Joints and Their Classification

Joints with a fluid-filled cavity between the articulating bones are called [1] joints. When such a joint is highly movable, such as the elbow, it is called a [2]; when it is only slightly movable, such as the joints between the carpal bones, it is an [3]. A joint that cannot move at all, such as a skull suture, is classified as a [4]. If two bones are bound together by collagen fibers, they form a [5] joint–for example, between the radius and ulna. The bodies of adjacent vertebrae are bound together by [6] joints, named for the kind of connective tissue found in the intervertebral discs.

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Fibrous, Cartilagenous, and Bony Joints

Fibrous joints are subdivided into [7], the joints between most skull bones; [8], the joints between a tooth and the mandibular or maxillary bone; and [9], where two bones are bound by a ligament. There are three types of 7; the type with an interlocking wavy line between bones is called a [10]. The fibers joining a tooth to its bony socket constitute the [11] membrane. Some long bones are bound to each other by an [12] membrane that extends along the shafts of the two bones, such as the radius and ulna and the tibia and fibula. In a synchondrosis, two bones are joined by [13]–for example, where the distal end of a rib is joined to the sternum. In a [14], the two bones are joined by fibrocartilage. The two halves of a baby's mandible are initially joined in this way, but in early childhood, they fuse seamlessly together and then form a joint called a [15].

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Form and Function of Synovial Joints

Synovial joints move relatively freely because of a lubricant called [16] in the joint cavity and a thin layer of [17] over the end of each opposing bone. The fluid is secreted and retained within the cavity by a two-layered [18]. The typical accessory structures of synovial joints include a muscle-to-bone connection called a [19], a fibrous bone-to-bone connection called a [20], and often a fluid-filled sac called a [21]. The six subclasses of a synovial joint include the [22] type represented by the shoulder and hip joints; the [23] type represented by the knuckles and knee; and the [24] type unique to the base of the thumb.

There is an extensive terminology for the types of motion seen at synovial joints. If you alternately make a fist and uncurl your fingers, your knuckles exhibit the motions called [25] and [26]. If you face forward and raise your arm to point to your right, your shoulder joint exhibits the motion called [27]. If you grind your teeth from left to right, as in chewing, your mandible exhibits lateral and medial [28]. If you rotate your forearm so your palm faces up, you [29] it; if you turn the palm down, you [30] the forearm. When you press on a gas pedal, your foot exhibits the motion called [31]. If you keep your heel on the floor and raise your toes, your foot exhibits [32].

The mobility of a joint is called its [33]. Our long bones act as levers and follow the same principles of physics as any other lever. The stationary point around which a bone or other lever swings is called the [34]. The portion between that point and the point where your muscle applies force to a bone is called the [35]. The portion between 34 and the point where the bone acts against a load or resistance is called the [36]. The ratio of 35 to 36 is called the [37] of the lever. If this value is less than 1.0, the lever will produce more [38] but less [39] than the input. This is always true of a [40]-class lever–one in which the effort is applied between the fulcrum and resistance. In a [41]-class lever, the fulcrum is between the effort and resistance, and value 36 can be either greater or less than 1.0.

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Anatomy of Selected Diarthroses

Although the diarthroses have a complex arrangement of ligaments and bursae, the names of these structures often indicate approximately where they are found. Thus, the sphenomandibular and temporomandibular ligaments stabilize the [42] joint. The rotator cuff tendons stabilize the [43] joint, but the most important stabilizer of this joint is the tendon of the [44] muscle. Although the socket of this joint is very shallow, it is slightly deepened by a fibrocartilage ring called the [45]. A similar ring called the [46] deepens the [47] joint, the body's only other ball-and-socket joint. From their names, we can infer that the iliofemoral and ischiofemoral ligaments are part of the [48] joint; the coracohumeral ligament belongs to the [49] joint; the tibiofibular ligament belongs to the [50] joint; and the oblique popliteal ligament is found at the [51] joint. The names of bursae are similarly informative. We would expect to find the popliteal and infrapatellar bursae at the [52] joint and the olecranon bursa at the [53] joint. The knee, which is especially vulnerable to sports injuries, exhibits two cartilages in the joint cavity called the lateral and medial [54] and an X-shaped pair of ligaments within the joint capsule called the anterior and posterior [55] ligaments.

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