in your hands and feet. Another example of a complex gliding joint is the acromioclavicular joint in your shoulder region. This gliding joint provides a pivot point for increased flexibility and allows the shoulder to be elevated or be depressed. Gliding joints These are also located in the axial skeleton (between your neck and trunk to improve regional flexibility). There are two sets of gliding joints in your thoracic region – one set is between the sternum and ribs by your intercostal joints, the other set is located between your vertebrae, ribs and vertebrocostal joints. They allow the ribs to elevate and depress, changing the volume of the thoracic cavity. These movements allow the normal and necessary process of breathing by working in alignment with your lungs. Your spine has 26 vertebrae and the gliding joints found here are located between these by the intervertebral joints.