Anatomy: Joints

Synovial (freely moveable) - diarthrotic joint

A synovial joint, also known as diarthrosis, joins bones with a fibrous joint capsule that is continuous with the periosteum of the joined bones, constitutes the outer boundary of a synovial cavity, and surrounds the bones' articulating surfaces. The synovial cavity/joint is filled with synovial fluid. Synovial joints fall into one of four categories: -Hinge, e.g. interphalangeal joints - fingers and toes. -Ball and socket, e.g. hip and shoulder joints. -Pivot, e.g. atlantoaxial joint between the atlas and axis - neck. -Gliding (= "Condyloid" joint), e.g. between radius, scaphoid and lunate bones - wrist

Ball and socket joint

Ball and socket joint: a natural or manufactured joint or coupling, such as the hip joint, in which a partially spherical end lies in a socket, allowing multidirectional movement and rotation

Cartilaginous (partially moveable) - amphiarthrotic joint:

Cartilaginous joints are connected entirely by cartilage (fibrocartilage or hyaline). Cartilaginous joints allow more movement between bones than a fibrous joint but less than the highly mobile synovial joint.

Pivot joint

Pivot joint: also called rotary joint, or trochoid joint, in vertebrate anatomy, a freely moveable joint(diarthrosis) that allows only rotary movement around a single axis. The moving bone rotates within a ring that is formed from a second bone and adjoining ligament

Six types of synovial joints

There are six types of synovial joints which allow varying types and ranges of movement to occur. The variation in the movements at these joints is because of the differences in their characteristics and limiting factors, as previously discussed. The six synovial joints are: -Glidding, Saddle, Hinge

Functional Classes of Joints

-Immoveable Joints -Slightly moveable joints -Freely moveable joints

The six key characteristics of synovial joints are listed below:

1. Articular cartilage: This is a smooth, white, shiny mass that covers the articular(joining) surfaces of bone. It protects bone tissue and reduces friction (rubbing) between bones when they move. 2. Articular capsule: The articular capsule attaches to the bones near the edge of their articulating surfaces. The capsule is made of strong fibrous tissue and surrounds the joint, adding stability and stopping unwanted material from entering and irritating the joint. The capsule also provides a cavity for synovial fluid to work within. 3. Synovial fluid: This is a yellowish oily fluid that lubricates the articulating surfaces. It forms a fluid cushion between surfaces, provides nutrients for cartilage and absorbs debris that is produced by friction between articulating surfaces. 4. Ligaments: These are strong fibrous bands that join articular surfaces, controlling movement and providing stability. In the adjacent diagram we see the collateral ligaments on each side of the knee which limit side to side movement of the knee and protect it from impact from the side. (Please note that the articular capsule has been removed in this diagram to show the 'inside' of the knee joint). 5. Articular discs (meniscus): Some synovial joints such as the knee have menisci. These are made of tough fibrous tissue and their function is to absorb shock and maintain joint stability. They lie between the articulating surfaces and as such protect the surfaces of articulating bone, as seen in the knee diagram above. 6. Bursae: These are found in some synovial joints. They are closed sacs filled with synovial fluid. Their role is to reduce friction that can occur with movement, such as when tendons rub over bones. An example of bursa is shown in the adjacent diagram of the shoulder (Note the articular capsule has also been removed in this diagram).

Gliding Joints

1. Gliding joints: The joint surfaces are flat and of approximately similar length. Movement occurs in a gliding or sliding of one bone against another. Side to side and back and forth movement is allowed with these joints. Examples of gliding joints are: between the carpals of the wrist and between the tarsals of the ankle.

Saddle Joints

2. Saddle joints: These joint surfaces resemble a saddle and allow side to side and back and forth movements. An example is the thumb joint shown adjacent between a carpal bone known as the trapezium and the first metacarpal.

Hinge Joints

3. Hinge joints: The joint surfaces are arranged to allow only back and forth movement such as bending and straightening. Examples of these joints are the elbow where the humerus and ulna join and the knee.

Pivot Joints

4. Pivot joints: These joints allow only one type of movement, the rotation of one bone on or around another. An example of a pivot joint is the joint between the atlas and axis (C1 & C2) vertebrae, the rotation around each other allows our heads to 'pivot' left and right.

Ball and Socket Joints

5. Ball and socket joints: This type of joint allows side to side, back and forth, and rotational movement. Examples of these joints are the hip or shoulder joints, where the head (ball) of one bone fits into the cavity (socket) of another.

Ellipsoid joints

6. Ellipsoid joints: This joint is also known as a 'condyloid joint'. Ellipsoid joints allow back and forth and side to side movement. Such joints occur between the metacarpals and phalanges (between the bones of the hand and the bones of the finger) as seen in the adjacent image.

Fibrous (immoveable) - synarthrotic joints:

Fibrous Joints: Fibrous joints are connected by dense connective tissue consisting mainly of collagen. These joints are also called fixed or immovable joints because they do not move. Fibrous joints have no joint cavity and are connected via fibrous connective tissue. The skull bones are connected by fibrous joints called sutures.

Force

Force can be described as "an influence applied by one object to another, which results in an acceleration or deceleration.*" Human movement is the interaction of the force that the muscles apply to the bones or the force of gravity. When muscles are instructed by the brain to create a movement, they play their respective parts (antagonist, or stabilizer) and move the bone, which acts as a lever to perform the movement. Most movements require many joints to move at once; this is called force-‐couple relationship.The term torque is "a force that produces rotation.*" Because the joints rotate, torque is used to describe the force applied to our joints by the bones. For instance, in standing poses where the legs are close together, lifting one leg requires less force (tadasana to vrksasana) than a transition between trikonasana and ardha chandrasana. This concept becomes important in a yoga class when we are building a strength base for our clients. For example, if we realize that shortening the distance in standing poses helps weaker student to execute a pose more safely, we can then encourage them, by increments, to widen their stances as they get stronger.

Gliding joint

Gliding joint: also known as a plane joint or planar joint, is a common type of synovial joint formed between bones that meet at flat or nearly flat articular surfaces. Gliding joints allow the bones to glide past one another in any direction along the plane of the joint — up and down, left and right, and diagonally.

Hinge joint

Hinge joint: a common class of synovial joint that includes the ankle, elbow, and knee joints. Hinge joints are formed between two or more bones where the bones can only move along one axis to flex or extend.

Factors limiting synovial joint movement

Some joints allow virtually unrestricted movement while others are very restricted. The range or degree of movements available at each synovial joint is determined by the following three factors: 1. Joint articulation (how the bones of the joint fit together): If we compare the shoulder and hip joint we can see some key differences in articulation. The shoulder has a very shallow socket which allows the humerus a greater range of movement before it comes in contact with the scapula. In comparison the deep socket in the hip joint allows far less range for the femur before its movement is stopped by contact with the pelvic bones. 2. Joint ligaments: Ligaments are tough, dense fibrous bands that join bone to bone and as you may remember they are a characteristic of synovial joints. Ligaments provide extra stability to joints and help to prevent 'undesirable movement'. An example of this is the collateral ligaments of the knee. The medial and lateral collateral ligaments work to help prevent side to side movement, while allowing the knee to flex and extend (bend and straighten) as normal. This can be seen in the below images of the knee joint. 3. Condition of muscles and tendons around a joint: Muscles and tendons also serve to protect joints from excessive movement. Examples can be seen at one of the most mobile joints in the body, the shoulder. Here the extra mobility of the shoulder, due to the shallow socket and lack of bony congruence, requires additional support from the muscles and tendons around it. The shoulder joint gets this additional support from the rotator cuff muscles and their tendons. This group of muscles provides stability and protection to make up for the lack of 'bony congruence', as can be seen in the below image.

Joints

Very simply, joints are the spaces where two bones meet. An articulation is a joint. They can be categorized in many ways. For the purposes of yoga, we can think of joints as ranging from the most limited mobility to the most mobile.

Structural Classes of Joints

fibrous, cartilaginous, synovial

Fibrous joints (also classified as synarthrotic joints):

immobile or extremely limited movement (like the sutures of the skull).

Cartilaginous joints (also classified as amphiarthrotic joints):

limited movement (like the pubic symphysis).

Synovial joints (also classified as diarthrotic joints):

most mobile (like the shoulder). -The joints that are most mobile are called synovial joints (they are diarthrotic joints), and make up about 80% of the joints in the body. Synovial joints have joint capsules, made out of connective tissue, which surround the joint and secrete synovial fluid, a nourishing lubricant for the joint. -Synovial joints are the most commonly occurring type of joint, which also produce the greatest range of movements. The movements created at synovial joints allow us to do everyday activities like walking, running, writing and typing.