Cheryl has taught veterinary and medical student for over 20 years and has a DVM and PhD degree in reproductive biology.
Joints, also called articulations, are areas where one bone contacts another bone. However, the range of motion between joints varies depending on the region of the body and the type of articulating bones. They include the sutures of the skull that show little movement, which helps protect the brain and sense organs, such as the eyes. In contrast, joints like the knees, shoulders, and elbows allow for extensive movement in various directions, as can be attested by those who do various sports that involve the arms and legs. These latter joints include joint (synovial) fluid to aid in lubrication and reduce friction as the two articulating bones glide past one another. While joints generally involve bone to bone, one exception is teeth insertion into the maxilla or mandible (jaw bone). This type of fibrous joint is termed gomphosis.
Structural Classification of Joints
Structural classification of joints is dependent upon the type of tissue that bridges the two bones. Based on this classification scheme, there are four types of joints: fibrous, cartilaginous, facet, and synovial joints.
In fibrous joints, the two bones are connected by dense regular connective tissue. The best example of this type is the sutures of the skull and the teeth insertion into the jaw.
In a cartilaginous joint, cartilage spans the gap between the two bones. Within this category, it can be classified as primary cartilaginous with hyaline cartilage bridging the area, or secondary cartilaginous (symphyses) with hyaline cartilage covering the bones, but fibrocartilage connecting them. Examples of primary cartilaginous are the growth plate (physis), spine, or ribs. Examples of secondary cartilaginous include intervertebral disks and pubic symphyses. These types of joints allow for more movement than fibrous, but less than synovial joints.
A facet joint is between two articular processes of two vertebrae in the spinal column.
A synovial (diarthrotic) joint is a type of joint where the two bones are not action adjoined by any specific tissue. Instead, the space between the edges of the bone, which are lined by articular (hyaline) cartilage, is occupied by a synovial cavity that is lined by a synovial membrane. All synovial joints thus have three main structures:
- A synovial cavity contains synovial fluid that is produced by the synovial cells of the inner synovial membrane
- A synovial membrane that is divided into the outer fibrous membrane and the inner synovial membrane that is lined by synovial cells. These synovial cells (fibroblasts) produce synovial fluid, which is similar to plasma, but contains high amounts of hyaluronic acid.
- Articular cartilage that is hyaline cartilage, but lacks a perichondrium. Thus, this cartilage can only regenerate via interstitial growth, which is slower than appositional growth.
The knee joint (as well as some other synovial joints) also has some other features.
- Articular disks or menisci are comprised of fibrocartilage and are situated on the appositional surfaces of the joint.
- Articular fat pads, such as the infrapatellar fat pad in the knee joint, act as further cushioning for the articular cartilage.
- Ligaments in the joint may be intracapsular or extracapsular, such as the anterior and posterior cruciate ligaments that help hold the bones in place during movement. The anterior cruciate ligament prevents the tibia from moving ahead of the femur.
- Bursae are sac or cyst-like structures that contain fluid of similar composition to synovial fluid. These act to reduce frictional forces from being generated in some joints, such as in the shoulder and knee.
- Tendons connect to muscles around the joint.
Functional Classification of Joints
Functional classification of joints is based on the range of movement. Based on this criterion, there are three types of joints: synarthrosis
Synarthrosis is typified by a joint where there is little to no mobility. This type of joint is seen in fibrous joints, including sutures of the skull.
Amphiarthrosis is characterized by a joint showing slight mobility. This category includes both types of cartilaginous joints, such as the intervertebral disk region.
A synovial (diarthrotic) joint has an excellent range of motion. Based on the range of motion and movement, synovial joints can be further classified as:
- A plane joint can be found in the carpal joint of the wrist. It only allows for gliding and sliding movements.
- A hinge joint is located in the elbow joint (humerus and ulna). It allows for flexion and extension, but just in a single plane like a door hinge.
- A pivot joint is located in the atlantoaxial joint and the proximal and distal radio-ulnar joints. This is where one bone rotates against the other.
- A condyloid joint is found in the wrist joint (radio-carpal region), and it allows for flexion, extension, abduction, and adduction.
- A compound joint is found in the knee joint (femur-tibia region and femur-patellar region). It allows for flexion and extension.
- A ball and socket joint is found in the shoulder and hip joints and allows for all movements except for a gliding action.
- A saddle joint is located in the carpometacarpal region (such as the thumb region), and this joint is where the concave end of one bone fits in the convex region of another, like a horse saddle. A saddle joint has an even greater range of motion than a condyloid joint.
Joints are essential connection points between bones. In some instances, little to no range of motion is preferred, such as the sutures of the head, whereas in other joints, the two bones need to glide past each other without generating any frictional forces that could wear away at the bones and overlying articular cartilage. Joints can be classified based on the tissue or material that bridges the two bones and the functional range of movement allowed by the joint. Based on the type of tissue/material occupying the joint region, joints can be classified as fibrous, where connective tissue connects to the two bones (sutures of the skull), cartilaginous, where either hyaline or fibrocartilage connects the two bones (the growth plate and intervertebral disk region), a facet joint (between articular processes of two vertebrae in the spinal column), and a synovial joint, which contains a synovial cavity region with synovial fluid. All synovial joints have a synovial cavity containing fluid (to cushion the joint and allow two bones to glide past one another), a synovial membrane (outer fibrous and inner synovial layer), and articular cartilage lining each end of the bone. Some synovial joints (i.e., the knee joint) also include tendons, ligaments, bursae, articular fat pads, and menisci.
Based on functional classification of joints or range of movement, there are three types of joints that equate to the type of tissue/material spanning the two edges of the bone. Synarthrosis is a joint where there is little to no mobility. This type of joint is seen in fibrous joints (sutures of the skull). Amphiarthrosis is characterized by a joint showing slight mobility (cartilaginous joints, such as the intervertebral disk region). A synovial (diarthrotic) joint has an extensive range of motion. Based on the range of motion and movement, synovial joints can be further classified as plane, hinge, pivotal, condyloid, compound, ball and socket, and saddle joint. Thus, the key for joints is that fibrous has little to no movement, cartilage some range of movement, and synovial has the most extensive range of movement possible.
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