CHAPTER 2 Structure and Function of Joints


43 D


Figure 2.11 cont’d (D) Nonaxial joints move in linear directions and not around an axis.


in the cervical spine as the head rotates occurs at this pivot joint.


Biaxial Joints


A biaxial synovial joint has two degrees of freedom: Motion occurs within two planes around two axes (see Fig. 2.11B). An ellipsoid joint is a biaxial joint in which one surface is a fl attened, convex, elongated ellipsoid and is paired with a concave trough articulation. The radiocarpal joint at the wrist is an ellipsoid joint, where fl exion, extension, abduction, and adduction can occur. In a condyloid joint, one joint surface is concave, and the other is convex. It is similar to a ball-and-socket joint except that the concave surface is shallow. An example of a condyloid joint is the metacarpophalangeal joint in the fi nger, where movement occurs in the sagittal plane during fl exion and extension and in the frontal plane during abduction and adduction as the phalanges move on the metacarpal bones. A saddle joint is a modifi ed condyloid joint with a sellar surface. Each of the articu- lar bones has both a convex and a concave surface; each surface is often at right angles to the other. This design allows motion in two planes and is similar in structure and function to a person astride a Western-style horse saddle. The carpometacarpal joint of the thumb is an example of a saddle joint.


Triaxial Joints


When a joint is capable of producing movement in three planes around three axes, it is classifi ed as a triaxial joint (see Fig. 2.11C). As its name implies, a triaxial joint has three degrees of freedom. A triaxial joint is also called a ball-and-socket joint because it features a convex ball- like surface and a concave socket. The confi guration of the surfaces allows a spinning accessory movement of one surface on the other, while maintaining the two sur- faces congruent with each other. The hip and shoul- der joints are examples of triaxial ball-and-socket joints moving in fl exion/extension, abduction/adduction, and


medial/lateral rotation or in diagonal patterns represent- ing two or more of these planes of movement. Func- tional movement of a body segment or the entire body during daily self-care, work, and sports activities requires the summation of two or more joints with their varying degrees of freedom of movement to achieve the tasks related to the activity. These multiple degrees of freedom provide the body with a large selection of movement pat- terns to achieve function. During the task of eating, the shoulder (three degrees of freedom) fl exes, the elbow (two degrees of freedom) fl exes, the forearm supinates, and the wrist maintains slight extension to allow the fi ngers to grasp the eating utensil to bring it to the mouth.


Nonaxial Joints


Some synovial joints, called plane or irregular joints, are nonaxial joints (see Fig. 2.11D). The surface of one bone glides in multiple planes along the surface of the oppos- ing joint surface. This gliding motion occurs within a plane but not around an axis. The joints between the carpal bones in the wrist and the facet joints of the spine are examples of nonaxial irregular joints.


JOINT MOVEMENT OSTEOKINEMATICS


Chapter 1 introduced the basic concepts of osteokinemat- ics; the movement of one body segment in relationship to another. These are the movements, under voluntary control, that occur as bony levers move in the cardinal planes allowing humans to perform self-care, work, and leisure activities. These motions are usually rotary move- ments of a bony shaft around a stationary bone. One example of an osteokinematic movement is the forearm bones—the radius and ulna—rotating around the humerus during elbow fl exion as the hand brings food to the mouth during eating. The femur rotating around a stationary tibia as a person moves from a sitting to a standing posi- tion is another example of an osteokinematic motion.


End-feels


Osteokinematic movement in terms of the available joint range of motion is typically quantifi ed by measuring the angle formed by the joint’s two bones with a goniometer or an inclinometer. The examiner often palpates the end- feel, or resistance to further motion, while examining range of motion. When motion is limited by bone in contact with bone, a hard or bony end-feel is present. This type of end-feel is often palpated when the ulna and humerus move into full elbow extension. As the elbow moves into end-range fl exion, the examiner may feel a soft end-feel when the forearm muscle bulk approxi- mates the muscle bulk of the biceps. A fi rm or capsular


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