Lesson: Joints

OverviewArticulation or Joint is the point of contact between bones. There are two classification methods used for joints. One is based the amount of movement possible (functional) and the other method of classifying joints is based on anatomy (structural). One thing to keep in mind – increasing the strength of a joint, decreases the flexibility, increasing the flexibility of a joint, decreases the strength.

Functional Classification of JointsFunctional Classification of Joints is based the amount of movement possible. Range of motion describes the amount of movement and the direction of movement. There are three categories:

Synarthroses joint is a function classification in which the joint is immoveable such as the sutures of the skull

Amphiarthroses joint is a functional classification in which the joint is slightly moveable such as the Symphysis of the pubis

Diarthroses joint is a functional classification in which the joint is freely moveable such as the knee.

Structural Classification of JointsStructural Classification of joints is based on the type of connective tissue that joins the bones together or by the presence of a fluid-filled joint capsule. There are three types: Fibrous, Cartilaginous and Synovial.

Fibrous Joint

These joints offer little to no movement between the bones. Most fibrous joints are also classified as a Synarthroses joint. There are three types of Fibrous joints:

Syndesmosis is a form of fibrous joint in which surfaces that are united by ligaments. This joint does allow for small movements due to the flexibility of the ligaments

Suture is only found in the skull. These joints form due a tooth like pattern with a thin layer of fibrous tissue. As adults, the sutures become ossified.

Gomphoses are only found in teeth. This unique joint is between a tooth and the alveolar process (sockets) of the mandible (inferior portion of the jaw) or the maxilla (superior portion of the jaw). The joint is slightly moveable (microscopically moveable) so nerves can “sense” the movement to let us know how hard we are biting or if something is stuck in our teeth.

Cartilaginous Joint

Cartilaginous joints allow for slight movement between the articulating bones. These joints form by either hyaline cartilage or fibrocartilage.

Synchondrosis is a joint formed using hyaline cartilage between the articulating bones. These joints are found between the first ribs and the sternum. They are also found between the epiphyses and diaphysis of a long bone.

Symphysis is a joint formed using fibrocartilage between the two articulating bones. This joint allows for slight movement when pressure is applied between the bones. The Symphysis pubis and the intervertebral disks are examples.

Synovial Joints

Synovial joints are freely moveable making them the most mobile and most numerous joint. These joints are enclosed in a fluid filled capsule. Synovial joints are Diarthroses as they allow free movement of the bones.

Structure of Synovial Joints

Joint capsule is a sleeve-like extension of the periosteum of each of the articulating bones. The capsule completely encases the ends of the bones.

Synovial membrane is a moist, slippery membrane that lines the inner surface of the joint capsule. This membrane secretes synovial fluid that lubricates and nourishes the inner joint surfaces.

Articular cartilage is a thin layer of hyaline cartilage that covers and cushions the articular surfaces of the bones

Joint cavity is a small space between the articulating surfaces of the bones of the joint. The cavity allows for extensive movement between the bones

Menisci (articular disks) are pads of fibrocartilage between the articulating ends of the bones for some joints. Many times the pad divides the joint cavity into two compartments such as in the knee.

Ligaments are strong cords of dense, white fibrous tissue found in most synovial joints. They grow between the bones causing the bones to be more tightly joined.

Bursae are closed pillow-like structures called bursa. Bursa are made of synovial membrane and filled with synovial fluid. Bursa functions in cushioning the joint and facilitating the movement of the tendons. This type of joint is typically found in associated with bony prominences in the elbow and knee. Bursitis is a common inflammation of the bursa due to excessive or repetitive exercise or trauma to the joints.

Types of Synovial Joints

There are three major types of synovial joints, each with two subtypes.

Uniaxial joints (Monaxial) are synovial joints that permit movement around only one axis and in one direction. These are hinge and pivot joints.

Hinge joints form a hinge-like shape that only allows for movement in one direction (back and forth) called flexion and extension. Example would be the elbow

Pivot Joints form when a projection of one bone articulates with a ring or notch of another bone such as the dens of the second cervical vertebrae with the first cervical vertebrae.

Biaxial joints permit movements around two perpendicular axes in two perpendicular planes. These are saddle and condyloid joints.

Saddle joints have the articulating ends of the bones form a miniature saddle. These are only found on the thumbs – the metacarpal bone articulates in the wrist with a carpel bone (trapezium). This joint arrangement allows the thumb to move in such a way as to touch the tips of the fingers. This joint is what gives primates their opposable thumbs.

Condyloid (ellipsoidal) joints have a condyle that fits into an elliptical socket. Example is the condyles of the occipital bone that fit into the elliptical depressions of the atlas.

Multiaxial joints permit movements around three (triaxial) or more axes and planes. These are Ball and Socket joints and Gliding joints.

Ball and socket joints (spheroid joints) are the most moveable joints with a ball shaped head of one bone fitting into the concave depression of another bone. Example would be the hip joint.

Gliding joint are flat articulating surfaces that allow limited gliding movements along various axis. Examples are the joints between vertebrae.