CHAPTER 2 Structure and Function of Joints Roll


The rolling of one articular surface across another is an arthrokinematic motion that results in a larger surface maintaining contact with a smaller surface during limb movement. Generally, a joint surface is moving on a rela- tively stable surface, and often these two surfaces do not correspond in size or shape. For example, the femoral condyles with a larger spherical convex shape move on the shallow tibia as the femur moves from fl exion into extension when a person moves from a sitting to a stand- ing position. For the femur to make complete contact with the articulating surface of the tibia, it must roll and glide in relation to the nonmovable tibia during knee fl exion or extension. This roll is similar to the motion that occurs when a ball is rolling across a table. The points of contact of each surface keep changing during the rolling motion (Fig. 2.12). Each subsequent point of the movement joint surface contacts a new point on the relatively stable joint surface.


Glide (Slide)


When a joint surfaces rolls in one direction, it often must glide (slide) to maintain alignment with its articulating surface. The sliding motion is analogous to a book sliding


Roll Roll


45


across a table; the point of contact of one surface remains the same as it slides across changing points of contact on the opposing surface. The motion of the moving joint surface is linear and remains parallel to the stable joint surface as it glides. The femur rolls anteriorly as the knee joint extends when a person moves from a sitting position into standing, and it glides posterior to maintain contact with the tibia. The same points on the femur slide poste- riorly on changing points on the tibia. This combination of rolling and sliding maintains the femoral condyles in alignment with the tibia and keeps them from rolling off of the tibia. An example of this gliding motion is depicted in Figure 2.13.


Spin


During the accessory motion of spinning, one surface turns on or around another in a rotary motion. The points of contact on each articulating surface remain the same throughout the motion. An example of an arthro- kinematic spinning motion is the “screw-home motion” when the femur spins in medial rotation on the tibia to


Slide


Figure 2.12 Rolling arthrokinematic motion. The points of contact change on each surface during the rolling motion. The femur rolls anteriorly on the tibia as the knee moves into extension during a sit-to-stand movement.


Figure 2.13 Gliding (sliding) arthrokinematic motion. During a gliding arthrokinematic motion, one point on the movable surfaces glides across different points of contact on the stationary surface. As the femur rolls anteriorly, it glides posteriorly to maintain contact with the tibia as the knee joint extends when moving from a sitting to standing position.


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