SKELETAL SYSTEM OUTLINE 9.1 Articulations (Joints) 253 9.1a Classification of Joints 253 9 9.2 Fibrous Joints 254 9.2a Gomphoses 254 9.2b Sutures 255 9.2c Syndesmoses 255 Articulations 9.3 Cartilaginous Joints 255 9.3a Synchondroses 255 9.3b Symphyses 256 9.4 Synovial Joints 256 9.4a General Anatomy of Synovial Joints 257 9.4b Types of Synovial Joints 258 9.4c Movements at Synovial Joints 260 9.5 Selected Articulations in Depth 265 9.5a Joints of the Axial Skeleton 265 9.5b Joints of the Pectoral Girdle and Upper Limbs 268 9.5c Joints of the Pelvic Girdle and Lower Limbs 274 9.6 Disease and Aging of the Joints 282 9.7 Development of the Joints 284 MODULE 5: SKELETAL SYSTEM mck78097_ch09_252-287.indd 252 2/14/11 2:58 PM Chapter Nine Articulations 253 ur skeleton protects vital organs and supports soft tissues. Its The motion permitted at a joint ranges from no movement (e.g., O marrow cavity is the source of new blood cells. When it inter- where some skull bones interlock at a suture) to extensive movement acts with the muscular system, the skeleton helps the body move. (e.g., at the shoulder, where the arm connects to the scapula). The Although bones are slightly flexible, they are too rigid to bend so they structure of each joint determines its mobility and its stability. There is meet at joints, which anatomists call articulations. In this chapter, an inverse relationship between mobility and stability in articulations. we examine how bones articulate and may allow some freedom of The more mobile a joint is, the less stable it is; and the more stable movement, depending on the shapes and supporting structures of a joint is, the less mobile it is. Figure 9.1 illustrates the “tradeoff” the various joints. between mobility and stability for various joints. 9.1a Classification of Joints Joints are categorized structurally on the basis of the type of con- 9.1 Articulations (Joints) nective tissue that binds the articulating surfaces of the bones, and Learning Objectives: whether a space occurs between the articulating bones: 1. Describe the general structure of articulations. ■ A fibrous (fı̄ ́brŭ s) joint occurs where bones are held 2. Discuss the connection between degree of movement and together by dense regular (fibrous) connective tissue. joint structure. ■ A cartilaginous (kar-ti-laj ́i-nŭ s; cartilago = gristle) joint 3. Identify both the structural and functional classifications of occurs where bones are joined by cartilage. joints. ■ A synovial (si-nō v́ ē -ă l) joint has a fluid-filled joint cavity that separates the cartilage-covered articulating surfaces of A joint, or articulation (ar-tik-ū -lā sh́ ŭ n), is the place of con- the bones. The articulating surfaces are enclosed within a tact between bones, between bone and cartilage, or between bones capsule, and the bones are also joined by various ligaments. and teeth. Bones are said to articulate with each other at a joint. The scientific study of joints is called arthrology (ar-throl ́ō -jē ; Joints may also be classified functionally based on the extent arthron = joint, logos = study). of movement they permit: ■ A synarthrosis (sin ar-thŕ ō sis;́ pl., sin ar-thŕ ō ś ē z; syn = joined together) is an immobile joint. ■ An amphiarthrosis (am ́fi-ar-thrō sis;́ pl., -sē z; amphi = Study Tip! around) is a slightly mobile joint. ■ = You can figure out the names of most joints by piecing together A diarthrosis (dı̄ -ar-thrō sis;́ pl., -sē z; di two) is a freely the names of the bones that form them. For example, the glenohumeral mobile joint. joint is where the glenoid cavity of the scapula meets the head of the The following discussion of articulations is based on their humerus, and the sternoclavicular joint is where the manubrium of the structural classification, with functional categories included as sternum articulates with the sternal end of the clavicle. appropriate. As you read about the various types of joints, it may help you to refer to the summary of joint classifications in table 9.1. Mobility Most mobile Immobile Glenohumeral joint Hip joint Elbow joint Intervertebral joints Suture (shoulder) Stability Very unstable Most stable Figure 9.1 Relationship Between Mobility and Stability in Joints. In every joint, there is a “tradeoff” between the relative amounts of mobility and stability. The more mobile the joint, the less stable it is. Conversely, the more stable the joint, the less mobile it is. Note how the glenohumeral (shoulder) joint is very mobile but not very stable, while a suture is immobile and yet very stable. mck78097_ch09_252-287.indd 253 2/14/11 2:58 PM 254 Chapter Nine Articulations Table 9.1 Joint Classifi cations Structural Structural Structural Category Example Functional Classifi cation Classifi cation Characteristics Fibrous Dense regular connective Gomphosis: Periodontal Tooth to jaw Synarthrosis (immobile) tissue holds together the membranes hold tooth to ends of bones and bone bony jaw parts; no joint cavity Suture: Dense regular Lambdoid suture (connects Synarthrosis (immobile) connective tissue connects occipital and parietal bones) skull bones Syndesmosis: Dense regular Articulation between radius Amphiarthrosis (slightly mobile) connective tissue fi bers and ulna, and between tibia (interosseous membrane) and fi bula between bones Cartilaginous Pad of cartilage is wedged Synchondrosis: Hyaline Epiphyseal plates in growing Synarthrosis (immobile) between the ends of bones; cartilage plate between bones; costochondral joints no joint cavity bones Symphysis: Fibrocartilage Pubic symphysis; Amphiarthrosis (slightly mobile) pad between bones intervertebral disc articulations Synovial Ends of bones covered Plane joint: Flattened or Plane joint: Intercarpal joints, Diarthrosis (freely mobile) with articular cartilage; slightly curved faces slide intertarsal joints joint cavity separates the across one another articulating bones; enclosed Hinge joint: Permits angular Hinge joint: Elbow joint by a joint capsule, lined by a movements in a single synovial membrane; contains plane synovial fl uid Pivot joint: Permits rotation Pivot joint: Atlantoaxial joint only Biaxial Diarthrosis (freely mobile) Condylar joint: Oval Condylar joint: MP articular surface on one (metacarpophalangeal) joints bone closely interfaces with a depressed oval surface on another bone Saddle joint: Saddle-shaped Saddle joint: Articulation articular surface on one between carpal and fi rst bone closely interfaces metacarpal bone with depressed surface on another bone Multiaxial (triaxial) Ball-and-socket joint: Diarthrosis (freely mobile) Ball-and-socket joint: Glenohumeral joint, hip joint Round head of one bone rests within cup-shaped depression in another bone WHATW DID YOU LEARN? the articulating bones). The three types of fibrous joints are gom- ●1 What type of articulation uses dense regular connective tissue to phoses, sutures, and syndesmoses (figure 9.2). bind the bones? 9.2a Gomphoses ●2 What term is used to describe immobile joints? A gomphosis (gom-fō sis;́ pl., -sē z; gomphos = bolt, osis = condi- tion) resembles a “peg in a socket.” The only gomphoses in the human body are the articulations of the roots of individual teeth with the sockets of the mandible and the maxillae. A tooth is held firmly in place by a fibrous periodontal (per ḗ -ō -don t́ ă l; peri = 9.29.2 FibrousFibrous JointsJoints around, odous = tooth) membrane. This joint is functionally clas- LearningLearning OObjectives:bjectives: sified as a synarthrosis. The reasons orthodontic braces can be painful and take a 1. DescribeDescribe thethe characteristics ofof the three types ofof fibrousfibrous joints.joints. long time to correctly position the teeth are related to the gom- 2. IdentifyIdentify locations ofof gomphoses, sutures, and syndesmoses phosis architecture. The orthodontist’s job is to reposition these in tthehe body.body. normally immobile joints through the use of bands, rings, and Articulating bones are joined by dense regular connective braces. In response to these mechanical stressors, osteoblasts and tissue in fibrous joints. Most fibrous joints are immobile or only osteoclasts work together to modify the alveolus, resulting in the slightly mobile. Fibrous joints have no joint cavity (space between remodeling of the joint and the slow repositioning of the teeth. mck78097_ch09_252-287.indd 254 2/14/11 2:58 PM Chapter Nine Articulations 255 Suture Ulna Radius Syndesmosis Root of (interosseous tooth membrane) Periodontal membranes Gomphosis Alveolar process of mandible (a) Gomphosis (b) Suture (c) Syndesmosis Figure 9.2 Fibrous Joints. Dense regular connective tissue binds the articulating bones in fibrous joints to prevent or severely restrict movement. (a) A gomphosis is the immobile joint between a tooth and the jaw. ( b) A suture is an immobile joint between bones of the skull. (c) A syndesmosis permits slight mobility between the radius and the ulna. 9.2b Sutures Sutures (soo choor;́ sutura = a seam) are immobile fibrous joints 9.3 Cartilaginous Joints (synarthoses) that are found only between certain bones of Learning Objectives: the skull. Sutures have distinct, interlocking, usually irregular edges that both increase their strength and decrease the number 1. Discuss the characteristics of the two types of cartilaginous of fractures at these articulations. In addition to joining bones, joints. sutures permit the skull to grow as the brain increases in size 2. Name locations of synchondroses and symphyses in the body. during childhood. In an older adult, the dense regular connec- The articulating bones in cartilaginous joints are attached tive tissue in the suture becomes ossified, fusing the skull bones to each other by cartilage. These joints lack a joint cavity. The together. When the bones have completely fused across the two types of cartilaginous joints are synchondroses and symphy- suture line, these obliterated sutures become synostoses (sin- ses (figure 9.3). os-tō ś ē z; sing., -sis).