Anatomy & Physiology: Joints Defined

Joints Defined

The intricate movements of a human, such as those performed in dance and athletics, are accomplished by using a wide variety of joints. Though joints allow the skeleton to be dynamic, they also play an important role in stability and protection. In fact, the mobility of a joint is often inversely proportional to its stability. For example, the sutures of the bones of the cranium are basically immovable in relationship to one another, but due to their stable nature, they serve to protect the brain throughout daily life and during incidents of trauma. On the other hand, the ball-and-socket of the shoulder enables a wide variety of complex movements. This increase in the amount of mobility leads to instability, which is why the shoulder is more susceptible to injury.

A joint (articulation) occurs wherever bones meet. Joints are classified both structurally and functionally, as shown in Table 1 .

TABLE 1

Joint Classification

Functional Class	Structural Class	Joint	Description Type	Example of Joint
Synarthrosis (immovable)	fibrous	suture	interlocking seams	between cranial bones
Synarthrosis (immovable)	fibrous	gomphosis	peg- and socket joint	between teeth & sockets
Synarthrosis (immovable)	cartilaginous	synchrondrosis	hyaline cartilage joint	between diaphysis & epiphysis in long bones
Amphiarthrosis (slightly movable)	fibrous	syndesmosis	ligament ordistal interosseous membrane	joint of tibia & fibula
Amphiarthrosis (slightly movable)	cartilaginous	symphysis	fibrocartilage acts as compressible cushion	intervertebral discs of vertebral column
diarthrosis (freely movable)	synovial	gliding	surfaces	between carpals
diarthrosis (freely movable)	synovial	hinge	concave surface with convex surface	between humerus & ulna
diarthrosis (freely movable)	synovial	pivot	rounded end fits into ring of bone and ligament	between altas (C₁) & axis (C₂) vertebrae
diarthrosis (freely movable)	synovial	condyloid	oval condyle with oval cavity	between metacarpals & phalanges
diarthrosis (freely movable)	synovial	saddle	each surface is both concave and convex	between carpus & the first metacarpal
diarthrosis (freely movable)	synovial	ball- and- socket	ball-shaped head with cup-shaped socket	between femur & pelvis