Types of Joints - Arthrology
The 206 bones in the human body give structural scaffolding, provide protection for internal organs and facilitate locomotion. However, in order for locomotion to be possible, it is important that these bones are able to articulate with each other. In contrast, protection of the visceral components is achieved by the immobility of adjacent bones. The point at which two bones lay adjacent to each other (with or without the ability to move) is called a joint. The joints of the human body have been classified based on the range of motion they exhibit and by the type of tissue that holds the neighboring bones together.
Classification of Joints
Synovial joints are most commonly found throughout the limbs. In order for the joint to be classified as synovial, both adjacent bones participating in the joint must be lined with hyaline cartilage. Additionally, the joint is encompassed in a capsule that encases the joint cavity. The interior of the capsule is covered with a synovial membrane that is responsible for producing and secreting synovial fluid that lubricates the joint, which aids in reducing the friction between the bones’ ends as they articulate with each other. Further reinforcement of the capsule is provided by ligaments, tendons and skeletal muscle. These joints are capable of a large range of motions and consequently, are the most susceptible to dislocations. The sternoclavicular, shoulder, elbow, hip, knee, radio-carpal and proximal tibiofibular joints are all examples of synovial joints that are capable of an eclectic arrangement of motions.
In contrast to synovial joints, fibrous joints are far simpler and less mobile. The articulating edges of bones are attached by fibrous connective tissue. Motion at these joints is negligible. Fibrous joints are found only in three areas throughout the body. In the skull, by three years of age, all the fontanelle (soft regions between cranial bones) would have fused. The remnants, referred to as cranial sutures, are fibrous connections (sutural ligaments) that occupy the joint space. The adjacent bones will completely ossify with time, which may result in obliteration of the suture lines. A second example of fibrous joints is those of the distal tibiofibular and the cuboideonavicular (cuboid and navicular bones) joints. They are held in place by interosseous ligaments and are called syndesmosis joints (translated from the Greek to mean “held together with a band”). The final fibrous joint is found in the mouth, where the pegged end of the teeth articulates with the dental alveoli. This joint is referred to as a gomphosis joint.
Cartilaginous joints are chiefly characterized by the fact that they connect with neighboring bones via cartilage. They exhibit a range of motion that falls between synovial and fibrous joints. There are two types of cartilaginous joints, synchondrosis and symphysis joints. Synchondrosis joints (translated from Greek meaning “with cartilage”) – also called primary cartilaginous joints – are joints in which hyaline cartilage meet with bone. These highly immobile joints can be observed at the costochondral joints of the anterior thoracic cavity and at the epiphyseal plates of long bones.
Symphysis (secondary cartilaginous) joints are the second group of cartilaginous joints. They are found primarily along the midline of the body. The joint features include adjacent bone surfaces lined with hyaline cartilage and connected by fibrous tissue with some degree of mobility. The intervertebral joints, pubic symphysis and the manubriosternal angle of Louis are all examples of symphysis joints. In some instances there may be joint cavities, but they are never synovial in nature.
Ranges of Motion
There are four general classifications of joint movements. Based on their location, joints can either move back and forth along a single axis (uniaxial), move about two distinct axes (biaxial), move through all three axes (polyaxial) or, in the case of flat bones, slide over each other (gliding movements). Uniaxial and biaxial joints can be further subdivided in relation to the movements of particular joints. For example, the elbows and knees are classified as hinge joints – only flexion and extension can occur about these joints. The radioulnar joints are referred to as pivot joints – the radius rotates about the ulna at the point of contact. While the temporomandibular joints are examples of condylar joints – the condyle of the mandible sits in the mandibular fossa of the temporal bone and motion is restricted to one plane.
Biaxial joints are either ellipsoidal or saddle joints. The ellipsoidal joint at the wrist permits independent ulnar or radial deviation, as well as flexion or extension of the hand. A combination of these motions gives the false impression that the wrist joint is polyaxial. The first carpometacarpal joint has an inverted saddle shape that permits movement in two axes, albeit not independently. These movements are vital for thumb opposition and subsequently, hand functionality.
The most mobile of all joint classifications – the polyaxial joints – are observed at the shoulder, hip and sternoclavicular joints. They are also called “ball and socket” joints due to the ball shape of one articular surface (the head of the humerus) and the socket shape of the other surface (the glenoid cavity). Limbs attached by these joints are capable of adduction, abduction, extension and flexion. The movements of the sternoclavicular joint differ such that it is undergoes protraction, retraction, elevation and depression. Finally, the gliding joints (or plane joints) can be found at the acromioclavicular, intermetacarpal, proximal tibiofibular, and some of the intertarsal joints.
Highly mobile joints are most susceptible to injury. One of the most common forms of joint related pathology is osteoarthritis (OA). Although the nomenclature implicates an inflammatory process, this degenerative joint disease (DJD) is most often of idiopathic aetiology and affects older individuals. It is most often described as the gradual deterioration of the cartilaginous surface of the joint. Repeated trauma to the joint or birth defects can also predispose younger individuals to OA.
The inflammatory variant of the above mentioned synovitis is rheumatoid arthritis (RA). Also of unknown origins, this DJD also results in obliteration of the articular cartilage of the joint with consequent distortion and stiffness of the joint. Even though the joints are predominantly affected by RA, the cardiovascular, respiratory and integumentary systems may also be susceptible to damage from the inflammatory process.
The inability to break down or effectively excrete uric acid can result in accumulation and crystalline deposition of the metabolite in the joints. This gouty arthritis is commonly caused by deficient or defective hypoxanthine guanine phosphoribosyl transferase (HGPRT) – an enzyme that facilitates the conversion of uric acid to useful purine bases. Build-up of gout crystals in the joint cavities will reduce the range of mobility of the joint and can result in damage to the cartilaginous lining.
Sprains and dislocations are also common injuries that occur at joints. Hyperextension or rupture of one of the ligaments, muscles or tendons supporting the joint will result in a sprain. However, the joint may not necessarily be displaced. If enough force is transmitted to the joint during trauma, adjacent bones may luxate (dislocate). The shoulder is the most frequently dislocated of the joints. It can be displaced anteriorly, posteriorly and most often, inferiorly (superior dislocation less likely due to the reinforcement of the deltoid muscle). Also common are hip, elbow, knee, ankle and digital dislocations.