152 Introduction to Anatomy & Physiology Pivot joints permit rotation around only one axis. (Think about moving around your stationary pivot foot in basketball.) At condyloid (KAHN-di-loyd) joints, one articulating bone surface is an oval, convex shape, and the other is a reciprocally shaped concave surface. Flexion, extension, abduction, adduction, and circumduction are permitted. Saddle joints are so named because their articulating bone surfaces are both shaped like the seat of a riding saddle. Movement capability is the same as that of the condyloid joint but greater range of movement is allowed. Ball-and-socket joints are the most freely movable joints in the body. In these joints the surfaces of the articulating bones are reciprocally convex and concave, with one bone end shaped like a “ball” and the other like a “socket.” Rotation is permitted in all three planes of movement. In cases in which the joint socket is relatively shallow, such as in the shoulder, a large range of motion is permitted, but at the sacrifice of joint stability. Alternatively, the deep socket of the hip joint maximizes stability but allows much less of a range of motion than at the shoulder. Two structures often associated with diarthrodial joints are bursae and tendon sheaths. Bursae (BER-see) are small capsules lined with synovial membranes and filled with synovial fluid that cushion the structures they separate. Most bursae separate tendons from bone, reducing the friction on the tendons during joint motion. Tendon sheaths are double-layered synovial structures surrounding tendons that are subject to friction because they are close to bones. These sheaths secrete synovial fluid to promote free motion of the tendon during joint movement. Many long muscle tendons crossing the wrist and finger joints are protected by tendon sheaths. Check Your Understanding 1. Which types of joints are most prevalent in the axial skeleton? Why? 2. What are the two main types of synarthroses? 3. What are the two main types of amphiarthroses? layer of hyaline cartilage. Examples include the sternocostal joints (between the sternum and the ribs) and the epiphyseal plates in the long bones. Symphyses (SIM-fi-seez) are joints in which thin plates of hyaline cartilage separate a disc of fibrocartilage from the bones. Examples include the vertebral joints and the pubic symphysis. Freely Movable Joints Freely movable joints are called diarthroses (digh-ar-THROH-seez). They are also referred to as synovial (si-NOH-vee-al) joints because each joint is surrounded by an articular capsule with a synovial membrane lining that secretes a lubricant known as synovial fluid (Figure 4.30). Each of the six different types of diarthroses is structured to permit different types of motion (Figure 4.31). At gliding joints the articulating bone surfaces are nearly flat. The only movement permitted is gliding. In hinge joints, one articulating bone surface is convex (curved outward), and the other is concave (curved inward). Strong ligaments restrict movement to a planar, hingelike motion, similar to the hinge on a door. © Body Scientific International Figure 4.30 Anterior view of the knee, which is a synovial joint. Synovial membrane Articular cartilage Medial meniscus Articular capsule Anterior view Articular capsule Patella Fat pad Junction of membrane with cartilage Lateral meniscus Copyright Goodheart-Willcox Co., Inc.