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

Fibrous joints are subdivided into [7] sutures , the joints between most skull bones; [8] gomphoses , the joints between a tooth and the mandibular or maxillary bone; and [9] syndesmoses , 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] serrate suture . The fibers joining a tooth to its bony socket constitute the [11] periodontal ligament. Some long bones are bound to each other by a/an [12] interosseous membrane that extends along the shafts of the two bones, such as the radius and ulna or the tibia and fibula. In a synchondrosis, two bones are joined by [13] hyaline cartilage —for example, where the distal end of a rib is joined to the sternum. In a [14] symphysis , 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 and then form a joint called a [15] synostosis, or bony joint .

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

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

The mobility of a joint is called its [33] range of motion (ROM) . 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] fulcrum . The portion between that point and the point where your muscle applies force to a bone is called the [35] effort arm . The portion between 34 and the point where the bone acts against a load or resistance is called the [36] resistance arm . The ratio of 35 to 36 is called the [37] mechanical advantage (MA) of the lever. If this value is less than 1.0, the lever will produce more [38] speed, or distance but less [39] force than the input. This is always true of a [40] third -class lever—one in which the effort is applied between the fulcrum and resistance. In a [41] first -class lever, the fulcrum is between the effort and resistance, and value 36 can be either greater or less than 1.0.

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] temporomandibular (jaw) joint. The rotator cuff tendons stabilize the [43] humeroscapular, or glenohumeral (shoulder) joint, but the most important stabilizer of this joint is the tendon of the [44] biceps brachii muscle. Although the socket of this joint is very shallow, it is slightly deepened by a fibrocartilage ring called the [45] glenoid labrum . A similar ring called the [46] acetabular labrum deepens the [47] coxal, or hip 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 47 joint; the coracohumeral ligament belongs to the [48] glenohumeral, or shoulder joint; the tibiofibular ligament belongs to the [49] talocrural, or ankle joint; and the oblique popliteal ligament is found at the [50] tibiofemoral, or knee joint. The names of the bursae are similarly informative. We would expect to find the popliteal and infrapatellar bursae at the 50 joint and the olecranon bursa at the [51] humeroulnar, or elbow joint. The knee, which is especially vulnerable to sports injuries, exhibits two cartilages in the joint cavity called the lateral and medial [52] menisci and an X-shaped pair of ligaments within the joint capsule called the anterior and posterior [53] cruciate ligaments.